Applying Feedforward Control Successfully

If feedforward control is not part of nor been successfully applied as part of your overall process control strategy, you may want to give Modeling and Control blog’s Greg McMillan‘s recent article a read. The Intech magazine article, Feedforward control enables flexible, sustainable manufacturing, offers the reasons why, the benefits and pitfalls, common opportunities that may exist in your plant, special application techniques, and problematic applications for feedforward control.

For those not as versed in control strategies, feedforward control is early warning from some control loops impacted earlier in time by a disturbance in the process to control loops, which see the disturbance after some period of delay. Greg describes how it helps in process control:

Feedforward can preemptively move a process to match the flows and conditions (e.g., temperature and composition) of all important process streams on a process flow diagram (PFD) for a given product and production rate, thereby enforcing material, component, charge, and energy balances for fed-batch and continuous processes. Plant-wide flow feedforward control provides the fastest possible and least disruptive transition to optimum operating conditions.

When this control strategy is effectively applied, it can:

…provide a preemptive correction of any disturbance that can be measured. The disturbance can be due to maintenance, abnormal conditions, operator actions, sequences, and changes in load (feed), raw materials, recycle streams, utilities, and environmental conditions.

The issue is in getting the amount of feedforward signal right and it’s timing right. Have you ever had the experience of receiving advice from someone that you’d wished you had received sooner? The timing wasn’t what it needed to be to have maximum benefit. The same is true with feedforward control. If the signal comes too early or too late in relation to the disturbance, it can amplify the disturbance. Greg notes that:

Feedforward dynamic compensation must insure the timing of the feedforward signal is right. If the feedforward signal arrives too soon, the initial response of the PV will be in the opposite direction of the final response from the disturbance. The result is an inverse response. Feedback action tries to back out the feedforward correction, which results in a second peak in PV. If the disturbance arrives too late, a second disturbance is created from feedforward action. In both cases, an oscillation develops, and the error from the disturbance is increased.

He describes ways through the application of deadtime blocks in the control strategy or changing the valve response time or secondary loop to get the timing to be right. He showed how to make these adjustments in our last web demo/seminar, How to Setup and Adjust the Dynamic Compensation of Feedforward Signals Webinar.

Flow control is the most common application for feedforward:

…because the process input manipulated by most process loops ends up being a flow. The disturbance is most often a feed flow.

Greg believes that Coriolis flow meters provide the best feedforward signals because of their high accuracy, rangeability, mass measurement, temperature measurement, and:

…an incredibly precise density measurement that could become an inferential composition measurement. Coriolis meters are capable of providing smarter and more accurate feedforward signals offering the opportunity for live process flow diagrams on the operator interface to show the true state of the process.

The benefits in faster transitions to optimum operating states, improved quality, reduced energy usage, and more, make feedforward control an important tool for process automation engineers. Greg’s article is worth a close read to understand how to make it work effectively as a part of your overall control strategy.

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