Current signal output automatic calibration circuit and temperature transmitter

Abstract

The invention discloses a current signal output automatic calibration circuit and a temperature transmitter, and aims to solve the problems that the precision of an existing current type transmitter is difficult to guarantee and an existing high-precision current signal output circuit needs multi-point manual calibration. The device comprises a signal input interface, an MCU module, a DAC module, a resistor R1, a signal output interface, a voltage / current detection module, an ADC module and a voltage reference source. The input end of the MCU module is connected with the signal input interface, and the output end of the MCU module is connected with the input end of the DAC module; the output end of the DAC module is connected to the signal output interface through a resistor R1; the non-inverting input end of the voltage / current detection module is connected with the positive electrode of the resistor R1, the inverting input end is connected with the negative electrode of the resistor R1, and the output end is connected with the first input end of the ADC module; the output current precision does not need to be manually adjusted, and the production efficiency is improved while the output precision is ensured.

Description

Technical Field

[0001] The present application relates to the technical field of temperature transmitters, and in particular to a current signal output automatic calibration circuit and a temperature transmitter. Background Art

[0002] With the increasing requirements for energy conservation and emission reduction, the precise measurement and control of ambient temperature has received more and more attention. Temperature sensors and transmitters, as the source of temperature control systems, play a key role in precise temperature control and are widely used in air conditioning and fresh air systems. In particular, some high-precision laboratories, libraries, museums, etc. have very high requirements for temperature and humidity.

[0003] Current type transmitters are widely used in various control systems due to their strong anti-interference ability and long-distance transmission. In existing building control systems, the output error accuracy of temperature transmitters reaches 0.5FS, which is already high precision. Current type temperature transmitters need to convert the collected temperature signal into a corresponding current signal output through an analog circuit. The analog conversion circuit itself has conversion errors, and the accuracy is difficult to guarantee.

[0004] The current signal output circuit is generally implemented with a transistor constant current source or a dedicated DAC chip. To ensure output accuracy and linearity, the circuit requires at least 2-point or multi-point manual calibration. For high-precision current-type temperature transmitters, most of them are manually adjusted before leaving the factory, and the linearity of the output circuit of each device is manually calibrated at multiple points. The production process is complicated and the production efficiency is low. Summary of the invention

[0005] The present application provides a current signal output automatic calibration circuit and a temperature transmitter, aiming to solve the problem that the accuracy of existing current type transmitters is difficult to ensure and the existing high-precision current signal output circuit requires multi-point manual calibration.

[0006] In a first aspect, a current signal output automatic calibration circuit includes a signal input interface, an MCU module, a DAC module, a resistor R1, a voltage / current detection module, an ADC module and a signal output interface;

[0007] The signal input interface, MCU module, DAC module, resistor R1, and signal output interface are connected in series in sequence; wherein the input end of the MCU module is divided into a target signal input end and a feedback signal input end, and the target signal input end is connected to the signal input interface;

[0008] The voltage / current detection module is used to detect the voltage difference across the resistor R1; the output end of the current detection module is connected to the input end of the ADC module, and the output end of the ADC module is connected to the feedback signal input end of the MCU module; the MCU module determines whether the output value is within the error accuracy range by comparing the feedback signal with the target signal, and automatically performs compensation calibration if it exceeds the error range to ensure that the output value is within the error range.

[0009] In the above solution, optionally, the automatic calibration circuit further includes a voltage reference source connected to the ADC module to ensure the accuracy of the acquisition level.

[0010] In the above solution, further optionally, the voltage reference source is selected according to the output range of the voltage / current detection module, and the output voltage of the voltage reference source is greater than the output range of the voltage / current detection module.

[0011] In the above solution, optionally, the signal input interface and the MCU module, the MCU module and the DAC module, and the ADC module and the MCU module are connected via a digital interface I2C or SPI.

[0012] In the above solution, optionally, the resolution of the ADC module is selected according to the actual range of the input signal.

[0013] In the above solution, optionally, the resistor R1 adopts a resistor with an accuracy of ≤0.1%, a resistance of ≤10Ω, and a temperature drift of <±10ppm / °C.

