JP6291930B2 - Sensor circuit - Google Patents

Description

  The present invention relates to a sensor circuit.

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

  For example, Patent Document 1 discloses a first output voltage of a series circuit of an infrared detection thermal element (thermistor) and a resistance element, and a second output voltage of a series circuit of a temperature compensation thermal element (thermistor) 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. In addition, 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 light) emitted from a heating roller (heat source) is output as a third output voltage that is a difference between the first output voltage and the second output voltage. This reflects the temperature difference between the amount of heat including the ambient temperature and the ambient temperature, that is, the amount of heat of pure infrared light emitted from the heating roller. The third output voltage is a characteristic having a peak value with respect to the ambient temperature, and the sensitivity (the third output voltage change with respect to the temperature change of the heating roller when the ambient temperature is the same) is also the ambient temperature. On the other hand, it has a peak value.

JP 2003-57116 A

  However, in the temperature detection method shown in Patent Document 1, there is an ambient temperature where the sensitivity of the third output is low. At this time, the third output voltage has low temperature detection accuracy because the change in output voltage per temperature is small. There was a problem.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a sensor circuit that can suppress a decrease in detection accuracy of a physical quantity to be measured.

In order to solve the above problems, a sensor circuit according to the present invention is connected in series to a first resistor connected to a first pole of a power source and the first resistor, and to a second pole of the power source. A first detection circuit having a first thermistor that is affected by a physical quantity to be connected;
A second thermistor connected in series to the first resistor and the second thermistor connected in series to the second resistor and reduced in the influence of the physical quantity of the measurement object connected to the second electrode A first bridge circuit including a second detection circuit; a third resistor connected to the first pole; and a third resistor connected in series to the third resistor and connected to the second pole. A third detection circuit having a third thermistor that is affected by a physical quantity to be measured; a fourth resistor connected to the first pole; and a fourth resistor connected in series to the fourth resistor and the second resistor. And a fourth detection circuit having a fourth thermistor in which the influence of the physical quantity of the measurement object connected to the pole of the measurement is reduced, and is an output of the first bridge circuit. Difference voltage between output of first detection circuit and output of second detection circuit The differential voltage between the output of the third detection circuit and the output of the fourth detection circuit that has the first peak value at the first ambient temperature and is the output of the second bridge circuit is the first ambient temperature. The output of the first and second bridge circuits having a second peak value at a second ambient temperature different from the temperature is input to an addition / subtraction circuit having a function of weighting an input voltage. .

  With the above configuration, the output voltage difference of the adder / subtracter circuit in two states with different physical quantities can have two peak values with respect to the ambient temperature, and a decrease in sensitivity can be suppressed over a wide ambient temperature range. The output voltage change per unit can be increased. The output of one bridge circuit using a thermistor has one peak value. When the temperature difference increases from the ambient temperature at this peak, the output voltage decreases, and the sensitivity is the output difference between two states with different physical quantities. Also decreases. By using two bridge circuits, weighting and adding to each output, it is possible to limit the ambient temperature region to which one bridge circuit corresponds, and as a result, suppress the decrease in the detection accuracy of the physical quantity to be measured Can do.

  Furthermore, the output voltage of the addition / subtraction circuit in two states with different physical quantities is obtained by inputting the output of the three or more bridge circuits to an addition / subtraction circuit having a function of weighting the input voltage. The difference can have three or more peak values with respect to the ambient temperature, can suppress a decrease in sensitivity even in a wider ambient temperature range, and can increase the output voltage change per physical quantity. By increasing the number of bridge circuits, the peak value per ambient temperature can be increased, and the sensitivity to the ambient temperature can be flattened.

The sensor circuit of the present invention includes a first resistance unit connected to a first pole of a power source and a physical quantity to be measured connected in series to the first resistor unit and connected to the second pole of the power source. A fifth detection circuit having a fifth thermistor that is affected by: a second resistor connected to the first pole; a second resistor connected in series to the second resistor; and the second pole And a sixth detection circuit having a sixth thermistor in which the influence of the physical quantity to be measured is reduced, and a resistance value of the first and second resistance means. When the switch is switchable and the switch is in the on state, the differential voltage of the output of the third bridge circuit has a third peak value at the third ambient temperature, and when the switch is in the off state The difference voltage at the output of the third bridge circuit is The output of the third bridge circuit has a fourth peak value at a fourth ambient temperature different from the third ambient temperature, and the output of the third bridge circuit is input to a differential amplifier circuit whose amplification factor changes in conjunction with the switch. A first digital value obtained by digitizing the analog output of the differential amplifier circuit when the switch is in an on state, and a digital value obtained by digitizing the analog output of the differential amplifier circuit when the switch is in an off state. It has a means for adding two digital values.

With the above configuration, since the configuration of the thermistor affected by the physical quantity to be measured and the thermistor in which the influence of the physical quantity to be measured is reduced suffices, the circuit can be configured with a small number of components.

Furthermore, the resistance value of the resistance means has three or more resistance values by the switch, and the peak value per ambient temperature can be increased by adding three or more digital values. It is possible to flatten the digitized sensitivity to.

In addition, when the ambient temperature region is equally divided into the sensitivity (output voltage difference of the addition / subtraction circuit in two states with different physical quantities) and the digitized sensitivity peak value, the peak value is applied to each divided ambient temperature region. By making it exist, the sensitivity can be further flattened.

The physical quantity to be measured may be temperature. In this case, the sensitivity is the difference in output voltage of the sensor circuit with respect to the change in the amount of heat of infrared rays, and a decrease in the temperature detection accuracy of the heat source can be suppressed.

  ADVANTAGE OF THE INVENTION According to this invention, the sensor circuit which can suppress the fall of the detection accuracy of the physical quantity of a measuring object can be provided.

It is a circuit block diagram which shows the sensor circuit which concerns on the 1st Embodiment of this invention. It is a graph which shows the temperature characteristic of the output of the addition circuit of the sensor circuit which concerns on 1st Embodiment, and the output of a differential amplifier circuit. The voltage obtained by subtracting the output of the addition circuit at 160 ° C. from the output of the addition circuit of the sensor circuit according to the first embodiment when the temperature to be measured is 180 ° C. and 160 ° C. and the output of the addition circuit at 180 ° C. It is a graph which shows a temperature characteristic. It is a circuit block diagram which shows the modification of the sensor circuit which concerns on the 1st Embodiment of this invention. It is a circuit block diagram which shows the sensor circuit which concerns on the 2nd Embodiment of this invention. It is a graph which shows the temperature characteristic of the output of the addition circuit of the sensor circuit which concerns on 2nd Embodiment, and the output of a differential amplifier circuit. The voltage obtained by subtracting the output of the addition circuit at 160 ° C. from the output of the addition circuit of the sensor circuit according to the first embodiment when the temperature to be measured is 180 ° C. and 160 ° C. and the output of the addition circuit at 180 ° C. It is a graph which shows a temperature characteristic. It is a circuit block diagram which shows the modification of the sensor circuit which concerns on the 2nd Embodiment of this invention. It is a circuit block diagram which shows the sensor circuit which concerns on the 3rd Embodiment of this invention. It is a circuit block diagram which shows the modification of the sensor circuit which concerns on the 3rd Embodiment of this invention. It is a timing chart which shows operation | movement of the logic circuit of the sensor circuit which concerns on the 3rd Embodiment of this invention. 5 is a circuit configuration diagram showing a sensor circuit according to Comparative Example 1. FIG. The output of the differential amplifier circuit at 160 ° C. from the output of the differential amplifier circuit of the sensor circuit according to Comparative Example 1 when the temperature to be measured is 180 ° C. and 160 ° C. and the output of the differential amplifier circuit at 180 ° C. It is a graph which shows the temperature characteristic of the pulled voltage.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the description, the same reference numerals are used for the same elements or elements having the same function, and redundant description is 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 a first embodiment of the present invention. In the present embodiment, description will be made 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.

  As shown in FIG. 1, the sensor circuit 100 includes a power source V1, a first bridge circuit 11, a second bridge circuit 12, a voltage follower (21, 22), and a differential amplifier circuit (31, 32). ) And a two-input adder circuit 41, and an A / D (analog / digital) converter circuit 51.

