Homemade Laser Encoder?

[KyleL]

I happened to stumble on to a Youtube video demonstrating a homemade laser interferometer, what surprised me was how simple the setup was and how accurate the sucker was.

For those that don't how laser interferometry works. Basically a laser of known wavelength creates a 'reference' beam (a beam thats not actively being interfered with). There are a number of configurations, but they all share the same common idea, the reference beam is split (or another beam is used) and bounced off a target object. The two beams interact with each other, the constructive/constructive interference from the beams is then typically bounced to some sort of detector. Conceptually, its 3 separate beams, the first is the reference beam beamed to the target, the second is the reflected beam from the target, and the third is the combination of the reference and reflected beams.

... So, uh, how does it help measure stuffs? Lets say the object being measured is exactly one wavelength away from the 'emitter' (the point where the reference beam 'ends') the reflected light will be directly in phase with the light from the reference beam, so they add together, and suddenly you have a light at the same wavelength that is twice as bright as it was when it was emitted. If the object is the moved for 1/2 the wavelength, (its not 1.5 y away from the emitter), the reflect light would be 180 degrees out of phase, thus canceling out the light from emitter entirely, the resulting beam would be non-existent. Each pulse (from full brightness to darkness) is 1/2 the wavelength of the light, so if the wavelength is known, and the number of pulses is known and their 'direction' you can precisely tell the position of an object down to 1/2 the wavelength of the light used ...aka really frig'n accurately!

I don't think it would be ... too ... hard to construct a laser interferometer, they've been built since the early 1900's to things in to perspective. It would require a laser of some known wavelength, a beam splitter, a photo-diode sensitive to the light wavelength, and some control electronics.

Anyone think we can design one of these things to help position a mill or lathe?

I don't believe the mechanical aspects are terribly difficult for someone to put together but the electronics are going to be the make or break point. A 1080nm laser would have a "resolution" of 0.00054mm, and every vibration from a machine tool is going to show up. However, assuming you can count all of the vibrations, they should cancel out in the long run. The real challenge is being able to poll the photo-diode fast enough so there is enough data to remove the noise from the machine vibrating.

For example at 1080nm, there would be 47,038 'pulses' per inch. At 100 IPM, that becomes 78,397 poll/second just to keep up with the movement of the object. Now, consider that your milling with a 8 flute end mill at 10k rpm (just throwing out worst case numbers), thats 80,000 impacts per minute, or 1,333 per second. By this point, almost 80k polls/second are needed just to be reasonably sure of the current position. As a factor of safety, it would be nice to increase the polls/second to somewhere around 160k-400k.

160khz - 400khz+ is not really that fast in the micro-controller/microprocessor world, but, in order to 'poll' the photo-diode you're going to need a really fast analog to digital converter, is this even feasible?

Thanks for reading, its kind of long post!

[note]Haha, just noticed my 'diagram' has the resultant beam and photo-diode on the wrong side of the beam splitter..[/note]

[YZF]

Originally Posted by KyleL 160khz - 400khz+ is not really that fast in the micro-controller/microprocessor world, but, in order to 'poll' the photo-diode you're going to need a really fast analog to digital converter, is this even feasible?

Not sure about the rest of it but A/D at 400Khz is not a problem - 1Mhz, 8 bit, $3 in Digikey (you can go into the 100's of Mhz and probably much further today, I'm not up to date...)

How about a link to the youtube video?

[KyleL]

Originally Posted by YZF Not sure about the rest of it but A/D at 400Khz is not a problem - 1Mhz, 8 bit, $3 in Digikey (you can go into the 100's of Mhz and probably much further today, I'm not up to date...) How about a link to the youtube video?

Sure, there's tons of them:
http://www.metacafe.com/watch/138154...memade_for_20/

YouTube- Michelson Interferometer

YouTube- Michelson Interferometer

YouTube- The Michelson Interferometer

YouTube- The Michelson Interferometer

YouTube- Homemade michelson interferometer

YouTube- Homemade michelson interferometer

YouTube- Lasermikrofon - Interferometer

YouTube- Lasermikrofon - Interferometer

[Zygoat]

it would be awesome. ill be watching.

[marzetti]

Very interesting. So what sort of detector would you use to measure the interference?

[Zig]

Electronically it is not hard at all to achive the sensing and interpretation of results.

the whole measurement system would require a fairly stable kind of a frame.

In fact You would not be using something like this during machine operation.

This is a reference instrument .
One would use it to calibrate an existing systemand store calibration points ( as is the case with EMC2 software)>

Onto Optics.

The interference patterns shown in photographs are stationary patterns. Under normal use they would be dynamically changing patternsand a photo detector ( probably PIN photodiode ) would be used to detect the the troughs an peaks of the moving interference pattern caused by relative lenght changes between refference and measurement paths.

Electronics to achieve this measurement and convert to logic level signals are fairly stright forward... PIN photo diode detector/pre amplifier followed by an analog comparator. Outcome is a way of counting wave length change s between two paths
A way to locate the second such channel a quarter wave length apart is needed so that a quadrature signal is obtained.

