A high-precision wide-range thermocouple resistance measurement circuit

Abstract

The present invention discloses a high-precision wide-range thermocouple resistance measurement circuit, which includes a chopping phase-changing analog switch U1, a low-voltage reference source Vref, a current-limiting resistor R1, a four-select-one analog switch U2, a high-precision resistor R2, a high-precision resistor R3, a high-precision resistor R4, a high-precision resistor R5 and a controller. The circuit device of the present invention measures the resistance values ​​of positive and negative phase thermocouples, and uses a controller to average and subtract the average value from the self-calibration deviation value to obtain a high-precision thermocouple resistance value.

Description

Technical Field

[0001] The invention relates to a precision resistance measurement technology, in particular to a high-precision wide-range thermocouple resistance measurement circuit. Background Art

[0002] Thermocouple is a commonly used temperature sensor. The thermoelectric potential it generates changes with temperature. The temperature of the measured medium can be obtained by measuring the output potential through an electrical instrument (secondary instrument). When the measuring end of the thermocouple is degenerated, the accuracy decreases, and its thermocouple resistance value changes. Measuring the resistance value of the thermocouple can preliminarily determine whether the thermocouple accuracy has decreased. The decrease in thermocouple accuracy is a slow development process, but as the thermocouple accuracy continues to decrease, its resistance value will gradually increase.

[0003] The resistance value of thermocouple can be measured in phase or in reverse phase. In phase measurement is to use high-precision low resistance meter for direct measurement, which is easily interfered by thermoelectric potential. In actual engineering, the measurement deviation of thermocouple resistance at 500 degrees Celsius can reach 40% when measured in phase. In addition, the long wire resistance may exceed the range of low resistance meter, so this method cannot meet the needs of engineering application. Therefore, the resistance value of thermocouple can be measured accurately by reverse phase measurement.

[0004] At present, the field thermocouple resistance value measurement mostly adopts multimeter to perform manual phase change measurement and record the value. Because the thermocouple is always in an unstable temperature state, manual measurement is very easy to introduce measurement errors, and the operation is very inconvenient, which is not conducive to judging the accuracy of the thermocouple. Summary of the invention

[0005] The present invention provides a high-precision wide-range thermocouple resistance measurement circuit, which can realize chopping self-calibration, calibrate the line resistance in a non-thermocouple circuit, measure the thermocouple resistance value by positive and negative phase switching and perform average processing, and subtract the self-calibration line resistance to improve the measurement accuracy and solve the problem of excessive system error caused by manual measurement.

[0006] The present invention provides the following technical solution: a high-precision wide-range thermocouple resistance measurement circuit, the circuit comprising a chopping phase-changing analog switch U1, a low-voltage reference source Vref, a current-limiting resistor R1, a four-select-one analog switch U2, a high-precision resistor R2, a high-precision resistor R3, a high-precision resistor R4, a high-precision resistor R5 and a controller, wherein:

[0007] The chopper commutation analog switch U1 has a K1 control terminal and a K2 control terminal. The K1 control terminal is enabled. The K1 control terminal measures the current circuit loop resistance Rf. According to the formula Rf=R2*(V1f-V2f) / V2f, a self-calibration deviation value is obtained and stored in the controller. The K2 control terminal is used to control the commutation switch K2-0 and the commutation switch K2-1 in the chopper commutation analog switch U1.

[0008] The thermocouple equivalent resistance Rx is the thermocouple resistance to be measured. The K1 control end is disabled, and the positive input end of the thermocouple equivalent resistance Rx is connected to the current limiting resistance R1 through the phase-changing switch K2-0 to obtain a voltage V1f. The negative input end of the thermocouple equivalent resistance Rx is connected to the input end IN of the four-to-one analog switch U2 through the phase-changing switch K2-1 to generate a voltage V2f. According to the formula Rx 正相 =R2*(V1f-V2f) / V2f to get the positive phase measurement Rx 正相 The resistance value is stored in the controller;

