Industrial process control system consists of four basic elements:
- A measurement to know the status of the condition of a process
- A controller to take action by considering the set value and measured value
- An output signal to manipulate the process that results from the controller
- The process itself that reacts to the signal (Input or output)
The two most important signals considered considered in industrial control systems are
- Process Variable (PV)
- Manipulating Variable (MV)
Process variable (PV) is an input signal to the automated controller, which is measured by field devices. Depending on this value, the controller action is decided by either the operator manually or through the automatic controller.
The manipulating variable (MV) is a variable to be manipulated in order to have control over the process variable. Suppose, for a particular flow control operation, the flow control valve position is the manipulated variable to control the flow rate (process variable PV).
The nature of the industrial control system is shown in the above figure wherein the process is controlled through sensory, controller and actuator systems. The controller gets the input signals from the process measurement devices like sensors. These values are compared with the set point values in the controller and the resultant is used to control the actuator in order to produce control input to control the process