KM-3 Z-axis troubleshooting help

I have a KM-3, with Ultimax 2 controls. I am having trouble with the z-axis. Here are the symptoms:

1. On occasion, the z axis will head at the current speed towards + or - Z, well beyond its commanded position and will then stop and report a "Motion Error on Z axis." If, for example, I turn the jog wheel just a little in the - direction, the spindle will continue down until it faults.
2. The problem seems to be temperature dependent (happens on warm days)
3. Resetting (Manual, Power, Reset Servos, Start) will bring things back on, but it will immediately fault again at the next command. My only hope is to shut down for the day and try again on a cooler morning.

Here is what I've done in trying to troubleshoot the problem:

1. I observed the tachometer signal coming from the z-axis encoder using an oscilloscope. I get the same signal at the encoder connector (blue and white wires) and at the DB-25 connector on the Z-axis card (pins 25 and 12 on PC1-A1), I'm not sure what the spec is for the encoder, but I do see a ~5V square wave that increases in frequency with increased Z-axis speed at both places (so I don't thing there is a cabling problem)
2. I can see the Marker signal on the green wire at the encoder (the z-axis usually calibrates correctly--but not always)
3. I have swapped the dual axis board (with the z-axis) board with a spare board that I have. Same behavior.
4. I have swapped the Z and X servo boards (I don't have a spare). Same behavior.

At this point I'm out of ideas. I assume that the problem is that the digital logic (i.e., dual-axis board) is commanding a movement, the servo board is creating a voltage on the motor, but the encoder signal is not getting back to the digital logic to close the loop. Based on that simple understanding, it seems that I've tested or swapped all of the relevant pieces. What am I missing?

Any help would be much appreciated,
,
-joel

Pop the top off the motor and check the encoder plug inside is ok. Also check the coupling is tight and not broken.

Thanks, Bloke. I've checked all that. As I say, I can see that the encoder is producing a square wave that increases frequency with motor speed. I haven't been able to find any specs for the Litton 84ABS-1000 encoder. I would like to compare my measurements with nominal values.

I guess the next step, failing that, would be to swap encoders with one of the other axes. Are they all identical?

-joel

Should be. There were two different types fitted in those motors though - a short metal cased encoder and a plastic cased model with a smaller standoff. The best way to find out is just to pop the cover off.

Sadly, I'm going from bad to worse with this. I swapped the encoders for the Z and Y axis and got the same behavior. I removed an reseated the Dual Axis boards a few more times, cleaned the connectors on all of the DB-25s on top of the boards and powered it up again. Now I have really strange behavior.

When I command an X-axis move via the console buttons, the Z-axis goes up and down (in reverse to what it would normally). As I'm doing this, the position counters for the X-axis changes on the screen, but Z-axis remains constant! Y moves uncontrollably (if I press the "+" or "=" button, it just goes to the limit, with no position feedback on the screen), and when I command Z, it gets a motion error immediately (no change on screen).

My first thought is that I somehow swapped the cables to the D-A cards, but I have not (checked them several times). I have also not changed any switches or PROMs on either of my D-A cards. So, why in the world is it so confused?

All I can think of is that the error is in the main CPU card as it is common to the two D-A cards. I've re-seated it as well, to no avail.

The only other thing that I have done is to swap servo amplifier cards (without re-tuning them), but I have put them back to their original positions with no change in the behavior.

Is now the time to start looking at a retrofit solution???

-joel

You have probably swapped x&z and y&spindle.
Looking at the rack, as you see it, left near plug is z, right near is spindle. Left furthest is x and right furthest is y.
Swap left near with left furthest and right near with right furthest.

Order should be:

AC2-P1. AC2-P2
AC1-P1. AC1-P2
MP-P1. MP-P2. MP-P3

Thanks, Bloke. That was my first thought, but I've checked and re-checked and I have the cables in their proper places (consistent with your description). Is there anything else that could cause such behavior? -joel

OK, so I am a little embarrassed. When I was checking the encoder signal on the AC1-P1 and AC2-P1 connectors, I inadvertently swapped the DB-25 covers (that had the labels on them). So, swapping back, I can get Z,S, and Y axes to work correctly. But, I still have the immediate motion error on X.

So, are AC1-P1, AC2-P1, and AC2-P2 (Z,X, and Y respectively) interchangeable without danger? In other words, could I have messed up the X-side of the X/Y D-A board by plugging Z into it? Something has gone wrong with X--which is new relative to my original problem.

-joel

My guess is that somewhere along the way you have inadvertently swapped some wires, inverting the encoder signal or the motor drive. Thus a disagreement between which way it goes and which way it 'thinks' it is going. Whichever is easier, swap the two motor output leads, or swap A- and A+ on the encoder input. Of course all changes while power is off.

I believe that I have it figured out. My X-axis problem was a red-haring. Apparently, I bumped the bulkhead connector for that axis going into the control cabinet as I was swapping the encoder from Z to Y. That was an easy fix. Then, it was clear that my Y axis was behaving the same way as my Z axis was before the switch (i.e., running away with not position feedback). At the same time, my Z axis was working fine with the Y-axis encoder. So, this pretty definitively points to the encoder as being the problem.

As I mentioned before, the signal on the "bad" encoder looks OK on the oscilloscope, but perhaps with a smaller amplitude than on a good one. And, it does work to find the marker. There are very few visible components on the upper PC board and I have sprayed each with circuit cooler spray to no effect. There are three little pots on the board. Is there any hope that mine is just out of adjustment? I still haven't been able to find any spec sheets for the device. It is a Litton Encoder 82ABZ-1000-5-1-E. Anybody have any documentation?

Thanks again,
-joel