DE19708011A1 - Method for detecting sensor element resistances - Google Patents

Abstract

Element resistance of an oxygen sensor is determined by setting a time constant corresponding to a frequency at which the impedance characteristics of the sensor are stable, applying a changing voltage having the time constant, and determining the resulting current and hence the sensor impedance. Air-fuel ratio sensor 30 generates linear signals proportional to concentration of oxygen in exhaust gas from an engine 11. A bias command signal vr is converted to an analog signal Vb by D/A converter 21 and high frequency components are removed by low pass filter LPF 22. The output voltage vc of the LPF 22 is provided to a bias control circuit 40. A single AC signal which has a predetermined frequency and which is provided with a predetermined time constant of about 159 ~s by the LPF 22 is applied to the air-fuel ratio sensor 30. Element resistance R of the air-fuel ratio sensor 30 is detected based on the voltage of the AC signal and the change in the current level Ip of the air-fuel ratio sensor 30 caused by the application of the AC signal.

Description

The invention relates to an oxygen concentration sensor or oxygen sensor for the detection of oxygen content, for example, in the exhaust gas from a vehicle engine. in the individual, the invention relates to a method for Detection of element resistance due to voltage and Current-frequency responses of the oxygen sensor.

In recent years, the regulation of the Air / fuel ratio in a vehicle engine for that Support lean burn and the like Demands made to improve the control accuracy. To the Fulfilling these requirements was for the linear registration of the Air / fuel ratio in one in the machine sucked-in mixture, namely the oxygen content in the Exhaust gas is a linear one called a lambda probe Air / fuel ratio sensor (oxygen concentration sensor or oxygen sensor) proposed and executed. To the To maintain the measuring accuracy of the probe, the lambda This type of probe should be kept in an active state. in the generally this makes this active state maintain that by controlling the turning on of an in the probe attached heating element a sensor element of Probe is heated.

With this switching control of the heating element hereinafter simply referred to as "element temperature" Detected temperature of the sensor element and then a Return control implemented such that the element finally a desired activation temperature (e.g. around 700 ° C). In this case, though the element temperature according to a measurement result at the Sensor element attached temperature sensor detected in real time be increased, but the attachment of the temperature sensor probably the total cost of the sensor device. That's why suggestions have been made, which are simply referred to below as “Element resistance” denotes the resistance of the sensor element to capture and based on a predetermined relationship between the element resistance and the element temperature  this way the element from the detected element resistance derive temperature. It should be noted that the captured Element resistance also for determining one, for example Deterioration of the properties of the probe can be.

FIG. 46A and 46B are diagrams illustrating a conventional method for detecting the resistance element, as disclosed for example in U.S. Patent No. 4,543,176. These figures illustrate a case in which a current limiting oxygen sensor is used as the lambda probe for machine control. As shown in FIG. 46A, a predetermined voltage, namely a positive voltage Vpos, is applied to the sensor element before a time t11 for measuring the air / fuel ratio. The air / fuel ratio is determined on the basis of a sensor current Ipos, which is produced in accordance with FIG. 46B in accordance with the applied voltage Vneg. Between time t11 and time t12, a negative voltage Vneg for measuring the element resistance is applied and a corresponding sensor current Ineg is detected. Then the negative voltage Vneg is divided by the sensor current Ineg in order to obtain the element resistance ZDC (ZDC = Vneg / Ineg). This method is commonly known as a method of sensing element resistance based on the DC properties of the lambda probe.

During this conventional method of detecting the Element resistance, namely the DC component of the Impedance by applying a DC voltage to the sensor element is used is in U.S. Patent 4,419,190 Method for detecting element resistance by applying an AC voltage to the sensor element described. At This process is continuously applied to the lambda probe AC voltage applied and the sensor output signal from the Lambda probe is passed through a low pass filter in order to Air / fuel ratio. The same Sensor output signal is passed through a high pass filter and averaged to measure AC impedance. This The method is generally considered to be the method of capturing the  Element resistance by using the alternating current properties of the lambda probe known.

In all of the conventional methods described above however, there are the following problems: DC impedance method using a negative voltage Vneg Rectangular waveform is applied, the sensor current changes Ineg very quickly and therefore it can be the peak current of the probe cannot be grasped exactly. For this reason it is Measuring the peak current until the sensor stabilizes suspend current. Accordingly, during a period of time, that Air / fuel ratio cannot be recorded. This problem, namely that the peak current during the application of the voltage is not possible with the rectangular curve shape probably also in the method of capturing the element resistance due to the AC properties.

Since the one disclosed in U.S. Patent 4,419,190 AC impedance method the sensor output signal to Detection of the air / fuel ratio via the low pass filter can also cause problems with the Ratio output signal such as a phase shift or a AC voltage interference signal occur. These problems occur especially when the machine is in one Transitional state is operated.

In view of the above problems with the Prior art, the invention is based on the object Detecting the element resistance of an oxygen sensor To create a process which is aimed at the Element resistance to be recorded accurately and the time for to shorten the detection of the element resistance.

To achieve the object, the invention results in one first aspect a method for detecting the element resistance of the oxygen sensor with steps where a time constant is set, that of a frequency corresponds in which the impedance properties of the Oxygen sensor are stable to the oxygen sensor voltage that changes with the time constant is applied, a current change in the oxygen sensor due to the  changing voltage flowing current is determined and due to the changing voltage and current change of the Element resistance of the oxygen sensor is determined.

Because in this way the oxygen sensor is one with voltage changing at a predetermined time constant , there may be sudden changes in the current value of the oxygen sensor can be prevented and the Level of current change can be detected in an accurate manner. As a result the element resistance of the oxygen sensor can do this exactly be recorded.

Preferably, when changing, the changing Voltage applied a voltage that matches the Has time constant changing only voltage curve. To this Way compared to conventional methods in which the element resistance due to a sequence of continued Voltage signals is detected, the element resistance of the Oxygen sensor can be detected in a short time.

In this method, preference is given to setting the time constant determines a frequency range in which the Impedance properties of the oxygen sensor are stable, and it a time constant is set which corresponds to a predetermined one Frequency within this frequency range corresponds. The Impedance properties of the sensor element of the current limitation Oxygen sensors are at voltage change frequencies of at least 1 kHz evenly. Hence the time constant can be set to 159 µs, which is a cutoff frequency of corresponds to not less than 1 kHz.

When the changing voltage is applied preferably applied a voltage having a section negative slope and a section with positive slope has, the current change is determined, which is due to the Section with the negative slope or with the positive Inclination is due, and element resistance due to the voltage changing with the negative inclination and the Current change in their determination from the section with the negative inclination or from which deals with the positive inclination changing voltage and the current change when determining it  determined based on the section with the positive slope becomes.

When the changing voltage is applied preferably when using the oxygen sensor Lean mixture ratio is detected, one with one Section of positive slope changing voltage applied, and then when using the oxygen sensor a fat mixture ratio is detected, one with a section of negative slope changing voltage applied, when when with the Oxygen sensor the lean mixture ratio is detected due to the negative tendency section Current change is determined, or when with the Oxygen sensor the fat mixture ratio that is detected on current change due to the positive slope section is determined. In this way, the flow of oxygen sensors can be detected precisely without the dynamic range of the Sensor current detector is exceeded.

In another aspect, the invention provides Method for detecting the element resistance of the Oxygen sensor with a step that selectively one first and / or a second measuring method is carried out. The first measuring method comprises steps for setting a Time constant that corresponds to a frequency at which the Impedance characteristic of the oxygen sensor is uniform to Apply a voltage that changes with the time constant the oxygen sensor, to detect one on the changing Voltage due to current change in the oxygen sensor flowing current and to determine the element resistance of the oxygen sensor from the changing Voltage and current change. The second measurement method includes Steps in which the oxygen sensor is switched off a change in the voltage on the oxygen sensor after the Shutdown of the same is detected, one on the voltage change based current change with respect to the current of the Oxygen sensor is detected and the element resistance from the Voltage change and the current change is determined.  

In this way, when the output current of the Oxygen sensor close to or center of the detectable current range is to determine the element resistance of the Oxygen sensor the first measurement method can be applied. On the other hand, if the output current is close to the maximum value or the minimum value of the current range can be used for the exact Detecting the element resistance of the oxygen sensor second measurement methods are used. So that can be for the Suitable detection of the current of the oxygen sensor Measuring method selected according to the current level of the same will.

The invention is based on execution examples explained with reference to the drawing.

Fig. 1 is a schematic representation of an air / fuel ratio measurement means according to a first embodiment of the invention,

Fig. 2 is a sectional view showing in detail the structure of an oxygen sensor of the measuring device,

Fig. 3 is a graph of voltage / current characteristics of the lambda probe,

FIGS. 4A and 4B show waveforms of an output voltage of a D / A converter and an output voltage of low-pass filters of the measuring device,

Fig. 5 is an equivalent circuit diagram of the lambda probe,

Fig. 6 is a graphical representation of impedance characteristics of the circuit of Fig. 5 with respect to the frequency of an input AC voltage,

Fig. 7 is a graph illustrating the relationship between the frequency of the AC voltage input to the lambda probe and the total impedance of the lambda probe,

Fig. 8 is a circuit diagram of a Vorspannungssteuer circuit according to the first embodiment,

Fig. 9 is a flowchart of a main routine, the fuel ratio is carried out measuring device according to the first embodiment by a microprocessor of the air /

Fig. 10 is a flowchart of a, by the micro-processor constructed in accordance with the first embodiment subroutine for detecting element resistance

FIG. 11A to 11D are graphs showing examples of changes in the voltage applied to the lambda probe voltage and caused by these voltage changes respective current changes show

Fig. 12 is a graph showing the resistance the relation between the element temperature and the member of the lambda probe illustrated

FIG. 13A to 13D, 14A to 15D are graphs showing the other variants of changes in the voltage applied to the lambda sensor voltage and current variations caused by these voltage changes are each show to 14D and 15A,

Fig. 16 is a flow chart of a subroutine for resistance measurement element according to a second execution of the invention,

Fig. 17 is a graph illustrating the relationship between voltage / current characteristics of the lambda probe and its dynamic range,

Fig. 18 is a graph showing the relationship between voltage / current characteristics of the lambda probe and its dynamic range, the range is set only for the lean mixture,

Fig. 19 is a flow chart of a subroutine for resistance measurement element according to a third example of the invention execution,

FIG. 20A and 20B are graphical representations of the voltage / current characteristics of oxygen sensors having mutually different element temperatures,

FIG. 21A and 21B are graphs illustrating the relationship between the voltage applied to the lambda probe voltage and the current sensor of the lambda probe,

FIG. 22A and 22B are graphs illustrating the relationship between the voltage applied to the lambda probe voltage and the sensor current of the lambda probe,

Fig. 23 is a graph illustrating the relationship between the frequency of an input into the lambda probe AC voltage and the AC impedance of the lambda probe,

Fig. 24 is a schematic diagram of a switching unit according to a fourth embodiment of the invention,

Fig. 25 is a schematic representation of the air / fuel ratio measurement means according to the fourth embodiment,

Fig. 26 is a circuit diagram of a Vorspannungssteuer circuit according to the fourth embodiment,

Fig. 27 is a flow chart of a subroutine for limiting current measurement according to the fourth embodiment,

Fig. 28 is a flow chart of a subroutine for the element resistance measurement according to the fourth execution,

Fig. 29 is a graph concretely illustrating the operation in the fourth embodiment,