  The power source V <b> 1 supplies a DC voltage to the first bridge circuit 11 and the second bridge circuit 12. As the power source V1, a stabilized constant voltage power source is used in order to suppress the influence of noise on each circuit output. The power source V1 has a first pole and a second pole. In the present embodiment, the first electrode is described as a positive electrode and the second electrode is described as a negative electrode. Hereinafter, the first electrode is referred to as “positive electrode”, and the second electrode is referred to as “negative electrode”.

  The first bridge circuit includes a first detection circuit for detecting infrared rays emitted from the heat source and a second detection circuit for detecting ambient temperature. The first detection circuit includes a series circuit of a first resistor R1 connected to the positive electrode of the power supply V1 and a first thermistor Th1 connected to the negative electrode of the power supply V1.

  The first thermistor Th1 is arranged so as to be affected by the amount of heat of infrared rays emitted from a heat source that is a physical quantity to be measured. That is, when the first thermistor Th1 is affected by the amount of heat of infrared rays radiated from the heat source, the resistance value changes due to the temperature of the first thermistor Th1 changing. The resistance value of the temperature of the first thermistor Th1 is determined by the temperature applied by the influence of the ambient temperature and the amount of heat of infrared rays emitted from the heat source.

As the first thermistor Th1, an NTC (Negative Temperature Coefficient) thermistor having a negative resistance temperature coefficient mainly composed of a metal oxide 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 B (B constant) is the thermistor constant. . In addition, B constant means that the resistance change rate with respect to a temperature change is so large that the value is large.
RA = RB * e ^{B (1 / TA-1 / TB)} Formula (1)

The first detection circuit outputs a voltage obtained by dividing the DC voltage supplied from the power supply V1 by the first resistor R1 and the first thermistor Th1 as the output P1. That is, the output P1 of the first detection circuit is expressed as follows, assuming that the direct-current voltage value supplied from the power supply V1 is Vr1, the resistance value of the first resistor R1 is Rr1, and the resistance value of the first thermistor Th1 is Rth1. The output P1 of the detection circuit satisfies the relationship of the following expression (3).
VO1 = Vr1 × Rth1 / (Rth1 + Rr1) Formula (3)
The output P1 of the first detection circuit is connected to the first differential amplifier circuit 31 via the voltage follower 21.

  The second detection circuit is composed of a series circuit of a second resistor R2 connected to the positive electrode of the power supply V1 and a second thermistor Th2 connected to the negative electrode of the power supply V1.

  The temperature of the second thermistor Th2 is the same as the ambient temperature, and the resistance value is determined by this temperature. That is, the second thermistor Th2 is arranged so that the influence of the amount of heat of infrared rays emitted from the heat source, which is the physical quantity to be measured, is reduced. Here, the second thermistor Th2 is preferably arranged at a position that is not affected by the amount of heat radiated from the heat source, but the first detection circuit and the second detection circuit are arranged close to each other in structure. If it is unavoidable, the second thermistor Th2 may be disposed at a position that is affected by the amount of heat of infrared rays radiated from the heat source, as long as there is no functional problem.

  As the second thermistor Th2, an NTC thermistor having a negative resistance temperature coefficient mainly composed of a metal oxide is used in the same manner as the first thermistor Th1.

The second detection circuit outputs a voltage obtained by dividing the DC voltage supplied from the power supply V1 by the second resistor R2 and the second thermistor Th2 as an output P2. That is,
The output P2 of the second detection circuit is expressed by the following formula (Vr1, the resistance value of the second resistor R2 is Rr2, and the resistance value of the second thermistor Th2 is Rth2): The relationship 4) is satisfied.
P2 = Vr1 × Rth2 / (Rth2 + Rr2) Formula (4)
The output P2 of the second detection circuit is connected to the first differential amplifier circuit 31 via the voltage follower 21.

  Similarly to the first bridge circuit 11, the second bridge circuit 12 includes two detection circuits, a third detection circuit for detecting infrared rays emitted from the heat source and a fourth detection circuit for detecting the ambient temperature. It consists of a detection circuit. The third detection circuit is configured by a series circuit of a third resistor R3 connected to the positive electrode of the power supply V1 and a third thermistor Th3 connected to the negative electrode of the power supply V1. The fourth detection circuit is configured by a series circuit of a fourth resistor R4 connected to the positive electrode of the power supply V1 and a fourth thermistor Th4 connected to the negative electrode of the power supply V1.

As the third thermistor Th3 and the fourth thermistor Th4, NTC thermistors having a negative resistance temperature coefficient having a metal oxide as a main component are used similarly to the first thermistor Th1 and the second thermistor Th2.

The second bridge circuit 12 has a circuit configuration similar to that of the first bridge circuit, and the output P3 of the third detection circuit has a DC voltage value Vr1 supplied from the power source V1 and a resistance of the third resistor R3. When the value is Rr3 and the resistance value of the second thermistor Th3 is Rth3, the relationship of the following formula (5) is satisfied.
P3 = Vr1 × Rth3 / (Rth3 + Rr3) Formula (5)
Similarly, the output P4 of the fourth detection circuit is as follows, assuming that the direct-current voltage value supplied from the power supply V1 is Vr1, the resistance value of the fourth resistor R4 is Rr4, and the resistance value of the fourth thermistor Th4 is Rth4. This satisfies the relationship of Equation (6).
P4 = Vr1 × Rth4 / (Rth4 + Rr4) Formula (6)
The output P3 of the third detection circuit and the output P4 of the fourth detection circuit are connected to the second differential amplifier circuit 32 via the voltage follower 22.

The voltage follower (21, 22) is a circuit that converts a high impedance signal into a low impedance signal. Since the outputs (P1, P2) of the first bridge circuit and the outputs (P3, P4) of the second bridge circuit are generally high impedance signals, they are converted into low impedance signals by passing through a voltage follower, and are output to the next stage. An unattenuated voltage can be transmitted to the first differential amplifier circuit 31 and the second differential amplifier circuit 32 to be connected. As voltage values, the outputs (P7, P8, P9, P10) corresponding to the inputs (P1, P2, P3, P4) of the voltage follower (21, 22) are the same. A voltage follower is not always necessary. Depending on the relationship with the input impedance of the next-stage circuit and the accuracy of the voltage to be transmitted, the voltage follower may be omitted.

  The differential amplifier circuits (31, 32) are circuits that amplify the difference between two input voltages with a constant coefficient. The first differential amplifier 31 receives the output (P7, P8) of the first voltage follower, takes the difference between the output voltage of the output P7 and the output voltage of the output P8, and amplifies only this difference. . That is, the difference between the outputs (P1, P2) of the first bridge circuit 11 is taken as a signal component, and only this difference is amplified. The output P13 of the first differential amplifier circuit 31 sets the constant of the first bridge circuit so that the ambient temperature region is divided into two and takes a peak value in the higher ambient temperature region. The first differential amplifier circuit 31 outputs a voltage obtained by amplifying the difference between the two input voltages as an output P13, and is input to the adder circuit 41 in the next stage.

  The second differential amplifier circuit 32 receives the output (P9, P10) of the second voltage follower, takes the difference between the output voltage of the output P9 and the output voltage of the output P10, and amplifies only this difference. . That is, the difference between the outputs (P3, P4) of the second bridge circuit 12 is taken as a signal component, and only this difference is amplified. The output P14 of the second differential amplifier circuit 32 sets the constant of the second bridge circuit so that the ambient temperature region is divided into two and takes a peak value in the lower ambient temperature region. The second differential amplifier circuit 32 outputs a voltage obtained by amplifying the difference between the two input voltages as an output P14, and is input to the adder circuit 41 in the next stage.

