JP5523370B2 - 4-wire RTD input circuit - Google Patents

Description

  The present invention relates to disconnection detection of a 4-wire resistance thermometer input circuit using a 4-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 schematic 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. The RTD 3 has both ends connected to the amplifying and 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 and adding circuit 5, and the A / D converter ( (Not shown) for conversion. The RTD 3 is connected to the offset resistor 4 via the external terminal 24 and grounded. However, such a conventional 4-wire resistance thermometer input circuit (for example, Patent Document 1) has a problem that it cannot detect disconnection.

  On the other hand, for example, Patent Document 2 discloses a three-wire temperature measurement circuit capable of detecting disconnection. FIG. 2 is a schematic diagram showing the configuration of this three-wire temperature measurement circuit. In this circuit, two constant current sources 61 and 62 are connected to a Pt100 sensor (RTD) 3 which is a resistance for temperature measurement via external terminals 22 and 23, respectively. Although the disconnection can be detected simultaneously with canceling the wiring resistance, there is a problem that it is difficult to perform high-precision measurement because the resistance difference appearing in the wiring resistance cannot be canceled because the value of the measurement current is large.

  Therefore, in the general four-wire RTD input circuit as shown in FIG. 1, a constant current source for detecting disconnection is separately provided, and wiring of the Pt100 sensor (RTD) 3 (wiring of the external terminals 22 and 23). A configuration for connecting to the terminal is conceivable. FIG. 3 is a schematic diagram showing the configuration of such a 4-wire RTD input circuit with a disconnection detection function. This circuit includes constant current sources 71 and 72 for detecting disconnection separately from the constant current source 1 for temperature measurement.

JP 2002-257877 A JP-A-9-105681

  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 In addition to the problem that various errors occur and the temperature cannot be measured in a highly accurate and stable state as originally expected, there is a problem that a disconnection cannot be detected. When the disconnection detection function is added, unlike the case of the 3-wire RTD input circuit, the disconnection detection constant current is combined with the temperature measurement constant current, resulting in an error in temperature measurement. There was a problem that it would end up.

  The present invention has been made to solve the above-described problems, and can measure temperature with high accuracy while having a simple structure, and also has a disconnection detection function. An object of the present invention is to provide a highly accurate 4-wire resistance thermometer input circuit that is free from errors due to secular change and free from errors due to disconnection detection.

  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 temperature measuring constant current sources for supplying a constant current to the reference constant resistance, and the connection of the at least two temperature measuring constant current sources to the temperature measuring resistor side or the reference And a cross switch for switching to the constant resistance side of the four-wire temperature measuring resistor input circuit, further comprising at least three constant current sources for detecting disconnection.

  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 for temperature measurement are provided, two constant resistors for reference are connected in series, a switch for switching the constant current for temperature measurement is provided, and three further constant current sources are provided. Although it has a simple structure with a constant current source for disconnection detection, it takes into account the surrounding environment, changes over time, and errors in the constant current for disconnection detection. Sensor measurement value) can be measured, and disconnection detection can also be performed.

It is the schematic which shows the structure of the conventional general 4-wire type resistance temperature detector input circuit. It is the schematic which shows the structure of the conventional 3 wire type resistance temperature detector input circuit with a disconnection detection function. It is the schematic which shows the structure of the conventional 4-wire type resistance temperature detector input circuit with a disconnection detection function. It is the schematic which shows the structure of the 4-wire type resistance temperature detector input circuit with a disconnection detection function 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. It is a correspondence table which shows a disconnection location and a disconnection judgment condition.

Embodiment 1 FIG.
FIG. 4 is a schematic diagram showing the configuration of a four-wire RTD input circuit with a disconnection detection function in the present invention. In this input circuit, two reference constant resistors 81 and 82 (R1, R2) for circuit error correction are connected in series, two constant current sources 11 and 12 for temperature measurement, and a constant current source. The constant current 1 from the current source 11 is supplied to the Pt100 sensor 3 which is a temperature measurement resistor connected to the outside, or two constant resistors 81 and 82 (R1, R2) for reference connected in series inside. And a cross switch 9 that can switch the flow to the side. 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 reference constant resistors 81 and 82 is channel 1 (CH1), and the constant for reference is set. A multiplexer 10 is provided for reading the voltage appearing at both ends of the low-side reference resistor R1 by the resistor 81 through the channel 2 (CH2). Are input to an A / D converter (not shown) to which the device is connected. Note that both ends of the Pt100 sensor 3 are connected to the multiplexer 10 via external terminals 22 and 23.

  Furthermore, the constant current sources 71, 72, 73 for detecting disconnection are connected to a negative voltage input line (wiring for the external terminal 23), a positive voltage input line (wiring for the external terminal 22), and a constant resistance 82 for reference. Connect to the wiring that supplies a constant current for temperature measurement. The disconnection detection constant currents i1 and i2 flowing from the constant current sources 71 and 72 cancel the measurement error due to the wiring resistance and increase the voltage when the disconnection occurs. The current i3 is for correcting the amount of error caused by the constant current i2 flowing through the Pt100 sensor 3. In addition, these disconnection detection constant currents i1 to i3 are very small currents (constant currents) compared to the temperature measurement constant currents 1 and 2 in order to suppress the influence of variations in wiring resistance (wire accuracy). It is assumed that i1 = i2 = i3).

