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When one is designing the control system of a new plant or updating the control system of an existing facility, the inexperienced might just make a quick I/O count and based on that figure, obtain bids and select the distributed control system (DCS) supplier. That's the wrong approach and is likely to result in endless extras and delays.
In order to develop an operator-friendly control system and guarantee a smooth start-up, one should first integrate the four main components of the total plant automation package: field instrumentation, safety systems, maintenance and simulation segments, and the plant-wide buses and networks.
Only after these four systems been defined and integrated, should one obtain bids from the DCS suppliers.
Field Instrument Integration
The first step in designing a new plant, should be to specify all the sensors, transmitters, final control elements, local controllers, motors and all other microprocessor-operated devices in the field, that are planned to be connected to the plant's networks or used to send information to, or receive commands from, the DCS system.
If the project is an addition or expansion of an existing plant, all the interfacing between the old and new must also be specified. The three types of interfaces most often used are OPC (OLE for Process Control), serial-to-highway-gateway and serial-to-serial port. Because digital field devices utilize a variety of communication protocols, the manufacturers provide interface cards to the common buses. Such cards are available for AS-interface, ControlNet, Ethernet TCP/IP, Foundation Fieldbus, HART, Modbus or Profibus.
This integration is not simple because some smart field devices are proprietary, and manufacturers implement the same task in different ways. To offer just one of the many examples, some manufacturers describe controller status by two parameters: Auto/Manual and Remote/Local, while others use a single parameter: Manual/Auto/Cascade/Remote-Cascade/Remote-Out.
The second step in the overall design is to specify the requirements for physical layer redundancy in terms of the number and type of network levels required. It is at this point that one must decide if the plant's safety instrumented systems (SIS) and the plant's control systems will be separate, and, if they are, what kind of interfacing to use.
If the safety and control systems will not be separated, a decision is required on prioritizing, redundancies, hard wiring, etc. Some suppliers provide redundancy by building two of everything in the system as standard, others provide redundancy on an optional basis for power supplies, data highways, traffic directors, remote controller electronics and workstation video terminals. Yet others provide 1 for 1, 1 for 4 or 1 for 8 backup of controller file cards. In addition to the redundancy method, there are also differences in back-up diagnostics and of switchover method: automatic or manual.
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