What is feedback and feedforward control system?

[sivaranjith]

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.

what is cascade control?

[vishal_jamdagni]

Hey there. Feed-forward, sometimes written feedforward, is a term describing an element or pathway within a control system that passes a controlling signal from a source in its external environment, often a command signal from an external operator, to a load elsewhere in its external environment. A control system which has only feed-forward behavior responds to its control signal in a pre-defined way without responding to how the load reacts; it is in contrast with a system that also has feedback, which adjusts the output to take account of how it affects the load, and how the load itself may vary unpredictably; the load is considered to belong to the external environment of the system.

A feedback loop is a common and powerful tool when designing a control system. Feedback loops take the system output into consideration, which enables the system to adjust its performance to meet a desired output response.

When talking about control systems it is important to keep in mind that engineers typically are given existing systems such as actuators, sensors, motors, and other devices with set parameters, and are asked to adjust the performance of those systems. In many cases, it may not be possible to open the system (the “plant”) and adjust it from the inside: modifications need to be made external to the system to force the system response to act as desired. This is performed by adding controllers, compensators, and feedback structures to the system.