HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Control Technology, Microelectronics and Nanotechnologysum. When an error exists over a long period of time, the integral cangrow to the maximum value the DCS memory can hold. This conditionis integral or reset windup.The controller output in this case will be dominated by the woundupvalue and will saturate with its control value either fully open orfully closed. Even when the error is removed or changes sign, thewound-up value continues to maintain its saturated output for a periodof time. The saturated value will continue until the negative error accumulationequals the previously accumulated positive error.When the controller is not in control, the integral will windup. Thiscan be initiated by the following:• failed control-loop communications,• sensor failure,• control constraint, and• process off or not working properly.Integral controllers should have an antiwind-up mechanism. Onetechnique is to specify a limit on the integral contribution to the totaloutput signal. Another strategy is to inhibit integral accumulation if theoutput reaches a limit. Either mechanism limits the integral windup toa value corresponding to full open or full closed. To accelerate the returnof the integral contribution to its proper value, some controllers makethe integral accumulation 10 to 20 times faster when coming off a limitingvalue.Reset FeedbackReset feedback is another technique where the integral of the erroris replaced with a filter on the output. In reset feedback there is no integralto windup. But, there can still be some windup to the upper orlower limits of the output.The filter can be on an external signal which would be identical tothe controller output if the control loop were closed. There is a windupto the edge of the saturation value and when control is recovered thereis no need for wind-down acceleration.©2001 by The Fairmont Press, Inc. All rights reserved.