  The adder circuit 41 is a circuit that adds and amplifies input voltages. The adder circuit 41 adds the two input voltages of the output P13 of the first differential amplifier circuit 31 and the output P14 of the second differential amplifier circuit 32. That is, the signal obtained by amplifying the difference voltage between the outputs (P1, P2) of the first bridge circuit 11 and the signal obtained by amplifying the difference voltage between the outputs (P3, P4) of the second bridge circuit 12 are added. . The output P16 of the adding circuit 41 is an output voltage obtained by adding a voltage having a peak value in the higher ambient temperature region and a voltage having a peak value in the lower ambient temperature region. This output voltage also has two peak values in sensitivity (change in output voltage with respect to the temperature change of the heating roller). The amplification factor of the differential amplifier circuit (31, 32) is set so that the peak values are substantially equal. With the above circuit configuration and constant setting, the sensitivity can be enhanced in each of the half ambient temperature regions different from each other in the ambient temperature region, so that there is no ambient temperature extremely far from the peak ambient temperature, and the decrease in sensitivity can be suppressed. Further, the amplification factor of the adder circuit 41 is appropriately set within the input voltage range of the circuit at the next stage. The output P16 of the adder circuit 41 is connected to the A / D conversion circuit 51. Although not shown in FIG. 1, the voltage value obtained by detecting the ambient temperature is also taken into the A / D conversion circuit 51. As this voltage value, a signal from a single detection circuit such as the path of the output P2 of the second detection circuit may be used, or the output P2 of the second detection circuit and the output P4 of the fourth detection circuit Alternatively, a synthesized signal may be used.

  The A / D conversion circuit 51 is a circuit that converts an analog value into a digital value. In the present embodiment, the output P16 of the adder circuit 51 is converted into a digital value. When converting from an analog value to a digital value, the temperature accuracy increases as the voltage for 1 bit, that is, in the case of a non-contact temperature sensor, the temperature for 1 bit decreases. In order to achieve high accuracy, it is conceivable to use the A / D conversion circuit 51 having a high resolution and to increase the input voltage. Therefore, if the input voltage is set as high as possible within the input voltage range of the A / D conversion circuit 51, the accuracy can be improved. Although not shown in FIG. 1, the value converted into a digital value by the A / D conversion circuit 51 is taken into a microcomputer and converted by a temperature conversion table or function to detect the temperature of the heat source.

  As described above, in the sensor circuit 100 according to the present embodiment, the output voltage of the output P16 of the adder circuit 41 has two peak values having substantially the same sensitivity, and these peak values are respectively divided into two equal ambient temperature regions. Since it exists, the fall of the sensitivity in specific ambient temperature can be suppressed. Regarding the resolution of the input voltage of the A / D conversion circuit 51 in the next stage, since the difference in sensitivity for each ambient temperature is reduced, it is possible to suppress a decrease in detection accuracy of the temperature of the heat source.

(Modification of the first embodiment)
Next, a configuration of a sensor circuit 200 that is a modification of the sensor circuit 100 according to the first embodiment of the present invention will be described with reference to FIG. FIG. 4 is a circuit configuration diagram showing a modification of the sensor circuit according to the first embodiment of the present invention.

The sensor circuit 300 according to this modification includes the first power supply V1, the first bridge circuit 11, the second bridge circuit 12, the voltage follower (21, 22), and the A / D conversion circuit 51. This is the same as the sensor circuit 100 according to the embodiment. The present modification is different from the sensor circuit 100 according to the first embodiment in that an addition / subtraction circuit 42 is provided instead of the differential amplifier circuits (31, 32) and the addition circuit 41. Hereinafter, a description will be given focusing on differences from the first embodiment.

The addition / subtraction circuit 42 is an analog arithmetic circuit that performs addition, subtraction, and amplification. In the present modification 200, the output P8 of the first voltage follower 21 and the output P10 of the second voltage follower 21 are added, the output P7 of the first voltage follower 21 and the output P9 of the second voltage follower are subtracted, The amplification factor can be set for each signal. In the sensor circuit 100 according to the first embodiment, the output voltage of the output P13 of the differential amplifier circuit 31 and the output of the second voltage follower 22 which are the differential voltages of the outputs (P7, P8) of the first voltage follower 21. After obtaining the output voltage of the output P14 of the differential amplifier circuit 32, which is the difference voltage of (P9, P10), these output voltages are added. That is, in the present modification 200, the same operation can be performed only by the addition / subtraction circuit 42, so that the circuit can be simplified. Note that either the second detection circuit of the first bridge circuit 11 or the fourth detection circuit of the second bridge circuit 12 may be omitted. That is, since the second detection circuit and the fourth detection circuit are circuits for measuring the ambient temperature, they can be shared.

(Second Embodiment)
Next, the configuration of the sensor circuit 300 according to the second embodiment of the present invention will be described with reference to FIG. FIG. 5 is a circuit configuration diagram showing a sensor circuit according to the second embodiment of the present invention. The sensor circuit 300 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. The sensor circuit 300 according to the second embodiment includes a third bridge circuit 13, a third voltage follower 23, a third differential amplifier circuit 33, and a three-input adder circuit 43. This is different from the sensor circuit 100 according to the embodiment. Hereinafter, a description will be given focusing on differences from the first embodiment.

  As shown in FIG. 5, the sensor circuit 300 includes a power source V1, a first bridge circuit 11, a second bridge circuit 12, a third bridge circuit 13, and a voltage follower (21, 22, 23). A differential amplifier circuit (31, 32, 33), a 3-input adder circuit 43, and an A / D (analog / digital) converter circuit 51.

  Similar to the first bridge circuit 11 and the second bridge circuit 12, the third bridge circuit 13 includes a fifth detection circuit for detecting infrared rays radiated from the heat source and a fifth detection circuit for detecting the ambient temperature. 6 of two detection circuits. The fifth detection circuit is configured by a series circuit of a fifth resistor R5 connected to the positive electrode of the power supply V1 and a fifth thermistor Th5 connected to the negative electrode of the power supply V1. The sixth detection circuit is configured by a series circuit of a sixth resistor R6 connected to the positive electrode of the power supply V1 and a sixth thermistor Th6 connected to the negative electrode of the power supply V1.

Similarly to the first thermistor Th1, the second thermistor Th2, the third thermistor Th3, and the fourth thermistor Th4, the fifth thermistor Th5 and the sixth thermistor Th6 are negative resistances mainly composed of a metal oxide. An NTC thermistor with a temperature coefficient is used.

The third bridge circuit 13 has a circuit configuration similar to that of the first bridge circuit 11 and the second bridge circuit 12, and the output P3 of the third detection circuit uses the direct current voltage value Vr1 supplied from the power source V1. When the resistance value of the fifth resistor R5 is Rr5 and the resistance value of the fifth thermistor Th5 is Rth5, the relationship of the following formula (7) is satisfied.
P5 = Vr1 × Rth5 / (Rth5 + Rr5) Formula (7)
Similarly, the output P6 of the sixth detection circuit is as follows, assuming that the direct-current voltage value supplied from the power supply V1 is Vr1, the resistance value of the sixth resistor R6 is Rr6, and the resistance value of the sixth thermistor Th6 is Rth6. This satisfies the relationship of Equation (8).
P6 = Vr1 × Rth6 / (Rth6 + Rr6) Formula (8)
The output P5 of the fifth detection circuit and the output P6 of the sixth detection circuit are connected to the third differential amplifier circuit 33 via the voltage follower 23.

The voltage followers (21, 22, 23) are circuits that convert a high impedance signal into a low impedance signal. Since the outputs (P1, P2) of the first bridge circuit, the outputs (P3, P4) of the second bridge circuit, and the outputs of the third bridge circuits (P5, P6) are generally high impedance signals, they are voltage followers. By passing the signal, the signal is converted into a low impedance signal, and a voltage that does not attenuate can be input to the differential amplifier circuit (31, 32, 33) connected to the next stage. As voltage values, the outputs (P7, P8, P9, P10, P11, P12) corresponding to the inputs (P1, P2, P3, P4, P5, P6) of the voltage follower (21, 22, 23) are the same. is there. When the bridge circuit output is lower by two digits or more than the input impedance of the next stage, a voltage follower is generally not particularly required.

