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When Cadbury needed to replace
an elderly DCS system at its Keynsham plant it was important that the
new system should be implemented as quickly as possible so as not to
interrupt the supply of its Crunchie Bars. The main production
facility for Cadbury's Crunchie chocolate bars in the UK is situated
at the old Fry's factory at Keynsham. Although the plant used to
produce the Crunchie bar is modern, the existing DCS control system
was found to be in need of replacement, having been originally
installed in 1981.
David Williams, Systems Engineer at Cadbury, explains why a new
control system was required: "While the existing DCS had served us
well for a number of years, we were starting to have problems with
support, against a history of increasing downtime and pressure to
continue to enhance the process performance through further
automation. Added to this, we had concerns over Y2K
compatibility."
Cadbury approached Aston Dane
plc for a possible solution. Harry Booker, project manager at
Aston Dane, takes up the story: "We have worked closely with Cadbury
on many critical projects, however, the Crunchie project was
exceptional in that the production downtime was continuously being
squeezed due to high demand for the product. Cadbury needed to
remove the old equipment and install a new control system quickly, so
as not to disrupt supplies."
Complex Process
Producing a Crunchie bar is a complex process. The bar
comprises two elements - the honeycombed centre and a milk chocolate
coating. The centre consists of sugar, glucose, syrup and
flavourings, which are mixed to produce high temperature foam.
This mixture is fed onto a moving conveyor, where it is monitored to
maintain a constant thickness. As the mixture cools, it sets,
and is then ready to be cut into individual pieces and coated in
chocolate. The time scale set for the new control system to be
installed was 10 weeks. In this time more than 3000 I/O needed
to be replaced.
PLC-based Control
The new system is based around four Allen Bradley PLC5/80C
controllers. Installed within the same panels, and connecting
via ControlNet™, is
the Allen Bradley 1771 range of I/O, which provides signal interface
to suit AC and DC applications. Other communication methods
employed at the plant are DeviceNet™
and Ethernet, which provides the backbone communication between the
PLC and PC's running Rockwell Software RSView32™.
Connected to the PLC's by
DeviceNet™,
are 24 Allen Bradley drives to provide speed control of motors
positioned all around the plant. The whole process is monitored
and controlled by two operators working the plant 24 hours a day.
Cadbury's Crunchie is one of the
leading confectionery brands in the UK. Careful planning and
execution of the old Ferranti replacement was crucial to ensure that
the famous Crunchie bars weren't compromised.
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