DIY encoder for digitizing

[duluthboat]

Question, how many CPR are on the smallest diameter of an everyday CD?
Question, can this cheap technology be used to build a high resolution encoder?

Gary

[ToyMaker]

Interesting idea.
Raw data comes off a CD at something like 44kHz. The drive motor is in a servo loop such that the disc passes under the head at a constant rate, a process called constant linear velocity (CLV). When the read head is near the inside edge the disc spins fast (>300 rpm?), and when near the outside edge it is slow (~100 rpm?). Because there is only one track on a CD (similar to a phonograph record) there would have to be a way to step the head out each revolution.
If you could record a (very) constant tone so that there were a known number of wavelengths per revolution, couple the shaft to your cnc machine, and count wavelengths, the whole thing just might work.
I think the index step each revolution might be the big gotcha.

robotic regards,

Tom
= = = = =
"Some politicians like to move their lips, and apparently, some lips like to move politicians."
- - Shan Jones

[pminmo]

How high of resolution do you want? Something I've wondered about is using the gear sets out of RC servos to increase resolution. I'm not sure what the ratio is but it could easily be greater than 32:1. It would be fairly easy to make a disk with say 32 quadrature cycles on it, at a ratio of 32:1 that would be 1/1024 or .0009. I'm sure there is some downfall to this idea, just don't know what it is. Nylon gearsets are available for less than $4.

Phil

[duluthboat]

I would like 100,000 pluses per revolution, 200,000 would be better. That makes the standard optical disk unrealistically large; under 2” would be OK for a diameter.

I looked at using gears and at first they look like a good option but even the ones that say they are zero backlash, have only 1.5 arc min. accuracy.

There are very small high resolution encoders, but I can not afford them. With all the inventive people on this forum we should be able figure something out.

Gary

[Graham S]

The fact that the dots on the CD form a spiral means it is a no-no as far as I can see. Although CD players are cheap they are modern marvels of control technology in the way they track etc.

Harmonic drives are one way to get true zero backlash gearing but looking at Faro patents it does seem that they use just small and expensive encoders.

One thought I had was to run a cord from the base of one arm portion to the tip of the next and wrap this multiple times around the shaft of an encoder that was springloaded, the extension and retraction of the cord would create multiple revolutions of the encoder, however this would be messy and possibly difficult to calibrate.

Graham

[ToyMaker]

The best trade-off for resolution v. price I can think of is an optical mouse. At <USD10 and 400 ticks/inch it's not too shabby (are there higher res OM out there?).
A 3.1416 inch diameter disk produces 4,000 counts/revolution at the perimeter. That's 11.11 counts/degree or about 5.4 arc-min/count (if my arithmetic is any good).
Even though that's only 1/25 of the goal of 100k counts/rev the price is attractive.

robotic regards,

Tom
= = = = =
"We have civil engineers; too bad we can't engineer civility."
- - Dennis R. Ridley

[Cliff_J]

A quick idea - has anyone found out more of the method of detection used by the higher resolution encoders? They don't use a simple slot system but rather a special pattern and multiple detectors to sense each movement. Is there a term for this science as some sort of geometric or physics to research?

How about some sort of 2D barcode scanner that could read a disc printed with a special pattern? If we could truly print at 1200dpi or 2400dpi on a laser printer on clear stock this sounds like a true budget dream, one person could print multiple wheels on a sheet for a few bucks and easy to mail. When I subscribed to Forbes magazine they sent out a barcode scanner, some sort of mouse device. You were supposed to be able to scan a barcode in ads and get web info from the advertiser. It was a flop and the units were sold as surplus later on (from what I can tell) but some high schools and colleges used the things to teach kids about bar codes and it was opensource. Another $10 idea but this one might be feasible to get real precise....