[0014] In a second aspect, a temperature transmitter includes a temperature sensor, characterized in that it also includes the above-mentioned current signal output automatic calibration circuit, the signal input interface of the automatic calibration circuit is connected to the signal output interface of the temperature sensor, and the signal output interface of the automatic calibration circuit serves as the output end of the temperature transmitter.

[0015] Compared with the prior art, this application has at least the following beneficial effects:

[0016] Based on further analysis and research of the existing technical problems, this application recognizes that the current output accuracy of the existing high-precision current signal output circuit needs manual adjustment, the production process is complicated, and the production efficiency is affected. In the existing high-precision current type temperature transmitter, although the output accuracy of the temperature sensor probe part can reach 0.1FS, due to the discreteness of the current conversion analog circuit components, conversion errors are common, and it is difficult to achieve an accuracy of 0.1FS. A high-precision resistor R1 is connected in series through the current signal output circuit, and the two ends of R1 are respectively connected to a high-precision current detection chip. When there is an output current to the outside, a voltage difference is generated at both ends of R1. The high-precision voltage / current detection module detects the voltage difference at both ends of R1, and outputs this voltage difference to the ADC module after proportional conversion. The ADC module sends the received voltage value to the MCU unit in the form of a digital quantity. The MCU compares the input value of the ADC with the input value of the sensor to determine whether the output value is within the error accuracy range (such as 0.1FS). If it exceeds the tolerance, the output of the DAC module is automatically compensated and calibrated to ensure that the output value is within the error range, thereby realizing a high-precision current signal output automatic calibration circuit. This circuit is used to realize a current type temperature transmitter with an error accuracy of 0.1FS.

[0017] This application also uses digital error adjustment, which is software-configurable and automatically adjusts the output error accuracy of the current signal. This application does not require manual adjustment of the output current accuracy, ensuring output accuracy while improving production efficiency. The solution of this application improves the output error accuracy of the current type temperature transmitter to 0.1FS. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A schematic diagram of a current signal output automatic calibration circuit provided in one embodiment of the present application.

[0019] Figure 2 A schematic diagram of the connection between the signal input interface and the MCU module provided for one embodiment of the present application.

[0020] Figure 3 A schematic diagram of the connection between the MCU module and the DAC module provided in one embodiment of the present application.

[0021] Figure 4 A schematic diagram of a DAC module provided in one embodiment of the present application being connected to a signal output interface via a resistor R1.

[0022] Figure 5 A schematic diagram of the connection relationship among a DAC module, a resistor R1, a signal output interface, and a current detection chip provided in one embodiment of the present application.

[0023] Figure 6A schematic diagram of the connection relationship between a current detection chip, an ADC module, and a voltage reference source provided in one embodiment of the present application.

[0024] Figure 7 A schematic diagram of the connection relationship between an ADC module, an MCU and a signal input interface provided in one embodiment of the present application.

[0025] Figure 8 A schematic diagram of a current signal output automatic calibration circuit provided in one embodiment of the present application. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0027] In the description of this application: unless otherwise specified, the meaning of "plurality" is two or more. The terms "first", "second", "third", etc. in this application are intended to distinguish the objects referred to, and do not have special meanings in terms of technical connotations (for example, they should not be understood as emphasizing the importance or order, etc.). Expressions such as "including", "comprising", "having", etc. also mean "not limited to" (certain units, components, materials, steps, etc.).

[0028] With the increasing requirements for energy conservation and emission reduction, the precise measurement and control of ambient temperature has received more and more attention. Temperature sensors and transmitters, as the source of temperature control systems, play a key role in precise temperature control and are widely used in air conditioning and fresh air systems. In particular, some high-precision laboratories, libraries, museums, etc. have very high requirements for temperature and humidity.

[0029] Current transmitters are widely used in various control systems due to their strong anti-interference ability and long-distance transmission.

[0030] The first purpose of the present application is to provide a current signal output automatic calibration circuit, which is software configurable, does not require manual calibration of the output current value, and the output error accuracy can reach 0.1FS. The second purpose of the present application is to provide a current type temperature transmitter.

