Efficient Plant July 2018 - 19

feature | maintenance strategies

TROUBLESHOOTING SKILLS can
be of great value when working with and
debugging control systems, but success can
sometimes be elusive. When an issue occurs
in a system, troubleshooting the problem
could be as straightforward as following a
simple trail of clues to a quick solution. Or, it
could mean struggling through a jungle of obscure, intermittent issues, i.e., occurring only
at certain times, that can leave even the best
problem solver scratching his or her head. In
either case, taking the following methodical
approaches can ease the process.

FIRST THINGS FIRST
Remember that understanding is key to
successful troubleshooting. Before digging
into a control-system issue, you must first
assess and categorize it. Try to reproduce it
and note what steps are required to do so.
While this task, in itself, can be difficult and
time-consuming, an effective solution cannot
be developed without a full understanding
of the problem.
When determining how to reproduce a
problem, you will also want to understand
how repeatable it is. For example, when the
issue is reproduced, does it occur 100% of the
time or only sometimes? If it repeats less than
100% of the time, perform some trials and
note the frequency of the recurrence.
Intermittent control-system issues can be
some of the most difficlt problems to troubleshoot and correct. While they are still
repeatable, the various factors responsible for
that repeatability are either outside a troubleshooter's control or extremely tough to

Left. Before digging into a control-system issue,
you must first assess and categorize it. Try to
reproduce it and note what steps are required
to do so.

JULY 2018

The trial-and-error method can be
very helpful. The best approach is
to change only one variable at a
time and observe the effect it has
on the issue.

identify. Regardless of the
issues that a control system
might present, the following three methods can help
lead to solutions.

DIVIDE AND
CONQUER

In a typical control system,
multiple components are working
together. When issues arise, they could
be caused by any one of those components or
the communications between them. If you are unsure of
an issue's root cause, devise a few tests to help divide the possibilities. Such tests might include replacing a sensor with a spare
or borrowed part from another machine. If the issue persists, you
would know that the sensor is likely functioning properly and,
thus, proceed to look at the rest of the system. Another test might
involve replacing a managed Ethernet switch with an unmanaged
switch. If that resolves the issue, you could assume the managed
switch is misconfigured and, therefore, rule out the rest of the
system. The tests you perform will be unique to every system
and issue, but results of a few good ones that divide the possible
sources of problems can help to quickly isolate the root cause.
In some circumstances, it may be possible to simplify the system or follow the data. This is particularly helpful when
debugging code or communications. To do this, simplify the
system as much as possible by disconnecting devices and/or disabling code. Is the issue resolved? If so, start adding back devices
or code slowly and checking each time. When the issue reappears,
you will have isolated the cause. Alternatively, you may be able to
follow the data or signals at each connection point to see if it is
getting lost somewhere along the line.
continued on p. 20

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Table of Contents for the Digital Edition of Efficient Plant July 2018