JPS60260842A - Heater temperature control circuit for air-fuel ratio sensor - Google Patents

Abstract

PURPOSE:To control the temp. in the heating part of a four-terminal type heater with good accuracy by providing a resistor for temp. setting in series to said heater and controlling the heater current in such a manner that the difference in the voltage drop between the heating part of the heater and the resistor for temp. setting is made zero. CONSTITUTION:The four-terminal type heater is constituted with the heating part (a) to heat an oxygen detecting part, lead parts (b), (c) for the part (a) and lead parts (d), (e) to detect the electric signal between the lead parts (b) and (c). The four-terminal type heater is connected in series to the resistor Rs for temp. setting. The resistor Rb in the lead part of the four-terminal type heater is connected at one end to the resistor Rs and the resistor Ra in the heating part of the four-terminal type heater is connected at one end to the lead part Rb. The resistor Rc at the lead part is connected at one end to the heating part Ra. The voltage drop of the temp. setting part Rs is connected to an operational amplifier 105 and the voltage drop of the heating part Ra is connected to an operational amplifier 110. The output voltages from these amplifiers are inputted to a differential amplifier 120 and the heater current is so controlled that the differential voltage between the resistor for temp. setting and the heating part is made zero.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a heater temperature control circuit for an air-fuel ratio sensor, and in particular, to a simple means for controlling the voltage drops between the heater and the temperature setting resistor to be equal. The present invention relates to a heater temperature control circuit for an air-fuel ratio sensor suitable for realizing highly accurate heater temperature control using a circuit configuration.

[Background of the invention]

Various types of sensors have been studied as air-fuel ratio sensors for detecting the oxygen concentration in exhaust gas, particularly for detecting the oxygen concentration in the S-lean region, and many patents have been filed. However, in either method, the ambient temperature has a large effect on the sensor characteristics, and it is necessary to control heating to a constant high temperature during use.

Figure 1 is an example of a lean sensor and is a structural diagram showing the principle of a single-hole diffusion type lean sensor.
be done.

In the figure, 1 is a zirconia solid electrolyte, 2゜3#'i
A platinum electrode, 4 an oxygen reference chamber, and d a gas diffusion hole serve as an oxygen detection section. Reference numeral 5 denotes a heater that heats the oxygen detection section, and 61-t an insulating layer that insulates the heater and the oxygen detection section, all of which are exposed to exhaust gas.

To detect the oxygen concentration in exhaust gas, when a constant voltage is applied between the platinum electrodes 2 and 3, the limit current Ip is determined from the amount of oxygen that diffuses into the oxygen reference chamber through the gas diffusion hole d due to the oxygen pump phenomenon.
z is obtained, which corresponds to the oxygen concentration in the exhaust gas. The relationship between the limiting current Ipt and the elemental concentration Po2 in the exhaust gas can be expressed by the following equation.

Here, F is the Faraday constant, D is the gas diffusion constant, S is the cross-sectional area of the diffusion hole, t is its length, and T is the absolute temperature. Further, the gas diffusion constant is a function of the absolute temperature T and the total mixture pressure P, and is expressed by the following equation, where Do is the diffusion constant under o'c and i atmospheric pressure.

+1), as is clear from equations (2), the limiting current characteristics have a temperature dependence proportional to the absolute temperature T to the 374th power, and it is also generally known that the lower limit temperature for operation in this method is 700°C or higher. It is being

Therefore, as a way to deal with these temperature effects, it has been considered to control the temperature to a certain constant temperature of 700'C or higher using a heater 5 integrated with the oxygen detection section.

FIG. 2 is a diagram showing an example of mounting the lean sensor shown in FIG. 1.

In the figure, 10 is the lean sensor shown in FIG.
12 is a protection tube, 12 is a flange for mounting on the exhaust pipe, 13 is a wire harness for connecting the lean sensor and its drive unit, 14 is a power supply, 15 is a drive circuit for the lean sensor, 16'#S - For example, it is a microprocessor that processes the sensor signals. - When mounted, the sensor is mounted on the protection pipe 11 side with the flange 12 as a border, inside the exhaust pipe, and on the other hand, the sensor is located on the atmosphere side.

In the operating state, the temperature of the exhaust gas may reach 900'C or more, and naturally the lean sensor itself is heated to that temperature, and the temperature on the atmospheric side is also correspondingly high.

The problem here is that in order to deal with the aforementioned temperature effect on the lean sensor output characteristics, the temperature is controlled using a heater to maintain a constant high temperature. At this time, since the heater uses a temperature-sensitive material such as platinum, lead parts other than the oxygen detection part are similarly heated and the resistance value changes, which affects the control temperature of the oxygen detection part. That is, the temperature control accuracy of the oxygen detection section deteriorates.

