I did some research and came to this conclusion. That you can’t say how accurate a given encoder can be.
And that a good rule of thumb is that you need a resolution of 2 to 5 times more than your desired accuracy.
So if I want an accuracy (=measured movement deviation of real movement) of 0.02 (1/50) mm then I need to select an
encoder that can deliver at least 100 counts per mm (0.01 mm gives me 1 count) and the best (5times) 250 counts per mm (0.004 mm gives me 1 count).
So an encoder that has a nominal resolution of 73 pulses/mm.
But to know the amount of counts per mm I need to chose the counting logic (X1,X2 or X4).
For example if I use an X4 counting logic then I have 73*4 = 292 counts/mm.
And if I use an X2 counting logic then I have 73*2 = 146 counts/mm.
so this encoder would do Is this correct ?
But this is only how much deviation i will have against the real value.
So if i wan tot calculate the smallest movement that the encoder can measure (very important detail). I need to know how many mm or inches 1 revolution is. This will give me the PPR for the encoder that i need to use. I found info about choosing the PPR value saying this :
Try to chose a PPR that is an even multiple of the value you are trying to measure or display. For example, if one revolution of the encoder equates to 12 inches of travel, you might chose a 1200 PPR encoder. This can eliminate or simplify the need for a calibration constant or scaling factor and more importantly, it eliminates the possibility of accumulating a rounding error over many cycles of the encoder. In this example you would be able to measure the travel to a resolution of 1/100 of an inch. You should also consider any 2x or 4x counting logic in your controller. If your controller can "see" pulses on both the A and B channels (2x logic), then it will count 2400 pulses for every 12 inches of travel in our example. If the controller counts both the leading edge and the trailing edge of each of the pulses on both channels (4x logic), then it will count 4800 edges per revolution and your effective resolution would increase to 1/400 of an inch per count.
So the smallest movement you can measure for this encoder is 0.0025 inch/count. (Conversion inch to mm *25,4)
Thank you for your time and help.
Pieter De Vos