JP2012168104A - Four-wire temperature measuring resistor input circuit - Google Patents

Abstract

Provided is a high-precision four-wire resistance thermometer input circuit that can measure temperature with high accuracy while having a simple structure and that has little error due to ambient environment and aging.
SOLUTION: While having a simple structure including two constant current sources, two constant resistors for reference are connected in series, and a switch for switching the constant current is provided, the ambient environment, secular change, etc. Therefore, it is possible to measure a highly accurate resistance value (measurement value of the temperature sensor) with few errors.
[Selection] Figure 2

Description

  The present invention relates to a four-wire resistance thermometer input circuit using a four-wire resistance thermometer that can measure temperature with high accuracy.

  For example, when it is necessary to measure temperature in a factory, plant, etc., a temperature measuring circuit for measuring temperature is usually obtained by obtaining an amplifier output voltage corresponding to temperature using a platinum resistance thermometer or thermistor as a temperature sensor. used.

  At this time, a resistance temperature detector is usually used as a temperature sensor in a field where a temperature needs to be measured with high accuracy and high speed, such as a chemical plant. The resistance temperature detector shows a resistance value corresponding to the ambient temperature. Therefore, the resistance value is converted into a voltage by supplying a constant current from a constant current source to the resistance temperature detector, and this value is input to the arithmetic circuit. The arithmetic circuit amplifies the voltage by the resistance temperature detector and outputs it. For this reason, the output voltage of the arithmetic circuit becomes a value corresponding to the temperature around the resistance temperature detector.

  There are two types of resistance temperature detectors, such as 2-wire, 3-wire, and 4-wire types. However, in order to eliminate errors due to temperature sensor lead wire resistance and constant current fluctuations, it is particularly highly accurate. In the field where measurement is required, a 4-wire RTD input circuit is used.

  FIG. 1 is a diagram showing a configuration of a general four-wire RTD input circuit. Here, the constant current source 1 is connected to a Pt100 sensor (RTD) 3 that is a resistance for temperature measurement via an external terminal 21, and supplies a constant current from the constant current source 1 to the RTD 3. Further, both ends of the RTD 3 are connected to the amplifying / adding circuit 5 via the external terminals 22 and 23. The voltage corresponding to the resistance change of the RTD 3 is read and amplified by the amplifying / adding circuit 5, and the A / D converter 6 To convert and output. The RTD 3 is connected to the offset resistor 4 via the external terminal 24 and grounded.

JP 2002-257877 A

  However, in a conventional 4-wire RTD input circuit, resistance error due to temperature characteristics, diode error, circuit offset, reference voltage error, etc., due to ambient temperature changes and component aging Various errors occur, and there is a problem that the temperature cannot be measured in a highly accurate and stable state as originally expected.

  The present invention has been made in order to solve the above-described problems, and can measure temperature with high accuracy while having a simple structure. Moreover, the present invention is highly accurate with no error due to the surrounding environment or aging. An object of the present invention is to provide a four-wire RTD input circuit.

  To achieve the above object, the present invention provides a four-wire resistance thermometer input circuit connected to a temperature measuring resistor for temperature measurement, and includes at least two constant resistors for reference, and the temperature measuring resistor. And at least two constant current sources for supplying a constant current to the reference constant resistance, and a cross switch for switching the connection of the at least two constant current sources to the temperature measuring resistor side or the reference constant resistance side It is characterized by comprising.

  Further, the present invention is characterized in that the at least two reference constant resistors are connected in series.

  According to the present invention, two constant current sources are provided, two constant resistors for reference are connected in series, and a switch for switching the constant current is provided. Considering changes and the like, it is possible to measure a highly accurate resistance value (temperature sensor measurement value) with few errors.

It is a block diagram of the conventional general 4-wire type resistance temperature detector input circuit. It is a block diagram of the 4-wire type resistance temperature detector input circuit in this invention. It is a flowchart which shows the process at the time of the shipping inspection in a factory. It is a flowchart which shows the normal process on the spot.

Embodiment 1 FIG.
FIG. 2 is a block diagram of a 4-wire resistance thermometer input circuit according to the present invention. In this input circuit, two reference constant resistors 81 and 82 (R1, R2) for circuit error correction are connected in series, and the two constant current sources 11, 12 and the constant current source 11 are connected to each other. Is supplied to the Pt100 sensor 3 which is a temperature measuring resistor connected to the outside, or to the two constant resistors 81 and 82 (R1, R2) for reference connected in series inside The cross switch 9 can be switched. When the cross switch 9 is set so that the constant current 1 flows to the Pt100 sensor 3 side (that is, externally), the constant current 1 flows to the Pt100 sensor 3 via the external terminal 21 and is offset via the external terminal 24. It flows to the ground through the resistor 4 for use. The constant current 2 flows through the two constant resistors 81 and 82 (R1, R2) for reference, and flows to the ground via the offset resistor 4.

