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

Control Technology, Microelectronics and Nanotechnologyules can be communicated to the supervisory computer when this informationis needed for computation.Supervisory TechniquesEarly supervisory control was used to command single-loop analogcontrollers in order to achieve a certain goal, such as to obtain auniform temperature or to determine the optimum blending of cold andhot decks. The functions that the analog controllers could not accomplishwere delegated to the digital computer to perform. These includedthe logic, sequence, or analytic computations to improve the controloperation.Coordinated supervisory control with integrated feedback or feedforwardcombined in a velocity control algorithm may be activated ordeactivated without creating perturbations. The velocity algorithmimplemented in the supervisory computer can be converted to the positionalgorithm which is usually applied at the DCS level. The controlcommand, in the supervisory computer at the current sampling time ismade up from the current computer position command and the pastsampled position command.Anti-windup ControlThe command from the supervisory computer to the setpoint forthe control loop at the DCS level is a form of cascade control. The primaryloop control command (supervisory computer command) requiresinformation from the lower or secondary loop at the DCS level in orderto command correctly. This information includes saturation status andlower loop current setpoint values at the DCS. This information at theDCS level requires constant monitoring by the supervisory computer ofthe real-time information on the DCS network. It is important to preventthe computer command from causing setpoint windup at the DCS level.Protecting the upper loop command from windup involves a freezingof the supervisory control command output to the DCS setpoint atits last value if the DCS output to the valve is saturated and the directionof the upper loop output increment is such that it would cause furthervalve saturation at the DCS level. Otherwise, the upper loop in the su-©2001 by The Fairmont Press, Inc. All rights reserved.