JP2002005754A - Temperature measuring resistor measuring circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a three-wire type temperature measuring resistor measuring circuit capable of making high-precision measurement in a specific temperature range without being affected by wire resistors. SOLUTION: This temperature measuring resistor measuring circuit measuring the resistance value of a temperature measuring resistor comprises a known resistor connected in series to first and second wire resistors connected at both ends of the temperature measuring resistor, a measuring means feeding a fixed current to a series circuit of the temperature measuring resistor, the first and second wire resistors, and the known resistor to measure a voltage drop, and a reference voltage adjusting means shifting the reference voltage used as a reference for measuring the voltage drop by the voltage drop of the first and second wire resistors.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistance thermometer measuring circuit, and more particularly, to a method for removing the influence of wiring resistance and improving the accuracy of measurement in a desired measuring temperature range.

[0002]

2. Description of the Related Art A temperature measuring resistor measures a temperature based on a change in resistance value of a resistor caused by a temperature change, and is widely used as an industrial temperature sensor in various fields. Conventionally, in a RTD measuring circuit, in order to measure the resistance value of the RTD with high accuracy, a voltage drop in a wiring resistance is removed by an analog arithmetic circuit using a three-terminal method. ing.

FIG. 5 shows an example of a conventional circuit disclosed in Japanese Patent Application Laid-Open No. 2000-74749. In the figure, r1 and r2 are wiring resistances, Rt is a resistance temperature detector, and the wiring resistances r1 and r2 are respectively connected to respective ends of the resistance temperature detector Rt. The other end of the wiring resistance r1 is connected to a non-inverting input terminal of an operational amplifier OP1 constituting the measuring means MES. A current source I is connected to a connection point between the wiring resistance r1 and the non-inverting input terminal of the operational amplifier OP1. An inverting input terminal of the operational amplifier OP1 is connected to an output terminal, and an output terminal of the operational amplifier OP1 is connected to, for example, an AD converter CNV via an amplifier circuit AMP.

[0004] A connection point between the resistance bulb Rt and the wiring resistance r2 is connected to an inverting input terminal of an operational amplifier OP2 which constitutes a voltage drop detecting means VDET. The non-inverting input terminal of the operational amplifier OP2 is connected to the connection point of the series circuit of the resistors R1 and R2 forming the reference voltage setting means VSET, and the output terminal is connected to the non-inverting input of the operational amplifier OP3 forming the voltage drop detecting means VDET. Connected to terminal. The other end of the wiring resistor r2 is connected to the non-inverting input terminal of the operational amplifier OP3.

The other end of the resistor R1 constituting the reference voltage setting means VSET is connected to an operational amplifier OP constituting the voltage drop detecting means VDET.
3 is connected to the output terminal. Reference voltage setting means VSET
Is connected to the common potential point.

[0006] The voltage drop detecting means VDET and the reference voltage setting means VSET constitute reference voltage adjusting means VADJ.

In FIG. 5, the measured value is Rt · I.
Here, the output voltage Vout of the operational amplifier OP1 is as follows: Vout = (r1 + Rt + r2) I + Vbase = (Rt + 2 · r2) I + Vbase (∵r1 = r2) (1) Also, the voltage drop detecting means VDET is configured Operational amplifier OP2
The voltage of the resistor R1 is equal to the voltage drop r2 · I generated by the wiring resistor r1 due to the action of OP3.

If R1 = R2, the voltage Vbase at the non-inverting input terminal of the operational amplifier OP3 becomes Vbase = −2 · r2 · I (2). From these two equations (1) and (2), And the target value Rt · I is obtained.

[0009]

By the way, in the circuit of FIG. 5, it is necessary to amplify the voltage of Vout in order to increase the measurement accuracy. However, the operational amplifier OP1
Is limited by the power supply voltage or the like, and it is difficult to set the amplification factor to a high value.

As a specific example, platinum PT10 is used as a resistance temperature detector.
Consider measuring 0Ω. 0 ° C using platinum PT100Ω
When measuring Rt = 100Ω and I = 1mA, Vou
t = 100mV. In order to measure this with high accuracy, for example, if the signal is amplified at an amplification factor of 100, the voltage becomes 10 V, and the power supply voltage cannot be reduced. In general, there are many demands for accurately measuring a temperature around 0 ° C.