FIG. 30A to 30C are graphs showing the voltage / current characteristic curves show the lambda probe outside the dynamic range of a current measuring circuit,

FIG. 31A to 31C are graphs showing the voltage / current characteristic curves show the lambda probe in which the probe is switched off,

Fig. 32 is a graph showing the result of the shutdown of the lambda probe detected element resistance Re illustrates a relationship between a detected due to a voltage change element resistance R and the characteristic curve,

FIG. 33A and 33B are graphs for comparing a member based on the voltage change resistance measurement method by a factor based on the deactivation of the lambda sensor element resistance measurement method,

Fig. 34 is a schematic illustration of an air / fuel ratio measurement means according to a fifth embodiment of the invention,

Fig. 35 is a circuit diagram of a Vorspannungssteuer circuit according to the fifth embodiment,

Fig. 36 is a flow chart of a subroutine for the element resistance measurement according to the fifth execution,

Fig. Is a flowchart of a routine for 37 based on the voltage change measuring the resistance element according to the fifth embodiment,

Fig. 38 is a flowchart of a routine for based on the deactivation of the lambda probe measuring the resistance element according to the fifth embodiment,

Fig. 39 is a voltage / current characteristic diagram showing the current ranges in which the based on the voltage change element resistance measurement method is applied and based on the temporary deactivation of the lambda sensor element resistance measurement method is applied,

Fig. 40 is a voltage / current characteristic diagram showing the current ranges in which the based on the voltage change element resistance measurement method is applied and based on the temporary deactivation of the lambda sensor element resistance measurement method is applied,

Fig. 41 is a sectional view showing in detail the construction of an integrated oxygen sensor according to a sixth embodiment of the invention,

Fig. 42 is a graph illustrating the relationship between an electromotive voltage of the integrated sensor lambda sensor and the air / fuel ratio according to the sixth embodiment,

Fig. 43 is a graph illustrating the relationship between a pumping current in the integrated lambda sensor and the air / fuel ratio,

Fig. 44 is a circuit diagram of a Vorspannungssteuer circuit according to the sixth embodiment,

FIG. 45A and 45B illustrate waveforms of a voltage change and a corresponding change in current according to the sixth embodiment,

FIG. 46A and 46B illustrate waveforms of a voltage change and a current change caused by such a change in voltage in a conventional resistive element measuring method.

The following is a first embodiment described in which the invention in an air / fuel Ratio measuring device is applied. The measuring device according to this embodiment, for a gasoline engine Electronic fuel injection control used in a vehicle is installed. An air / fuel ratio control system from The machine controls the amount of fuel to be injected into the machine Fuel according to that detected by the measuring device Air / fuel ratio to a desired one To achieve air / fuel ratio. Be below detailed the procedures for detecting the air / fuel Ratio A / F using a lambda probe and for measuring the Element resistance using the AC properties of the lambda probe explained.

Fig. 1 is a schematic diagram showing a configuration of the air / fuel ratio measurement means of this embodiment. Referring to FIG. 1, the measuring device includes a designated hereinafter referred to as lambda probe current limiting air / fuel ratio sensor 30 serving as oxygen concentration sensor. The lambda probe 30 is installed in an exhaust pipe 12 which extends away from an engine block 11 of an engine 10 . In response to the application of a voltage as guided by a microprocessor 20 , the lambda probe 30 generates a linear lambda measurement signal with a value that is proportional to the oxygen content in the exhaust gas. The microprocessor 20 contains a central processing unit CPU, a read-only memory ROM, a read / write memory RAM and the like. The microprocessor 20 controls a bias control circuit 40 and a heating control circuit 25 according to a predetermined control program, which will be described below.

FIG. 2 is a schematic sectional view of the lambda probe 30. Referring to FIG. 2, the oxygen sensor 30 protrudes into the interior of the exhaust pipe 12. The lambda probe 30 mainly consists of a cover 31 , a main sensor part 32 known as a sensor element and a heating element 33 . The cover 31 has a U-shaped cross section. In the periphery of the cover 31 , a large number of small holes 31 a for forming the connection between the inside and the outside of the cover 31 is formed. The sensor element 32 generates a limit current which corresponds to the oxygen concentration in a region in which the air / fuel mixture is lean or in a region in which the mixture is rich, the concentration of unburned gases (CO, HC, H₂ and the like) corresponds.

The structure of the sensor element 32 is described in detail below. In the sensor element 32 of a solid electrolyte layer 34 has the U-shaped cross section, an exhaust-side electrode layer 36 is applied to the outer surface, while layer on the inner surface of the solid electrolyte 34, an atmosphere-side electrode layer is applied 37th Furthermore, a diffusion resistance layer 35 is formed on the outer surface of the exhaust gas side electron layer 36 by plasma spraying or the like. The dry electrolyte layer 34 has a sintered body made of an oxide conductive for oxygen ions, which is obtained by adding ZrO₂, HfO₂, ThO₂, Bi₂O₃ or the like with CaO, MgO, Y₂O₃, Yb₂O₃ or the like as a stabilizer. The diffusion resistance layer 35 consists of a heat-resistant inorganic material such as aluminum oxide, magnesium oxide, silicate material. Spinel, mullite or the like. The two electron layers 36 and 37 on the part of the exhaust gases and on the part of the atmosphere are formed from a noble metal which has high catalyst properties such as platinum or the like, and the surface of which is coated with porous chemical materials. The area and the thickness of the exhaust-side electron layer 36 are respectively about 10 to 100 mm² and about 0.5 to 2.0 mm, while the atmosphere-side electron layer 37 has an area of less than 10 mm² and a thickness of about 0.5 to 2.0 mm.

The heating element 33 is arranged within the atmosphere-side electrode layer 37 and generates thermal energy for heating the sensor element 32 (which consists of the atmosphere-side electron layer 37 , the dry electrolyte layer 34 , the exhaust-side electron layer 36 and the diffusion resistance layer 35 ). The heating element 33 has a heat generation capacity which is sufficient for activating the sensor element 32 .

In the lambda probe 30 configured in the manner described above, the sensor element 32 calls a limit current corresponding to the oxygen concentration in a range in which the air / fuel ratio is in the lean mixture range with respect to a theoretical value of the air / fuel ratio. In this case, the limit current corresponding to the oxygen content is determined by the area of the electrode layer 36 on the exhaust gas side and by the thickness, the porosity and the average hole diameter of the diffusion resistance layer 35 . The sensor element 32 is indeed suitable for linear detection of the oxygen concentration, but it is impossible to control the activation area solely by using the heat of the exhaust gases from the machine 10 , since the temperatures required for activating the sensor element 32 are high temperatures above 600 ° C. are and the temperature range for keeping the sensor element 32 active is relatively narrow. Therefore, in this embodiment, the sensor element 32 is heated by heat control with the heating element 33 up to the activation temperature range. On the other hand, in the area where the air / fuel ratio is in relation to the stoichiometric air / fuel ratio in the rich mixture area, the concentration of unburned gases such as carbon monoxide CO and the like changes substantially linearly with respect to changes in the air / Fuel ratio. In this case, the sensor element 32 produces a limit current which corresponds to the concentration of CO and the like.

The voltage / current characteristics of the sensor element 32 are described with reference to FIG. 3. According to FIG. 3, a linear relationship can be observed between the current flowing into the dry electrolyte layer 34 of the sensor element 32 , which is proportional to the air / fuel ratio detected by the lambda probe 30 , and the voltage applied to the dry electrolyte layer 34 . The limit current of the sensor element 32 is represented by the straight curve sections parallel to the voltage axis V. The limit current (sensor current) becomes stronger or lower in proportion to the ratio A / F, namely depending on whether the air / fuel mixture is leaner or richer. That is, the larger the air / fuel ratio A / F, the stronger the limit current. On the other hand, the lower the A / F ratio, the weaker the limiting current.

In the figure representing the voltage / current characteristic curves, the voltage ranges below the straight curve sections parallel to the voltage axis V are resistance-dependent ranges in which the gradient of a straight line, e.g. B. a line L1 is determined by the internal resistance of the dry electrolyte layer 34 of the sensor element 32 , which corresponds to the element resistance. This element resistance changes in proportion to the sensor temperature and therefore this gradient becomes smaller as the element resistance increases as the temperature of the sensor element 32 becomes lower.

As shown in Fig. 1, the microprocessor 20 to a D / A converter 21 outputs a digital bias control signal Vr for applying voltage to the lambda probe 30 , which by the D / A converter 21 becomes an analog signal Vb implemented and then delivered to a low-pass filter TPF 22 . An output voltage Vc is then output to the bias voltage control circuit 40 , which is generated by the low-pass filter 22 by eliminating high-frequency components from the analog signal Vb. This bias control circuit 40 is designed to apply to the lambda probe 30 either a voltage to sense the air / fuel ratio A / F or a voltage to measure the element resistance. When the ratio A / F is detected, the characteristic curve L1 shown in FIG. 3 is used to set the applied voltage Vp in accordance with the corresponding ratio A / F. On the other hand, when the element resistance is detected, a single voltage signal having a predetermined time constant, which is a predetermined frequency for changing the applied voltage, is applied to the lambda probe 30 .

The bias control circuit 40 includes a current measurement circuit 50 that detects the current that is generated after voltage is applied to the lambda probe 30 . An analog signal, which indicates the current value detected by the current measuring circuit 50 , is fed to the microprocessor 20 via an A / D converter 23 . A configuration of the bias control circuit 40 is described in detail below.

The heating control circuit 25 controls the function of the heating element 33 which is installed in the lambda probe 30 . That is, the heating control circuit 25 controls the turn-on factor of the electric power supplied to the heating element 33 from a battery power source (not shown) according to the element temperature of the lambda probe 30 or the heating element temperature to regulate the heating function of the heating element 33 .

The control voltage applied to the lambda probe 30 for sensing the element resistance is described in detail below. The microprocessor 20 generates the digital bias control signal Vr. This bias control signal Vr is converted after passing through the D / A converter 21 and the low pass filter 22 into an analog single or pulse voltage signal with a predetermined time constant. In Fig. 4A and 4B, examples of signal waveforms of the output voltage Vb from the D / A converter 21 and the output voltage Vc from the low-pass filter 22 are shown in detecting the element resistance. In this case, according to FIGS. 4A and 4B, at a time t1, the output voltage Vb from the D / A converter 21 is switched to a value which is higher by ΔV than the voltage Vp applied at time t1, namely the voltage for the Detect the A / F ratio. At a point in time t2, the output voltage is switched to a value which is lower than the above-mentioned applied voltage Vp only for a short period of time which is shorter than the period between times t1 and t2. The output voltage Vb is then reset to its original voltage value Vp at a time t3. On the other hand, the output signal Vc from the low-pass filter 22 is a signal whose high-frequency components are eliminated by introducing a predetermined time constant.

In this embodiment, as a voltage with a predetermined time constant denotes a signal which only contains a single frequency component. The determination of this Frequency component is described below.

The Fig. 5 is an equivalent circuit diagram of the lambda probe 30. The equivalent circuit of the sensor element 32 is shown in detail in FIG. 5. In this circuit, the particle resistance of the dry electrolyte layer 34 to oxygen ions with Rg is the particle resistance and the intergranular capacity of the dry electrolyte layer 34 at their grain interfaces, respectively with Ri and Ci, and the electrode interface resistance and the electrode limit capacity of the platinum electrode layers 36 and 37 with Rf and Cf, respectively.

