[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?

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.

[NC Cams]

Bridgeport Machines tried the glass scale/servo interface on their early Extrak machines and ended up replacing them with motor mounted servo encoders.

Since there always will be some hysterisis and/or lost motion between the servo and the glass scale, it is pretty much impossible to get accuracy and/or vibration free motion due to the lost motion issues.

Both DZASTR"s and ZUMBA's admonitions are pretty much spot on. The use and interfacing of glass scales is tempting (especially with DRO's as they are) but it has been proven that this sort of feedback is not a wheel that needs to be reinvented.

[CoolHand]

It's not a problem that's impossible to solve. I have a Moog VMC manufactured 15 yrs ago with Heidenhain glass scales on it that will still repeat down to a tenth. I know it's had a less than easy life, so it can't be said that the machine has no wear at all either.

It IS possible, and has been done with great success by a number of manufacturers.

Perhaps it's out of the realm of what a DIY'er can handle, but I'm not even convinced of that.

[Al_The_Man]

Here is a Galil video that shows how it is done by tuning the PID in an Advanced dual-loop system.
http://www.galilmc.com/training/v_du...mp_method.html

[js412000]

Using scales for feedback could be a way to get by with less than perfect ballscrews. I used to run a 1980 vintage mill with 11 feet of x axis travel, and heidenhain glass scales, and a point to point control. The ball screws were not very good. The z axis had .100 inch backlash in the ball screw, but the head counterweight was on the light side, so the backlash was always in the down direction, and was not a problem. The y axis would sometimes overshoot .010 to .012 inch if you tried to rapid up to the desired position. The x axis would occasionally get "stuck" .001 away from the position, the servo motor would just sit there humming for ,what seemed like a minute, until it got there. If the table overshot, it would not go back. It was not a very good machine, by todays standards, but much better than a manual machine.

[CoolHand]

Here is a Galil video that shows how it is done by tuning the PID in an Advanced dual-loop system.
http://www.galilmc.com/training/v_du...mp_method.html

Bingo.

I was just about to post that when I replaced the X Axis drive motor on that machine that the servo had an encoder lead, meaning that the control was using BOTH encoders to control the oscillation and overshoot generated by any backlash in the system.

It is very easy to see how a system with only the linear scales and a lot of backlash would hunt for it's end point, and that video lecture (which was quite well done BTW) explains how the control can make use of the second encoder to damp the unwanted motion out.

[NC Cams]

The isssue of whether or not it can be done is essentially a moot point - Reason: someone can and will have the skill and technology and take the time and effort to do it. The problem comes when you start dealing with the how to and how much does it cost.

Any number of times that I've explained how to get milling machines (ala Bridgeports and/or the clones) to get essentially zero backlash/hysterisis axis', an issue is raised about the cost. Sadly, the cost for precision, zero/near zero compliance ball screw bearings is typically out of the range of affordability for the DIY'er.

Yes, you can use less costly bearings BUT they can not/will not have the performance that the "good ones" do. Result: you will end up with compliance/backlash. Depending on your feedback logic, you may or may not be able to deal with/manage/eliminate the resultant lost motion from the compliance.

Then you get into the issue of motor response. ANY backlash can be dealt with UNTIL/UNLESS you want rapid/high speed moves and do a lot of rapid direction changes. Keeeping up with bidirectional slop becomes a never ending nightmare after a while.

We tried it that way (tuning) and finally gave up. We then set out to get rid of the slop/hysterisis and, after spending lots of time and money, we still had to tune the motors. However, at that point, motor tuning basically dotted the I's and crossed the T"s with regard to smooth and deadly accurate system motion.

I suspect that factory equipped with glass scale equipped servo systems that are deadly accurate with no backlash were engineered to be that way from the get go. They probably have preloaded ball screws and well fitted/adjusted ways or gibbs. They also probably have a very good set of servos and servo drives.

Yes, it has and can be done. It has also been tried by seasoned pro's and abandoned because they couldn't get it to work adequately/properly and/or economically. Like most successful endeavors, it is all in the planning and execution.

[CoolHand]

Now that I will certainly agree with.

My machine has preloaded ball screws and huge end block mounts, which makes for very nearly zero backlash.

Even in that system you do have to tune the servo drives to each motor (which was a rather large PITA I will add).

If I were to build a machine from scratch I would bypass the linear scales in favor of a conventional rotary encoder and a drive system that eliminates as much lost motion as possible. In the end it will cost just as much, but it'll be quite a lot easier to set-up and tune software wise.

I've been pondering taking a large swing manual lathe and converting it to linear guides, ball screws, etc, but I am afraid that by the time I do a good job on the conversion I will has spent at least as much as what a good used slant bed costs, and I will still be out the ages of time spent on the project.

I guess sometimes you just gotta know when to back off and punt.

[wizard]

Does anyone have experience using linear glass scale as servo encoders on vertical milling machines....

Well not vertical milling machines but diamond turning lathes yes. While still machining a huge difference in technology.

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.

Garbage! It is a question of where the vibration is, but if the gain is high enough vibration impacts rotary encoders just as much as linear. Now this doesn't mean that specific mounting arrangements with linear encoders won't make issues with vibration worst, just that it is another engineering problem.

Given all of that vibration would kill the whole machining task in a diamond turning lathe so great lengths are taken to isolate the machine form the environment. Even so when the lathe switched over to rotary encoders it still would react to vibration.

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

I'm not sure why you would want to do this, in fact I'd think it would be regressive. A properly mounted linear encoder is the only way to go with a DRO as it eliminates the problems associated with wear and backlash.

Thanks,
Dennis

The equipment I use to work on had both types of encoders mounted on each axis. The linear encoders for gross movements with resolution of about a micron and the rotary encoders to provide very high resolution at a much slower speed. When running on the rotary encoders the resolution was out to the nanometer range even if the lathe didn't resolve that in the real world.

In any event the control lops had very high gain. Have either one of the encoders go bad ( develop a bas spot on the engraving) and the drives would react to the faulty device. ... And do very bad things!!! Same thing with vibration, at the wrong time and in the wrong place the drives would react to the vibration.

Dave