Laser inteferometer in feedback loop

[csirac2]

Is that done, or deemed unnecessary by most?

I understand there's a lot of variables to accuracy: I've read the ball screw primer thread, which emphasises good bearings as well as high quality lead screws and nuts/backlash setup. I've read the epoxy frame thread to death, for temperature stable rigid, vibration damped performance. High-res encoders limit feed rates for servo systems; not a concern for me (100 inches per minute would be plenty).

I want to build a large format imaging device, on the scale of 2m x 3m x 0.5m travel. I'd like to achieve 0.0003" positioning resolution in the hope that I could get 0.0006" repeatability anywhere in the x/y plane. For my application it's mostly the repeatability that I want, but accuracy would come with it. I suppose I could easily afford to lose/gain 0.0003" over 5 or 10 inches if that was useful.

I have contacts at Agilent but I know there's just about nothing they have in their catalogues that'd like the price of as a hobbyist individual :-) And nor would Optodyne, Reinshaw, etc.

How are people using interferometers in the servo feedback loop? What about the measurement delay/lag? I'm particularly interested in being able to measure squareness of gantry and tool travel.

Lasertex seem to sell a product called "LaserScaler", which would replace glass scales, it seems:

http://www.lasertex.com.pl/index.php...=products&id=1

Given that this an imaging application, and I'm not actually loading up my servos with any kind of cutting etc., this makes things much easier for me - for example rigidity requirements of the design are much reduced. I think that if I also used something like the LaserScaler in my feedback loop, or as part of a start-up automatic calibration routine, I could also contemplate making my machine frame "portable" (maybe relocatable is a better word: being able to transport and then spend maybe one day to rig-up the machine would be immensely valuable).

Thoughts appreciated... especially insights into the cost :-)

[BillTodd]

Since you're only moving an imaging device. Rather than worry about positioning your heavy gantry to that kind of accuracy, why not have a secondary high accuracy XYZ positioning device carried by the gantry? (high accuracy small movements are cheaper than large movements usually)

[Geof]

I looked up your link and found this:

Measurement accuracy:
- without meteo sensors 10ppm
- with meteo sensors 1ppm
- in vacuum 0.01ppm


I guess it is safe to rule out the 'in vacuum' figure is being not applicable.

However I cannot help thinking even 10ppm is a bit of overkill; unless you are planning on doing everything in a very tightly controlled environment, temperature, humidity and vibration.

[csirac2]

Originally Posted by BillTodd Since you're only moving an imaging device. Rather than worry about positioning your heavy gantry to that kind of accuracy, why not have a secondary high accuracy XYZ positioning device carried by the gantry? (high accuracy small movements are cheaper than large movements usually)

I'm aware of this concept, used in CD/DVD drives for example. However I didn't think it'd be that expensive to have fine positioning over such a large area? A lead screw with 6 turns per inch gives 500 stops per revolution to achieve 0.0003" resolution. If the travel is smooth enough and my servo motor is geared slow enough, that kind of resolution should be possible - accepting the slow travel?

I could even use steppers, seeing as there's no cutting loads to consider, and I can accept slow scanning speeds because that's going to be slow anyway. It's just that the startup autocal procedure I have in mind (approx. 1600 inches travel at the moment which I'd tolerate up to 30 minutes to complete) would benefit greatly from a fast servo setup.

[csirac2]

Originally Posted by Geof However I cannot help thinking even 10ppm is a bit of overkill; unless you are planning on doing everything in a very tightly controlled environment, temperature, humidity and vibration.

Well, if the price is right, overkill is nice isn't it? :-) Still waiting on a price back from them. Also, 10ppm over 3m/120 inches is only about 0.0012"... not sure if that is how that spec actually works (parts per million of full scale?), still waiting on documentation.

It will be working most of the time in a temp/humidity controlled environment actually, 24-27 degC, 40-60% RH and I'll be doing my best to eliminate vibration if it becomes necessary.

I was just curious if anybody out there was fixing any kind of permanent laser distance measurement onto their CNC machines in conjunction with or in lieu of glass scales.

[Geof]

Originally Posted by csirac2 .....I'll be doing my best to eliminate vibration if it becomes necessary.....

Move into the Outback, way out back .

[csirac2]

Originally Posted by Geof Move into the Outback, way out back .

It's not perfect, there's still the problem of Kangaroos hopping by, and drop-bears

[CarbideBob]

Hope you are sitting down when you see the price. Wouldn't want you to fall down and hurt yourself.

Laser interferometers are used as feedback devices in ultra-precision machine tools (like diamond turning lathes for making optics), high end CMMs, and semiconductor processing.
Commonly used to calibrate machine tools but there is no way you are going to map out a 3 axis machine in 30 minutes. More like 6-10 hours.

You can get glass scales with resolutions down to 1/10 of a micron and they are much more forgiving. Accuracy without external calibration can approach 1 micron.

Are you trying to build something like a video CMM. http://www.acu-gage.com/

When building this type of equipment pitch, yaw, roll, straightness, and squareness errors from your slides will be your biggest design challenges.
Maintaining less than .0003 abbe error over a 2 meter assembly will demand very high end bearings and a very rigid base.
Bob

[csirac2]

Originally Posted by CarbideBob Hope you are sitting down when you see the price. Wouldn't want you to fall down and hurt yourself.

That's okay. I was really curious as to whether these things had five or six digit pricing, that's all :-) I am hopeful that Lasertex's LaserScaler product would be significantly cheaper, given what it's supposed to replace.

Commonly used to calibrate machine tools but there is no way you are going to map out a 3 axis machine in 30 minutes. More like 6-10 hours.

Actually my 30 minute happy dance routine would be registering the imaging device against standard targets and establishing optimum scanning parameters, not mapping out axis positioning errors. But 6-10 hours to map out the machine has given me pause for thought... This is to map out rotary encoder counts vs actual distance travel? Mapping backlash? I had assumed that if the laser was in the feedback loop, you wouldn't need to map out anything.

You can get glass scales with resolutions down to 1/10 of a micron and they are much more forgiving. Accuracy without external calibration can approach 1 micron.

Glass scales will be my next avenue of investigation, the attraction to the lasers was being able to measure up for squareness. Also, how "transport friendly" are glass scales when mounted...

Are you trying to build something like a video CMM. http://www.acu-gage.com/

Yes, somewhat similar.

Thanks for your thoughts, I appreciate it. I'll let you all know how much the LaserScalers come to, if they ever get back to me :-)

[csirac2]

Well, I got back an example system that they recently put together for 6-axis and I stripped out the components that I thought I would not need in my application (which was really just guessing, I need to talk to them more about what accessories I need), and it would appear that it comes to 9450 (euro?) without a Wollastone straightness kit (3000 by itself).

So, this price is actually far better than I would have expected from Agilent.

The full six-axis system with environmental monitoring/compensation was 31,000.