Design of pressure measurement control system based on single chip microcomputer

At present, my country's coal production mechanization is developing rapidly. The application of fully mechanized mining equipment is a measure to improve efficiency and safety.

One of the main factors affecting the start-up rate is the control of the roof of the working face by the support. Therefore, it is necessary to monitor and control the mine pressure of the fully mechanized mining face. To do this, it is necessary to first monitor the actual working conditions of the hydraulic support of the underground working face, process and analyze the detection data, evaluate its effect, and take corresponding measures to improve the start-up rate and increase production. This paper takes monitoring the pressure of the fully mechanized mining hydraulic support as the research content, and develops a pressure measurement and control system based on a single-chip microcomputer.

2. Functional design of pressure measurement 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, balance jack, and oil pressure chamber of the front beam jack through 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 of the four channels and transmits it to the communication measurement and control system, which then transmits it to the ground. The pressure measurement and control system is equipped with a button. When pressed, the pressure values ​​of the four channels can be cyclically displayed in the LCD display window.

3 Structural design of pressure measurement control system

Figure 1 Block diagram of pressure measurement control system

The structure of the pressure measurement control system is shown in Figure 1. It is based on the 80C51 single-chip microcomputer and includes sensors, optocouplers, multi-channel data switches, LCD liquid crystal displays, SRAM, EPROM, automatic reset circuits, RS-485 interface circuits, and high-efficiency power supply circuits. The following introduces their respective design features.

3.1 Sensors

The sensor uses the piston pressure transmission large-range resonant string hydraulic sensor designed previously, and the sensor outputs a rectangular wave with an amplitude of 5 volts.

3.2 Signal input circuit

(1) Multiplexer (Multiplexer) 74HC151

74HC 151 is an 8-to-1 data selector, which 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 3 data selection terminals A, B, C change from 000 to 111, different channels can be selected.

(2) Signal input circuit: Taking D0 channel as an example, its circuit is shown in Figure 2.

Figure 2 Signal input circuit

As shown in Figure 2, the sensor frequency signal is coupled to the input pin D0 of 74HC151 through the optocoupler 6N139. The P1.0 and P1.1 of 80C51 output control codes, select one of the D0--D3 inputs, and use T0 of the microcontroller to measure the signal frequency. [page]

3.3 80C51 external expansion 8KB EPROM and 8KB SRAM

In the pressure measurement 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 operating frequency of the 80C51 microcontroller, which can meet the data acquisition requirements and reduce the power consumption of the microcontroller. The expansion system connection diagram is shown in Figure 3.

Figure 3 Pressure measurement control system 80C51 expansion system

In the 80C51 microcontroller, the external I/O port is expanded as an external RAM, and the address selection method is exactly the same as the method of expanding the external RAM. In addition to the external RAM, this circuit also has an LCD display as an external I/O device. Therefore, it is not enough to use only the line address selection method, and the decoding address selection method should be used.

3.4 LCM dot matrix liquid crystal display module interface design

LCM dot matrix liquid crystal display module can display many characters and Chinese characters, so it has been widely used in intelligent measurement and control instruments. This system uses the EA-D20040AR dot matrix liquid crystal display module produced by EPSON. It consists of a TN type liquid crystal display, a CMOS driver and a CMOS controller. The module integrates a character generator and data storage, and is powered by a single ±5V power supply. It has a character library that can display 96 ASCII characters and 92 special characters. The interface circuit between EA-D20040AR and the single-chip microcomputer 80C51 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 of this system and all the pressure measurement and control systems share the same intrinsically safe power supply, and the line loss should be reduced as much as possible to improve the power supply efficiency. To this end, in addition to controlling the power supply of the sensor, the pressure measurement and control system also uses the high-efficiency, +5V output adjustable step-down voltage regulator MAX639 from MAXIM of the United States, which can convert the battery voltage between +5.5~+11.5V to +5V output and provide 100mA output current in the entire voltage range, with a quiescent current of only 10μA and an efficiency of more than 90%.

MAX639 requires fewer peripheral devices, namely a small, cheap inductor, an input bypass capacitor, a filter capacitor, and a Schottky diode, and does not require any compensation components. 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, and the current through the inductor ramps up, thereby storing energy in the inductor's electric field. This current also flows into the output filter capacitor and the load; when the switch is disconnected, this current flows through the inductor in the same way, but because the switch is disconnected at this time, it must flow through the diode. When the switch is disconnected, the inductor only supplies 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.

