FEEDFORWARD CONTROL vs FEEDBACK CONTROL

Feedforward control and feedback control are the two predominant process control systems used in industries. Let's check the differences that exist between the feedforward and feedback control.

Feedforward control	Feedback control
Feedforward control measures one or more inputs of a process, calculates the required value of the other inputs and then adjusts it.	Feedback control measures the output of a process, calculates the error in the process and then adjusts one or more inputs to get the desired output value.
Feedforward control has to predict the output as it does not measure output. So, it is sometimes called as PREDICTIVE CONTROL.	The feedback control reacts only to the process error (the deviation between the measured output value and set point). So, it is called as REACTIVE CONTROL.
Feedforward control does not check how the adjustments of inputs worked in the process. So, it is referred to as OPEN LOOP CONTROL.	Feedback control measures the output and verifies the adjustment results. So, it is called as CLOSED LOOP CONTROL.
Feedforward control is not self-correcting. If the input adjustments failed to produce correct output, then the process will continue to produce the wrong output.	If the input adjustments failed to produce correct output, then the feedback control can identify the error and make further adjustments accordingly. This process will continue till the desired output is achieved. Therefore, feedback control is self-correcting.
Feedforward control takes corrective action before the disturbances entering into the process.	Feedback control takes corrective action only after the disturbances has affected the process and generated an error.
The feedforward control requires to measure and control more inputs. The process has to be stimulated quickly in order to predict the process output for effective control which demands sophisticated equipments. The additional instruments and control equipment makes it expensive.	As there is no need for any simulation of process and measurement of inputs, the feedback control requires less measuring instruments and control equipments comparatively. The expense is also less.
The feedforward control does not affect the stability of the system. A stable system without feedforward control would remain stable even after including it.	The feedback control may create instability of the system. The controller corrects the output deviations by trial and error method which results in the oscillatory response of the feedback loop.

Also read other control schemes

Inferential Control 

Self-Adaptive Control

Programmed or Gain Scheduled Adaptive Control

Adaptive Control - An Introduction

Override Control

Feedforward Control

Feedback Control

Cascade Control

Ratio Control