Control system maintenance to minimise
downtime
On New Year’s Day 1968, the
programmable logic controller (PLC) was first designed. Whilst most were busy
celebrating and making resolutions, Dick Morley was planning his invention. The
PLC has been used ever since to make logic based decisions in automated
industrial processes. Despite their resilience and rugged design, PLC-based
control systems can still break down and their failure can lead to costly
downtime.
Here, James Davey, service manager of systems integrator Boulting Technology,
discusses the top causes of PLC control system failure and how the risks can be
minimised.
PLCs use
microprocessors to digitally control industrial automated processes. Early PLCs
replaced relay logic systems, but advancements over the years enabled them to
cope with a larger number of inputs, processes and outputs. In essence, a PLC
is a set of hardware and sequence of coded instructions that enables equipment to
perform complex and reliable electromechanical functions.
A PLC will
usually run constantly, despite the harsh industrial environment it operates in.
Unfortunately, even this robust control system can fail sometimes, which leads
to serious consequences, such as a production line or processes stopping
altogether.
Downtime
is extremely costly and, not only can it seriously affect plant output,
with the recent advances in PLC technology that embrace safety-related
functions, it can occasionally create a hazardous situation that needs
immediate attention. To ensure this does not happen, businesses need to follow
a planned maintenance routine.
When
a PLC control system does break down,
identifying the cause can be tricky. Often, a copy of the PLC software, a
laptop, programming lead and a multimeter are the only tools necessary for
diagnosing the fault, along with some knowledge of the processes. Sounds
straightforward? In many cases it is, but the trap of complacency has a habit
of biting. Below is a list of common reasons why PLC control systems
fail.
I/O modules and field devices
About 80 per
cent of PLC failures are a result of field devices, Input/Output (I/O) module
failure or power supply issues. Typically, these defects manifest themselves as
a sudden process stop or irregularity of performance. This is because the PLC
control system is waiting for a signal to allow it to step through its program
sequence. In this situation, the engineer usually determines where the sequence
has stopped by interrogating the software ‘on-line’, with the aim of tracing
the problem to a specific I/O module and input or output point.
By identifying
the I/O point, the engineer can then trace the problem to its root cause. This could
be a PLC configuration error, tripped circuit breaker, loose terminal block,
failure of a 24 VDC supply or issues with wiring. It may be that the I/O module
itself needs replacing. This relies on having a readily available supply of
replacements, something that is becoming increasingly difficult for legacy
systems.
Erratic
behaviour or failures of groups of inputs indicate there is an internal PLC
error or issue with a common power source. If the I/O module is not the reason
for the failure and power and wiring issues have been eliminated, then
attention should be paid to the field devices — the components external to the
I/O module. These could be incorrectly configured, mechanically damaged or they
could have failed electrically, for example due to water ingress.
Ground integrity
Proper grounding
is important in protecting both the PLC and maintenance personnel. A well
grounded enclosure can also act as a barrier to outside electrical noise. During
maintenance or diagnosis, the engineer can perform a visual check of ground
wiring to establish if there has been any damage or if there are any loose connections.
The engineer can
test the integrity of the ground with a multimeter. By checking the resistance
of the PLC ground terminal to a main earth bonding point in the equipment
enclosure, we can establish if this is the root of the problem.
Power supply issues
The reliability
of PLC-based control systems is dependent on having an uninterrupted power
source. Power supply issues can result from a range of causes including loose
or corroded cables and power supply failure.
In addition, many
manufacturing facilities, utilities and infrastructure companies will usually
have redundant power systems or install uninterruptable power supplies (UPS). This
way, a part of the plant continues to function even in the event of a mains power
failure, thus providing control of essential items in order to maintain its safe
operation.
Even if an
industrial plant considers a UPS nonessential and a complete process stop is
manageable in the event of a power outage, a PLC’s memory can be lost when the
power fails. This can lead to loss of process data, but also complete loss of
operational programs. To prevent this, a PLC sometimes employs its own backup
battery to ensure the device restarts correctly when power is restored.
Failure to
maintain and replace the batteries in a PLC or UPS can lead to a major system
failure in the event of a power outage.
It is vital to
back up the PLC software regularly and store it securely. If an industrial
plant fails to back up the system, it makes it incredibly difficult to resume
normal function in the event of PLC memory loss. Furthermore, it turns a minor
power loss incident into a major downtime issue.
Dealing with interference
Electromagnetic
interference (EMI) and radio frequency interference (RFI) are common in
industrial environments that contain a variety of electrical equipment. Anything
from handheld radio transmitters used by maintenance staff, to a large motor
starting can cause interference.
Companies need
to control electrical noise as much as possible, because it can lead to
intermittent faults or unusual behaviour and even PLC failure.
There are many ways
to mitigate the risk of downtime caused by electrical noise through design. A Boulting
Technology service engineer can recommend ways to minimise noise by relocating
sensitive equipment, segregating systems with high power components and adding barriers,
grounding, or shielding cable between sensitive equipment.
Network and communications
Most PLC control
systems need to communicate with periphery devices such as Human Machine
Interfaces (HMIs) and other ‘intelligent’ equipment. A typical communication
medium will consist of an industrial network, which industrial plants
increasingly base around industrial Ethernet. A loss of communication between
devices will often result in immediate plant downtime.
Engineers can
mitigate against communications failures by ensuring the physical network
infrastructure is correctly installed and terminated, that network devices are
suitable for purpose — especially when more and more devices are added — and
firmware patches are regularly installed to maintain reliable and secure
operation.
Heat
The environment
is a critical factor in the life of equipment and control systems. Failure to service
air filtration components in the control cabinet can cause insufficient airflow
and cooling within the control panel. This can lead to equipment overheating
and the acceleration of component failure.
Equipment will
fail at high temperatures or humidity, particularly above the limits of maximum
temperatures recommended by manufacturers. A high humidity can also lead to
condensation forming on electrical components and, in turn, this can lead to
failure. Industrial plants can mitigate by using panel-cooling systems or by
considering where the control panel will be located during installation design.
Managing the risks
By following a
simple best practice routine, companies can minimise the chance of PLC control
system failure. Engineers should ensure the environment in which the control
system operates is sound. Through systematic inspections, engineers can
identify any overheating or electrical noise problems.
By regularly
checking and testing batteries and UPS systems, companies ensure that in the
event of a power fault, their system is reliable and operates continuously.
Other maintenance activities include checking the wiring integrity, grounding,
terminals, field devices, Ethernet and other industrial networks. Plant
managers should regularly back-up software and install firmware patches.
Last, but not
least, obsolescence management is also important because PLC manufacturers
regularly cycle their product ranges. If you are operating with a component
that is several years old, it is important to have a readily available
replacement for it. Businesses can manage this internally or by using a third
party, such as Boulting Technology, who is able to highlight the risks,
indicate which areas of a control system are more likely to lead to failure and
put a contingency plan in place to mitigate that risk, including sourcing
legacy components.
Control system
best practice is not all about the hardware. Regularly backing up PLC software
ensures that if downtime does occur, normal function can resume quickly.
Upgrading firmware also makes the system more secure because patches and
upgrades eliminate known software vulnerabilities.
Although it has
been around for a long time, the PLC is not invincible. However, with proactive
maintenance, environment control and contingency plans, your PLC control system
will be there to keep your operations up and running every day of the year, including
New Year’s Day.