Optimal control systems require responsive controllers to manipulate the control variables found in the physical plant. On large industrial control systems operating over plant-wide communications networks, delays associated with data transmission can present formidable problems to the control system designer. Although these problems can be minimized through the use of high-speed data-transmission methods, an alternative is the use of a distributed control arrangement. For many facilities, distributed control arrangements exist in the form of programmable logic controllers (PLCs) linked via local area networks (LANs). Although such systems are primarily intended to deal with logical operations, interfacing with continuous-time systems, such as adjustable-speed drives, is usually required. Responsive closed-loop-drive operation using the PLC as the controller is usually not practical without extensive hardware additions to the PLC system. However, by embedding a local controller within the adjustable speed drive and allowing the PLC to act as a system controller, the distributed control system can be implemented. A description is given of such a system implementation incorporated into an AC adjustable frequency drive controller. Some basic characteristics and attributes of such a system are described.
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