Sensor circuit

The present invention relates to a sensor circuit.

A sensor circuit that detects a physical quantity by using a change in the resistance of a thermistor is known. This type of sensor circuit has a detection thermister that is affected by the physical quantity of the measurement target and a compensation thermister that is not affected by the physical quantity of the measurement target, and the resistance value of the detection thermista is the physical quantity of the measurement target and the measurement. Although it is affected by physical quantities other than the target, the resistance value of the compensation thermista is affected only by the physical quantities other than the target. Therefore, the physical quantity to be measured is detected by the difference in the resistance values of these two thermistors. Based on such a principle, various physical quantities such as temperature, gas concentration, humidity, and flow velocity can be detected.

For example, Patent Document 1 describes a first output voltage of a series circuit of an infrared detection heat-sensitive element (thermista) and a resistance element, and a second output voltage of a series circuit of a temperature compensation heat-sensitive element (thermista) and a resistance element. Of the third output voltage that outputs the difference between the first output voltage and the second output voltage, the first and third output voltages are converted into digital values, and these two digital values are used as the basis. A temperature detection method for detecting the temperature of the heating element has been proposed.

In the temperature detection method described in Patent Document 1, infrared light (infrared) in which a third output voltage that outputs the difference between the first output voltage and the second output voltage is radiated from a heating roller (heat source). It reflects the temperature difference between the amount of heat including the ambient temperature (external ambient temperature) and the ambient temperature, that is, the amount of heat of pure infrared light radiated from the heating roller. Since this third output voltage is smaller than the first output voltage that reflects the atmospheric temperature, it is amplified by using a differential amplifier circuit.

<p><patcit num="1"><text>Japanese Unexamined Patent Publication No. 2003-57116</text></patcit></p>

<p num="0006"> By the way, as in Patent Document 1, when the temperature of a heat source is measured in a non-contact manner, the temperature of the measurement target is used as a reference temperature, and the fluctuation amount from that temperature is measured. At this time, the fluctuation of the third output voltage becomes an effective signal component.</p><p num="0007"> However, in the temperature detection method shown in Patent Document 1, in addition to the fluctuation of the third output voltage, a voltage component reflecting the temperature difference between the reference temperature and the atmospheric temperature is superimposed, and the voltage component depends on the atmospheric temperature. It fluctuates greatly. Therefore, the amplification factor of the differential amplifier circuit needs to be within the allowable input voltage range of the A / D converter connected to the next stage, assuming that the superimposed voltage is maximized. Therefore, the amplification factor with respect to the fluctuation of the third output voltage, which is an effective signal component, is limited to be relatively low, and the change in output voltage per temperature becomes small. As a result, there is a problem that the temperature detection accuracy is low.</p><p num="0008"> The present invention has been made in view of such a problem, and an object of the present invention is to provide a sensor circuit capable of suppressing a decrease in detection accuracy of a physical quantity to be measured.</p>

<p num="0009"> In order to solve the above problems, the sensor circuit according to the present invention is connected in series with the first resistance connected to the first pole of the power supply and the second pole of the power supply. To the first detection circuit having the first thermistor affected by the physical quantity to be measured, and the second resistance and the second resistance which are substantially equal to the resistance value of the first resistance connected to the first pole. The influence of the physical quantity of the measurement target connected in series and connected to the second pole has been reduced. The influence of the additional resistance connected in parallel to the second thermistor and the second thermistor and the physical quantity of the measurement target has been reduced. A second detection circuit having a series circuit of the additional thermistor is provided, and the resistance value of the first thermistor and the combined resistance value of the second thermistor, the additional resistance, and the additional thermistor are measured by the first thermistor. It is characterized by being approximately equal when influenced by physical quantities.</p><p num="0010"> With the above configuration, when affected by the physical quantity to be measured, the output voltage of the first detection circuit and the output voltage of the second detection circuit become substantially equal, and the output voltage of the first detection circuit and the second detection circuit become substantially equal. The difference in the output voltage of the detection circuit is close to 0V. Here, since the output of the first detection circuit and the output of the second detection circuit are signals for differential input, when this output voltage difference is input to the differential amplifier circuit, the output of the differential amplifier circuit becomes It becomes close to 0V. That is, even if the physical quantity fluctuates based on the physical quantity to be measured, only the fluctuation amount is output based on 0V as the output of the differential amplifier circuit, so that the amplification factor of the differential amplifier circuit is efficiently increased. Therefore, the change in output voltage per physical quantity can be increased. As a result, it is possible to suppress a decrease in the detection accuracy of the physical quantity to be measured.</p><p num="0011"> It is preferable that the second detection circuit includes a plurality of additional resistors and a plurality of additional thermistors connected in parallel to the second thermistor, and the number of the plurality of additional resistors and the number of the plurality of additional thermistors are the same. Thereby, the resistance value of the first thermistor of the first detection circuit, the second thermistor of the second detection circuit, the plurality of additional resistors, and the combined resistance value of the plurality of additional thermistors can be further brought closer to each other. That is, the difference between the output voltage of the first detection circuit and the output voltage of the second detection circuit becomes extremely close to 0V. Therefore, when the physical quantity of the measurement target is constant, the physical quantity of the measurement target can be accurately detected even in the binarization process in which the threshold voltage is set near 0V, which does not require a complicated circuit, and the circuit is simplified. Is possible.</p><p num="0012"> The physical quantity to be measured may be temperature. In this case, the output voltage of the differential amplifier circuit that amplifies the difference between the output voltage of the first detection circuit and the output voltage of the second detection circuit is the output voltage corresponding to the amount of heat of infrared rays, so that the differential amplifier circuit Amplification rate can be increased efficiently. As a result, it is possible to suppress a decrease in the detection accuracy of the temperature of the heat source.</p>

<p num="0013"> According to the present invention, it is possible to provide a sensor circuit capable of suppressing a decrease in detection accuracy of a physical quantity to be measured.</p>

<figref num="1">It is a circuit block diagram which shows the sensor circuit which concerns on 1st Embodiment of this invention.</figref><figref num="2">It is a circuit block diagram which shows the sensor circuit which concerns on 2nd Embodiment of this invention.</figref><figref num="3">It is a circuit block diagram which shows the modification of the sensor circuit which concerns on 2nd Embodiment of this invention.</figref><figref num="4">It is a graph which shows the temperature characteristic of the output of the 1st detection circuit and the output of the 2nd detection circuit of the sensor circuit which concerns on 1st Embodiment.</figref><figref num="5">It is a graph which shows the temperature characteristic of the output of the differential amplifier circuit of the sensor circuit which concerns on 1st Embodiment.</figref><figref num="6">It is a graph which shows the temperature characteristic of the output of the 1st detection circuit and the output of the 2nd detection circuit of the sensor circuit which concerns on 2nd Embodiment.</figref><figref num="7">It is a graph which shows the temperature characteristic of the output of the differential amplifier circuit of the sensor circuit which concerns on 2nd Embodiment.</figref><figref num="8">6 is a graph showing the temperature characteristics of the output of the first detection circuit and the output of the second detection circuit of the sensor circuit according to Comparative Example 1.</figref><figref num="9">It is a graph which shows the temperature characteristic of the output of the differential amplifier circuit of the sensor circuit which concerns on Comparative Example 1.</figref><figref num="10">It is a circuit block diagram which shows the sensor circuit which concerns on the comparative example 1. FIG.</figref>

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the description, the same reference numerals will be used for the same elements or elements having the same function, and duplicate description will be omitted.

(First Embodiment) First, the configuration of the sensor circuit 100 according to the first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a circuit configuration diagram showing a sensor circuit according to the first embodiment of the present invention. In this embodiment, a sensor circuit for measuring the temperature of the heat source in a non-contact manner will be described. That is, the measurement target is a heat source, and the physical quantity of the measurement target is temperature.