  The differential amplifier circuits (31, 32, 33) are circuits that amplify the difference between two input voltages with a constant coefficient. The third differential amplifier circuit 33 operates in the same manner as the first differential amplifier circuit 31 and the second differential amplifier circuit 32, and receives the outputs (P11, P12) of the third voltage follower as inputs. The difference between the output voltage of the output P11 and the output voltage of the output P12 is taken and only this difference is amplified. Since the voltage values before and after the voltage follower are the same, the difference between the outputs (P5, P6) of the third bridge circuit 13 is taken and only this difference is amplified.

The output P13 of the first differential amplifier circuit 31 sets the constant of the first bridge circuit so that the ambient temperature region is divided into three and takes a peak value in the intermediate ambient temperature region. The first differential amplifier circuit 31 outputs a voltage obtained by amplifying the difference between two input voltages as an output P13 and is input to the adder circuit 43 in the next stage.
The output P14 of the second differential amplifier circuit 32 sets the constant of the second bridge circuit so that the ambient temperature region is divided into three and takes a peak value in the lower ambient temperature region. The second differential amplifier circuit 32 outputs a voltage obtained by amplifying the difference between the two input voltages as an output P14 and is input to the adder circuit 43 in the next stage.
The output P15 of the third differential amplifier circuit 33 sets the constant of the third bridge circuit so that the ambient temperature region is divided into three and the peak value is obtained in the higher ambient temperature region. The third differential amplifier circuit 33 outputs a voltage obtained by amplifying the difference between the two input voltages as an output P15 and is input to the adder circuit 43 in the next stage. The ambient temperature region may be divided into three, and any combination of the ambient temperature regions handled by the three differential amplifier circuits may be used. That is, the constant of the first bridge circuit is set so that the output P13 of the first differential amplifier circuit 31 takes a peak value in the higher ambient temperature region, and the output P14 of the second differential amplifier circuit 32 is set. However, the constant of the second bridge circuit is set so that the peak value is taken in the intermediate ambient temperature region, and the output P15 of the third differential amplifier circuit 33 takes the peak value in the lower ambient temperature region. As described above, the constant of the third bridge circuit may be set.

  The adder circuit 43 is a circuit that adds and amplifies input voltages. The adder circuit 43 adds the three input voltages of the output P13 of the first differential amplifier circuit 31, the output P14 of the second differential amplifier circuit 32, and the output P15 of the third differential amplifier circuit 33. That is, the signal obtained by amplifying the difference voltage of the outputs (P1, P2) of the first bridge circuit 11, the signal obtained by amplifying the difference voltage of the outputs (P3, P4) of the second bridge circuit 12, and the third bridge The signal obtained by amplifying the differential voltage of the outputs (P5, P6) of the circuit 13 is added. The output P19 of the adding circuit 43 adds a voltage having a peak value in the higher ambient temperature region, a voltage having a peak value in the intermediate ambient temperature region, and a voltage having a peak value in the lower ambient temperature region. Output voltage. This output voltage also has three peak values in sensitivity (change in output voltage with respect to the temperature change of the heating roller). The amplification factors of the differential amplifier circuits (41, 42, 43) are set so that the peak values are substantially equal. Further, the amplification factor of the adder circuit 43 is appropriately set within the input voltage range of the next-stage circuit. As a result, even when the ambient temperature region is wide, the sensitivity is equalized over the entire ambient temperature region. Therefore, there is no place where the temperature of one bit of the A / D conversion circuit 51 connected to the next stage is extremely high, and the heat source It is possible to suppress a decrease in the temperature detection accuracy.

(Modification of the second embodiment)
Next, a configuration of a sensor circuit 400 that is a modification of the sensor circuit 300 according to the second embodiment of the present invention will be described with reference to FIG. FIG. 8 is a circuit configuration diagram showing a modification of the sensor circuit according to the second embodiment of the present invention.

The sensor circuit 400 according to this modification includes a power source V1, a first bridge circuit 11, a second bridge circuit 12, a third bridge circuit 13, a voltage follower (21, 22, 23), A The / D conversion circuit 51 is the same as the sensor circuit 300 according to the second embodiment. The present modification is different from the sensor circuit 300 according to the second embodiment in that an addition / subtraction circuit 44 is provided instead of the differential amplifier circuit (31, 32, 33) and the addition circuit 43. Hereinafter, a description will be given focusing on differences from the second embodiment.

The addition / subtraction circuit 44 is an analog arithmetic circuit that performs addition, subtraction, and amplification. In the present modification 400, the output P8 of the first voltage follower 21, the output P10 of the second voltage follower 22, and the output P12 of the third voltage follower 23 are added, and the output P7 of the first voltage follower 21 and the first By subtracting the output P9 of the second voltage follower 22 from the output P11 of the third voltage follower 23, the amplification factor can be set for each signal. In the sensor circuit 300 according to the second embodiment, the output voltage of the output P13 of the differential amplifier circuit 31 and the output of the second voltage follower 22 which are the differential voltages of the outputs (P7, P8) of the first voltage follower 21. The output voltage P15 of the differential amplifier circuit 33, which is the difference voltage between the output voltage P14 of the differential amplifier circuit 32 (P9, P10) and the output voltage (P11, P12) of the third voltage follower 23. After obtaining the voltage, these output voltages were added. That is, in the present modification 400, the same operation can be performed only by the addition / subtraction circuit 43, so that the circuit can be simplified.

(Third embodiment)
Next, the configuration of a sensor circuit 500 according to the third embodiment of the present invention will be described with reference to FIG. FIG. 9 is a circuit configuration diagram showing a sensor circuit according to the third embodiment of the present invention. The sensor circuit 500 according to the third 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.

  As shown in FIG. 9, the sensor circuit 500 includes a power supply V1, a bridge circuit 14, a voltage follower 21, a differential amplifier circuit 34, an A / D conversion circuit 51, a first storage device 52, and a first storage device 52. 2 storage devices 53, an adder 54 and a logic circuit 55.

  The bridge circuit 14 includes two detection circuits and a switch, a seventh detection circuit for detecting infrared rays emitted from the heat source and an eighth detection circuit for detecting the ambient temperature. In this embodiment, a P-type MOS (metal-oxide-semiconductor) switch is used as this switch, but other switches such as a transfer gate may be used. The seventh detection circuit includes a series circuit of first resistance means connected to the positive electrode of the power supply V1 and seventh thermistor Th7 connected to the negative electrode of the power supply V1. The first resistance means includes a resistor (R27, R29) connected in series and a first switch M1, and the first switch M1 is between the contact point of the resistor R27 and the resistor R29 and the positive electrode of the power source V1. Connected to. The eighth detection circuit is constituted by a series circuit of a second resistance means connected to the positive electrode of the power supply V1 and an eighth thermistor Th8 connected to the negative electrode of the power supply V1. The second resistance means is composed of a resistor (R28, R30) connected in series and a second switch M2, and the second switch M2 is between the contact point of the resistor R28 and the resistor R30 and the positive electrode of the power source V1. Connected to.

The seventh thermistor Th7 and the eighth thermistor Th8 are NTC thermistors having a negative resistance temperature coefficient mainly composed of a metal oxide, similarly to the thermistors (Th1, Th2, Th3, Th4, Th5, Th6). .

