JPS6048518A - Heater temperature control circuit for air fuel ratio sensor - Google Patents

Abstract

PURPOSE:To prevent the generation of a rush current by raising a reference voltage according to a prescribed time constant when a power source is turned on. CONSTITUTION:A heater 4 and resistance 11-13 form a bridge circuit, and a power transistor (TR) 10 is controlled with the output of a differential amplifier 14 to control the heater 4 to fixed temperature. In this case, resistances 17-19, a capacitor 20, and a diode 21 are provided centering on a TR16 for control, and the rush current in power-on operation, etc., is suppressed. When a voltage is applied to a power source terminal 15, the capacitor 20 begins to be charged with a time constant tau determined by a resistance 19, and the base voltage of the TR16 varies following up it. This variation is transmitted to a differential amplifier 14, whose reference voltage is raised according to the time constant tau. Consequently, the current flowing to the heater 4 through the TR10 also increases from zero after the power-on operation according to the time constant tau.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a temperature control circuit for an air-fuel ratio sensor used to control an automobile Ganlin engine, and particularly relates to an air-fuel ratio sensor equipped with a heater using a thick film process. Concerning a suitable heater temperature control circuit.

[Background of the invention]

Automotive engines are subject to strict exhaust gas regulations, and therefore must be operated in a state where the ratio of the amount of fuel to the amount of air in the air-fuel mixture, the so-called air-fuel ratio, is always appropriately controlled.

For this reason, air-fuel ratio feedback control has come to be widely used in which the components of the gas are detected and the air-fuel ratio of the air-fuel mixture is controlled accordingly.

By the way, as an air-fuel ratio sensor necessary for such air-fuel ratio feedback control, a so-called 02 sensor, which gives a step-like detection signal near the stoichiometric air-fuel ratio, has traditionally been mainly used, but in recent years The air-fuel ratio sensor, the so-called Lienssen height, has come to be used, which allows detection approximately proportional to the oxygen concentration in the air-fuel ratio range (No. 4).

Various types of lean sensors have been proposed, but to date, even in the lean sensors, the sensor characteristics have a large temperature dependence, and in particular, the sensor characteristics are highly dependent on temperature. Since the sensor function does not function until the sensor reaches a certain temperature, it is necessary to maintain the sensor at a constant high temperature during use, and a heater is essential for practical use.

An example of such a lean sensor is shown in FIG.

The senna shown in Fig. 1 is of the single-hole diffusion method and is manufactured mainly by a thick film process. , 4 is a heater, 5 is an insulating layer made of ceramic or the like, 6 is a diffusion hole, and 7 is a diffusion chamber.

When main body 1 is kept at a high temperature of 700°C or higher and a DC voltage of a predetermined polarity is applied between platinum electrodes 2 and 3, oxygen in diffusion chamber 7 is ionized by platinum electrode 2, and this ionizes between electrodes 2 and 3. The electric field causes the metal to move through the main body 1 and be transferred to the platinum electrode 3, where it is neutralized and released to the outside. This is called the so-called oxygen pump phenomenon, and as a result, the oxygen concentration within the diffusion chamber 7 decreases over time.

Therefore, if this entire sensor is exposed to the exhaust gas to measure the oxygen concentration, the reporter pump phenomenon will cause the diffusion chamber 7 to
After the completion of construction in the exhaust gas (- concentration decreases), oxygen in the exhaust gas diffuses into the diffusion chamber 7 from the diffusion hole 6.

On the other hand, at this time, the current flowing between the platinum electrodes 2 and 30 is
It corresponds to the amount of oxygen ions supplied from the diffusion chamber 7.

Therefore, when the amount of oxygen carried to the outside from the diffusion chamber 7 by the oxygen pump phenomenon is balanced with the amount of oxygen supplied from the exhaust gas to the diffusion chamber 7 through the diffusion hole 6, the platinum cap surface The current flowing between 2 and 3 is determined, and at this time, the conditions of the diffusion hole 60 are constant, so the amount of oxygen supplied from the exhaust gas into the diffusion chamber 7 per unit time is determined by the oxygen concentration of q of the exhaust gas. In the end, the oxygen concentration in the exhaust gas can be detected by the current between the electrodes 2 and 3 of the platinum r.