FIG. 3 shows an example of a two-terminal heater, and FIG. 4 shows an example of a heater control circuit for controlling the heater shown in FIG.

As a means of improving the accuracy of the temperature-side fence of the oxygen detecting section, it is conceivable to increase the resistance concentration ratio of a so that the resistance ratio between a, which is the oxygen detecting section, and the lead section becomes a) b. (At this time, it is ideal to bring the resistance of b close to zero.) To achieve this, the resistance value can be reduced by increasing or thickening the cross-sectional area between b of the lead part in the process process, but conversely, The amount of platinum material used has also increased, and printing industry!
This increases the number of resistors, which leads to an increase in cost, and there are limits to this resistance concentration method.

Next, as a method of controlling heater temperature using a two-terminal type heater, a bridge type circuit as shown in FIG. 4 is generally known.

In the figure, vI+ is a power supply, TR is a transistor, and 50
．． 51 and 52 are resistors, and 53 is a differential amplifier.

Now, set the resistance values of resistors 50 and 51.52 to Rso
+Rs+, resistance value of R1121 heater 5'eRis
- Then the equilibrium condition in this bridge circuit is RHll-
RSl=RIIO-Rs2, and power is supplied to the heater 5 to control heating so that both sides are equal.

However, as mentioned above, in addition to the resistance a of the oxygen detection section, the heater 5 has a resistance at the lead section, so the temperature ss is a.
The temperature is controlled so as to be balanced at a resistance value of -4-b.

Therefore, for example, when the ambient temperature rises, the resistance value of the lead section also increases, and the heater resistance value RH5 under equilibrium conditions.
Since a is always constant, the resistance value of a becomes lower accordingly, which causes the temperature of the oxygen detection section to be lower than the target control temperature, which has a negative effect on the output characteristics.

[Purpose of the invention]

The object of the present invention is to provide a simple heater control circuit in which a temperature setting resistor is arranged in series with a heater having a temperature detection lead, and the voltage drop difference between the two is controlled to be zero. The present invention provides a heater temperature control circuit for a fuel ratio sensor.

[Summary of the invention]

Air-fuel ratio sensors manufactured using a thick film process have a high operating temperature, and their characteristics are greatly affected by temperature, so it is desirable to use a heater integrated with the sensor to heat the sensor to a constant temperature.

The temperature control method using a two-terminal heater has unavoidable problems such as an increase in cost due to an increase in the resistance concentration ratio of the heating section and a decrease in temperature control accuracy due to the influence of temperature on the resistance value of the lead section.

To deal with this, we used a four-terminal heater to detect the temperature of the heater from the heating part, and by making it possible to ignore the temperature effect on the resistance value of the lead part, we were able to ignore the effect of the ambient temperature on the sensor. This makes it possible to control temperature with high precision without being affected by heat.

In addition, it is theoretically possible to control a 4-terminal heater using the bridge method, which is considered to be the simplest method to control the temperature of the heater because it has the least number of circuit components. Since it is assembled using resistance values, the power supply is doubled and all the resistance parts generate heat, making it impractical.

Based on the above background, in order to deal with these problems, the present invention provides a four-terminal heater with a temperature setting resistor in series, on the condition that the resistance-temperature characteristics of the heating part of the heater is known, to set the desired control temperature. A heater that can accurately control the temperature of the heating part of the heater by controlling the potential difference between the voltage drop that occurs in the heating part of the heater and the voltage drop that occurs in the temperature setting resistor part to zero. This also made it possible to implement the control circuit.

[Embodiments of the invention]

FIG. 5(a) shows an example of a four-terminal heater made by a thick film process used in the heater temperature control circuit for an air-fuel ratio sensor according to the present invention, and Q3) shows a specific example of the heater control circuit. .

In FIG. 5(a), a is a heating part that heats the oxygen detection part, b and c are lead parts of heating part a, and d and e are heating part a1.
That is, a four-terminal type heater is configured by a detection lead part that detects an electric signal between leads A and C. According to the four-terminal heater described above, the heating state of the heating part a is detected by the lead parts d and e.
Since it is detected as an electrical signal in the heating area, the temperature effect on the resistance value of the leads A and C due to the ambient temperature, which was a problem with conventional two-terminal heaters, can be ignored, and only the heating state at the heating area A can be detected. This makes it possible to control the temperature with high accuracy.