  Further, the voltage appearing at both ends of the Pt100 sensor 3 is channel 0 (CH0), the voltage appearing at both ends of the high-side reference resistance R1 + R2 by the two constant resistances 81 and 82 for reference is the channel 1 (CH1), and the constant for reference is used. A multiplexer 10 that reads the voltage appearing at both ends of the low-side reference resistor R1 by the resistor 81 is read by the channel 2 (CH2), and the voltage appearing at both ends of each resistor is taken in by the amplifying and adding circuit 5, and the ADC reference voltage 7 for reference is obtained. Input to the connected A / D converter 6 for conversion. Note that both ends of the Pt100 sensor 3 are connected to the multiplexer 10 via external terminals 22 and 23.

  Next, processing at the time of shipping inspection in a factory will be described with reference to the flowchart of FIG. FIG. 3 is a flowchart showing a process for obtaining the resistance values R1 and R2 of the reference constant resistances 81 and 82 at a constant temperature at the time of shipping inspection in a factory.

  First, the cross switch 9 is set on the side where the constant current source 11 is connected to the external terminal 21 (the constant current 1 is set to the outside) (step ST31). At this time, an inspection resistor Ra having a known resistance value is connected to a location to which the Pt100 sensor 3 that is a temperature measurement resistor is normally connected (step ST32). Then, the voltage appearing at both ends of the inspection resistor Ra is switched to CH0 of the multiplexer, amplified by the amplification / addition circuit 5, and A / D converted by the A / D converter 6 and read (step ST33). Note that this voltage reading operation is performed after a sufficient thermal equilibrium is achieved. Thereafter, instead of the inspection resistor Ra, a known inspection resistor Rb having a resistance value different from Ra is connected (step ST34), and the same processing as described above is performed (step ST35).

  As a result, a linear expression of circuit correction: Y = aX + b (Y is a value of the resistor R, X is a value after A / D conversion of a voltage appearing at both ends of the resistor R, a is a slope, and b is an offset) is inspected. Since two equations are obtained when the use resistor Ra is used and when the inspection resistor Rb is used, the unknown numbers a and b can be obtained (step ST36).

  Next, in order to measure the resistance values R1 and R2 of the two constant resistances 81 and 82 for reference, the cross switch 9 is connected to the side where the constant current source 11 is connected to the constant resistance 82 for reference (constant current). 1 is switched to the inside (step ST37). At this time, since the coefficients a and b of Y = aX + b can use the values obtained earlier, the voltage appearing at both ends of the low-side reference resistor R1 is switched to CH2 of the multiplexer 10 to be amplified and A / D converted ( Step ST38). And the value of R1 can be calculated | required from primary expression: Y = aX + b (step ST39). Similarly, the voltage appearing at both ends of the high-side reference resistor R1 + R2 is switched to CH1 of the multiplexer 10 and amplified and A / D converted (step ST40), so that the value of R1 + R2 can be obtained. The value R2 of the constant resistance 82 can also be calculated (step ST41).

  These constant resistances 81 and 82 (R1, R2) for reference are high-precision precision resistors that do not change in value due to changes in ambient temperature (changes are sufficiently small). Therefore, the values of R1 and R2 measured at this time are recorded in the nonvolatile memory (step ST42). Note that the coefficients a and b of the above-mentioned primary expression: Y = aX + b are values that change depending on the ambient temperature and the like, and therefore need to be measured each time at the site. Do not use when measuring temperature in

  Next, normal processing in the field will be described with reference to the flowchart of FIG. FIG. 4 shows the assumption that the resistance values R1 and R2 of the reference constant resistors 81 and 82 are accurate, and the linear expression Y = aX + b slope a and offset b are calculated on site. It is a flowchart which shows the process for calculating | requiring and measuring temperature from the resistance value of Pt100 sensor 3 which is calculated | required and is a resistance for temperature measurement.

  First, the resistance values R1 and R2 of the reference constant resistors 81 and 82 recorded in the nonvolatile memory in the above-described step ST42 are read (step ST43). Then, the cross switch 9 is switched (the constant current 1 is inside) to the side where the constant current source 11 is connected to the reference constant resistor 82 (step ST44). In this state, the voltages appearing at both ends of the low-side reference resistor R1 and the high-side reference resistor R1 + R2 are read and amplified, respectively, and A / D converted (steps ST45 and 46). Then, using these values and R1 and R2, the unknowns a and b of the primary expression: Y = aX + b are obtained (step ST47). Since there are two unknowns to be obtained here, it is impossible to obtain the two unknowns a and b with only one constant resistance for reference. Therefore, two or more constant resistances for reference are necessary.