The present invention focuses on such a problem, and an object of the present invention is to provide a three-wire type resistance thermometer capable of performing high-accuracy measurement in a specific temperature range without being affected by wiring resistance. It is to provide a measuring circuit.

[0012]

According to a first aspect of the present invention, there is provided a temperature measuring resistor measuring circuit for measuring a resistance value of a temperature measuring resistor, which is connected to both ends of the temperature measuring resistor. A constant current is supplied to a known resistance connected in series with the first and second wiring resistances, and a series circuit of these resistance temperature detectors, the first and second wiring resistances, and the known resistance to supply a voltage. Measuring means for measuring the voltage drop; and reference voltage adjusting means for shifting a reference voltage serving as a reference for measuring the voltage drop by a voltage drop in the first and second wiring resistances. .

According to a second aspect of the present invention, in the resistance thermometer measurement circuit according to the first aspect, the measuring means includes a series circuit of the resistance thermometer, the first and second wiring resistors, and a known resistor. It is characterized by comprising a constant current source for supplying a constant current and a buffer circuit for detecting the voltage drop.

According to a third aspect of the present invention, in the resistance temperature measuring circuit of the first aspect, the reference voltage adjusting means detects a voltage drop in the first or second wiring resistance. And reference voltage setting means for setting the reference voltage based on the voltage drop detected by the voltage drop detecting means.

According to a fourth aspect of the present invention, in the resistance temperature measuring circuit according to the third aspect, the voltage drop detecting means includes an inverting input terminal at a connection point between the temperature measuring resistor and the second wiring resistance. And a second operational amplifier which is a buffer circuit having a non-inverting input terminal connected to the other end of the second wiring resistance and an output terminal of the first operational amplifier. And a non-inverting input terminal of the first operational amplifier and an output terminal of the second operational amplifier are connected to the reference voltage setting means.

According to a fifth aspect of the present invention, in the resistance temperature measuring circuit of the fourth aspect, the reference voltage setting means has one end grounded and the other end connected to the non-inverting input terminal of the first operational amplifier. And a second resistor having one end connected to the other end of the first resistor and the other end connected to the output terminal of the second operational amplifier. I do.

Thus, the resistance value of the resistance temperature detector can be measured with a simple circuit configuration that does not use an arithmetic circuit as in the related art, without being affected by the wiring resistance.

According to a sixth aspect of the present invention, there is provided the temperature measuring resistor measuring circuit according to the first aspect, further comprising a calibration means.

According to a seventh aspect of the present invention, in the temperature measuring resistor measuring circuit according to the sixth aspect, the calibration means comprises: a reference resistance;
A first switch circuit for switching a constant current supplied by the measuring means to the reference resistance or the resistance temperature detector and the wiring resistance, and a voltage at one end of the first wiring resistance and at both ends of the reference resistance is applied. A second switch circuit having an output connected to the measuring means, a voltage at a connection point between the temperature measuring resistor and the second wiring resistance, and a voltage at the other end of the second wiring resistance as the reference. It is characterized by comprising third and fourth switch circuits applied to the voltage adjusting means.

According to an eighth aspect of the present invention, in the temperature measuring resistor measuring circuit according to the sixth or seventh aspect, the calibration means sets the same voltage to be applied to the measuring means and the reference voltage adjusting means. It is characterized by performing zero point calibration.

According to a ninth aspect of the present invention, in the resistance thermometer measurement circuit according to the seventh aspect, a voltage obtained by subtracting a voltage drop at the reference resistor from the voltage applied by the calibration means to the measurement means is provided as the reference voltage. The present invention is characterized in that the span point is calibrated by applying the voltage to the adjusting means.

Thus, the offset component of the operational amplifier can be compensated.

According to a tenth aspect of the present invention, in the resistance temperature measuring circuit of the first aspect, the resistance value of the known resistance is equal to the resistance value of the resistance temperature detector at a desired measurement temperature. .

Thus, the amplification factor at a desired measurement temperature determined by the resistance value of the resistance bulb equal to the resistance value of the known resistance can be increased, and highly accurate measurement can be performed.