The switches (M1, M2) are turned on or off at the same timing. When the switches (M1, M2) are on, the resistor R27 and the resistor R28 are short-circuited to the positive electrode of the power source V1, so that the seventh detection circuit that is a series circuit of the resistor R29 and the thermistor Th7, the resistor R30, and the thermistor Th8 The bridge circuit is configured by an eighth detection circuit which is a series circuit. Note that the on-resistance of the switch is ignored. The output P21 of the seventh detection circuit is as follows. The DC voltage value supplied from the power source V1 is Vr1, the resistance value of the resistor R29 is Rr29, the resistance value of the seventh thermistor Th7 is Rth7, and the on-resistance of the switch is 0Ω. The relationship of the following formula (9) is satisfied.
P21 = Vr1 × Rth7 / (Rth7 + Rr29) Formula (9)
Similarly, the output P22 of the eighth detection circuit is expressed by the following formula (Vr1, the resistance value of the resistor R30 is Rr30, and the resistance value of the eighth thermistor Th8 is Rth8): 10) is satisfied.
P22 = Vr1 × Rth8 / (Rth8 + Rr30) Formula (10)
When the MOS switches (M1, M2) are off, the first resistor means includes a resistor R27 and a resistor R29 in series. The output P21 of the seventh detection circuit at this time is represented by Vr1 as the DC voltage value supplied from the power supply V1, (Rr27 + Rr29) as the resistance value of the first resistance circuit, and Rth7 as the resistance value of the seventh thermistor Th7. Therefore, the relationship of the following formula (9) is satisfied.
P21 = Vr1 × Rth7 / (Rth7 + (Rr27 + Rr29)) Formula (11)
Similarly, when the switch (M1, M2) is OFF, the output P22 of the eighth detection circuit is Vr1 as the DC voltage value supplied from the power supply V1, (Rr28 + Rr30) as the resistance value of the second resistance means, When the resistance value of the thermistor Th8 is Rth8, the relationship of the following expression (12) is satisfied.
P22 = Vr1 × Rth8 / (Rth8 + (Rr28 + Rr30)) Formula (12)
By turning on and off the switches (M1, M2), the fourth bridge circuit can output a voltage having a peak value in the higher ambient temperature region and a voltage having a peak value in the lower ambient temperature region. The outputs (P21, P22) of the bridge circuit 14 are connected to the differential amplifier circuit 34 via the voltage follower 21.

The voltage follower 21 is a circuit that converts a high impedance signal into a low impedance signal. When the output impedance of the bridge circuit 14 is not sufficiently lower than the input impedance of the differential amplifier circuit 34, a voltage signal can be transmitted without being attenuated by placing a voltage follower.

The differential amplifier circuit 34 is a circuit that can vary the amplification factor with two switches (T1, T2) and amplifies a difference between two input voltages. In this embodiment, a transfer gate is used for the switches (T1, T2), but a P-type MOS switch or the like may be used.

The switches (T1, T2) are turned on or off at the same timing. When the switches (T1, T2) are on, both ends of the resistor R31 and both ends of the resistor R34 are short-circuited, so that the gain is low. As for the amplification factor of the differential amplifier circuit 34, assuming that the resistance values of the resistors R33 and R36 are the same Rra, the resistance values of the resistors R32 and R35 are the same Rrb, and the amplification factor is Av1, the following equation (13) is satisfied. Will be met.
Av1 = Ra / Rrb Formula (13)
When the switches (T1, T2) are off, both ends of the resistor R31 and both ends of the resistor R34 are open, so that the resistance value increases and the amplification factor increases. The amplification factor of the differential amplifier circuit 34 is the same as the combined resistance value in which the resistance values of the resistors R33 and R36 are the same Rra, the resistance R31 and the resistance R32 are connected in series, and the combined resistance value in which the resistance R34 and the resistance R35 are connected in series. When (Rrb + Rrc) and the amplification factor are Av2, the relationship of the following formula (14) is satisfied.
Av2 = Rra / (Rrb + Rrc) Formula (14)

In the sensor circuit 100 according to the first embodiment, when the amplification factor of the differential amplifier circuit 31 is Av3, the amplification factor of the differential amplifier circuit 32 is Av4, and the amplification factor of the adder circuit 41 is Av5, Av1 and Av2 are as follows: The relational expressions (15) and (16) can be satisfied.
Av1 = Av3 * Av5 Formula (15)
Av2 = Av4 * Av5 Formula (16)
That is, since the resistance value can be changed by turning on and off the switch to control the level of sensitivity, the same operation as that of the sensor circuit 100 can be performed with a simplified circuit, particularly with only one pair of thermistor pairs. Expressions (15) and (16) are for explaining the amplification relationship between the sensor circuit 100 and the sensor circuit 500, and the amplification factor of the sensor circuit 100 and the amplification factor of the sensor circuit 500 are not necessarily the same. Not necessarily.

The output P17 of the differential amplifier circuit 34 is converted from an analog value to a digital value by the A / D conversion circuit 51. This digital value is stored in the storage device (52, 53).

  The storage devices (52, 53) are devices for temporarily storing digital values. In the present embodiment, a digital value that is an output of the A / D conversion circuit 51 is stored. When the switch of the bridge circuit 14 and the switch of the differential amplifier circuit 34 are on, the analog value output from the differential amplifier circuit 34 is converted into a digital value by the A / D conversion circuit 51, and this digital value is stored in the storage device. 52. When the switch of the bridge circuit 14 and the switch of the differential amplifier circuit 34 are OFF, the analog value output from the differential amplifier circuit 34 is converted into a digital value by the A / D conversion circuit 51, and this digital value is stored. The data is stored in the device 53. That is, data when the switch of the bridge circuit 14 and the switch of the differential amplifier circuit 34 are on and off are alternately stored in the storage devices (52, 53). The outputs of the storage devices (52, 53) are connected to the adder 54.

The logic circuit 55 is a circuit that generates a signal for turning on / off the switch and latching in the memory device using an external clock signal. It is composed of a data flip-flop DF1 and two ANDs (AND1, AND2). The data flip-flop is a logic circuit that outputs the same value as the data input to the output Q and the inverted value to the output Qb when the clock input becomes 1. The AND is a logic circuit that outputs 1 when the values of all inputs are 1. The switch of the bridge circuit 14 and the switch of the differential amplifier circuit 34 are switched at the same timing. The latch timing of the storage device is stored in the storage device 52 when the switch is on, and stored in the storage device 52 when the switch is off.

  The adder 54 is an arithmetic device that performs addition. In the present embodiment, the digital values of the first storage device 52 and the second storage device 53 are added. In the sensor circuit 100 according to the first embodiment, analog addition is performed by the adder circuit 41 and then the digital value is changed by the A / D conversion circuit 51. In the sensor circuit 500 according to the third embodiment, signal addition from each bridge circuit is performed after the A / D conversion circuit. Thereby, the output OUT1 of the A / D conversion circuit 51 of the sensor circuit 100 according to the first embodiment and the output OU2 of the adder 54 of the sensor circuit 500 according to the third embodiment obtain equivalent signals. Is possible. Arithmetic processing such as storage and addition can be performed by a microcomputer or the like. Although not shown in the sensor circuit 100, since a microcomputer is used in the subsequent stage, it is possible to increase the number of circuits in the digital processing portion without using the microcomputer by using this microcomputer. Also in the sensor circuit 500, by using a microbridge computer at the subsequent stage, only two thermistors used for the bridge circuit can be used and can be handled by switching, so that the circuit configuration can be very simplified.

(Modification of the third embodiment)
Next, a configuration of a sensor circuit 600 which is a modification of the sensor circuit 500 according to the third embodiment of the present invention will be described with reference to FIG. FIG. 10 is a circuit configuration diagram showing a modification of the sensor circuit according to the third embodiment of the present invention.

The sensor circuit 600 according to this modification includes a power supply V1, a voltage follower 21, an A / D conversion circuit 51, a first storage device 52, a second storage device 53, an adder 54, and a logic circuit 55. This is the same as the sensor circuit 500 according to the third embodiment. This modification is different from the sensor circuit 500 according to the third embodiment in that a bridge circuit 15 and a differential amplifier circuit 35 are provided instead of the bridge circuit 14 and the differential amplifier circuit 34. Hereinafter, a description will be given centering on differences from the third embodiment.

The bridge circuit 15 includes two detection circuits, a ninth detection circuit for detecting infrared rays emitted from the heat source, and a tenth detection circuit for detecting ambient temperature, and a switch. In this embodiment, a P-type MOS (metal-oxide-semiconductor) switch is used as this switch, but other switches such as a transfer gate may be used. The ninth detection circuit includes a series circuit of third resistance means connected to the positive electrode of the power supply V1 and a seventh thermistor Th7 connected to the negative electrode of the power supply V1. In the third resistor means, a resistor R37 and a resistor 39 connected in series with the switch M3 are connected in parallel. The tenth detection circuit is constituted by a series circuit of a fourth resistance means connected to the positive electrode of the power supply V1 and an eighth thermistor Th8 connected to the negative electrode of the power supply V1. In the fourth resistance means, a resistor R38 and a resistor 40 connected in series with the switch M3 are connected in parallel. The switch M3 is used in common for the third resistance means and the fourth resistance means.

