DIFFERENCE BETWEEN FEEDBACK AND FEEDFORWARD CONTROL LOOPS

INTRODUCTION

There are so many control loops in the industries nowadays.In this session we are going to discuss about difference between feedback and feedforward controls loops

FEEDFORWARD CONTROL LOOPS

A feedback control loop is reactive in nature and represents a response to the effect of a load change or disorder. A forward control loop, on the other hand, responds directly to load changes and, therefore, provides improved control.
In the feedforward control, a sensor is used to detect changes in the process load or disturbances as they enter the system. The figure shows a block diagram of a typical control cycle.

The sensors measure the values of the load variables and a computer calculates the correct control signal for the existing load conditions and the process setpoint. The Feedforward control presents some important problems. Its configuration assumes that the disturbances are known in advance, that they will have sensors associated with them and that there will be no significant disturbances not detected. Therefore, feedforward control is more complicated and more expensive, and requires that the operator have a better understanding of the process than a standard feedback loop. Therefore, the anticipatory control is generally reserved for critical and well understood applications.

Advantages

  • Compensates for a disturbance before the process output is affected
  • Does not affect the stability of the control system

Disadvantages

  • Can not eliminate steady-state offset
  • Requires a sensor and model for each disturbance

 

FEEDBACK CONTROL LOOPS

In a feedback control loop, the variable to be manipulated is measured. This measured process value (PV) is then compared to an established point (SP) to generate an error signal (e = PV – SP). If there is a difference or error between the actual value and the desired value of the process, a process controller will take the necessary corrective action to return the process to the desired value. The figure shows a block diagram of a simple feedback control loop.

The measured process variable is detected or measured by the appropriate instrumentation, such as temperature, flow, level or analytical sensors. This measured value is then compared with the set point. The controller uses this comparison to properly adjust the manipulated variable by generating an output signal. The output signal is based in turn on any control strategy or algorithm that has been selected. Because in the process industries, the manipulated variable is usually a flow, the output from the controller is usually a signal for a flow control valve, as shown in the Figure. During the operation of the process, disturbances can enter the process and handle the process variable in one direction or another. The only manipulated variable is used to compensate for all the process changes produced by the disturbances. In addition, if changes occur in the set point, the manipulated variable is altered to produce the necessary change in the output of the process

Advantages

  • Very simple technique
  • Effective for all disturbances
  • Provides zero offset
  • Works with minimum knowledge of the process

Disadvantages

  • Does not take control action until the process output has deviated from set point
  • Affects the closed-loop stability

 

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