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Process Computer Implementation Strategies

Part of a set of study notes on Digital Control
M. Tham

The use of digital computers to aid process plant operations dates back to the mid-1960's. Then, not only were digital computers expensive, they were also not very reliable.  Thus the number of installations were few and far between. The situation has changed dramatically since those early years. Nowadays, it is rare not to find a process computer in a process plant, where they are deployed to perform a wide variety of manufacturing tasks; from simple process monitoring to plant wide optimisation.

Examples of some of the typical uses of digital computers in the process industries are shown in the following schematic diagrams.

Data Acquisition System

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In this simple application, process signals are transmitted to a process computer, where either the raw data or values calculated based on the raw data are stored and / or displayed.

Operator Guideline System

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An operator guideline system is very similar to the data acquisition system shown above. However, in addition to the storage and display of data, the computer could be programmed to advise on plant operation; notify that equipment need maintenance; propose changes that should made to operating conditions; suggest manufacturing schedules, etc. These guidelines are made based on knowledge of the process,  encoded in the computer either as an algorithm; as a knowledge based system that can be queried, or both.

Note that the process is usually being controlled by local analog controllers. Any changes to operating conditions will be achieved by changing the set-points of these controllers, and it is possible to carry this out remotely via another computer.

This scheme is therefore a cascaded 'closed-loop' type strategy, where the inner loop comprises the local analog controllers while the computerised system forms the outer, 'supervisory', level.

However, whether the supervisory loop is closed depends ultimately on the decision of the operator. That is, the operator may decide to ignore or overide the 'suggestions' of the computer.

Supervisory Control System

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The operator guidline system can be fully automated such that there is no need for operator intervention. With this strategy, the process computer takes in signals from the process, and uses either the raw or derived data to calculate set-points to local analog controllers. Set-point changes are effected automatically by the process computer. This set-up is commonly known as a supervisory control scheme and is typically used to optimise the operating conditions of the plant.

Direct Digital Control (DDC) with Analog Backup

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In  supervisory control, the outputs of the process computer are the set-points of local controllers. However, there is no reason why the outputs of the computer could not be sent directly to the final control elements. If this is done, then a direct digital control (DDC) scheme results. However, to ensure continuity of plant operation in the event of computer failure, a bank of (usually analog)  controllers are often used as backup.

Hierarchical Computer Based Operation
Note that the above classifications are used merely to indicate some of the singular tasks that process computers can be employed to do. In practice, a combination is always encounterd.

In a modern plant, it is usual to find mixed mode operations, where some parts of a plant may be under DDC while others are under supervisory control, each running off a different process computer. Strategic units may also have a computer based operater guide system attached. That is, the task of running the whole plant is distributed to several computing units rather than a single one. Reliability is increased as a result because the operation of the plant is no longer reliant on a single computer.

Such networks of process computers are usually arranged in a hierarchical manner, where each level of the hierarchy reflects the closeness to the shop floor.

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Additionally, the network can be extended to include computers used in the analysis laboratories; personnel office, and those holding the plant inventory database, all linked to provide a plant information system. Indeed, the computers on different plants may be networked, so that employees at different geographical locations can share information, and managers can co-ordinate and optimise production effort.

With the current technology, computers can clearly be put to a multitude of uses on a process plant. The only limit is costs. In this set of notes, we shall focus only on the use of computers for process control, within either a DDC or supervisory control framework. The principles of controller design are, however, identical in both cases and typical scenarios where computer based process control may be beneficial are:

Plants with large throughputs - because utilities consumption is approximately proportional to throughput, a small improvement can result in large savings

Plants subject to frequent upsets - some plants requires quick responses to process upsets that operators can not provide

Complex plants -some processes are too complex for operators to deal with the relationships between process variables and hence are unable to determine the best operating conditions, and maintaining consistent operation is difficult

Batch processes - some batch processes require frequent cycling or changes in product specification, and computer control can increase production rate and decrease labour cost

New processes - in plants with 40-50 control loops, a DCS may prove to be cheaper than an equivalent analog system.

Next we will cover some of the more important mathematical tools that are commonly used in the the design and analysis of digital control systems.

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