FIG. 6 shows the complex impedance characteristics of the lambda probe 30 shown in FIG. 5. In this Fig. 6, a real part of the complex impedance Zreal is shown on the X-axis, while an imaginary part of the complex impedance Zimaginär is shown on the Y-axis. The impedance ZAC is expressed by the following equation:

ZAC = Zreal + jZimaginary (1)

In FIG. 6, the impedance is shown with a change of the applied voltage with a frequency of 1 kHz at a point A. If the frequency is lower than 1 kHz, the impedance characteristic is that to the right of point A, whereas if the frequency is above 1 kHz, the impedance characteristic is that to the left of point A. That is, if the frequency is close to 1 kHz, the sum of Rg and Ri is measured as the impedance.

FIG. 7 is a modification of FIG. 6, the frequency being shown on the X axis and the impedance ZAC being plotted on the Y axis. As shown in FIG. 7, the impedance ZAC of the lambda probe converges in the frequency range from 1 kHz to 10 MHz to the determined value Rg which is less than (Rg + Ri). Thus, the frequency range from 1 kHz to 10 MHz is best suited because in this range the impedance ZAC is constant regardless of the frequency f of changing or switching the applied voltage. In this embodiment, by setting the frequency to 1 kHz, the time constant is set by the low-pass filter 22 to approximately 159 μs in order to obtain the correspondingly increasing curve shape (time t1 to t2 in FIG. 4B). The lower limit of the time constant, namely the upper limit of the frequency, depends on the processing capability of the D / A converter 21 or the A / D converter 23 . The lower limit of the time constant can be further reduced by using faster circuits.

For these reasons, when the voltage to be applied to the lambda probe 30 is changed, the microprocessor 20 generates a digital signal which has frequency components with approximately 1 kHz. After passing through the D / A converter 21 and the low-pass filter 22, this digital signal is converted into a signal with a predetermined time constant (of approximately 159 μs). The control signal output by the microprocessor 20 can be generated relatively easily since it has a rectangular shape.

On the other hand, if the voltage for sensing the element resistance is switched back to the voltage for sensing the ratio A / F and this switching to the voltage for sensing the ratio is done directly, the sensor current reaches immediately after this switching due to the effects of Electrical charges stored in the capacitances Ci and Cf described above immediately peak, so that the time required for the current to converge to its original value is therefore increased. As a result, in this embodiment, when switching from the voltage for sensing the element resistance to the original voltage, which is the voltage for sensing the ratio A / F, a voltage becomes short (from t2 to t3 in FIG. 4A) with a polarity opposite to the polarity of the voltage previously applied (in the time t1 to t2 according to FIG. 4A), in order to completely discharge the electrical charges in the capacitances Ci and Cf within a short period of time and also to increase the time for stabilizing the sensor current shorten. In this case, the aim should be to control the voltage in such a way that essentially the same amount of electrical charges is moved in the sensor element if the voltage applied is to one polarity (which can be the positive or negative polarity) and if thereafter that applied voltage is switched to the other polarity. For this purpose, when switching the applied voltage, this voltage can have positive and negative waveforms that are symmetrical to each other.

Next, the structure of the bias control circuit 40 will be described with reference to the circuit diagram shown in FIG. 8. According to Fig. 8, the bias control circuit 40 includes mainly a reference voltage switching circuit 44, a first voltage supply circuit 45, a second voltage supply circuit 47 and the current sensing circuit 50. The reference voltage circuit 44 divides a constant voltage Vcc with voltage dividing resistors 44 a and 44 b down to a constant reference voltage Va.

The first voltage supply circuit 45 is formed by a voltage follower circuit. This first voltage supply circuit 45 supplies a voltage, which is equal to the reference voltage Va of the reference voltage circuit 44 , to a terminal 42 of the lambda probe 30 , which is connected to the atmosphere-side electrode layer 37 shown in FIG. 2. In detail, the first voltage includes supplying circuit 45 includes an operational amplifier 45 a, its non-inverting input terminal is connected to a voltage dividing point between the voltage dividing resistors 44 a connected to b and 44 and its inverting input terminal connected to one terminal 42 of the oxygen sensor 30 is connected, a resistor 45 b, which is connected at one terminal to the output terminal of the operational amplifier 45a, an NPN transistor 45c and an PNP transistor 45 d. The bases of the NPN transistor 45 c and the PNP transistor 45 d are connected to the other terminal of the resistor 45 b. The collector of the NPN transistor 45 c is connected to the constant supply voltage Vcc and the emitter of the same is connected via a current measuring resistor 50 a to the one connection of the lambda probe 30 . This current measuring resistor 50 a represents the current measuring circuit 50. The emitter of the PNP transistor 45 d is connected to the emitter of the NPN transistor 45 c, while the collector of the PNP transistor 45 d is connected to ground.

The second voltage supply circuit 47 is also formed by a voltage follower circuit. This second voltage supply circuit 47 supplies a voltage, which is equal to the output voltage Vc from the low-pass filter 22 , to the other connection 41 of the lambda probe 30 , which is connected to the exhaust-side electrode layer 36 shown in FIG. 2. In detail, the second voltage supply circuit 47 includes an operational amplifier 47 a, its non-inverting input terminal is connected to the output terminal of the lowpass filter 22 and the inverting input terminal is connected to the other terminal 41 of the oxygen sensor 30, a resistor 47 b, connected to one terminal an NPN transistor 47 c and a PNP transistor 47 is connected to the output terminal of the operational amplifier 47 a, d. The bases of the NPN transistor 47 c and the PNP transistor 47 d are both connected to the other terminal of the resistor 47 b. The collector of the NPN transistor 47 c is connected to the constant voltage Vcc and the emitter of this transistor 47 c is connected to the other connection of the lambda probe 30 . The emitter of the PNP transistor 47 d is connected to the emitter of the NPN transistor 47 c, while the collector of the transistor 47 c is connected to ground.

With the structure described above, the one terminal 42 of the lambda probe 30 is constantly supplied with the constant voltage Va. If the other terminal 41 of the lambda probe 30 is supplied with a voltage Vc which is lower than the constant voltage Va via the low-pass filter 22 , the lambda probe 30 receives a positive bias. If a voltage Vc is supplied to the other connection of the lambda probe 30 via the low-pass filter 22 , which is higher than the constant voltage Va, the lambda probe 30 is negatively biased.

The operation of the air-fuel ratio measuring device having the structure described above will be explained below. The Fig. 9 is a flowchart of a control procedure according to this embodiment. This control procedure is carried out upon the start of the power supply to the microprocessor 20 . According to FIG. 9, the microprocessor 20 determines in step 100 whether or has expired after the last detection of the air / fuel ratio A / F for a predetermined time T1. The predetermined time T1 corresponds to the frequency for detecting the ratio A / F. Preferably T1 is set to e.g. B. set 2 to 4 ms. If the predetermined time T has elapsed after the last detection of the ratio A / F, the microprocessor 20 receives an affirmative answer in step 100 and control proceeds to step 110. In step 110, the microprocessor 20 receives the sensor current Ip (limit current or threshold current) detected by the current measuring circuit 50 and, using a predetermined characteristic field, determines a corresponding value of the air / fuel ratio of the machine 10 , which is the sensor current Ip corresponds. Using the characteristic curve L1 shown in FIG. 3, the microprocessor 20 applies a voltage Vp to the lambda probe 30 which corresponds to the detected ratio A / F.

Furthermore, the microprocessor 20 determines in a step 120 whether a predetermined time T2 has expired after the last detection of the element resistance. The predetermined time T2 corresponds to the frequency of the element resistance detection. This predetermined time is set selectively, for example, according to the operating state of the machine 10 . In this embodiment, when a relatively small change in the ratio A / F occurs, namely, the machine operating condition is normal, the predetermined time T2 is set to 2 s. On the other hand, in the event of a sudden change in the ratio A / F, namely a transitional operating state of the machine, the time T2 is set to 128 ms. If the microprocessor 20 receives a negative determination at step 120, the ratio A / F is detected each time the predetermined time described above elapses. If the microprocessor 20 receives a positive answer in step 120, the element resistance is subsequently measured in step 130.

A procedure for measuring element resistance will be described with reference to a subroutine shown in FIG. 10. Referring to FIG. 10, the microprocessor 20 determines in a step 131, is whether the mixture with the current ratio A / F, a rich or a lean mixture. If the ratio A / F indicates a lean mixture, the microprocessor 20 switches the applied voltage Vp, namely the ratio measurement voltage, from negative to positive in a step 132. If the ratio A / F corresponds to a rich mixture, the applied voltage Vp changes from positive to negative in a step 133, whereby the bias control signal Vr is changed.

Then, in a step 134, after switching the applied voltage, the microprocessor 20 records the amount ΔV of the voltage change and the amount ΔI of the change in the sensor current detected by the current measuring circuit 50 . In a subsequent step 135 the element resistance R is calculated by the microprocessor 20 from AV and ΔI (R = ΔV / ΔI). After step 135, control returns to the original main routine.

In FIGS. 11A to 11D are waveforms of voltages applied to the oxygen sensor 30 (output voltages Vc 22 after passing through the low pass filter) and waveforms of the current sensor shown, the flow at the application of these voltages. That is, if the ratio A / F corresponds to the lean mixture, e.g. B. is equal to 18, the voltage applied to the lambda probe 30 is switched by a voltage amount .DELTA.V toward the negative polarity, and a corresponding current change .DELTA.I of the current value is detected toward the negative polarity, as shown in FIGS . 11A and 11B. The applied voltage a [V] and the sensor current b [A] shown in Figs. 11A and 11B correspond to points a and b in Fig. 3, respectively . On the other hand, if the ratio A / F corresponds to the rich mixture, e.g. . B. is equal to 13, as shown in FIGS. 11C and 11D, the voltage applied to the lambda probe 30 is switched over to the positive polarity by the voltage amount .DELTA.V and a corresponding current change .DELTA.I of the current value becomes the positive polarity detected. The applied voltage c and the sensor current d shown in FIGS . 11C and 11D correspond to points c and d in FIG. 3, respectively.

Since the sensor current is determined for the lean mixture ratio A / F according to a voltage change to the negative side or for the fat mixture ratio A / F according to a voltage change to the positive side, the sensor current never comes from the (in Fig. 3 shown) dynamic range of the current measuring circuit 50 out.

On the other hand, as shown in FIG. 12, there is a certain relationship between the sensor element resistance R, which can be determined in the manner described above, and the sensor element temperature. That is, as soon as the element temperature drops, the element resistance R increases considerably. As shown in this figure, an element resistance R of 90 Ω corresponds to an element temperature of 600 ° C which is a temperature at which the lambda probe 30 is activated to a certain extent, while an element resistance R of 30 Ω corresponds to an element temperature of 700 ° C corresponds to a temperature at which the lambda probe 30 is fully activated. In the case of the heating element control, the amount of power is calculated which is to be supplied to the heating element 33 in order to cancel a difference between the calculated element resistance R and a resistance setpoint (of, for example, 30 Ω) at which the lambda probe 30 is fully activated, and for controlling the Activation factor of the power supply to the heating element 33 is used. That is, feedback control based on the element temperature is carried out.

