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

Control Technology, Microelectronics and NanotechnologyFeed-forward control should yield much faster correction thanfeedback control. In the ideal case compensation takes place in such amanner that the effect of the disturbance is never seen in the output.A competent operator could use feed-forward to offset the changesin inlet water temperature of a direct contact water heater. Detecting achange in inlet water temperature, they would increase or decrease theheat rate to annul the change. The same compensation could be doneautomatically with an inlet temperature detector designed to initiate theproper corrective response in the heat value.The notion of feed-forward control is forceful, but it can be hard toimplement in a pure form in most control applications. In many casesdisturbances cannot be accurately measured and this makes pure feedforwarddifficult to use.A major limitation of feed-forward is the inability to generate perfectmodels or to make accurate measurements. Because of these limitations,pure feed-forward would accumulate the errors in its model andwould eventually self-destruct.The main limitation of feedback control is that it cannot predictupsets but can only make corrections after the upsets have occurred. Itmakes this correction in an oscillating, cycling way.The best use of feedback and feed-forward is to combine them insuch a way that the imperfect feed-forward model corrects for about90% of the upset as it occurs and the task of correcting for the remaining10% is done with the bias generated by the feedback loop. Using thisapproach, the feed-forward component is not pushed beyond its abilitiesand the load on the feedback loop is reduced by an order of magnitude,allowing much tighter control.Proportional Control ModeControllers generally use different control modes for control management.The proportional mode is the simplest of these. It is characterizedby a continuous linear relationship between the controller inputand output. Other terms in common usage include proportional action,correspondence control, droop control, and modulating control. A proportionalcontrol loop is shown in Figure 3-2.The adjustable parameter of the proportional mode, Kc, is knownas the proportional gain, or proportional sensitivity. It may be expressed©2001 by The Fairmont Press, Inc. All rights reserved.