Process Control Basics: Feedforward and Closed Loop Control ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Process Control Basics: Feedforward and Closed Loop Control

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Feedforward Control
Feedforward control is a control system that anticipates load disturbances and controls them before they can impact the process variable (PV). It is a form of open loop control, as the process variable is not used in the control action. In feedforward control the major process variables(A1, A2 & A3) are fed into a model to calculate the manipulated variable (MV) required to control at setpoint (SP). For feedforward control to work effectively, the user must have a mathematical understanding of how the manipulated variable(s) will impact the process variable(s)
Below is a block diagram of a feedforward control loop. As can be seen here, the PV (controlled variable C) is a result of the control action:

In practical applications, feedforward control is never used in isolation rather it is used in combination with feedback or closed loop control. In the hybrid feedforward control, the imperfect feedforward control corrects up to 90% of the upsets, leaving the feedback system to correct the 10% bias left by the feedforward component.

Closed Loop Control
Closed loop control also known as feedback control is the most popular and common control system used for process control. In closed loop control, the process variable (PV) is used to determine control action. The block diagram below illustrates the basic principle of closed loop control:

The main goal of closed loop control is to measure the PV (process variable). Compare this with the SP (setpoint), which is the desired or target value; and determine an appropriate control action that results in a change of the OP (output) value of an automatic controller. The controller output(OP) acts on the manipulated variable (MV) which then acts on the process  with load disturbances (A1, A2 & A3) to correct any deviation or error from the set point (SP). A detailed description of how an automatic (feedback) control loop works is described in: How a Process Control Loop Works in Automatic Control Systems.
In most cases, the ERROR (ERR) term is used to calculate the output (OP) value.
          ERR = PV - SP
If ERR = SP -PV has to be used, the controller has to be set for REVERSE control action.

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