Most basic process control systems consist of a control loop as shown in the figure.
This has four main components which are:
- A measurement of the state or condition of a process
- A controller calculating an action based on this measured value against a pre-set or desired value (set point)
- An output signal resulting from the controller calculation which is used to manipulate the process action through some form of actuator
- The process itself reacting to this signal, and changing its state or condition
Two of the most important signals used in process control are called
- Process Variable or PV
- Manipulated Variable or MV
In industrial process control, the Process Variable or PV is measured by an instrument in the field and acts as an input to an automatic controller which takes action based on the value of it. Alternatively, the PV can be an input to a data display so that the operator can use the reading to adjust the process through manual control and supervision.
The variable to be manipulated, in order to have control over the PV, is called the Manipulated Variable. If we control a particular flow for instance, we manipulate a valve to control the flow. Here, the valve position is called the Manipulated Variable and the measured flow becomes the Process Variable.