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Several control methods for pneumatic actuators I. Introduction As an actuator, the pneumatic motor plays a very important role in industrial production and industrial control. Pneumatic motors use air instead of electricity or hydraulics to generate power; they enable stepless speed adjustment, can start, stop, and reverse instantly, have an automatic cooling function, produce no electric sparks, and can operate in flammable and explosive environments with humid, dusty conditions and the presence of chemical, flammable, or volatile substances. They are used for driving purposes in industries such as mining, tunnels, paint factories, chemical plants, the petrochemical industry, biotechnology, pharmaceuticals, wafer manufacturing, semiconductors, fiber optics, armories, ships, and aquaculture. Since air is used as the power source, it is easily available, and the air used can be discharged directly into the atmosphere without causing pollution. Compressed air can also be supplied centrally and controlled from a distance. II. Working principle of pneumatic valve actuators: Compressed air is used to drive multiple sets of pneumatic pistons within the actuator, thereby transmitting force to the crossbeam and the internal curved track; this in turn causes the hollow main shaft to rotate. The compressed air is delivered to various cylinders, and by changing the positions of the inlet and outlet ports, the rotation direction of the main shaft can be altered. Depending on the rotational torque required by the load (valve), the number of cylinders used can be adjusted to enable the load (valve) to function properly. III. Control methods for pneumatic valve actuators As there are an increasing number of control methods and techniques available today, there are also many ways to control pneumatic actuators in practical industrial applications and industrial control systems. The most common methods include the following. (1) Control via an intelligent display unit developed based on a microcontroller. The intelligent display unit is used to monitor the operating status of valves and to control their operation; it monitors the valve’s status using two position sensors to determine whether the valve is open or closed. It records the numerical values indicating whether the valve is open or closed through programming, and it also features two 4–20mA outputs corresponding to the valve’s opening degree, as well as two normally open and normally closed output contacts. These output signals are used to control the opening and closing of the valve. According to the system requirements, the intelligent valve indicator can be designed in three hardware components: the analog section, the digital section, and the button/display section. 1. The analog circuit section mainly consists of three parts: the power supply, the analog input circuit, and the analog output circuit. The power supply section provides energy to the entire circuit, including the power needed for analog circuits, digital circuits, and the display. To achieve remote control of the valve opening, it is necessary to transmit the valve’s opening degree information to other control instruments; simultaneously, these control instruments must be able to set the valve to a specific opening degree from a distance. The system requires 1 analog input signal of 4–20mA and 1–2 analog output signals of 4–20mA. The analog input signal is converted into a digital signal corresponding to the valve opening degree through A/D conversion, and then sent to the microcontroller in the digital section; after filtering it in the microcontroller, the signal can be outputted. The valve opening information is converted via D/A conversion into an analog signal for output, which is used to display the valve opening on a meter or to connect to other control devices. In this design system, all digital data are processed via serial input/output. To save chip resources and space, the 4–20mA analog signals received as input are converted to digital form by utilizing existing 4-channel DA chips in combination with the microcontroller’s system resources, thereby employing an 8-bit ADC. 2. The digital circuit section mainly includes: a microcontroller, power loss protection, two input signals for monitoring pulses, and two output contacts that can switch between open and closed states. In the design scheme, the commonly used 51-series microcontroller AT89C4051 is selected. The AT89C4051 is a low-voltage, high-performance CMOS 8-bit microcontroller that features 4K bytes of erasable, reprogrammable read-only flash memory. By integrating a multi-functional 8-bit CPU flash on a single chip, it is fully compatible with the 80C51 and 80C52 in terms of performance, instruction set, and pins. Given that it is necessary to retain certain valve parameters previously set in the instrument in the event of a system power failure or restart, and since the data memory in the microcontroller does not have power-loss retention capabilities, a chip X5045 with such retention functionality was added externally. The X5045 is a programmable circuit that combines three functions: watchdog, power monitoring, and serial EEPROM. This integrated design helps reduce the amount of space required on the circuit board. The watchdog in the X5045 provides protection for the system; when a fault occurs and the specified time limit is exceeded, the watchdog within the circuit responds to the CPU via a RESET signal. X5045 provides three time values for users to choose from. Its voltage monitoring function also protects the system from low voltages; when the power supply voltage drops below the allowable level, the system will reset until the voltage returns to a stable value. The memory of X5045 and the CPU can be interfaced via serial communication. There are 4,069 in total, and the data can be stored in blocks of 512×8 bytes. Figure 1 shows the pin arrangement of X5045. It has a total of 8 pins, and the functions of each pin are as follows: CS: Circuit selection terminal, active at low level ； SO: Serial data output terminal ； SI: Serial data input terminal ； SCK: Serial clock output terminal ； WP: Write protection input, active low ； RESET: Reset output terminal ； Vcc: Power supply terminal ； Vss: Ground terminal. The circuit for detecting the pulse input signal is shown in Figure 2. Figure 2: Figure 2 shows the circuit for one detection pulse input signal (the other one is exactly the same). INA is the input signal, which is the valve pulse signal (>10mA) captured by the photoelectric sensor. After being filtered by the bypass capacitor, this signal is sent to the optocoupler, where it is converted into an OUT voltage signal that is then fed to the microcontroller. The output voltage can be directly connected to the MCU’s I/O ports. In control, it is required that both pulses from paths A and B be received before it is considered to be a signal input; AB indicates forward rotation while BA indicates reverse rotation. Counting is not performed when there is only one signal input. Two channels of normally open and normally closed switching contact outputs. It is used to connect the solenoid valve, and by controlling the activation of the solenoid valve, it enables the pneumatic actuator to perform the corresponding actions of opening or closing the valve. 