[Gdeaver]

I'll throw this out as something completely different. A good encoder will probably cost 50 to 60$. The problem with encoders is that they require the use of interrupts in programming. Two encoders isn't too bad. 4 gets a little tighter to implement. Add a couple timer routines and things get complicated. I've just started playing around with a hacked optical mouse as a alternative to an encoder. Basically an optical mouse can track movement of a surface. The Aligent 2610 chip that I'm using can track up to 10 inches per second, however I've found its better to keep the disk that the mouse is sensing to 6-8 inches per second max. Using 2 wire serial communications the mouse returns the number of counts since the last request. The 2610 resolution is 400 dpi. That's a high resolution non-contact tachometer for about 12$ and no interrupts to program and debug. The chip does the counting for you, just poll it often enough that the registers don't overflow. I've only just started playing but it looks good. There were some threads on here about optical mice. Note that the Aligent mice chips are optimized for an almond colored texture Formica surface. The mouse lens needs to be 2mm from the surface. At higher velocities it's critical. I'm looking to use it to track and log valve linkage movement.