Re: [CAD_CAM_EDM_DRO] Limit and Homing switches

Hi Alan,

I was wondering if you could check me out on something. I'm currently
using servo motors with Gecko G340's. The encoders have an index pulse
(or at least a pin for one, should check that out I guess) and I was
hoping to use some sort of an AND gate to generate the home signal. I
think I may have a problem though because I'm using the G340's at a
X5 multiplier. For some reason I think this means the index pulse MAY
occur between "step pulses" from the computer. In other words it may
be the case that the index pulse happens "between" step pulses. Has
anyone else tried to use an index pulse with G340s in something other
than X1 mode?

Drew

Alan Marconett KM6VV wrote:

>
> 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

--
Drew Rogge
drew@...