  Thereafter, in order to measure the temperature from the resistance value of the Pt100 sensor 3 which is a temperature measurement resistor, the cross switch 9 is switched to the side where the constant current source 11 is connected to the external terminal 21 (the constant current 1 is set to the outside). Step ST48). Thereafter, the voltage appearing at both ends of the Pt100 sensor 3 is read and amplified, and converted by the A / D converter (step ST49). Then, the resistance value of the Pt100 sensor 3 is calculated by inputting the linear expression using a and b obtained in step ST47: Y = aX + b (step ST50).

  The linear expression Y = aX + b slope a and offset b are values that change according to changes in ambient temperature and the like, and therefore, a 4-wire RTD input circuit that requires highly accurate temperature measurement. In order to eliminate the temperature measurement error in step ST44 to ST50, the processes in steps ST44 to ST50 described above are performed each time the temperature is measured. However, the number of times of calculation of a and b may be performed every predetermined period, or may be thinned out to some extent in consideration of the environment used.

  Here, functions of the constant current source 12 and the constant current 2 in FIG. 2 will be described. In the above-described processing at the time of shipping inspection at the factory (see FIG. 3) and normal processing at the site (see FIG. 4), if the constant current source is only the constant current source 11, only the constant current 1 is the temperature. If it is used by switching whether to connect to the Pt100 sensor 3 side (external) or to the constant resistances 81 and 82 for reference (internally), which is the measurement resistance, each time it switches It becomes necessary to wait until the state is reached. In other words, the constant current 2 from the constant current source 12 is always supplied to the side to which the constant current 1 is not connected, thereby shortening the warm-up time and supplying a stable current. It is intended.

More specifically, although self-heating is Q = I ² R, for example, there is one constant current source (only the constant current source 11), and the heat dissipation constant of the Pt100 sensor 3 is 2 mW / ° C. When the measurement current is 0.001 A and 100 ohms, the temperature that rises due to self-heating when converted to temperature is 50 mK. In the case of high-precision temperature measurement with an accuracy of several tens of mK, the detected temperature in such a state where the thermal equilibrium is unstable is an error itself and cannot be used.

  On the other hand, when two constant current sources 11 and 12 are used as in the present invention, the matching, which is the difference between the two, naturally affects the accuracy, but general non-accurate parts are used as the constant current source. Even if it is used, the matching error between the constant current sources 11 and 12 is about 3%, which is not a problem. When the heat dissipation constant of the Pt100 sensor 3 is 2 mW / ° C and the measurement current is 1 mA, the temperature error amount is 0.045 mK. This value is very small and can be ignored. By using the two constant current sources 11 and 12 in this way, since the warming up is possible, the measurement can be started immediately, and the temperature measurement can be stably and highly accurately performed.

  As described above, according to the present invention, the linear equation Y: aX + b slope a and offset b are obtained each time while maintaining a stable current supply state by two constant current sources and switches. Since temperature measurement can be performed using a and b taking into account the influence of ambient temperature at the time of measurement and aging of parts, etc., two constant current sources are provided and two constant resistors for reference are provided. Measures highly accurate resistance values (temperature sensor measurement values) with few errors in consideration of the surrounding environment and changes over time, while having a simple structure in which a switch for switching a constant current is provided while connecting in series. can do.

  In the embodiment of the present invention, two reference constant resistors are connected in series and two constant current sources are used. However, when the reference constant resistors are connected in parallel, a constant current source is used. By setting the number to 3, the same effect can be obtained. Further, the number of reference constant resistors may be any number as long as it is two or more. However, since the number of constant current sources needs to be increased accordingly, from the viewpoint of the number of parts and cost, the embodiment of the present invention. Thus, it is optimal to use two constant resistors for reference and two constant current sources.

  Further, in the present invention, any constituent element of the embodiment can be modified or any constituent element of the embodiment can be omitted within the scope of the invention.

1, 11, 12 Constant current source 21, 22, 23, 24 External terminal 3 Pt100 sensor for measurement 4 Resistance for offset 5 Amplification addition circuit 6 A / D converter 7 ADC reference voltage 81, 82 Constant resistance for reference 9 Cross switch 10 Multiplexer

Claims (2)

In the 4-wire RTD input circuit connected to the RTD for temperature measurement,
At least two constant resistors for reference;
At least two constant current sources for supplying a constant current to the resistance temperature detector and the reference constant resistance;
A cross switch for switching the connection of the at least two constant current sources to the resistance temperature detector side or the reference constant resistance side;
A four-wire resistance thermometer input circuit comprising:   The four-wire RTD input circuit according to claim 1, wherein the at least two reference constant resistors are connected in series.

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