High-precision platinum resistor temperature sampling system

Abstract

The invention, which belongs to the technical field of platinum resistor temperature sampling, discloses a high-precision platinum resistor temperature sampling system comprising a micro-processing unit, an analog-to-digital conversion unit, a signal processing unit and a constant current unit. A high-precision constant-current source is generated through the constant-current unit and flows through the platinum resistor, voltage signals are obtained from the two ends of the platinum resistor, the voltage signals are processed through the signal processing unit and then sent into the analog-to-digital conversion unit to be sampled by an ADC, the micro-processing unit obtains data sampled by the ADC, and the temperature value of the platinum resistor position is obtained after calculation through a conversion formula. According to the invention, high-precision temperature sampling is realized through the combination of the operational amplifier and the ADC in a constant-current driving mode, the linearity is high, the self-heating power of the resistor is small, the response speed is high, and the sampling precision is high; the platinum resistor has a two-wire system connection method, a three-wire system connection method and a four-wire system connection method, and can be compatible with any one connection method, so that high-precision temperature sampling in different scenes can be realized; and meanwhile, multi-channel temperature sampling can also be realized.

Description

Technical Field

[0001] The invention relates to the technical field of platinum resistance temperature sampling, and in particular to a high-precision platinum resistance temperature sampling system. Background Art

[0002] Currently, the platinum resistance temperature sampling on the market has integrated chip sampling, but this chip with slightly higher sampling accuracy is expensive and difficult to purchase, and the sampling accuracy of ordinary integrated chips is very low. There is also a half-bridge sampling method. This simple voltage sampling method also brings problems of low sampling accuracy and poor linearity.

[0003] The above problems need to be solved urgently. Therefore, a high-precision platinum resistance temperature sampling system is proposed. Summary of the invention

[0004] The technical problem to be solved by the present invention is how to solve the problems of low sampling accuracy in the existing platinum resistance temperature sampling technology, and provides a high-precision platinum resistance temperature sampling system.

[0005] The present invention solves the above technical problems through the following technical scheme, which includes a microprocessing unit, an analog-to-digital conversion unit, a signal processing unit and a constant current unit; a high-precision constant current source is generated by the constant current unit to flow through a platinum resistor, a voltage signal is obtained from both ends of the platinum resistor, and after being processed by the signal processing unit, it is sent to the analog-to-digital conversion unit to be sampled by ADC, the microprocessing unit obtains the sampling signal of the ADC, and the temperature value of the position of the platinum resistor is obtained after calculation by a conversion formula.

[0006] Furthermore, the microprocessing unit is an MCU, which obtains the sampling signal of the ADC through a serial interface, converts the sampled CODE value into a voltage signal Vd across the platinum resistor, calculates the resistance Rpt of the platinum resistor at t°C through a formula, and converts the resistance Rpt of the platinum resistor into a real-time temperature according to the conversion formula, thereby obtaining the temperature value at the position of the platinum resistor.

[0007] Furthermore, in the microprocessing unit, the calculation formula of the resistance value Rpt of the platinum resistor at t°C is as follows:

[0008] Rpt=Vd / Iset

[0009] Where, Iset is the current flowing through the platinum resistor;

[0010] The conversion formula for converting the resistance value Rpt of the platinum resistor to the real-time temperature t is as follows:

[0011] Rpt=R0(1+At+Bt 2 )

[0012] Among them, R0 represents the resistance value of the platinum resistor at 0°C, and A and B are factors related to the temperature coefficient TCR of the platinum resistor.

[0013] Furthermore, the analog-to-digital conversion unit includes an ADC, which converts the analog signal output by the signal processing unit into a digital signal, and achieves different levels of precision sampling by selecting ADCs with different bit numbers; when performing multi-channel temperature sampling, the ADC has the ability of multi-channel input and samples the voltages at both ends of multiple platinum resistors at the same time.

[0014] Furthermore, in the signal processing unit, the voltage across the platinum resistor is differentially amplified and converted into a single-ended signal relative to the ground, so that the amplitude of the converted single-ended signal meets the sampling range of the ADC; the signal processing unit includes a filtering circuit, a following circuit and a differential amplifier circuit, the filtering circuit is used to remove high-frequency noise, the following circuit is used to achieve impedance matching, and the differential amplifier circuit is used to differentially amplify the voltage across the platinum resistor.

