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

Control Technology, Microelectronics and NanotechnologyIf the controlled process is a flow process, the process gain is high,maybe 10.0. In this case the controller gain should be 0.05 (10.0 × 0.05 =0.5), so that the total loop gain is 0.5.The higher the controller gain, the more corrective action the controllercan apply to the process in response to a small deviation fromsetpoint, and the better the quality of the resulting control will be.When the gain product of controller and process reaches unity, theprocess becomes unstable and undampened oscillations (cycling) occur.It is more difficult to tightly control fast (high-gain) processes withoutsome oscillating. It is far easier to obtain tight control on slow, low-gainprocesses, since the use of high-gain controllers will not affect stabilityas much.The loop gain is the product of all the gains in the loop, includingsensor, controller, control valves and process. In a properly tuned loop,the product of all these gains is 0.5. What makes tuning difficult is thatthe process gain often varies with load.In heat transfer processes, when the heat load is low and the heattransfer surface available to transfer the heat is large, the transfer of heatis performed efficiently. This type of process is a high-gain process. Asthe load rises, the heat transfer process becomes a low-gain process sincethe fixed heat transfer area becomes less and less sufficient to transferthe heat. This is why the gain of a heat transfer process varies with load.Tuning such a system can be difficult since in order to arrive at anoverall loop gain of 0.5, the controller needs to apply a high gain whenthe load is high and a low gain when the load is low. Standard controllerscannot do that, because they are tuned to provide a single gain.If the loop is tuned at high loads, the loop can cycle when the loaddrops. If the loop is tuned at low loads, the loop will not be able to holdthe process on setpoint when the load rises. It will be slow or sluggishto respond.One way to compensate for the variable load effect is to use avariable gain control valve in the loop, which increases its gain as theload rises. If the gain of the process drops, the gain of the valve increasesand the total loop gain remains relatively about the same.Feedback ControlAutomatic control systems can use either feedback (closed-loop)control or feed-forward (open-loop) control. Feedback control is more©2001 by The Fairmont Press, Inc. All rights reserved.