[0009] The K1 control terminal is disabled, the negative input terminal of the thermocouple equivalent resistor Rx is connected to the current limiting resistor R1 through the phase-changing switch K2-1, generating a voltage V3f, and the positive input terminal of the thermocouple equivalent resistor Rx is connected to the input terminal IN of the four-to-one analog switch U2 through the phase-changing switch K2-0, generating a voltage V4f. According to the formula Rx 反相 =R2*(V3f-V4f) / V4f to get the reverse phase measurement Rx 反相 The resistance value is stored in the controller;

[0010] The value of the thermocouple resistance Rx is calculated by the controller according to the formula Rx=(Rx 正相 +Rx 反相 ) / 2-Rf to get the accurate thermocouple resistance value.

[0011] Further, the four-to-one analog switch U2 has a control terminal K3 and a control terminal K4, the resistance values ​​of the high-precision resistors R2, R3, R4 and R5 increase in sequence, and the high-precision resistors R2, R3, R4 and R5 are correspondingly connected to the OUT1, OUT2, OUT3 and OUT4 pins in the four-to-one analog switch U2;

[0012] Connect the input terminal IN of the four-to-one analog switch U2 to OUT1 to obtain voltage V1 and voltage V2. According to the formula Rx 正相 =R2*(V1-V2) / V2 to calculate Rx 正相 Resistance value;

[0013] When the initial measurement (V1-V2) / V2≥2, the controller controls the control terminal K3 and the control terminal K4, and sequentially connects the input terminal IN of the four-choice analog switch U2 to OUT2 or OUT3 or OUT4, and measures the voltage V1 and the voltage V2 until (V1-V2) / V2<2. The controller records the positive phase thermocouple resistance value Rx at this time. 正相 ; If the initial measurement (V1-V2) / V2≤0.1, the controller controls the control terminal K3 and the control terminal K4, and connects the input terminal IN of the four-choice analog switch U2 to OUT3 or OUT2 or OUT1 in turn, and measures the voltage V1 and the voltage V2 until (V1-V2) / V2>0.1. The controller records the positive phase thermocouple resistance value Rx at this time 正相 .

[0014] Furthermore, the input terminal IN of the four-to-one analog switch U2 is connected to OUT1 to obtain voltages V3 and V4. According to the formula Rx 反相 = R2*(V3-V4) / V4 to calculate Rx 反相 Resistance value;

[0015] When the initial measurement (V3-V4) / V4≥2, the controller controls the control terminal K3 and the control terminal K4, and sequentially connects the input terminal IN of the four-choice analog switch U2 to OUT2 or OUT3 or OUT4, and measures the voltage V3 and the voltage V4 until (V3-V4) / V4<2. The controller records the positive phase thermocouple resistance value Rx at this time. 反相 ; If the initial measurement (V3-V4) / V4≤0.1, the controller controls the control terminal K3 and the control terminal K4, and connects the input terminal IN of the four-choice analog switch U2 to OUT3 or OUT2 or OUT1 in turn, and measures the voltage V3 and the voltage V4 until (V3-V4) / V4>0.1. The controller records the positive phase thermocouple resistance value Rx at this time 反相 .

[0016] Furthermore, the controller is based on the formula Rx=((Rx 正相 +Rx 反相 ) / 2)–Rf, and calculate the thermocouple resistance.

[0017] Furthermore, the high-precision resistor R2, high-precision resistor R3, high-precision resistor R4, and high-precision resistor R5 are resistors with high-precision resistance values, and the resistance values ​​marked on their factory labels can be directly used as their actual resistance values, with an error of no more than ±0.1%. The high-precision resistor R2 is 100 ohms, the high-precision resistor R3 is 1000 ohms, the high-precision resistor R4 is 10 kiloohms, and the high-precision resistor R5 is 100 kiloohms. The resistance values ​​of the high-precision resistor R2, high-precision resistor R3, high-precision resistor R4, and high-precision resistor R5 have been pre-stored in the controller.

[0018] Furthermore, the chopper commutation analog switch and the four-to-one analog switch can be a controllable switch device such as a MOS tube, an optical coupling switch, a magnetic isolator or a relay.

