As there are more and more control methods and means, there are many methods used to control pneumatic actuators in actual industrial daily life and industrial control. The following are commonly used.
(1) Intelligent display control developed based on single-chip microcomputer The intelligent display is an instrument used to monitor the working status of the valve and control the valve during execution. It monitors the working status of the valve through two-way position sensors and determines whether the valve is open or closed. In the valve closing state, the number of the valve switch is recorded through programming, and there are two 4-20mA outputs corresponding to the valve opening and two normally open and normally closed output contacts. Through these output signals, the switching action of the valve is controlled. According to the requirements of the system, the intelligent valve display can be designed in three parts in terms of hardware: analog part, digital part, and button/display part.
1. The analog circuit part mainly includes three parts: power supply, analog input circuit, and analog output circuit. The power supply section supplies energy to the entire circuit, including energy supply for analog circuits, digital circuits and displays. In order to realize remote control of valve opening, it is necessary to transmit the valve opening information to other control instruments. At the same time, the control instrument can set the valve to a certain opening from a distance. The system requires a 4-20mA analog input signal and 1~2 channels of 4~20mA analog output signals. The analog input signal is converted into a digital signal corresponding to the valve opening through A/D conversion and then sent to the digital part of the microcontroller. It can be output after filtering in the microcontroller. The valve opening information is converted into an analog signal output through D/A conversion, which is used to connect a display to display the valve opening or connect to other control equipment. In this design system, all digital data adopts the serial input and output method. In order to save chip resources and space, the existing 4-channel DA chip is used when the input 4~20mA analog quantity is converted into digital quantity. Combined with the system resources of the microcontroller, it can be used by the 8-bit AD.
2. The digital circuit part mainly includes: single chip microcomputer, power-down protection, two channels of monitoring pulse input signals, and two channels of normally open and normally closed conversion contact outputs. In the design plan, the currently commonly used 51 series microcontroller AT89C4051 is selected. The AT89C4051 is a low-voltage, high-performance CMOS 8-bit microcontroller with 4K bytes of erasable and reprogrammable read-only flash memory. By combining a multi-functional 8-bit CPU flash memory in a single chip, it is fully compatible with 80C51 and 80C52 in terms of performance, instruction settings and pins.
Considering that when the system is powered down or restarted, some valve parameters previously set in the instrument need to be maintained, and the data memory in the microcontroller does not have a power-down storage function, so a chip with a power-down storage function was expanded outside the chip. X5045. X5045 is a programmable circuit that integrates three functions: watchdog, power monitoring and serial EEPROM. This combined design can reduce the circuit’s demand for circuit board space. The watchdog in X5045 provides protection for the system. When the system sends a fault and exceeds the set time, the watchdog in the circuit will respond to the CPU through the RESET signal. X5045 provides three time values for users to choose. Its voltage monitoring function can also protect the system from low voltage. When the power supply voltage drops below the allowable range, the system will reset until the power supply voltage returns to a stable value. The memory and CPU of X5045 can be interfaced through serial communication. There are 4069 bits in total, and data can be placed in 512×8 bytes. The pin arrangement of X5045 is shown in Figure 1. It has a total of 8 pins. The functions of each pin are as follows: CS: circuit selection terminal, active low level; SO: serial data output terminal; SI: serial data input terminal; SCK: serial clock output terminal; WP: write protection input terminal, active low level; RESET: reset output terminal; Vcc: power supply terminal; Vss: ground terminal. INA is the input signal, which is the valve pulse signal (<10mA) collected by the photoelectric sensor. The signal is filtered by the bypass capacitor and then sent to the optocoupler, which is converted into an output OUT voltage signal and sent to the microcontroller. The output voltage can directly enter the I/O port of the microcontroller. In the control, it is required that when both A and B pulses are received, it is considered to be a signal input. AB means forward rotation and BA means reverse rotation. It does not count when there is only one signal input. Two normally open and normally closed switching contact outputs. It is used to connect the solenoid valve and control the pneumatic actuator to open or close the valve accordingly by controlling the suction of the solenoid valve.