3. The display section mainly includes: a microcontroller, a 4-digit LED display, 3 status indicator lights (auto, forward, reverse), and 3 buttons (MODE/SET button, up button, down button). The display section uses the AT89C4051 microcontroller to control a 4-digit LED display; it also communicates with the microcontroller in the digital section, as well as enabling the selection and control of the controller’s modes. The display is equipped with 3 status indicator lights to show the status of the actuator: forward, reverse, automatic ； 3 buttons: MODE/SET button, up button, down button, used to control the operating mode of the actuator and the initialization of certain parameters. These 3 components are connected through interfaces to form a complete control system that can control actuators such as pneumatic motors. In practical applications, the various performance metrics required in advance have been basically achieved. (II) Systems controlled by PLCs The use of PLCs in control systems is becoming increasingly widespread. Since this solution is developed based on OMRON’s PLCs, OMRON’s PLCs will be used as an example here. Hardware components: 1 computer, 1 set of PLC (including CPU, I/O modules, ID212, OC224, AD003 modules), 2 relays, 2 solenoid valves, 1 pneumatic valve actuator. Its working principle is as follows: A PC connects to the OMRON PLC via an RS-232 serial interface to program and monitor the PLC. The I/O modules of the PLC are connected to the input and output signals respectively; the input module is linked to the two position sensors on the valve, and the on/off status of the valve is indicated by the sequence in which the indicator lights of the PLC’s input module ID211 turn on. The input module receives two sets of valve detection pulse inputs, namely Pulse A and Pulse B. In operation, when pulse A is applied, indicator light A lights up; when pulse B is applied, indicator light B lights up. An input sequence of AB indicates opening the valve. An input sequence of BA indicates closing the valve. The pulse A and B signals for valve detection must be partially superimposed; otherwise, the valve opening cannot be detected properly. Two relays are controlled through the PLC’s output module OC225; these relays have two sets of normally open and normally closed output contacts – one set for opening the valve and the other set for closing it. When the valve is opened, the output contact for opening the valve activates when the valve’s opening degree is greater than or equal to the set limit value; it also activates when the valve’s opening degree is less than that set limit value. Meanwhile, the output contact for opening the valve resets when the valve’s opening degree is below the set limit value. When the valve is closed, the closing output contact activates once the valve reaches the closed position and no pulse is input within 21 seconds ； If a pulse is input within 21 seconds, the valve-shutting output contact will act after a 21-second delay. The operation of the two solenoid valves is controlled by the engagement of a relay; once the solenoid valves are activated, it is possible to control the pneumatic valve actuator to cause the valve to open or close accordingly. At the same time, the proximity sensor transmits the opening/closing status of the valve to the PLC, and compares it with the required valve opening degree until the requirement is met. Automatic zeroing and automatic full adjustment: The control system features automatic zeroing and automatic full adjustment functions. When the valve opening is below the zeroing range value, or when the distance from the valve opening to the full scale is within the fullness adjustment range and the time elapsed is equal to or greater than the set stabilization time, the PLC automatically controls the valve to perform zeroing or automatic full adjustment. In the experiment, the valve opening is calculated by the position sensor on the valve. When the valve leaves sensor A first and then sensor B, it indicates that the valve is closing. When the valve leaves sensor B first and then sensor A, it indicates that the valve is open. The sensor receives a pulse signal, and the on/off status of the valve is recorded based on the signal collected by the position sensor. The ladder diagram that records the open and close valve pulses is shown in Figure 3. Figure 3 shows that the ladder diagram is created using the programming software CX-programmer on the host computer, and then downloaded to the PLC for execution. Control and monitoring are carried out through the configuration software on the host computer, and the number of on/off cycles for the valves can be determined by entering the corresponding value in the interface of this configuration software. Once the configuration interface is ready, operations such as opening and closing valves, stopping the system, and controlling the main switch can be carried out in a straightforward and intuitive manner right on the configuration interface. IV. Conclusion The two control methods for pneumatic motors discussed earlier have been applied in the production control processes of various petrochemical plants, achieving the desired functional controls. The systems developed using microcontrollers feature a simple structure and are easy to control. Control systems developed using PLCs require a higher initial investment, but they offer better stability and control accuracy compared to systems developed with microcontrollers. This post was last edited by Mobei Yihai on 2009-4-6 09:39.]