At this time, the current flowing between the platinum electrodes 2 and 3 while keeping the voltage between them at a predetermined constant condition is called the limiting current , t.
X ri J L P O! The relationship with can be expressed by the following formula.

Here, F is Faraday constant, D is gas diffusion constant, S
is the cross-sectional area of the diffusion hole 6, t is its length, and 'r is the absolute temperature.

Moreover, FIG. 2 is a characteristic diagram showing the limiting current ■μ and r) element (P<02> and [sigma]-)+5.

As is clear from these equations and Figure 2, the detection performance of this lean sensor is temperature dependent, and the temperature required for operation is also 700°C or higher3. To use lean senna,
It is necessary to operate the device at a constant high temperature of 700°C or higher, and for this reason, it has traditionally been necessary to use electric heating with a force of 1.
J heat is mainly used, and this is the heater 4 in FIG.

By the way, at this time, as is clear from the above explanation, it is necessary to control the heating temperature by the heater 4 to a constant temperature of 700 degrees Celsius or higher. .

The circuit shown in FIG. 3 utilizes the temperature dependence shown in FIG. The voltage drop occurring in this heater 4 is determined by g+++, and the heating voltage ')W+ji
It is possible to obtain a constant heating temperature by controlling the magnitude of t, and it is also called a bridge circuit system because a resistor bridge is used for this purpose. In the figure, 10 is a power transistor, 11,1
2.13 is a resistor, 14 is a differential amplifier, and the heater 4 is as shown in FIG.

The heater 4, the resistor 11, and the resistors 12 and 13 constitute a bridge circuit, and a current is supplied from the power source via the power transistor 10. The unbalanced voltage generated in the bridge circuit at this time is input to the differential amplifier 14, and the output of the differential amplifier 4 controls the power transistor 10, thereby controlling the magnitude of the current supplied to the bridge circuit. be done.

Next, the operation of this circuit will be explained.

First, a target temperature T0 to be controlled is determined, and this temperature T
The resistance value RHe of the heater 4 at 0 is determined based on the characteristics shown in FIG.

Next, the resistance values of resistors 11, 12, and 13 are set to R1, respectively.
1? R, 11N(11), and to these resistance values, I(12,R
1.

At this time, it is necessary that the resistance values of these resistors 11, 12, and 13 have almost no temperature dependence, or even if they do have a temperature dependence, it is conveniently small.

As a result, the magnitude of the voltage difference between the ten inputs of the differential amplifier 14 and the single power is determined by the resistance value of the heater 4, that is, its temperature, and when the temperature of the heater 4 is lower than the target temperature T0, When the output becomes ten, the transistor 10 causes a large current to flow through the bridge circuit, increasing the current of the heater 4 and increasing its heat generation amount. As the temperature of the heater 4 approaches the target temperature T, 'l' of the heater 4! The current decreases, and the temperature of the heater 4 is kept constant at the target temperature T0 within a residual deviation determined by the amplification degree of the differential amplifier 14, etc.

Therefore, if the circuit shown in Fig. 3 is applied to the lean sensor shown in Fig. 1, the lean sensor will be kept constant at a predetermined temperature, for example, 900°C, by the heater 4, and will fully perform its function as a lean sensor. It will be possible.

The circuit shown in Fig. 3, which controls the temperature based on the temperature dependence of the heater 4, has a small number of parts, has a simple circuit configuration, and can provide temperature control characteristics sufficient for practical use. , has been widely used.

However, in this conventional temperature control circuit, when the temperature of the heater is low, a large current flows through the heater, so when the power is turned on, a large inrush current flows through the heater as shown in Figure 5. This has the drawback of causing deterioration and disconnection of the heater, shortening the service life of the lean sensor.

In particular, in lean sensors, the operating temperature is high, and the resistance fluid fB of the heater is large between room temperature and operating temperature, so the maximum value of the above-mentioned input/extraction current tends to increase, causing heater deterioration and = There is a significant possibility that x will occur.

