Re: [CAD_CAM_EDM_DRO] Re: Encoder index and homing

> >
> > In the last few days there has been discussion about using the
>index
> > pulse on an encoder "anded" with a home switch to get ultra precise
> > home repeatability.

For the Stewart platform / Hexapod I'm working with, it is important to
have a precisely defined reference position, where the lengths of the legs
are certain to be repeatable. For those unfamiliar with hexapods, they
differ from regular 3-axis mills in that movement along X, Y, and Z
requires simultaneous and non-linear movement of all six actuators. If you
don't know exactly where you are, you can't decide how to get anywhere else.

The absolute position is less important than the repeatability, since
calibration corrects for the constant errors.

I'm using inexpensive plunger switches intended for safety interlocks,
coupled with an index pulse from the encoders. The switches are not very
repeatable, with a measured variation of about .040" in the "make"
position. This is way too much to provide a good reference position, but
it is high enough resolution to be a good reference point for fine
positioning with the index mark.

The switch must close roughly between index pulses, to avoid the case where
sometimes the switch closes on one side of the index, and sometimes on the
other, which creates a variability of exactly one turn of the encoder.

In my case, the switches and the encoder outputs are brought into the
computer separately, so that the controller can plan the motion to assure a
repeatable home position.

It would be challenging to make a circuit that would "AND" the encoder and
home switches and create a single output. I think that doing so would
require information about which direction the axis is moving, which could
be derived from the encoder quadrature signals. For me, it was easier to
being both into the controlling computer, and leave the motion control up
to it.

-- Carl


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