As shown in FIG. 1, the sensor circuit 100 includes a power supply V1, a first detection circuit 10, a second detection circuit 20, a differential amplifier circuit 30, and an A / D (analog / digital) conversion circuit. It has 40 and.

The power supply V1 supplies a DC voltage to the first detection circuit 10 and the second detection circuit 20. As the power supply V1, a stabilized constant voltage power supply is used in order to suppress the influence of noise on each circuit output. Further, the power supply V1 has a first pole and a second pole. In the present embodiment, the first pole will be described as a positive electrode, and the second pole will be described as a negative electrode. Hereinafter, the first electrode is referred to as a "positive electrode" and the second electrode is referred to as a "negative electrode".

The first detection circuit 10 is a circuit for detecting infrared rays radiated from a heat source. The first detection circuit 10 is composed of a series circuit of a first resistor R1 connected to the positive electrode of the power supply V1 and a first thermistor Rth1 connected to the negative electrode of the power supply V1.

The first thermistor Rth1 is arranged so as to be affected by the amount of heat of infrared rays radiated from the heat source, which is the physical quantity to be measured. That is, when the first thermistor Rth1 is affected by the amount of heat of infrared rays radiated from the heat source, the resistance value changes as the temperature of the first thermistor Rth1 changes. The resistance value of the temperature of the first thermistor Rth1 is determined by the temperature applied by the influence of the external environmental temperature and the amount of heat of infrared rays radiated from the heat source.

As the first thermistor Rth1, an NTC (Negative Temperature Cooperative) thermistor having a negative temperature coefficient containing a metal oxide as a main component is used. The characteristics of the thermistor are approximated by the following equation (1), where RA and RB are the resistance values of the thermistor at arbitrary temperatures TA [K] and TB [K], and the thermistor constant is B (B constant). .. The larger the value of the B constant, the larger the rate of change in resistance to temperature change. RA = RB × e^{B (1 / TA-1 / TB)} Equation (1) Further, since the thermistor does not have a linear relationship between the temperature and the resistance characteristic, in the first detection circuit 10, the first resistor R1 is connected in series to the first thermistor Rth1 to bring the temperature and the output voltage closer to the linear relationship. There is. The resistance value Rr1 of the first resistance R1 is Rthl for the resistance value of the thermistor at the lower limit temperature of the external environmental temperature range, Rthm for the resistance value of the thermistor at the intermediate temperature of the external environmental temperature range, and the upper limit temperature of the external environmental temperature range. Assuming that the resistance value of the thermistor in is Rthh, the relationship of the following equation (2) is satisfied. Rr1 = {2 x Rthl x Rth-Rthm (Rthl + Rthh)} / {2 x Rthm- (Rthl + Rthh)} Equation (2) Therefore, the resistance value Rr1 of the first resistor R1 that brings the temperature and the output voltage closer to each other can be calculated from the above equation (2).

The first detection circuit 10 outputs the voltage obtained by dividing the DC voltage supplied from the power supply V1 by the first resistor R1 and the first thermistor Rth1 as the output VO1. That is, the output VO1 of the first detection circuit 10 has a DC voltage value supplied from the power supply V1 as Vr1, a resistance value of the first resistor R1 as Rr1, and a resistance value of the first thermista Rth1 as Rthr1. The output VO1 of the detection circuit 10 of 1 satisfies the relationship of the following equation (3). VO1 = Vr1 × Rthr1 / (Rthr1 + Rr1) Equation (3) The output VO1 of the first detection circuit 10 is connected to the differential amplifier circuit 30.

The second detection circuit 20 is a circuit for detecting the external environmental temperature. The second detection circuit 20 includes a second resistor R2 that is substantially equal to the resistance value of the first resistor R1 connected to the positive electrode of the power supply V1, a second thermistor Rth2 connected to the negative electrode of the power supply V1, and a second. It is composed of an additional resistance R3 connected to the midpoint between the resistance R2 of 2 and the second thermistor Rth2, and an additional thermistor Rth3 connected to the negative electrode of the power supply V1. More specifically, the second resistor R2 and the second thermistor Rth2 are connected in series to form a series circuit, and the additional resistor R3 and the additional thermistor Rth3 are connected in series to form a series circuit, and the additional resistor is formed. The series circuit of R3 and the additional thermistor Rth3 is connected in parallel to the second thermistor Rth2. In the present embodiment, the additional resistor R3 is connected to the midpoint between the second resistor R2 and the second thermistor Rth2, and the additional thermistor Rth3 is connected to the negative electrode of the power supply V1, but the additional thermistor Rth3 is the first. It may be configured so that it is connected to the midpoint between the resistor R2 of 2 and the second thermistor Rth2, and the additional resistor R3 is connected to the negative electrode of the power supply V1.

The temperature of the second thermistor Rth2 and the additional thermistor Rth3 is the same as the external environmental temperature, and the resistance value is determined by this temperature. That is, the second thermistor Rth2 and the additional thermistor Rth3 are arranged so as to reduce the influence of the amount of heat of infrared rays radiated from the heat source, which is the physical quantity to be measured. Here, it is preferable that the second thermistor Rth2 and the additional thermistor Rth3 are arranged at positions that are not affected by the amount of heat radiated from the heat source, but the first detection circuit 10 and the second detection circuit 20 are arranged. If structurally close to each other, the second thermistor Rth2 and the additional thermistor Rth3 may be placed at positions affected by the amount of infrared rays radiated from the heat source, to the extent that there is no functional problem. good.

As the second thermistor Rth2 and the additional thermistor Rth3, similarly to the first thermistor Rth1, an NTC thermistor having a negative temperature coefficient containing a metal oxide as a main component is used.

The second detection circuit 20 outputs a voltage obtained by dividing the DC voltage supplied from the power supply V1 by the combined resistance of the second resistor R2, the second thermistor Rth2, the additional resistor R3, and the additional thermistor Rth3 as the output VO2. To do. That is, the combined resistance value Rcb of the second thermistor Rth2, the additional resistor R3, and the additional thermistor Rth3 is such that the resistance value of the second thermistor Rth2 is Rthr2, the resistance value of the additional resistance R3 is Rr3, and the resistance value of the additional thermistor Rth3 is Rr3. Assuming Rthr3, the following equation (4) is obtained. Rcb = Rthr2 × (Rr3 + Rthr3) / (Rthr2 + Rr3 + Rthr3) Equation (4) Further, the output VO2 of the second detection circuit 20 satisfies the relationship of the following equation (5), where Vr1 is the DC voltage value supplied from the power supply V1 and Rr2 is the resistance value of the second resistor R2. Become. VO2 = Vr1 × Rcb / (Rcb + Rr2) Equation (5) The output VO2 of the second detection circuit 20 is connected to the differential amplifier circuit 30 and the A / D conversion circuit 40.

In the present embodiment, the resistance value Rthr1 of the first thermistor Rth1 of the first detection circuit 10 and the combined resistance value Rcb of the second thermistor Rth2, the additional resistance R3, and the additional thermistor Rth3 of the second detection circuit 20 However, when the first thermistor Rth1 receives the heat of infrared rays emitted from the heat source, they are substantially equal. That is, the output VO1 of the first detection circuit 10 and the output VO2 of the second detection circuit 20 when the first thermistor Rth1 is affected by the amount of heat of infrared rays radiated from the heat source are at least within the external environmental temperature range. Can be the same output voltage. At this time, the operation similar to that of the first detection circuit 10 when the second detection circuit 20 is affected by the amount of heat of infrared rays radiated from the heat source is exhibited, and the output VO1 of the first detection circuit 10 is exhibited. The difference between the output voltage of the output VO2 of the second detection circuit 20 and the output voltage of the output VO2 of the second detection circuit 20 is close to 0V in the external environmental temperature range. When the temperature of the heat source changes with reference to the temperature of the heat source in this state, the difference between the output voltage of the output VO1 of the first detection circuit 10 and the output voltage of the output VO2 of the second detection circuit 20 is output only by the fluctuation amount. .. That is, it is possible to obtain an output voltage that reflects only the temperature change of the heat source. Here, the second thermistor Rth2 can adjust the upper limit temperature of the external environmental temperature range. However, with only the second thermistor Rth2 capable of adjusting the upper limit temperature of the external environmental temperature range, the resistance value of the second thermistor Rth2 is set to the resistance value of the first thermistor Rth1 when receiving the amount of heat radiated from the heat source. It is difficult to make them equal. On the other hand, as in the present embodiment, in addition to the second thermistor Rth2, an additional resistance R3 capable of finely adjusting the intermediate temperature in the external environmental temperature range and an additional thermistor Rth3 capable of adjusting the lower limit temperature in the external environmental temperature range are provided. Since the entire temperature range of the external environment temperature range can be adjusted, the combined resistance value Rcb of the second thermistor Rth2, the additional resistance R3, and the additional thermistor Rth3 is made substantially equal to the resistance value of the first thermistor Rth1. be able to.