In addition, many of these sensors are manufactured using a thick film process as explained in FIG. In most cases, the heater is formed as a single piece of platinum, but since the heater is formed as a thin platinum film, an inrush current may cause deterioration or abandonment, and Thermal run (a phenomenon in which when a local temperature rise occurs in a heat-generating conductor film, the resistance value of that part increases, causing a concentration of heating and gravity there, causing an accelerated temperature rise, leading to wire breakage) The influence of the above-mentioned inrush current is particularly significant.

[Purpose of the invention]

The purpose of the present invention is to eliminate the drawbacks of the prior art described above, and to provide a lean sensor that can prevent the generation of rush current that occurs when the power is turned on, and eliminate the risk of heater deterioration and disconnection. Located in providing temperature control properties.

[Summary of the invention]

In order to achieve this objective, the present invention is characterized in that the reference voltage for controlling the current that does not flow through the heater rises with a predetermined time constant when the power is turned on, in comparison with the voltage drop of the heater. 1. [Embodiments of the Invention] Hereinafter, a heater temperature control circuit for an air-fuel ratio sensor according to the present invention will be explained in detail with reference to illustrated embodiments.

Fig. 6 shows an embodiment of the present invention, where 15 is a power supply terminal, 16 is a control transistor, 17° is a current limiting resistor, 19 is a time constant circuit resistor, and 20 is a time constant. Circuit capacitor 2 is a diode for discharge, heater 4, power transistor 10, resistor 11
.about.13, the difference F9 amplifier 14 is the same as the conventional example shown in FIG.

In the next section 1, the operation of this embodiment will be explained.

The resistance value of the heater 4 is set to R10, as in the conventional example shown in FIG.
The resistance values of resistors 11, 12, and 13 are R, 1, R, □, R1
3, and the voltage of the single output of the differential amplifier 14 is V), and the voltage of the 10 inputs is V+, then these voltages V, V+.

That is, the output voltage of the bridge circuit is as follows.

V = E - f',, / (R,□ +R1□)...
...(SV+= E -kg, /' (R,,2+
R, 3) -- (here, E is the emitter voltage of the power transistor 10, that is, the voltage applied to the bridge circuit).

On the other hand, the stability condition for the differential amplifier E4 is V-=ke, which is also the equilibrium condition for the bridge circuit 'a':'t, and this and (
(From the equation, the equilibrium condition of the bridge circuit is as follows.

Then, from this equation, the equilibrium condition of the bridge circuit (R, E ・1 − 2) = (R, □ ・”+3) is derived.

As already explained, this condition is satisfied when the temperature of the heater 4 is the target temperature 'I'. become,
This occurs only when the resistance value H of the heater 4 becomes 0.

Therefore, the power transistor 10 is controlled by the output of the differential amplifier 14, changes the input voltage E of the bridge circuit, and controls the current flowing to the heater 4 so that its resistance value R becomes R, . That is, the temperature is the target temperature T
. It operates to provide temperature control functions. Note that the above operation is the same as the conventional example shown in FIG.
-2, that is, the voltage drop of the heater 4, and by making the two match, temperature control can be achieved.

By the way, if this continues, a rush current will flow to the heater 4 when the power is turned on, as in the conventional example shown in FIG. 3, but with this implementation, ia! A resistor 19, a capacitor 20, etc. are installed around the transistor 16, which effectively suppresses the inrush type 61E caused by the '1d large input. I will explain about it.

The base of the control transistor 16 is connected via a resistor 18 to a capacitor 20, which is further connected via a resistor 19 to the ttE4 terminal 15.

Therefore, if the 'IJI source is turned on and the voltage is applied to the power supply terminal 15 (t), the capacitor 2° begins to receive light from the '4L source terminal 15 via the resistor 19.
The terminal pressure VC of the capacitor 20 rises from zero to the power supply voltage 1 in accordance with the time constant τ determined by the static voltage t of the capacitor 20 and the resistance value of the resistor J9. Since the capacitor 2° is connected to the base of the transistor 16 via the resistor 18, the base voltage of the transistor 16 also changes in accordance with the terminal voltage (2) of the capacitor 20.

