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

HVAC Control in the New MillenniumDerivative ModeThe proportional mode considers the present state of the processerror and the integral mode uses its past history. The derivative modeanticipates its future state and acts on this forecast. This third controlmode became needed as the size of equipment increased with a greatermass and thermal inertia.A large process has more inertia or momentum which makes itdifficult to stop or reverse a control trend. The derivative mode tries topredict process errors before they evolve and take corrective action inadvance of the occurrence.When the error is constant, the derivative contribution to the outputsignal is zero. This is because the derivative contribution is based onthe rate at which the error is changing. When that rate is zero the derivativeis zero. The derivative contribution always corresponds to the slopeof the error curve.Derivative TimeThe derivative setting uses a derivative time (Td) which is thelength of time the derivative mode looks into the future. If the derivativemode is set for Td, it will supply a correction that will equal the correctionwhich the proportional mode would have generated at Td. Thelonger the Td setting, the further into the future the derivative-modepredicts and the larger is the corrective value. The derivative time canbe adjusted from a few seconds to hours.PD control loops are sometimes used in the slave controller in temperaturecascade systems when there is a need to increase the sensitivityof the slave loop. PID control is more widely used and applicationsinclude many temperature control systems.Limitations of Derivative ModeThe derivative mode acts upon the rate of error signal change andit can cause unnecessary upsets. It tends to react to sudden setpointchanges and will amplify noise. The control algorithm can be altered sothat the derivative acts on the measurement and not on the error. Thiswill reduce upsets.©2001 by The Fairmont Press, Inc. All rights reserved.