[0015] Furthermore, the filtering circuit includes a resistor R4, a capacitor C2, a resistor R5, and a capacitor C3, one end of the resistor R4 and the resistor R5 are respectively connected to the two ends of the platinum resistor, one end of the capacitor C2 and the capacitor C3 are grounded, and the other end is respectively connected to the other end of the resistor R4 and the resistor R5.

[0016] Furthermore, the follower circuit includes an operational amplifier U2 and an operational amplifier U3, the positive input terminals of the operational amplifier U2 and the operational amplifier U32 are connected to the other ends of the resistors R4 and R5 respectively, and the negative input terminals are connected to their own output terminals.

[0017] Furthermore, the differential amplifier circuit includes resistor R6, resistor R7, resistor R8, resistor R9, capacitor C4 and operational amplifier U4, one end of the resistor R6 and resistor R7 are respectively connected to the output end of the operational amplifier U2 and the operational amplifier U3, and the other end is respectively connected to the reverse input end and the positive input end of the operational amplifier U4; one end of the resistor R8 is grounded, and the other end is connected to the positive input end of the operational amplifier U4; the resistor R9 is connected to the reverse input end of the operational amplifier U4, and the other end is connected to the output end of the operational amplifier U4; the capacitor C4 is connected across the reverse input end and the output end of the operational amplifier U4, and the output end of the operational amplifier U4 is also connected to the ADC.

[0018] Furthermore, the constant current unit includes a resistor R1, a resistor R2, a resistor R3, a capacitor C1 and an operational amplifier U1, the reverse input terminal of the operational amplifier U1 is grounded through the resistor R1, and the forward input terminal is connected to a reference voltage VREF generated from a reference source; one end of the resistor R2 is connected to the reverse input terminal of the operational amplifier U1, and the other end is connected to one end of the platinum resistor; one end of the resistor R3 is connected to the output terminal of the operational amplifier U1, and the other end is connected to the other end of the platinum resistor; the capacitor C1 is connected across the reverse input terminal of the operational amplifier U1 and the output terminal of the operational amplifier U1.

[0019] Compared with the prior art, the present invention has the following advantages: the high-precision platinum resistor temperature sampling system utilizes a constant current driving mode and realizes high-precision temperature sampling through a combination of an operational amplifier and an ADC, and has high linearity, low resistance self-heating power, fast response speed, and high sampling accuracy; platinum resistors have two-wire, three-wire, and four-wire connection methods, and the present invention is compatible with any of the above connection methods to realize high-precision temperature sampling in different scenarios; and the present invention can also realize multi-channel temperature sampling. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a schematic diagram of the structure of a high-precision platinum resistance temperature sampling system in an embodiment of the present invention;

[0021] Figure 2 : is a circuit schematic diagram of a high-precision platinum resistance temperature sampling system in an embodiment of the present invention;

[0022] Figure 3 Schematic diagram of different wire connection methods of platinum resistance in the embodiment of the present invention, wherein (a) is a two-wire system, (b) is a three-wire system, and (c) is a four-wire system;

[0023] Figure 4 It is a schematic diagram of the multi-channel temperature acquisition principle in an embodiment of the present invention. DETAILED DESCRIPTION

[0024] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiment.

[0025] like Figure 1 and Figure 2 As shown, this embodiment provides a technical solution: a high-precision platinum resistance temperature sampling system, including an MCU (microprocessing unit), an analog-to-digital conversion unit, a signal processing unit and a constant current unit;

[0026] MCU samples the ADC data through the SPI interface, converts the sampled CODE value into the voltage signal Vd across the platinum resistor, and then calculates the resistance value Rpt of the platinum resistor at t℃ through the formula Rpt=Vd / Iset. There is a fixed conversion formula between the resistance value of the platinum resistor and the temperature. Generally, Rpt=R0(1+At+Bt 2 ), Rpt represents the resistance of the platinum resistor at t℃, R0 represents the resistance of the platinum resistor at 0℃, when the temperature coefficient of the platinum resistor TCR is 0.003851, A=3.9083*10 -3 ℃ -1 , B=-5.775*10 -7 ℃ -2 The resistance value Rpt of the platinum resistor can be converted into the real-time temperature t by table lookup or formula calculation.

