1. Introduction Currently, China's coal production mechanization is developing rapidly. The application of fully mechanized mining equipment is a measure to improve efficiency and safety. A major factor affecting the operating rate is the effectiveness of the support's control over the roof of the working face. Therefore, it is essential to monitor and control the mine pressure at the fully mechanized mining face. To achieve this, it is necessary to first monitor the actual working condition of the hydraulic supports at the underground working face, process and analyze the detection data, evaluate its effectiveness, and take corresponding measures to improve the operating rate and increase production. This paper focuses on monitoring the pressure of fully mechanized hydraulic supports and develops a pressure measurement and control system based on a single-chip microcomputer. 2. Functional Design of the Pressure Measurement and Control System The pressure measurement and control system is used to monitor the support pressure. Each measurement and control system is equipped with four sensors, which can be connected to the support column, the balance jack, and the hydraulic chamber of the front beam jack via high-pressure oil pipes. After receiving the data acquisition command from the communication measurement and control system, the pressure measurement and control system collects the pressure from the four channels, transmits it to the communication measurement and control system, and then transmits it to the surface. The pressure measurement and control system is equipped with buttons. When pressed, the pressure values ​​of the four channels can be displayed in a cycle on the LCD display window. 3 Structural design of the pressure measurement and control system Figure 1 Block diagram of the pressure measurement and control system The structure of the pressure measurement and control system is shown in Figure 1. It is based on the 80C51 microcontroller and includes sensors, optocouplers, multiplexers, LCD displays, SRAM, EPROM, automatic reset circuit, RS-485 interface circuit and high-efficiency power supply circuit. The design features of each component are introduced below. 3.1 Sensor The sensor adopts the piston-transmitting large-range resonant string hydraulic sensor designed above. The sensor output amplitude is a 5-volt rectangular wave. 3.2 Signal input circuit (1) Multiplexer (multi-channel data selector) 74HC151 The 74HC151 is an 8-to-1 data selector. It has 8 data input terminals D0-D7, 1 strobe terminal S, 3 data selection terminals A, B, C and 2 output terminals Y, W. When the three data selection terminals A, B, and C change from 000 to 111, different channels can be selected. (2) Signal input circuit: Taking channel D0 as an example, its circuit is shown in Figure 2. Figure 2 Signal input circuit As can be seen from Figure 2, the sensor frequency signal is coupled to the input pin D0 of 74HC151 through optocoupler 6N139. The P1.0 and P1.1 output control codes of 80C51 select one of the D0-D3 inputs and use the T0 of the microcontroller to measure the signal frequency. 3.3 80C51 external expansion of 8KB EPROM and 8KB SRAM In the pressure measurement and control system, 80C51 expands 8K EPROM (27C64) and 8K SRAM (6264) as external program memory and data memory. The lower 6MHz is selected as the working frequency of 80C51 microcontroller, which can meet the data acquisition requirements and reduce the power consumption of microcontroller. The connection diagram of the expansion system is shown in Figure 3. Figure 3. Pressure Measurement and Control System 80C51 Expansion System. In the 80C51 microcontroller, external I/O ports are expanded as external RAM, and the addressing method is exactly the same as that for expanding external RAM. In addition to the external RAM, this circuit also has an LCD display as an external I/O device. Therefore, line addressing alone is insufficient; decoding addressing should be used. 3.4 LCM Dot-Matrix LCD Module Interface Design. LCM dot-matrix LCD modules can display many characters, including Chinese characters, and are therefore widely used in intelligent measurement and control instruments. This system uses the EA-D20040AR dot-matrix LCD module manufactured by EPSON. It consists of a TN-type LCD display, a CMOS driver, and a CMOS controller. The module integrates a character generator and data storage, uses a single ±5V power supply, and has an internal character library capable of displaying 96 ASCII characters and 92 special characters. The interface circuit between the EA-D20040AR and the 80C51 microcontroller is shown in Figure 4: Figure 4 Interface circuit between EA-D20040AR and 80C51 3.5 Power Supply Circuit The communication measurement and control system and all pressure measurement and control systems of this system share the same intrinsically safe power supply. Therefore, line losses should be minimized and power supply efficiency improved as much as possible. For this purpose, in addition to the power supply for the control sensors, the pressure measurement and control system also uses the high-efficiency, +5V adjustable step-down regulator MAX639 from MAXIM Corporation of the United States. This regulator can convert the battery voltage between +5.5V and +11.5V to +5V and provide 100mA output current across the entire voltage range, with a quiescent current of only 10μA and an efficiency higher than 90%. The MAX639 requires few external components: a small, inexpensive inductor, an input bypass capacitor, a filter capacitor, and a Schottky diode. No compensation components are needed. It is essentially a step-down DC-DC converter. When the switch is closed, the voltage applied to the inductor is equal to V+ minus VOUT. The current through the inductor ramps up, thus storing energy in the inductor's electric field. This current also flows into the output filter capacitor and the