Rotary Encoder Problem?

Here is the scoop. 2 axis Takasawa CNC Lathe. There is a z-axis overshoot (It will travel .050 too far on average at rapid speed and bounce back like it is on a rubber ban). Slow feeds or using the handle wheel it works fine (checked with indicator). The movements are smooth going slow, but rough and noisy at anything higher than turning metal speeds. Ball screw, couplings, bearings and rails are fine. I swapped cables to use the x button to jog the z axis, and the z was still the same. I figured this eliminates anything to do with circuit boards. The position on the read out does not reflect positioning errors. I have no way to check the encoder and to get a tech here is probably $800 just to show up. I can buy an encoder for $625. Money is tight right now, what would you guys do?

It might be a dc motor full of carbon dust, assuming you have a dc servo. If so take the brushes out and blow air to clean the commutator. I don't think it's the encoder. I could be wrong though.

Regards,

Stan-

Thanks for the reply. It is an AC Fanuc red cap motor with to my knowledge are damn near bullet proof. Been around 100's of them in my machining life time and never had to replace one and I played back up maintenance man at a few large shops. I hope it isn't that as it needs a coupling removed that is hard to get at to pull it. Also it is probably $1500. I figured if it was a bad motor it would act up at all speeds. I was looking for a majority vote for me to buy the encoder so far it is one vote against. I am B grade in the tech department, but running against a lot of problems that take an A grade guy to diagnose.

I used to do work on Universal Instruments equipment and they had a coupler that attached an optical motor to a shaft. The coupler had tree this, stamped discs with rivets joining the discs in a pattern. Sometimes these would fatigue at the rivet, breaking part of the disc. When this happened, the symptom was much as you describe. It was also difficult to spot unless you removed the coupler!
For any encoder directly coupled to a motor shaft, it seems unlikely (at least in my limited experience) that you would see your results unless the motor itself were damaged. Some part of the control loop is failing you.
If you swapped the motor cables to make the X button work to drive the Z, that might imply that the drive itself was ok and you are back to looking at the motor. You didn't say if the X axis worked ok if pressing the Z button. All of this assumes X and Z are somehow similar in their loading.
My 2 cents...

Does this machine have a separate encoder or use the one on the red cap motor? If it uses the red cap motor's encoder it's not the encoder. If the red cap encoder fails the motor will buzz, hunt all over, vibrate like crazy, etc. Does not sound like this is happening so I don't think it's the encoder - unless there's a second encoder just for the control.
If you'd like to check the red cap's encoder just pull off the red cap. The encoder is held on with 3 screws which should be tight. If they're loose then the encoder could be moving a bit and losing your 0.050. The middle of the encoder has a rubber plug under which is the screw holding the encoder to the motor shaft. You can pull the plug and check tightness here, too.
If the encoder is loose there's a procedure to time it back to the motor or the drive won't commutate correctly. Let me know if you need this, it's in the Fanuc AC Servo book (54765E) which is at home.

X axis worked ok pressing the Z button. I think we are down to the encoder or motor being the problem. The machine position readout does not reflect the bounce, I thought the encoder was responsible for correct positioning? The readout is not recording the error.

If the registers counting encoder pulses say an axis has hit the right destination, the readout will simply reflect the "right" coordinate. As far as the control is concerned, there is nothing to correct. If there is a flaw with the encoder (dirt, breakage, or even a glass encoder wheel separating from its aluminum hub), the control will still drive until either an e-stop hits or the axis pops a breaker (the mechanical stops either work or they don't).

The connection to the encoder is suspect. I have also seen the set screws on the coupling work loose. Then, if there is a flat on the encoder shaft, I have seen the coupling wobble back and forth because of the play as the set screw catches on either side of the flat.

I have also seen where the bearings begin to fail in an encoder. This is unusal because the encoder isn't supposed to see any torque to speak of. Sometimes it does, especially when a coupler is installed improperly. When the bearings are good in the encoder, there is so little encoder induced drag that the coupler easily turns the encoder, even when the set screws are not tight. When the bearings are bad, and the coupler slips for any reason, trouble.

So - How is the encoder attached to the Z drive? Is it on the end of the motor housing (and thus often part of the back end shaft)? If so, look to the motor as having the highest odds of being the problem. Otherwise, the encoder is usually the easiest thing to investigate.

Some systems will drive a leadscrew through a belt and have the encoder attached to the leadscrew instead of the motor. These types of systems will then have a coupler going to the leadscrew that attaches the encoder.

The encoder bolts directly to the back of the motor which is directly coupled to the shaft. No belts involved. I haven't pulled the encoder, but have cleaned the plug ins on both ends.

If it overshoots and corrects itself and it's mechanically sound, I would be leaning toward a servo drive issue instead of an encoder issue. IF the X and Z axis are the same drive type, I would swap X and Z servo drives and see if it follows the drive.