[0027] The analog-to-digital conversion unit mainly includes ADC, which converts the analog signal output by the signal processing unit into a digital signal. Different levels of precision sampling can be achieved by selecting ADCs with different bits. In the case of high-precision sampling, it must not be less than 16 bits. If multi-channel temperature sampling is required, the ADC can select a multi-channel input model to sample the voltages at both ends of multiple platinum resistors simultaneously.

[0028] If the platinum resistor is PT100, because the temperature coefficient of the platinum resistor is 0.385Ω / ℃, when the excitation current is 1mA, the voltage change of the platinum resistor is 385uV / ℃, that is, for every 1℃ change in the target temperature, the voltage across PT100 will change by 385uV. If the differential gain of the signal processing unit is 20 times, the voltage change at the ADC input end will be 7.7mV (385uV*20) for every 1℃ change in the target temperature. If the ADC selects 16 bits and the reference source is set to 4096mV, the ADC can resolve 0.0625mV per LSB, that is, it can resolve 0.008℃ per LSB.

[0029] The signal processing unit is used to differentially amplify the voltage across the platinum resistor and convert it into a single-ended signal to ground, so that the amplitude of the converted single-ended signal meets the sampling range of the ADC. This unit is mainly divided into three circuits: filter circuit, follower circuit, and differential amplifier circuit;

[0030] The filter circuit includes resistor R4, capacitor C2, resistor R5, and capacitor C3. Because the lead length of the platinum resistor varies for different products, and the longest may exceed more than ten meters, a lot of high-frequency noise may be coupled on the lead. Resistor R4, capacitor C2, resistor R5, and capacitor C3 form two groups of RC circuits to remove high-frequency noise. The cutoff frequency of the filter circuit should not be set too low, which will affect the response time of the signal. Because temperature changes are generally slow-changing signals, the cutoff frequency can be set between 1kHz and 10kHz. Resistors R4 and R5 also protect the input pins of operational amplifiers U2 and U3, ensuring that the positive input terminals of U2 and U3 are not directly exposed to the connectors connected to the platinum resistors, which are easily damaged by static electricity.

[0031] The follower circuit includes operational amplifiers U2 and U3. Since the output impedance of the filter circuit in the front stage is equivalent to R4 and R5, and the differential circuit in the back stage requires the signal input impedance of the positive and negative input terminals to be infinitely close to 0, the follower circuit is inserted to achieve impedance matching. U2 and U3 need to select low-noise and low-bias operational amplifiers, otherwise operational amplifier noise will be introduced here, affecting the accuracy of temperature sampling.

[0032] The differential amplifier circuit includes resistors R6, R7, R8, R9, capacitor C4, and operational amplifier U4. It is a standard differential amplifier circuit built by an operational amplifier, in which the resistance values ​​of R6 and R7 are equal, and the resistance values ​​of R8 and R9 are equal. The signal amplification factor G = R8 / R7; the signal amplification factor is related to the model of the platinum resistor, the Iset value, the maximum temperature value of the sampling, and the sampling range of the ADC. When the platinum resistor is PT100, Iset is set to 1mA, and the maximum temperature is 200℃, then at 200℃, the voltage across PT100 is about 176mV. If the sampling range of the ADC is 0-4096mV, the amplification factor of the differential circuit can be configured to be about 20 times, so that the maximum output of the differential amplifier circuit is close to the full scale of the ADC under the maximum temperature condition. In order to improve the accuracy of the differential amplifier circuit, resistors R6, R7, R8, and R9 need to be precision resistors (for example, 0.1% accuracy and 25ppm temperature coefficient), and U4 needs to be a low-noise, low-bias operational amplifier like U2 and U3. In order to ensure the stability of the loop, capacitor C4 is added across the reverse input and output of operational amplifier U4. This capacitor forms an RC low-pass filter circuit with R9 at the same time. Capacitor C4 should not be too large, which will increase the response time of the circuit.