In FIG. 5(b), VB is the power supply voltage, Tr is a transistor that connects its collector to the power supply voltage Vi, R8 is a temperature detection resistor whose one end is connected to the emitter end of the transistor Tr, and Rb is a temperature detection resistor whose one end is connected to the power supply voltage Vi. The resistance of the lead part of the four-terminal heater connected to the setting resistor Rs, Ra is the resistance of the heating part of the four-terminal heater whose one end is connected to the lead part Rh, Rc is the resistance of the heating part of the four-terminal heater connected to the lead part Rh, and Rc is the resistance of the heating part of the four-terminal heater connected to the lead part Rh. A resistance of a lead part whose other end is grounded, 101 is a resistor whose one end is connected to the emitter of the transistor Tr, 102 is a resistor whose one end is connected to the resistor 101,
The other end of the resistor is grounded, and one end is the temperature setting resistor R1! The resistor 104 has one end connected to the resistor 1
The resistor 105 connected to 03 has its positive input end connected to the midpoint of resistors 101 and 102, and its negative input end connected to resistor 1.
At the midpoint of 03 and 104, a differential amplifier whose output end is connected to the resistor 104, 106 is a resistor whose one end is connected to the resistor of the heating part of the heater, and 107 is a resistor whose one end is connected to the resistor 106.
, a resistor whose other end is grounded, 108 is a resistor whose one end is connected to the resistor Rc of the lead part of the heater, 109 is a resistor whose one end is connected to the resistor 108, and 110 is a resistor whose positive input end is connected to the resistor 108. Connect the minus input end to the midpoint of resistor 108°109, and the output end to resistor 10.
A differential amplifier 120 has a positive input terminal connected to the output terminal of the differential amplifier 105, a negative terminal to the output terminal of the differential amplifier 110, and an output terminal connected to the base of the transistor Tr. Configure the heater control circuit with a dynamic amplifier.

The operation here is as follows.

As is generally known, the relationship between the resistance Ri of a heater made of platinum and the ambient temperature T is expressed by the following formula (3) where RIHo is the resistance of the heater at zero degrees, and α is the temperature coefficient of platinum. It is.

In the present invention, it is assumed that after the heater is fabricated, the relationship between the resistance value RH of the heater and the ambient temperature T, particularly the characteristics of the resistance R of the heating section, are known in advance.

Therefore, by determining the control temperature of the heater heating section, the heater resistance value R1 at that time is also determined.

Now, when the control temperature is determined to be t"Tst, the resistance value of the heater becomes Rm from equation (3), and the heater control circuit controls the resistance value of the heating part to be constant for 1, but at this time, The control target for controlling the resistance value of the heating section to BJaT uses a signal of a voltage difference caused by a voltage drop across the temperature setting resistor R8.

Since the same heater current II flows through the temperature setting resistor R8 and the heating section resistor R, the following voltage drops occur in each of them.

The differential voltage V Rs due to the voltage drop across the temperature setting resistor is Vns = Rm ・II --- (4) The differential voltage Vy+a due to the voltage drop across the resistor R of the heating section is VR-=R-・In... ...... (5).

Therefore, from equations (4) and (5), if the differential voltages Vns HVisa of both equations at the control temperature are equal, the resistance ratio a of the heating section? and the temperature setting resistor R1 have the same value, and the heating section is controlled at the control target temperature. Therefore, the heater control circuit should perform feedback control so that the difference between Vns and VRTH becomes zero.

The operational amplifiers 101 to 105, the operational amplifiers 2110 to 110, and the differential amplifiers 120 are for this purpose.

The output v1 of the operational amplifier 1 has the resistance value of the resistors 101 to 104 respectively as Riot + Rsoz * R103, RI
04 Toshinashi &o+ = R103=几t R162=
Let 14 = Rz, where el is on the resistor R,,,, e2 is on the resistor R1
This is the input voltage input to the oll side.

Therefore, if R+ -R is set, it becomes a differential amplifier with an amplification factor of l.

The same applies to the output voltage v2 of the operational amplifier 2 composed of 106 to 110.

The differential amplifier 120 operates to make this output voltage V1r Vz + (that is, the difference voltage between the temperature setting resistor R3 and the heating section becomes zero), and its output voltage V3 is equal to the difference between Vl and V2. The higher the voltage, the higher the voltage, the collector-emitter current of the transistor Tr, that is, the heater current IR value, is increased, heating the heater part, and conversely, when the potential difference is reversed, the heater current is repeatedly controlled to eliminate VI+V2. The potential difference between the heating parts is made zero, that is, the resistance value of the heating section is kept constant at the controlled temperature.

