Encoder Expansion - ALLin1DC

[andrecn]

Hello everyone, my name is André, and I'm writing from Portugal.

I already posted on this forum but unfortunately something happen, and my account was deleted.

I'm finishing a retrofit of a 3 axis Mill with the ALLin1DC and why question is the following:

The controller has 6 encoder ports, I'm using 3 encoders at the back of each of the 3 axis DC servo motors which were adapted with special parts. I'm looking to use another encoder for the spindle position feedback which will enable rigid tapping, there is also the ATC carousel which also requires a drive expansion plus another encoder. At the end of the day, it will leave me with just one encoder port left.

The machine has originally per axis a feedback encoder directly at the ball screws, they are old and low res and not compatible with Centroid controllers. My idea was to get high resolution encoders for true position control. Is there any expansion card which enables adding more encoders, and will it be possible to use those readings as the true XYZ position? What are the advantages and disadvantages?

I hope you can help me with this one.

Cheers,

André

[tblough]

If you are looking at position feedback, then you need glass scales and not additional rotary encoders. Assuming you used zero backlash couplings on your axis drives, then adding an additional encoder to the ballscrew will not gain anything because you still have all of the mechanical inaccuracies between the ballscrew and your part that are being ignored.

For position feedback to be effective, that feedback needs to come from as close to the workpiece as possible. This means expensive, high resolution, glass scales attached to the table. This accounts for ballscrew mounting bearing slop, ballscrew mount deflection, ballscrew nut backlash, some way backlash, and a host of other mechanical tolerances.

To be effective you are looking at 0.1 micron scales at $1500 for each axis, and the resulting system will end up with some dither. Scales are not a cheap cure-all for mechanical issues. You'd be better off investing that money in better ballscrews, ABEC7 ballscrew bearings, and precision scraping on your ways.

[Centroid188]

Actually if you put DC encoders directly on the ballscrew you do not need encoders on the motors with the ALLINONEDC. This is the exact solution our Hardinge CHNC / HNC kits use..

[andrecn]

Hello, thank you for your reply.

The ballscrews are driven on 2:1 ratio belt by the servo motors. I'm currently setting up the backlash compensation and I must admit I'm quite happy with the results. The original DC motors had a tachogenerator and the encoder, as I mention before, were at the ballscrews. Is it OK to leave them at the motor or changing them to the ballscrew position give any precision improvement? At the motors I'm using 8000 counts encoder, if diretly attached to the ballscrew, it would require I higher resolution?! Right?

Regarding all that, what if one of the belts snaps ? How do the controller knows ?

Thanks in advance,

Cheers

[cnckeith]

the controller doesn't know if a timing belt snaps. this is quite rare if they are installed properly.
more often i see motor shafts break (due to overtightening the belt!!) than timing belts themselves.

i would leave the encoder on the motor and add scales on the axes to make up for any mechanical positioning problems.
number one is to improve the mechanicals as much as possible and then you can add scales if you still need more accuracy.

uwe just posted a video on how he did it with AcornSix. (cnc12 for allin1dc also has this feature)

[cnckeith]

and create a photo album of your machine so we can see if and give you the best advice. put the photos in a google photo album and post the link along with your questions.

here is how.