When the switch M3 is on, the resistor R37 and the resistor R38 are connected to the positive electrode of the power source V1, so that the resistor R37 is connected in parallel with the resistor R39. Similarly, since the resistor R38 and the resistor R40 are connected to the positive electrode of the power source V1, the resistor R38 is connected in parallel with the resistor R40. The ninth detection circuit has a configuration in which the resistors (R37, R39) connected in parallel and the thermistor Th7 are connected in series. The output P23 of the ninth detection circuit has a DC voltage value supplied from the power source V1 as Vr1, a resistance value of the third resistance circuit as Rr37 * Rr39 / (Rr37 + Rr39), a resistance value of the seventh thermistor Th7 as Rth7, When the on-resistance of the switch is 0Ω, the relationship of the following formula (17) is satisfied.
P21 = Vr1 × Rth7 / (Rth7 + Rr37 * Rr39 / (Rr37 + Rr39)) Formula (17)
Similarly, the output P24 of the tenth detection circuit has the DC voltage value supplied from the power source V1 as Vr1, the resistance value of the fourth resistance circuit as Rr38 * Rr40 / (Rr38 + Rr40), and the resistance value of the eighth thermistor Th8. When Rth8 is satisfied, the relationship of the following formula (18) is satisfied.
P23 = Vr1 × Rth8 / (Rth8 + Rr38 * Rr40 / (Rr38 + Rr40)) Formula (18)
The bridge circuit 15 is a circuit corresponding to change the resistance value with only one switch M3. By turning on / off the switch M3, the fifth bridge circuit can output a voltage having a peak value in the higher ambient temperature region and a voltage having a peak value in the lower ambient temperature region. In the sensor circuit 500 according to the third embodiment, the resistance means indicates a bridge circuit when connected in series, and the sensor circuit 600 according to the present modification shows a bridge circuit when connected in parallel. This is an example, and a circuit that changes other resistance values may be used. The outputs (P23, P24) of the bridge circuit 15 are connected to the differential amplifier circuit 35 via the voltage follower 21.

The differential amplifier circuit 35 is a circuit that can vary the amplification factor with two switches (T3, T4) and amplifies the difference between the two input voltages. In this embodiment, a transfer gate is used for the switches (T3, T4), but a P-type MOS switch or the like may be used.

The switches (T3, T4) are turned on or off at the same timing. When the switches (T3, T4) are on, the resistors R41 and R42 are connected in parallel. Similarly, the resistor R44 and the resistor R45 are also connected in parallel. This lowers the amplification factor. As for the amplification factor of the differential amplifier circuit 34, the resistance values of the resistors R43 and R46 are the same Rrd, the resistance values of the resistors R41 and R44 are the same Rre, the resistance values of the resistors R42 and R45 are the same Rrf, and the amplification factor is Av3. Then, the relationship of the following formula (19) is satisfied.
Av3 = Rd / (Rre * Rrf / (Rre + Rrf)) Formula (19)
When the switch (T3, T4) is off, one end of the resistor R41 and the resistor R44 is opened, so that the resistance value of the combined resistor composed of the resistor (R41, R42) and the switch T3 and the resistor (R44, R45) The resistance value of the combined resistor formed by the switch T4 is increased and the amplification factor is increased. As for the amplification factor of the differential amplifier circuit 35, the resistance values of the resistors R43 and R46 are the same Rrd, the resistance values of the resistors R42 and R45 are the same Rrf, and the amplification factor is Av4. Will be met.
Av4 = Rd / Rrf Formula (20)
As shown in Expression (19) and Expression (20), the amplification factor is changed by changing the resistance value of the denominator. The resistance value of the denominator shows an example in which resistors are connected in series in the sensor circuit 500 according to the third embodiment, and an example in which the sensor circuit 600 according to this modification is connected in parallel. For example, a circuit that changes other resistance values may be used.

  Hereinafter, Examples 1 and 2 and Comparative Example 1 will specifically show that this embodiment can suppress a decrease in the detection accuracy of the temperature of the heat source. However, the present invention is not limited to these. In Examples 1 and 2 and Comparative Example 1, the output of the bridge circuit, the output of the differential amplifier circuit, the output of the adder circuit, and the temperature characteristics of sensitivity were simulated.

  In Example 1, the sensor circuit 100 according to the first embodiment described above was used. In Example 2, the sensor circuit 300 according to the second embodiment described above was used. In Comparative Example 1, the sensor circuit 700 shown in FIG. 12 was used. FIG. 12 is a circuit configuration diagram illustrating a sensor circuit according to the first comparative example.

  First, the configuration of the sensor circuit 700 according to Comparative Example 1 will be described. As illustrated in FIG. 12, the sensor circuit 700 includes a power supply V <b> 1, a bridge circuit 11, a voltage follower 21, a differential amplifier circuit 31, and an A / D conversion circuit 51.

  The bridge circuit includes a first detection circuit for detecting infrared rays emitted from the heat source and a second detection circuit for detecting ambient temperature. The first detection circuit is configured by a series circuit of a first resistor R1 connected to the positive electrode of the power supply V1 and a first thermistor Th1 connected to the negative electrode of the power supply V1. The second detection circuit is configured by a series circuit of a second resistor R2 connected to the positive electrode of the power supply V1 and a second thermistor Th2 connected to the negative electrode of the power supply V1. The voltage follower 21 is a circuit that converts a high impedance signal into a low impedance signal. The output (P1, P2) of the bridge circuit 11 which is a high impedance output is converted to a low impedance in order to prevent the voltage from being attenuated and connected to the differential amplifier circuit 31 at the next stage. That is, the signal voltage has the same voltage value at the output P1 of the bridge circuit 11 and P7 of the voltage follower 21. Similarly, the output P2 of the bridge circuit 11 and P8 of the voltage follower 21 have the same voltage. The differential amplifier circuit 31 is a circuit that amplifies the difference between two input voltages with a constant coefficient. The output P13 of the differential amplifier circuit 31 is connected to the A / D conversion circuit 51. Although not shown in FIG. 11, the voltage value obtained by detecting the ambient temperature is also taken into the A / D conversion circuit 51. The A / D conversion circuit 51 is a circuit that converts an analog value into a digital value. That is, the analog value of the output P13 of the differential amplifier circuit 31 is converted into a digital value. Although not shown in FIG. 1, the value converted into a digital value by the A / D conversion circuit 51 is taken into a microcomputer and converted by a temperature conversion table or function to detect the temperature of the heat source.

  Next, the temperature characteristics of the sensor circuit 100 according to the first embodiment will be described with reference to FIGS. FIG. 2 shows an output obtained by differentially amplifying two outputs of the first bridge circuit of the sensor circuit according to the first embodiment, and a differential amplification of the two outputs of the second bridge circuit. It is a graph which shows the temperature characteristic of the output of the addition circuit which added the output of two differential amplifier circuits, and the output which took the difference and amplified by the circuit.

  Each constant of each bridge circuit of the sensor circuit 100 has a resistance value of 7 kΩ and a thermistor (Th1, Th2) of the resistance (R1, R2) of the first bridge circuit 11 that has a peak value in the higher ambient temperature region. The resistance value at 25 ° C. is 100 kΩ, the B constant is 4485 K, the resistance value of the resistor (R3, R4) of the second bridge circuit 12 having a peak value in the lower ambient temperature region is 45 kΩ, and the thermistor (Th3, Th4). ) At 25 ° C. was set to 10 kΩ, and the B constant was set to 4100K. The amplification factor of the first differential amplifier circuit 31 is set to 1.95 times, the amplification factor of the second differential amplifier circuit 32 is set to 1.01 times, and the amplification factor of the adder circuit 41 is set to 1.00 times. .

