Multichannel thermal resistance, thermocouple multiplexing measurement and common-mode bias voltage circuit

Abstract

The utility model belongs to the technical field of industrial production process automatic measurement, and particularly discloses a multichannel thermal resistance and thermocouple multiplexing measurement and common mode bias voltage circuit, which mainly comprises multiplexers U1 and U2, a two-way switch U3 and constant current sources I1 and I2. The thermal resistor and thermocouple multiplexing measurement and bias voltage circuit is simple and reliable, is not limited to a measurement device for an 8-channel thermal resistor and thermocouple, is also suitable for a 16-channel measurement device, a 24-channel measurement device and the like, and enables differential signal measurement of an analog-to-digital converter (ADC) to work in an optimal common-mode bias voltage mode.

Description

Technical Field

[0001] This utility model relates to the field of automated measurement technology in industrial production processes, and in particular to multi-channel RTD, thermocouple multiplexing measurement and common-mode bias voltage circuit. Background Technology

[0002] In industrial process control, resistance temperature detectors (RTDs) and thermocouples are widely used for measuring temperature parameters in industrial production processes due to their advantages such as high measurement accuracy, stable performance, high reliability, and ease of processing. Because of the large number of RTDs and thermocouples used in industrial processes, single temperature monitoring devices often employ multi-channel signal measurement methods, such as 8-channel, 16-channel, or 24-channel configurations.

[0003] For resistance signal measurement, since a constant current flows through the resistor being measured, generating a voltage across the resistor, the signal measurement has strong resistance to interference in industrial environments. Multi-channel measurement devices generally use this constant current source measurement method. Thermocouple millivolt signals are generally directly introduced into an analog-to-digital converter (ADC) for conversion and measurement.

[0004] Considering both the accuracy of resistance temperature detector (RTD) measurements and the cost of laying signal cables in industrial settings, RTD measurements in industrial production often employ a three-wire connection method.

[0005] A typical three-wire dual constant current source resistance temperature measurement circuit, such as Figure 1 As shown, the circuit signal conversion process is as follows:

[0006] 1. The constant current source I1 flows in from terminal A, through the line resistor R LA Resistance temperature detector (RTD) R t Line resistance R LC It flows out from terminal C, then through resistor R0 back to the reference ground.

[0007] 2. Constant current source I2 passes through line resistor R LB R LC It flows out from terminal C, then through resistor R0 back to the reference ground.

[0008] 3. Voltage U A =(R LA +R t )*I1+(R LC +R0)*(I1+I2),U B =R LB *I2+(R LC +R0)*(I1+I2),

[0009] U AB =U A -U B =(R LA +Rt )*I1-R LB *I2;

[0010] Since the three signal connection lines in a three-wire system generally use three wires from the same multi-core cable, the line resistance is the same, i.e., R. LA =R LB And with two constant current sources I1 = I2, therefore U AB =R t *I1, i.e., thermal resistance R t The voltage at both ends.

[0011] 4. Resistance temperature detector (RTD) R t Voltage U at both ends AB After being amplified by the programmable gain amplifier (PGA) of the analog-to-digital converter U1, the voltage V AB =G*R t *I1 (G is the gain of PGA) is then used for A / D analog-to-digital conversion.

[0012] 5. The function of resistor R0 is to provide common-mode bias voltage to the PGA of analog-to-digital converter U1 by the voltage generated by the constant current source. The resistance value is generally 100Ω.

[0013] 6. In order to measure the thermal resistance R t In addition, a high-precision reference resistor R is required. G The measurement. Similarly, the measurement of R. G Voltage V across the terminals RG =G*R G *I1, from formula V AB =G*R t *I1 and V RG =G*R G *I1 can be used to obtain the resistance value R of the thermal resistor. t =R G *V AB / V RG .

[0014] like Figure 2 As shown, the measurement of thermocouple millivolt voltage signals is relatively simple, and the process is as follows:

[0015] 1. Figure 2 Medium thermocouple E t The millivolt differential voltage U at both ends AB After passing through an anti-aliasing RC filter circuit composed of resistors R1 and R2 and capacitors C1, C2, and C3, the voltage V is amplified by the programmable gain amplifier (PGA) of the analog-to-digital converter U1. AB =G*U AB (G is the gain of PGA) Then perform A / D analog-to-digital conversion.

[0016] 2. In order to measure the thermocouple millivolt voltage value, a high-precision reference millivolt voltage U is also required. G The measurement. Similarly, the voltage V is measured. G =G*U G From formula V AB =G*U AB and V G =G*U G The millivolt voltage U of the thermocouple can be obtained. t =U AB =U G *V AB / V G .

[0017] 3. A 2.5V reference voltage provides common-mode bias voltage to the PGA of analog-to-digital converter U1.

