Troubleshooting confusion: What is the real problem?

Happy new year everyone.

Well, not so happy for me though, because of a problem that I am really confuse with.

Last friday, one of my client called me late in the evening saying their carrier plating machine is acting really "weird". The carrier plating machine has 3 carriers, the problem started with carrier 1 (C1) over-running. During its horizontal movement, it should run in 50 hz (@ normal speed) then 20 hz (when nearing the target position) before stopping. They observed it ran @ 80 hz without stopping. After sometimes, the same thing happened to carrier 3 (C3).

Me & my technician came in early saturday morning. We have been observing the whole system. Each carrier is connected to an inverter, each inverter is set to function in multi-speed. With the inverter's feature, you can run 16 different frequencies depending on the logic combination of 4 contacts (X1, X2, X3 & X4). We tested the machine in its manual mode (as it's auto operation has been long bypassed) & here's our observations: 1.) Horizontal testing: C1 is running good horizontally, same as with C3, 2.) Vertical testing: C1 is running good vertically, but C3 horizontal movement became erroneous. It is over-running & is not following the set speed, i.e. it is running @ 80hz instead of 50. To prevent erroneous movement of C3, you need to move C1 1 horizontal position before moving C3, 3.) If C3 is moved vertically without moving it 1 position horizontally, C1 becomes erroneous.

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You didn't specify which inverter, anyway

Start at the set points in the inverter, see which set point is set at 80Hz. Say it is saved inside set point 3 [Binary 2 + 1]

Then trace which outputs which are triggering 2 & 1, and why it was triggered while it was not supposed to.

Regards

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I checked the program, logic combination of X1-X4 is correct however, during vertical movement of any 2 carrier, the output corresponding to X4 closes even @ program it's OFF. This happens only during vertical movement of any 2 carrier.

Our action:
My first suspect is a damage CPU. We replaced it, problem is still there. Second guess is back board so we replaced but no luck. I transferred the 2 carrier's X4 to a different output, the problem disappeared. Therefore, it is an output module problem I thought.

Till it happened again the following day..

What they did? They reset the power supply, then the fault disappeared.

Can someone help me figure this one out?

Maybe the output is in the program in two separate places?

Since you said that you have already checked this then I would program some type of indicator such as a light or buzzer to another output and make it come on when the suspect output is on. If the buzzer or light is on when the fault is present then I'd have to think the problem is in the program since that should have eliminated all external devices. If the output comes on when the buzzer or light is off then I'd have to think that the problem is external to the PLC, noise? ground? shorted wiring? bad drive?

Good luck.

I checked the program, logic combination of X1-X4 is correct however, during vertical movement of any 2 carrier, the output corresponding to X4 closes even @ program it's OFF. . . I transferred the 2 carrier's X4 to a different output, the problem disappeared.

Click to expand...

The last statement indicates an intermittent problem that only occurs in certain situations, at certain times - maybe when some other AC circuit is active. Are your wires from the PLC to your VFD preset speed inputs shielded cable or in a metal conduit? If not, you can easily get enough induced AC voltage on a wire to cause a false voltage TRUE level on the VFD. Make sure the PLC and VFD has good grounding, reroute the X1-X4 wires if possible, if not, get a shielded 4-conductor cable for these signals.

Ripley
You said in original post
"We tested the machine in its manual mode (as it's auto operation has been long bypassed)

I find this bothersome. IF running in manual how will you ever know when you are not there to observe that problems are due to operator?

Maybe another way of looking at it
Why put in all this automation when they run in manual??

Dan Bentler

I transferred the 2 carrier's X4 to a different output, the problem disappeared. Therefore, it is an output module problem I thought.

Till it happened again the following day..

Click to expand...

Your "Moving the X4 outputs" fix indicates you are on the right path. Having it only work sometimes and not other times indicates that X4 is at times being triggered by something other than the PLC X4 output. A bare spot on two wires could do that, or induced voltage from other sources, or a ground loop between the two devices, where one has an adequate ground path, and the other one only has a ground through one of its input terminals back to the other device.

I'm thinking a sticking RELAY output.

A sticking relay contact is a very good possibility. I wonder if the PLC has relay outputs or if there are external interposing relays.

Cable

If the drive motor is part of the carrier assembly and therefore moves with it I would replace the motor cable at least in the section that moves.

leitmotif said:

Ripley
You said in original post
"We tested the machine in its manual mode (as it's auto operation has been long bypassed)

I find this bothersome. IF running in manual how will you ever know when you are not there to observe that problems are due to operator?

Maybe another way of looking at it
Why put in all this automation when they run in manual??

Dan Bentler

Click to expand...

I was wondering also. I once offered them we do the repair for their auto operation. I just couldn't remember their reply, might as well ask again.

Lancie1 said:

A sticking relay contact is a very good possibility. I wonder if the PLC has relay outputs or if there are external interposing relays.

Click to expand...

There is no interposing relay going to the inverter. During our investigation, I took out the connection from PLC output to inverter & by the use of a meter, directly read from the PLC output to monitor any changes while observing the program online. The PLC output turned ON without the program triggering it.

I believe they meant a "Relay output card" and not an interposing relay.
Also, since the output turns on, did you do a cross-reference see what turns the output on? (besides the multiplexer or whatever they use for the 4 bit combo?)
There may be some kind of logic that does indeed turn the output on when it goes high.
I would investigate this prior to spedning any time observing motion and run the system in manual and such.

Lancie1 said:

Having it only work sometimes and not other times indicates that X4 is at times being triggered by something other than the PLC X4 output.

Click to expand...

I checked this a couple of times, I am convinced that X4 is triggered by the PLC out. I conducted a test after taking out the connection between PLC output & inverter. Using a meter, I directly monitor any status change on X4 with respect to the PLC program. The program is not turning the X4 on, but the latter is being triggered. That's why I thought it was a CPU or backboard fault, I never suspected the output module. I was thinking if the PLC out has stucked, it should have the same status always. How come the X4 is " weirdly" triggered only when I start moving any 2 carrier vertically? This is such a headache.

bkottaras said:

Also, since the output turns on, did you do a cross-reference see what turns the output on? (besides the multiplexer or whatever they use for the 4 bit combo?)

Click to expand...

The output turns ON without the program doing so. I took out the connection between PLC & inverter, just to verify that it is not a faulty wire thing. I am convinced it's not because I checked it with a meter. How can the PLC out turn ON without it triggered on the program? Then, what confuses me more is when I transfer X4 to another output, the problem disappeared. How can it be a faulty PLC out?