Which is characterized by the execution described above Examples of advantages are listed below:

• (a) In this embodiment, the voltage applied to the lambda probe 30 for detecting the air / fuel ratio is changed to the voltage for measuring the sensor element resistance of the lambda sensor 30 by setting a predetermined time constant for this voltage change, and the element resistance of the sensor element of the lambda probe 30 is determined from the change in voltage and a change in current caused by the change in voltage. Therefore, according to this embodiment, the steep current rise, which is a problem observed in the prior art, can be prevented at the oxygen sensor during the switching from the applied voltage to the voltage for measuring the element resistance. As a result, the sensor current values can be measured accurately, which enables highly accurate detection of the element resistance of the lambda probe 30 . In this case, since the element resistance is detected by a single AC wave applied during the air-fuel ratio detection process, the time required for element resistance detection is shortened. Furthermore, the accuracy of the detection of the ratio A / F is not impaired and remains at a considerably high value even when the machine is operated in its transient state.
• (b) In this embodiment, the above-mentioned time constant is set to a value (less than 159 µs) that sets the cutoff frequency for switching the voltage for measuring the element resistance to 1 kHz. As a result, the frequency response of the lambda probe 30 becomes uniform as shown in FIG. 7. To further stabilize the impedance curve, the aim should be to limit the time constant within a range from 32 to 53 µs, which corresponds to the frequency range from 3 kHz to 5 kHz.
• (c) In this embodiment, the low-pass filter 22 is used to apply an AC signal having a predetermined time constant to the lambda probe 30 . As a result, it is possible to achieve the desired purpose with a much simpler structure. In this case, the microprocessor 20 only has to generate digital signals and does not have to perform any high-quality calculations. It is thus possible to provide an air / fuel ratio measuring device which can be manufactured in a simple manner.
• (d) In addition, in the design according to this embodiment, the element resistance is determined from a current change caused by the application of a voltage to the negative side when the ratio A / F corresponds to a lean mixture. On the other hand, the element resistance is detected from a current change caused by the application of a voltage to the positive side when the ratio A / F corresponds to a rich mixture. In any case, the element resistance is detected by using a current change which is directed into the dynamic range which is set for the current measuring circuit 50 from the outset. A deterioration in the measurement accuracy caused by current changes outside the dynamic range can thus be prevented. Another advantageous effect is that the dynamic range can be kept to a minimum range, that is, the dynamic range can be set within a narrow range in order to ensure a high measuring accuracy of the current measuring circuit 50 .
• (e) The voltage waveform is set so that when a positive voltage and a negative voltage of the same magnitude are applied to the lambda probe 30 in the sensor element, substantially the same amount of electric charge is moved. In this way, the convergence of the sensor current can be accelerated after the element resistance measurement has ended.
• (f) If, as described above, the element resistance can be detected in a very precise manner, it is also possible to increase the accuracy in the rules of activating the lambda probe 30 , namely controlling the power supply to the heating element 33 using the detected element resistance improve. In addition, the detected element resistance can also be used effectively to judge deterioration in the functions of the probe.

The switching of the voltage applied when measuring the element resistance can be modified as shown in FIGS. 13A to 15D. In the variant shown in FIGS. 13A to 13D, the voltage applied for measuring the element resistance is switched to both positive and negative voltages, as in the exemplary embodiment described above. In this variant, however, the extent of the voltage change ΔV and the extent of the current change ΔI are measured according to the second voltage curve and not according to the representation in FIGS. 11A to 11D according to the first voltage curve. If the ratio A / F corresponds to the lean mixture, e.g. B. is 18, as shown in FIGS. 13A and 13B, the voltage is changed from positive to negative voltage with respect to the voltage immediately before, namely the voltage for detecting the ratio A / F and the element resistance from the amount ΔV of the negative Voltage change and the amount ΔI of the negative current change are calculated. On the other hand, if the ratio A / F corresponds to the rich mixture, namely, for example, is equal to 13, the voltage is switched from negative to positive voltage with reference to the voltage applied immediately before, and the element resistance from the amount ΔV of the positive, as shown in FIGS. 13C and 13D Voltage change and the amount ΔI of the positive current change are calculated.

Furthermore, in the variant shown in FIGS. 14A to 14D, the applied voltage is changed only to one side, namely either to a positive or to a negative voltage. Then the amount ΔV of the voltage change and the amount ΔI of the current change are measured. If the ratio A / F corresponds to the lean mixture, as shown in FIGS . 14A and 14B, as in the respective previous exemplary embodiments, the applied voltage is set to a negative voltage and the element resistance is determined from the magnitude of the current change and the magnitude of the voltage change. On the other hand, when the ratio A / F corresponds to the rich mixture, as shown in FIGS. 14C and 14D, the applied voltage is set to a positive voltage and the element resistance is determined from the amount of the current change and the amount of the voltage change. In this case, although the time required for the sensor current to converge to its original limit current level is extended, the accuracy is not affected for the detection of the element resistance.

In the variants shown in FIGS. 13A to 14D, as in the preceding first exemplary embodiment, the negative sensor current change is measured when the ratio A / F corresponds to the lean mixture, whereas the positive sensor current change is measured when the ratio A / F corresponds to the rich mixture . Thus, the sensor current can be measured precisely within the dynamic range of the current measuring circuit 50 . Furthermore, the dynamic range of the current measuring circuit 50 can be set to a minimum range from the beginning.

Furthermore, in a further variant shown in FIGS. 15A to 15D, the applied voltage is set to both the positive and the negative voltage, and the amount ΔV of the voltage change and the amount ΔI of the current change are set according to a difference between the maximum value and the minimum value measured which are obtained with the voltage change. The element resistance can be measured most precisely in comparison to the other cases, but a relatively large dynamic range must be set for the current measuring circuit 50 . In this case, regardless of the value of the air / fuel ratio, the applied voltage may first be changed to either the positive or the negative voltage, namely either the embodiment shown in FIG. 15A or the embodiment shown in FIG. 15C.

Below, the invention is according to a second to sixth embodiment described. Here is the Explanation of parts or the like omitted with those in the first embodiment are identical. The following description is therefore only on sub differed from the first embodiment.

The second embodiment will be described below with reference to FIGS. 16 to 18. According to the description of the first embodiment, the element resistance is unconditionally detected according to a current change which is caused by the application of negative voltage in the lean mixture ratio and by the application of positive voltage in the fat mixture ratio. However, the predetermined dynamic range of the current measuring circuit 50 is not always set with respect to the stoichiometric air / fuel ratio, namely the ideal ratio. As a result, in this second exemplary embodiment the sequence of switching the applied voltage to positive and negative voltage is determined on the basis of a predetermined current value acting as a reference value within the dynamic range.

FIG. 16 is a flow chart of a subroutine for measuring the resistance element according to the second embodiment. This flowchart is a modification of the flowchart shown in FIG. 10 for the first embodiment. That is, in the flowchart in FIG. 16, a step 131 in FIG. 10 is replaced by a step 231. Specifically, in step 231, the microprocessor 20 compares the sensor current (limit current) Ip detected in step 110 of FIG. 9 with a predetermined reference value Ipo, which is set within the dynamic range. If Ip Ipo results, the process proceeds to step 132, where the microprocessor 20 first switches the applied voltage Vp to a negative voltage and then to a positive voltage. In Ip Ipo of the micro-processor 20 at step 133 changes the voltage applied to the first positive voltage, and then to the negative voltage. The other steps are the same as those shown in the flowchart in Fig. 10 and are therefore not explained here.

The reference value Ipo can be set in such a way that it is approximately in the vicinity of an average value of the dynamic range. For example, the reference value Ipo can be set as shown in FIGS . 17 and 18. If the dynamic range is set such that, as shown in Fig. 17, the ratio A / F can be detected over a range extending from the rich mixture range to the lean mixture range, the reference value Ipo comes close to "zero" (near the stoichiometric value). On the other hand, if the dynamic range is set such that, as shown in Fig. 18, the ratio A / F is detected only over the lean mixture range, the reference value Ipo can be set to a few mA, which corresponds to an A / F ratio of about 20 .

In the second embodiment, the output current Ip of the lambda probe 30 can always be measured without a deviation from the dynamic range and therefore the element resistance R can be detected in a precise manner. Here, even if the dynamic range is not set with respect to the stoichiometric value, as is the case in FIG. 18, there is no decrease in accuracy in detecting the element resistance R.

The third embodiment of the invention will be described with reference to Figs. 19 to 20B.

In the foregoing embodiments, to measure the element resistance, the applied voltage Vp is changed with a predetermined time constant. In the preceding exemplary embodiments, however, the size of the voltage change ΔV is fixed (only an upper limit value for the voltage change ΔV is given in US Pat. No. 4,419,190). Therefore, when the element temperature of the lambda probe 30 drops or the like and the voltage change ΔV is kept at a fixed value, the amount ΔI of the current change becomes small. As a result, there is a possibility that errors will occur in the detection of the element resistance P, which may lead to deterioration in the accuracy of the element resistance measurement.

The reason for this will be explained with reference to Figs. 20A and 20B. FIG. 20A shows voltage / current characteristics at a relatively high temperature member of the lambda probe 30, z. B. at the element temperature of about 700 ° C. FIG. 20B shows voltage / current characteristics at a relatively low temperature member, z. B. at the element temperature of about 600 ° C.

Comparing FIGS. 20A and 20B, in a resistance-dependent region according to FIG. 20B, namely in a voltage range which lies below the straight section parallel to the voltage axis V, the slope of a first straight section is smaller than the slope of the first straight section in the resistance dependent area of Fig. 20A. This means that the internal element resistance (Ri as shown in FIGS. 20A and 20B) of the sensor element according to FIG. 20B is greater than the internal element resistance of the sensor element according to FIG. 20A. Therefore, if the voltage applied to the two sensor elements according to FIGS. 20A and 20B is changed to the same extent, a current change ΔI 'in the sensor element according to FIG. 20B is only half as large as the current change ΔI in the sensor element according to FIG. 20A . Thus, there is a likelihood that errors will occur in the detection of the output current, which can lead to an incorrect detection of the element resistance.

Accordingly, in this embodiment, the amplitude of the voltage change ΔV is changed according to the element resistance R to ensure that the same level of the output current that is obtained at the high element temperature can be obtained even when the element temperature is low, namely, the internal resistance Ri is large is. According to FIG. 20B, the current change .DELTA.I can be achieved in that the amount of the voltage change of the DV 'is set (.DELTA.V' .DELTA.V).

Fig. 19 is a flowchart of an element resistance measurement subroutine according to the present invention. This flowchart is a partial modification of the flowchart shown in FIG. 16 in the second embodiment. That is, step 230 has been added. According to the flowchart shown in Fig. 19, at step 230, the microprocessor 20 first multiplies the element resistance R detected in the previous process by a predetermined proportional coefficient k to obtain the voltage change ΔV (ΔV = k × R). Then, at step 132 or 133, the voltage Vp is changed according to the voltage change ΔV calculated at step 230. The processes in steps 231 and 132 to 135 of FIG. 19 are the same as those described above and are therefore not explained here.

The proportional coefficient k is a constant which is determined in advance according to the following concept: It may be advantageous to increase the proportional coefficient k in order to improve the accuracy in the detection of the element resistance R. However, the current change .DELTA.T cannot be detected precisely if it comes out of the dynamic range of the current measuring circuit 50 . This means that ΔI must not exceed half the width of the dynamic range. ΔI is preferably set to approximately a quarter of the width of the dynamic range.

In the third embodiment of the invention, the Voltage change ΔV for measuring the element resistance set to be larger when this element resistance R increases. Thus the amount of electricity change ΔI is not drastically reduced even if for example, the element temperature is low, namely Internal resistance of the element is large, and thus one Deterioration in accuracy when detecting the element resistance R prevented.

