PLC failure and loss of communications on a network are every engineer's
nightmare, but the latest drives with built in intelligence know exactly
what to do when this happens. Mark Daniels explains.
Figure 1. The factory shown produces reels of cable and uses a wire
extrusion machine to coat wire with a plastic jacket.
There are many advantages associated with using a communications network
for industrial control systems. Installation and commissioning times are
greatly reduced, wiring is simplified and complex runs of control cabling
are replaced by a single network connection. But perhaps more
importantly, these networks allow information from the factory floor to
be accessed from any location, either local or remote. Engineers have
access to powerful diagnostics, enabling them to rectify many faults
without having to visit the site, and managers have access to real time
production data to aid their decision making. But what happens if the
network connection is lost due to a network failure or loss of a PLC?
A catastrophic network failure can be the result of accidental damage
caused by a fork lift truck or perhaps a local fire, while the loss of
the PLC may be due to a hardware fault or power failure. Either way, the
devices that rely on the PLC and communications network for their
instructions, are suddenly on their own.
Drive products from individual manufacturers react in different ways to
this situation and whilst some will carry on using the last set of
instructions, others detect the fault condition and coast to a halt.
However, every application is different and whilst coasting to a halt may
be ideal for a conveyor system, serious damage could be caused to an
extruder in a continuous process. The option of a motor continuing to run
is equally problematic causing packages to fall off a conveyor system, or
hoppers and storage tanks to overflow causing damage and creating mess.
Figure 2. Loss of control causes the conveyor to stop and the hopper
quickly overflows.
Recognising how serious these consequences can be, Rockwell Automation
has built intelligence into its range of Allen-Bradley PowerFlex drives
enabling them to be configured to continue to operate in a pre-determined
way. Different holding parameters can be programmed to cover either the
loss of the network, or failure of the PLC.
In the example illustrated (Figure 1), a wire extrusion machine is
coating wire with a plastic jacket using an injection moulding method.
The process comprises a simple conveyor with DOL starter and a DeviceNet
Starter Auxiliary I/O, monitoring a sensor and actuating a solenoid. The
conveyor is used to feed plastic pellets into the extruder and a
PowerFlex Variable Speed Drive controls the extrusion screw motor.
Heating elements in the screw are used to melt the plastic pellets.
All is operating normally until a fork lift truck damages the
communications cable, causing a catastrophic network failure. Using a
conventional starter and drive without the benefit of built-in
intelligence, the loss of PLC control would cause the conveyor to stop
and the hopper to quickly overflow (Figure 2). The drive would trip,
causing the extruder to stop; the plastic pellets would solidify and the
extruder would be jammed (Figure 3). The result would be lost production
and costly delays while the machine was stripped down and cleaned out.
Figure 3. The extruder stops and the plastic pellets solidity resulting
in lengthy down time.
Alternatively, using products with built in local intelligence would
enable the conveyor with DeviceNet Starter Auxiliary I/O to sense the
network fault and stop. It would also close the hopper. The PowerFlex
drive would use its 'network failure' parameters to continue to run,
clearing product and pellets from the extruder. By using this approach,
an extruder jam is avoided, lost production and wastage are mi