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

HVAC Control in the New MillenniumThe control objective is to have the process output move to thesetpoint (X sp ) within some specific time t1. This specifies above derivativeand the control law becomes the model inverse which determines mfrom the following:f(x,m,p) ± x sp ± xt1=0Since x is measured, p is set, and x sp and t1 are user-defined, the equationcan be solved for m. The tuning parameter is t1. These equations areoften nonlinear, and the solution may require iterative techniques suchas Newton’s.A steady-state offset occurs if the model is not perfect. Feedbackadjustment includes biasing the setpoint and adjusting a process parameter.Adjustable parameters represent a real process mechanism, not ahypothetical disturbance. These adjustable parameters can include heatexchanger fouling factors and ambient heat losses.MultivariablesThere are often measurable loads which affect the output in HVACas well as other inputs in addition to the manipulated variable. Theireffects on the output can be modeled to achieve feedforward action. Ifthere are more manipulatable inputs than controlled outputs, a decisionmechanism is used to associate all controlled variables with at least onemanipulated variable.The new information technology in use today enables control systemsto function on nonproprietary networks. The controllers often arePC platforms. Typically, the user interfaces are the familiar, commerciallyavailable operating systems.Data and InformationAll of this ability to communicate large quantities of data in a shortperiod of time means data overload can become a likely problem. Thisis the task of control PCs’ software interface. It must create useful operatorinformation from the plant data.©2001 by The Fairmont Press, Inc. All rights reserved.