The fourth embodiment will be described below with reference to FIGS. 21A to 29. In the exemplary embodiments described above, the bias control signal Vr for the voltage to be applied to the lambda probe 30 is always passed via the D / A converter 21 and the low-pass filter 22 . According to the above explanation, the time constant of the low-pass filter 22 is appropriately set for measuring the internal resistance of the sensor element. However, if the time constant of the low-pass filter 22 is used unchanged for the detection of the air / fuel ratio, considerable errors could occur in the detection of the ratio A / F. The reason for this will be explained with reference to Figs. 21A and 21B.

In Fig. 21A and 21B changes the applied voltage Vp and the sensor current Ip at the detection of the ratio A / F shown. Referring to FIG. 21A and 21B, the ratio A / F changes from the ratio for the rich mixture to the ratio for the lean mixture. This means that the values of Vp and Ip move along the straight characteristic curve L1 shown in FIG. 3 to the top right in tracking with a limit current range. Referring to FIG. 21A, the applied voltage Vp at each time interval Ta is gradually larger. In Fig. 21A, the output voltage Vb of the D / A converter 21 of FIG. 1 shown by dashed lines, while the output voltage Vc of the low pass filter 22. namely the voltage applied directly to the lambda probe 30 is represented by a solid line. In this case, as shown in FIG. 21B, the sensor current Ip deviates from the true value of the limit current shown by a broken line and becomes larger. This deviation is based on the frequency response of the lambda probe 30 , which was explained with reference to FIGS. 5, 6 and 7. The extent of the deviation is determined by the time constant of the low-pass filter 22 .

If, when the ratio A / F is changed, the sensor current Ip is detected immediately after the applied voltage Vp has been switched, namely at the times shown by full circles in FIG. 21B, no measurement error occurs because the sensor current is close to the true value of the limit current lies. However, if the ratio A / F is detected at intervals shorter than the time period Ta in which the applied voltage Vp is changed, the sensor current Ip has the values shown by blank circles in Fig. 21B which differ from the true values . As a result, errors occur in the detection of the ratio A / F.

In this fourth embodiment, which has been designed in view of the above-mentioned problem, when the ratio A / F is detected, the error that arises as shown in FIG. 21B is minimized by using another low-pass filter whose time constant is larger than that of the low-pass filter used for the detection of the element resistance.

The reason why the error in detecting the ratio A / F becomes smaller becomes smaller as the time constant of the low-pass filter is increased. Increasing the time constant of the low-pass filter means reducing the frequency of a change in the voltage applied to the lambda probe 30 . As the voltage application frequency is decreased, as shown in FIG. 23, the impedance ZAC becomes larger. As the impedance ZAC increases, the amount of current change decreases as the applied voltage changes. Therefore, as shown in FIGS. 22A and 22B, the deviation of the limit current from its true value as shown in FIG. 21B becomes smaller, thereby reducing the error in detecting the ratio A / F. A point A (f = 1 kHz) in Fig. 23 represents a cut-off frequency corresponding to a time constant of the low-pass filter suitable for measuring the element resistance, and a point B (at the frequency f = 10 Hz) represents a cut-off frequency represents a time constant of the low-pass filter, which is suitable for the detection of the ratio A / F.

From the above description it can be seen that this problem can be solved by changing the time constant of the low-pass filter 22 according to FIG. 1 depending on whether the element resistance or the air / fuel ratio A / F is to be detected. A simplified design of the low-pass filter 22 is shown in FIG. 24. That is, in the embodiment according to Fig. 24, the time constant is switched to a switch 26 by connecting a resistor of the low pass filter 22 on R1 or R2 (wherein R2 is R1). Specifically, when the ratio A / F is detected, the switching state shown in Fig. 24 is maintained to increase the time constant. On the other hand, when measuring the element resistance, the switch 26 is switched to reduce the time constant. Alternatively, the capacitance of a capacitor can be changed to set the time constant.

However, it may be difficult to accurately detect the ratio A / F or the element resistance due to the disturbance caused when the switch 26 is turned on and off, the variation in the time constant resulting from the forward resistance of the switch 26, and the like. Therefore, a circuit without a switch as shown in FIG. 25 can be used in this embodiment.

Fig. 25 is an illustration similar to that in Fig. 1 for the first embodiment. An important difference between the system shown in FIG. 25 and the system according to FIG. 1 is that two low-pass filters 22 a and 22 b and two D / A converters 21 a and 21 b are provided in the system according to FIG. 25. It should be noted that the low-pass filter 22 a must have a time constant which is substantially equal to that of the low-pass filter 22 shown in FIG. 1, and that the low-pass filter 22 b must have a time constant which is greater than that of the low-pass filter 22 a.

Here, the microprocessor 20 generates a first bias control signal Vr1 for measuring the element resistance R and a second bias control signal Vr2 for detecting the ratio A / F. The Vorspannungssteuer signals Vr1 and Vr2, respectively to D / A converters 21 a and b are supplied to the 21st Through the D / A converter 21 a and 21 b, the bias control signals Vr1 and Vr2 are converted into respective analog signals Vb1 and Vb2, the high-frequency components of which are suppressed by the low-pass filters 22 a and 22 b. The output voltages Vc1 and Vc2 then respectively obtained are fed to the bias control circuit 40 . By the bias control circuit 40, the output voltages Vc1 and Vc2 are amplified and then applied to the terminals which are respectively connected to the atmosphere-side and the exhaust-side electrode of the lambda probe 30th

FIG. 26 is a circuit diagram of Vorspannungssteuer circuit 40 according to this embodiment. A difference of the bias control circuit 40 according to this embodiment example compared to the bias control circuit 40 shown in Fig. 8 according to the first embodiment is that no reference voltage circuit 44 is provided for the delivery of a fixed voltage to the computing amplifier 45 a of the first voltage supply circuit 45 and the non-inverting Input terminal of this computing amplifier 45 a, the output voltage Vc2 is supplied from the additional low-pass filter 22 b. The output voltage Vc2 of the low-pass filter 22 b is applied to the connection 42 , namely the connection of the lambda probe 30 connected to the atmosphere-side electrode layer 32 .

The operation of the microprocessor 20 will now be described with reference to the flowcharts shown in FIGS. 27 and 28.

FIG. 27 shows the flowchart of a subroutine for the detection of a limit current, namely an air / fuel ratio A / F. This flowchart shows step 110 according to FIG. 9 in detail . According to FIG. 27, in a step 111 the microprocessor 20 first detects a limit current Ip flowing via the lambda probe 30 according to a measurement result of the current measuring circuit 50 shown in FIG . Then, in a step 112, the microprocessor 20 determines whether a predetermined time T3 has passed since the last change in the applied voltage Vp. That is, the predetermined time T3 is the interval between times for changing the voltages to detect the ratio A / F. This time period T3 need only be longer than the predetermined time T1 for the detection of the ratio A / F (see FIG. 9). The predetermined time T3 is preferably set to 2 to 10 ms.

In this case, if the answer to step 112 is negative because the predetermined time T3 has not yet expired, the microprocessor 20 ends this subroutine. If step 112 gives an affirmative answer because the predetermined time T3 has elapsed, control proceeds to step 113.

At step 113, the microprocessor 20 determines the voltage Vp to be applied to the lambda probe 30 in accordance with the element resistance R and the limit current Ip detected during the previous process (Vp = R * Ip + Ve). The equation at step 113 corresponds to the straight line L1 in FIG. 3, where Ve corresponds to the intersection of the straight line L1 with the V-axis and is initially set to a value of approximately 0.4 V near the center of the limit current range .

Then, in a step 114, the voltage Vp is applied to the lambda probe 30 . The bias control circuit 40 is applied as the control voltage, the second bias control voltage Vr2 for the low-pass filter 22 b, which is connected to the terminal 42 of the lambda probe 30 . The low-pass filter 22 b eliminates the high-frequency components of the voltage actually applied to the lambda probe 30 according to its time constant. Here, the first bias control voltage Vr1 for the low-pass filter 22 a only needs to be set to a predetermined voltage.

The determination of the time constant of the low-pass filter 22 b is described below. This time constant is preferably set to the greatest possible value in order to improve the accuracy of the detection of the ratio A / F. However, if the time constant is large, there arises another problem: If the time constant of the low pass filter 22 b is greater than a certain value, to be applied to the lambda probe 30 voltage can not follow a sudden change of the limiting current value Ip. As a result, the voltage Vp to be applied to the lambda probe 30 deviates greatly from the straight line L1 according to FIG. 3. If this time constant is increased further, the applied voltage Vp comes out of the limit current range and therefore the limit current Ip or the ratio A / F cannot be detected exactly. Thus, taking into account the accuracy of detecting the ratio A / F and responding to changes in the limit current Ip, the time constant of the low-pass filter 22 b is preferably set such that the cut-off frequency of this low-pass filter 22 b is approximately 10 Hz as shown in FIG. 23.

FIG. 28 is a flowchart corresponding to the element resistance measurement subroutine shown in FIG. 16 for the second embodiment. Although the function of the subroutine shown in Fig. 28 is basically the same as that of the subroutine of Fig. 16, it should be noted that in steps 132 and 133, the control voltage which is the first bias control voltage Vr1 is changed. In this case, the high-frequency components are eliminated by the low-pass filter 22 a from the voltage actually applied to the lambda probe 30 . The time constant of the low-pass filter 22 a is substantially the same as that in the first embodiment, but less than the time constant of the low-pass filter 22 b.

The above-described operation is described with reference to the graph in FIG. 29. The upper curve, in Fig. 29, represents the difference (Vr1-Vr2) between the two voltages generated by the microprocessor 20 , the middle curve represents the voltage actually applied to the lambda probe 30 , and the lower curve represents the sensor current The points on the sensor current curve marked with a full circle correspond to the limit current Ip detected in step 111 in FIG . The points marked with an empty circle on the sensor voltage curve and the sensor current curve correspond to the values .DELTA.V and .DELTA.I recorded in step 134 in FIG. 28. As described above, the limit current Ip is detected at predetermined time intervals T1, while the element resistance R (= ΔV / ΔI) is measured at predetermined time intervals T2. Furthermore, the sensor voltage control value, namely the second bias control voltage Vr2 for detecting the ratio A / F is changed at predetermined time intervals T3.

Furthermore, since the air / fuel ratio A / F is changed toward the lean mixture ratio according to the exhaust gas measurement in FIG. 29, the sensor current or limit current Ip increases and accordingly the control voltage (Vr1-Vr2) also increases.

In this case, the time constant for the Suppress the high frequency components of the control voltage Low pass filter used changed depending on it whether the ratio A / F or the element resistance is detected shall be. Therefore, in these two cases, the response is the sensor voltage to the control voltage different. On in this way the limit current Ip and the values ΔV and ΔI to be grasped exactly.

In the air / fuel ratio level means the ratio A / F is generated to be applied to the terminal 42 of the oxygen sensor 30 second bias voltage Vr2, and for measuring the element resistance to be applied to the terminal 41 first bias voltage Vr1 for detecting. However, the air / fuel ratio control device is not limited to this embodiment. That is, other designs can be applied so long as the air / fuel control device supplies a voltage that changes with the detection of the ratio A / F with a time constant that is larger than the time constant when the element resistance is measured.

