0 Introduction Sewage flowmeters have been rapidly applied in industrial fields in recent years, especially in the measurement of river water conveyance and sewage discharge flow. Because of its flexible and convenient measurement and high sensitivity, it is widely used in industry to measure various types of water flow. Slurry flow and other media. However, due to the special requirements of coal mine safety, there are few sewage flow meters that can be used in underground coal mines at present, for example, they cannot meet the mine explosion-proof and intrinsic safety requirements, the narrow measurement range, and the transmission signals are vulnerable to external magnetic fields. In response to these problems, a sewage flow meter circuit suitable for mine flow measurement was developed. It adopts a new type single-chip microcomputer to control, plus a dedicated intrinsically safe excitation circuit, with real-time detection and timely display and other functions, after a number of companies trial, performance and technical indicators have basically reached the advanced level of similar products at home and abroad.
1 Design Plan
1.1 Design Concept At present, the safety type of mine explosion-proof electrical products in China is mainly flameproof and intrinsically safe. Intrinsically safe design is to strictly control the spark energy generated by various parts of the circuit. Therefore, a dedicated intrinsically safe excitation circuit is used in the design to prevent the excitation current from causing sparks; a new low-power single-chip microcomputer is used to control the operation of the entire device to ensure Its stability and reliability; using a variety of electromagnetic shielding measures to prevent static electricity and electromagnetic interference to ensure the stability of the circuit.
1.2 System Circuit Hardware (1) The block diagram of the device uses a high-speed microcontroller module circuit (STM32F407), which has a strong floating-point operation, ultra-low power consumption, and power consumption of only 38.6mA.
It is used to control the whole sewage flow meter, including generating excitation pulse square wave signal, receiving voltage signal reflecting the flow from the probe, outputting 4~20 mA current signal for analog display instrument, CPU module with various interface circuits RS232, High-speed USB interface, etc.), operating keyboard and display interface circuit. The block diagram is shown in Figure 1. 
(2) Sensor In order to ensure the accuracy of measurement, the probe of the sensor is required to be streamlined. When the probe is inserted, the influence on the fluid is small, and it is considered that the sensor is in a non-blocking state. Specifically, a 25.4 mm stainless steel tube is used as the probe housing. The core of the excitation coil is made of a soft magnetic steel core. The coil is wound and sealed in a high-strength plastic shell that is a streamlined hemisphere with a pair of stainless steel electrodes. The excitation coils are connected. In order to avoid the interference caused by the inductive emission signal in the probe, the signal transmission lead, the coil and the electrode connection are shielded.
(3) Special circuit design 1 Excitation signal and drive circuit In order to ensure that the probe operates in a non-blocking state, the probe size should be as small as possible, so that the generated excitation current is also small, and the subsequent receiving circuit must have a sufficiently high input impedance. To ensure the sensitivity and anti-jamming of the sensor; In order to prevent AC 50 Hz power frequency interference, square wave current is selected as the excitation current, and its frequency can select 1/4 frequency frequency of 12.5 Hz, effectively suppressing power frequency interference.
The excitation signal is generated internally by the microcontroller. It outputs a 12.5 Hz square wave signal from pin 26 and receives a 3-pin signal from IC5 (LMD18200T). It is the drive module circuit of the excitation coil. The internal H bridge is used to drive the excitation current. The excitation coil between its 2 feet and 10 feet. A positive and negative symmetrical square wave current of 20 to 30 mA is formed on the excitation coil L, which is synchronized with the control square wave voltage. The square wave voltage and the exciting current generated by the fluid motion cutting magnetic flux line are completely synchronized, that is, they are synchronized with the square wave voltage of the control switch network to facilitate the synchronous demodulation of the signal in the receiving circuit. The circuit is shown in Figure 2. 
2 Intrinsically safe excitation coil circuit excitation current in the sensor is a dangerous source of electrical spark, sewage flow meter to be used in the mine, to ensure that the excitation circuit to achieve intrinsic safety requirements. When the current flows through the excitation coil, a magnetic field is generated inside the coil, and the magnetic field stores energy. When the circuit is disconnected, the energy stored in the coil is released in the form of a spark. Therefore, the energy stored in the coil must be discharged. Meet intrinsic safety requirements. A bidirectional diode (TVS tube P6KE6.8CA) is connected in parallel across the coil as a freewheeling branch to discharge magnetic energy stored in the coil, reducing the spark at the time of switching off, absorbing current, and releasing energy. In order to achieve intrinsic safety.
3 sensor amplifier circuit shown in Figure 3. The IC2 (SL28617) preamplifier is used to amplify the voltage signal sent by the sensor to reflect the flow rate, changing the Rin and Rfb in the figure.
The magnitude of the resistance can change the gain of the op amp. S1 is the excitation signal source, and R17 and R18 are the input bias resistors. The 9 and 16 pins of the op amp supply ±5 V respectively. The IC3 (ADS8320) connected to the output of op amp IC2 is a 16-bit high-speed A/D converter. The converter has a conversion speed of up to 16 kHz/s. The IC12 (ISL21090) connected between the preamplifier and the A/D converter is a three-terminal regulator.
1.3 The software software flow chart is shown in Figure 4. 
In the process of compiling the electromagnetic flow software, it is required to initialize the system first and then initialize the CPU. The CPU outputs the excitation pulse square wave signal to control the excitation current generated by the LM8200 to drive the excitation coil. Collect the sensor signal, send the sensor to reflect the magnitude of the voltage signal, after the IC2 preamplifier to amplify the signal, and then through the 16-bit A / D converter and SPI serial port into the microcontroller for processing, in the microcontroller to A / D real-time Acquisition, real-time filtering, and data splicing and data transfer, and finally after D/A conversion, are converted into a 4 to 20 mA current signal by IC4 directly or output as a voltage.
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