Re: [CAD_CAM_EDM_DRO] Limit and Homing switches

Hi Carlos,

I have used a "phase" of the motor signal before in an RDF project. The
Optical switch must be narrow enough to only "gate" one phase signal or
index signal. So if the optical or mechanical switch was 3 steps wide,
one could position the switch so that the combined signal (sw AND index)
would now only be as wide as the index.

The index only occurs once every rev of the motor, so in a simple case,
it's easy to "fit" it in 8000 steps! If you use a phase of the motor
(full step), then the switch must be a little smaller then 4 steps.
Half step? it would seem that you'd have 8 steps, but the resultant
width would be two steps.

Encoders will give you 4 phases, if you gate: (A AND B AND sw). If the
switch is TOO WIDE (more then 4 phases), then you'd get multiple
"homes", which may or may not work for you. It appears it WOULD be
useable/helpful, if you always approach the home switch in the same
direction).

I have some timing belt 9:1 reductions, and there is a disk with a
narrow slot on the output shaft, AND the motor shaft (two 3:1
reductions). They turn in opposite directions, and when they align,
there is a very short "index" pulse as a result (optical interrupter
sw).

For a limit switch, I don't think it would be important (or even a good
idea) to use this. The limit doesn't have to be accurate, and the
additional logic would degrade the "fail safe" features of just switches
in series (N/C). For home switches, the extra TTL logic would not be as
much of a problem, IMO.

HTH

Alan KM6VV

Carlos Guillermo wrote:

>
> Hi Alan -
>
> The idea of AND'ing a low-repeatability switch with the encoder
> index pulse sounds like a great low-cost method of finding a
> precise home. Is it really that simple? You just look for the
> AND'ed signal and set that as home? Wouldn't you have to
> calibrate the alignment of the index pulse to the homing/limit
> switch? Or, can you just have, a short ON-time of the coarse
> switch, which would necessarily be less than the axial travel of
> one revolution of the encoder? Could you elaborate? I'd like to
> leave room for something like this on my interface PCB, if it's
> simple enough.
>
> TIA,
>
> Carlos Guillermo
> VERVE Engineering & Design
>
> -----Original Message-----
> From: Alan Marconett KM6VV [mailto:KM6VV@...]
> Sent: Wednesday, March 06, 2002 5:38 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] Limit and Homing switches Over 2200
> members today
>
> Hi Bill,
>
> Small lever switches like Microswitch are available surplus, they
> are
> fine for limits. They may not be repeatable enough for limit
> switches.
> Optical switches (optical interrupter) are fairly small also, look
> like
> a 'U' with feet. A thin blade passes through the 'U', and
> actuates the
> switch. Normally they are TTL output, which is easy to use, but
> you
> can't "daisy chain" them. Optical switches can be found on HP
> laser
> printers ;>) and floppy disk drives. They can be quite
> accurate for
> home position, if their output is AND'ed with a phase of an
> encoder or
> stepper motor phase. TTL logic should be fine for this.
>
> Congrats on the 2200++ members!
>
> Alan KM6VV
> Need Pix?
>
> wanliker@... wrote:
> >
> > I am converting an EMCO COMPACT 5PC lathe to more modern drives,
> G-320, with
> > servos, instead of the old steppers.
> > I have now come to the part where i have to think about Limit,
> and Home
> > switches.
> > Has anyone on the list got the answers to what I should use.
> This is a 5 x
> > 12 lathe so something bit would be very hard to mount and use.
> >
> > So any suggestions, and part numbers would be welcome, with the
> emphasis on
> > small, being a high priority.
> >
> > I have noticed on several surplus, (not lathes,) that they are
> using optical
> > switches, any comment on repeatability and how to tie them into
> the system?
> >
> > Help,
> > bill