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

HVAC Control in the New MillenniumThe logical product of a fuzzy logic function in response to a specificset of inputs always occurs at a point in time. These calculations aresimilar to the scan cycle of a PLC.The logical sum combines the results of the rules. The fuzzy devicedoes not make a quick decision. The logical sum may suggest that thecontrolled parameter should be decreased, but whether this should bedone quickly or slowly is left fuzzy unless the result is TRUE or FALSE.This is sometimes called the conclusion of the fuzzy interference.DefuzzificationThe defuzzification operation calculates the center of gravity of thefuzzy interference. This value becomes the output. When the output isa single value determining the required change, the defuzzification isknown as the Mamdani method and is the most common techniqueused.For multivariable control, the final control action is calculated fromthe weighted individual membership values of each active rule.Based on the rules programmed into the fuzzy controller, it willoutput the most valid value corresponding to the variable input conditions.With fuzzy control, the steps are performed continuously, while ininformation processing, such procedures are executed only when theinput data varies. In one fuzzy control system application the variabilityin product properties was reduced by about 30%.Temperature Overshoot ControlA fuzzy logic controller can reduce the temperature overshoot thatoccurs during heat-up. The ideal control system might start with a lowsetpoint and then gradually adjust it toward the correct setpoint whilewatching the control performance. Fuzzy logic can be used for this typeof control (Figure 6-8).Autotuning is used to obtain the PID constants and the process characteristicssuch as the dead time and time constant. The controller manipulatesthe heat input based on the deviation from the setpoint. It automaticallychanges the internal setpoint to a lower value when the anticipatorylogic predicts an overshoot. The controller continues to monitor©2001 by The Fairmont Press, Inc. All rights reserved.