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automation.isa.org http://automation.isa.org/2011/12/how-do-you-know-when-feed-forward-is-needed/

Block di agram of a typical PID control system. But how do

you know when feed -forward is nee de d?

How do you know when feed-forward is needed?

This guest post is authored by Greg McMillan.

In the ISA Automation Week Mento r Program I am providing guidance for extremely talented individuals f rom

Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a quest ion and the

answers each week. If you would like to pro vide additional answers, please send them to Susan Colwell at

ISA.

The third quest ion f rom Hecto r Torres in Mexico is:

How do you know when a f eed-f orward signal is

needed? What characterist ics should a feed-f orward

signal have? How does the lag and dead t ime get

taken into account?

1. If on a t rend recording a change in an input f low

or speed (load) asso ciated with a primary process

loop causes a signif icant change in the primary

process variable, there is an opportunity f or

f eedforward if the primary loop manipulates a secondary loo p f low or speed. If the primary loo p

manipulates a valve, the f eedforward gain is generally to o nonlinear unless there is the valve has

linear trim and a constant pressure drop. If the f eedback time delay (primary loop deadtime) minus

the f eedfo rward time delay is posit ive, a feedfo rward correction arrives early possibly causing

inverse response. The primary feedback time delay can be observed by momentarily putt ing the loo p

in manual and making a step change in output and then immediately putt ing the primary controller

back in auto and noting the time delay t ill a change in the primary process variable. The feedforward

time delay can be observed as t he time delay between the start of the load change and the start o f

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 f eedfo rward signal should be passed through a

deadtime block whose deadtime is the dif f erence between feedback and f eedfo rward time delays f or

dynamic compensation of the f eedforward for deadtime. If the diff erence is negative (f eedback time

delay smaller than f eedforward delay), the f eedforward arrives late and dynamic compensation is not

possible to make it arrive sooner. For a late f eedforward, the f eedfo rward gain should be reduced by

the ratio o f this negative dif f erence to the f eedback loo p deadtime. As the diff erence approaches

the loop deadtime, the feedforward gain approaches zero .

2. The computation of lead-lag dynamic compensat ion is more complicated than realized. Mos t o f the

contro l 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 f or dynamic compensat ion by lead-lag is more complicated. I would f irst

get the deadtime dynamic compensation right. If the response to a load change is in the same

direction without the f eedfo rward, the f eedback lag is larger than the f eedforward lag. A lead-lag can

be added to reduce the deviation f or the load change. The lead should be gradually increased. A lag

should be f irst added that is 1/10 of the lead to smooth out noise that is amplified by the lead.

Pages 222 232 in Shinskeys Process Contro l Systems 4thedition of f ers details on the dynamic

compensation o f f eedf orward signals f or load disturbances.

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3. The f eedf orward gain is the ratio of the change in controller output (without f eedf orward) to the

change in the feedfo rward signal. The ratio calculation should be in the engineering units o f the

f eedforward and contro ller output . The feedfo rward signal scale should be set to match the

controller output scale. The operator should be allowed to enter a ratio setpoint that becomes the

f eedforward gain. The operator should see the actual ratio as well. The ratio setpoint should be

adjusted to eliminate any persistent dif f erence between the actual ratio and the ratio setpoint.

4. Contrary to what is of ten portrayed in the contro l literature, a f eedforward multiplier is of ten more of

a problem than a solution because of scaling and measurement errors and the nonlinearityintroduced. The poss ible exception is the ratio cont rol o f inline plug flow systems (e.g. extruders)

and sheet lines with high accuracy and rangeability f low and/or speed measurements. Here the

nonlinearity of the feedforward multiplier compensates f or the nonlinearity of the processs

composition and temperature response. For mos t o ther systems, a feedf orward summer is more

f orgiving, compensates f or bias erro rs in measurements, and keeps the cont roller gain more

constant. The contro ller gain is proport ional to the ratio of the process time constant to the open

loop gain. Mos t people dont realize that in the cont rol o f temperature and composition in volumes

with some degree of mixing, the process time constant and the open loop gain are both inversely

proportional to to tal f low. A f eedfo rward summer simply biases the computed f eedfo rward

(f eedforward signal af ter dynamic compensat ion and multiplication by the feedforward gain) so that

the process gain retains the inverse relationship with f low.

For more inf ormation check out the Match/April 2011 InTech article Feedforward enables f lexible

sustainable manufacturing

Demo- Seminar Feedf orward Control (Deminar#11)

Post Flexible Manufacturing

Post Smart Adaptive Feedforward

Post Universal Concept Bias

http://modelingandcontrol.com/2011/01/universal_concept_-_bias_corre/http://modelingandcontrol.com/2011/01/smart_adaptive_feedforward_con/http://modelingandcontrol.com/2011/04/flexible_manufacturing/http://modelingandcontrol.com/deminars/http://www.isa.org/InTechTemplate.cfm?Section=Control_Fundamentals1&template=/ContentManagement/ContentDisplay.cfm&ContentID=85575