how do you control a pneumatic actuator
The Fundamental Principle
By directing compressed air into one side of the actuator's piston or diaphragm and allowing air to exhaust from the other, you operate the actuator. A directional control valve is the term used to describe the device that accomplishes this.
- Two-position, straightforward on/off control
This is the most prevalent method for opening and closing a valve, such as a ball, butterfly, or gate valve.
Constituents:
3/2-Way Solenoid Valve: For single-acting spring-return actuators.
5/2-Way or 5/3-Way Solenoid Valve: For double-acting actuators.
A solenoid is an electromagnetic coil that, when activated, influences the internal filament of the valve.
Mechanism of Operation:
For a 5/2 valve on a double-acting actuator:
Signal turned on: The solenoid is activated. The actuator is relocated to Position A (e.g., OPEN) as air is ported to Side A, thereby discharging air from Side B.
Signal OFF: The solenoid de-energizes (or a second solenoid energizes). The actuator is relocated to Position B (e.g., CLOSED) as air is ported to Side B, thereby discharging air from Side A.
For a 3/2 valve on a spring-return actuator:
Signal ON: The spring is compressed by the air that is ported to the actuator.
Signal OFF: The actuator is returned to its fail-safe position by the spring after air is exhausted.
Control Signal: A discrete electrical signal, typically 24V DC or 120V AC, that is simple and on/off, and is generated by a PLC, relay, or switch.
- Local Control and Manual Override
Required for failure modes, testing, or maintenance.
Manual Override on Solenoid Valve: A toggle or knob on the solenoid valve that allows for physical movement without the use of electricity.
Lock-Up/Block Valves: Manual valves that are installed in the air lines to isolate the actuator and secure it in place.
Needle valves are installed on the exhaust ports of the control valve to regulate the pace of actuator movement, thereby preventing water hammer.
- Throttling/Regulating Control (Modulating)
Utilized for precise positioning, such as the regulation of flow with a globe valve. This necessitates the continuous regulation of atmospheric pressure.
Components: An I/P converter (current-to-pressure) is the primary component. It receives an analog electrical signal (e.g., 4-20 mA) and then produces a proportional pneumatic pressure (e.g., 3-15 psi or 0.2-1.0 bar).
Positioner: A closed-loop device that is more sophisticated. The actual position is compared to the demanded position (derived from a 4-20 mA signal) using a feedback lever. It modifies the air supply to the actuator until the actual position corresponds to the demand. This accounts for variations in friction and load.
Mechanism of Operation:
The desired valve position (0-100%) is represented by a 4-20 mA signal sent by the control system (PLC, DCS, controller).
This signal is received by the I/P converter or positioner.
It precisely modulates the air pressure that is directed to the actuator chamber, thereby ensuring that it is located at the precise corresponding location.
Control Signal: An analog electrical signal that is nearly always 4-20 mA.
- Specialized and Advanced Control
Smart/Digital Positioners: (e.g., PROFIBUS, Foundation Fieldbus, HART). These positioners are microprocessor-based and communicate digitally, enabling the configuration, diagnostics, and sophisticated control algorithms to be accessed from a central system.
Trip valves and solenoid lock-up systems are utilized for safety shutdowns (ESD). On a trip signal, a solenoid valve releases air from the actuator, causing it to either return to its fail-safe position (spring-return) or be powered by auxiliary air.
Double solenoid latching mechanisms: The actuator is moved by a pulse signal and remains in that position without continuous power, making it a practical solution for battery-backed systems.