The differential amplifier circuit 30 is a circuit that amplifies the difference between two input voltages with a constant coefficient. In the present embodiment, the output VO1 of the first detection circuit 10 and the output VO2 of the second detection circuit 20 are set as two input voltages, and the difference between the output voltage which is the output VO1 and the output voltage which is the output VO2 is taken. Only this difference is amplified. That is, the output VO1 of the first detection circuit 10 and the output VO2 of the second detection circuit 20 are signals for differential input. The differential amplifier circuit 30 outputs a voltage obtained by amplifying the difference between the two input voltages as an output VO3. As the output voltage of the differential amplifier circuit 30, a voltage obtained by adding a voltage obtained by amplifying the difference between the two input voltages to the reference voltage is output. At this time, the reference voltage is set so as to be within the input voltage range of the circuit of the next stage. For example, if the input voltage range of the circuit in the next stage is 0V to 1V, the amplification factor of the differential amplifier circuit 30 is 20 times, and the reference voltage of the differential amplifier circuit 30 is set to 0.5V, the output of the differential amplifier circuit 30 is output. The voltage is 0.5V when the difference between the two input voltages is 0V, 0.7V when the difference between the two input voltages is 0.01V, and 0 when the difference between the two input voltages is -0.01V. It becomes .3V. The amplification factor of the differential amplifier circuit 30 is appropriately set within the input voltage range of the circuit in the next stage. The output VO3 of the differential amplifier circuit 30 is connected to the A / D conversion circuit 40.

The A / D conversion circuit 40 is a circuit that converts an analog value into a digital value. In the present embodiment, the output VO3 of the differential amplifier circuit 30 and the output VO2 of the second detection circuit 20 are converted into digital values. When converting from an analog value to a digital value, the voltage for one bit, that is, in the case of a non-contact temperature sensor, the smaller the temperature for one bit, the higher the temperature accuracy. In order to achieve high accuracy, it is conceivable to use an A / D conversion circuit 40 having a high resolution and to increase the input voltage. Therefore, the accuracy can be improved by setting the input voltage as large as possible within the input voltage range of the A / D conversion circuit 40. Although not shown in FIG. 1, the value converted into a digital value by the A / D conversion circuit 40 is taken into a microcomputer and converted by a temperature conversion table or a function to detect the temperature of the heat source.

As described above, in the sensor circuit 100 according to the present embodiment, when the physical quantity to be measured is the temperature and is affected by the heat quantity of the infrared rays emitted from the heat source, the output voltage of the first detection circuit 10 And the output voltage of the second detection circuit 20 become substantially equal, and the difference between the output voltage of the first detection circuit 10 and the output voltage of the second detection circuit 20 becomes close to 0V. Here, since the output VO1 of the first detection circuit 10 and the output VO2 of the second detection circuit 20 are signals for differential input, when this output voltage difference is input to the differential amplifier circuit 30, it is differential. The output of the amplifier circuit 30 is close to 0V. That is, even if the temperature of the heat source changes based on the amount of heat of infrared rays radiated from the heat source and the amount of heat of infrared rays radiated from the heat source fluctuates, the output of the differential amplifier circuit 30 outputs only the fluctuation amount based on 0 V. Therefore, the amplification factor of the differential amplifier circuit 30 can be efficiently increased, so that the change in output voltage per temperature becomes large. As a result, it is possible to suppress a decrease in the detection accuracy of the temperature of the heat source.

(Second Embodiment) Next, the configuration of the sensor circuit 200 according to the second embodiment of the present invention will be described with reference to FIG. FIG. 2 is a circuit configuration diagram showing a sensor circuit according to a second embodiment of the present invention. The sensor circuit 200 according to the second embodiment will also be described using a sensor circuit that measures the temperature of the heat source in a non-contact manner. That is, the measurement target is a heat source, and the physical quantity of the measurement target is temperature. In the sensor circuit 200 according to the second embodiment, the second detection circuit 120 includes an additional resistor R4 and an additional thermistor Rth4, and a window comparator 50 is provided instead of the A / D conversion circuit 40. , It is different from the sensor circuit 100 according to the first embodiment. Hereinafter, the points different from the first embodiment will be mainly described.

As shown in FIG. 2, the sensor circuit 200 includes a power supply V1, a first detection circuit 10, a second detection circuit 120, a differential amplifier circuit 30, and a window comparator 50.

The second detection circuit 120 is a circuit for detecting the external environmental temperature, similarly to the second detection circuit 20 of the first embodiment. The second detection circuit 120 includes a second resistor R2 connected to the positive electrode of the power supply V1, a second thermistor Rth2 connected to the negative electrode of the power supply V1, a second resistor R2, and a second thermistor Rth2. An additional resistor R3 connected to the midpoint, an additional thermistor Rth3 connected to the negative electrode of the power supply V1, an additional resistor R4 connected to the midpoint between the second resistor R2 and the second thermistor Rth2, and a power supply. It is composed of an additional thermistor Rth4 connected to the negative electrode of V1. More specifically, the second resistor R2 and the second thermistor Rth2 are connected in series to form a series circuit, and the additional resistor R3 and the additional thermistor Rth3 are connected in series to form a series circuit, and the additional resistor is formed. R4 and the additional thermistor Rth4 are connected in series to form a series circuit, the series circuit of the additional resistor R3 and the additional thermistor Rth3 is connected in parallel with the second thermistor Rth2, and the series circuit of the additional resistor R4 and the additional thermistor Rth4 It is connected in parallel to the second thermistor Rth2. That is, the second detection circuit 120 includes a plurality of additional resistors (R3, R4) and a plurality of additional thermistors (Rth3, Rth4), respectively, and has a number of the plurality of additional resistors (R3, R4) and a plurality of additional thermistors (Rth3, Rth4). The numbers of Rth3 and Rth4) are the same. In the present embodiment, the additional resistor R4 is connected to the midpoint between the second resistor R2 and the second thermistor Rth2, and the additional thermistor Rth4 is connected to the negative electrode of the power supply V1, but the additional thermistor Rth4 is the first. It may be configured so that it is connected to the midpoint between the resistor R2 of 2 and the second thermistor Rth2, and the additional resistor R4 is connected to the negative electrode of the power supply V1.

The temperature of the additional thermistor Rth4 is the same as the external environment temperature, and the resistance value is determined by this temperature. That is, the additional thermistor Rth4 is arranged so as to reduce the influence of the amount of heat of infrared rays radiated from the heat source, which is the physical quantity of the measurement target. Here, it is preferable that the additional thermistor Rth4 is arranged at a position that is not affected by the amount of heat radiated from the heat source at all, but the first detection circuit 10 and the second detection circuit 120 are arranged close to each other in terms of structure. If there is no choice but to do so, the additional thermistor Rth4 may be arranged at a position affected by the amount of heat of infrared rays radiated from the heat source to the extent that there is no functional problem.