load. When the switch is open, this current flows through the inductor in the same way, but since the switch is now open, it must flow through the diode. When the switch is open, the inductor only supplies the load current, and this current decreases to zero as the energy stored in the inductor's magnetic field is transferred to the output filter capacitor and the load. 3.6 RS-485 Interface Circuit In this system, the communication measurement and control system and each pressure measurement and control system belong to a master-slave communication network. To suit this long-distance, multi-point, and highly interference-prone communication environment, an RS-485 interface circuit is adopted. The MAX483 interface circuit selected in this system is a low-power transceiver manufactured by MAXIM specifically for RS-485 communication. It contains one driver and one receiver, and its features include a driver with reduced slew rate, minimizing EMI (electromagnetic interference) and reducing the impact of improper cable termination. It can transmit data error-free at speeds up to 250kb/s. 3.7 Automatic Reset Circuit Since the pressure measurement and control system operates continuously downhole, an automatic reset circuit, i.e., a watchdog circuit, is introduced to prevent "crashes" caused by unexpected interference. Many types of watchdog circuits exist; to reduce the number of components, this system selects the MAX706. The MAX706 ensures reset during power-on and prevents microprocessor code execution errors under power-down or voltage-drop conditions. During power-on, once Vcc reaches 1V, a low-level output is guaranteed. When Vcc rises above the reset threshold voltage, an internal timer takes approximately 200ms before the output is allowed to return to the low level. As soon as VCC drops below the reset threshold voltage, the output returns to the low level. The MAX706 watchdog circuit also monitors the microprocessor's operation. If the microprocessor does not trigger the watchdog input (WDI) within 1.6 seconds and WDI is not in a tri-state, WDO will go low. 4. Pressure Measurement Control System Software Design The pressure measurement control system program consists of a main program and several subroutines. The subroutines mainly include a frequency measurement subroutine, a pressure calculation subroutine, a data display subroutine, and a serial communication subroutine. 4.1 Frequency Measurement Subroutine: This system uses the second method, the fixed TM multiple-period measurement method, for two scenarios of multi-cycle synchronous measurement by the microcontroller. 4.2 Pressure Calculation Subroutine: After the microcontroller measures a signal frequency, it retrieves the corresponding pressure and constants A, B, and f0, and calculates the pressure P (in megapascals) using the formula or converts it to other units. 4.3 Data Display Subroutine: The microcontroller outputs the calculated pressure value to the LCM LCD display module for display. The initialization program for the LCM LCD display module EA-D20040AR is as follows: START: MOV DPTR, #2000H; Set instruction register address MOV A, #38H; Set function, data is operated in 8-bit mode, 2-line display, 5X7 dot matrix MOVX @DPTR, A MOV A, #06H; Set input mode, set AC to incremental mode, display does not move MOVX @DPTR, A MOV A, #OEH: Set display on/off control, display on, display cursor, flashing off MOVX @DPTR, A MOV A, #01H; Total clear, clear screen, set AC address to zero MOVX @DPTR, A RET 4.4 Pressure Measurement Control System Serial Communication Subroutine In this system, the communication measurement control system and the communication measurement control system are multi-machine communication. The communication measurement control system is the master, and each pressure measurement control system is a slave. The press sends data, and the communication measurement control system receives data. The interrupt-mode communication program design for the pressure measurement and control system (slave unit) is as follows: After setting up the initialization work related to serial communication reception and interrupts in the main program, it waits for an interrupt. Upon receiving an address frame, it initiates an interrupt and enters the communication service program. Subsequent reception or transmission uses a polling method until the current communication ends, at which point it returns to the main program via an interrupt. If the slave unit is not ready to transmit or an illegal command occurs, it also returns from the interrupt and prepares in the main program. The master unit should re-establish contact with the slave unit, causing the slave unit to re-enter the serial port interrupt. The author's innovation lies in the fact that the pressure measurement and control system designed in this paper uses an 80C51 microcontroller as its core, including sensors, optocouplers (6N139), data switches (74HC151), a dot-matrix LCD module (EA-D20040AR), RS-485 interface circuits, and high-efficiency power supply circuits. After receiving the data acquisition command from the communication sub-unit, the measurement and control system collects the pressure data from four channels, transmits it to the communication sub-unit, and then the communication sub-unit transmits it to the ground, achieving rapid monitoring, processing, and timely feedback. References: [1] Tang Huiqiang, Kong Zhaolin. Design of Foundation Fieldbus Pressure Measurement System [J]. Microcomputer Information, 2007, 6-1: 194-195 [2] Deng Hongbin. MSC121X System-Level Microcontroller Principles and Applications. 1st Edition, Beijing: Machinery Industry Press, 2004, 1-6 [3] Tang Huiqiang. Development of Precision Pressure Transmitter. Measurement and Control Technology, 1999, 6-18: 63-64. [4] SensoNor, An In Infineon Technologies Company, Product Specification, SP12 TYRE PRESSURE SENSOR [J], www.infineon.com, 2003