[0033] Some ADCs have differential input and internal gain configuration functions, so the differential amplifier circuit of the signal processing unit can be omitted and the output from the follower circuit can be directly output to the ADC input port.

[0034] The constant current unit is used to provide a high-precision constant current source for the platinum resistor, and mainly includes resistor R1, resistor R2, resistor R3, capacitor C1, and operational amplifier U1. In order to avoid the inaccurate temperature measurement caused by the self-heating effect of the platinum resistor, when the platinum resistor is PT100, the current value of the constant current source should not exceed 1mA; when the platinum resistor is PT1000, the current value of the constant current source should not exceed 0.5mA. This unit realizes the constant current function through a precision operational amplifier. The reverse input terminal of the operational amplifier U1 is grounded through the resistor R1, and the forward input terminal is connected to a reference voltage VREF, which is required to be very precise and generated from a reference source; one end of the resistor R2 is connected to the reverse input terminal of the operational amplifier U1, and the other end is connected to one end of the platinum resistor; one end of the resistor R3 is connected to the output terminal of the operational amplifier U1, and the other end is connected to the other end of the platinum resistor; the capacitor C1 is connected across the reverse input terminal of the operational amplifier U1 and the output terminal of the operational amplifier U1. Through the virtual short principle of the operational amplifier, the voltage of the reverse input terminal of the operational amplifier U1 is equal to the reference voltage VREF, then the current IR1 flowing through R1 = VREF / R1, R1 needs to select a precision resistor (for example, 0.1% accuracy and 25ppm temperature coefficient), because VREF is also a precision voltage source, so the current of IR1 is very precise, and the reverse input terminal of U1 is a high resistance, then the current will only flow from the output terminal of the operational amplifier U1 through R3, the platinum resistor, and R2 to R1, then the current Iset flowing through the platinum resistor is equal to the resistance IR1 flowing through R1. C1 is used as a phase compensation capacitor to ensure the stability of the loop. U1 also needs to use a precision low-noise operational amplifier to avoid introducing operational amplifier noise.

[0035] In order to obtain a more accurate temperature sampling signal, there are certain requirements for the system power supply. The operational amplifiers of the system can be uniformly powered by a single power supply. The power supply voltage is recommended to be set to 5V, which can be generated by the LDO module. If it is generated from the DCDC module, it needs to be filtered by LC before it can be used as the system power supply voltage.

[0036] like Figure 3 As shown, this system is compatible with two-wire, three-wire and four-wire connection methods. When the lead length is short, the error caused by the lead resistance is small, and the two-wire system can be used directly. In order to avoid the influence of lead resistance, it is recommended to use a three-wire or four-wire connection method.

[0037] like Figure 4 As shown in the figure, under the requirement of multi-channel detection, multiple constant current units and signal processing units can be connected to a multi-channel ADC, and the MCU can realize the function of multi-temperature acquisition by sampling the multi-channels of the ADC. Because temperature is a slow-changing signal, it is necessary to note that when multiple channels of the ADC are sampled at the same time, the switching frequency between channels should not be too fast, because the circuit has a response time, which will introduce interference between channels.

[0038] In summary, the high-precision platinum resistance temperature sampling system of the above embodiment utilizes a constant current driving method and realizes high-precision temperature sampling through a combination of an operational amplifier and an ADC. It has high linearity, low resistance self-heating power, fast response speed, and high sampling accuracy. Platinum resistance has two-wire, three-wire, and four-wire connection methods. The present invention is compatible with any of the above connection methods to realize high-precision temperature sampling in different scenarios. At the same time, the present invention can also realize multi-channel temperature sampling.

[0039] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A high-precision platinum resistance temperature sampling system, characterized in that: It includes a microprocessing unit, an analog-to-digital conversion unit, a signal processing unit and a constant current unit. The constant current unit generates a high-precision constant current source to flow through the platinum resistor, and a voltage signal is obtained from both ends of the platinum resistor. After being processed by the signal processing unit, it is sent to the analog-to-digital conversion unit for ADC sampling. The microprocessing unit obtains the sampling signal of the ADC, and obtains the temperature value of the platinum resistor position after calculation through a conversion formula.