As the additional thermistor Rth4, like the first thermistor Rth1, the second thermistor Rth2, and the additional thermistor Rth3, an NTC thermistor having a negative temperature coefficient containing a metal oxide as a main component is used.

The second detection circuit 120 divides the DC voltage supplied from the power supply V1 by the combined resistance of the second resistor R2, the second thermistor Rth2, the additional resistor R3, the additional thermistor Rth3, the additional resistor R4, and the additional thermistor Rth4. The compressed voltage is output as output VO4. That is, the combined resistance value Rcb2 of the second thermistor Rth2, the additional resistance R3, the additional thermistor Rth3, the additional resistance R4, and the additional thermistor Rth4 is such that the resistance value of the second thermistor Rth2 is Rthr2 and the resistance value of the additional resistance R3 is Rr3. Assuming that the resistance value of the additional thermistor Rth3 is Rthr3, the resistance value of the additional resistance R4 is Rr4, and the resistance value of the additional thermistor Rth4 is Rthr4, the following equation (6) is obtained. Rcb2 = Rthr2 × (Rr3 + Rthr3) × (Rr4 + Rthr4) / (Rthr2 × (Rr3 + Rthr3) + (Rr3 + Rthr3) × (Rr4 + Rthr4) + Rthr2 × (Rr4 + Rthr4)) Equation (6) Further, the output VO4 of the second detection circuit 120 satisfies the relationship of the following equation (7), where Vr1 is the DC voltage value supplied from the power supply V1 and Rr2 is the resistance value of the second resistor R2. Become. VO4 = Vr1 × Rcb2 / (Rcb2 + Rr2) Equation (7) The output VO4 of the second detection circuit 120 is connected only to the differential amplifier circuit 30.

The window comparator 50 is a kind of voltage comparison circuit element, and has a function of combining two comparators. Specifically, a circuit element that compares one input voltage with two reference voltages and digitally outputs a high level if the input voltage is within the reference voltage range and a low level if the input voltage is outside the reference voltage range. Is. In the present embodiment, the output VO3 of the differential amplifier circuit 30 is used as an input voltage, and a high level or a low level is digitally output as compared with two reference voltages. That is, by setting the two reference voltage ranges of the window comparator 50 to be the heat source temperature to be detected, it is possible to detect whether or not the temperature is a specific heat source temperature. That is, it is possible to discriminate between two states, a specific heat source temperature and another heat source temperature.

Similar to the first embodiment, the present embodiment also has the resistance value Rthr1 of the first thermistor Rth1 of the first detection circuit 10 and the second thermistor Rth2, the additional resistor R3, and the additional thermistor of the second detection circuit 120. The combined resistance values Rcb2 of Rth3, the additional resistance R4, and the additional thermistor Rth4 are substantially equal when the first thermistor Rth1 receives the heat of infrared rays emitted from the heat source. That is, the output VO1 of the first detection circuit 10 and the output VO4 of the second detection circuit 120 when the first thermistor Rth1 is affected by the amount of heat of infrared rays radiated from the heat source are at least within the external environmental temperature range. Can be the same output voltage. In the present embodiment, since the second detection circuit 120 includes a plurality of additional resistors (R3, R4) and a plurality of additional thermistors (Rth3, Rth4), the intermediate temperature and the lower limit temperature in the external environmental temperature range are set. It can be adjusted more closely. Therefore, the difference between the output voltage, which is the output VO1 of the first detection circuit 10, and the output voltage, which is the output VO4 of the second detection circuit 120, is extremely close to 0V.

As described above, the sensor circuit 200 according to the present embodiment includes a plurality of additional resistors (R3, R4) and a plurality of additional thermistors (Rth3, Rth4) connected in parallel to the second thermistor Rth2, respectively, and has a plurality of additional thermistors (Rth3, Rth4). The number of additional resistors (R3, R4) and the number of a plurality of additional thermistors (Rth3, Rth4) are the same. As a result, the resistance value Rthr1 of the first thermistor Rth1 of the first detection circuit 10 and the second thermistor Rth2 of the second detection circuit 120, a plurality of additional resistors (R3, R4), and a plurality of additional thermistors (Rth3, Rth3). The combined resistance value Rcb2 of Rth4) can be further approached. That is, the difference between the output voltage of the first detection circuit 10 and the output voltage of the second detection circuit 120 becomes extremely close to 0V. Therefore, when the physical quantity of the measurement target is constant, the physical quantity of the measurement target can be accurately detected even in the binarization process in which the threshold voltage is set near 0V, which does not require a complicated circuit, and the circuit is simplified. Is possible.

(Modified example of the second embodiment) Subsequently, with reference to FIG. 3, the configuration of the sensor circuit 300, which is a modification of the sensor circuit 200 according to the second embodiment of the present invention, will be described. FIG. 3 is a circuit configuration diagram showing a modified example of the sensor circuit according to the second embodiment of the present invention.

The sensor circuit 300 according to this modification is the same as the sensor circuit 200 according to the second embodiment for the power supply V1, the first detection circuit 10, the second detection circuit 120, and the differential amplifier circuit 30. This modification is different from the sensor circuit 200 according to the second embodiment in that the first comparator 60 and the second comparator 70 are provided instead of the window comparator 50. Hereinafter, the points different from the second embodiment will be mainly described.

The first comparator 60 and the second comparator 70 are circuit elements that compare one input voltage with a reference voltage and digitally output the comparison result. In this modification, the first comparator 60 uses the output VO3 of the differential amplification circuit 30 as an input voltage, and when the input voltage exceeds the first reference voltage, the low level is set, and the input voltage is equal to or lower than the first reference voltage. When it outputs a high level. The second comparator 70 uses the output VO3 of the differential amplification circuit 30 as an input voltage and outputs a high level when the input voltage is equal to or higher than the second reference voltage and a low level when the input voltage is lower than the second reference voltage. To do. Here, the first reference voltage of the first comparator 60 is set to the upper limit of the error range of the specific heat source temperature, and the second reference voltage of the second comparator 70 is set to the lower limit of the error range of the specific heat source temperature. When set, it can be determined that the temperature is a specific heat source temperature when both the output of the first comparator 60 and the output of the second comparator 70 are high level, and when the output of the first comparator 60 is low level, it is specific. It can be determined that the heat source temperature is higher than the heat source temperature, and when the output of the second comparator 70 is at a low level, it can be determined that the heat source temperature is lower than the specific heat source temperature. As described above, in this modification, since the first comparator 60 and the second comparator 70 are provided, a specific heat source temperature, a heat source temperature higher than the specific heat source temperature, and a heat source temperature higher than the specific heat source temperature are provided. Three states of low heat source temperature can be discriminated. For example, assuming that the specific heat source temperature is 180 ° C. ± 1 ° C., the first reference voltage is 181 ° C., and the second reference voltage is 179 ° C., the output of the first comparator 60 and the output of the second comparator 70 are the same. The relationship between the heat source temperatures that can be detected in the modified example is as shown in Table 1.

<tables num="1"></tables>

Hereinafter, it will be specifically shown by Examples 1 and 2 and Comparative Example 1 that the decrease in the detection accuracy of the temperature of the heat source can be suppressed by this embodiment. However, the present invention is not limited to these. In Examples 1 and 2 and Comparative Example 1, the temperature characteristics of the output of the first detection circuit and the output of the second detection circuit and the temperature characteristics of the output of the differential amplifier circuit were measured.

In the first embodiment, the sensor circuit 100 according to the first embodiment described above was used. In the second embodiment, the sensor circuit 200 according to the second embodiment described above was used. In Comparative Example 1, the sensor circuit 400 shown in FIG. 10 was used. FIG. 10 is a circuit configuration diagram showing a sensor circuit according to Comparative Example 1.

First, the configuration of the sensor circuit 400 according to Comparative Example 1 will be described. As shown in FIG. 10, the sensor circuit 400 includes a power supply V2, a first detection circuit 210, a second detection circuit 220, a differential amplifier circuit 230, and an A / D conversion circuit 240. ..

