[6789den]

I ‘m new to CNC building.
I would like to use a DRO / linear encoder for controlling the axis’ on my first DIY router build for a closed loop system. My build so far consists of a steel frame with THK linear rails. I’m leaning towards the Mach4 CNC software. I plan to use NEMA 23 stepper motors.
I really don’t want to use motors with built in rotary encoders. I have heard that you can use linear scales in place of the encoders on the motors.
I’ve heard good things about doing it this way:
backlash free from the get go, linear scales read the actual table position, easily configured, connect the motor to controller, you can tie the motor encoder to the drive as normal but reads from the linear scales. more failsafe if I break a motor shaft or there is inaccuracy in the ballscrew
And bad things:
impossible to do any kind of feedback loop, instability cause by too much delay between the motor and carriage.
blah blah blah. Too much conflicting info and technical stuff to sort through.
I’m on a budget and was wondering if I could make these work? https://www.ebay.com/itm/800MM-Linea...l/233243351657
Just in case the link doesn’t work they are

Linear Scale For Milling Lathe Machine Linear Glass Scale Double Seal
Specs:
Voltage: 5V DC
Current: 50mA
Travel Length: 800mm
Grating Pitch: 0.02mm (50LP/min)
Resolution: 5μm (0.005mm) 0.0002"
Accuracy: ±0.005 mm or 0.0002" (at 20 deg C)
Response Speed: 60m/min or 40inch/sec
Cable Length: 3m
Sealing: Double seal
IP Class: 53
Product Dimention(LxWxH): 94 x 2 x 5cm (37.01" x 0.79" x 1.97")

If they will work will they just connect to the encoder input on the motor drive and off I go or is there more to it? Do I need some type of motor control board or what?
Any help would be greatly appreciated.

[Jim Dawson]

My best guess is that the closed loop stepper drives want to see the encoder that is installed on the motor for the scaling to be correct, unless the drive can be adjusted for different encoder resolutions.

Using linear scales with steppers is not very easy, I have done it, but not using hobby class hardware. Maybe a Dynomotion Kflop would work, or maybe the Kflop/Kanalog board. You need to close the loop at the controller rather than at the drive.

Rather than Nema 23 steppers, why not go with some Nema 23 analog servos. That way your linear scales would play nice your Dynomotion Kflop/Kanalog system.

My personal preference is magnetic scales rather than glass. Much more compact and very robust.

[machinedude]

Using linear scales with steppers is not very easy, I have done it, but not using hobby class hardware.

My personal preference is magnetic scales rather than glass. Much more compact and very robust.

this is something i am looking at right now for myself. and it looks like Linux cnc with some Mesa hardware can provide the feed back for position from linear encoders. at first i was thinking about glass scales until i dug up more information on the types available and quickly seen why magnetic scales are probably a much better choice on a large machine.But Mesa hardware is reasonable in price if you can get past the Linux cnc learning curve. i have not done much of anything with it yet but it looks like the direction i'm headed in most likely.

[Deluminati]

Can a digital caliper be stripped to be a suitable linear encoder for position feedback on axes? I'm trying to put a little machine together on a budget...

[Jim Dawson]

Can a digital caliper be stripped to be a suitable linear encoder for position feedback on axes? I'm trying to put a little machine together on a budget...

If you're clever and can figure out how to interface the caliper electronics with your controller then it might work. But there are most likely easier ways to do it.

[joeavaerage]

Hi,

backlash free from the get go, linear scales read the actual table position, easily configured, connect the motor to controller, you can tie the motor encoder to the drive as normal but reads from the linear scales. more failsafe if I break a motor shaft or there is inaccuracy in the ballscrew

Mach4 is not natively a feedback controller, so closing the loop with Mach4 is not possible. In fact its not possible with ANY Windows based CNC software, Windows all but precludes realtime operation
and therefore the comms time delay precludes high bandwidth closed loop operation..

There are a few alternatives.
1) Use Mach4 but partner it with either a Vital Systems Hicon or a CSLab CSMIO/A controller. Both of those controllers can close the loop. This would be a potentially complex undertaking.
2) Use Mach4 and a Galill controller. Galill is a very good and very capable controller that will handily accommodate a linear scale. Galill are not cheap, about $2000 for a new 3axis controller.
3) Use LinuxCNC, the Linux distro used is very close to realtime and therefore can be used to close the loop.
4) Use Delta A2 series servos, these have dual encoder channels, the one regular and essential rotary encoder built into the servo but can accept a second load mounted encoder, a
linear scale for instance, for position loop control. Very elegant solution but requires A2 servos/drives at around $500 each
5) Use decent ballscrews which don't have any lash, then the rotary encoder would report the actual position as faithfully as a linear encoder. Many, in fact the majority of industrial CNC work this way.


There is an increasing use of linear scales in top end machines, and the advantages that accrue from doing so are very much inline with your quote. Using servos and drives that accommodate dual encoders
have been instrumental in this development. The Delta A2 series is a good example but all the top end servo manufacturers have the same capabilty.

My new build mill, literally commissioned over the last few weeks uses Delta B2 series servos that have a 160,000 count per rev rotary encoder, but do not accommodate a secondary encoder. The
ballscrews I have used are 32mm THK double nut ground C5 spec units. As much as I might like linear scales there is little need, the ballscrews are accurate enough.

Craig