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

HVAC Control in the New Millenniumpervisory computer will not be frozen at its last value.For supervisory control, the upper-level control algorithm must beinitialized when the output from the supervisory computer commandingthe lower loops in the DCS is initiated or when an emergency conditionrequires that the supervisory control be prevented.System FailureWhen the supervisory computer fails, the control generated at thecomputer level cannot be transferred to the DCS level. An integrated controlsystem should use a watchdog timer or communication link messageto flag the lower DCS loops. This is needed to prevent an unnecessary disturbanceto the system due to a supervisory computer failure.Fault-tolerant control can be achieved in the event that the computerfails. One approach to fault tolerance is to implement a generalizedoutput control algorithm at the DCS level.Supervisory DisplaysThe display on the CRT screen becomes the main interface whichcan be used for system performance evaluation. There are usually hundredsof display page screens provided in both DCS and supervisorycomputer software. These display pages are easily accessed through akeyboard, touch screen, mouse, or track ball.A typical display access method is to use a tree-structured, multileveldisplay architecture. A two-level structure with 16 elements (pages)in each level provides 16 × 16 = 256 pages. Each screen page at the secondlevel can cover 16 elements and each element can be a digital oranalog input/output combination. This provides a total of 4096 combinedI/O points that can be displayed at the second level. If the propersearch path is selected, only two keystrokes are needed to arrive at thedesired point.Traditionally analog controllers have been arranged in clusters ofrows or columns. In DCS, controllers are usually arranged in groups of8, 12, 16, or 32 for screen displays. One of the simplest ways to representinformation for supervisory controls is to use a multi-input/multi-outputdisplay.©2001 by The Fairmont Press, Inc. All rights reserved.