Automating Manufacturing Systems - Process Control and ...

plc software - 32.3
32.2.1 Troubleshooting
After a system is in operation it will eventually fail. When a failure occurs it is
important to be able to identify and solve problems quickly. The following list of steps
will help track down errors in a PLC system.
1. Look at the process and see if it is in a normal state. i.e. no jammed actuators,
broken parts, etc. If there are visible problems, fix them and restart the process.
2. Look at the PLC to see which error lights are on. Each PLC vendor will provide
documents that indicate which problems correspond to the error lights. Common
error lights are given below. If any off the warning lights are on, look for
electrical supply problems to the PLC.
HALT - something has stopped the CPU
RUN - the PLC thinks it is OK (and probably is)
ERROR - a physical problem has occurred with the PLC
3. Check indicator lights on I/O cards, see if they match the system. i.e., look at
sensors that are on/off, and actuators on/off, check to see that the lights on the
PLC I/O cards agree. If any of the light disagree with the physical reality, then
interface electronics/mechanics need inspection.
4. Consult the manuals, or use software if available. If no obvious problems exist
the problem is not simple, and requires a technically skilled approach.
5. If all else fails call the vendor (or the contractor) for help.
32.2.2 Forcing
Most PLCs will allow a user to force inputs and outputs. This means that they can
be turned on, regardless of the physical inputs and program results. This can be convenient
for debugging programs, and, it makes it easy to break and destroy things! When forces
are used they can make the program perform erratically. They can also make outputs occur
out of sequence. If there is a logic problem, then these don’t help a programmer identify
these problems.
Many companies will require extensive paperwork and permissions before forces
can be used. I don’t recommend forcing inputs or outputs, except in the most extreme circumstances.
32.3 PROCESS MODELLING
There are many process modeling techniques, but only a few are suited to process
control. The ANSI/ISA-S5.1-1984 Piping and Instrumentation Diagram (P&ID) standard
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