[0031] In one embodiment, reference Figure 1 , provides a current signal output automatic calibration circuit, including a signal input interface, an MCU module, a DAC module, a resistor R1, a voltage / current detection module, an ADC module and a signal output interface;

[0032] The signal output interface signal input interface, MCU module, DAC module, resistor R1, and signal output interface are connected in series in sequence; wherein the input end of the MCU module is divided into a target signal input end and a feedback signal input end, and the target signal input end of the signal output interface is connected to the signal input interface of the signal output interface;

[0033] The signal output interface voltage / current detection module is used to detect the voltage difference across the resistor R1; the output end of the current detection module is connected to the input end of the signal output interface ADC module, and the output end of the ADC module is connected to the feedback signal input end of the signal output interface MCU module; the MCU module determines whether the output value is within the error accuracy range by comparing the feedback signal with the target signal. If it exceeds the tolerance, compensation calibration is automatically performed to ensure that the output value is within the error range.

[0034] refer to Figure 1 In this embodiment, the sensor input signal is sent to the MCU in the form of digital quantity, and the MCU sends the acquired value to the digital-to-analog conversion DAC chip in the form of digital quantity, and the DAC outputs the corresponding 4-20mA current signal. The current signal output circuit is connected in series with a high-precision resistor R1, and the two ends of R1 are respectively connected to a high-precision current detection chip. When there is an output current to the outside, a voltage difference is generated at both ends of R1. The voltage / current detection module detects the voltage difference at both ends of R1, and outputs this voltage difference to the ADC module after proportional conversion. The ADC module sends the received voltage value to the MCU unit in the form of digital quantity. The MCU compares the input value of the ADC with the input value of the sensor to determine whether the output value is within the error accuracy range (such as 0.1FS). If it exceeds the tolerance, the output of the DAC module is automatically compensated and calibrated to ensure that the output value is within the error range.

[0035] In one embodiment, the signal input interface and the MCU module, the MCU module and the DAC module, and the ADC module and the MCU module are connected via a digital interface I2C or SPI.

[0036] refer to Figure 2 , the external high-precision temperature sensor sends the detected temperature value to the MCU module through the digital interface (I2C or SPI, I2C is used in this example). Because digital transmission is used between the MCU and the sensor, there is no conversion error in the middle, which well ensures the accuracy of the high-precision value.

[0037] refer to Figure 3 The MCU module receives the temperature value (within 0.1FS accuracy error) sent by the high-precision sensor through the digital interface (I2C or SPI, I2C is used in this example), and sends the obtained temperature value to the DAC module without distortion.

[0038] refer to Figure 4The DAC module receives the measurement value input by the MCU module through a digital interface (I2C or SPI, I2C is used in this example), and converts the measurement value into a corresponding analog current signal output.

[0039] In one embodiment, the resistor R1 is a resistor with high precision, small resistance and low temperature drift.

[0040] In this embodiment, the resistor R1 has a precision of ≤0.1%, a resistance of ≤10Ω, and a temperature drift of <±10ppm / °C.

[0041] refer to Figure 5 , the shunt resistor R1 is connected in series at the output end. When the output current passes through R1, a voltage difference V1 is formed at both ends of the resistor. Assuming the output current is Io, then V1=Io*R1. R1+ of R1 is connected to the same-direction input end of the voltage / current detection module, and R1- is connected to the reverse input end of the voltage / current detection module. The voltage / current detection module receives the value of V1. To ensure high-precision detection, the resistance value of R1 needs to be high-precision, small resistance, and low temperature drift. R1 is configured to be 2R*0.01%. When Io is 4mA, V1=4mA*2R=8mV. The voltage is very weak. To facilitate the detection of the back-end ADC, V1 is amplified to V1* after passing through IC1. The amplification factor is selected as 50V / V, then V1*=50*V1. When the output current is 4-20mA, the corresponding V1 is 8-40mV, and the corresponding V1* is 0.4-2V.

[0042] In one embodiment, the automatic calibration circuit further includes a voltage reference source connected to the signal output interface ADC module to ensure the accuracy of the acquisition level.

[0043] In one embodiment, the voltage reference source is selected according to the output range of the voltage / current detection module, and the output voltage of the voltage reference source is greater than the output range of the voltage / current detection module.

[0044] In one embodiment, the resolution of the ADC module is selected according to the actual range of the input signal.

[0045] refer to Figure 6 ADC chip IC2 receives the voltage value V1* input by IC1. To ensure the accuracy of the acquisition level, IC2 is connected to a voltage reference source with low temperature drift or zero temperature drift. Since the input range of V1* is 0.4V-2V, we select a 2.5V voltage reference source.