FIG. 2 shows a case where the heat source temperature is 180 ° C. The output P13 of the differential amplifier circuit 31 that amplifies the difference between the outputs of the first bridge circuit is the entire ambient temperature region of −40 ° C. to 100 ° C., and is divided into two parts between 30 ° C. and 100 ° C. As shown in the characteristic, it reaches a peak value around 70 ° C. The output P14 of the differential amplifier circuit 32 that amplifies the difference between the outputs of the second bridge circuit is a peak value in the vicinity of −20 ° C. between the remaining two parts of −40 ° C. to 30 ° C. as shown by 62 characteristics. become. The result of adding these two differential amplifier circuits is 61 characteristics. In FIG. 2, each amplification factor is set so that the voltage peak value of 61 is 1V.

  FIG. 3 shows the temperature characteristics of sensitivity. The output of the adder circuit when the heat source temperature is 180 ° C. has 61 characteristics as in FIG. 2, but the 65 characteristic of the adder circuit output when the heat source temperature is 160 ° C. is added here. Sensitivity is the difference between when the heat source temperature is 180 ° C. and 160 ° C., and has a characteristic like 66. Sensitivity peak values exist in the ambient temperature region divided into two, and the peak values are substantially equal. The sensitivity characteristic 66 in the entire ambient temperature range is between 0.12 V and 0.16 V, and the sensitivity change due to the ambient temperature is small. This means that the temperature change per bit in the subsequent A / D conversion circuit is small with respect to the ambient temperature. That is, since the sensitivity to the ambient temperature is averaged, the temperature change does not become small at a specific ambient temperature, and the detection accuracy of the temperature of the heat source can be made uniform in the entire ambient temperature region.

  Subsequently, temperature characteristics of the sensor circuit 300 according to the second embodiment will be described with reference to FIGS. 6 and 7. FIG. 6 shows an output obtained by differentially amplifying the two outputs of the first bridge circuit of the sensor circuit according to the second embodiment and a differential amplification of the two outputs of the second bridge circuit. The output temperature of the adder circuit obtained by adding the difference between the output of the third bridge circuit, the output of the third bridge circuit amplified by the differential amplifier circuit, and the output of the three differential amplifier circuits. It is a graph which shows a characteristic.

  Each constant of each bridge circuit of the sensor circuit 300 has a resistance value of 33 kΩ for the resistance (R1, R2) of the first bridge circuit 11 that gives a peak value in the intermediate ambient temperature region, and 25 of the thermistor (Th1, Th2). The resistance value at 100 ° C. is 100 kΩ, the B constant is 4485 K, the resistance value of the second bridge circuit 12 having a peak value in the lower ambient temperature region (R3, R4) is 75 kΩ, and the thermistor (Th3, Th4) The resistance value at 25 ° C. is 10 kΩ, the B constant is 4100 K, the resistance value (R5, R6) of the third bridge circuit 13 having a peak value in the higher ambient temperature region is 3.6 kΩ, thermistor ( The resistance value at 25 ° C. of Th5 and Th6) was set to 330 kΩ, and the B constant was set to 4750K. Further, the amplification factor of the first differential amplifier circuit 31 is 1.08 times, the amplification factor of the second differential amplifier circuit 32 is 0.85 times, and the amplification factor of the third differential amplifier circuit 33 is 2. The amplification factor of the adder circuit 43 was set to 38 times and 1.00 times.

FIG. 6 shows a case where the heat source temperature is 180 ° C. The output P13 of the differential amplifier circuit 31 obtained by amplifying the difference between the outputs of the first bridge circuit has a total ambient temperature region of −40 ° C. to 140 ° C., and is divided into three parts between 20 ° C. and 80 ° C. It becomes a peak value around 40 ° C. like the characteristic. The output P14 of the differential amplifying circuit 32 that amplifies the difference between the outputs of the second bridge circuit is peaked at around −20 ° C. like the characteristic of 72 between the lower divided −40 ° C. to 20 ° C. Value. The output P15 of the differential amplifier circuit 33 obtained by amplifying the difference between the outputs of the third bridge circuit reaches a peak value at around 110 ° C. as shown by 74 characteristics between the higher 80 ° C. and 140 ° C. divided into three. Become. The result of adding the three differential amplifier circuits is the characteristic 71. In FIG. 6, each amplification factor is set so that the peak value of the voltage of 71 is 1V.

  FIG. 7 shows the temperature characteristics of sensitivity. The output of the adder circuit when the heat source temperature is 180 ° C. has 71 characteristics as in FIG. 6, but 75 characteristics of the output of the adder circuit when the heat source temperature is 160 ° C. are added here. The sensitivity is the difference between when the heat source temperature is 180 ° C. and 160 ° C., and has a characteristic like 76. Despite the fact that the ambient temperature range has become wider from -40 to 140 ° C, the output of the three bridge circuits has peak values at high, medium, and low temperatures, and the gain is adjusted and added. Thus, the sensitivity characteristic 76 in the entire ambient temperature range is between 0.14 V and 0.16 V, and it is possible to further reduce the sensitivity change due to the ambient temperature. Since the sensitivity to the ambient temperature is further averaged, the detection accuracy of the temperature of the heat source can be made uniform in the entire ambient temperature region, and the decrease in detection accuracy due to a decrease in sensitivity at a specific ambient temperature can be suppressed. it can.

  Subsequently, a timing chart and operation of the logic circuit of the sensor circuit 500 according to the third embodiment will be described with reference to FIGS. 11 and 2. FIG. 11 is a timing chart of the logic circuit 55 of the sensor circuit 500 according to the third embodiment. A switch (M1, M2) signal for switching the resistance value of the resistance means of the block circuit 15 and a switch (T1, T2) for switching the amplification factor of the differential amplifier circuit 35 using the clock signal CK from the outside. T2) signal and a data latch signal of the storage device (52, 53) are output. When the output Qb of DF1 is 0, the switches (M1, M2, T1, T2) are short-circuited. At this time, the output voltage of the output P17 of the differential amplifier circuit 34 is stored in the storage device 52 by the rising waveform of the output of the AND1. When the output Qb of DF1 is 1, the switches (M1, M2, T1, T2) are opened. At this time, the output voltage of the output P17 of the differential amplifier circuit 34 is stored in the storage device 53 by the rising waveform of the output of the AND2. When the ambient temperature is 10 ° C., the voltage value at point a in FIG. 2 is stored in the storage device 52 as a digital value, and the voltage value at point b is stored in the storage device 53 as a digital value. In the adder 54, a digital value at the point c obtained by adding the points a and b is output to the output OUT 2 of the addition circuit 54. As a result, it is possible to obtain an output equivalent to the output OUT1 of the A / D conversion circuit 51 of the first embodiment.

  Next, with reference to FIG. 13, the temperature characteristics of the sensor circuit 700 of Comparative Example 1 are shown. The constants of the sensor circuit 700 are: the resistance (R1, R2) of the bridge circuit is 18 kΩ, the resistance value of the thermistor (Th1, Th2) at 25 ° C. is 100 kΩ, the B constant is 4485 K, and the amplification factor of the differential amplifier circuit is 2. Set 20 times.

The differential amplifier circuit output when the heat source temperature is 180 ° C. has 81 characteristics, and the differential amplifier circuit output when the heat source temperature is 160 ° C. has 85 characteristics. The difference between the differential amplifier circuit output when the heat source temperature is 180 ° C. and the differential amplifier circuit output when the heat source temperature is 160 ° C. is 86 characteristics. The output of the differential amplifier circuit has a peak value when the ambient temperature is 50 ° C., and the sensitivity decreases as the ambient temperature moves away from 50 ° C. around 50 ° C. In the region where the ambient temperature is 0 ° C. to 100 ° C., the sensitivity characteristic 86 is between 0.09 V and 0.21 V, but when the temperature range is expanded from −40 ° C. to 140 ° C., the sensitivity characteristic 86 is 0.01 V. To 0.21V, and the sensitivity to the ambient temperature is significantly reduced. Since the sensitivity varies depending on the ambient temperature, the temperature change per bit of the A / D conversion circuit is large, and the ambient temperature is low, and the temperature detection accuracy is extremely low. When the difference between the heat source temperature of 180 ° C. and the heat source temperature of 160 ° C. is 20 ° C., the ambient temperature is 50 ° C., there is a voltage change of 0.21V, and when the ambient temperature is −40 ° C., there is a voltage change of 0.01V. From this, when the ambient temperature is 50 ° C., it becomes 0.10 ° C./mV, and when the ambient temperature is −40 ° C., it becomes 1.60 ° C./mV. That is, when the 1-bit resolution of the A / D conversion circuit is 1 mV, when the ambient temperature is −40 ° C., there is no temperature detection accuracy of 1.60 ° C. or less.

