Scheme design of 0-20mA current generator based on RT-Thread

Overview

The signal current of general instruments is 4-20mA, which means that the minimum current is 4mA and the maximum current is 20mA. When transmitting a signal, consider that there is resistance on the wire. If voltage transmission is used, a certain voltage drop will be generated in the wire, and the signal at the receiving end will have a certain error, so the current signal is used as the standard transmission of the transmitter. Why choose 4-20mA instead of 0-20mA? 4ma instead of 0ma is used to detect an open circuit. If 0 is the minimum, then the open circuit fault will not be detected. In order to solve the above problems and avoid the influence of related noise, we use current to transmit signals, because current is not sensitive to noise. The 4-20mA current loop uses 4mA to represent the zero signal, and 20mA to represent the full scale of the signal, and signals lower than 4mA and higher than 20mA are used for various fault alarms.

Many controllers accept 0-20mA or 4-20mA current from various measuring instruments, and make a 0-20mA signal generator, which can calibrate or test many instruments.

2. Overall scheme design

2.1. Design goals

The single-chip microcomputer controls 4 PWM signals, and the output PWM signal controls the output voltage to be between 0-3.0V, which turns into a current signal through the current-voltage-to-current circuit, and the output current is between 0-20mA. Press the button to adjust the output current, oled displays the current value of 4 channels in real time.

2.2. Design ideas

The chip selects STM32F030C8T6, with 4 PWM outputs, which is cost-effective and supports rt-thread operating system. Here we choose the 2.1 version. OLED selects Zhongjingyuan Electronics 0-96 inch OLED.

2.2.1. Hardware circuit principle

Figure 1 is a typical circuit that converts voltage to current. The single-chip microcomputer outputs PWM to control the Vi voltage level between 0-3V, and the current flowing through RL is 0-20ma.

Figure 1 Voltage to current circuit

Figure 2 OLED display circuit

Since the MCU can be configured with a pull-up resistor, the buttons can be directly connected to the MCU.

Figure 3 Button circuit

2.2.2. Software design flow diagram

2.2.4. RTT usage

The initialization function of the key does not need to be added in the main function. Instead, the following function can be called INITDEVICEEXPORT(rthwkey_init) in key.c. The code looks clean and tidy. Through scons to tailor the configuration system, there is no need to manually delete the code, which is very easy to use. Finsh components are convenient for debugging and for customizing and adding serial commands. The code is linux programming style. Provide POSIX standard interface, easy to transplant upper code.

3. Physical display

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