In the same way as described above Embodiments is in the fourth embodiment the invention the exact measurement of the element resistance R enables. Of course, with this execution example solved the object of the invention. Also revealed yourself the following benefits:

• (a) In this embodiment, the time constant for changing the voltage to be applied to the lambda probe 30 is set depending on whether the element resistance or the A / F ratio is to be detected. Here, in comparison to the time constant when the element resistance is detected, the time constant of the voltage change when the ratio A / F is detected is set to a larger value. As a result, even if the ratio A / F is detected in a period that is shorter than the period for switching the applied voltage Vp used to detect the ratio A / F (e.g., T1 T3 in FIG. 29 ), Errors with regard to the limit current Ip of the lambda probe 30 can be switched off and a deterioration in the accuracy of the detection of the ratio A / F can be prevented.
• (b) Furthermore, two low-pass filters 22 a and 22 b are connected to the two connections 41 and 42 , which are connected to the dry electrolyte layer 34 of the lambda probe 30 , each of which has its own time constant. For the detection of the element resistance and the ratio A / F, the two low-pass filters 22 a and 22 b are used selectively. In this case, in comparison with the case where a switch is provided which is switched on and off to change the time constant, no measurement error caused by the resistance of the switch or no interference signal caused by the switching process occurs. As a result, the measurement accuracy can be prevented from being reduced.

A fifth embodiment of the invention will be described with reference to FIGS. 30A to 40.

First, the principle underlying the function in this embodiment will be briefly described with reference to Figs. 30A to 31C. If the current change ΔI caused by changing the applied voltage Vp is detected in the measuring method according to the exemplary embodiments described above, the magnitude of the current change ΔI cannot be measured in the vicinity of a maximum or minimum value of the dynamic range. This consequently leads to an incorrect measurement of the element resistance P. If, as shown in FIG. 30A, the ratio A / F is close to a minimum value of the dynamic range and a voltage change ΔV is caused in such a way that it exceeds the minimum value of the dynamic range, falls as shown in FIG. 30C, the current change .DELTA.I from the field and therefore it can not be measured, the element resistance R. To some extent, this problem can be solved by adjusting the direction of the voltage change according to the ratio A / F or according to the sensor current, this fifth embodiment providing an alternative solution to this problem.

To solve the problem described above, in this embodiment, as a method of detecting ΔI near the minimum or maximum value of the dynamic range, the voltage supply to the lambda probe 30 is temporarily interrupted, and then the element resistance R due to the electromotive sensor voltage and the current change is detected that arise during the current interruption of the voltage supply. In this way, the element resistance R can be accurately detected by combining this measuring method with the method for measuring the element resistance due to the voltage change over the entire area.

The method for measuring the element resistance R using the voltage of the lambda probe 30 will be described below. FIG. 31A to 31C are graphs which illustrate the underlying principle of this measurement method. When the air / fuel ratio is detected, the voltage Vp is applied to the lambda probe 30 and the sensor current or limit current is detected as Ip when the voltage Vp is applied. Then, when measuring the element resistance, that circuit is interrupted for a moment 36727 00070 552 001000280000000200012000285913661600040 0002019708011 00004 36608hen through which the sensor current Ip flows, namely the circuit for supplying the voltage to the lambda probe 30 . The sensor current Ip thus immediately becomes "zero" and the lambda probe 30 generates a voltage Ve which corresponds to a difference in the partial oxygen pressure between the inside and the outside of the dry electrolyte layer 34 . The element resistance Re can be determined on the basis of a relationship between the voltage change amount ΔV (= Ve-Vp) and the current change amount ΔI, "Re" being used to distinguish this resistance from the element resistance R measured on the basis of the voltage change. The element resistance Re detected in this way is substantially equal to the element resistance R (Re ≈ R) detected according to the voltage change.

The reason for this correspondence of the element resistance Re detected when the circuit is broken with the element resistance R detected when the voltage change is explained with reference to FIG. 5, in which the equivalent circuit of the lambda probe 30 is shown. Since this circuit is in its normal state when the ratio A / F is detected, current flows through Rg, Ri and Rf. If the circuit is briefly switched off under these conditions, namely the current flow is temporarily interrupted, the potential difference across the resistor Rg is immediately eliminated because it is a DC resistor. With regard to the resistor Ri, the electrical charge accumulated in the capacitance Ci immediately flows through the resistor Ri due to the small time constant as described above with reference to FIGS. 6 and 7, so that the potential difference across the resistor Ri immediately disappears. Therefore, the ratio between the voltage change caused by the circuit break and the current change caused becomes the sum of Rg and Ri, and thus the element resistance Re detected when the circuit is broken takes the same value as the element resistance R that according to the voltage change is detected.

In fact, however, the lambda probe 30 does not have the exact same circuit as that shown in FIG. 5. Therefore, according to FIG. 32, the element resistance R detected according to the voltage change does not completely agree with the element resistance Re detected when the lambda probe 30 is switched off at the moment, and certain errors can occur. As a result, a discontinuity in the measured values of the element resistance could arise when switching between these two measuring methods. Consequently, in this discontinuity part in the heating element control, for example, for keeping the element resistance constant, the element temperature cannot be stabilized.

Therefore, in this embodiment, are eliminated this discontinuity in terms of the measured element resist these two element resistance measurement methods, the on the voltage change or on the temporary Interrupt the circuit based within one Area applied with a predetermined width. Then it will be from a difference between those according to the two measurement methods detected element resistances a correction coefficient ka calculated. With this correction coefficient is due of the temporary break in the circuit Element resistance Re corrected.

If, on the other hand, the limit current Ip lies outside the dynamic range, none of the measuring methods described above can be used to detect the exact value of the element resistance. Even in this case, however, the measurement method based on the temporary interruption of the circuit can be used to detect the element resistance, although it could cause certain errors. In Fig. 33A and 33B are flow characteristics of the oxygen sensor 30 is shown in a region limiting current Ip outside the dynamic voltage /. FIG. 33A shows the element resistance R detected due to the voltage change, while FIG. 33B shows the element resistance Re detected due to the current interruption of the circuit. The true values of the element resistances R and Re are shown in the respective figures by dashed lines.

Referring to FIG. 33A and 33B 33A can Elementwiderstand- with the measuring method according to Fig., The current change amount .DELTA.I not be measured (.DELTA.I = 0). Therefore, even if the element resistance changes, this change cannot be detected. On the other hand, with the element resistance measurement method shown in Fig. 33B, a usable value of ΔI can always be calculated because the measured current always becomes 0 mA, and so can always Changes in element resistance can be detected. For example, even if the control of the heating element of the lambda probe 30 is performed to keep the element resistance constant, even if the limit current Ip is outside the dynamic range, by using the element resistance measurement method based on the current circuit break, the occurrence of a most critical one Situation in which the heating cannot be regulated.

The concrete design of this exemplary embodiment is described below, which is based on the functional principle described above. Fig. 34 shows the overall construction of the air / fuel ratio measurement means of this embodiment. A difference in the design of the air / fuel ratio measuring device according to this embodiment compared to the design in the first embodiment shown in Fig. 1 is that the microprocessor 20 of the bias control circuit 40 signals for turning on and off a switch for the intermittent interruption of the sensor circuit leads.

Fig. 35 shows a configuration of the bias control circuit 40. A difference in the design of the bias control circuit 40 according to this embodiment example compared to the bias control circuit shown in Fig. 8 according to the first embodiment is that between the resistor 45 b and the bases of the transistors 45 c and 45 d of the first voltage supply circuit 45, an analog switch 51 is attached. This analog switch 51 is switched by on / off signals from the microprocessor 20 . When the analog switch 51 is turned off, the two transistors are switched off 54 c and 54d. Consequently, the emitter connections of the two transistors 45 c and 45 d have high impedance and therefore the circuit connected to the sensor connection 42 is immediately interrupted.

The operation of the microprocessor 20 in this embodiment will be described below with reference to the flowcharts in Figs. 36, 37 and 38. FIG. 36 shows a subroutine at step 130 of the main routine of FIG. 9, and FIGS. 37 and 38 show subroutines at steps 140 and 150 of FIG. 36.

When the routine of Fig. 36 is started, the microprocessor 20 determines at step 231 which element resistance measurement method should be used based on the previously measured limit current Ip. There are three alternatives for the microprocessor 20 :
Applying only the element resistance measurement method based on the voltage change, applying only the element resistance measurement method based on the momentary switching off of the circuit, and applying the two element resistance measurement methods described above.

In this exemplary embodiment, as shown in the voltage / current characteristic diagram in FIG. 39, two predetermined values Ip0 and Ip1 with Ip0 <Ip1 are set near the minimum value of the dynamic range. Ip0 and Ip1 determine a reference current range. If Ip <Ip0, the element resistance is detected due to the voltage change, while if Ip <Ip1, the element resistance is detected due to the circuit break. Furthermore, if Ip has a value Ip0 Ip Ip1, namely Ip is within the reference current range, the element resistance is detected both due to the voltage change and due to the current circuit break.

Therefore, if Ip <Ip1 is determined in step 231 of FIG. 36, the program of the microprocessor 20 proceeds to step 140, in which the element resistance R is detected due to a change in the applied voltage. That is, at step 140, in steps 141 to 143 of FIG. 37, the same operations as in steps 132, 134 and 135 of the routine shown in FIG. 10 in the first embodiment are carried out by the element resistance R = ΔV / ΔI to eat.

If Ip <Ip0 is determined in step 231, the program of the microprocessor 20 proceeds to step 150, in which the element resistance Re is detected in accordance with the method introduced here, namely on the basis of the current circuit break. Fig. 38 shows a routine for measuring the element resistance Re based on the circuit break. That is, according to FIG. 38, the off by the microprocessor 20 in Fig. Switch 51 shown 35, to turn off the first voltage supply circuit 45th Subsequently, in a step 152, the microprocessor 20 detects a voltage Eo (difference between Va and Vc according to FIG. 35) on the lambda probe 30 caused by the circuit interruption.

Thereafter, the switch 51 is switched on by the microprocessor 20 in a step 153 and then in a step 154 the element resistance Re is calculated from the previously measured electromotive voltage Eo, the previously measured limit current Ip and the voltage Vp applied before the voltage was temporarily switched off ( Re = (Vp - Eo) / Ip).

The program of the microprocessor 20 then proceeds to a step 232 according to FIG. 36, in which to calculate the element resistance R, the element resistance Re determined in step 150 is corrected with a correction coefficient ka (R = ka · Re). This correction coefficient ka is used to eliminate the discontinuity between the element resistance R detected due to the voltage change and the element resistance Re detected due to the temporary circuit break and is calculated at a limit current Ip in the range from Ip0 to Ip1.

The process to be performed when the limit current Ip is in the range from Ip0 to Ip1 will be described below. That is, if Ip0 Ip Ip1 is determined at step 231, the microprocessor 20 executes step 140 (for the process of FIG. 37) to detect the element resistance R according to the change in the applied voltage, and then step 150 (for the process of Fig. 38) to detect the element resistance Re due to the open circuit. The microprocessor 20 determines the correction coefficient ka (ka = R / Re) according to the relationship between the element resistance values R and Re. This correction coefficient ka is used in the above-mentioned step 232.

The predetermined values Ip0 and Ip1 in the routine according to FIG. 36 can be set in any way to values between a minimum value of the dynamic range and 0 mA. It must be noted that the measuring accuracy of the element resistance measuring method is deteriorated due to the voltage change in the vicinity of the minimum value of the dynamic range. On the other hand, since the measuring accuracy of the element resistance measuring method is deteriorated in the vicinity of 0 mA, a margin of not less than 1 to 2 mA between 0 mA and Ip1 is preferably provided.

