Sequencing software used to automate mixing process. Adds great flexibility, visibility and reliability to factory automation.

1. MIXER INGREDIENT FEED USING OUR SEQUENCING SOFTWARE
This brochure describes how our plc
sequencing system is used to control a
mixer ingredient feed system. This
particular system has both loss-in-
weight and gain-in-weight weighers for
the 12 ingredients. The system is fully
automatic and can mix up to three
tonnes an hour of biscuit dough to
ingredient weighment accuracies of
under 1%.
This particular system, and similar
systems for other plants, have been
running without problems since 1984.
(Screen graphics are prior to the
Windows graphic software now used).
The plc code is highly structured, with major
areas for alarm handling, plant unit control,
tasks, and system utilities. This is explained in
our brochure about the sequencing software.
The plant unit is a key concept – it comprises
all the equipment (vessels, valve, motors,
pipelines, etc) that makes up the system for, say,
feeding flour. In our simplified graphics, each
plant unit is shown as a box. The colour of the
box gives an instant display as to the status of
the unit – on the picture to the right we can see
the mixer is red (hold) due to an alarm, sugar
and fat are pink (pause) due to their feeding into
the mixer at the time of the alarm, and other
units are green (running) or white (standby).
On the right is the screen for a (fictional)
recipe. This particular system also required a
simple acid premix recipe to be set up as well.
Recipe actions are placed within stages. The
plc initiates all actions within a stage and then
waits at the end of stage step until they have all
completed. The exception to this rule are
preweigh actions, which prepare weighments
for the following batch. The recipe does not
wait for preweigh actions to complete.
(However, a feed action, which empties a
holding vessel, cannot complete if it has been
booked by a preweigh or weigh action that has
not completed).
In this scheme, there are 32 steps with one
parameter – other schemes are more complex.
www.etprocessdesign.co.uk +44 (0)1404 823650 or +44 0775 444 2578

2. Once a recipe has been downloaded to the plc
and started, the first scan of the recipe kicks
off preweigh tasks only. The next scan then
kicks off all tasks within the first stage. This is
the point that the system status screen on the
right shows, with active steps shown in green.
Two of the three preweigh tasks have
completed and are shown, with achieved
weights, in white.
The menus can be touch screen buttons or, in
this case, function buttons. The first button at
the top right allows any target weight to be
altered in the current recipe. The bottom line
shows the current alarm.
The alarm screen shows which alarms are
active and in hold (red), which are dead and
awaiting reset (yellow) and dead, reset historic
alarms (white). Alarms must be individually
reset.
Individually resetting alarms ensures that only
selected plant units are restarted, and thus
other, unexpected faults (such as transitory
alarms – eg fat tank overfull) can be
investigated before being restarted.
Some faults on investigation will be seen to be
due to switches going out of adjustment and
causing an alarm, even though the plant is
working correctly.
In this instance, maintenance can decide to
inhibit the alarm via a password protected
screen. This will prevent the alarm from being
triggered and give maintenance time to
schedule the repair, whilst permitting the plant
to continue to work normally.
Alarm inhibits are automatically reset every
day at a specific time. Safety alarms cannot be
inhibited, of course.
www.etprocessdesign.co.uk +44 (0)1404 823650 or +44 0775 444 2578

3. There are instances where the graphics
terminal may be remote from the plc
controlling the plant, or where the atmosphere
restricts the opening of the cabinet.
For these plants we provide an additional
screen that shows the state of the plc’s inputs
and outputs (I/O).
I/O that operate for a very short time cannot
be displayed, but the system is very useful for
verifying that the program is actually driving
an output, or that another section of plant is
powering an input.
The more advanced systems are fitted with
simulation software. This permits the plant to
be used for training purposes outside
production periods.
A supervisor keyswitch is used to put the plant
into simulation mode when it has shutdown.
Simple simulation consists of downloading a
recipe and showing new staff how the plant
operates in real life. The plc drives the plant as
per normal, except that all outputs are masked
off, thus preventing drives from operating.
Inputs are simulated, including the weighment
of ingredients and the discharge of vessels.
More complex simulation involves downloading a list of simulated alarms. These will occur at the
correct point in the operation and operators can be trained in the actions to take. An engineering version
also creates alarms, but does not display the alarm message – the engineers must decide which alarm has
occurred and inhibit it before the plant can continue to operate.
Further enhancements include:
- Calibration weighments
- Proof of process reports
- SMS texting of alarms and events
- Trending graphs
- On-screen maintenance diagrams
- Faultfinding charts with photographs
- Downtime analysis
- Ingredient usage reports
- Statistical process control
www.etprocessdesign.co.uk +44 (0)1404 823650 or +44 0775 444 2578