High-precision temperature measurement and control system based on STM32

High progress temperature measurement system based on STM32, this article is a schematic design analysis article

• Design schematic diagram of high-precision temperature measurement and control instrument
• High-precision temperature measurement and control instrument design PCB articles (not updated)
• High-precision temperature measurement and control instrument design STM32 code articles (not updated)
• High-precision temperature measurement and control instrument design host computer articles (not updated)

Schematic analysis of high-precision temperature measurement and control system based on STM32

1. Overall analysis

Circuit realization function: stm32 master controls the constant current source to output current to the temperature sensor (PT100), returns the differential signal, performs analog-to-digital conversion after filtering, the single-chip microcomputer collects the signal, processes the display, and controls the peripheral circuit processing, sending information to the host computer, Control LED and buzzer buzzer (The temperature measurement and control system works under strong magnetic interference, and the device selection and design should pay attention to anti-interference!)

Circuit configuration:

2. Partial design analysis

2.1 Power supply design

The power supply is 220v input, and working in a strong magnetic environment, the design is more troublesome, so directly use the finished module of Jinshengyang, plus an isolation module, connect a fuse to protect the live wire, and 220V output +/-12V, and do a good job For isolation, +/-12V to +/-5V is used to supply power to the core acquisition circuit, which directly affects the acquisition accuracy, so use an isolated and low-noise LDO, as follows:

• 220 conversion +/-12V circuit:

I connected a 104 and a 120uf electrolytic decapacitor filter to the two output terminals.

• +12 to +5V circuit

Using TPS7A4701, a chip produced by TI, the low-dropout voltage regulator dedicated to high-precision AD acquisition DC-DC output voltage noise is only 4.17 µVRMS (10 Hz, 100 kHz), the schematic design is changed according to the official typical circuit, and the filter uses 0.1 Uf+10uf tantalum capacitor, the output voltage changes the output resistance value of the output terminal and the output mode of the upper and lower ends.

• -12 to -5V circuit

Using TPS7A3301, a negative linear low-dropout voltage regulator dedicated to high-precision AD acquisition DC-DC output voltage noise 16μVRMS (10Hz to 100kHz);

• Turn 12V to 5V and then turn to 3V to power the microcontroller

In the process of designing the core circuit and peripherals of the single-chip microcomputer, 5V and 3.3V need to be used. If the +/- 5V of the differential signal is directly used, it may interfere with the accuracy of the core acquisition circuit, so two new chips are used. For step-down, they are 7805 and AMS1117, which are enough for the microcontroller and some of its peripheral modules. (The cost of the chip is about 5 yuan)

• Modular isolation

The digital power supply and the analog power supply should be isolated, and the magnetic beads are used for isolation here.

• The reference voltage

The reference voltage has a great impact on the acquisition accuracy. The reference paper used here is the ADR421ARZ chip, which has high accuracy (0.025%) and low noise (0.1 Hz to 10 Hz: 1.75 μV peak-to-peak value). The specific implementation circuit is as follows:

Because of the high precision requirements, there are many filtering and decoupling capacitors to prevent interference.

2.2 Analog-to-digital conversion circuit design

The analog-to-digital conversion circuit mainly uses a constant current source to excite the sensor. The differential response voltage generated at different temperatures is different. The constant current source circuit outputs a set current value to excite the four-wire temperature sensor. The output voltage of the temperature sensor is differentiated. The channel circuit selects one of the outputs and outputs to the analog-to-digital conversion part through the differential buffer. Inside the analog-to-digital conversion circuit, the ADC chip comes with a programmable gain circuit and a low-pass filter circuit. STM32 controls ADC to perform analog-to-digital conversion, and calculates the obtained digital quantity to obtain the voltage value or resistance value of the sensor. The main design points are: 1. Constant current source design, 2. Conversion circuit design, 3. Control circuit design

• Constant current source design

The design of the constant current source is to follow the voltage, so that the constant voltage acts on the resistor to generate current, which can supply power to the sensor. With a multiplexer, multiple resistors can be selected to generate currents of different sizes. Here are 1ma, 100ua. , 10ua, 1ua, single channel as shown below, we need a total of 8 channels (single channel chip cost about 65, 8 channels about 500)

• Analog-to-digital conversion circuit design

The principle of the analog-to-digital conversion circuit is to use ADG1409 to gate A and B, input the filtered differential signal, and perform analog-to-digital conversion. The microcontroller reads the information through the protocol. The design circuit is as follows:

2.2 SCM core control circuit

• STM32 core control circuit

The core control chip of the measurement system selects the STM32 series chip STM32F407ZGT6. The core circuit of STM32 is shown in the figure, which mainly includes a network that identifies the circuit connection relationship, 12MHz and 32.768Hz crystal oscillator circuits, and a reset circuit.

• RS232 communication circuit

Convert TTL to 232 and communicate with host computer, refer to the paper design, connect to 100 for impedance matching

• Ethernet communication circuit

The single-chip microcomputer communicates with the computer through Ethernet. This design is more troublesome. It directly uses the finished module serial port of the human cloud to Ethernet, connects to an isolated network port RJ45, and the differential impedance is 100. Here, the Jiali Chuang assistant is used to match the line width.

• Button design

A paper for reference is very strange, hardware anti-shake, it feels like adding a filter capacitor according to the traditional button connection, there is no need to make it so complicated, the revision is as follows, even if the filter capacitor is unstable, it can be solved.

• Display circuit

The design here is very simple, nothing special, just pay attention to the OLED connection plug selection is basically no problem (OLED cost 10)

• Download circuit

It’s very simple, connect 22 for protection, you can also connect a TVS

The schematic design is basically as above, the next PCB design

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