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

HVAC Control in the New MillenniumThe DCS system does not continuously evaluate each measurement,but checks them intermittently. It makes its approximations on thebasis of the present and the previous error. One type of calculation inuse is called positional where the full output signal is recalculated everytime the measurement is checked. The derivative is approximated by thechange between the previous and the present value of the error. Theintegral is approximated by the sum of all previous errors between timezero and the present time.Another type of calculation is called a velocity algorithm. When itis used, the value of the previous output signal is held in memory, andonly the required change in that output signal is calculated. The changein the error between the previous and the present scan period is calculated(second derivative).The positional algorithm is used with noisy measurements since itworks with the error and not the rate of error change. Velocity algorithmscan provide bumpless transfer, less reset windup and are bettersuited for motor-driven devices. Their main problems are noise sensitivityand oscillation.Setpoint ResponseWhen the error is the deviation from setpoint, a sudden setpointchange causes a sudden error change, which can cause a controlleroutput spike. When both proportional and derivative control act only onmeasurements, the output spiking due to setpoint changes is preventedwhile the tuned controller performance is not affected. Some controllershave setpoint ramping to prevent output spiking by eliminating thesudden setpoint change.Setpoint StationsSetpoint stations are used to interface a digital control computerwith analog controllers and actuators (Figure 3-4). They allow the computerto make changes in operating conditions. In supervisory controlapplications, the computer adjusts the setpoint of an analog controller.In direct digital or DCS control, the computer may adjust the actuatordirectly. The signals from the computer must be translated into the signalsrequired by the receiving device.©2001 by The Fairmont Press, Inc. All rights reserved.