Mechatronics - Control design: Pervasive and perplexing

Feedback, feedforward, and a disturbance observer get the job done.


Kevin C. Craig, Ph.D., Marquette UniversityControl is a hidden, enabling technology that is present in almost every engineered system today. Despite this fact, control system design is still mysterious and often falls in the domain of a specialist. Today, every engineer must know how to create, implement and integrate a control system into a design from the start of the design process. An engineer needs to understand how to balance performance, low cost, robustness and efficiency to effectively accomplish these goals.

Evaluating a design concept is best done through modeling, not by building and testing, as modeling provides true insight on which to base design decisions. There is a hierarchy of models possible of varying complexity and fidelity, but a simple design model which captures essential attributes is the most useful, i.e., dominant dynamics. An integrated control system can enhance a design through stabilization, command following, disturbance and noise rejection, and robustness. All of this can be accomplished through a combined approach, rather than trying to accomplish all with a single feedback controller, as is too often the case.

Mechatronics: Feedback and feedforward diagram. Control Engineering, Plant Engineering, and Design NewsTo best understand this combined approach, I had extensive discussions with Dr. Rob Miklosovic, a leading mechatronics innovator at Rockwell Automation in Cleveland, OH. The diagram illustrates just such an approach.

The design model is typically used for both feedback and feedforward controller design. However, in practice, the physical system will deviate from that design model. A disturbance observer regards any difference between the physical system and the design model as an equivalent disturbance applied to the model. It estimates the disturbance and uses it as a cancellation signal. So in addition to enhancing disturbance rejection, the disturbance observer makes the physical system behave like the design model over a certain frequency range, thereby simplifying the design of the feedback and feedforward controllers. Since the design model inverse is not realizable, a unity gain, low-pass filter, specifying the observer bandwidth, is added.

Next, the feedback controller is designed solely to force dynamic consistency by mitigating the effects of model uncertainty and disturbances, usually with high gain and integral control. A common mistake is made in designing the feedback controller for desired output with no regard for robustness, only to find poor performance when applied to the physical system. However, once consistency is enforced, the desired output can be augmented with a feedforward controller, typically the dynamic model inverse, to recover the dynamic delay of the closed-loop system with no effect on stability or properties of the closedloop system.

The combination of a disturbance observer with both feedback and feedforward controllers is not new and many researchers have demonstrated its effectiveness. What needs to be done now is to bridge that theory/practice gap and put this technique into the hands of the mechatronics engineers responsible for creating the innovative systems we all need.

- Kevin C. Craig, Ph.D., Robert C. Greenheck, Chair in Engineering, Design & Professor of Mechanical Engineering, College of Engineering, Marquette University.

Visit the Mechatronics Zone for the latest mechatronics news, trends, technologies and applications.

Consulting-Specifying Engineer's Product of the Year (POY) contest is the premier award for new products in the HVAC, fire, electrical, and...
Consulting-Specifying Engineer magazine is dedicated to encouraging and recognizing the most talented young individuals...
The MEP Giants program lists the top mechanical, electrical, plumbing, and fire protection engineering firms in the United States.
Boiler basics; 2017 Product of the Year winners; Manufacturing facilities Q&A; Building integration; Piping and pumping systems
2017 MEP Giants; Mergers and acquisitions report; ASHRAE 62.1; LEED v4 updates and tips; Understanding overcurrent protection
Integrating electrical and HVAC for energy efficiency; Mixed-use buildings; ASHRAE 90.4; Wireless fire alarms assessment and challenges
Power system design for high-performance buildings; mitigating arc flash hazards
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Commissioning electrical systems; Designing emergency and standby generator systems; VFDs in high-performance buildings
As brand protection manager for Eaton’s Electrical Sector, Tom Grace oversees counterfeit awareness...
Amara Rozgus is chief editor and content manager of Consulting-Specifier Engineer magazine.
IEEE power industry experts bring their combined experience in the electrical power industry...
Michael Heinsdorf, P.E., LEED AP, CDT is an Engineering Specification Writer at ARCOM MasterSpec.
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me