[0019] The present invention has the following beneficial effects: the positive and negative input terminals of the chopper-commutated analog switch U1 are connected to the positive and negative terminals of the thermocouple, and the control terminals K1, K2, K3, and K4 are controlled by a controller. First, the K1 control terminal is enabled, and the current circuit loop resistance is measured to obtain a self-calibration deviation value and store it in the controller; then the K2 control terminal is enabled or disabled, and the resistance values ​​of the positive and reverse phase thermocouples are measured. The controller takes the average and subtracts the average value from the self-calibration deviation value to obtain a high-precision thermocouple resistance value. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of a high-precision wide-range thermocouple resistance measurement circuit provided by an embodiment of the present invention;

[0021] Figure 2 It is a specific measurement flow chart of the circuit provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0022] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present application.

[0023] The high-precision wide-range thermocouple resistance measurement circuit provided by the embodiment of the present invention can be applied to various types of thermocouple resistance measurements. It should be pointed out that the thermocouple resistance is not a specific resistance element, but a virtual resistance used to preliminarily determine whether the thermocouple accuracy is within the allowable range. For example, the resistance of the thermocouple sensor measured at the factory is 120 ohms. After a period of use, the resistance rises to 200 ohms. At this time, the thermocouple sensor is preliminarily judged to have undergone serious degeneration and the accuracy is no longer within the required range.

[0024] Figure 1 This is a schematic diagram of a high-precision wide-range thermocouple resistance measurement circuit in an embodiment of the present invention. As shown in the figure, the thermocouple resistance measurement circuit in an embodiment of the present invention may at least include a chopper commutation analog switch U1, a low-voltage reference source Vref, a current limiting resistor R1, a four-select-one analog switch U2, a high-precision resistor R2, a high-precision resistor R3, a high-precision resistor R4, a high-precision resistor R5 and a controller, wherein:

[0025] Enable the K1 control terminal in the chopper commutation analog switch U1, enable the K1 control terminal, measure the current circuit loop resistance Rf, obtain the self-calibration deviation value according to the formula Rf=R2*(V1f-V2f) / V2f and store it in the controller;

[0026] The K1 control terminal in the disabled chopper commutation analog switch U1, the thermocouple equivalent resistance Rx is the thermocouple resistance to be measured, the positive input terminal of the thermocouple equivalent resistance Rx is connected to the current limiting resistance R1 through the commutation switch K2-0, and the voltage V1f is obtained. The negative input terminal of the thermocouple equivalent resistance Rx is connected to the input terminal IN in the four-choice analog switch U2 through the commutation switch K2-1 to generate a voltage V2f. According to the formula Rx 正相 =R2*(V1f-V2f) / V2f to get the positive phase measurement Rx 正相 The resistance value is stored in the controller; the K1 control end of the disabled chopper commutation analog switch U1, the negative input end of the thermocouple equivalent resistor Rx is connected to the current limiting resistor R1 through the commutation switch K2-1, generating a voltage V3f, and the positive input end of the thermocouple equivalent resistor Rx is connected to the input end IN of the four-choice analog switch U2 through the commutation switch K2-0, generating a voltage V4f. According to the formula Rx 反相 =R2*(V3f-V4f) / V4f to get the reverse phase measurement Rx 反相 The resistance value is stored in the controller; the value of the thermocouple resistance Rx is calculated by the controller according to the formula Rx = (Rx 正相 +Rx 反相 ) / 2-Rf to get the accurate thermocouple resistance value.

