Re: [CAD_CAM_EDM_DRO] Re: Encoder index and homing

Hello Carl,

Here is one way to do it with half of a LS74 dual D flip flop, two 1
kohm resistors and a normally open limit switch.

The normally open limit switch is wired with one contact to ground and
the other in series with a 1000ohm resistor to +5v. The junction of the
switch and resistor is the switch output and goes to the flip flop Reset
line and D input.

The encoder index output goes to the flip flop clock input.

The second resistor is used to pull up the unused Set input. One end of
the resistor to +5v and the other end to flip flop Set input. Some folks
don't use a resistor and just wire +5v to the Set input. My early days
of EE was designing to meet mil spec and we tied NOTHING directly to
power except power pins. Unusued TTL inputs were pulled up via
resistors.

Be sure to wire the flip flop power and ground pins to the power supply
also. There should be a 0.1ufd, a 0.001ufd and a 10ufd electrolytic in
parallel with the flip flop power and ground pins. These capacitors
should be located very close to the flip flop.

How it works: Travel toward the limit switch until the limit switch
closes. The flip flop Q output will immediately go low when the limit
switch closes. Then travel away from the limit switch. On the first
rising edge of the encoder index after the limit switch opens, the flip
flop Q output will go high as the encoder index rising edge clocks the
flip flop.

When you first power up you should home twice since the flip flop may
power up either high or low. This will guarantee a valid home. You
should NOT be within one index pulse of the end of the travel range that
is opposite the limit switch.

Now this is not the ideal solution since you would probably prefer a
normally closed switch. That way the default for a disconnected limit
switch is safer. I did this for somebody several months ago. I'll have
to dig up my notes to see how I did that with a N/C limit switch. I may
just have used the other half of the dual flip flop as an inverter.

Tom Hubin
thubin@...

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Carl Mikkelsen wrote:

>
> > >
> > > 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