>I implemented it in software on an 16F84A, I know they are
>outdated, but I have lots of them!. When the shaft accelerates
>fast then I start to miss about 3 to 10% of the pulses
>depending on the acceleration, getting offsets in position.

I suspect that using any micro is going to cause you problems. I think you
may have to resort to something like Direct Digital Synthesis, or a proper
phase lock loop system as a minimum. Whatever you do there will be a time
lag between the encoder speeding up and your multiplied output catching up,
as the multiplier cannot predict when the next edge is going to occur that
it will need to measure.

Your other problem is going to occur with the lowest rotation rate of the
encoder that you wish to lock to. I cannot recall you stating this, but I
seriously doubt you will be able to go right down to 0 rpm, and I get the
feeling that is what you want to do.

When you refer to offsets in position, are you trying to make an x-y plotter
or something similar? It seems to me that what you really need to do is get
an encoder that has more steps per revolution. Your only other way to option
that I can think of is to remove the encoder from its current connection,
and have a suitable gearbox between it and the drive shaft to increase the
shaft speed into the encoder. One of these two solutions is going to be the
only way you will get a satisfactory low speed performance.

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