This signal then could be used in conjunction with a couple of D type flipflops to decode DIRECTION signal and some further logic to decode the four unique states within one wavelength ( given quadrature signals ) so that the resolution of measurement changes from half a wavelength to a quarter wavelength.

This is all predicated on a sturdy stable frame and some data filtering to eliminate noise.

[YZF]

I think the OP was thinking of digitizing the analog signal from the photo-diode and processing it in software on a micro-controller. Should be doable.

It wouldn't surprise if commercial interferometers modulate the laser to improve the signal to noise ratio. You can have many different ambient lighting situations in different environments.

By the way, these sort of devices can be used for real time feedback. In fact you can buy interferometer based feedback systems off the shelf, e.g.:
http://www.renishaw.com/en/laser-encoders--6404

[vger]

I think the head end of the detector could be fairly simple. Just use 2 photo diodes sensing the rings (fringes) spaced so that they output a sine/cosine function. If the ring spacing is 1" at the target plane, then space your photo diodes 1/4" apart. Then you would have an output like a standard encoder. Connect that to a hardware up/down counter of say 24 or 32 bits. Just disreguard a few of the least significant bits and sample the count at what ever rate you would like.

added:
You will need an additional latch circuit to freeze the count so you can grab the count 1 byte at a time. If you are counting full cycles (of 1080 nm) and use a 24 bit counter you will have a maximum count that relates to 18.11939328 meters. Your least significant bit would be worth your 1080 nm and if you throw away the 4 least significant bits, your new least significant bit (bit 4) would be worth 0.01728 mm or about .000680315 inches.

As for vibration causing problems... it's not so much the frequency of the vibration but the velocity. In vibration work a machine with less than .005 inches/sec velocity reading is considered very smooth running. At 900 RPM, a peak to peak displacement of .0001 inches will give you a peak velocity of .0049 inches/sec.
At 30K RPM, that same .0001 inch peak to peak (total displacment) will give you a peak velocity of a little more than .157 inches/sec and would be considered slightly rough operation. This would give you a maximum of about 4 Khz of noise at .157 inches/sec velocity. Of course other things can cause error noise in and interferometer reading.... if the angle of the moving mirror changes or oscillates perhaps because the bed of the lathe is resonating, or the slide is racking even the slightest amount. Another issue is your laser frequency drift due to temperature changes. Helium Neon lasers are much more stable than LED lasers without temperature compensation getting into the equation.

Steve

[YZF]

My bet is that a simple, comparator based system, will not work. Especially not with your laser pointer style, low power lasers, where the ambient light will swamp the laser. Note how in all the videos the target mirror moves very short distances, only a few microns. When you move that mirror a few orders of magnitude higher distances you will start seeing your laser diverge and move. Even in those videos there's a fair variation in how the fringes look on the image plane.

KyleL: Why don't you try an experimental setup on your CNC? just put a white screen where your sensor would eventually go and don't worry about the electronics for now. Then you can jog your axis around and get a better feel for how simple (or not) this system really is.

[vger]

You would want to house the beam to eliminate interferance from objects and temperature differentials in the air. Using a bellows (small flexi air duct) supported by a wire or cable would work. Interferometer encoders are available comercially like these... http://www.renishaw.com/en/rle-system-overview--6594 ... but are pretty pricy. In Kylel's original drawing there is one element missing... a path for the reference beam. The beam from the laser is split by the beam splitter (small ones are available in the read head of a CD rom drive) and the reflected beam goes to a stationary mirror refelcting the beam back through the beam splitter (see attached). The Renishaw system uses fiber optics to deliver the beam from the laser to the detector head which contains the interferometer and detection sensors.

Steve

[KyleL]

Originally Posted by YZF My bet is that a simple, comparator based system, will not work. Especially not with your laser pointer style, low power lasers, where the ambient light will swamp the laser. Note how in all the videos the target mirror moves very short distances, only a few microns. When you move that mirror a few orders of magnitude higher distances you will start seeing your laser diverge and move. Even in those videos there's a fair variation in how the fringes look on the image plane. KyleL: Why don't you try an experimental setup on your CNC? just put a white screen where your sensor would eventually go and don't worry about the electronics for now. Then you can jog your axis around and get a better feel for how simple (or not) this system really is.

I'll give it a shot once I find a laser pointer, might take me a bit before I get a chance but I'll let you know what I find out.

I had planned on enclosing the whole system in to some sort of container that would not allow ambient light (or maybe a low amount of it) in btw.

Thanks for all the replies this is rather interesting!

[vger]

As for ambient light... if the sensor optics are enclosed you shouldn't have much problem as most of the ambient will be off axis of the laser return beam. Just use a flat black paint on the inside of the enclosure if needed. You could also add a small tube at the apeture, say a 1/4" brass tube smoked or painted on the inside to further reduce any ambient.

Steve