[0025]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an example of an embodiment of the present invention, and portions common to FIG. 5 are denoted by the same reference numerals. The difference between FIG. 1 and FIG. 5 is that in FIG. 1, the first and second wiring resistances R1 and R connected to both ends of the resistance bulb Rt are shown.
That is, a known resistor Rs is connected in series with 2.

In the configuration shown in FIG. 1, the output signal Vout of the operational amplifier OP1 is given by: Vout = (r1 + Rt + r2 + Rs) I + Vbase = (Rt + 2 · r2 + Rs) I + Vbase (∵r1 = r2) (4) Operational amplifier OP2 that constitutes detection means VDET
Due to the action of OP3, a voltage equal to the wiring resistance r2, the voltage drop (r2 + Rs) × I generated by the known resistance Rs is generated in the resistor R1.

Here, since R1 = R2, the voltage Vbase at the non-inverting input terminal of the operational amplifier OP3 becomes Vbase = −2 · (r2 + Rs) · I (5) From these two equations (4) and (5), Becomes

This voltage Vout = (Rt−Rs) · I can be suppressed lower than the voltage Vout = Rt · I generated in the conventional circuit of FIG. Then, for example, by making the resistance value of the known resistance Rs equal to the resistance value of the resistance bulb Rt at 0 ° C.,
Since the Vout voltage at 0 ° C. becomes 0, it is possible to set the amplification factor of the amplifier circuit AMP high when measuring a temperature near 0 ° C., and it is possible to perform highly accurate measurement.

As is apparent from the above, the wiring resistance of the three-wire type RTD measuring circuit can be reduced with a simple circuit configuration similar to the conventional one and without using an arithmetic circuit for removing the influence of the wiring resistance. The effect can be eliminated. By adding the known resistance Rs, the amplification factor of the amplifier circuit AMP in a specific temperature measurement range can be increased, and high-precision measurement can be performed. Since the drive voltage for obtaining the same measurement accuracy is reduced, the power supply voltage can be reduced.

FIG. 2 is an example of an actual measurement circuit based on the basic circuit of FIG. 1, showing an example of one channel having an internal calibration circuit. At the contact of each switch in FIG.
s represents a measurement position, Cal represents a calibration position, Full represents a full span point calibration position, and Zero represents a zero point calibration position. Vos1, Vos2, Vo
s3 represents the offset voltage of each of the operational amplifiers OP1, OP2, OP3.

The switches at the time of measurement are all switched to the Mes side. The measured value in this case is Vout (Mes) = (Rt−Rs) · I−2 · Vos2−Vos3−Vos1 (7) based on the circuit operation similar to FIG.

The switches at the time of zero point calibration are Cal
Switch to the side and Zero side. The measured value in this case is Vout (Zero) = − 2 · Vos2-Vos3-Vos1 (8) based on the circuit operation similar to FIG.

The switch for full span point calibration is as follows:
Switch to Cal side and Full side respectively. The measured value in this case is Vout (Full) = Rf.I-2.Vos2-Vos3-Vos1 (9) based on the circuit operation similar to FIG.

As is clear from the above equations, the measurement data of equation (7) is corrected by the measurement data of zero point calibration of equation (8) and the measurement data of full span point calibration of equation (9). Thereby, the offset voltages of the operational amplifiers OP1, OP2, OP3 can be compensated, and highly accurate measurement can be performed. Specifically, the following operation is performed. M = [Vout (Mes) −Vout (Zero)] / [Vout (Full) −Vout (Zero)] = (Rt−Rs) · I / Rf · I = (Rt−Rs) / Rf (10)

FIG. 3 shows an embodiment in which the circuit of FIG. 2 is developed into a multi-channel switching configuration. In FIG. 3, one known resistor Rs is used.
Are connected so as to be used in common for each channel, and the channels are not insulated.

FIG. 4 also shows an embodiment in which the circuit of FIG. 2 is developed into a multi-channel switching configuration. In FIG. 4, the known resistors Rs are individually connected to the respective channels, and the channels are completely insulated. .

