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

HVAC Control in the New MillenniumThe integral mode eliminates the offset which exists in proportionalcontrol. Proportional control results in an offset since it disregardsthe past history of error and leaves the accumulated effect of past errorsuncorrected.The integral mode continues to use the total past history of theerror by integrating the area under the error curve. This integral correctionacts to eliminate the offset.The integral time is usually given in repeats/minute or in minutes/repeat.The integral setting of control loops in DCS systems isusually set from 0 to 300 repeats/minute, or from 0.2 seconds to about60 minutes or more in terms of minutes/repeat. The shorter the integraltime, the more often the proportional correction is repeated (more repeats/minute).Pure integral control or floating control is rarely used. In verynoisy valve position or flow control systems, the PI loop may be tunedwith low gain and high reset (integral). The proportional mode tends toact as a noise amplifier, while the integral mode integrates the areaunder the noisy error curve and provides a smoothed average.Reset WindupWhile the integral mode solves the problem of offset, it introducesanother problem. When the loop is idle, if the integral mode is allowedto stay active, it will eventually saturate, and its output will either dropto zero or rise to the maximum value of its power supply. Once saturated,the controller is not able to control and can upset the process bytrying to introduce a correction for the error, which it experienced duringits idle state.In these installations the controller must be provided with eitheran external reset, which prevents it from becoming idle, or with antiresetwindup, which protects it from saturating in the idle state.In some control systems, when a windup limit is reached, the integraltime (repeats/minute) is increased to speed the unwinding andreturn the system to normal operation. In DCS systems these operationsare implemented in software.In digital systems the integral is approximated by an accumulatorwhich continuously adds the deviation from setpoint to the previous©2001 by The Fairmont Press, Inc. All rights reserved.