Glass scales as encoders

[dcprecision]

Does anyone have experience using linear glass scale as servo encoders on vertical milling machines.... I recall reading that dro scales such as Anilams will cause servo fault problems due to machine vibration but motor shaft encoders are not affected.

Also has anyone used their servo motor rotary encoders with an Anilam DRO. If so,, how is it done........

Thanks,
Dennis

[Al_The_Man]

Heidenhain have a mill system that uses their Scales for position FB. You may get problems if the backlash is high, for precision ground ball screws it is not usually a problem.
Electronically, the scales are the same as rotary encoders.
Al.

[DareBee]

Having a scale system is regarded as more accurate than encoders.
Encoders read off of the motor or screw and do not actually give a "real" measurement - if there is backlash the system will not compensate (by itself) and parts will be wrong.
Scale system gives direct measurement of the table travels so moves are always the right size. If machines get too much backlash then the servo tries to position the table, normally the table (when sloppy) will overshoot the target repeatedly back and forth and things get kinda crazy, but at least you know there is a problem.

[dcprecision]

So it seems that there is some possibility here.. I have a RF 45 equipped with an Anilam wizard 800 dro. I am currently installing ground ball screws that I obtained a while back on the x - y, and will eventually also add servos to cnc it....

Has anyone actually built a system using the linear encoders. If so, can the DRO operate in parallel with the servo boards.. What kind of isolation may be needed ???

[DareBee]

You will have to throw the DRO in the garbage and send the scale signals into a controller card and go from there.

[dcprecision]

Woh,,,

Can't throw a $ 1500 programmable dro in the trash.. Is it possible to read the quadrature output of the glass scales with both devices ???? The scale system is running on 5VDC regardless of the dc source.. The principal idea in this is to capitalise on both the dro as a backup or manual use and also use linear encoder direct measurement for better servo accuracy... If it cant be done I would still leave the dro intact and use rotary encoders on the servo motors...

[Al_The_Man]

It could be done, but it involves some fancy (electronic) foot work.
I would power the scales from the DRO, this would mean the DRO would be on at all times, and make a TTL buffer circuit fed by the output of the scales and output the buffer signals and common to the input of my control, keep in mind that the two systems would not be isolated.
It might pay however to use the two independantly and plug the scales into whatever system happens to be in use.
As you would only be using one or the other.
Al.

[NC Cams]

Bridgeport tried the linear scale route to feedback the Z axis on their Extrak mill. Ultimately, they had to go to motor encoders, supposedly due to vibration, hunting and oscillation problems.

Pretty sure it was because they did NOT have a true ball screw drive on Z but rather a rack and pinion as I recall.

The problem you always have to deal with is hysterisis and/or slop.

If you monitor the motor position, the servo will go where you tell it and stay there. The table might not be exactly where you want it due to slop/backlash/whetever but that is usually manageable.

If you use linear encoders and have backlash in system, the motor will tend to hunt as the table is going to move and the motor WILL try to keep up with fixing/adjusting for the movement.

I'd contend that a sloppily/uneven worn gibb based system would be especially problematic with a scale system due to the slop potential.

If you have linear guides that are slop free, the scales would probably be dead on - but then too, so would a well tuned motor mounted encoder based system.

[RICHARD ZASTROW]

We had a similar but exagerated problem. 1957 Niles 30 ft. dia. vertical boring mill remanufactured to 4-axis with Siemens 840D control. The size of the machine & workpieces caused the Heidenhain glass scales to vibrate and cause loss of feedback. We replaced the glass scales with Newell Spherosine round tubular rod running thru a reader block that stabilized the rod at the measuring point cancelling the vibration problem. It worked so well that we replaced the other scales on all the VBM.s in that shop, even the manual machines with DRO's

[unterhaus]

I suspect that successful application of linear scales probably includes a sensor on the motor and careful tuning or sensor fusion. I know people are using sensor fusion for this problem.

[rcduncan]

Can I ask did you get the ball screws cut for you to size or are you machining them yourself? Where did you get them?

Originally Posted by dcprecision So it seems that there is some possibility here.. I have a RF 45 equipped with an Anilam wizard 800 dro. I am currently installing ground ball screws that I obtained a while back on the x - y, and will eventually also add servos to cnc it.... Has anyone actually built a system using the linear encoders. If so, can the DRO operate in parallel with the servo boards.. What kind of isolation may be needed ???

[Zumba]

IMHO, a linear encoder on a CNC is just a way around doing things right... that is having a rigid, vibration damping frame, accurate linear rails/ways, and an accurate ballscrew system.

If all the components are functioning the way they should, even a cheap no-feedback stepper system should give you excellent results with error that's difficult to measure.