Re: [CAD_CAM_EDM_DRO] Re: Encoder index and homing

One way of making a limit more precise is by using a belcrank. A short leg and a long leg pivoting where the two intersect. Table touches the short leg and moves the long leg at a much fater rate and greater distance thereby increasing the precision. Works with microswitches, prox sensors, etc. Homing switches on a 3 axis mill are generally uneccesary.

Ron

braidmeister <braidmeister@...> wrote:


I was thinking last night...scary I know...What about a set of
ignition points that are dialed in with a .001" gap? Seems that some
of the switches out there are either really expensive or too sloppy.
Points are cheap, available and adjustable.

Any thoughts?

-Brady

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Tom Hubin wrote:
> 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@e...
>
> ***********************************
>
> 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





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