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The site’s existing control system (> 3000 I/O) had become difficult to maintain considering the limited number of PLC spares available including racks, power supplies, processors, I/O and communications cards – not to mention the HP Monitrol SCADA system which was a single point of failure for the entire wheat receival, milling and flour distribution operations. Weston Milling at Enfield operate a rotating 24 hr / 7 day shift pattern with out loading trucks, delivery trucks and trains coming at all times of the day, any system downtime available for commissioning.
A Dickinson in house developed manufacturing execution engine provided the flexible and scheduled control over the plant, ensuring traceability and appropriate visibility of all work orders currently running and planned within the system. One of the key features of the design of the control system was the building of individual process sequences into distinct process control ‘phases’ using ISA 88 guidelines. Each control phase defines a sequence and a set of input and output parameters. These provide the building blocks for the complete manufacturing and material management process. This design allows a high degree of flexibility and customisation of each process, as the sequencing of the control phases is done within the production execution system rather than being completely embedded in PLC logic. would be limited.
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The commissioning phase of a project is often one of the risky periods. To manage the level of this risk, a staged commissioning approach was used, breaking the overall system into three main areas and because of this, additional functionality within the replacement system was required to interface both new and old providing for the use of both systems concurrently for extended periods of time. To minimize the amount of downtime applied during changeover operations, maintenance periods were utilized to install double terminal wiring which would allow for a quick and easy plug and socket style swap over – also providing the option of swap back if required. This option was envisaged to be exercised for the larger stage involving some 1500 I/O to be commissioned in one time – allowing Weston’s to return to regular operations using the existing system after only short periods of inactivity. Also, the comprehensive FAT Dickinson had carried out prior to the staged installation of the system was able to be used as leverage in the installation and operational testing thus minimising the time and effort required. Thus the final system will have minimized production interference and risk involved with the upgrade process through comprehensive design, FAT and flexible implementation processes.
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In the implementation phase, the standard control modules simplified the development of PLC code, tightly integrated SCADA faceplates, and streamlined system configuration, thus minimising development time and reducing the risk of errors. Custom applications were developed by Dickinson to simulate the plant equipment, allowing for comprehensive Factory Acceptance Testing (FAT) of all areas of the system – this also ensured that all functionality required by the customer is included and no significant surprises are encountered during the commissioning and handover phases of the project.
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Weston’s now have a vastly improved information management and distributed reporting system, removing the requirement for several of their existing cumbersome paper based historical production logs that were available in the original system.
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