The power supply V2 supplies a DC voltage to the first detection circuit 210 and the second detection circuit 220. The power supply V2 has a first electrode (hereinafter, referred to as "positive electrode") and a second electrode (hereinafter, referred to as "negative electrode").

The first detection circuit 210 is a circuit for detecting infrared rays radiated from a heat source. The first detection circuit 210 is composed of a series circuit of a first resistor R21 connected to the positive electrode of the power supply V2 and a first thermistor Rth21 connected to the negative electrode of the power supply V2. The first detection circuit 210 outputs the voltage obtained by dividing the DC voltage supplied from the power supply V2 by the first resistor R21 and the first thermistor Rth21 as the output VO21. The output VO21 of the first detection circuit 210 is connected to the differential amplifier circuit 230.

The second detection circuit 220 is a circuit for detecting the external environmental temperature. The second detection circuit 220 is composed of a series circuit of a second resistor R22 connected to the positive electrode of the power supply V2 and a second thermistor Rth22 connected to the negative electrode of the power supply V2. The second detection circuit 220 outputs the voltage obtained by dividing the DC voltage supplied from the power supply V2 by the second resistor R22 and the second thermistor Rth22 as the output VO22. The output VO22 of the second detection circuit 220 is connected to the differential amplifier circuit 230 and the A / D conversion circuit 240.

The differential amplifier circuit 230 is a circuit that amplifies the difference between two input voltages with a constant coefficient. In the first comparative example 1, the output VO21 of the first detection circuit 210 and the output VO22 of the second detection circuit 220 are used as two input voltages, and the difference between the output voltage of the output VO21 and the output voltage of the output VO22 is taken. , Only this difference is amplified. The differential amplifier circuit 230 outputs a voltage obtained by amplifying the difference between the two input voltages as the output VO23.

The A / D conversion circuit 240 is a circuit that converts an analog value into a digital value. In Comparative Example 1, the output VO23 of the differential amplifier circuit 230 and the output VO22 of the second detection circuit 220 are converted into digital values. Although not shown in FIG. 10, the value converted into a digital value by the A / D conversion circuit 240 is taken into a microcomputer and converted by a temperature conversion table or a function to detect the temperature of the heat source.

Subsequently, with reference to FIGS. 4 and 5, the temperature characteristics of the sensor circuit 100 of the first embodiment are shown. FIG. 4 is a graph showing the temperature characteristics of the output of the first detection circuit and the output of the second detection circuit of the sensor circuit according to the first embodiment. FIG. 5 is a graph showing the temperature characteristics of the output of the differential amplifier circuit of the sensor circuit according to the first embodiment.

First, the circuit constant of each detection circuit of the sensor circuit 100 is set. The circuit constant of the first detection circuit 10 is the resistance value of the first resistor R1 from the equation (2) when the resistance value Rthr1 of the first thermistor Rth1 at 25 ° C. is set to 33 kΩ (B constant is 4600 K). Rr1 becomes 3.3 kΩ. The circuit constant of the second detection circuit 20 sets the resistance value Rr2 of the second resistor R2 to 3.3 kΩ, which is the same as the resistance value Rr1 of the first resistor R1. Further, the combined resistance value Rcb is the resistance value Rthr1 when the first thermistor Rth1 receives the heat of infrared rays radiated from the heat source of 180 ° C. in the temperature range of the external environment temperature of 0 ° C. to 120 ° C. Set each constant so that they are approximately equal to. Specifically, the resistance value Rthr2 of the second thermistor Rth2 at 25 ° C. is 25 kΩ (B constant is 4380 K), the resistance value Rr3 of the additional resistance R3 is 10 kΩ, and the resistance value Rthr3 of the additional thermistor Rth3 at 25 ° C. When is set to 82 kΩ (B constant is 3400 K), the combined resistance value Rcb becomes substantially equal to the resistance value Rthr1 of the first thermistor Rth1 in the temperature range of the external environment temperature from 0 ° C. to 120 ° C. Here, Table 2 shows the relationship between the combined resistance value Rcb and the resistance value Rthr1 of the first thermistor Rth1 in the temperature range of 0 ° C. to 120 ° C. when the external environment temperature is 180 ° C. when the heat source temperature is 180 ° C.

<tables num="2"></tables>

As shown in Table 2, in the temperature range where the external environmental temperature is 0 ° C. to 120 ° C., the combined resistance value Rcb and the resistance value Rthr1 of the first thermistor Rth1 are substantially equal, and the accuracy thereof is the first. The combined resistance value Rcb is in the range of 100% ± 1% with respect to the resistance value Rthr1 of the thermistor Rth1.

Then, with respect to the sensor circuit 100 in which the circuit constant is set as described above, the output VO1 of the first detection circuit 10 when the heat source temperature is 160 ° C, 180 ° C, 200 ° C when the external environment temperature is 0 ° C to 120 ° C. The temperature characteristics and the temperature characteristics of the output VO2 of the second detection circuit 20 when the external environment temperature was 0 ° C. to 120 ° C. were measured. Here, the voltage value of the power supply V1 was set to 1V.

The measurement results are shown in FIG. As shown in FIG. 4, the output VO1 of the first detection circuit 10 when the heat source temperature is 180 ° C. has a voltage characteristic 43. When the heat source temperature is 200 ° C., the output VO1 of the first detection circuit 10 is the resistance of the first thermistor Rth1 according to the temperature obtained by adding the increase in the amount of infrared heat emitted from the heat source when the external environment temperature is the same. Since the value Rthr1 is determined, the voltage characteristic is 44. When the heat source temperature is 160 ° C., the output VO1 of the first detection circuit 10 is the resistance of the first thermistor Rth1 by the temperature obtained by subtracting the decrease in the amount of infrared heat emitted from the heat source when the external environment temperature is the same. Since the value Rthr1 is determined, the voltage characteristic is 42.

On the other hand, the output VO2 of the second detection circuit 20 has a voltage characteristic 41, which is close to the voltage characteristic 43. This is because the combined resistance value Rcb and the resistance value Rthr1 of the first thermista Rth1 receiving the heat of infrared rays having a heat source temperature of 180 ° C. are substantially equal to each other, and the resistance value Rr1 of the first resistance R1 and the second resistance value Rr1 Since the resistance value Rr2 of the resistor R2 also has the same resistance value, the output VO1 obtained by dividing the DC voltage supplied from the power supply V1 by the first resistance R1 of the first detection circuit 10 and the first thermistor Rth1. Because the output VO2 obtained by dividing the DC voltage supplied from the power supply V1 by the combined resistance of the second resistor R2 of the second detection circuit 20 and the second thermistor Rth2, the additional resistor R3, and the additional thermistor Rth3 becomes equal. Is. In other words, the combined resistance value is such that the output VO1 of the first detection circuit 10 and the output VO2 of the second detection circuit 20 have the same output voltage when the heat source temperature receives the heat of infrared rays of 180 ° C. This is because Rcb is adjusted so as to be equal to the resistance value Rthr1 of the first thermistor Rth1.

Next, with respect to the sensor circuit 100 set to the circuit constant described above, the temperature characteristics of the output VO3 of the differential amplifier circuit 30 when the heat source temperatures are 160 ° C., 180 ° C., and 200 ° C. when the external environment temperature is 0 ° C. to 120 ° C. Was measured. That is, the voltage characteristics 42 to 44, which are the output VO1s of the first detection circuit 10, and the voltages, which are the output VO2s of the second detection circuit 20, when the heat source temperatures shown in FIG. 4 are 160 ° C., 180 ° C., and 200 ° C. This means that the output VO3 of the differential amplifier circuit 30 when the difference from the characteristic 41 is amplified is measured. Here, the amplification factor of the differential amplifier circuit 30 is increased 25 times so that 1V can be effectively used, assuming that the voltage width of the input voltage allowable range of the A / D conversion circuit 40 connected to the next stage is 1V. I set it. The reference voltage of the differential amplifier circuit 30 was set to 0V. When using with a positive power supply, the reference voltage of the differential amplifier circuit 30 may be set to 0.5V. In this case, the output VO3 of the differential amplifier circuit 30 can be detected in the range of 0V to 1V.