2. A high-precision platinum resistance temperature sampling system according to claim 1, characterized in that: The microprocessing unit is an MCU, which obtains the sampling signal of the ADC through the serial interface, converts the sampled CODE value into a voltage signal Vd across the platinum resistor, calculates the resistance value Rpt of the platinum resistor at t°C through a formula, and converts the resistance value Rpt of the platinum resistor into real-time temperature according to the conversion formula, that is, obtains the temperature value at the position of the platinum resistor.

3. A high-precision platinum resistance temperature sampling system according to claim 2, characterized in that: In the microprocessing unit, the calculation formula of the resistance value Rpt of the platinum resistor at t°C is as follows: Rpt=Vd / Iset Where, Iset is the current flowing through the platinum resistor; The conversion formula for converting the resistance value Rpt of the platinum resistor to the real-time temperature t is as follows: Rpt=R0(1+At+Bt 2 ) Among them, R0 represents the resistance value of the platinum resistor at 0°C, and A and B are factors related to the temperature coefficient TCR of the platinum resistor.

4. A high-precision platinum resistance temperature sampling system according to claim 3, characterized in that: The analog-to-digital conversion unit includes an ADC, which converts the analog signal output by the signal processing unit into a digital signal, and achieves different levels of precision sampling by selecting ADCs with different bits; when performing multi-channel temperature sampling, the ADC has the ability of multi-channel input and simultaneously samples the voltages at both ends of multiple platinum resistors.

5. A high-precision platinum resistance temperature sampling system according to claim 4, characterized in that: In the signal processing unit, the voltage across the platinum resistor is differentially amplified and converted into a single-ended signal relative to the ground, so that the amplitude of the converted single-ended signal meets the sampling range of the ADC; the signal processing unit includes a filtering circuit, a following circuit and a differential amplifier circuit, the filtering circuit is used to remove high-frequency noise, the following circuit is used to achieve impedance matching, and the differential amplifier circuit is used to differentially amplify the voltage across the platinum resistor.

6. A high-precision platinum resistance temperature sampling system according to claim 5, characterized in that: The filter circuit includes a resistor R4, a capacitor C2, a resistor R5, and a capacitor C3. One end of the resistor R4 and the resistor R5 are respectively connected to two ends of the platinum resistor. One end of the capacitor C2 and the capacitor C3 are grounded, and the other end is respectively connected to the other end of the resistor R4 and the resistor R5.

7. A high-precision platinum resistance temperature sampling system according to claim 6, characterized in that: The follower circuit includes an operational amplifier U2 and an operational amplifier U3. The positive input terminals of the operational amplifiers U2 and U32 are connected to the other ends of the resistors R4 and R5 respectively, and the reverse input terminals are connected to their own output terminals.

8. A high-precision platinum resistance temperature sampling system according to claim 7, characterized in that: The differential amplifier circuit includes resistor R6, resistor R7, resistor R8, resistor R9, capacitor C4 and operational amplifier U4, one end of the resistor R6 and resistor R7 are respectively connected to the output end of the operational amplifier U2 and the operational amplifier U3, and the other end is respectively connected to the reverse input end and the positive input end of the operational amplifier U4; one end of the resistor R8 is grounded, and the other end is connected to the positive input end of the operational amplifier U4; the resistor R9 is connected to the reverse input end of the operational amplifier U4, and the other end is connected to the output end of the operational amplifier U4; the capacitor C4 is connected across the reverse input end and the output end of the operational amplifier U4, and the output end of the operational amplifier U4 is also connected to the ADC.

9. A high-precision platinum resistance temperature sampling system according to claim 8, characterized in that: The constant current unit includes a resistor R1, a resistor R2, a resistor R3, a capacitor C1 and an operational amplifier U1. The reverse input terminal of the operational amplifier U1 is grounded through the resistor R1, and the forward input terminal is connected to a reference voltage VREF generated from a reference source; one end of the resistor R2 is connected to the reverse input terminal of the operational amplifier U1, and the other end is connected to one end of a platinum resistor; one end of the resistor R3 is connected to the output terminal of the operational amplifier U1, and the other end is connected to the other end of the platinum resistor; the capacitor C1 is connected across the reverse input terminal of the operational amplifier U1 and the output terminal of the operational amplifier U1.

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