In the fifth embodiment of the invention, it is possible, as with the previously described Embodiments the element resistance R with a high Measure degree of accuracy. With this embodiment not only enables the object of the invention to be achieved, but it also offers the following additional benefits:

• (a) In this embodiment, the element resistance measurement method based on the voltage change and the element resistance measurement method based on the temporary open circuit are selectively applied in accordance with the limit current Ip of the lambda probe 30 . In detail, in the vicinity of the minimum value of the dynamic range in which the measurement result in the element resistance measurement method based on the voltage change is likely to be faulty, the element resistance measurement method based on the open circuit is used. On the other hand, in the vicinity of 0 mA where the element resistance measuring method based on the open circuit is likely to fail, the element resistance measuring method based on the voltage change is applied. As a result, the element resistance in any current measuring range can be detected accurately.
• (b) Furthermore, for turning on these two Element resistance measuring method near predetermined values a minimum value (or a maximum value) of the dynamics area. That way it can be done on the Element resistance measurement methods for voltage change a wider current measurement range can be used.
• (c) By setting the predetermined values for determining the limit current Ip of the lambda probe 30 with a predetermined distance (Ip0 to Ip1) between them, both element resistance measurement methods are carried out when the limit current Ip of the lambda probe 30 is within the predetermined Distance between the predetermined values. The correction coefficient ka is determined from the relationship between the results in the two measurement methods, with which the element resistance Re detected during the temporary interruption of the circuit is corrected.

Therefore, even if there are differences between the Characteristic curves of the one based on the voltage change Element resistance measuring method detected element resistance R and the characteristics of the after on the circuit break based element resistance measurement method resisted Re, the differences between the Results in the two measurement methods are eliminated in order to eliminate the discontinuity. As a result, the execution of the heating control due to the element resistance R bring about a uniform regulation of the element temperature be performed. The embodiment described above is also for correcting errors in the event of deviations between different probes to be used and at one Deterioration in the quality of the probes useful.

This embodiment can also be implemented in the following way:
In the routine shown in FIG. 36, steps 140 and 150 are carried out when the limit current Ip is in the range from Ip0 to Ip1, and the correction coefficient ka is calculated from the values R and Re obtained here. However, it is also possible to calculate the R / Re characteristic of the lambda probe 30 and then to determine the correction coefficient ka from such a characteristic according to a predetermined equation or table. It is also possible to correct one of the element resistances according to the characteristic curves for both resistors. Specifically, if it is known from the outset that R and Re have a relationship like that shown in Fig. 32, from Re1, which is the element resistance based on the circuit break, the resistance R1, which is due to a change in the applied voltage based element resistance.

While in this embodiment the element resistance R is calculated from the current change ΔI when the applied voltage Vp is switched from negative to positive and then the current change ΔI caused by the change in the applied voltage Vp is calculated, it is of course possible to measure the element resistance R by To change the applied voltage Vp from positive to negative. In this case, in step 231, the determination of the current Ip according to the values Ip0 and Ip1 can be carried out in such a way that Ip0 and Ip1 are set to values which are symmetrical with respect to the horizontal V-axis to the values thereof according to FIG. 39 are.

When the limit current Ip comes out of the dynamic range, the measurement accuracy in the element resistance measurement method based on the voltage change is worse than that in the element resistance measurement method based on the open circuit. Furthermore, if the limit current Ip deviates very much towards the lean mixture area, the element resistance cannot be detected. This problem can be solved in that, as shown in Fig. 40, both on the negative side and on the positive side Ip0 and Ip1, which determine a reference current range, or Ip0 'and Ip1', which apply a further reference current range determine. In this case, as shown in FIG. 40, the element resistance measurement method based on the voltage change is applied at a limit current Ip close to 0 mA. If the limit current Ip is close to the maximum value or close to the minimum value of the dynamic range, the element resistance measurement method based on the open circuit is applied.

If the difference between the element resistance R detected according to the voltage change and the element resistance Re detected according to the circuit interruption is so small that no corrections are required, or if the difference between R and Re should be disregarded to simplify the processing, Ip0 are set equal to Ip1, so that the above-mentioned measurement methods are not carried out simultaneously. In this case, the correction coefficient ka used in the flowchart in Fig. 36 is not required.

Next, a sixth embodiment of the invention will be described with reference to Figs. 41 to 45B. While in the respective embodiments described above, the air / fuel ratio measuring device is constructed with a cup-shaped lambda probe 30 , so that the air / fuel ratio can be determined from the limit current which flows when voltage is applied to this probe 30 In this sixth exemplary embodiment, the air / fuel ratio measuring device is designed with an integrated lambda probe 60 instead of the aforementioned lambda probe 30 . The structure and features of the integrated lambda probe 60 will be described below with reference to the drawing.

FIG. 41 is a sectional view showing the structure of the integrated lambda sonde 60. The integrated lambda probe 60 contains two dry electrolyte layers 61 and 62 made of zirconium dioxide, which are generally referred to as a pump cell (dry electrolyte layer 61 ) or measuring cell (dry electrolyte layer 62 ). A diffusion gap 63 provided below the dry electrolyte layer 61 acts as an oxygen concentration measuring chamber. On the other hand, an air channel 64 provided below the dry electrolyte layer 62 acts as an atmospheric pressure chamber. Pinholes 65 are formed in the dry electrolyte layer 61 so that exhaust gas can penetrate into the diffusion gap 63 . Heating elements 66 are used to heat the probe 60 .

On the upper and lower surfaces of the dry electrolyte layer 61 . namely the pump cell, platinum electrodes 67 and 68 are attached, while on the upper and lower surface of the dry electrolyte layer 62 , namely the measuring cell, platinum electrodes 69 and 70 are attached. The electrode 67 is connected to a connection 71 , the electrodes 68 and 69 are connected to a connection 72 and the electrode 70 is connected to a connection 73 .

The principle of operation of this integrated lambda probe 60 is explained below. Fig. 42 is a graph showing the characteristic of an electromotive sensor voltage Vs generated between the terminals 72 and 73 . The electromotive sensor voltage Vs is determined by an oxygen concentration Pv in the diffusion gap 63 and an oxygen concentration Po in the air channel 64 , which is equal to the oxygen concentration in the outside air:

Vs = (RT _* In [Po / Pv]) / 4F (2)

In equation (2), R is a gas constant, T is the absolute temperature and F the Faraday constant.

The oxygen concentration Pv in the diffusion gap 63 is usually equal to the oxygen concentration PA in the exhaust gas. Therefore, when the air / fuel ratio becomes richer and the oxygen concentration PA in the exhaust gas becomes lower, the oxygen concentration Pv in the diffusion gap 63 also becomes lower, and thus the electromotive sensor voltage Vs increases. On the other hand, if the air / fuel ratio becomes leaner, the oxygen concentration Pv in the diffusion gap 63 becomes larger and the electromotive sensor voltage Vs becomes smaller. This electromotive sensor voltage Vs is measured at the terminal 73 .

Furthermore, by applying a voltage Vp to the terminal 71 for the flow of a pump current Ip, oxygen ions pass through the solid electrolyte layer 61 . In this way, the oxygen concentration Pv in the diffusion gap 63 can be controlled as desired. If, according to the principle described above, the electromotive sensor voltage Vs is measured and the voltage Vp applied to the terminal 71 is controlled such that Vs becomes constant, the oxygen concentration in the exhaust gas or the air / fuel ratio A / F can be regulated in accordance with the pump current Ip .

That is, in order to regulate a constant electromotive sensor voltage Vs, the oxygen concentration Pv in the diffusion gap 63 must be constantly regulated to a constant oxygen concentration Pvo. For this purpose, an amount of oxygen equivalent to a difference between the oxygen concentration PA in the exhaust gas and the oxygen concentration Pvo must be supplied. In this case, the amount of oxygen supplied corresponding to the difference between PA and Pvo is determined by the strength of the pump current Ip. The oxygen concentration in the exhaust gas or the ratio A / F can thus be determined from the pump current Ip. As shown in Fig. 42, when the electromotive sensor voltage Vs is adjusted to a predetermined value (Vs = 0.45 V) when the ratio A / F is 14.7, namely equal to the stoichiometric value A / F that is changes slightly depending on the type of machine, the relationship shown in FIG. 43 results between the pump current Ip and the ratio A / F, where Ip is zero if the ratio A / F is 14.7. The characteristic diagram in FIG. 43 shows that a positive pump current Ip flows when the air / fuel mixture is lean. On the other hand, a negative pump current Ip flows with a rich air / fuel mixture.

FIG. 44 is a diagram showing 40 according to the design of the bias control circuit to this embodiment. Only the differences of this embodiment from those in which the cup-shaped probe is used are described below.

The integrated lambda probe 60 has, in addition to the connections 71 and 72 , which correspond to the connections 41 and 42 according to FIG. 8 and are used to apply voltage to the sensor element, the further connection 73 for measuring the electromotive voltage. The electromotive voltage Bs is detected at the connection 73 and the electromotive sensor voltage Vs is compared with a reference voltage Vso by a computing amplifier 75 and an amplified comparison result is output. Furthermore, the amplified signal is supplied to the second voltage supply circuit 47 in order to obtain a difference between this amplified signal and a signal output by the low-pass filter 22 . That is, compared to the first embodiment in which the cup-shaped probe is used, the second voltage supply circuit 47 acts as a differential amplifier instead of a voltage follower circuit.

The reference voltage Vso at the arithmetic amplifier 75 is set in this circuit as follows: In this embodiment, the reference voltage Vso is set such that Ip is zero when the ratio A / F is 14.7. That is, at air / fuel ratio 14.7 , the voltage at port 71 must be equal to the voltage at port 72 . Assuming that the output voltage of the low-pass filter 22 and the voltage to be used for the detection of the ratio A / F is Vp, the reference voltage Vso is set such that an output voltage Vx of the computing amplifier 75 is Vp-Va.

If the exhaust gas corresponds to a rich mixture in the circuit with this construction, the electromotive sensor voltage Vs at the connection 73 becomes larger and the output voltage of the computing amplifier 75 thus becomes higher. At the same time, the output voltage Vc of the second voltage supply circuit 47 and thus the voltage applied to the terminal 71 become lower. As a result, the pump current Ip flows in a direction opposite to that shown in Fig. 41, that is, a pump current flows in the negative direction. In this way, oxygen is drawn into the diffusion gap 63 . On the other hand, if the exhaust gas becomes a lean mixture exhaust gas, a pump current Ip flows in the positive direction and oxygen is pumped out of the diffusion gap 63 .

In the air / fuel ratio measuring device according to this embodiment with the structure described above, the element resistance of the integrated lambda probe 60 is measured with the same procedure as in the first embodiment, for example. The outline of this procedure will be described with reference to the timing chart shown in Figs. 45A and 45B. FIG. 45A shows a voltage to be applied to the lambda probe 60 , namely the output voltage of the low-pass filter 22 , while FIG. 45B shows the sensor current, ie the pump current Ip, which flows as a result of the application of this voltage.

When measuring the element resistance, the application voltage is applied to the integrated lambda probe 60 as a signal which has a predetermined time constant as a result of passing through the low-pass filter 22 . Here, the applied voltage is changed with respect to a voltage for detecting the ratio A / F to both the positive and the negative side. If the ratio A / F corresponds to a lean mixture, the element resistance R is determined from a negative voltage change ΔV and a negative current change ΔI (R = ΔV / ΔI). The reason for applying the measured negative value is the same as that in the first embodiment, namely that this setting enables the dynamic range of the lambda probe 60 to be set to a minimum. The process for detecting the element resistance R is carried out in predetermined cycles T4, where T4 can be a fixed value or a value which changes depending on the operating state of the machine. Although this is not shown graphically, the element resistance R for the rich mixture A / F is determined from the positive voltage change ΔV and the positive current change ΔI.

