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Re: Spindle encoder with transmission reduction
Posted: Wed Aug 18, 2021 12:27 pm
by Dave_C
tblough wrote: ↑Wed Aug 18, 2021 11:22 am
In a 1:2 pulley setup you are getting 2 encoder pulses for every turn of the spindle. That will not work as the re-entrant tap could start at 0 or 180 degrees out from the first pass. With a 2:1 pulley setup you are getting 1 pulse every two spindle revolutions. That pulse is always in the same spot so that will work.
I.e. if you are doing a speed reduction between the motor and spindle, you must attach your encoder to the spindle. If you are doing a speed increase, you can attach the encoder to the motor only if the increase is an integral amount (2:1, 3:1, 4:1, ...) and if you use a synchronous belt and pulley setup for the encoder. You have to use a toothed belt for the encoder because v-belts, orings, etc have some slip and therefore can't guarantee that the encoder pulse will occur in the exact same spindle orientation each time.
Thanks Tom I see what the difference is!
Dave C.
Re: Spindle encoder with transmission reduction
Posted: Wed Aug 18, 2021 1:40 pm
by Muzzer
The OP said "I have big pulley on motor side and small on the spindle side", so there is no issue with a 2:1 speed increase.
Dave_C says " In a 2 to 1 overdriven setup you are getting 2 Z pulses per one turn of the spindle. So how can you hit the thread start on the second pass? You might and you might be 180 degrees off!"
Surely "overdriven" means the spindle is running faster than the motor? In which case I think he has his logic wrong. If the motor-mounted encoder is at its index mark, you know exactly where the spindle is. Perhaps introducing a term such as "overdriven" is confusing matters if it is ambiguous.
Re: Spindle encoder with transmission reduction
Posted: Wed Aug 18, 2021 2:05 pm
by eng199
Dave_C wrote: ↑Wed Aug 18, 2021 10:24 am
It will not generate an error if encoder pulses are more or less than expected or if an index pulse is missed or spurious.
IF your A/B pulses do not come in in the correct order you will get an error and the spindle will stop! I have an encoder that needs a proper shielded cable installed because it gets a false pulse once in a while. I had to disable the error tolerance from a default of 3 to a 256 in order to make it run.
The only incorrect "order" of A and B pulses is both A and B transitioning at the same time. This is "quadrature error" in Centroid systems.
Systems (except ACORN) also detect if wire /A is the opposite state of A and /B is the opposite state of B. Failure of this is "differential error" or "encoder connection bad".
There is no checking of the number of encoder counts between index pulses.