Maintaining various older servos, drives and PLC's

In the plant I work in we have numerous pieces of equipment that have various PLC's drives and servos. The issue is that we are having random failures due unknown reasons - mostly age, sometimes lightning or power surges or just plain unexplained. In many cases the motor, drive or PLC is obsolete and not available for replacement.

In some cases it is a matter of finding a broken wire or a short. In others it might be a I/O board failure, etc. In other cases the entire drive/servo may need to be replaced because it is smoked and there is no direct replacement.

I know every situation is going to be different. I am looking for advice to get a handle on troubleshooting and repairing equipment like this. Other than a multimeter are there a tool set that would help in these situation? A software suite? A data collection tool?

Just an example...A specific situation I currently have is a PLC that is sending a signal to a drive to tell the drive to advance or retract a certain distance. We have no way of telling of the PLC is actually sending the right signal. Is the PLC bad or the drive? We are trying to eliminate the drive/motor first in this case by working with the manufacturer. The drive and motor are both obsolete.

Any help or general pointers would be appreciated in dealing with the general issue of aging equipment with old and obsolete failing controls.

Evaluate your vulnerability. Rate all your equipment in terms of criticality to the process and obsolesence.

Criticality:
1 - Major production loss
2 - Some production loss
3 - Minimal effect

Obsolesence:
1 - Obsolete, no parts available from recyclers (ebay, radwell...)
2 - Obsolete, parts available from recyclers
3 - Obsolete, new parts available
4 - In production


Plan to replace the highly critical items that have no parts available soon. Prioritize the other upgrades.

flycast said:

Just an example...A specific situation I currently have is a PLC that is sending a signal to a drive to tell the drive to advance or retract a certain distance. We have no way of telling of the PLC is actually sending the right signal. Is the PLC bad or the drive?

Click to expand...

Did you use a multimeter to see if you are getting a voltage at the PLC output which is connected to the motor encoder when that output turns on ?

Of all the buzzwords and catch phrases I have been exposed to in 20 years as a industrial tech, the only akronym I ever really liked and bought into was COTS.

Common
Off
The
Shelf
(parts).

Replace that junk on a case by case basis as it fails. That requires:
1) Identify the big hitters (most critical to production).
2) Plan the replacement systems; this can be tedious...and the payback postponed.
3) Leapfrog. Skip a generation if you can get your old craq repaired. Then get the latest and greatest a few years down the road when rev. 2.0 comes out.
4) As the opportunities to install the updated design present themselves, take advantage of that to expand and knock out other affected systems working towards COTS.

Example: If a particular drive fails and its safety system is also known to be substandard, it is almost always a rubber stamp approval to update safety controls.

Another: A motor with a weird mounting face is no longer repairable, making it COTS, means the gear reducer's gotta go too. *Maybe it has other weakness, like an RPM limit. *Replace the drive motor and reducer with common, modern ones and make the machine 10% more productive.

This improved efficiency may afford more scheduled downtime to perform planned maintenance and upgrades.

If you take downtime, take advantage. What I mean by that, is if you have a plan and a pile of parts and something goes poof, and you can whip up the manpower to upgrade it, turn a minimum of what might only be one hour of downtime every few months into 4 hours once, and put that chunk to bed for a decade.

Common off the shelf motor frames, common control schemes for VFDs, contactors. If you can modify the design, you also get the chance to improve it's overall performance as well as get away from specialized and special order parts.

This is not so easy to do with servo control, my best advice there is to keep a good handle on the source code and machine constants, so you totally understand them when it's time to retire the old controllers.

All the new 'lektronik stuff is smaller faster and cheaper, but I foresee the longevity (duration of availability) getting much shorter.

This seems more true with VFDs, although the reliability is very good. I predict in five years, more than 90% of the drives bought today will be obsolete, so aim for standard mounting and standard control schemes.

The good news is the "smaller" part when it comes to electrical gear. This often means at the end of the mods, you have lots of free panel space, less heat, less power waste...

I really like to open the doors of a really old panel and see that it has been neatly updated with modern controls with all kinds of space all around the devices. it is a pleasure to work in a panel with 50% free space evenly distributed by a gradual but well planned upgrade.

To avoid redundant labor and wiring, get a design for the end result of the whole system up front. This way, you don't end up redoing a bunch of stuff. As much as humanly possible, you want to "go as you planned, not plan as you go".

Bottom line: You will always be in the business of replacing a really old part with a really new one. To make that task simple and efficient really has more to do with the interfaces of these components than the components themselves.

You first need to determine what kind of signal the PLC is supposed to send. Is it an on/off digital signal? An analog speed reference? A pulse train? Some sort of fieldbus? There is no universal software for troubleshooting, so you will need the PLC software for online debugging, and if the servo drive has communications, then that software also. Some drives can be run in a test mode. There is also usually some kind of display for showing any faults or alarms.

As far as tools, a multilmeter and a scope are all you usually need. If you want to post more details on your current problem, maybe we can help. Post the models of PLC and servo drive, and how they are wired together.