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
of some of the typical uses of digital computers in the process industries
are shown in the following schematic diagrams.
|Data Acquisition System
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
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
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
Digital Control (DDC) with Analog Backup
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.
of process computers are usually arranged in a hierarchical
manner, where each level
of the hierarchy reflects the closeness to the shop floor.
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
large throughputs - because utilities consumption is approximately
proportional to throughput, a small improvement can result in
to frequent upsets - some plants requires quick responses
to process upsets that operators can not provide
-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
- some batch processes require frequent cycling or changes in
product specification, and computer control can increase production
rate and decrease labour cost
- in plants with 40-50 control loops, a DCS may prove to be
cheaper than an equivalent analog system.
we will cover some of the more important mathematical tools that are commonly used
in the the design and analysis of digital control systems.