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

Control Technology, Microelectronics and Nanotechnology 87Chapter 3Control Technology,Microelectronics andNanotechnologyControl systems require an understanding of the characteristicsof the process and involve the following:• controlled variable,• manipulated variable and• uncontrolled load.The control loop is closed by installing some type of automaticcontroller onto the process. The control loop must be able to measure thecontrolled variable and compare that measurement to some desired targetvalue such as a temperature setpoint. If there is an error and thecontrolled variable (temperature) does not equal the temperature setpoint,then the output is modified and this changes the manipulatedvariable in such a way to reduce the deviation from setpoint.The oldest control approach is to use a simple switch for control.This switch might turn on the heat source when the temperature is lowand turn it off when the temperature is high. This type of system iscalled two-position control and is shown in Figure 3-1.Controllers have different operating modes to meet the needs ofthe particular process. These modes are tuned to optimize the controlfunction. Temperature control is a relatively slow, nonlinear (variablegain) process. The process is not noisy. Gas pressure control has nonoise, and it is linear. A proportional-only controller can be used. Whenproportional-integral (PI) control is used, the integral is set for 1/3 to 1/5 of the period of oscillation which is usually in seconds.©2001 by The Fairmont Press, Inc. All rights reserved.