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

PC-based Control, Software and Bus Trendsa traffic cop between the hardware and the application software programs.The operating system controls basic tasks such as directing datato and from devices.As operating systems have evolved, they have taken over functionsthat the older applications software used to handle. During the lastfew decades, the quality of operating systems available for small computershave increased with the demand for better user interfaces. One ofthe most popular operating systems for control systems is Windows NT.PLCs and PCsWhen PLCs were introduced over 30 years ago, relay ladder logic(RLL) was the standard programming language. Ladder diagrams identifythe order of operation of devices. The logic can be rewired by programmingon the CRT display and tested, without any physical rewiringof the banks of relays. The displays show a ladder diagram with contacts,coils and circuit connection very similar to a schematic diagram.RLL was not designed to support a wide range of control functions.The main input devices to PLCs are push-buttons, limit switchesand other command devices. The output devices are contactors, relays,solenoids and indicating lamps.PLCs have been moving towards commodity hardware items, asPCs already have, and the trend is towards providing more completeapplication solutions. This means more open systems and commercialoff-the-shelf technologies to meet user demands for performance, ease ofuse and lower cost of ownership.Continued optimization of overall plant performance requires leveragingplant information with automation applications. Even PLC architecturesare increasingly being dominated by the flow of information.The use of object-based technologies, industrial Ethernet and Internet accessto information are examples of this trend. PLCs along with PCs arechanging from closed-control-specific devices to open information serversproviding access to previously untapped operational information.Flowchart ProgrammingFlowchart programming is an advance that helps to realize the fullpower of PC-based control systems. A simple, intuitive graphical de-©2001 by The Fairmont Press, Inc. All rights reserved.