Cliff

[Al_The_Man]

I am not sure if you mean incremental encoders above around 100p/turn, below around 100p/turn the light/photo source can differentiate between the individual slots, above that a light/photo source does not work due to the very narrow slot, high resolution encoders still use a slot grating chrome plated or photo etched into the glass This applies to both rotary and linear types.
Above a certain resolution a phenomena known as the moire effect is used. ( I dont profess to know the physics behind the moire effect).
But you may have observed this when light shines through a fine mesh or sheer drapes etc, you can observe a swirling pattern effect ( no I wasn't smoking anything)!.
In a fine graticule encoder e.g. 1000 p/rev, a small piece of grating material is carried by the moving photo head assy and is slightly skewed in relation to the main grating. The effect is, that due to the moire effect, the photo reciever now *sees* an alternating light-dark band, the width of each band is around half the verticle height of the grating, the alternating bands move like a shutter, perpendicular to the direction of motion, i.e. on a linear scale, if the head is moving in a horizontal direction the 'shutter' appear to be rotating up or down,depending on the direction of movement.
This is obviously much wider than the space between the grating, the band will change from light to dark for one increment of the scale.
I hope I have put this in some meaningfull way, but it is one of those
things that is easier with pictures.
Al

[KissSys]

Well I've been reading this thread from day one and have enjoyed it greatly, I guess its time I put in my 2 bits worth to keep this going.

If some computer guru could burn the same 2bit grey code onto a pocket CD we could mount two laser diodes to read the bits just as in a typical optical encoder. The CD rom itself would not be used and the disc would move just like an optical encoder but hopefully with more steps.

Even better would be to burn a 16 bit grey code radially and have an absolute encoder.

Steve

[ESjaavik]

Al, I think the moiré effect is also used by having a graticule with a slightly different spacing than the disk. If you have 2 disks, one with 1000 lines/rev, and one with 1001 lines. When viewing through both one side will be completely dark 0% light, the other side 50% dark. In between you will have 0-50% dark. Here is an example of a linear scale:

http://www.sjaavik.com/CNC/images/moire_effect.gif
(Why doesn't IMG tags work here?)

The upper row of lines are spaced 4.08mm, the lower one 4.00mm and the lines are 2mm wide. There are the same number of lines in each row. Now if you scan over the lines left to right, within 25 lines there is complete darkness and going further 25 lines you're back to 50% darkness. (There is no brighter than 50% because the lines and the spacings are equal.

Do you know why people wearing striped suits or ties make the TV production crew cry? It's because they know the moiré effect will make it look awful with lots of strange color artifacts.

[ESjaavik]

You want high resolution? Well then take the signal from the phototransistor in the encoder and don't shape it to a logic Hi/Low, but buffer it and run it to an analog input of a microcontroller (or PC). Now you will be able to distinguish not only bright (line 0-12 in my example) or dark (line 13-25). You can also find how dark it is. The position resolution could in theory approach number-of-lines times bits-of-DAC-resolution.

So with an AVR ATtiny15 chip costing $3-4 you multiply the encoder resolution with 1024. This is assuming a lot of things not being there in real life, but let's say you can multiply it by 200, even that is not bad. You would probably have to make a lookup table as the output from the phototransistor during constant speed motion will be more like a sinewave than a triangular wave.

[Al_The_Man]

Originally Posted by ESjaavik Al, I think the moiré effect is also used by having a graticule with a slightly different spacing than the disk. If you have 2 disks, one with 1000 lines/rev, and one with 1001 lines. When viewing through both one side will be completely dark 0% light, the other side 50% dark. In between you will have 0-50% dark.

Evidentally it will also work with gratings of the same spacing as shown in this technical paper, where you can copy two exact gratings for demonstration purposes. http://claymore.engineer.gvsu.edu/~j.../cont_sens.pdf
Also the shutter effect can be seen if a 'dead' encoder with the disc exposed but the head still present is held up to reflective light and the shaft is turned.
I still remember working on old linear scales by Ferranti that had a 15watt incandescant lamp in them and every time you changed a lamp the sine/cosine relationship between the heads had to be adjusted using a 'scope.
Al