3. The display part mainly includes: microcontroller, 4-digit LED display, 3 status indicators (automatic, forward, reverse), 3 buttons (MODE/SET button, up button, down button). The display part uses AT89C4051 microcontroller, which is used to control the 4-digit LED display and communicate with the digital part of the microcontroller. It also needs to select and control the mode of the controller accordingly. The display is designed with 3 status indicators to show the status of the actuator: forward, reverse, and automatic; 3 buttons: MODE/SET button, up button, and down button to control the working mode and some parameters of the actuator. Initialization. These three parts are connected through interfaces to form a complete control system, which can control some actuators such as pneumatic motors. In practical applications, various performance indicators required in advance have been basically achieved.
(2) Systems controlled by PLC PLC is used more and more widely in control systems. Since this solution is developed on OMRON’s PLC, OMRON’s PLC will be used as an introduction. Hardware composition: 1 computer, 1 set of PLC (including CPU, I/O module, ID212, OC224, AD003 module), 2 relays, 2 solenoid valves, 1 pneumatic valve actuator. Its composition principle is: the PC is connected to OMRON’s PLC through RS-232 serial communication to program and monitor the PLC. The I/O modules of the PLC are respectively connected to input and output signals. The input module is connected to the two position sensors on the valve. The switching status of the valve is displayed in the order in which the indicator lights of the PLC input module ID211 light up. The input module receives two valve detection pulse inputs, namely pulse A and pulse B. In the running state, indicator light A lights up when pulse A is input, and indicator light B lights up when pulse B is input. The input sequence is AB, which means opening the valve. The input sequence is BA, which means closing the valve. The valve detection pulse A and B signals must be partially superimposed, otherwise the valve opening cannot be detected normally. Two relays are controlled through the output module OC225 of the PLC. The relay has two sets of normally open and normally closed output contacts, one set is the valve opening output contact, and the other set is the closing valve output contact. When opening the valve, when the valve opening is greater than or equal to the set valve limit value, the valve output contact will act. When the valve opening is less than the set valve limit value, the valve output contact will act. If the valve opening is smaller than the set valve limit value, the valve output contact will act. When the limit value is reached, the valve output contact is reset. When closing the valve, when the valve is closed to the zero position and there is no pulse input within 21s, the valve closing output contact will act; if there is a pulse input within 21s, the valve closing output contact will act after a delay of 21s. The switching of the two solenoid valves is controlled by the pull-in of the relay. After the solenoid valve is opened, the pneumatic valve actuator can be controlled to make the valve open or close accordingly. At the same time, the proximity sensor transmits the switching status of the valve to the PLC and compares it with the required valve opening until it meets the requirements.
Automatic zero return and automatic full adjustment: The control system has automatic zero return and automatic full adjustment functions. When the valve opening is less than the zero return range value or the valve opening distance from the full scale range is less than the full scale adjustment range value, and the time is greater than or equal to the set When the value reaches the stable time value, the PLC automatically controls the valve to return to zero or automatically adjust to full.
In the experiment, the opening of the valve is calculated by the position sensor on the valve. When the valve leaves sensor A first and then sensor B, it means the valve is closing. When the valve leaves sensor B first and then sensor A, it means the valve is opening. What the sensor receives is a pulse signal, and the switching status of the valve is recorded through the signal collected by the position sensor. Use the programming software CX-programmer to write the ladder diagram in the host computer, and then download the ladder diagram to the PLC for running. Control and monitor in the configuration software of the host sheet metal fabrication aluminum 6061 tablet computer housing. The valve switching amount can be input by the circle on the configuration software interface. The value is determined. After the configuration interface is completed, the control and actions of valve opening, valve closing, stop, main switch and other controls can be directly operated intuitively in the configuration interface. The working principle of the pneumatic valve actuator uses compressed air to promote the movement of multiple groups of pneumatic pistons in the actuator, transmitting the force to the cross beam and the characteristics of the inner curved track, driving the hollow spindle to rotate, and the compressed air disc is transported to each cylinder to change the inlet and outlet. The position of the cylinder can be adjusted to change the rotation direction of the spindle. According to the rotation torque required by the load (valve), the number of cylinder combinations can be adjusted to drive the load (valve) to work. The two-position five-way solenoid valve is usually used with a double-acting pneumatic actuator. The two-position position is controllable in two positions: on-off. The five-way solenoid valve has five channels for ventilation, one of which is connected to the air source, and two are connected to the double-acting actuator. The air inlet and outlet of the outer air chamber of the action cylinder are connected, and the two air inlets and outlets of the inner air chamber are connected. The specific working principle can be referred to the working principle of the double-acting pneumatic actuator.