Further, when heating the heater at a high temperature, the heater current Isi also increases accordingly. Therefore, the temperature setting resistor R1 also generates heat, resulting in power loss. Therefore, the smaller the resistance value of the temperature setting resistor R11, the better the efficiency. This problem can be solved by adjusting the gain of the operational amplifier (R*/Rt in equation (6)). That is, GAINe of the operational amplifier 1 may be increased to compensate for the reduction in the resistance value of the temperature setting resistor R3.

FIG. 6 shows the temperature dependence of the lean sensor output-excess air ratio characteristic.

In the figure, the horizontal axis is the excess air ratio λ, and the vertical axis is the pump current I.
Show P money. In addition, in the figure, Tt # Tz r Ts indicates the temperature of the lean sensor, and the magnitude relationship is Ts > T2
>T3.

As is clear from this, the output pump current It of the lean sensor has a proportional relationship when the excess air ratio λ is λ≧1, but (
As shown in equation 2), the output is proportional to the 3z4th power of the absolute temperature T, so that it has temperature dependence such that as the temperature T increases, the characteristics increase, and conversely, as the temperature T decreases, the characteristics decrease.

FIG. 7 shows a comparative example of the output-temperature characteristics of lean sensors in the prior art and in the present invention.

In the figure, the horizontal axis shows the temperature of the lean sensor, and the vertical axis shows the output value of the lean sensor. In addition, in the figure, X is without a heater,
y is the case when the temperature is controlled by a two-terminal heater, and Z is the case when the temperature is controlled by a four-terminal heater. In this measurement, the surrounding atmospheric gas was held constant and the temperature was varied. In addition, the characteristic y1 z with heater is the control temperature of heater 800
It was adjusted as °C.

As a result, as mentioned above, the output of the lean sensor is affected by temperature in proportion to the 374th power of the absolute temperature, and the characteristic without a heater x Jd lower operating limit temperature is approximately 60 (1 or more, the output value changes greatly, and below the lower operating limit temperature Becomes insensitive to atmospheric oxygen.

Furthermore, in the case of y controlled using a two-terminal heater, compared to the characteristics of the ambient temperature SOOυ, the output decreases when the ambient temperature is 800° C. or higher, and the output increases when the ambient temperature is 800° C. or lower. This is due to the temperature effect of the resistance value of the heater lead part. Above 800T, the resistance of the lead part increases and the control temperature of the detection part decreases accordingly. Conversely, below 800"C, the resistance value of the lead part increases. This is due to the fact that the control temperature of the detection section increases as the temperature decreases, and the characteristics increase.

On that point, Z
Although the output increases when the control target temperature is 800° C. or higher, the output remains constant over the entire temperature range below the ambient temperature of 800° C., and there is no temperature influence on the lean sensor output. This is the control temperature detection of the heating part as shown in Figure 5(a).
This is possible because the temperature effect on the lead part can be ignored since it is performed using the detection lead shown in FIG.

FIG. 8 shows another embodiment of a heater control circuit for controlling a four-terminal heater.

In the figure, 200 is a constant current source, 201 is a constant current source 20
0, 202fi voltage source, 203 a switch connected to voltage source 202, 204 a sawtooth wave generation circuit, 205 a switch connected to the output of operational amplifier 110, 206 a capacitor connected to switch 205. , 207 is a resistor connected to the capacitor 206, 2
08 is a capacitor connected to the resistor 207, 209 is an operational amplifier connected to the resistor 207 and the capacitor 208, 2
10 is an input terminal for inputting information for determining the control temperature of the heater; 207 to 210 constitute an integrator; 211 is a comparator connected to the operational amplifier 209 and the sawtooth wave generation circuit 204; and 212 is an input terminal to the comparator 211. The connected inverter elements 81 and 82 constitute a pulse heating type heater control circuit using the switches 203 and 201t - control signal.

The operation here is as follows.

Control target temperature TsFi heating section resistance value R, and constant current source 2
It is determined by the product of constant voltage construction 1 supplied from 00, and the corresponding temperature control voltage V? is input to the integrator from the input terminal 210.

When the power is turned on when the ambient temperature of the sensor is lower than the control target temperature Ts, the comparator 211 compares the level of the output Vl of the integrator with the sawtooth wave height value VN repeatedly generated at a certain period by the sawtooth wave generation circuit 204. The output goes to "High" V when VN>Vl, VlI<
When it is Vl, it becomes a "LOW" level, and this becomes an on-off signal (pulse width time ratio=duty ratio) within a - period.

