Feedforward Vs Feedback Control

A feedforward, also spelled feedforward, is a component or conduit in a control system that transmits a controlling signal from a source in the system’s external environment to a load in another part of the system’s external environment.

This is frequently an external operator’s command signal. In contrast to a system with only feed-forward behavior, which adjusts the input to take account of how it affects the load and how the load itself may vary.

The control variable adjustment in a feed-forward system is not error-based. Instead, it is based on process knowledge in the form of a mathematical model of the process, as well as knowledge about, or measurements of, process disturbances.

For a pure feed-forward without feedback control method to be reliable, some criteria must be met: an external command or controlling signal must be accessible, and the effect of the system’s output on the load must be known (that usually means that the load must be predictably unchanging with time).

Because of the lack of feedback, pure feed-forward control is sometimes referred to as ballistic. It cannot be modified anymore; any corrections must be made via a fresh control signal. Cruise control, on the other hand, uses a feedback system to alter the output in reaction to the load it faces.

Control theory, physiology, or computing could all be related to these systems. Disturbances are measured and accounted for before they have a chance to affect the system via feed-forward or feedforward control. In the case of the house, a feed-forward system may detect the fact that the door has been opened and switch on the heater before the house becomes too cold.

If a window that wasn’t being measured was opened, the feed-forward-controlled thermostat might allow the home to cool down. Within the field of CPU-based automatic control, the phrase has a special meaning.

The subject of feedforward control as it relates to current, CPU-based automatic controls is often studied, but it is rarely used due to the difficulty and cost of establishing or delivering the mathematical model required to enable this sort of control. The terms open-loop control and feedback control are frequently used interchangeably.

Feedback control

The controller’s control action is dependent on feedback from the process in the form of the value of the process variable in closed-loop control (PV). This generates a controller output that switches the boiler on and off to keep the building at the proper temperature.

A feedback loop assures that the controller exerts a control action to modify the process variable to be the same as the Reference input or set point in a closed-loop controller. Closed-loop controllers are also known as feedback controllers because of this.

Feedback control is a common approach in the process industries.

The following are its key benefits.

  • Regardless of the source or type of disturbance, corrective action is taken as soon as the controlled variable deviates from the set point.
  • Feedback control necessitates only a basic understanding of the process to be regulated; in particular, a mathematical model of the process is not essential, but it can be highly beneficial in the design of control systems.
  • The PID controller is a versatile and reliable controller. Retuning the controller when the process circumstances change usually results in satisfactory control.

Feedback control, on the other hand, has a number of drawbacks:

  • After a deviation in the controlled variable occurs, no remedial action is conducted. Theoretically, complete control, in which the controlled variable does not vary from the set point during disturbance or set-point changes, is unattainable.
  • To compensate for the lack of predictive control action, feedback control does not give predictive control action.
  • For processes with large time constants and/or extensive time delays, it may not be sufficient. If substantial and frequent disruptions occur, the process may remain in a transitory state for the rest of its life, never reaching the desired steady state. Feedback control is not possible in some instances because the controlled variable cannot be assessed on-line.

Feedforward Control

Figure 1

The primary idea behind feedforward control is to monitor critical disturbance variables and take remedial action before they disrupt the process.

Feedforward control has a number of drawbacks:

On-line measurements of the disturbance variables are required. This isn’t possible in a lot of cases.

At the very least, an approximate process model should be available to employ feedforward control effectively.

We need to know how the controlled variable reacts to changes in the disturbance and manipulated variables, in particular. The correctness of the process model determines the quality of feedforward control. Theoretically, perfect feedforward controllers are capable of producing perfect c.

Example:

Figure 1 shows a boiler drum with a traditional feedback control mechanism. The feedwater flow rate is adjusted based on the level of the boiling liquid.

Because of the tiny liquid capacity of the boiler drum, this control system is very sensitive to quick changes in the disturbance variable, steam flow rate. As a result of steam demand, rapid disturbance shifts might occur.

The liquid level can be better controlled using the feedforward control system shown in the diagram below. The feedforward controller changes the feedwater flow rate after measuring the steam flow rate.

Feedforward control is usually used in conjunction with feedback control in practical applications.

Feedback trim compensates for flaws in the process model, measurement error, and unmeasured disturbances, whereas feedforward control reduces the consequences of quantifiable disturbances.

Difference between Feedback and Feed Forward control systems

1. Feedback Control System:

The feedback control system is basically a control system where the output depends on the feedback signal generated. The feedback control system is responsible for processing the feedback signal which further serves as input to the system.

Feedback control systems consist of various components such as resistors, transistors, and other electrical devices that perform basic functions. Feedback can be further divided into positive feedback and negative feedback.

2. Feed Forward Control System:

The feedforward control system is a system that transmits the signal to some external load. It rejects disturbances before they affect the controlled variables. It controls major disturbances and is sometimes used in conjunction with feedback systems.

Feedforward systems are vulnerable to modeling errors. It makes a direct measurement of disturbances in the system.

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