[0027] The four-to-one analog switch U2 has a control terminal K3 and a control terminal K4, and the resistance values ​​of the high-precision resistors R2, R3, R4, and R5 increase in sequence, and the high-precision resistors R2, R3, R4, and R5 are correspondingly connected to the OUT1, OUT2, OUT3, and OUT4 pins in the four-to-one analog switch U2;

[0028] Connect the input terminal IN of the four-to-one analog switch U2 to OUT1 to obtain voltage V1 and voltage V2. According to the formula Rx 正相 =R2*(V1-V2) / V2 to calculate Rx 正相 Resistance value;

[0029] When the initial measurement (V1-V2) / V2≥2, the controller controls the control terminal K3 and the control terminal K4, and sequentially connects the input terminal IN of the four-choice analog switch U2 to OUT2 or OUT3 or OUT4, and measures the voltage V1 and the voltage V2 until (V1-V2) / V2<2. The controller records the positive phase thermocouple resistance value Rx at this time. 正相; If the initial measurement (V1-V2) / V2≤0.1, the controller controls the control terminal K3 and the control terminal K4, and connects the input terminal IN of the four-choice analog switch U2 to OUT3 or OUT2 or OUT1 in turn, and measures the voltage V1 and the voltage V2 until (V1-V2) / V2>0.1. The controller records the positive phase thermocouple resistance value Rx at this time 正相 .

[0030] Connect the input terminal IN of the four-to-one analog switch U2 to OUT1 to obtain voltages V3 and V4. According to the formula Rx 反相 = R2*(V3-V4) / V4 to calculate Rx 反相 Resistance value;

[0031] When the initial measurement (V3-V4) / V4≥2, the controller controls the control terminal K3 and the control terminal K4, and sequentially connects the input terminal IN of the four-choice analog switch U2 to OUT2 or OUT3 or OUT4, and measures the voltage V3 and the voltage V4 until (V3-V4) / V4<2. The controller records the positive phase thermocouple resistance value Rx at this time. 反相 ; If the initial measurement (V3-V4) / V4≤0.1, the controller controls the control terminal K3 and the control terminal K4, and connects the input terminal IN of the four-choice analog switch U2 to OUT3 or OUT2 or OUT1 in turn, and measures the voltage V3 and the voltage V4 until (V3-V4) / V4>0.1. The controller records the positive phase thermocouple resistance value Rx at this time 反相 .

[0032] High-precision resistor R2, high-precision resistor R3, high-precision resistor R4, and high-precision resistor R5 are resistors with high-precision resistance values. The resistance values ​​marked on their factory labels can be directly used as their actual resistance values, with an error of no more than ±0.1%. The high-precision resistor R2 is 100 ohms, the high-precision resistor R3 is 1000 ohms, the high-precision resistor R4 is 10 kiloohms, and the high-precision resistor R5 is 100 kiloohms. The resistance values ​​of high-precision resistor R2, high-precision resistor R3, high-precision resistor R4, and high-precision resistor R5 have been pre-stored in the controller.

[0033] The chopper commutation analog switch and the four-to-one analog switch can be a controllable switch device in a MOS tube, an optical coupling switch, a magnetic isolator or a relay, but other available controllable switch devices are not excluded.

[0034] like Figure 2 As shown, the specific working steps of the controller in this embodiment are as follows:

[0035] (1) Chopping zero initialization

[0036] The controller enables the K1 control terminal in the chopper commutation analog switch U1, disables the K2 control terminal in the chopper commutation analog switch, controls the K3 and K4 control terminals in the four-to-one analog switch U2, and connects the input terminal IN of the four-to-one analog switch U2 to OUT1. Measure the voltage values ​​of V1 and V2, calculate the self-calibration deviation value according to the formula Rf=R2*(V1-V2) / V2, and store it in the controller.

