Re: [CAD_CAM_EDM_DRO] repeatable vs accurate switch

Don Rogers wrote:

>
> I am working on a machine design that needs a signal that is repeatable to
> a very tight tolerance. I don't care if it trips at 0.250" or at
> 0.000250", as long as it is the same each time. My thoughts now are
> focusing on optical sensors. First is just a plane flag through an optical
> coupler, and the second is a mechanical amplified version, IE a 10X gear
> driven optical aperture.
>
> Any pitfalls or ideas that come to mind??

Typically you'd use a limit switch (which might have a 1/100" inch or
better accuracy) combined with the index pulse on a rotary encoder.

Example: 200 CPR (cycles per revolution) quadrature rotary encoder with
index pulse, directly mounted on a 5 TPI (thread per inch) leadscrew

This means 200*4 = 800 pulse per revolution in quadrature, so
1 index pulse = 1/800th revolution resolution. Since a 5 TPI screw,
thats 1/(800*5) = 1/4000 = 0.000 250 inch resolution.

The machine goes until it hits the low resolution limit switch, then
it slows down and then <usually> continues in the same direction until
the index pulse is encountered. If the momentum carries it past the index
pulse, the motion is slowly reversed until the index pulse is hit once again.

Thus the condition of: limit switch ON AND index pulse HI
signals home (or zero).

Obviously there are 4 logical variations of this.


Doug Fortune
http://www.cncKITS.com











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