'What to know about PID process controllers
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Control Valve Actuators Failure Modes. Control valves can be built from various combinations of valve actuator and valve body. The combination of actuator and valve body is usually chosen to provide a particular failure mode should the instrument air supply fail for any reason. The most common control valve actuator used in the industry is the diaphragm actuator. Diaphragm actuators, as in the case of valve bodies, can be classified as either direct or reverse acting. Any failure mode can be obtained with a combination of direct or reverse acting. The two most common failure mode of control valves are :.
Ryan G. Rosandich Jul 01, Hundreds of thousands of PID controllers are used each year to control industrial processes, and it is important to become familiar with their operation. What is a process controller? In simple terms, it's a device that measures an input and then tries to maintain it at a desirable value by adjusting an output device. Two controllers that we are familiar with in everyday life are the home thermostat and the automotive cruise control. The quantity that is measured and controlled is called the process variable PV.
Q: As I understand it, if the controller output increases when the measurement value rises, it is a direct-acting controller, and if controller output decreases when the measurement rises, it is a reverse-acting controller. In addition, at minimum output of the controller, the desired fail-safe action must be achieved.
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The output of a proportional controller is "directly" proportional to the error. The error is the difference between the measured variable and the set point. Therefore, if the error increases, the output of the controller is expected to increase. The relationship between the controlled variable and the measured variable is considered here. The actuator manipulates the controlled variable. If the control output is increased by a positive error, the controller is said to be direct acting. In the opposite case, the controller is said to be reverse acting when a positive error decreases the control output.
Control Hardware Electric-Pneumatic Actuation
In reverse acting control, the actuator output decreases if the error increases. Consider again level control of a tank but with an inflow valve.
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