[0037] (2) Forward measurement

[0038] The controller disables the K1 control terminal in the chopper commutation analog switch U1, disables the K2 control terminal in the chopper commutation analog switch, controls the K3 and K4 control terminals in the four-to-one analog switch U2, connects the input terminal IN of the four-to-one analog switch U2 to OUT1, measures the voltage values ​​of V1 and V2, and uses the formula Rx 正相 =R2*(V1-V2) / V2 to calculate the resistance value. If (V1-V2) / V2≥2, the control terminals K1 and K2 of the four-choice analog switch U2 are controlled by the controller to switch to the large resistance position, that is, to shift up; that is, the input terminal IN of the four-choice analog switch U2 is connected to OUT2 or OUT3 or OUT4, and to one end of the high-precision resistor R3 or high-precision resistor R4 or high-precision resistor R5, and the other end of the corresponding high-precision resistor is grounded to GND, and the voltage values ​​of V1 and V2 are measured until (V1-V2) / V2<2, and the resistance value Rx of the positive phase thermocouple is recorded. 正相 ; If (V1-V2) / V2≤0.1, the control terminals K1 and K2 of the four-to-one analog switch U2 are controlled by the controller to switch to the small resistance position, that is, to shift up; that is, the input terminal IN of the four-to-one analog switch U2 is connected to OUT3 or OUT2 or OUT1, and is connected to one end of the high-precision resistor R4 or high-precision resistor R3 or high-precision resistor R2, and the other end of the corresponding high-precision resistor is grounded to GND; measure the voltage values ​​of V1 and V2 until (V1-V2) / V2>0.1, and record the resistance value Rx of the positive phase thermocouple 正相 .

[0039] (3) Reverse measurement

[0040] The controller disables the K1 control terminal in the chopper commutation analog switch U1, enables the K2 control terminal in the chopper commutation analog switch, controls the K3 and K4 control terminals in the four-to-one analog switch U2, connects the input terminal IN of the four-to-one analog switch U2 to OUT1, measures the voltage values ​​of V3 and V4, and calculates the voltage according to the formula Rx. 正相=R2*(V3-V4) / V4 to calculate the resistance value. If (V3-V4) / V4≥2, the control terminals K1 and K2 of the four-choice analog switch U2 are controlled by the controller to switch to the large resistance position, that is, to shift up; that is, the input terminal IN of the four-choice analog switch U2 is connected to OUT2 or OUT3 or OUT4, and to one end of the high-precision resistor R3 or high-precision resistor R4 or high-precision resistor R5, and the other end of the corresponding high-precision resistor is grounded to GND, and the voltage values ​​of V3 and V4 are measured until (V3-V4) / V4<2, and the resistance value Rx of the reverse thermocouple is recorded. 反相 ; If (V3-V4) / V4≤0.1, the control terminals K1 and K2 of the four-to-one analog switch U2 are controlled by the controller to switch to the small resistance position, that is, downshift; that is, the input terminal IN of the four-to-one analog switch U2 is connected to OUT3 or OUT2 or OUT1, and is connected to one end of the high-precision resistor R4 or high-precision resistor R3 or high-precision resistor R2, and the other end of the corresponding high-precision resistor is grounded GND; measure the voltage values ​​of V3 and V4 until (V3-V4) / V4>0.1, and record the resistance value Rx of the reverse thermocouple 反相 .

[0041] (4) Calculate the thermocouple resistance value

[0042] The controller uses the formula Rx=((Rx 正相 +Rx 反相 ) / 2)–Rf, and calculate the thermocouple resistance.