  Next, processing at the time of shipping inspection in a factory will be described with reference to the flowchart of FIG. FIG. 5 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 amplifying and adding circuit 5, and A / D converted 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). In addition, a and b calculated | required here become a value after considering the error part of the constant currents i1-i3 for disconnection detection, respectively.

  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 previously, 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. 6 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 the flowchart which shows the process for calculating | requiring and measuring temperature from the resistance value of Pt100 sensor 3 which is calculated | required and resistance for temperature measurement, and the process for disconnection detection.

  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 measured by reading CH0 of the multiplexer 10 (step ST49).

  Here, the disconnection of the 4-wire RTD input circuit is detected based on the voltage value read in step ST49 (step ST50). FIG. 7 is a correspondence table showing the disconnection location and the disconnection determination condition based on the measured voltage value when each line of the circuit is disconnected. Here, for convenience of explanation, the current supply line (wiring of the external terminal 21) in FIG. 4 is A line, the positive voltage input line (wiring of the external terminal 22) is B line, and the negative voltage input line (wiring of the external terminal 24) in FIG. Are referred to as C line and ground line (wiring of external terminal 24) as D line, the input range is between 100 to 150 ohms, and constant currents 1 and 2 are 1 mA. At this time, the measured value of the voltage appearing at both ends of the Pt100 sensor 3 which is a temperature measurement resistor is 100 to 150 mV in a normal state.

  When the A line is disconnected, 0 mV is detected, so that the underrange (below 100 mV) is reached. Further, when the B line is disconnected, the value is considerably higher than 150 mV, and a plus overrange (above 150 mV) is obtained. On the contrary, when the C line is disconnected, the value is considerably lower than 0 mV, which is an underrange (below 100 mV). Further, when the D line is disconnected, 0 mV is detected, so that the underrange (below 100 mV) is obtained. In addition, various combinations are conceivable. For example, when the B line and the C line are disconnected at the same time, 0 mV is detected, which results in an underrange (below 100 mV).

  That is, if the measured voltage value read in step ST49 is underrange (a value smaller than 100 mV) or plus overrange (a value larger than 150 mV), one of the wires may be disconnected. It can be detected.

  On the other hand, in order to calculate the resistance value, the voltage value read in step ST49 is amplified and A / D converted (step ST51). Then, the resistance value of the Pt100 sensor 3 is calculated by inputting the linear expression Y = aX + b using a and b obtained in step ST47 (step ST52).

  In addition, since the slope a and the offset b of the primary equation: Y = aX + b are values that change according to changes in the ambient temperature and the errors of the disconnection detection constant currents i1 to i3, highly accurate temperature measurement In order to eliminate the temperature measurement error in the four-wire RTD input circuit that requires the above, the processes in steps ST44 to ST52 described above are performed each time the temperature is measured. However, the number of times a and b are calculated may be determined 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. 4 will be described. In the above-described processing at the time of shipping inspection at the factory (see FIG. 5) and normal processing at the site (see FIG. 6), the constant current source 11 is the only constant current source for temperature measurement. If only 1 is connected to the Pt100 sensor 3 side which is a temperature measuring resistor (externally) or connected to the constant resistances 81 and 82 side for reference (internally) Every time, it becomes necessary to wait until a steady 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 for temperature measurement are used as in the present invention, the matching which is the difference between them naturally affects the accuracy, but a general non-accurate component is required. Even if it is used as a constant current source, 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. Thus, by using the two constant current sources 11 and 12 for temperature measurement, since warming up is possible, the measurement can be started immediately, and stable and highly accurate temperature measurement can be performed. It is.

  As described above, according to the present invention, while maintaining a stable current supply state by the two constant current sources for temperature measurement and the switch, the linear equation: Y = aX + b slope a and offset b are obtained every time. Therefore, it is possible to perform temperature measurement using a and b taking into account the influence of ambient temperature at the time of measurement, aging of parts, and errors of disconnection detection constant currents i1 to i3. Two constant current sources for temperature measurement are provided, two constant resistors for reference are connected in series, a switch for switching the constant current is provided, and three constant current sources for detecting disconnection are further provided. It is possible to measure highly accurate resistance values (temperature sensor measurement values) with few errors in consideration of the surrounding environment, secular change, and errors in constant current for disconnection detection. Able to detect disconnection It can be also carried out.

  In the embodiment of the present invention, two constant resistances for reference are connected in series and two constant current sources for temperature measurement are used. However, when the constant resistance for reference is connected in parallel, The same effect can be obtained by using three constant current sources for temperature measurement. However, in that case, four constant current sources for detecting disconnection are required. 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. As described above, it is optimal to use two constant resistors for reference, two constant current sources for temperature measurement, and three constant current sources for disconnection detection.

  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, 61, 62 Constant current source (constant current source for temperature measurement)
21, 22, 23, 24 External terminal 3 Pt100 sensor for measurement 4 Resistor for offset 5 Amplifying and adding circuit 71, 72, 73 Constant current source for disconnection detection 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 temperature measuring constant current sources for supplying a constant current to the resistance temperature detector and the reference constant resistance;
A cross switch for switching connection of the at least two constant current sources for temperature measurement to the resistance temperature detector side or the reference constant resistance side;
A four-wire RTD input circuit comprising:
A four-wire resistance thermometer input circuit further comprising at least three constant current sources for detecting disconnection.   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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