On the other hand, when the power is turned on and the power supply voltage is applied to the power supply terminal 15 as described above, the temperature of the heater 4 changes to the target temperature T.
If it is lower than 0, the resistance value RB of the heater 4 is lower than the value RH0 at the time of gradual heating, and the bridge circuit becomes unbalanced, causing a difference in its output voltage V-, and the differential booster 14 The input voltage E of the bridge circuit is increased by the action of the power transistor 10, and a large current is passed through the heater 4, raising the temperature of the heater 4 and increasing its resistance value, thereby increasing the voltage V- to the voltage V+. Raise it so that both are one piece.

Therefore, if the temperature of the heater 4 is at a low temperature near normal temperature at this time, a large inrush current will flow to the other heater 4 as described above, resulting in a wire breakage or the like.

However, in this embodiment, the emitter and collector of the control transistor 16 are connected in parallel with the bridge circuit 13, and the terminal voltage vc of the capacitor 20 is applied between the base and collector of the transistor 16. Therefore, the voltage ■+ is the transistor 16
K is set so that it changes to follow its base potential while it is in a conductive state.

The base potential of this transistor 16 is the terminal voltage V of the capacitor 20 as described above. Therefore, in this embodiment, when the power is turned on, the output voltage of the bridge circuit is initially at zero potential, and then the capacitor 20 is charged through the resistor 19, and the terminal voltage VC changes according to the time constant τ. As the voltage rises, the button changes accordingly, and the differential amplifier 14 and power transistor 10 operate to match the voltage V- by following this rising change from zero. Therefore, the current flowing through the heater 4 also increases from zero according to the time constant τ after the power is turned on, and it is the effective VCC positive pressure that causes the rush current.

In this way, after a predetermined time determined by the time constant τ has passed after the power is turned on, the terminal voltage VC of the capacitor 20 almost reaches the power supply voltage, and as a result, the voltage VC
When the voltage becomes higher, the base-emitter of the transistor 160 becomes reverse biased, and thereafter, the transistor 16 is kept non-conductive until the power is turned off. So,
It has no effect on the operation of the temperature control circuit, and there is no adverse effect on the function of the temperature control circuit, such as control accuracy and response characteristics for keeping the temperature of the heater 4 at the target value T.

Note that in the above, the resistors 17 and 18 are used to control the base currents of the power transistor 10 and the control transistor 16, respectively, and are merely provided to stabilize the operation, and have no particular relation to the above operation. .

Next, the function of the diode 210 will be explained.

As is clear from the above description, in this embodiment, when the power is turned on, the charge on the capacitor 20 is zero, and its terminal voltage is ■.

is required to be zero.

However, once the power is turned on and a predetermined time has elapsed, the capacitor 20 is in a top-charged state, and its terminal voltage Vc has almost reached the 'ifl voltage of the power supply. However, in this state -C- & etran raster 16 is non-conducting, so if there was no evening diode 21, after turning off the power, the capacitor? There is no low-impedance discharge path for the capacitor 20, and if the power is turned on again after a short period of time after the power is turned off (well, the terminal voltage V of the capacitor 20 becomes zero and the・Sometimes, there is a possibility that the above-described time constant operation cannot be obtained and the inrush current cannot be suppressed sufficiently.

Therefore, a diode 21 is connected in parallel with the resistor 19 with the polarity shown in the figure, and when the power is turned off, this diode 21 provides a low impedance discharge circuit for the capacitor 20, and the charge on the capacitor 20 is immediately discharged after the power is turned off. This is to cause the terminal voltage to return to zero by discharging it.

Note that the discharge path of the capacitor 20 formed by the diode 21 at this time is also formed by a circuit consisting of the power transistor 10 in the on state, the resistors 11 to 13, and the heater 4, which is connected in parallel to the power supply terminal 15. It may also be formed by other load circuits that are connected to each other.

FIG. 7 shows the temperature rise characteristics of the heater 4 obtained by this example. ir, 76, and the characteristics of the conventional example in FIG. 3 are shown by broken lines for comparison.

As is clear from FIG. 7, the embodiment shown in FIG. 6 not only suppresses the medical current when the power is turned on, thereby reducing heater deterioration and disconnection, but also reduces the temperature of the heater. The rise characteristic can be made more gradual, and the occurrence of heater deterioration and disconnection in this aspect can be reduced.

