Re: Encoder report w/oscilloscope

On Tue, 04 Oct 2005 22:55:13 -0400, john <amdinc@xxxxxxxxxxxxxxx>
wrote:

>
>
>Eric R Snow wrote:
>>
>> On Tue, 04 Oct 2005 19:15:41 GMT, "Mike Young"
>> <boat042-spam@xxxxxxxxx> wrote:
>>
>> >"R. O'Brian" <rasek@xxxxxxx> wrote in message
>> >news:JBy0f.407$784.353@xxxxxxxxxxxxx
>> >>> next. However, I think the problem is hysteresis in the rubber belt
>> >>> driving the encoder from the carriage. We'll see.
>> >>> Eric
>> >>
>> >> You probably need a metal belt, a thin flat endless ribbon of stainless
>> >> steel or some such alloy. Dimensional stability is excellent at constant
>> >> temp. See
>> >> http://www.machinedesign.com/BDE/mechanical/bdemech1/bdemech1_20.html
>> >
>> >Or phosphor bronze... I can see that working for partial turns, but not
>> >multiple turns as Eric is looking for. Two opposing straps wound not quite
>> >the full circumference, fixed firmly at the ends, will track like a worm and
>> >rack.
>> >
>> >It would be instructive to disassemble a dial indicator and look inside.
>> >Mebbe I would just find a way to mount the encoder to the de-faced
>> >indicator, rather than mess with mirrors, lasers, belts, oscilloscopes,
>> >quadrature waveforms, ... unless the idea is to mess, to reinvent, rather
>> >than measure.
>> I know how indicators work. And they involve anti-backlash gears. But
>> that misses the point. The encoder should have NO mechanical lash. The
>> disc with the lines is mounted directly to the shaft. A metal ribbon
>> if the friction was high enough. I ordered some fine cable today from
>> McMaster-Carr today. It will be wrapped a few turns around the encoder
>> shaft. I hope that this will provide enough friction while at the same
>> time have low enough tension to keep the friction low in the pulleys
>> the cable will travel over.
>> ERS
>
>
>The turn-on and turn-off positions do not happen at exactly at the same
>position when the shaft is reversed. This is where your "backlash" is
>coming from. If you want better accuracy, use a higher count per rev.
>encoder.
>
>John
John,
This had better not be the case. At the most there should be only 1
count backlash, not 9.
Eric
.

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