Feedback control system:
A feedback system measures a value in a process and reacts to the change in the value measured.
The output of the process is measured with the help of a sensor and the sensor value is given to the controller to take a proper controlling action. A controller compares this sensor signal with a set point and generates a control signal. Actuators are the controlling devices used in a process so that the controller output is the actuator input signal. Actuators effect the plant directly by varying the manipulating variable. Controller action will be zero until the process variable meets setpoint.
In a feedback control system controller takes control action only after the process variables and disturbance effects the process and the control action is also given to the process directly, which is different in feedforward control.
Feedforward control system:
A feedforward controller avoids the slowness of the feedback control.
Using feedforward control the performance of control systems can be enhanced greatly.
Process variables such as pressure, level, flow, temperature are interrelated and so one variable may affect another as a disturbance in the process. Feedforward system measure important disturbance variables and take corrective action before they upset the process.
Here the setpoint is fixed in the feedforward controller after doing little complex mathematical derivations. The feedforward controller determines the needed change in the manipulated variable, so that, when the effect of the disturbance is combined with the effect of the change in the manipulated variable, there will be no change in the controlled variable at all. The disturbance is measured at the input side of the process and the manipulating variable also, so the controlling process is done before a disturbance affects the process.