Mitsubishi AC-servo makes loud noice and vibrations

[CNC Viking]

Today my Y-axis AC-servo started to make loud noice and vibrations at random positions during Y-axis travel.

The noice is a strong humming noice and it vibrates through the whole machine.

I first encountered it during programmed machining, but it appears during jogging and manual cranking as well. When operating the Y-axis there is a constant background rattle and the random strong noices I mentioned, but every 3mm (1/4 servo-rev.) it is completely silent. As I continue, the rattle starts again and soon I run into those noice / vibration spots again.

No alarms are shown on the display, and the part I was cutting came out fine, with correct dimensions.

The X-axis has a weak background rattle and has had so as long as I have owned the machine. The Z-axis is completely silent.

The machine only has 4500 machine-on-hours with far less working hours so I am a bit puzzled.

Anyone that can share some information so I can fix this? I don't dare to run again it before I have fixed this.

[Docter Don]

My experience with that kind of sudden-onset vibration is that the coupler from the encoder to the motor or the ball screw is loose or broken, or else that the encoder has a bad spot, so check the mechanical coupling, and then replace the encoder.

Hope this helps.

[Tom Antrim]

Do you have an extra encoder? If not does one of the other drives have the same encoder? You might exchange Y with one of the other ones and see if you chase the problem.

[CNC Viking]

Thanks for your input Doc.,

This morning I started by switching the X- and Y- servo amplifiers. The Y-axis run smoothly but the X-axis started a constant high pich howling immediately. It moved when I cranked the manual dial, but I had to shut off the control because of the loud noise. I switched back the amplifiers and started up the machine and both X- and Y- axis were fine for a couple of hours, then Y started getting worse again, and after 5 hours I had to give up since it was as bad as yesterday.

I removed the Y-axis waycover and started checking things. The flexible coupling had 1 screw that was very easy to loosen and the flange coupling towards the ballscrew had 1 screw of 4 that was completely loose and the other 3 could be additionally tightened. When I had tightened everything up, the servo motor started the same constant high pitch howling as the X-motor did earlier this morning and it also vibrated.

I removed the servo motor and then the cover of the encoder housing. Now I don't dare to go any further before I know exactly what to do.

Mitsubishi servo HA80NT-E33, built 1996.
Sorry no camera with me today, so I have to explain.

The first thing you see is a circuit board screwed to 3 columns of the metal base plate. The screw heads are sealed with paint. Underneath the circuit board is a rotating glass disc with some delicate coupling at the centre. Underneath the disc is the baseplate which is secured to the motor housing by 4 screws. If I loosen those screws, I can rotate the baseplate but not easily lift it off the motor end.

Waiting for further advise before I remove it from the motor...

EDIT: Sorry Tom, you just answered while I was writng mine, but I think that my answer works for you as well... I will bring home the motor, take some pics of the encoder and post later tonight.

[Tom Antrim]

Are the servos inside the control cabinet the same? If they are swap them and see if it chases the problem

[mactec54]

CNC Viking

You really should not mess with the encoder, you are dealing with a very precisely installed encoder, on the Misusbishi motor

What has happened is now the machine has had some running, it is now moving easier, than when new, you need to,Tune the drive, start with a little less gain & this will solve your problem

There may be just some filters that are in your control that need ajusting,if not you have to do it in the drives

[Al_The_Man]

On most Mitsubishi AC servo's the encoder is keyed on to the shaft in order to correctly register the commutation angle, I had a problem on some Mitsubishi's of the encoder disc coming loose causing an error on the commutation and usually resulted in excess current.
Al.

[Brian Polley]

If you determine that the servo motor needs service, maybe the encoder has gone defective--which is common, or the axis card is bad, I have a source for repair in Canada-Oakville, Ontario- Accu Electric Accu Electric Motors - Industrial Repair Services | Call: 1-888-932-9183. They have a New York office that US customers can ship motors into. You can contact me at brian@accuelectric.com for details.

[onthebumper]

Try running the motor while it is disconnected from the ball screw. If it still vibrates then you know the mechanics of the machine are good. You can try to reduce the gains and sometimes this will help as some manufacturers crank the gains as high as they can but this causes problems down the road when the machine gets a little looser mechanicaly. What type of control do you have so we can tell you what servo parameter to change?

[CNC Viking]

Tom Antrim:
The X- and Y- servo amplifiers inside the control cabinet are the same MDS-A-SVJ-10, but for the Z-axis there is a slightly different one MDS-A-SVJ-20 with an internal fan as well. As for testing, I have been told to stay away from the Z-axis amplifier. Don't mess with that one!!, he said.

I would like to move the X-axis motor to the Y-axis position and try again for some hours with the original Y-axis amplifier. Maybe it does not scream there.
If it does, should I suspect the Y-axis amplifier then?

Can this be done without any motor present on the X-amplifier while that amplifier is still being powered?

mactec54/Al_The_Man:
This is really over my head. I will have to consult some experienced CNC-technician here in Sweden when it comes to that.

OK, I may have done something stupid by loosening the 4 corner screws for the encoder baseplate and thereby messing up the rotational tuning against the motor. If this is no good, how can I retune it again? However I have not used any force attempting to pull it off the motor.

The servo amplifiers. From left to right, X, Y, Z.


X- and Y-axis servo motor labels.



The encoder inside. There is an inner plastic cover as well, not shown here. Note the separate white connector for the 2 wires snaking into the motor.

A closer look at the encoder's reading department.


The rotating glass disc with its special hub.


When rotating the shaft, everything feels OK. Firm bearings, smooth rotation and no axial play. I don't think anything is bad mechanically with the motor.

[Tom Antrim]

You'll have to write down all the wiring numbers then remove the servo amp swap them and wire them back the proper places. Put the connectors in as they were before.
I worked as a service engineer for Mazak but I wouldn't have taken apart the encoder. If you haven't damaged it it should go back together.

[CNC Viking]

onthebumper:

I will test-run the motor asap, disconnected mechanically from the ballscrew. The Y-axis is really smooth , I can easily move the table by rotating the ballscrew by hand.

My control is a Mitsubishi Meldas 520AMR built 1996.

I welcome any info re. where to find the relevant parameters in the menu's and what values to go for. First I will look it up and then report back the existing values for your advise.