[0046] To ensure 0.1FS high precision, the ADC module needs to select the corresponding resolution according to the actual range. The full range of the transmitter is 0℃-100℃, so the 0.1FS accuracy error needs to be 100℃*0.1%=±0.1℃. The current output range is 4-20mA, and the corresponding accuracy error is (20mA-4mA)*0.1%=0.016mA. The actual design accuracy is doubled to 0.008mA, and the corresponding ADC recognition accuracy needs to be at least: 0.008mA*2R*50=0.8mV=0.0008V. The ADC resolution is increased by one bit to 0.00001, so we choose 16bit, and the resolution can reach 1 / 2^16, that is, 0.00001.

[0047] refer to Figure 7 , the ADC chip sends the collected voltage value V1* to the MCU module through the digital I2C interface without loss, and the MCU module then converts V1* into the corresponding current value Io. The current conversion formula is Io=V1*÷50÷2; assuming that the actual input temperature value of the input end is Ti, and the full range of 0℃-100℃ corresponds to 4-20mA output, then the current value per 1℃ is (20-4)÷100=0.16mA, and the ideal output current I=0.16*Ti+4mA. Io-I=ΔI, if ΔI is within the range of ±0.1℃, that is, ±0.016mA, no compensation is required. If it exceeds the tolerance, corresponding compensation is performed. Compensation current value: ΔI>0.016mA, compensation current I*=ΔI-0.016mA; ΔI<-0.016mA, compensation current I*=ΔI+0.016mA.

[0048] Because the ADC and MCU communicate through the I2C interface and do not participate in the conversion of signal quantities, the system has low requirements for the MCU and an 8-bit low-cost microcontroller can meet the requirements.

[0049] In one embodiment, a temperature transmitter is provided, including a temperature sensor and the above-mentioned current signal output automatic calibration circuit, wherein the signal input interface of the automatic calibration circuit is connected to the signal output interface of the signal output interface temperature sensor, and the signal output interface of the automatic calibration circuit serves as the output end of the temperature transmitter.

[0050] In summary, the present application realizes a high-precision current signal output automatic calibration circuit. The present circuit is used to realize a current type temperature transmitter with an error accuracy of 0.1FS.

[0051] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A current signal output automatic calibration circuit, characterized in that: It includes a signal input interface, an MCU module, a DAC module, a resistor R1, a voltage / current detection module, an ADC module and a signal output interface; The signal input interface, MCU module, DAC module, resistor R1, and signal output interface are connected in series in sequence; wherein the input end of the MCU module is divided into a target signal input end and a feedback signal input end, and the target signal input end is connected to the signal input interface; The voltage / current detection module is used to detect the voltage difference across the resistor R1; the output end of the current detection module is connected to the input end of the ADC module, and the output end of the ADC module is connected to the feedback signal input end of the MCU module.

2. The current signal output automatic calibration circuit according to claim 1, characterized in that: The automatic calibration circuit also includes a voltage reference source connected to the ADC module to ensure the accuracy of the acquisition level.

3. The current signal output automatic calibration circuit according to claim 2, characterized in that: The voltage reference source is selected according to the output range of the voltage / current detection module, and the output voltage of the voltage reference source is greater than the output range of the voltage / current detection module.

4. The current signal output automatic calibration circuit according to claim 1, characterized in that: The signal input interface is connected to the MCU module, the MCU module is connected to the DAC module, and the ADC module is connected to the MCU module via a digital interface I2C or SPI.

5. The current signal output automatic calibration circuit according to claim 1, characterized in that: The resolution of the ADC module is selected according to the actual range of the input signal.

6. The current signal output automatic calibration circuit according to claim 1, characterized in that: The resistor R1 has a precision of ≤0.1%, a resistance of ≤10Ω, and a temperature drift of <±10ppm / ℃.

7. A temperature transmitter, comprising a temperature sensor, characterized in that: It also includes the current signal output automatic calibration circuit as described in claim 1, the signal input interface of the automatic calibration circuit is connected to the signal output interface of the temperature sensor, and the signal output interface of the automatic calibration circuit serves as the output end of the temperature transmitter.

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