According to this example, as a result of suppressing the inrush current, the heat generation of the power transistor 10 and the bridge circuit resistor 11 can be reduced, making it possible to reduce the capacity and size of these components. , it is possible to obtain effects such as cost reduction.

Next, FIG. 8 shows another embodiment of the present invention.

In this embodiment, a switching element that is periodically turned on and off is used to control the current supplied to the heater 4, and the ratio of the on time and off time of this switching element is changed. The present invention is applied to a pulse control type temperature control circuit, and in FIG.
0 is a constant 'ε current power supply for supplying heating current to the heater 4, 110 is a switching element that turns on and off the mixed current flowing to the heater 4, and 1.20 is a switching element for detecting the voltage of the heater 4. , 1.30 is a capacitor for holding the voltage of the heater 4 taken out by the switching element 120 for a predetermined period; 140. 150.
160 is a resistor, 170 is a capacitor, 180 is a differential amplifier, 190 is a comparator, 200 is a sawtooth wave generating circuit, and the rest is the same as the embodiment shown in FIG. By the way, if you exclude the circuit inside the broken line, it becomes the temperature control circuit of the pulse control method described in "2", and this circuit is, for example, the one described in Japanese Patent Publication No. 52-899
These are known from Japanese Patent Publication No. 99, Japanese Patent Publication No. 56-6504, and the like.

Next, the operation of this embodiment will be explained.

As already explained, once the control target temperature T0 is determined, the resistance value RHD of the heater 4 at that temperature is known, and if the current supplied from the constant current source 100 to the heater 4 is constant? &Otsu4, by detecting the terminal voltage vH of the heater 4, it is possible to detect whether the temperature d of the heater 4 is higher or lower than the target temperature T0.

On the other hand, the pressure a, which serves as a reference for controlling the target temperature at this time, is created by dividing the pressure by resistors 150 and 160.

Therefore, the switching element 120 and the capacitor 130 are taken out from the heater 4 (JJ pressure 8 is taken out by the resistor 140
is supplied to one input terminal of a differential amplifier 180 having a capacitor 170, and is integrated, and compared with a reference voltage applied to its ten input terminals, resulting in an output voltage V0 commensurate with the potential difference between the two. Therefore, the differential amplifier 180 with
■Output voltage. is the voltage V,...! , V+, the larger the voltage becomes.

The output voltage V of the differential amplifier 180 is supplied to one input of the comparator 190, and is compared with the sawtooth wave signal V supplied from the sawtooth wave generation circuit 200 to the other input, and the comparison signal S is Output from comparator 200. At this time, the signal S, power, and voltage ■. The relationship with is shown in FIG.

That is, v0-8=A, Vo<Otori→S=='σ'.

This signal S is supplied to switching elements 110 and 120, and these switching elements 110 . These elements are driven so that the heater 4 turns on and turns off at the level 10''.
As shown in FIG. 9, the direct current rIl flowing through the
During the T1 period when is at level A, the constant current source becomes constant current III '11 due to the constant current power supply 100, and the signal S becomes level %QW.
This means that it is possible to perform rectangular wave control that becomes zero during the period (T, -T1) where the current is zero. As is clear from FIG. 9, the duty ratio of this signal S is the output voltage (2), which represents the difference between the voltage (2) and the voltage (2). As a result, the current I□ flowing through the heater 4 is large when the temperature of the heater 4 is lower than the target temperature T, and decreases as it rises and approaches the target temperature T0. When the temperature matches the temperature T, the temperature is controlled to be zero, and a temperature control function is obtained.

Note that at this time, the period 'r0. In other words, the switching element 110. The length of the period that determines the 1200 on/off cycle is determined to be sufficiently short compared to the heat tolerance of the heater 4, so that the force Dlh temperature by the heater 4 is It is necessary to avoid pulsation.

Further, since the voltage 4 appearing on the heater 4 also changes in a rectangular waveform in response to the change in the current IIf, the voltage 4 on the heater 4 changes by the switching element 120 and the capacitor 130.
It is designed to sample and hold and detect.

