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

Control Technology, Microelectronics and NanotechnologyExcessive noise and step changes in the measurement can be correctedby filtering out any change in the measurement that occurs fasterthan the maximum speed of response of the process. DCS systems providesoftware with adjustable filters for each variable. The time constantof these filters is usually adjustable from 0 to 100 seconds. In analogcontrol systems, inverse derivative modules are often used.Inverse Derivative ModeThis control action is used on fast processes. The inverse derivativemode is the opposite of the derivative mode. While the output of thederivative mode is directly proportional to the rate of change in error,the output of the inverse derivative mode is inversely proportional tothe rate of change in error.Inverse derivative is used to reduce the gain of the controller athigh frequencies and is useful in stabilizing a control loop. The dynamicgain of the derivative function is selected as 0.5 or less. The gain of theinverse derivative controller decreases from the proportional gain at lowfrequency to a limiting value of the proportional gain divided by thisfactor at high frequency.A proportional-plus-inverse-derivative controller provides highgain to minimize offset at low frequency and low gain to stabilize theloop at high frequency. Inverse derivative can also be added to a proportional-plus-integralcontroller to stabilize loops requiring very low proportionalgain for stability.Inverse derivative should only be added when the loop is unstableat the minimum gain setting of the proportional-plus-integral controller.It is available in a separate unit and can be added to the loop whenstability problems occur. The addition of inverse derivative when properlytuned has little effect on the natural frequency of the loop.Digital Control ModesWhen control mode algorithms are implemented in digital systems,integration is replaced by finite summations and time differentialsbecome finite differences. The scan period of DCS systems is usuallyfixed at 0.5 seconds or selectable for each loop at 0.1 to 30 seconds.©2001 by The Fairmont Press, Inc. All rights reserved.