  On the other hand, in the sensor circuit 100 of Example 1, since it is 0.12 V even at −40 ° C. which is the lowest ambient temperature, it is 0.17 ° C./mV, which is compared with the sensor circuit 700 of Comparative Example 1. A significant decrease in the detection accuracy of the heat source can be suppressed. Furthermore, in the sensor circuit 300 of the second embodiment, when the ambient temperature with the lowest sensitivity is −40 ° C., it is 0.14 V, so that it is about 0.15 ° C./mV, and the sensitivity is 0.13 ° C./peak at the peak value. It is greatly improved so that it is not different from mV. In the entire ambient temperature region, it has a high temperature detection accuracy of 0.15 ° C./mV or less.

  The preferred embodiments of the present invention have been described above. However, the present invention is not necessarily limited to the above-described embodiments, and various modifications can be made without departing from the scope of the present invention. The constituent elements described include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the described constituent elements can be appropriately combined.

  Moreover, although the said embodiment demonstrated the example which applied the sensor circuit which concerns on this invention to what is called a non-contact temperature sensor which measures the temperature of a heat source non-contact, it is not limited to this. For example, the present invention can be applied to a gas sensor, a humidity sensor, and a flow rate sensor that detect a physical quantity using a temperature difference between two thermistors.

  An optical gas sensor, called NDIR (non-dispersive infrared detector), irradiates the first thermistor with infrared light that has passed through the gas to be measured, and the second thermistor. Irradiates infrared rays that have passed through a standard gas that does not contain the gas to be measured, and detects the gas concentration in the gas to be measured from the difference in temperature rise between the two thermistors. That is, the first thermistor is affected by a physical quantity called gas concentration, and the second thermistor is reduced in its influence. Even in such a gas sensor using two thermistors, by applying the sensor circuit according to the above-described embodiment, it is possible to mitigate a decrease in detection accuracy of a physical quantity to be measured, that is, a 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 sealed 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 the humidity, but the second thermistor is not affected by the humidity. The temperature difference between the two thermistors reflects humidity. That is, the first thermistor is affected by a physical quantity called humidity, and the second thermistor is reduced in its influence. Even in a humidity sensor using such two thermistors, a decrease in detection accuracy of a physical quantity to be measured, that is, humidity, can be suppressed by applying the sensor circuit according to the embodiment.

  A flow rate sensor using two thermistors is one in which the first thermistor is exposed to the fluid to be measured and the second thermistor is not exposed to the fluid. When these two thermistors are heated under the same conditions, the first thermistor loses heat and changes its temperature in accordance with the flow velocity, but the second thermistor is not affected. The temperature difference between the two thermistors reflects the flow rate. That is, the first thermistor is affected by a physical quantity called the flow velocity, and the second thermistor is reduced in its influence. Even in a flow rate sensor using two such thermistors, the sensor circuit according to the above embodiment can be applied to suppress a decrease in detection accuracy of a physical quantity to be measured, that is, a flow rate.

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

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

V1, power supply, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13, R14, R15, R16, R17, R18, R19, R20, R21, R22, R23, R24, R25, R26, R27, R28, R29, R30, R31, R32, R33, R34, R35, R36, R37, R38, R39, R40, R41, R42, R43, R44, R45, R46 ... resistors, Th1, Th2, Th3, Th4, Th5, Th6, Th7, Th8 ... Thermistor, M1, M2, M3, M4 ... P-type MOS switch, T1, T2, T3, T4 ... Transfer gate, OP1, OP2, OP3, OP4, OP5 OP6, OP7, OP8, OP9, OP10 ... operational amplifier, 11, 12, 13, 14, 15 ... bridge Path 21, 22, 23 ... voltage follower 31, 32, 33, 34, 35 ... differential amplifier circuit, 41, 43 ... adder circuit, 42, 44 ... adder / subtractor circuit, 51 ... A / D converter circuit, 52, 53 ... Storage device, 54 ... Adder, P1, P2, P3, P4, P5, P6, P21, P22, P23, P24 ... Output of bridge circuit, P7, P8, P9, P10, P11, P12 ... Voltage follower Output, P13, P14, P15 ... Output of differential amplifier circuit, P16, P19 ... Output of adder circuit, OUT1 ... Output of A / D converter circuit, OUT2 ... Output of adder, 100, 200, 300, 400, 500 , 600, 700 ... sensor circuit, 61, 65, 71, 75 ... voltage characteristics of the output of the adder, 62, 63, 72, 73, 74 ... voltage characteristics of the output of the differential amplifier circuit, 66, 76 Voltage temperature minus the voltage characteristics of the output of the temperature 160 ° C. during the addition circuit being measured from the voltage characteristic of the output of the adder circuit at 180 to be measured

Claims (6)

A first thermistor connected to the first pole of the power supply and the first thermistor connected in series to the first resistance and affected by the physical quantity of the measurement object connected to the second pole of the power supply; A first detection circuit comprising:
A second thermistor connected in series to the first resistor and the second thermistor connected in series to the second resistor and reduced in the influence of the physical quantity of the measurement object connected to the second electrode A second detection circuit;
A first bridge circuit comprising:
A third resistor having a third thermistor connected to the first pole and a third thermistor connected in series to the third resistor and affected by a physical quantity to be measured connected to the second pole A detection circuit of
A fourth resistor connected to the first pole and a fourth thermistor connected in series to the fourth resistor and having a reduced influence of the physical quantity of the measurement target connected to the second pole A fourth detection circuit;
Comprising a second bridge circuit comprising
The difference voltage between the output of the first detection circuit and the output of the second detection circuit, which is the output of the first bridge circuit, has a first peak value at a first ambient temperature, and the second bridge circuit The difference voltage between the output of the third detection circuit and the output of the fourth detection circuit, which is the output of, has a second peak value at a second ambient temperature different from the first ambient temperature,
An output of the first and second bridge circuits is input to an addition / subtraction circuit having a function of weighting an input voltage. The sensor circuit according to claim 1, comprising three or more bridge circuits. A first resistance means connected to the first pole of the power source and a fifth resistor connected in series to the first resistance means and affected by the physical quantity of the measurement target connected to the second pole of the power source A fifth detection circuit having a thermistor;
A second thermistor connected in series to the first pole and the sixth thermistor connected in series to the second resistor and reduced in the influence of the physical quantity of the measurement object connected to the second pole A sixth detection circuit having:
Comprising a third bridge circuit comprising
A switch capable of switching a resistance value of the first and second resistance means;
When the switch is on, the differential voltage at the output of the third bridge circuit has a third peak value at a third ambient temperature, and when the switch is off, the differential voltage of the third bridge circuit is The differential voltage of the output has a fourth peak value at a fourth ambient temperature different from the third ambient temperature,
The output of the third bridge circuit is input to a differential amplifier circuit whose amplification factor changes in conjunction with the switch,
A first digital value obtained by digitizing the analog output of the differential amplifier circuit when the switch is in an on state, and a second digital value obtained by digitizing the analog output of the differential amplifier circuit when the switch is in an off state. A sensor circuit comprising means for adding digital values. 4. The sensor circuit according to claim 3, wherein the resistance value of the resistance means has three or more resistance values by the switch. 5. The sensor according to claim 1, wherein when the ambient temperature region is equally divided by the number of outputs of the bridge circuit, the peak value exists in each of the divided ambient temperature regions. circuit. The sensor circuit according to claim 1, wherein the physical quantity to be measured is a temperature.

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