IGaging Magnetic Scales, Yuri's TouchDRO & LinuxCNC

Looking for experience and comments on the feasibility of my rough idea...

I'm converting an LMS 5500 (Seig SX2.7) bench mill to CNC with the following:

1. Ball screws from a machine-specific kit that have more backlash than I'd prefer (but I'm stuck with them for now)
2. An existing setup of Yuri's Bluetooth TouchDRO interface board with IGaging magnetic scales and a 9" tablet for display
3. A headless Raspberry Pi3 controller running LinuxCNC
4. An MPG pendant for using the mill manually on a regular basis

There will be no display at the machine for Linux CNC, it will be controlled from a PC across the shop. I'll have dedicated program start/stop and touch-off buttons on a control panel at the machine.

My questions are:

1. Has anyone used these magnetic scales as encoder inputs in LinuxCNC? The sparse comments in the forum here are old and if anything, indicate the scales may be too slow. I'm curious if anyone has implemented these on a "slow" machine and succeeded. My systems will not be doing anything high speed for sure.
2. Is anyone familiar with the Yuri's interface board and whether it might be capable of sharing the DRO signal to both the TouchDRO display and LinuxCNC?
3. Is there anything about this approach that immediately fails a basic sanity check?

I guess the first question is: has anyone used iGauging-type cheap Chinese scales as closed loop position feedback hard-wired?

Ideally, I was hoping to share the signal coming from scales to feed position to a Mesa card (hard wired) for closed loop, and maintain the existing tablet display via Bluetooth. The scales connect to the Bluetooth board via mini-USB, so I'd have to create a physical split - if the approach could even work.

For now I definitely need manual control, since I'm dependent on some fixtures that will require a lot of R&D to replace with anything automated. I do get what you're saying though - don't engineer-in crutches without a reason.

The connector is USB, but the protocol is not - it's a distinct 21-bit protocol all its own. I've looked at some of the project notes at Yuri's site and it gives the pinouts as 3.3V, Data, Clock and Ground. I was hoping someone had investigated this in Linuxcnc, even if they hit a dead and. I have the system and I'd like to use it in the refit if possible. It's not what I'd buy if starting from scratch, but since I've got a few hundred dollars in it and no other use for the equipment I figured I'd ask around.

Thanks for giving me those links - very helpful. I'll spend some time on it this week and see what I can learn.

DanMN wrote:

Thanks for giving me those links - very helpful. I'll spend some time on it this week and see what I can learn.

There was a machinekit driver for beaglebone and rpi for all those cheap scales, but I would guess those scales arenot suitable for what you want (do not let that discourage you from trying though! ).

you will need level converter (I think the scales have max tolerant of 1.2volt input), its a simple serial protocol (there are about 5 variations of the protocol), you will need to bitbang the data from 2 gpio pins via some amplifier.

its very slow (update at about 3Hz), seems there are errors from noise if using any length of cable above 100mm(I found would get a good readin with filtering once every 2 seconds with a 2m cable)

I could post links but I really do not have the time, google will giver plenty details if you search for the right keywords.

Thanks for the additional keywords - that should get me going.

Also, according to a post I just found on Yuri's blog, the scales I'm using have a clock of 2KHz and 250 Hz refresh -- so they may be a bit more useful than the older, cheaper version. They are the " iGauging DRO+ " scales.

I believe Yuri has also given the recipe for the level converter here

I'll be checking it all out this week.