ExperTune Library

White Paper: Modeling and Building a Feedforward Compensator with ExperTune's Software

Modeling the System

Modeling The Feedback Process

1. First collect data on the feedback process. Collect data from a setpoint change, or setpoint pulse, with the controller in automatic. Or collect data with the controller in manual and from a step test, pulse or doublet pulse test.
2. Click the Analysis button and observe the window showing the Identified Process Model. Note this process; it is the Feedback Process.

Modeling The Feedforward Process

1. Allocate two ExperTune trends: One to the load variable, and one to the process variable (see Extra Trends Setup ). The process variable is the variable that is being controlled to the loop setpoint. The load variable is the variable that has an effect on the process variable. For example, in a pH control loop, the incoming pH is the load variable and the outgoing pH is the process variable.
2. Allocate a new ExperTune loop (see Extra Loops Setup ). The CO of the new loop should be the load variable and the PV of the new loop should be the process variable.
3. Put the controller in Manual mode and collect data as though you were tuning this new loop. Arrange for a change in the load variable to cause a change in the process variable. Do not change the controller output during the test.
4. "Tune" the new ExperTune loop, but disregard the tuning values. Click the Analysis button and observe the window showing the Identified Process Model. Note this process; it is the Feedforward Process.

Use ExperTune Loop Simulator to design and simulate a Feedforward control scheme

1. In ExperTune Loop Simulator, select the Feedforward Loop tab on the main window.
2. In the loop window, select the Load Process tab. This is the Feedforward Process as defined above. Enter the Gain, Dead Time and Lags that were identified as described above.
3. In the ExperTune Loop Simulator loop window, select the Process tab. This is the process you obtained when you were tuning the feedback loop in the normal way. Enter the Gain, Dead Time and Lags of the feedback process.
4. In the ExperTune Loop Simulator loop window, select the Controller tab and set up your controller details.
5. In the ExperTune Loop Simulator loop window, select the Compensator tab. This is the suggested design of the feedforward compensator.
   In practice it is usually better to start out with a conservative compensator. It is better to under compensate than over-compensate. To make the design more conservative, reduce the compensator gain by roughly 1/3 and reduce the compensator lead by roughly half.
6. Simulate the entire feedforward system, until you have satisfactory Compensator values. Transfer these values to your control system. Remember to start out conservative in your design on the real system.