Multi-mode temperature measurement system

0 Introduction
The measurement range, measurement method and measurement accuracy of most temperature measurement tools on the market are fixed at the factory. Their measurement method is single, the measurement range is fixed, and the sensing method can only adapt to certain occasions. Therefore, they cannot be well applied to some occasions with multiple measurement methods and measurement ranges. In addition, the data storage of the data acquisition card has been solidified, which cannot be applied to some occasions with special requirements. This paper uses field programmable gate array FPGA to process data, and the VHDL program can be modified online. The multi-mode temperature measurement system has extremely strong plasticity, and its program and lookup database can be improved and updated in a timely manner, so that the performance of the system can be upgraded and the system can meet the needs of different occasions.

1 Hardware design of multi-mode temperature measurement system
The multi-mode temperature measurement system uses three types of temperature sensors: PN junction (IN4007), thermal resistor (PT100), and thermocouple (nickel-chromium-nickel-silicon K type) for temperature measurement. The PN junction (IN4007) is connected to a single-arm DC bridge (unbalanced), and the output of the bridge is connected to an amplifier (amplified 100 times), and then connected to a 12-bit analog/digital converter through a multi-way analog switch; the thermal resistor (PT100) is connected to the DC bridge in a three-wire system, and the output of the bridge is connected to an amplifier (amplified 70 times), and then connected to a 12-bit analog/digital converter through a multi-way analog switch; the thermocouple (nickel-chromium-nickel-silicon K type) is connected to the cold end compensator (the four bridge arms of the homemade bridge are all 1 Ω resistors, three of which are wound with manganese copper wire and one with copper wire), and its output is connected to an amplifier (amplified 200 times), and then connected to a 12-bit analog/digital converter through a multi-way analog switch. The outputs of the three sensors are finally converted into a voltage of 0 to 10 V, which meets the conversion requirements of the 12-bit analog/digital converter. The converted data is sent to the FPGA, which then processes the data and displays the output. The hardware block diagram of the multi-mode temperature measurement system is shown in Figure 1.

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1.1 PT100 thermal resistor temperature measurement principle hardware circuit
Since the PT100 thermal resistor generates a resistance signal as the temperature changes, the resistance value increases when the temperature rises. Therefore, the thermal resistor must be connected into a single-arm DC bridge to convert its resistance change into a voltage change signal. Then the voltage signal is amplified to the range of 0-10 V and sent to the A/D conversion circuit. The circuit diagram is shown in Figure 2.
1.2 12-bit analog/digital converter ADC1674
In order to meet the measurement accuracy requirements, this multi-mode temperature measurement system uses a 12-bit A/D converter and a unipolar input method. Thus, the temperature can be accurate to the second decimal place. The circuit schematic is shown in Figure 3.
1.3 Single-chip microcomputer control circuit (AT89C51)
In order to reduce the program burden of FPGA, its peripheral control circuit is controlled by a single-chip microcomputer. The single-chip microcomputer controls the multi-channel analog switch (CD4051) and the scale table memory (2716) and FPGA, so that the FPGA controls the processing of different measurement data and the temperature display output. The circuit schematic is shown in Figure 4.

1.4 FPGA Programmable Gate Array
This multi-mode temperature measurement system uses FPGA programmable gate array to process the data after A/D conversion, compares or calculates the temperature data through internal table lookup, and then decodes and outputs the temperature value. The circuit schematic is shown in Figure 5.

2 Multi-mode temperature measurement system software design
The multi-mode temperature measurement system software is divided into single-chip microcomputer program design and FPGA program design. The single-chip microcomputer program is written in assembly language to realize the control of peripheral circuits; the FPGA is written in VHDL language to realize data processing and temperature display output.
2.1 Single-chip microcomputer control
The single-chip microcomputer control source program is as follows:

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2.2 FPGA data processing
In the thermocouple or thermal resistor measurement mode, the FPGA searches for the data of the corresponding scale table in turn and compares and calculates with the data converted by A/D, and finally obtains its temperature value. The program flow chart is omitted. In the PN junction measurement mode, the FPGA calculates the data according to the temperature voltage change function of the PN junction (for every 1°C increase in temperature, the forward conduction voltage drop of the PN junction decreases by 1 mV), thereby obtaining the corresponding temperature value; the program flow chart is shown in Figure 6.

3 Conclusion
From system design to debugging, the measurement system has high requirements for signal and circuit stability. At the beginning, the system output display was very unstable due to neglect of stability. Finally, the measurement conversion circuit was separated from the system processing circuit to improve the problem. The temperature measurement system can realize temperature measurement in three ways: PN junction, thermal resistor (PT100), and thermocouple (nickel-chromium-nickel-silicon K type). It can meet the needs of different measurement ranges, different measurement accuracy and different occasions. This design uses EDA as a development tool, with single-chip microcomputer control, making it possible to cooperate with single-chip microcomputer and EDA. The system uses single-chip microcomputer for simpler implementation, and EDA for simpler implementation. For example, keyboard operation is much simpler with single-chip microcomputer than EDA, which makes the whole design have a newer design idea. The use of 12-bit ADC analog/digital converter has greatly improved the measurement accuracy. Field programmable gate array FPGA is used for data processing. Its extremely high program execution speed makes the system respond faster and more accurately. This article can be used for multi-mode temperature measurement with high accuracy and speed requirements.