Switches 201 and 202 are control signals S1 and 8z
When the output of the comparator 211 is at the ``)iigh'' level, the switch 203 is conductive, and when it is at the ``LOW'' level, the switch 203 is conductive, and when it is at the ``LOW'' level, the switch 203 is conductive. 201 becomes conductive.

Now, when the output of the comparator 211 is at the "°)high" level, the switch 203 is turned on and power is supplied to the heater from the voltage source 202, heating the heater.
Since the output of switch 1 is at "LOW" level, switch 20
3 becomes non-conductive, and conversely, the switch 201 becomes conductive, supplying constant current ■1 to the heater. and 106-1
The operational amplifier 10 uses the potential difference across the resistor Ra of the heating section -7, and its output charges the capacitor 206 via the switch 205 which becomes conductive at the same time as the switch 201. This charged value corresponds to the resistance value of the heater heating section, that is, the temperature, and is compared with the control voltage corresponding to the control target temperature input from the input terminal 210, and the integrator integrates the difference between the two. When this difference is large, the output of the integrator is high, and conversely, when it is small, the output is low. Therefore, the heating time width for supplying electric power to heat the heater changes depending on the change in the output of the integrator, and the duty ratio between the heating time width and the cutoff time width becomes narrower as the temperature difference becomes larger. In addition, inverse ζ, the closer the temperature is to the target temperature, the shorter the heating time width becomes.

What has been described above is the operation within the sawtooth wave cycle, but as mentioned above, the sawtooth wave from the sawtooth wave generation circuit is generated repeatedly, and temperature control is performed by repeating the above-mentioned operation. be.

Therefore, even in a heater control circuit using the pulse heating method, by using a 4-terminal heater and making it possible to detect signals from only the heating section, the influence of the ambient temperature on the heater lead section can be ignored and highly accurate temperature measurements can be achieved. .

〔Effect of the invention〕

According to the present invention, there are the following effects.

(1) Place a temperature detection resistor in series with the 4-terminal heater,
By controlling the voltage drop at the heater heating section to be equal to the voltage drop across the resistor, heater control can be achieved with a simple circuit configuration.

(2) The configuration allows the detection sensitivity of the voltage drop detection unit at the temperature detection resistor to be freely adjusted, which has the effect of preventing heat generation at this resistor and eliminating power loss at sources other than the heater.

(3) Also in the pulse heating heater control circuit, adding a four-terminal heater and a circuit for detecting signals from the heating section has the effect of improving temperature control accuracy and reducing power loss in areas other than the heater.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the principle structure of a lean sensor made using a thick film process, Fig. 2 is a diagram showing an example of the mounting type of a lean sensor, Fig. 3 is a diagram showing a conventional two-terminal heater, and Fig. 4 is a control circuit diagram for controlling a two-terminal heater, Fig. 5(a)r
/'i A diagram showing a four-terminal type heater according to the present invention, Φ) is a heater control circuit diagram according to the present invention that controls a four-terminal type heater, and Fig. 6 is a diagram showing the ambient temperature dependence of lean sensor characteristics.
FIG. 7 is a diagram showing the influence of heater temperature control accuracy on lean sensor characteristics, and FIG. 8 is a diagram showing an application example of a control circuit for a four-terminal heater. a... Heating part, b, c... Lead part, d, e...
Detection lead section, VB...Power supply voltage, Tr...Transistor, Rs...Resistor, 101-104...Resistor, 105...Differential amplifier, Gil I device. #1 Atrocities 2 Diagram 3 A Granny 4 Hard Sac S 圀(L) End Otsu Ward 1 Item k degree T C'Cノ

Claims (1)

[Claims] 1. In a heater temperature control circuit consisting of a heater having a temperature detection lead part specially provided in order to improve temperature control accuracy of the heater and means for controlling the heater, the heater has a temperature setting function. A heater temperature control circuit for an air-fuel ratio sensor, characterized in that the accuracy of temperature control of the heater can be improved with a simple control circuit configuration in which resistors are arranged in series and feedback control is performed so that the voltage drop between the two is the same value. 2. In claim 1, by making it possible to adjust the detection sensitivity t of the voltage drop in the temperature setting resistor, heat generation in the resistor is prevented, and power loss in areas other than the heater and the power transistor is reduced. 1. A heater temperature control circuit for an air-fuel ratio sensor, which is characterized by being able to eliminate this, and to achieve miniaturization and cost reduction. 3. In claim 1, by adding a detection circuit that detects the signal of the heating part even when controlling the heater using a pulse heating method, high-precision temperature control can be realized and power loss other than the heater can be eliminated. A heater temperature control circuit for an air-fuel ratio sensor.