Feedforward control

Here's how to augment feedback control with feedforward control.

03/01/2010


 

 

Feedback control

Driving a bumper car in reverse forces the driver to wait until after a disturbance has occurred before making a course correction.

Driving a bumper car in reverse forces the driver to wait until after a disturbance has occurred before making a course correction.


Source: Control Engineering


Driving forward allows the driver to visually measure an impending disturbance, anticipate its effect on the car

Driving forward allows the driver to visually measure an impending disturbance, anticipate its effect on the car's future trajectory adn correct course.

Source: Control Engineering

Traditional feedback control has been likened to driving a bumper car backwards. Without a view of the track ahead, the driver must look backward to decide where to go. Steering correction can only compensate after the car is off course.

 

Feedforward control is more akin to driving the car forwards so that the driver can make steering corrections just before reaching the next curve. When the driver sees that an obstacle ahead will disturb the desired trajectory, preemptive action can minimize the impending deviation.

 

In industrial applications, a feedforward controller can similarly minimize the effects of a disturbance, but only if the disturbance can be measured or calculated before it affects the process variable. A classic example is a steam distribution system where a central boiler provides steam at a constant pressure to plant equipment.

 

When an idle machine comes on line and starts drawing steam from the boiler, the pressure controller can preemptively turn up the heat and inject more water into the boiler if it knows how much steam the machine will need. If it were to rely strictly on feedback, the pressure controller would have to wait until the boiler pressure dropped before attempting to compensate for the new load.

 

The trick to effective feedforward control is accurately predicting the effects of measured disturbances. An experienced bumper-car driver can easily decide how to steer around an upcoming obstacle. With steam distribution, a feedforward pressure controller would need to know exactly when a particular machine will come on line and how much steam it will need.

 

Mathematical models can predict how a process responds to measurable disturbances and to the controller’s efforts. Since a model never can be 100% accurate and because other unmeasurable disturbances may also affect the process variable, a feedforward controller is almost always combined with a feedback controller, for the best of both worlds.

 

The feedforward controller makes its best guess about how to modify the control effort to compensate for an impending disturbance, and the feedback controller takes up the slack. It measures the net effect of the disturbance and the feedforward controller’s preemptive efforts then compensates for any deviations in the process variable that the feedforward controller could not prevent.

 

Feedforward controllers can be difficult to implement, especially if process behavior is not well understood or if disturbance variables are hard to measure or too many. A poorly-designed feedforward controller can also amplify a disturbance and make the feedback controller’s job harder.

 

Still, feedforward controllers can be worth the effort if disturbances are so frequent or so large that a feedback controller alone cannot keep up. A well-designed feedforward controller can reduce the effects of major load changes to mere blips in the process variable.

 



 

Author Information

Vance VanDoren, Ph.D., P.E., is Control Engineering consulting editor, at controleng@msn.com .



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Hannover Messe 2016: Taking hold of the future - Partner Country status spotlights U.S. manufacturing; Honoring manufacturing excellence: The 2015 Product of the Year Winners
Inside IIoT: How technology, strategy can improve your operation; Dry media or web scrubber?; Six steps to design a PM program
World-class manufacturing: A recipe for success: Finding the right mix for a salad dressing line; 2015 Salary Survey: Manufacturing slump dims enthusiasm
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
Warehouse winter comfort: The HTHV solution; Cooling with natural gas; Plastics industry booming
Managing automation upgrades, retrofits; Making technical, business sense; Ensuring network cyber security
Designing generator systems; Using online commissioning tools; Selective coordination best practices

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
click me