Difference between Feedback and Feed Forward Control Systems

A system of components that is used to provide the desired response by controlling different parameters in the process is referred to as a control system. In a control system, the final output or response of the system is controlled by varying the input, and hence it named so.

Based on the control mechanism used, the control systems can be classified into the two types namely, feedback control system and feed forward control system.

The fundamental difference between the feedback control system and feed forward control system is that in the case of feedback control system, the corrective action is taken after the output gets affected by the disturbance, while in the case of feed forward control system, the corrective action is taken before the disturbance of the output.

In this article, we will discuss all the important differences between feedback control system and feed forward control system. But before discussing the differences, let’s have a look into their basics.

What is a Feedback Control System?

A type of control system in which the corrective action is taken after the occurrence of disturbance in the output, it is called a feedback control system. The feedback control system is a type of closed loop control system in which a feedback signal is used to regulate the behavior of the system. The block diagram of a feedback control system is shown in Figure 1.

In the case of a feedback control system, the system output is monitored and compared with a reference value. If there is any difference between the actual output and the desired output, then a control signal is generated to regulate the system to produce the desired output.

In simple words, a feedback control system is one in which the output of the system is sampled and fed back to the input side so that the system can produce the desired output.

Feedback control systems are widely used industrial automation and process control applications.

What is a Feed Forward Control System?

A feed forward control system is a type of control system in which the corrective action is taken before the occurrence of disturbances in the output of the system.

In the case of a feed forward control system, the control mechanism anticipates the variations in the input signal of the system and regulates the output of the system in advance of these variations. For making adjustments in the output, the feed forward control system does not relay on the feedback signal.

In other words, the control system which can predict variations in the input signals in advance and can adjust the system output based on the these predictions is referred to as a feed forward control system.

The block diagram of a typical feed forward control system is shown in Figure 2.

This type of control system is mainly used where the relationship between the input and output of the system is either known or predictable. One of the important features of the feed forward control systems is that they respond quickly to variations in the input signal. A heating system is a typical example of a feed forward control system.

After discussing the basics of feedback control system and feed forward control system, let us now discuss the important differences between them.

Difference between Feedback Control System and Feed Forward Control System

The following table highlights all the major differences between feedback control system and feed forward control system −

Parameter	Feedback Control System	Feed Forward Control System
Definition	A type of control system in which the corrective action is taken after the occurrence of disturbance in the output of the system is referred to as a feedback control system.	A type of control system in which the corrective action is taken before the occurrence of disturbance in the output of the system is referred to as a feed forward control system.
Adjustment in output	In the feedback control system, the output is adjusted according to the feedback signal.	In the feed forward control system, the output is adjusted according to the predication of variations in the input signal.
Nature of system	Feedback control system is reactive in nature, which means it responds to variations in the output after they have occurred.	Feed forward control system is proactive in nature. That means it responds to changes in the output before they have occurred.
Response speed	Feedback control systems are relatively slower to respond to variations in the input signal.	Feed forward control systems are faster to respond to changes in the input signal.
Corrective actions	Feedback control systems take the necessary corrective actions after the occurrence of disturbances in the output.	Feed forward control systems take the necessary corrective actions before the occurrence of disturbances in the output.
Adjustment of control variables	In the feedback control system, the control variables are adjusted based on errors occurred in the output.	In the feed forward control system, the control variables are adjusted based on predication of variations in the input signal.
Effectiveness against disturbances	Feedback control systems are more effective at dealing with disturbances that affect the system output.	Feed forward control systems are relatively less effective in compensating the disturbances that affect the system output.
Implementation ease	Feedback control systems are easier to implement.	Feed forward control systems are relatively complex to implement.
Type of system	Feedback control system is a type of closed loop control system because it measures changes in the output.	Feed forward control is a type of open loop control system as it does not measure changes in the output.
System stability	If the feedback loop of the system is not properly implemented, then it can cause instability of the feedback control system.	Feed forward control system is often a stable system.
Regulating system	Feedback control systems can be employed for regulating linear as well non-linear systems.	Feed forward control systems can be employed for regulating only the linear systems.
Suitability	Feedback control systems are better suited for controlling systems that have unpredictable disturbances in the output.	Feed forward control systems are better suited for controlling systems that have predictable disturbances in the output.
Applications	Feedback control systems are used in engineering and automation in different industries such as robotics, manufacturing, process control, etc.	Feed forward control systems are mainly used in special applications like in defense, aerospace, etc.

Conclusion

The most significant difference between feedback control system and feed forward control system is in the regulating action taken. In the feedback control system, the regulating action is taken after disturbances occurred in the output; whereas in the feed forward control system, the regulating action is taken before disturbances occurred in the output.

Overall, both feedback and feedforward control systems are used to regulate and control the behavior of dynamic systems to obtain the desired results.