IV.1.5 Final Control Element

The hardware component of a control loop that resides between the process and the controller and implements the control action is called the Final Control Element (FCE). It receives the control signal from the controller and regulates the value of the manipulated variable accordingly. An ideal FCE should be an instantaneously operating hardware that does not induce any time lag between process and controller. However, in reality it is impossible to find such an instantaneously operating FCE. Nevertheless any FCE, whose time constant is very small, is considered to be a good hardware for a control system.

Various types of FCEs are available in process industries that are widely used in control applications. They can be largely classified based on their energy source, viz . Pneumatic, Hydraulic and Electric, etc .

A few of them are discussed in Module VII.

IV.2 Dynamic behaviour of feedback controller

Consider the generalized closed loop process given in the Fig IV.5.

Fig. IV.5: Generalized block diagram of feedback control loop of a chemical process