One of the fundamental limitations of single-loop control is the fact that all control action is based on the error between the setpoint and the controlled parameter. If there is no error, then the control output will remain at its last value. To correct for process disturbance input using single-loop control, the disturbance input must impact the controlled parameter. Only after a change is seen in the controlled parameter will the manipulated parameter be adjusted to correct for the disturbance. However, if a measurement of the disturbance is available to the control system, then through the application of feedforward control, it is possible to anticipate the impact of the disturbance. Measured disturbances may be fed forward to the control system so that action can automatically be taken to correct for the measured disturbance before it impacts the controlled parameter, as shown below.
In practice, feedforward control is always applied in combination with feedback control because there are always some disturbances that aren’t measured, and for those, the feedback is still required. A key point when considering the use of feedforward control is that the process disturbance must be a measured input to the control system. In a basic feedforward strategy, a scaled value of the measured disturbance input is simply added to the feedback controller contribution to determine the manipulated parameter value.
The PID function blocks in Foundation Fieldbus devices and in many control systems support a feedforward input. The scaling of the input and addition of this input to the feedback contribution is done internally within the PID block. If the process response to changes in the manipulated and disturbance inputs can be characterized as a first order plus deadtime, then the difference in the process response may be compensated for by using a deadtime block and a lead-lag block in the feedforward path. Dynamic compensation may be added using a deadtime block and lead/lag block in the feedforward path as illustrated below.
The feedforward control workshop included in Chapter 13 of Control Loop Foundation – Batch and Continuous Processes, provides several exercises that may be used to further explore feedforward control. A steam heater process is used in the feedforward control workshop. By accessing the book’s web site, you may complete this feedforward control workshop using your web browser. The viewer below may be used to see the solution to this exercise.