How to create full sequencing software with standard programmable logic controller (eg Allen Bradley)

1. SIMPLIFIED PLC CODE USING A SEQUENCING SYSTEM
Ariadne has been writing plc and graphics code since 1976. We realised early on that the designer
has a responsibility to provide well structured code in order to ensure low long-term costs.
We developed a system of writing plc code that splits
the complete operation into a series of small tasks.
Individual tasks are self-contained modules of code
that are easy to test and document, and software
change control is simplified. Adjusting the design of
an individual task does not require any other part of
the code to be tested again.
Sets of tasks are formed into sequences of operation
within the plc in a very simple way. A sequence is no
more than a list of task numbers with any associated
parameters. Changing a list of tasks (through a
graphics terminal, for instance) may completely
change the operation of the plant, but no additional
testing of code is required, as all the individual tasks
have been tested already. Verification of the process
controlled by the sequence is still required, of course.
On the left is a very simplified
example of a task list. Note that
some tasks are used to control
sequences – tasks can also be used to
make another sequence skip some
tasks, or repeat a task, or even adjust
the parameter of a task.
All tasks are self-contained and have
designated interface areas for the
overall sequence executive control.
The plant to be controlled is also
split into conceptual plant units.
PLC code assigns a set of actions to be taken whenever a
problem occurs within a plant unit. Alarms are assigned
as being either hold or informative – the presence of a
hold alarm initiates a predetermined shutdown of the
plant unit. Other plant units feeding that plant unit are
paused and perform a predetermined pause routine.
All these action routines need only to be tested once.
Additional hold alarms invoke the same routine and so
new alarms can be added to a plant unit without the
need for testing other than verifying that the input
causes the alarm to occur.
The screenshot shows the effect of the mixer being opened whilst sugar and fat are being fed in.
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2. The diagram on the left shows a very simple
sequence that empties plant unit C. (In real code,
such a simple action would all be one task – it is
simplified here to show the principles clearly).
The next diagram shows the incorporation of the
Empty plant unit C sequence into another sequence.
Our sequence system allows us to use this piece of
code as often as we like in the entire project, but
does not increase the programming overhead or
require further testing. The central code that
overseas all tasks and sequences ensures that all
code modules run individually – if a task or
sequence is called a second time whilst running, the
second call waits until the first call has completed.
Sequence 3 also shows that tasks can have parameters
assigned to them. These are usually process variables,
but can also be used to control other parts of the plc
code. One use is where an operator checks the product
conformance to quality standards, and determines
whether to repeat an earlier step using a different
parameter value. This is particularly useful in
pharmaceutical plant control.
Sequence control of modular task code is particularly
useful in prototype processes, where the code requires
frequent changes. Our system allows a plc code to be
very flexible, changed in seconds, yet requires no
retesting of code. (As said before, the process flowchart
must be correct – the plc can be told to empty a tank
before it is filled, for instance).
Sequencing is also very useful where multiple
mixers are fed from a central weighing system, or
where repeated ingredient feeds are required within
a recipe.
Perhaps the greatest benefit of the plant unit and
sequencing system comes when a plant needs to be
modified. Complete new areas of plant can be
added or removed with ease, as the code is totally
modular. Care must be undertaken, of course, but
retesting is not required – the remaining plant will
operate as before providing the process
requirements are still met.
Programming methodology and datatable structures
are identical across different plants and factories,
which is a great benefit for maintenance staff.
www.etprocessdesign.co.uk +44 (0)1404 823650 or +44 0775 444 2578