[0043] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0044] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A high-precision wide-range thermocouple resistance measurement circuit, characterized in that: The circuit includes a chopper commutation analog switch U1, a low voltage reference source Vref, a current limiting resistor R1, a four-select-one analog switch U2, a high-precision resistor R2, a high-precision resistor R3, a high-precision resistor R4, a high-precision resistor R5 and a controller, wherein: The chopper commutation analog switch U1 has a K1 control terminal and a K2 control terminal. The K1 control terminal is enabled. The K1 control terminal measures the current circuit loop resistance Rf. According to the formula Rf=R2*(V1f-V2f) / V2f, a self-calibration deviation value is obtained and stored in the controller. The K2 control terminal is used to control the commutation switch K2-0 and the commutation switch K2-1 in the chopper commutation analog switch U1. The thermocouple equivalent resistance Rx is the thermocouple resistance to be measured. The K1 control end is disabled, and the positive input end of the thermocouple equivalent resistance Rx is connected to the current limiting resistance R1 through the phase-changing switch K2-0 to obtain a voltage V1f. The negative input end of the thermocouple equivalent resistance Rx is connected to the input end IN of the four-to-one analog switch U2 through the phase-changing switch K2-1 to generate a voltage V2f. According to the formula Rx 正相 =R2*(V1f-V2f) / V2f to get the positive phase measurement Rx 正相 The resistance value is stored in the controller; The K1 control terminal is disabled, the negative input terminal of the thermocouple equivalent resistor Rx is connected to the current limiting resistor R1 through the phase-changing switch K2-1, generating a voltage V3f, and the positive input terminal of the thermocouple equivalent resistor Rx is connected to the input terminal IN of the four-to-one analog switch U2 through the phase-changing switch K2-0, generating a voltage V4f. According to the formula Rx 反相 =R2*(V3f-V4f) / V4f to get the reverse phase measurement Rx 反相 The resistance value is stored in the controller; The value of the thermocouple resistance Rx is calculated by the controller according to the formula Rx=(Rx 正相 +Rx 反相 ) / 2-Rf to get the accurate thermocouple resistance value; The four-to-one analog switch U2 has a control terminal K3 and a control terminal K4, the resistance values ​​of the high-precision resistors R2, R3, R4 and R5 increase in sequence, and the high-precision resistors R2, R3, R4 and R5 are correspondingly connected to the OUT1, OUT2, OUT3 and OUT4 pins in the four-to-one analog switch U2; Connect the input terminal IN of the four-to-one analog switch U2 to OUT1 to obtain voltage V1 and voltage V2. According to the formula Rx 正相 =R2*(V1-V2) / V2 to calculate Rx 正相 Resistance value; When the initial measurement (V1-V2) / V2≥2, the controller controls the control terminal K3 and the control terminal K4, and sequentially connects the input terminal IN of the four-choice analog switch U2 to OUT2 or OUT3 or OUT4, and measures the voltage V1 and the voltage V2 until (V1-V2) / V2<2. The controller records the positive phase thermocouple resistance value Rx at this time. 正相 ; If the initial measurement (V1-V2) / V2≤0.1, the controller controls the control terminal K3 and the control terminal K4, and connects the input terminal IN of the four-choice analog switch U2 to OUT3 or OUT2 or OUT1 in turn, and measures the voltage V1 and the voltage V2 until (V1-V2) / V2>0.

1. The controller records the positive phase thermocouple resistance value Rx at this time 正相 ; Connect the input terminal IN of the four-to-one analog switch U2 to OUT1 to obtain voltages V3 and V4. According to the formula Rx 反相 = R2*(V3-V4) / V4 to calculate Rx 反相 Resistance value; When the initial measurement (V3-V4) / V4≥2, the controller controls the control terminal K3 and the control terminal K4, and sequentially connects the input terminal IN of the four-choice analog switch U2 to OUT2 or OUT3 or OUT4, and measures the voltage V3 and the voltage V4 until (V3-V4) / V4<2. The controller records the positive phase thermocouple resistance value Rx at this time. 反相 ; If the initial measurement (V3-V4) / V4≤0.1, the controller controls the control terminal K3 and the control terminal K4, and connects the input terminal IN of the four-choice analog switch U2 to OUT3 or OUT2 or OUT1 in turn, and measures the voltage V3 and the voltage V4 until (V3-V4) / V4>0.

1. The controller records the positive phase thermocouple resistance value Rx at this time 反相 .

2. A high-precision wide-range thermocouple resistance measurement circuit according to claim 1, characterized in that: The high-precision resistor R2, high-precision resistor R3, high-precision resistor R4, and high-precision resistor R5 are resistors with high-precision resistance values. The resistance values ​​marked on their factory labels can be directly used as their actual resistance values, with an error of no more than ±0.1%. The high-precision resistor R2 is 100 ohms, the high-precision resistor R3 is 1000 ohms, the high-precision resistor R4 is 10 kiloohms, and the high-precision resistor R5 is 100 kiloohms. The resistance values ​​of the high-precision resistor R2, high-precision resistor R3, high-precision resistor R4, and high-precision resistor R5 have been pre-stored in the controller.

3. A high-precision wide-range thermocouple resistance measurement circuit according to claim 2, characterized in that: The chopper commutation analog switch and the four-to-one analog switch are controllable switch devices such as MOS tubes, optical coupling switches, magnetic isolators or relays.

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