I'm not familiar with the Fanuc motors, but, my DC Fadal motors have an encoder and a tachogenerator. The encoder tells it where it is, the tachogenerator tells the
driver board how fast its going.

The tachogenerator puts out X volts per rpm, in my case it was 14v per thousand. Had a short in the hoodickey with wires wrapped all around it, it was bouncing all over
the place, so, took the magnets out also, BIG mistake, they don't stay energized if you pull them apart. So now I was getting about 4 volts per 1000rpms (new hoodickey).
With the low voltage, control thought it was moving slower than it was, and it would overshooot and snap back.

I don't know if that will help at all, or if I even explained it well enough to make sense.

Bobw your explanation makes sense, though Dualkit's servos don't have a tach. The servomotor has an encoder only directly coupled with the back of the motor's shaft. This encoder has a 4-bit absolute position encoder the drive uses for commutation which the control can't see, and a 2000, 2500, or 3000 PPR incremental encoder that the drive uses for velocity feedback and the control shares it for position feedback.
Thinking back I do remember a case where the glass disk in the encoder had a crack in it which caused trouble. Didn't this machine get moved recently - if so has it worked since? If it hasn't worked I will change my mind and say the encoder MAY be at fault.
I agree with Tony that the servo amp is more likely and I'd swap the X and Z amps to check. I'd even consider swapping the X and Z motors if the servo amp swap doesn't change it.
Do you have the part# of the encoder? I may have a spare on hand. Part# will be something like A860-0304-T111.

Tonytn36 said:

If it overshoots and corrects itself and it's mechanically sound, I would be leaning toward a servo drive issue instead of an encoder issue. IF the X and Z axis are the same drive type, I would swap X and Z servo drives and see if it follows the drive.

Click to expand...

Maybe I wasn't clear in my explanation in the first post. That is what I did. When I used the x-drive to run the z-axis it acted exactly the same.

The machine was moved and probably dropped as it ended up with a dented cover and a misplaced cabinet hinge pin. The guys on this end were not equipped to move things with low door openings and I never saw them pull the machines out of the box truck they traveled 2600 miles in and reload them on to their flat bed. They half dropped one of my screw machines right in front of me. I had two customers of mine with some tech knowledge and me thinking encoder and all you guys are saying motor.

Pull the red cap off and check the encoder part#. If it's what I wrote above I can loan you my spare so you can try it out. They're simple to exchange, only gotcha is the 2 wires for the thermal sensor buried in the motor windings need to be cut & spliced.

You might also look for your motor and/or encoder on ebay. Perhaps it would be worth having a complete spare? It's possible that the same encoder is used on a wide variety of their motors, in which case the cheapest "red cap" motor available might give you the same encoder...?

Good luck!

Can you swap the X and Z encoders?

Doo they have plug in connections, or are they soldered fast at the fitting?


---------------

Think Snow Eh!
Ox

Dualkit said:

Maybe I wasn't clear in my explanation in the first post. That is what I did. When I used the x-drive to run the z-axis it acted exactly the same.

Click to expand...

yep some confusion as to what u really swapped... so u swapped the CNC axis and the drive but not the motor? if so and problem stayed on z then z motor or z mechanical is the problem. ur description of overshoot tells me 99.9% that if you spend the money on a new z encoder you will have totally wasted your money. check for broken motor to screw coupling or bad thrudst bearing on screw. what u describe as only happening at stop from hi speed tells me it is a torque issue; u elimated the drive or cnc axis tuning by the swap. an ac servo motor very very seldom changes its torque output characteristics without lots of visible smoke. the encoder really cannot do anything to cause these symtoms but a loose or broken encoder coupling can. or m otor coupling. but most likely a loose or broke thrust bearing that allows the screw to actually move linearly - when it happens can u tell if the backup is rotary motion or linear motion of the axis? if ur descriptiojn of the cnc not seeing the m otion is accurate then it seems most likely the SCREW is moving and the encoder does not see it. can u push on the screw to see if it moves? it would require a lot of force since it only happens when you stop from a high speed where T is highest at T=Jw/t. can u stick an indicator on the screw to see if it is moving? but save ur encoder money.

Thanks for the offer Fasto. Pulled the encoder apart, there is a pit that doesn't look like it came that way on the reflective wheel on the perimeter where there are other timing marks or whatever you call them. Ordered an encoder yesterday, for $625 and a few hours labor it is worth the gamble. Breaking the motor lose from the coupling will be a nightmare it isn't very assessable even with the way cover pulled as far out of the way as possible.

ur description of overshoot tells me 99.9% that if you spend the money on a new z encoder you will have totally wasted your money.

Click to expand...

Mike I have to agree with your reasoning, but because the CNC gets it's velocity information form the motor encoder, IMO that number is much smaller.


With a open loop the CNC has no lost motion detection capabilities.

Tommy

Just put in the encoder, it is a little better but not fixed, looks like the people saying it was the servo motor were right, I sure hope so.