The measurement results are shown in Table 3 and FIG. As shown in Table 3, the output VO3 of the differential amplifier circuit 30 when the heat source temperature is 180 ° C. is 0V ± 0.03V in the temperature range where the external environment temperature is 0 ° C. to 120 ° C. That is, the output VO3 of the differential amplifier circuit 30 when the heat source temperature is 180 ° C. has a substantially constant output voltage. Further, as shown in FIG. 5, the output VO3 of the differential amplifier circuit 30 when the heat source temperature is 180 ° C. is the voltage characteristic 43 which is the output VO1 of the first detection circuit 10 and the second detection circuit 20. Since the voltage characteristic 41, which is the output VO2, is similar, the voltage characteristic 56, which is a substantially constant output voltage close to 0V, is obtained in the temperature range of the external environment temperature of 0 ° C. to 120 ° C. The output VO3 of the differential amplifier circuit 30 when the heat source temperature is 200 ° C. is the difference between the voltage characteristic 44, which is the output VO1 of the first detection circuit 10, and the voltage characteristic 41, which is the output VO2 of the second detection circuit 20. The voltage characteristic 57, which is the output voltage obtained by amplifying the above. That is, the voltage characteristic 57 is a voltage characteristic in which only the increase in the amount of infrared heat when the heat source temperature is increased to 200 ° C. is output based on the amount of heat of infrared rays when the heat source temperature is 180 ° C. The output VO3 of the differential amplifier circuit 30 when the heat source temperature is 160 ° C. is the difference between the voltage characteristic 42, which is the output VO1 of the first detection circuit 10, and the voltage characteristic 41, which is the output VO2 of the second detection circuit 20. The voltage characteristic 55, which is the output voltage obtained by amplifying the above. That is, the voltage characteristic 55 is a voltage characteristic in which only the decrease in the amount of infrared heat when the heat source temperature is reduced to 160 ° C. is output based on the amount of heat of infrared rays when the heat source temperature is 180 ° C.

<tables num="3"></tables>

As described above, the voltage characteristics 55 and 57, which are the output VO3s of the differential amplifier circuit 30, are substantially constant output voltages when the heat source temperature is 180 ° C. in the temperature range of the external environment temperature of 0 ° C. to 120 ° C. Since only the fluctuation of the amount of heat of infrared rays from the heat source temperature of 180 ° C. is reflected in the output based on the voltage characteristic 56, the amplification factor of the differential amplifier circuit 30 can be efficiently increased.

Subsequently, with reference to FIGS. 6 and 7, the temperature characteristics of the sensor circuit 200 of the second embodiment are shown. FIG. 6 is a graph showing the temperature characteristics of the output of the first detection circuit and the output of the second detection circuit of the sensor circuit according to the second embodiment. FIG. 7 is a graph showing the temperature characteristics of the output of the differential amplifier circuit of the sensor circuit according to the second embodiment.

First, the circuit constant of each detection circuit of the sensor circuit 200 is set. The circuit constant of the first detection circuit 10 is the resistance value of the first resistor R1 from the equation (2) when the resistance value Rthr1 of the first thermistor Rth1 at 25 ° C. is set to 33 kΩ (B constant is 4600 K). Rr1 becomes 3.3 kΩ. The circuit constant of the second detection circuit 120 sets the resistance value Rr2 of the second resistor R2 to 3.3 kΩ, which is the same as the resistance value Rr1 of the first resistor R1. Further, the combined resistance value Rcb2 is the resistance value Rthr1 when the first thermistor Rth1 receives the heat of infrared rays radiated from the heat source of 180 ° C. in the temperature range of the external environment temperature of 0 ° C. to 120 ° C. Set each constant so that they are approximately equal to. Specifically, the resistance value Rthr2 of the second thermistor Rth2 at 25 ° C. is 25 kΩ (B constant is 4370 K), the resistance value Rr3 of the additional resistance R3 is 32 kΩ, and the resistance value Rthr3 of the additional thermistor Rth3 at 25 ° C. Is set to 100 kΩ (B constant is 3200 K), the resistance value Rr4 of the additional resistance R4 is set to 16 kΩ, and the resistance value Rthr4 of the additional thermistor Rth4 is set to 310 kΩ (B constant is 4800 K). In, the combined resistance value Rcb2 becomes extremely equal to the resistance value Rthr1 of the first thermistor Rth1. Here, Table 4 shows the relationship between the combined resistance value Rcb2 and the resistance value Rthr1 of the first thermistor Rth1 in the temperature range of 0 ° C. to 120 ° C. when the external environment temperature is 180 ° C. when the heat source temperature is 180 ° C.

<tables num="4"></tables>

As shown in Table 4, in the temperature range where the external environmental temperature is 0 ° C. to 120 ° C., the combined resistance value Rcb2 and the resistance value Rthr1 of the first thermistor Rth1 are extremely equal, and the accuracy thereof is the first. The combined resistance value Rcb2 is in the range of 100% ± 0.1% with respect to the resistance value Rthr1 of the thermistor Rth1.

Then, with respect to the sensor circuit 200 set to the above circuit constant, the temperature characteristic of the output VO1 of the first detection circuit 10 and the external environment temperature are 0 when the heat source temperature is 180 ° C. when the external environment temperature is 0 ° C. to 120 ° C. The temperature characteristics of the output VO4 of the second detection circuit 120 from ° C. to 120 ° C. were measured. Here, the voltage value of the power supply V1 was set to 1V.

The measurement results are shown in FIG. As shown in FIG. 6, the output VO1 of the first detection circuit 10 when the heat source temperature is 180 ° C. has the voltage characteristic 43 as in the first embodiment. On the other hand, the output VO4 of the second detection circuit 120 has a voltage characteristic 61, which is very close to the voltage characteristic 43. This is because the combined resistance value Rcb2 and the resistance value Rthr1 of the first thermista Rth1 receiving the heat of infrared rays having a heat source temperature of 180 ° C. are extremely equal to each other, and the resistance value Rr1 of the first resistance R1 and the second resistance value Rr1 Since the resistance value Rr2 of the resistor R2 also has the same resistance value, the output VO1 obtained by dividing the DC voltage supplied from the power supply V1 by the first resistance R1 of the first detection circuit 10 and the first thermistor Rth1. Divides the DC voltage supplied from the power supply V1 by the combined resistance of the second resistor R2 of the second detection circuit 120, the second thermistor Rth2, the additional resistance R3, the additional thermistor Rth3, the additional resistance R4, and the additional thermistor Rth4. This is because the output VO4s produced are extremely equal. In other words, when the heat source temperature receives the heat of infrared rays of 180 ° C., the combined resistance is such that the output VO1 of the first detection circuit 10 and the output VO4 of the second detection circuit 120 have extremely equal output voltages. This is because the value Rcb2 is adjusted to be equal to the resistance value Rthr1 of the first thermistor Rth1.

Next, with respect to the sensor circuit 200 set to the circuit constant described above, the temperature characteristics of the output VO3 of the differential amplifier circuit 30 when the external environment temperature was 0 ° C. to 120 ° C. and the heat source temperature was 180 ° C. were measured. That is, the difference between the voltage characteristic 43, which is the output VO1 of the first detection circuit 10, and the voltage characteristic 61, which is the output VO4 of the second detection circuit 120, when the heat source temperature shown in FIG. 6 is 180 ° C. is amplified. It means that the output VO3 of the differential amplifier circuit 30 at that time was measured. Here, the amplification factor of the differential amplifier circuit 30 is set to 25 times. The reference voltage of the differential amplifier circuit 30 was set to 0V.