With the sixth embodiment, the same can be done Effects as in the previously described embodiment Examples can be achieved and therefore the element resistance can be detected with a high degree of accuracy, whereby the object of the invention is achieved.

The invention was made in connection with the preferred embodiments with reference to the Drawing fully described, but it should be noted that  for the expert various changes and modifications are evident.

For example, in the element resistance measurement process of FIG. 10, the sequence of setting the applied voltage to negative and positive voltage in the first embodiment is set depending on whether the air / fuel mixture is rich or lean at a certain time. however, this sequence can be determined regardless of the air / fuel ratio A / F. In this case, the applied voltage is always changed from the positive to the negative or from the negative to the positive voltage, but the negative voltage change ΔV and the negative current change ΔI are preferably measured in the case of lean air / fuel mixture, while in the case of rich air / Fuel mixture, the positive voltage change ΔV and the positive current change ΔI can be measured. The same applies to the integrated lambda probe 60 according to the sixth embodiment.

Furthermore, in the second to fifth embodiments example the integrated probe as described in the sixth embodiment can be used. In this case can have the same functions and effects as that described above can be achieved.

In the fifth embodiment described above example becomes a first predetermined value for the selective Apply either the one based on the voltage change Element resistance measurement method or that on the circuit interruption-based element resistance measurement method in the Proximity of the maximum or minimum value of the Dynamic range placed during a second predetermined value offset to the central section of the dynamic range is placed. The second predetermined value can be selective Setting the element based on the voltage change resistance measuring method can be used. That is, if with the second predetermined value as the boundary line the detected Current is higher than the second predetermined value, the  Element resistance only due to the current change on the positive or negative side to the inside of the Dynamic range detected and if the measured current is lower than the second predetermined value, the Element resistance due to the creation of Voltage resulting total current change to both the positive as well as the negative side. In this Fall will be the accuracy of the element resistance measurement closer to the center of the dynamic range larger. It can be used for Eliminate the discontinuity between the measurements for the Element resistance by selectively applying the respective measurement method is caused, a measurement result on the middle part of the dynamic range to correct the respective measured values used in the two methods will.

When the integrated probe described in the sixth embodiment is used, according to the description of the variants of the first embodiment ( Figs. 13A to 15D), it is also possible to change the waveform of the applied voltage or its position for measuring ΔV and ΔI.

Although in the previously described embodiment examples of the invention in a lambda probe for the Detect the oxygen concentration in the exhaust gas Motor vehicle machine or for detecting the Air / fuel ratio is applied of the invention not only on the lambda probe for vehicles limited. Rather, the invention can be used for others Purposes. For example, it is possible to Invention in an oxygen concentration sensor for the Measuring the concentration of oxygen e.g. B. in combustible Use gases (methane gas, ethane gas and the like).

In the above-described embodiments, by transforming the digital signals generated by the microprocessor 20 with a predetermined time constant, the signals are converted into sinusoidal signals which are used for measuring the element resistance. However, the microprocessor 20 can also generate sinusoidal signals which are used for measuring the element resistance.

A lambda probe generates in response to an Control signal from a microprocessor linear measurement signals for a Air / fuel ratio leading to the concentration of Oxygen in the exhaust gas from an engine are proportional. On bias control signal generated by the microprocessor a D / F converter, which converts it into an analog signal implements. The signal is then a low pass filter Eliminate the high frequency components of the analog signal fed. The output voltage of the low pass filter becomes one Bias control circuit supplied. To the lambda probe through the low-pass filter a single AC signal at a predetermined frequency and a predetermined Time constant created. From the voltage of the AC voltage signals and that by applying the AC voltage signal caused change in the current level of the lambda probe Element resistance of the lambda probe determined.

Claims (22)

1. A method for measuring the element resistance of an oxygen sensor, which is used for determining the oxygen concentration in a gas sample, characterized in that
a time constant is set which corresponds to a frequency at which the course of the impedance (ZAC) of the oxygen sensor ( 30 ) is uniform,
a voltage (Vp) is applied to the oxygen sensor which changes with the time constant,
a current change (ΔI) of the current flowing in the oxygen sensor caused by the voltage change is determined and
the element resistance (R) of the oxygen sensor is determined from the voltage change (ΔV) and the current change (ΔI). 2. The method according to claim 1, characterized in that a voltage is applied as the changed voltage (Vp) which in a single voltage waveform with the Time constant changes. 3. The method according to claim 1 or 2, characterized in that when setting the time constant, a frequency range is determined in which the course of the impedance (ZAC) of the oxygen sensor ( 30 ) is uniform, and the time constant corresponding to a certain frequency within the frequency range is set. 4. The method according to any one of claims 1 to 3, characterized characterized in that the time constant is approximately 159 µs is set. 5. The method according to any one of claims 1 to 4, characterized characterized in that a voltage as the changed voltage (Vp) is invested with a positive inclination or with a negative tendency changes. 6. The method according to any one of claims 1 to 4, characterized in that
a voltage is applied as the changed voltage (Vp) which changes with a section of negative inclination and a section of positive inclination,
the current change caused by the section negative slope or the section positive slope is determined as current change (ΔI) and
the element resistance (R) is determined from the voltage change (ΔV) in the negative slope section and the current change (ΔI) when the current change based on the negative slope section is determined, or from the voltage change in the positive slope section and the current change is determined when the current change based on the positive slope portion is determined. 7. The method according to any one of claims 1 to 6, characterized in that
a current value (Ip0) is set within a detectable current range of the oxygen sensor ( 30 )
whereby as the changed voltage (Vp) a voltage with a negative inclination is applied if the current (Ip) of the oxygen sensor is at least equal to the current value (Ip0), or a voltage with a positive inclination is applied when the current of the Oxygen sensor is less than the voltage value, and
the current change (ΔI) is the current change caused by the voltage change of negative inclination if the current of the oxygen sensor is at least as large as the current value, or the current change caused by the voltage change of positive inclination is determined if the current of the oxygen sensor is lower than that Current value. 8. The method according to claim 7, characterized in that a current value is set as the current value (Ip0) which is approximately in the middle of the detectable current range of the oxygen sensor ( 30 ). 9. The method according to any one of claims 1 to 6, characterized in that a voltage is applied as the changed voltage (Vp) when an air / fuel ratio (A / F) corresponding to a lean mixture is detected with the oxygen sensor ( 30 ) is, which makes the current in the oxygen sensor negative, or when an air / fuel ratio corresponding to a rich mixture is detected with the oxygen sensor, a voltage is applied which makes the current in the oxygen sensor positive. 10. The method according to any one of claims 1 to 4, characterized characterized in that a voltage as the changed voltage (Vp) is created which has a section of negative inclination and a Section has a positive tendency, and as current change (ΔI) caused by both the section negative slope as well as the section of positive slope changed voltage caused current change is determined. 11. The method according to any one of claims 1 to 4 and 10, characterized in that the changed voltage (Vp) with a section of positive inclination and a section of negative inclination is set such that one in the oxygen sensor ( 30 ) by applying the Voltage with the positive tendency moved amount of electric charges is equal to an amount of electric charges moved in the oxygen sensor by applying the voltage with the negative tendency. 12. The method according to any one of claims 1 to 11, characterized characterized in that the amplitude of a next changed  Voltage (Vp) according to the determined element resistance (R) is set. 13. The method according to any one of claims 1 to 12, characterized in that a time constant is set as the time constant, which when measuring the element resistance (R) of the oxygen sensor ( 30 ) and when detecting the air / fuel ratio (A / F ) has different values with the oxygen sensor. 14. The method according to claim 13, characterized in that two different time constant values are provided in the setting of the time constant and the time constant value is selected for setting as a time constant depending on whether the element resistance (R) of the oxygen sensor ( 30 ) is measured or with the air / fuel ratio (A / F) is detected by the oxygen sensor. 15. The method according to claim 14, characterized in that when selecting the time constant, the larger value of the time constant values is selected if the air / fuel ratio (A / F) is detected with the oxygen sensor ( 30 ), or the smaller value of time constant values is selected when the element resistance (R) of the oxygen sensor is measured. 16. A method for measuring the element resistance of an oxygen sensor, which is used for determining the oxygen concentration in a gas sample, characterized in that selective
a first measuring method in which a time constant is set which corresponds to a frequency at which the course of the impedance (ZAC) of the oxygen sensor is uniform,
a voltage (Vp) is applied to the oxygen sensor which changes with the time constant,
a current change (ΔI) of the current flowing in the oxygen sensor (Ip) caused by the changed voltage is determined and
the element resistance (R) of the oxygen sensor is determined from the voltage change (ΔV) and the current change (ΔI), and / or
a second measuring method is carried out in which the oxygen sensor is switched off,
a voltage (Eo) at the oxygen sensor that is changed by switching off the oxygen sensor is detected,
a current change (ΔI) of the current (Ip) of the oxygen sensor caused by the changed voltage (Eo) is detected and
the element resistance (Re) is determined from the changed voltage (Eo) and the current change (ΔI). 17. The method according to claim 16, characterized in that
at least one reference current level (Ip0, Ip1, Ip0 ′, Ip1 ′) is applied within a detectable current range of the oxygen sensor ( 30 ),
the output current (Ip) of the oxygen sensor is detected and
an optimal current level corresponding to a stoichiometric air / fuel ratio (A / F) is set,
wherein the first measurement method is selected if the output current level is closer to the optimal current level than the reference current level, or the second measurement method is selected if the output current level is not closer to the optimal current level than the reference current level. 18. The method according to claim 17, characterized in that that the reference current level (Ip0, Ip1, Ip0 ', Ip1') is close to either a minimum value or a maximum value of the detectable current range is applied. 19. The method according to claim 16, characterized in
that a first reference value (Ip0, Ip1) which is greater than a minimum value and close to the minimum value of the detectable current range of the oxygen sensor, and a second reference value (Ip0 ', Ip1') is set which is smaller than a maximum value and close to the maximum value of the detectable current range,
wherein the first measurement method is selected if the value of the output current (Ip) is greater than the first reference value and less than the second reference value, and the second measurement method is selected if the value of the output current is not greater than the first reference value and not less than is the second reference value. 20. The method according to claim 16, characterized in that
that a first reference range (Ip0, Ip1), which is greater than a minimum value and close to the minimum value of the detectable current range of the oxygen sensor, and / or a second reference range (Ip0 ', Ip1') is set, which is smaller than a maximum value and close is at the maximum value of the detectable current range,
wherein both the first measuring method and the second measuring method are selected if the value of the output current (Ip) lies within the first reference range or the second reference range. 21. The method according to any one of claims 16 to 20, characterized characterized in that when executing both the first Measuring method and the second measuring method is selected, from the determined after the first measurement process Element resistance (R) and that after the second measurement method determined element resistance (Re) a correction coefficient (ka) is determined and either the one determined by the first measurement method Element resistance (R) or after the second measurement method determined element resistance (Re) by means of the correction coefficient (ka) is corrected. 22. The method according to any one of claims 16 to 20, characterized in that
the course of the impedance (ZAC) of the oxygen sensor ( 30 ) is determined using the first measuring method,
the course of the impedance of the oxygen sensor is determined using the second measuring method,
a correction is determined from the impedance curves determined according to the first measuring method and the second measuring method and either the element resistance (R) determined according to the first measuring method or the element resistance (Re) determined according to the second measuring method is corrected using the correction coefficient (ka).