[0018] However, when Figure 1 and 2 When the circuit for measuring resistance temperature detectors (RTDs) and thermocouples shown is designed as a multi-channel RTD / thermocouple multiplexing circuit, a problem arises:

[0019] 1. Single-channel RTD t During continuous measurement, the common-mode bias voltage generated across resistor R0 is stable, and the signal conversion of analog-to-digital converter U1 is also stable. However, when the multi-channel RTD... t During measurement, the differences between each channel and the switching between channels will cause fluctuations in the common-mode bias voltage across resistor R0, resulting in fluctuations and jumps in the measured values ​​of the same signal in the same channel and differences in the measured values ​​of the same input signal in different channels.

[0020] 2. The common-mode bias voltage for RTD measurement is generated by resistor R0, while the common-mode bias voltage for thermocouple measurement is provided by a 2.5V reference voltage. When RTD and thermocouple are used together for measurement, the channel measuring RTD will force the 2.5V reference voltage to drop, resulting in a large change in the 2.5V bias voltage and an error in the AD signal conversion. Utility Model Content

[0021] The purpose of this invention is to solve the technical problems existing in the background art. To this end, a multi-channel RTD, thermocouple multiplexing measurement and common mode bias voltage circuit is provided.

[0022] To achieve the above objectives, the technical solution adopted by this utility model is as follows:

[0023] Multi-channel RTD, thermocouple multiplexing measurement and common-mode bias voltage circuit, including multiplexers U1 and U2, dual-channel switch U3, and constant current sources I1 and I2;

[0024] The output terminal of constant current source I1 is electrically connected to the common input terminal of multiplexer U1 through one switch of dual-channel switch U3. One or more output terminals of multiplexer U1 are electrically connected to resistor R. LA One end, resistor R LA The other end is electrically connected to a thermal resistor R. t One end, thermal resistance R t The other end is electrically connected to resistor R LC One end, resistor R LC The other end is electrically connected to one end of resistor R0, and the other end of resistor R0 is grounded;

[0025] The output of constant current source I2 is electrically connected to the common input terminal of multiplexer U2 via another switch of dual-channel switch U3. One or more output terminals of multiplexer U2 are electrically connected to resistor R. LB One end, resistor R LB The other end is electrically connected to the thermal resistor R. t and resistance R LC On the line between them.

[0026] The following is a further defined technical solution of this utility model: one or more output terminals of the multiplexer U1 are electrically connected to a thermocouple E. t One end of the multiplexer U2 is electrically connected to one or more output terminals via thermocouple E. t The other end;

[0027] The common input terminal of multiplexer U1 is electrically connected to one end of resistor R1, and the other end of resistor R1 is connected to the non-inverting input terminal of gain amplifier PGA. The common input terminal of multiplexer U2 is electrically connected to one end of resistor R2, and the other end of resistor R2 is connected to the inverting input terminal of gain amplifier PGA.

[0028] The following is a further defined technical solution of this utility model: the non-inverting input terminal of the gain amplifier PGA is electrically connected to one end of capacitor C1, and the other end of capacitor C1 is grounded; the inverting input terminal of the gain amplifier PGA is electrically connected to one end of capacitor C2, and the other end of capacitor C2 is grounded.

[0029] The following is a further technical solution of this utility model: a capacitor C3 is connected between the non-inverting input terminal and the inverting input terminal of the gain amplifier PGA.

[0030] The following is a further defined technical solution of this utility model: the inverting input terminal of the gain amplifier PGA is connected to a reference voltage source.

[0031] The following is a further defined technical solution of this utility model, which also includes a dual-channel switch U5;

[0032] The output of constant current source I1 is electrically connected to reference resistor R through one of the switches of dual-channel switch U5. G One end, reference resistor R G The other end is electrically connected to one end of resistor R0, and the other end of resistor R0 is grounded;

[0033] The output of constant current source I2 is electrically connected to the reference resistor R through the other switch of dual-channel switch U5. G On the line between resistor R0 and resistor R0.

[0034] Compared with the prior art, the present invention has the following technical effects:

[0035] The RTD and thermocouple multiplexing measurement and bias voltage circuit designed in this utility model is simple and reliable. The circuit is not limited to the measurement device of 8-channel RTD and thermocouple, but is also suitable for 16-channel, 24-channel and other measurement devices, and makes the analog-to-digital converter (ADC) output signal measurement work in the optimal common-mode bias voltage mode.

[0036] The present invention will be further described below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of this utility model or the prior art, the drawings used in the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a connection diagram of the existing resistance temperature detector (RTD) measurement circuit.