Now, even in this pulse control type temperature control circuit,
When the power is turned on when the temperature of the heater 4 is much lower than the target temperature T, the voltage vt of the heater 4 is
As the difference between
The duty ratio of the output signal S of 0 becomes 100%, and the heater 4 is heated up quickly, causing a risk of deterioration of the heat and the wire breakage.

Therefore, in this 'f: example, the transistor 16 for control 1111. Resistor 1B, 19. A capacitor 20 is provided in the capacitor 20, which gives a predetermined time constant τ to the rise of Kyo (ia 'i, j pressure V), and suppresses the heating λπ degree of the heater 4 when the power is turned on.As a result, This is to prevent the heater 4 from deteriorating or being stored. Incidentally, since the bending and stretching of this circuit is the same as that of the actual 77m example shown in FIG. 6, detailed explanation thereof will be omitted.

Figure 10 shows the operation of this embodiment in comparison with the operation of the conventional example. After turning on the four power sources, the duty ratio of the conventional example shown by the broken line immediately becomes 100%, and this state remains for a while. (However, in the embodiment of the present invention shown in the actual inspection, the duty ratio increases slowly from the time when the power is turned on, then shifts to a steady state, and the heater 4 suddenly increases φ.
There is no risk of causing deterioration or the result of iθ.

According to the embodiment shown in FIG. 8, while making full use of the feature of the pulse control method that it is possible to eliminate power loss caused by the power transistor for controlling the heater current, Deterioration of the heater and occurrence of disconnection can be reliably prevented.

In the above embodiments, an air-fuel ratio sensor using a thick film process was explained. However, since any type of heater is affected by the inrush current when the power is turned on, the present invention applies to a lean sensor using a thick film process. Needless to say, the present invention is not limited to sensors, but can be applied to any type of air-fuel ratio sensor that is equipped with a heater, and can exert the necessary effects.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to effectively suppress the inrush current that occurs when the power is turned on, thereby eliminating the risk of heater deterioration and Ml generation in air-fuel ratio sensors and the like due to the thick film process. It is possible to sufficiently lengthen the service life of the air-fuel ratio sensor, reduce running costs, and easily provide efficient heater temperature control for the fuel-fuel ratio sensor once.

[Brief explanation of the drawing]

Figure 1 shows side 1 of an example of an air-fuel ratio sensor equipped with a heater.
A new view, Figure 2 is a percentage diagram showing its temperature dependence, Figure 3 is a circuit diagram showing a conventional example of a heater temperature 1ijlJ control circuit,
Fig. 4 is a characteristic diagram showing the resistance temperature dependence of the first current, Fig. 5 is a characteristic diagram showing an example of the first inrush current, and Fig. 6 is an implementation of the heak temperature control circuit for the air ratio sensor according to the present invention. A circuit diagram showing an example, No. 71, a characteristic diagram explaining the effect of the mask, FIG. 8 a circuit diagram showing another embodiment of the present invention, FIG. 9 a waveform diagram for explaining its operation, and FIG. 10. is a characteristic diagram of effect explanation 10. 4... Heater, 10.../lower transistor, 11-13... Resistor for bridge circuit, 14... Differential amplifier, 16... ...Control transistor, 19...Resistance for time constant circuit, 2
0... Time constant circuit Figure 1, Figure 2, Figure 3, Figure 4. Continued from page 1 ■ Inventor Toshitaka Suzuki @ Inventor Nobuo Sato 3-1-1 Saiwai-cho, Hitachi City Hitachi Research Institute, Hitachi, Ltd. 3-1-1 Saiwai-cho, Hitachi, Ltd. Hitachi Research Laboratory, Hitachi, Ltd.

Claims (1)

[Claims] 1. In a heater temperature control circuit that controls the heating current supplied to the heater according to the voltage difference between the voltage drop of the heater and the reference voltage, when the heater temperature control circuit starts operating by turning on the power supply voltage. 1. A heater temperature control circuit for an air-fuel ratio sensor, characterized in that a constant circuit is used so that the reference voltage rises to a predetermined value with a predetermined delay time after the power supply voltage is turned on. 2. Claim 1 is characterized in that the heating current supplied to the heater is controlled by controlling the duty ratio of on and off switching operations. Heater temperature control circuit for air-fuel ratio control.