I picked up one of those cheap Chinese incremental rotary encoders from Amazon to use for measuring linear speed... I'm timing the gap between pulses to come up with a speed reading. I am seeing a variance of 15%, even when I turn the encoder at a very precise rate. I've attached a wheel to the encoder and mounted it to the table of my (pro, not hobbyist) CNC machine, and hooked up the encoder output to my oscilloscope and I see a variance of +/- 15% in pulse width (and time between pulses) as the encoder turns. It isn't random... it seems to cycle from -15% to +15% and back, which seems to suggest slop in manufacturing.
I'm wondering if this is normal for these cheap Chinese encoders or is this possibly just a bum unit? I don't want to buy a handful more if they could all be expected to be equally bad. What about the $50-100 range encoders from the likes of Grayhill, Copal, and Honeywell - should I expect more accuracy from those units over the Chinese encoders from Amazon?
Thanks for any input.