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3.6 RS-485 Interface Circuit

In this system, the communication measurement control system and each pressure measurement control system belong to the master-slave communication network. In order to adapt to this long-distance, multi-point, and high-interference communication environment, the RS-485 interface circuit is used. The MAX483 interface circuit selected in this system is a low-power transceiver produced by MAXIM specifically for RS-485 communication. It contains a driver and a receiver. Its characteristics are that it has a driver that reduces the conversion rate, which can minimize EMI (electromagnetic interference) and reduce the impact caused by improper cable terminals. It can transmit data without errors at a rate of up to 250kb/s.

3.7 Automatic reset circuit

Since the pressure measurement and control system runs continuously underground, in order to prevent "freeze" caused by unexpected interference, an automatic reset circuit, namely a watchdog circuit, is introduced into the pressure measurement and control system. There are many types of watchdog circuits. In order to reduce the number of components, this system uses MAX706.

The MAX706 ensures reset during power-up and prevents microprocessor code execution errors in power-down or voltage drop conditions. Once Vcc reaches 1V at power-up, the output is guaranteed to be low. When Vcc rises above the reset threshold voltage, it takes about 200ms for an internal timer to allow it to become the current level. As long as VCC drops below the reset threshold voltage, it becomes a low level.

The MAX706 watchdog circuit also monitors the operation of the microprocessor. If the microprocessor does not trigger the watchdog input (WDI) within 1.6S and WDI is not in a three-state state, WDO will become a low level.

4. Pressure measurement control system software design

The program of the pressure measurement control system consists of a main program and several subprograms. The subprograms mainly include frequency measurement subprogram, pressure calculation subprogram, data display subprogram, serial communication subprogram, etc.

4.1 Frequency measurement subroutine

There are two situations of single-chip microcomputer multiple-cycle synchronization method measurement. This system uses the second method, that is, the multiple-cycle measurement method of fixed TM.

4.2 Pressure calculation subroutine

When the single chip microcomputer measures a signal frequency, the corresponding pressure box constants A, B, f0 are taken out, and the formula

Calculate the pressure P (in MPa) or convert it into other units of pressure.

4.3 Display Data Subroutine

The single chip microcomputer outputs the pressure value calculated previously to the LCM liquid crystal display module for display. The initialization procedure of the LCM liquid crystal display module EA-D20040AR is as follows:

START: MOV DPTR, #2000H; set the instruction register address

MOV A, #38H; reset function, data is operated in 8 bits, 2 lines display, 5X7 dot matrix

MOVX @DPTR, A

MOV A, #06H; Set input mode, set AC to incremental mode, the display screen does not move

MOVX @DPTR, A

MOV A, #OEH: Set display on/off control, display on, display cursor, flash off

MOVX @DPTR, A

MOV A, #01H; Clear all, clear the screen, set AC address to zero

MOVX @DPTR, A

RIGHT

4.4 Serial communication subroutine of pressure measurement control system

In this system, the pressure measurement and control systems and the communication measurement and control systems are multi-machine communications. The communication measurement and control system is the host, and the pressure measurement and control systems are slaves. The press sends data, and the communication measurement and control system receives data.

The design idea of ​​the pressure measurement control system (slave) interrupt mode communication program is: after setting up the initialization work related to serial communication reception and interruption in the main program, wait for the interruption, start the interruption to enter the communication service program after receiving a frame of address information, and use the query mode in the subsequent reception or transmission communication until the communication ends and returns to the main program from the interruption. If the slave is not ready to send or an illegal command appears, it will also return from the interruption and prepare in the main program. The host should reconnect with the slave to make the slave enter the serial port interruption again.

The author's innovation points:

The pressure measurement and control system designed in this paper is based on the 80C51 single-chip microcomputer, including sensors, optocouplers 6N139, data switches 74HC151, dot matrix LCD display modules EA-D20040AR, RS-485 interface circuits and high-efficiency power supply circuits. After receiving the data acquisition command from the communication extension, the measurement and control system collects the pressure of the four channels and transmits it to the communication extension, which is then transmitted to the ground by the communication extension, realizing rapid monitoring and processing and timely feedback of information.