The measurement results are shown in Table 5 and FIG. As shown in Table 5, the output VO3 of the differential amplifier circuit 30 when the heat source temperature is 180 ° C. is 0V ± 0.00V in the temperature range where the external environmental temperature is 0 ° C. to 120 ° C. That is, the output VO3 of the differential amplifier circuit 30 when the heat source temperature is 180 ° C. has a constant output voltage. As shown in FIG. 7, the output VO3 of the differential amplifier circuit 30 when the heat source temperature is 180 ° C. is the voltage characteristic 43 which is the output VO1 of the first detection circuit 10 and the output VO4 of the second detection circuit 120. Since the voltage characteristic 61 is extremely similar, the voltage characteristic 76 is a constant output voltage extremely close to 0 V in the temperature range of the external environment temperature of 0 ° C. to 120 ° C. As described above, the voltage characteristic 76, which is the output VO3 of the differential amplifier circuit 30, has a constant output voltage in the temperature range of the external environment temperature of 0 ° C. to 120 ° C., and therefore is close to 0V which does not require a complicated circuit. Since the temperature of the heat source can be detected accurately even in the binarization process in which the threshold voltage (reference voltage) is set to, the circuit can be simplified. That is, the sensor circuit 200 of the second embodiment can detect the temperature of the heat source with the window comparator 50 having a small circuit scale without using the A / D conversion circuit or the temperature conversion table.

<tables num="5"></tables>

Subsequently, with reference to FIGS. 8 and 9, the temperature characteristics of the sensor circuit 400 of Comparative Example 1 are shown. FIG. 8 is a graph showing the temperature characteristics of the output of the first detection circuit and the output of the second detection circuit of the sensor circuit according to Comparative Example 1. FIG. 9 is a graph showing the temperature characteristics of the output of the differential amplifier circuit of the sensor circuit according to Comparative Example 1.

First, the circuit constant of each detection circuit of the sensor circuit 400 is set. The circuit constant of the first detection circuit 210 is the resistance value of the first resistor R21 from the equation (2) when the resistance value Rthr21 of the first thermistor Rth21 at 25 ° C. is set to 33 kΩ (B constant is 4600 K). Rr21 becomes 3.3 kΩ. The circuit constant of the second detection circuit 220 is the resistance value of the second resistor R22 from the equation (2) when the resistance value Rthr22 of the second thermistor Rth22 at 25 ° C. is set to 33 kΩ (B constant is 4600 K). Rr22 is 3.3 kΩ, which is the same as the resistance value Rr21 of the first resistor R21. When the first detection circuit 210 does not receive the heat of infrared rays emitted from the heat source, that is, when there is no temperature difference between the first detection circuit 210 and the second detection circuit 220, the first detection circuit 210 Since the circuit constants of the first resistor R21 and the first thermista Rth21 and the second resistor R22 and the second thermista Rth22 of the second detection circuit 220 are equal, the first detection circuit 210 and the second detection The circuit 220 is in a thermal equilibrium state. That is, since the output VO21 of the first detection circuit 210 and the output VO22 of the second detection circuit 220 have the same output voltage, the output VO23 of the differential amplifier circuit 230 has the same output voltage regardless of the external environmental temperature. , 0V constant voltage. On the other hand, when the first detection circuit 210 receives the heat of infrared rays emitted from the heat source, the thermal equilibrium state of the first detection circuit 210 and the second detection circuit 220 is disrupted, and the resistance of the first thermistor Rth21 The resistance value Rthr22 of the value Rthr21 and the second thermistor Rth22 will be significantly different. Here, Table 6 shows the relationship between the resistance value Rthr21 of the first thermistor Rth21 and the resistance value Rthr22 of the second thermistor Rth22 in the temperature range of the external environment temperature of 0 ° C. to 120 ° C. when the heat source temperature is 180 ° C. ..

<tables num="6"></tables>

As shown in Table 6, in the temperature range where the external environmental temperature is 0 ° C. to 120 ° C., the resistance value Rthr21 of the first thermistor Rth21 and the resistance value Rthr22 of the second thermistor Rth22 are significantly different. The resistance value Rthr22 of the second thermistor Rth22 is about 200% at the maximum with respect to the resistance value Rthr21 of the thermistor Rth21. As described above, in the sensor circuit 400 of Comparative Example 1, the first detection circuit 210 and the second detection circuit 220 are in a thermal equilibrium state when the first detection circuit 210 receives the heat of infrared rays radiated from the heat source. It does not become. On the other hand, as in the sensor circuit 100 of the first embodiment, when the first detection circuit 10 receives the heat of infrared rays radiated from the heat source, the combined resistance value Rcb and the resistance value of the first thermistor Rth1. When Rthr1 is made substantially equal, the first detection circuit 10 and the second detection circuit 20 are in a thermal equilibrium state when the first detection circuit 10 receives the amount of heat of infrared rays radiated from the heat source. Further, as in the sensor circuit 200 of the second embodiment, when the first detection circuit 10 receives the heat of infrared rays radiated from the heat source, the combined resistance value Rcb2 and the resistance value Rthr1 of the first thermistor Rth1 are extremely set. If they are equal, the first detection circuit 10 and the second detection circuit 120 are in a thermal equilibrium state when the first detection circuit 10 receives the heat of infrared rays radiated from the heat source.

Then, with respect to the sensor circuit 400 set to the circuit constant described above, the temperature characteristics of the output VO21 of the first detection circuit 210 when the heat source temperatures are 160 ° C., 180 ° C., and 200 ° C. when the external environment temperature is 0 ° C. to 120 ° C. The temperature characteristics of the output VO22 of the second detection circuit 220 were measured when the external environmental temperature was 0 ° C to 120 ° C. Here, the voltage value of the power supply V2 was set to 1V.

The measurement results are shown in FIG. As shown in FIG. 8, the output VO21s of the first detection circuit 210 when the heat source temperatures are 160 ° C., 180 ° C., and 200 ° C. have voltage characteristics 42 to 44, respectively. That is, it is the same as the voltage characteristics 42 to 44 which are the output VO1 of the first detection circuit 10 when the heat source of the first embodiment is 160 ° C., 180 ° C., and 200 ° C. On the other hand, the output VO22 of the second detection circuit 220 has a voltage characteristic of 81, which is significantly different from the voltage characteristics 42 to 44. This is because the thermal equilibrium state of the first detection circuit 210 and the second detection circuit 220 is broken when the first detection circuit 210 receives the heat of infrared rays radiated from the heat source.

Next, with respect to the sensor circuit 400 set to the above circuit constant, the temperature characteristics of the output VO23 of the differential amplifier circuit 230 when the heat source temperature is 160 ° C., 180 ° C., and 200 ° C. when the external environment temperature is 0 ° C. to 120 ° C. Was measured. That is, the voltage characteristics 42 to 44, which are the output VO21s of the first detection circuit 210, and the voltages, which are the output VO22s of the second detection circuit 220, when the heat source temperatures shown in FIG. 8 are 160 ° C., 180 ° C., and 200 ° C. This means that the output VO23 of the differential amplifier circuit 230 when the difference from the characteristic 81 is amplified is measured.