[0039] Figure 2 This is a diagram showing the connection relationships of thermocouple measurement circuits in existing technology;

[0040] Figure 3 This is a circuit connection diagram for multi-channel resistance temperature detectors (RTDs) and thermocouple multiplexing measurements according to this utility model.

[0041] Figure 4 This utility model relates to multi-channel resistance temperature detectors (RTDs), thermocouples, and reference resistors R. G Circuit connection diagram for multiplexing measurements. Detailed Implementation

[0042] To make the above-mentioned objects, features, and advantages of this utility model more apparent and understandable, the specific embodiments of this utility model will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a full understanding of this utility model. However, this utility model can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this utility model. Therefore, this utility model is not limited to the specific embodiments disclosed below.

[0043] like Figure 3 and 4 As shown, this embodiment provides a multi-channel RTD, thermocouple multiplexing measurement and common-mode bias voltage circuit, which mainly consists of 8-to-1 multiplexers U1 and U2, dual-channel switch U3, resistor R0 (100Ω), dual constant current sources I1 and I2, reference voltage 2.5V and analog-to-digital converter U4, etc.

[0044] Figure 3 The circuit works as follows:

[0045] 1. Measurement of resistance temperature detector (RTD) signals

[0046] (1) When the dual-way switch U3 is closed, the first switch of the 8-to-1 multiplexer U1 and U2 is selected;

[0047] (2) The constant current source I1 flows into the A1 terminal of the first switch of the 8-to-1 multiplexer U1 through one of the two-way switches U3, and then flows through the line resistor R. LA1 Resistance temperature detector (RTD) R t1 Line resistance R LC1 It flows out from terminal C, then through resistor R0 and back to RGND reference ground.

[0048] (3) The constant current source I2 flows into the B1 terminal of the first switch of the 8-to-1 multiplexer U2 through one of the two-way switches U3, and then through the line resistor R. LB1 R LC1 It flows out from terminal C, then through resistor R0 and back to RGND reference ground.

[0049] (4) Voltage U A1 =(R LA1 +R t1 )*I1+(R LC1 +R0)*(I1+I2),U B1 =R LB1 *I2+(R LC1 +R0)*(I1+I2),

[0050] U A1B1 =U A1 -U B1 =(R LA1 +Rt1 )*I1-R LB1 *I2;

[0051] Since the three signal connection lines in a three-wire system generally use three wires from the same multi-core cable, the line resistance is the same, i.e., R. LA1 =R LB1 And dual constant current sources I1 = I2. Therefore, the voltage U A1B1 =R t1 *I1, i.e., thermal resistance R t1 The voltage at both ends.

[0052] (5) Thermal resistance R t1 Voltage U at both ends A1B1 The voltage V is amplified by the first switch of the 8-to-1 multiplexers U1 and U2 and then introduced into the programmable gain amplifier (PGA) of the analog-to-digital converter U4. A1B1 =G*R t1 *I1 (G is the gain of the preamplifier) ​​is then processed for A / D conversion.

[0053] (6) The measurement process for other channels of resistance temperature detectors is the same as described above.

[0054] 2. Thermocouple signal measurement

[0055] (1) When the dual-way switch U3 is open, the 8th switch of the 8-to-1 multiplexer U1 and U2 is selected;

[0056] (2) Thermocouple E t8 The millivolt differential voltage U at both ends A8B8 After passing through an anti-aliasing RC filter circuit composed of resistors R1 and R2 and capacitors C1, C2, and C3, the signal is amplified by the programmable gain amplifier (PGA) of the analog-to-digital converter U4, with voltage V. A8B8 =G*U A8B8 (G is the gain of PGA) Then perform A / D analog-to-digital conversion.

[0057] (3) When other channels are connected to thermocouple signals, the measurement process is the same as described above. High-precision reference millivolt voltage U G The measurement process is the same as described above, but U G Measurements are typically performed during channel full-scale and zero-point signal self-calibration, i.e., V. GF =G*U GF V GZ =G*U GZ , generally full scale U GF 75mV can be taken, zero position U GZ Take 0mV.

[0058] Therefore, according to V A8B8 =G*U A8B8 VGF =G*U GF V GZ =G*U GZ The thermocouple millivolt signal value U is obtained. A8B8 =U GF *(V A8B8 -V GZ ) / (V GF -V GZ )=75*(V A8B8 -V GZ ) / (V GF -V GZ ).

[0059] 3. High-precision reference resistor R G Measurement, such as Figure 4 As shown, when the dual-channel switch U3 is closed, both the 8-to-1 multiplexers U1 and U2 are open, and the dual-channel switch U5 is closed.

[0060] The constant current source I1 is connected to one of the two-way switches U5, and then to the reference resistor R. G The current flows through resistor R0 back to RGND reference ground. The constant current source I2 flows through one of the two-way switches U5, through resistor R0 back to RGND reference ground.

