Increasing CNC precision

[hani_a]

I have a CNC router I'm trying to make more precise using a feedback system. I have done some research on the types of encoders I might need and found that linear encoders might be more suitable for the job at hand. So far, I found several companies that claim to have "low-cost, high resolution" encoders and was wondering if anyone knew more about where I could get decent linear encoders. I'm looking for resolution in the 100 um or less range. During my search, I came across the following linear encoder and wanted to know if anyone knows how they work or what the maximum distance could be..

http://cgi.ebay.com/ws/eBayISAPI.dll...065305880&rd=1

Thanks in advance!

[ger21]

You need to make sure your controller can take the encoder feedback and compensate for it.

[Al_The_Man]

To me that looks like a reading head only, you need the magnetic tape, they profess to go to a usable 70m.
Renishaw is usually very expensive, top end equipment.
Al.

[Mat-C]

http://cgi.ebay.co.uk/GS300-370-Stan...QQcmdZViewItem
(Europe only, but you might ask them nicely...)

http://cgi.ebay.co.uk/Meister-Linear...QQcmdZViewItem
(Worldwide)

I've been looking at these too... anyone have any experience?

[CarbideBob]

Has anyone here done this? Actually "close the loop" in real time while in motion using a linear encoder and not just correct at the end of a move? What kind of controller? How much better did it work?

[Splint]

I was always under the impression that using linear encoders was undesirable because of the "backlash" effect, would high resolution encoders on the motor shaft be a better option?

Splint

[DennisCNC]

Carbide,

That would be a expensive system to set up, you will need to use a motion controller like Galil and have code written for it and have very minimal backlash. I think many machine makers have experimented with this idea but nice ballscrews and linear rails solve the problem for most of them.

[Al_The_Man]

Originally Posted by CarbideBob Has anyone here done this? Actually "close the loop" in real time while in motion using a linear encoder and not just correct at the end of a move? What kind of controller? How much better did it work?

I think you will find an explanation video on the Galil tuning site that covers this subject using dual input, encoder on the motor and a linear scale, the PID is split between the two.
Al.

[CarbideBob]

Yes I use Galil controllers (and I love them). When Galil first introduced this in the early 90s thinking this was the "magic bullet" I ran out to the shop, mounted up some encoders and tried it (dismal failure). Since then I've tried it on several machines but have not been impressed with the performance. This of course uses servo motors. It seems to me that this is impossible with steppers because if you miss a step you are past the ability of the motor to respond and pushing extra steps to it won't help.

[Mat-C]

It seems that when you get as far as linear encoders, manufacturers use zero backlash gearing, ground ballscrews and/or linear motors. All jolly expensive.

Given that glass encoders are relatively cheap I'm wondering if there is a decent homebrew solution using steppers and software compensating for any backlash etc.