The measurement results are shown in FIG. As shown in FIG. 9, the output VO23 of the differential amplifier circuit 230 when the heat source temperatures are 160 ° C., 180 ° C., and 200 ° C. has voltage characteristics of 95 to 97, respectively. Specifically, the first detection circuit 210 outputs a voltage that reflects the temperature obtained by adding the temperature generated by the amount of infrared rays radiated from the heat source to the external environment temperature, and the second detection circuit 220 outputs a voltage. Outputs a voltage that reflects the external ambient temperature. That is, the voltage characteristics 95 to 97, which are the output VO23 of the differential amplifier circuit 230, are outputs that reflect the temperature generated by the amount of heat of infrared rays radiated from the heat source. In the sensor circuit 400 of Comparative Example 1, each resistance (R21, R22) and each thermistor (Rth21) so that the output VO21 of the first detection circuit 210 and the output VO22 of the second detection circuit 220 are approximately linearly approximated. , Rth22), but in this case, the output VO23 of the differential amplifier circuit 230 when the first detection circuit 210 receives the heat of infrared rays emitted from the heat source at the same external ambient temperature. In the temperature range of the external environment temperature from 0 ° C. to 120 ° C., the voltage becomes the largest at the intermediate temperature of 60 ° C., and the voltage becomes the smallest at the upper limit temperature of 120 ° C. or the lower limit temperature of 0 ° C. Therefore, since the sensor circuit 400 of Comparative Example 1 is a fluctuation amount from the external environment temperature, the fluctuation amount is large, and the fluctuation amount is the output voltage of the output VO23 of the differential amplifier circuit 230 at 60 ° C. which is an intermediate temperature of the external environment temperature. Since the difference between the output voltage of the output VO23 of the differential amplifier circuit 230 at the upper limit temperature of 120 ° C. and the lower limit temperature of 0 ° C. is large, the amplification factor of the differential amplifier circuit 230 is A / Assuming that the voltage width of the input voltage allowable range of the D conversion circuit 240 is 1V, even if 1V is increased so that it can be used effectively, it will be 10 times at the maximum.

On the other hand, in the sensor circuit 100 of the first embodiment, the combined resistance value Rcb is the first thermistor when the heat source temperature is 180 ° C. and the first detection circuit 10 receives the heat of infrared rays emitted from the heat source. Since the resistance value of Rth1 is substantially equal to Rthr1, the voltage characteristics 55 to 57, which are the output VO3 of the differential amplifier circuit 30, are based on the output VO3 of the differential amplifier circuit 30 when the heat source temperature is 180 ° C. Only the fluctuation amount is output. In this case, in the temperature range of the external environment temperature from 0 ° C. to 120 ° C., the voltage becomes the largest when the heat source temperature is 200 ° C. and the external environment temperature is 60 ° C., and the heat source temperature is 160 ° C. and the external environment temperature is 60 ° C. When, the voltage becomes the smallest. Therefore, the amplification factor of the differential amplifier circuit 30 is such that the voltage characteristic 56, which is the output VO3 when the heat source temperature is 180 ° C., is constant at 0 V, and the voltage characteristics 55 and 57, which are fluctuations of the voltage characteristic 56, are amplified. Therefore, it can be set as large as 25 times.

As described above, in the sensor circuit 400 of Comparative Example 1, the amplification factor of the differential amplifier circuit 230 is up to 10 times, and the differential amplifier circuit 30 of the sensor circuit 100 (200) of Example 1 (Example 2). The amplification factor cannot be increased efficiently, as in the case of 25 times the amplification factor of (30). As a result, the output voltage per temperature becomes small, and the accuracy of detecting the temperature of the heat source becomes low.

Further, as shown in FIG. 9, the sensor circuit 400 of Comparative Example 1 detects the temperature per 1 mV at 0.1 ° C./mV at an external environmental temperature of 60 ° C. and 0.4 ° C. at an external environmental temperature of 0 ° C. While the temperature is about ° C./mV, the sensor circuit 100 of the first embodiment detects the temperature per 1 mV at 0.04 ° C./mV at an external environmental temperature of 60 ° C., as shown in FIG. Since the ambient temperature is about 0.16 ° C./mV at 0 ° C., the temperature detection accuracy of the sensor circuit 100 of Example 1 is improved 2.5 times as compared with the sensor circuit 400 of Comparative Example 1. ..

Although the preferred embodiment of the present invention has been described above, the present invention is not necessarily limited to the above-described embodiment, and various modifications can be made without departing from the gist thereof. In addition, the described components include those that can be easily assumed by those skilled in the art and those that are substantially the same. Further, the described components can be combined as appropriate.

For example, a buffer such as a voltage follower may be added between each circuit. In this case, the voltage signal is attenuated especially when the output impedance of the first detection circuit 10 and the second detection circuit 20 is not sufficiently lower than the input impedance of the differential amplification circuit 30 and the A / D conversion circuit 40. You will be able to convey it.

Further, in the above embodiment, an example in which the sensor circuit according to the present invention is applied to a so-called non-contact temperature sensor that measures the temperature of a heat source in a non-contact manner has been described, but the present invention is not limited thereto. For example, the present invention can also be applied to a gas sensor, a humidity sensor, and a flow velocity sensor that detect a physical quantity by using the temperature difference between two thermistors.

An optical gas sensor called NDIR (non-dispersive infrared sensor) irradiates the first of the two thermistas with infrared rays that have passed through the gas to be measured, and causes the second thermista to receive infrared rays. Is to irradiate infrared rays transmitted through a standard gas containing no gas to be measured, and detect the gas concentration in the gas to be measured from the difference in temperature rise between the two thermistas. That is, the first thermistor is affected by the physical quantity of gas concentration, and the second thermistor is less affected by the physical quantity. Even in a gas sensor using such two thermistors, by applying the sensor circuit according to the above embodiment, it is possible to suppress a decrease in the detection accuracy of the physical quantity to be measured, that is, the gas concentration.

In the humidity sensor using two thermistors, the first thermistor of the two thermistors is exposed to the atmosphere to be measured, and the second thermistor is arranged in a closed dry air. When these two thermistors are heated under the same conditions, the temperature of the first thermistor is affected by the change in the thermal conductivity of the atmosphere due to humidity, but the temperature of the second thermistor is not affected by the humidity. The temperature difference between the two thermistors reflects the humidity. That is, the first thermistor is affected by a physical quantity called humidity, and the second thermistor is less affected by the physical quantity. Even in a humidity sensor using such two thermistors, by applying the sensor circuit according to the above embodiment, it is possible to suppress a decrease in the detection accuracy of the physical quantity to be measured, that is, the humidity.

In the flow velocity sensor using two thermistors, the first thermistor of the two thermistors is exposed to the fluid to be measured, and the second thermistor is arranged at a position not exposed to the fluid. When these two thermistors are heated under the same conditions, the first thermistor is deprived of heat according to the flow velocity and the temperature changes, but the second thermistor is not affected by this. The temperature difference between the two thermistors reflects the flow velocity. That is, the first thermistor is affected by the physical quantity called the flow velocity, and the second thermistor is less affected by the physical quantity. Even in a flow velocity sensor using such two thermistors, by applying the sensor circuit according to the above embodiment, it is possible to suppress a decrease in the detection accuracy of the physical quantity to be measured, that is, the flow velocity.

As described above, in various sensors that detect a physical quantity as a temperature difference between two thermistors, by applying the sensor circuit according to the above embodiment, it is possible to suppress a decrease in the detection accuracy of the physical quantity to be measured.

The sensor circuit according to the present invention can be used in air conditioners, copiers, microwave ovens, and the like.

V1, V2 ... Power supply, R1, R21 ... 1st resistor, R2, R22 ... 2nd resistor, R3, R4 ... Additional resistance, Rth1, Rth21 ... 1st thermista, Rth2, Rth22 ... 2nd thermista, Rth3 , Rth4 ... additional thermistor, VO1, VO21 ... first detection circuit output, VO2, VO4, VO22 ... second detection circuit output, VO3, VO23 ... differential amplifier circuit output, 42-44 ... first Voltage characteristics of the output of the detection circuit, 41,61,81 ... Voltage characteristics of the output of the second detection circuit, 55-57,76,95-97 ... Voltage characteristics of the output of the differential amplifier circuit, 30,230 ... Difference Dynamic amplifier circuit, 40, 240 ... A / D conversion circuit, 50 ... window comparator, 60 ... first comparator, 70 ... second comparator, 10, 210 ... first detection circuit, 20, 120, 220 ... first 2 detection circuits, 100, 200, 300, 400 ... Sensor circuits.