Bartosz:
I have just printed out a circular array of filled triangluar shaped spokes on my HP-4MV 600 dpi laser printer. The array has 720 of these spokes, each with bar width of 0.25 degrees and the spaced around the circular array at 0.50 degrees. This would provide a 720 cycles per revolution out of a single photo emitter / pickup pair. I found, just as you did, that the printing does not produce a usable pattern until the diameter is rather large. My finding is a diameter greater than six inches (1500 mm).

Here is a link to a .PDF file that contains the graphic for this pattern. When I use Adobe Acrobat and print the PDF file to the same laser printer the resulting pattern is nearly the same as the original output from AutoCad. This should allow anybody who wants to look at this pattern from their printer to evaluate it just printing from this link.

720 Spoke Encoder Wheel

You may still find it valuable to print the wheel even larger and then photo reduce it like I described before.

It is also necessary to place a slit aperture in front of the optical pickup device that is in close alignment with the spokes on the encoder wheel. Ideally the slit is narrower than the width of the spoke by a factor of 1/5 to 1/10th.

In my prior working experience with lower resolution homemade wheels with 30 spokes I did not use a slit aperture but instead made a small collimator. It was made as shown in the picture below:



The hole in the collimator was drilled to a diameter about 1/3 the encoder wheel spoke width. Depth of hole should be about 4 times its diameter. The BLACK ABS collimator was then glued to the IR detector with a drop or two of ABS-Weld solvent. Use care to not get solvent in the hole or on the lens surface of the detector. After bonding I painted the assembly with a black acryllic paint to keep stray light away from the detector coming in through on the sides and edges of the IR detector.

Here is a data sheet for the PhotoLogic type of IR detector.
http://rocky.digikey.com/WebLib/Optek/Web%20Data/opl550.pdf


Michael Karas