To the left are some pictures of the final encoder. It consists of a custom magnet and a rapid prototyped SLA shaft. The SLA shaft reduced machining cost greatly, and allowed for a much easier assembly.

This encoder allows several hundred RPM and absolute position, with better than .1 degrees of angular resolution. All that and the parts cost about $10.

The lower picture shows the flux density indicating how strong the field is when it's penetrating the encoder chip. This give me a flux density to try and maintain in my final design. A weaker flux density would likely cause inaccuracies in the sensed position, as well as allow outside fields to raise the noise floor. Both bad things that need to be prevented for a good quality encoder.

This shows my first attempt at this design. I did this design before I knew about FEMM, I designed it completely based on gut feel. I sure am glad I did the FEA. This could have cost me a pretty penny if I had gone with it. I had thought that even though my magnet was a smaller diameter, the extra length would make the flux a near equivalent of the above vendor recommendation. Boy was I wrong.

This design used a custom magnet that was indented to replace the original shaft. This shaft had a feature to align the encode chip and circuit board. The design intent here should have allowed for just about anybody to modify an off the shelf servo. However it simply wouldn't have worked.

This is the final design. I did an asymmetrical FEA with FEMM. This allowed me to confirm the magnet flux should be similar to what I saw with the vendor recommended magnet. This prediction indicates this magnet is going to be strong enough to allow the chip to function correctly. The chip will report if the flux field is to strong or too weak, and the FEA landed it right in the middle on the first prototype, not to strong and not to weak.

This design uses a custom magnetic ring. The shaft is made via SLA rapid prototype plastic machine, and the PCB incorporates an alignment mechanism. Such that no expensive alignment equipment is required, and the parts can be assembled very rapidly and cost effectively.