Linear Feedback using a HeNe Laser

Description

CNC relies methods for determining where an axis is. To do this methods from open feedback methods like stepper motors to closed feedback like servos provide the essential ability to determine where a machine is. Ultimately, the precision of the machine is determined by how accurately you can determine where an axis is. A traditional CNC is good to about .0001 inches, which is fine for manufactured goods, but horrible for nanotechnology and other emerging manufacturing processes that need ever more precision. By using a simple Michelson interferometery setup, precision on the order of wavelength of visible light can be achieved. Demonstrated is a simple axis positioning system using a HeNe laser and a simple light detector.

Details

Basics of the Michelson interferometer.

When you move the one of the mirrors, you are changing the distance one beam has to travel, and therefore the interference pattern changes. As you move the mirror, it appears to move outwards (or inwards). By using a simple photo detector, you can easily count the number of "peaks" and "valleys" to determine the distance the movable mirror moved. The distance moved is n * lambda / 2 . Therefore you can resolve distances moved on the order of the wavelength of light used.

Components

1 × HeNe Laser

1 × HeNe Laser Power Supply

1 × Fixed Mirror

1 × Movable Mirror

1 × Beam splitter

Discussions

Anthony Tricarichi wrote 02/11/2015 at 14:45

The control circuitry can be rather simple, to translate fringe movement into distance all you need to do is count. However, its very hard to ascertain direction, so the controller must be a little smart to which direction you want to move the axis.

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Kert wrote 02/11/2015 at 16:19

Fortunately as far as direction goes, if the counting is done in the software keeping the hardware side as simple as possible for CNC axis there is preliminary data about in which direction the step was attempted. Although after thinking about it a little bit more one of the potential issues that pop into my mind is vibration - with the sensitivity being quite high it might be able to generate a lot of readings if the amplitude of these vibrations is significant enough - then again this might be sorted with some kind of dampening or by having a very rigid construction.

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Kert wrote 02/11/2015 at 09:31

That is certainly interesting way of doing it. What kind of circuitry or software is involved to translate these measurements usable for a CNC machine/controller if the project is already far enough to think about that?

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androng wrote 09/13/2014 at 05:13

Ah I see, so it would be exactly like this, except instead of the entire apparatus moving, only one mirror would move! http://upload.wikimedia.org/wikipedia/commons/7/7c/MichelsonMorleyAnimationDE.gif

I would like to build this. I would not like to pay $1000 for a HeNe laser. But I can afford a $70 beamsplitter. Where can I buy a cheaper laser that's still coherent enough to discern a phase difference? And I can just use a common LDR right, like this one? http://www.taydaelectronics.com/photo-conductive-cell-resistor-ldr-650nm-radial-ke-10720.html

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Anthony Tricarichi wrote 09/13/2014 at 05:50

HeNe lasers arent typically that expensive, my laser and supply was about $80, also a beam splitter should be $5. Laser coherence is a topic that is really hard to get a straight answer from anyone, supposedly you can use a common laser pointer, with Nd:YAG giving pretty good results. For a detector anything will do really, a LDR is more than sufficient.

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Joshua Conway wrote 08/05/2014 at 20:47

This project sounds interesting, however I'm confused. I did a bit of wiki reading regarding this. When you say detector, what kind of device are you talking about? A webcam or something like, or something much more intensive?

Also, could one make a beam splitter in a home lab or similar workplace?

I'm really curious, but I'm kind of at a loss due to scientific instrument language.

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Anthony Tricarichi wrote 08/05/2014 at 21:06

With a michelson interferometer you create an interference pattern due the phase difference of the laser beams in the setup. This pattern when created sucessfully looks like this: http://mcuplace.com/dump/Interfearence.jpe

When you move the one of the mirrors, you are changing the distance one beam has to travel, and therefore the interference pattern changes. As you move the mirror, it appears to move outwards (or inwards). By using a simple photo detector, you can easily count the number of "peaks" and "valleys" to determine the distance the movable mirror moved. The distance moved is n * lambda / 2 . Therefore you can resolve distances moved on the order of the wavelength of light used.

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Joshua Conway wrote 08/05/2014 at 21:12

Thank you for responding Anthony. Like I said, I'm not terribly well versed in the hardware side, although I think I understand the principle.

What kind of hardware would we need to replicate this setup? Are we talking junk webcams or light time of flight cameras for the detection? Can we also use a standard red laser to generate the coherent light, or does it have to be HeNe? It would be rather nice to have these work: http://www.ebay.com/itm/70pcs-per-lot-650nm-6mm-3V-5mW-Laser-Dot-Diode-Module-Red-Copper-Head-/140953935882?pt=LH_DefaultDomain_0&hash=item20d182600a

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Anthony Tricarichi wrote 08/05/2014 at 21:21

To detect the interference pattern, a simple LDR can be used, you only need to count the peaks. Optics wise, the most expensive part is the beam splitter, but you can use anything that works.

For the laser, you "need" a coherent laser source. The coherence length of the laser limits how far the beam can travel before it no longer has suitable properties. Now, as for what laser you can use, HeNes are some of the most stable laser and produce a very coherent output, they are generally preferred for this type of experiment. However, there is some debate on whether you can use regular red laser diodes, temperature compensated green lasers are also suppose to have decent coherence length.

Its actually a very interesting topic and there really aren't any good answers whether you can get away with solid state lasers, its one of the questions i hope to document.

Ideally if a stable setup using a low cost diode can be achieved it would prove to be a very cheap and decent linear feedback system.

Beam splitters are bought or found, you can probably salvage one from a laser pointer kit or just buy a cheap one off ebay, making optics at home can be very annoying and not worth the trouble.

I bought all the optics from Anchor Optics
http://www.anchoroptics.com/

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ganzuul wrote 08/05/2014 at 01:33

Very cool. I'm building a piezo-based actuator to control mirrors for a laser and I'm about to do something very similar to your project!

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PointyOintment wrote 07/28/2014 at 17:27

What's going on in the comments here?

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Anthony Tricarichi wrote 07/28/2014 at 17:38

i have no clue, i woke up to find this guy spamming my project, any idea on who i can contact?

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Linear Feedback using a HeNe Laser

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Linear Feedback using a HeNe Laser

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