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Maintenance Management of your company's PLC (Programmable Logic Controller).By Don Fitchett
(This article also featured in November 15, 2007 Chemical Engineering Magazine)
If you could
not answer with confidence or you answered ‘No’ to any of the above
questions, you need to read this article
on maintenance management of PLCs. Why? Because the PLCs
(Programmable Logic Controllers) are the brains of your operation. When the
PLC is not functioning properly, lines shut down, plants shutdown, even city
bridges and water stations could cease to operate. Thousands to millions could
be lost by one little PLC in an electrical panel that you never even knew
existed. But most importantly, damage to machine and personnel could result from
improper maintenance management of
your company’s PLCs.
First I’d
like to explain in the most non-technical
terms possible, What a PLC is. As this article is not just for the maintenance
technician, but for maintenance managers, plant managers and corporate managers.
A PLC (Programmable Logic Controller) is the type of computer that controls most
machines today. The PLC is used to control AND
to troubleshoot the machine.
The PLC is the brain of the machine. Without it, the machine is dead. The
maintenance technicians we train, are the brain surgeons. That is how I explain
it to my doctor any way. (His mouth drops open, “... you train brain
surgeons?”)
Important Note:
Just as a doctor asks the patient questions to figure out what is wrong, a
maintenance technician asks the PLC questions to troubleshoot the machine. The
maintenance technician uses a laptop computer to see what conditions have to be
met in order for the PLC to cause an
action to occur (like turn a motor on). In a reliable maintenance management
environment, the maintenance technician will be using the PLC as a
troubleshooting tool to reduce downtime. A
little more detailed definition of a PLC: A programmable controller is a small industrial strength
computer used to control real world actions, based on its program and real world
sensors. The PLC replaces thousands of relays that were in older electrical
panels, and allows the maintenance technician to change the way a machine works
without having to do any wiring. The program is typically in ladder logic, which
is similar to the wiring schematics maintenance electricians are already
accustomed to working with. Inputs to a PLC can be switches, sensors, bar
codes, machine operator data, etc. Outputs from the PLC can be motors,
air solenoids, indicator lights, etc.
My company has
had an ongoing PLC related global maintenance
survey since the year 2000. The majority of the participants back in 2001,
reported 3-6 PLCs in their facility, that they know of. Granted most
participants are managers and don't open electrical panels much, but many of the
participants are from fortune 500 companies having hundreds of employees. The
odds are most of them have 12-30 PLCs in their facilities. Currently the average
is 6-9 reported, so the good news is the industry as a whole is becoming more
PLC aware. It is common
to only learn about a PLC once the machine is down and the clock is ticking at a
thousand dollars an hour, or more. Unfortunately, it is also common that after
the fire is out, it's on to the next fire, without fully learning what can be
done to avoid these costly downtimes in the future, and in other similar
machines in a company or corporation. Some older
electrical panels may only have relays in them, but most machines are controlled
by a PLC. A bottleneck machine in your facility may have a PLC. Most plant air
compressors have a PLC. How much would it cost if the bottleneck or plant air
shut down a line, a section of your facility, or even the entire plant?
The first step
to take is to perform a PLC audit. Open every electrical panel, and write down
the PLC brand, model, and other pertinent information. Then go the next two
steps. Analyze the audit information and risk, then act on that analysis. To
help you out, I want to share with you our
company PLC audit form.
Once you have
collected the basic information in your Plant wide and/or corporate audit, you
need to analyze the information to develop an action plan based on risk
analysis. In the risk analysis, bottlenecks and other factors will help you
assess priorities. Starting with the highest priority PLC, you will need to ask
more important questions. o
Do we have the most common spares for the PLC? o
Is the OEM (Original Equipment Manufacturer) available 24/7? Or even in business any more? o
Do we have a back up copy of the PLC program? o
Does our program copy have descriptions so we can work with it reliably and efficiently? o
Do we have the software needed to view the PLC program? Are our
maintenance personnel trained on that PLC brand? These are some
of the questions our managers must ask, to avoid unnecessary risk and to insure
reliability.
Is
your maintenance staff trained on the PLC? (Silly to squander over a couple
thousand in maintenance training when the lack of PLC knowledge could cost you
10 thousand an hour.
Then the
question should be asked, what should we look for in training. Well I have been
training individuals for over a decade and could easily write another article on
just PLC training alone. I can tell you here, that you should seek training with
two primary objectives. 1.
The training you decide on, should stress working with PLCs in a
Safe and Reliable way. (not just textbook knowledge or self learned knowledge) 2.
Secondly, the training should be actually centered around the PLC
products you are using or plan to use in your facility. I feel the two
criteria above are the most important. Some other good ideas to get more out of
your PLC training investment would be to get hands on training using the actual
PLC programs and software the maintenance technician will be working with in the
facility. Insure your personnel have the software, equipment and encouragement
to continue with self education. PLC Training CBT (Computer Based Training) CDs
are a great way for employees to follow up 6 months after the initial training.
Some other ideas you could do is to provide them with simulation software and/or
a spare PLC off the shelf to practice with.
It seems that
in our industrial culture, if policy and procedures are not written and
enforced, we eventually stray back to the old unreliable ways. I have
reviewed many policy and procedures as well as books on the topic matter and
hardly ever see maintenance management of the PLCs included. It amazes me how an
organization can write guidelines for what they believe is the health of the
entire organization’s body, and leave out the brain (the PLC :>). Once
again, a complete PLC policy and procedure manual is out of the scope of this
article. However, I will donate a few random items below to get you started. 1.
Write PLC policies and procedures into your existing maintenance
policy and procedures. (SOP) 2.
All personnel working with PLCs will be trained on that PLC
equipment. 3.
Backup copies of the PLC programs will be made every 6 months
regardless of change status. 4.
If a PLC program has been changed ... a.
It will be documented in the software copy, in the printed copy and
in the CMMS program. b.
Copies of the PLC program will be stored on a media more reliable
than floppy disk (CD, USB, etc.). c.
Multiple copies will be stored on laptop, maintenance manager’s
office and off site (corporate). d.
If available, EEPROM will be updated with new changed program.
i. If outside vendor changes, a-d will be performed by maintenance
personnel 5.
Future equipment purchases ... a.
A common PLC brand in all equipment will be sought out
(Standardization of PLC types) b.
OEM will be required to provide a descriptor copy of PLC programs
in the customer’s native language. c.
All PLC 110v control voltage will have a line filter on it. d.
All PLCs will have the backup EEPROM option for zero downtime in
some failure modes. 6.
Forcing inputs and outputs on or off shall be treated as a Safety
issue. (See safety SOP) 7.
Inputs and outputs shall not be forced on or off with out a clear
understanding of complete effect on PLC program and a second opinion. a.
If forces are installed, they shall be removed with in 24 hours and
a more permanent solution found. b.
All forces should be documented in software and a written log
before being enabled. 8.
Online programming is somewhat of a safety risk, normal procedure
is to change offline and download to the PLC. Hope this
helps, if you have a specific question you can find me in our PLC discussion
area at the PLC
Discussion Forum. Don Fitchett
(President) Business
Industrial Network PLC Training -
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