[0037]

As described above, according to the present invention,
In a three-wire RTD measuring circuit, the influence of wiring resistance can be removed with a simple circuit configuration, and the resistance value of the RTD at a desired temperature (for example, 0 ° C.) can be reduced using a known resistor Rs. By selecting the same value as the resistance value, the output voltage near a specific temperature (for example, 0 ° C.) can be set to 0 V, so that the amplification factor of the output voltage can be set high, and highly accurate measurement can be realized.

[Brief description of the drawings]

FIG. 1 is a basic circuit diagram illustrating an example of an embodiment of the present invention.

FIG. 2 is an example of an actual measurement circuit based on the basic circuit of FIG.

FIG. 3 is a diagram illustrating an example of a measurement circuit obtained by expanding the circuit of FIG. 2 into multiple channels.

FIG. 4 is a diagram illustrating another example of a measurement circuit in which the circuit of FIG. 2 is expanded to multiple channels.

FIG. 5 is a circuit diagram of a conventional circuit.

[Explanation of symbols]

 Rt RTD Rs Known resistance r1, r2 Wiring resistance OP1, OP2, OP3 Operational amplifier

Claims (10)

[Claims] 1. A resistance thermometer measurement circuit for measuring a resistance value of a resistance thermometer, wherein a known resistance connected in series with first and second wiring resistances connected to both ends of the resistance thermometer. A resistance, a measuring means for supplying a constant current to a series circuit of the resistance thermometer, the first and second wiring resistances, and the known resistance to measure a voltage drop, and a reference as a reference for measuring the voltage drop A reference voltage adjusting means for shifting a voltage by a voltage drop in the first and second wiring resistances. 2. A constant current source for supplying a constant current to a series circuit of the resistance temperature detector, first and second wiring resistors, and a known resistor, and a buffer circuit for detecting the voltage drop. 2. The resistance thermometer measurement circuit according to claim 1, comprising: 3. The reference voltage adjusting means includes: a voltage drop detecting means for detecting a voltage drop in the first or second wiring resistance; and the reference voltage adjusting means based on the voltage drop detected by the voltage drop detecting means. 2. The resistance temperature measuring circuit according to claim 1, further comprising a reference voltage setting means for setting a voltage. 4. The first operational amplifier having an inverting input terminal connected to a connection point between the temperature measuring resistor and the second wiring resistance, the second operational amplifier comprising:
A second operational amplifier which is a buffer circuit having a non-inverting input terminal connected to the other end of the wiring resistance and an output terminal of the first operational amplifier, and a non-inverting input terminal of the first operational amplifier. 4. The resistance thermometer measurement circuit according to claim 3, wherein an output terminal of said second operational amplifier is connected to said reference voltage setting means. 5. A reference voltage setting means comprising: a first resistor having one end grounded, the other end connected to a non-inverting input terminal of the first operational amplifier, and one end connected to the other end of the first resistor. 5. The resistance thermometer measuring circuit according to claim 4, further comprising: a second resistor connected to an output terminal of the second operational amplifier. 6. The resistance temperature measuring circuit according to claim 1, further comprising calibration means. 7. A first switch circuit for switching a reference current, a constant current supplied by the measuring means to the reference resistance or the resistance temperature detector and the wiring resistance, and the first wiring. A second switch circuit to which a voltage at one end of a resistor and both ends of the reference resistor is applied and whose output is connected to the measuring means, a voltage at a connection point between the temperature measuring resistor and the second wiring resistance, 7. The resistance thermometer measurement circuit according to claim 6, further comprising third and fourth switch circuits for applying a voltage at the other end of said second wiring resistance to said reference voltage adjusting means. 8. The resistance thermometer according to claim 6, wherein the calibrating means performs zero point calibration by setting the same voltage to be applied to the measuring means and the reference voltage adjusting means. Body measurement circuit. 9. A span point calibration by applying a voltage obtained by subtracting a voltage drop at the reference resistor from a voltage applied to the measuring means by the calibrating means to the reference voltage adjusting means. 7. The resistance thermometer measurement circuit according to 7. 10. The resistance temperature measuring circuit according to claim 1, wherein the resistance value of the known resistance is equal to the resistance value of the resistance temperature detector at a desired measurement temperature.