The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the U.S. This question comes from Hector Torres.

Hector Torres is a senior process and control engineer for Eastman Chemical Company in Tlaxcala, Mexico, with more than 20 years of experience.

Hector Torres’ Question

How do you know when a feedforward signal is needed? What characteristics should a feed-forward signal have? How does the lag and dead time get taken into account?

Greg McMillan’s Answer

If on a trend recording a change in an input flow or speed (load) associated with a primary process loop causes a significant change in the primary process variable, there is an opportunity for feedforward if the primary loop manipulates a secondary loop flow or speed. If the primary loop manipulates a valve, the feedforward gain is generally too nonlinear unless there is the valve has linear trim and a constant pressure drop. If the feedback time delay (primary loop deadtime) minus the feedforward time delay is positive, a feedforward correction arrives early possibly causing inverse response. 

The primary feedback time delay can be observed by momentarily putting the loop in manual and making a step change in output and then immediately putting the primary controller back in auto and noting the time delay till a change in the primary process variable. The feedforward time delay can be observed as the time delay between the start of the load change and the start of the process variable change with the primary loop in manual. To ensure the correction arrives in the process at the same time as the load change, the feedforward signal should be passed through a deadtime block whose deadtime is the difference between feedback and feedforward time delays for dynamic compensation of the feedforward for deadtime. 

If the difference is negative (feedback time delay smaller than feedforward delay), the feedforward arrives late and dynamic compensation is not possible to make it arrive sooner. For a late feedforward, the feedforward gain should be reduced by the ratio of this negative difference to the feedback loop deadtime. As the difference approaches the loop deadtime, the feedforward gain approaches zero.

The computation of lead-lag dynamic compensation is more complicated than realized. Most of the control literature incorrectly shows the measured disturbance entering downstream of the process directly into the measured primary process variable. Since most load disturbances enter as a process, the computations for dynamic compensation by lead-lag is more complicated. I would first get the deadtime dynamic compensation right. If the response to a load change is in the same direction without the feedforward, the feedback lag is larger than the feedforward lag. A lead-lag can be added to reduce the deviation for the load change. The lead should be gradually increased. A lag should be first added that is 1/10 of the lead to smooth out noise that is amplified by the lead. Pages 222–232 in Shinskey’s Process Control Systems fourth edition offers details on the dynamic compensation of feedforward signals for load disturbances.

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The feedforward gain is the ratio of the change in controller output (without feedforward) to the change in the feedforward signal. The ratio calculation should be in the engineering units of the feedforward and controller output. The feedforward signal scale should be set to match the controller output scale.

The operator should be allowed to enter a ratio setpoint that becomes the feedforward gain. The operator should see the actual ratio as well. The ratio setpoint should be adjusted to eliminate any persistent difference between the actual ratio and the ratio setpoint.

Contrary to what is often portrayed in the control literature, a feedforward multiplier is often more of a problem than a solution because of scaling and measurement errors and the nonlinearity introduced. The possible exception is the ratio control of inline plug flow systems (e.g. extruders) and sheet lines with high accuracy and rangeability flow and/or speed measurements. Here the nonlinearity of the feedforward multiplier compensates for the nonlinearity of the process’s composition and temperature response.

For most other systems, a feedforward summer is more forgiving, compensates for bias errors in measurements, and keeps the controller gain more constant. The controller gain is proportional to the ratio of the process time constant to the open loop gain. Most people don’t realize that in the control of temperature and composition in volumes with some degree of mixing, the process time constant and the open loop gain are both inversely proportional to total flow. A feedforward summer simply biases the computed feedforward (feedforward signal after dynamic compensation and multiplication by the feedforward gain) so that the process gain retains the inverse relationship with flow.

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Additional Mentor Program Resources

See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

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