[0061] U RG =R G *I1+R0*(I1+I2)-R0*(I1+I2)=R G *I1, voltage U RG The voltage is converted and processed by dual switches U5 and U3 and resistors R1 and R2, then amplified by analog-to-digital converter U4. The amplified voltage is V. RG =G*R G *I1 (G is the gain of PGA), therefore according to V RG =G*R G *I1, V A1B1 =G*R t1 *I1

[0062] Obtain the measured value R of the thermal resistance. t1 =R G *V A1B1 / V RG .

[0063] Figure 3 , Figure 4The key to stable and reliable multi-channel RTD and thermocouple multiplexing measurements in the circuit is that, for the analog-to-digital converter U4, the common-mode bias voltage of the programmable gain amplifier (PGA) is always a 2.5V reference voltage. Because the loop reference ground RGND through which the constant current sources I1 and I2 flow is isolated from the ground of the analog-to-digital converter U4 and the 2.5V reference voltage reference ground AGND, the voltage generated on resistor R0 during RTD signal measurement will not affect the 2.5V reference voltage. Furthermore, for an analog-to-digital converter operating at 5V, the optimal operating point for the common-mode bias voltage of the differential input signal of the programmable gain amplifier (PGA) is a stable 2.5V reference voltage.

[0064] The above description is merely a preferred embodiment of this utility model and is not intended to limit the utility model in any way. Any person skilled in the art can make many possible variations and modifications to the technical solution of this utility model using the disclosed methods and techniques, or modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of this utility model. Therefore, all equivalent changes made based on the shape, structure, and principle of this utility model without departing from its technical solution should be covered within the protection scope of this utility model.

Claims

1. A multi-channel RTD / thermocouple multiplexing measurement and common-mode bias voltage circuit, characterized in that, Includes multiplexers U1 and U2, dual-channel switch U3, and constant current sources I1 and I2; The output terminal of constant current source I1 is electrically connected to the common input terminal of multiplexer U1 through one switch of dual-channel switch U3. One or more output terminals of multiplexer U1 are electrically connected to resistor R. LA One end, resistor R LA The other end is electrically connected to a thermal resistor R. t One end, thermal resistance R t The other end is electrically connected to resistor R LC One end, resistor R LC The other end is electrically connected to one end of resistor R0, and the other end of resistor R0 is grounded; The output of constant current source I2 is electrically connected to the common input terminal of multiplexer U2 via another switch of dual-channel switch U3. One or more output terminals of multiplexer U2 are electrically connected to resistor R. LB One end, resistor R LB The other end is electrically connected to the thermal resistor R. t and resistance R LC On the line between them.

2. The multi-channel RTD / thermocouple multiplexing measurement and common-mode bias voltage circuit as described in claim 1, characterized in that, One or more output terminals of the multiplexer U1 are electrically connected to thermocouple E. t One end of the multiplexer U2 is electrically connected to one or more output terminals via thermocouple E. t The other end; The common input terminal of multiplexer U1 is electrically connected to one end of resistor R1, and the other end of resistor R1 is connected to the non-inverting input terminal of gain amplifier PGA. The common input terminal of multiplexer U2 is electrically connected to one end of resistor R2, and the other end of resistor R2 is connected to the inverting input terminal of gain amplifier PGA.

3. The multi-channel RTD / thermocouple multiplexing measurement and common-mode bias voltage circuit as described in claim 2, characterized in that, The non-inverting input of the gain amplifier PGA is electrically connected to one end of capacitor C1, and the other end of capacitor C1 is grounded. The inverting input of the gain amplifier PGA is electrically connected to one end of capacitor C2, and the other end of capacitor C2 is grounded.

4. The multi-channel RTD / thermocouple multiplexing measurement and common-mode bias voltage circuit as described in claim 3, characterized in that, A capacitor C3 is connected between the non-inverting and inverting input terminals of the gain amplifier PGA.

5. The multi-channel RTD / thermocouple multiplexing measurement and common-mode bias voltage circuit as described in claim 2, characterized in that, The inverting input of the gain amplifier PGA is connected to a reference voltage source.

6. The multi-channel RTD / thermocouple multiplexing measurement and common-mode bias voltage circuit as described in claim 1, characterized in that, It also includes the dual-channel switch U5; The output of constant current source I1 is electrically connected to reference resistor R through one of the switches of dual-channel switch U5. G One end, reference resistor R G The other end is electrically connected to one end of resistor R0, and the other end of resistor R0 is grounded; The output of constant current source I2 is electrically connected to the reference resistor R through the other switch of dual-channel switch U5. G On the line between resistor R0 and resistor R0.

Images