Real time production data acquisition is a potent tool for optimising
control of production processes such as assembly and packaging,
and Richard Armstrong of Ritech Systems has developed a low cost
system based on standard Mitsubishi control components that brings
this ideal within reach of many plant operators' budgets
The viability of many industrial processes is based on the production
of large quantities of the product. If the manufacturing volumes
drop, perhaps due to plant downtime or slow product changeovers
the overall margins can be eroded severely. Excessive product giveaway,
reworking and substandard quality can also be equally compromising.
Conversely, a small gain multiplied through the production run can
bolster bottom line profits considerably.
It is not uncommon in the food packing industry for instance to
reckon that a 0.5-1% improvement in productivity will put thousands
of pounds an hour onto the bottom line, giving payback in months
rather than years. Ritech has developed its system from a modest
base. We were talking to a customer about extracting production
data automatically instead of using the conventional paper trail
method, when it struck me that as the data would be "live", there
was no reason for waiting until the end of the day to calculate
performance. Further, displaying the information on an immediate
basis would highlight production problems easily.
We also started creating hourly Excel reports, initially thinking
that this would help us with commissioning, but soon realised that
the production managers were diving on the reports as soon as they
appeared. This initial system was based simply on communicating
with a pair of weighers at the beginning of a packaging line, and
a check weigher at the end using serial comms blocks under the local
control of modest Mitsubishi FX1N micro PLCs. The PLCs in turn were
linked together via a CC-Link, to a larger Mitsubishi A1SJ supervisory
PLC in the production control office. The distance from the packaging
hall to the production office was considerable, so the CC-Link's
twisted-pair fieldbus was an economical choice as well as providing
fast and secure data transfer. CC-Link also provides the expandability
that will be required to eventually service all the production lines
across the factory.
The architecture is such that much of the calculation load is
undertaken by the local micro PLCs so that the central A1SJ is free
to communicate with a SCADA system monitoring the whole site.
At this point, we looked to further enhance the system, so that
comprehensive data could be collected and used effectively. One
of the first ideas was to use another RS232 communications block
to drive a giant LED display above the packaging line, so that the
operatives could monitor their own progress.
This shows information such as quantity packed, percentage giveaway,
yield and efficiency and had the immediate effect of encouraging
the line workers to constantly improve their own productivity. Data
are actually fed to the display in ASCI code which it converts to
update the display every 10s. The LEDs are tricolour, red, amber
and green; with the colours cascading as performance tolerances
are approached and exceeded.
Another idea was to install a Mitsubishi E910T HMI (human machine
interface) at the checkweigher end of the packaging line and to
connect it via Ethernet to the office based PC in order to update
the latest production information centrally.
When complete, every line will be connected to a central hub and
addressed using its own individual TCP/IP address.
Program modifications to the attached micro PLC can also be performed
transparently across the Ethernet from the office without disturbing
line personnel. Refinements to the system include a reworking loop
for efficient handling of packs and raw materials that are out of
tolerance, and the recording of downtime events along with the appropriate
report. The system is very flexible and new ideas can be incorporated
and trialled in very short timescales.
