Re: [CAD_CAM_EDM_DRO] Rotary Axis home and/or limit switches

Have you though about somehow attaching an absolute encoder to your rotary head?

Nathan



________________________________
From: Stephen Wille Padnos <spadnos@...>
To: CAD_CAM_EDM_DRO@yahoogroups.com
Sent: Fri, February 5, 2010 3:48:14 PM
Subject: Re: [CAD_CAM_EDM_DRO] Rotary Axis home and/or limit switches


David LeVine wrote:

> Stephen Wille Padnos wrote:
>
>> ya_nvr_no wrote:
>>
>>
>>> I have added a couple of rotary axis's But am having an issue with coming up with a good way to home them. One switch gives me a home position but if I rotate one revolution the switch is made again. I am using CNCpro, it uses a limit switch as the home switch.(would another program handle this better?)
>>>
>>>
>>>
>> EMC2 would ignore the home/limit input when it's not actively homing.
>> Having a limit switch on a rotary axis that can revolve> 360 degrees
>> doesn't make much sense, since there is no real limit to travel.
>>
>>
> Actually, there can be if one uses something which is indexed to the
> rotary shaft. For example, imagine the effect of a milling head which
> must be "trammed" manually every time it is used vs. having a zero index
> pulse to the index to the trammed position. The encoder/motor could
> easily be driving a worm to locate the Bridgeport-type head and it
> easily could make 180 turns per revolution of the head.

In that case, the head angle would be the B or C axis, not the physical
motor position. This is equivalent to a motor driving a ballscrew,
where the motor makes many turns (and therefore there are many index
marks) to traverse the table.

> The limit
> switch should not be the home switch since home wants to be vertical and
> driving the head 45º or 90º each cycle would be a poor choice, whereas
> driving it to the "home" position might be easy, generally you would
> already be there. However the home switch might not have enough
> resolution, and the real "home" position would have to be the
> motor/encoder home zero index pulse plus the home switch transition.
>

Sure. There are reasons to share limit and home, and other reasons not
to. If you want a home switch somewhere other than at a travel limit,
that's a good reason to use two switches. Don't confuse home with limit
with index though. It doesn't matter how many index pulses there are
over the full range of travel, as long as the transition from "home
switch deactivated" to "home switch activated" is guaranteed to be
between two of them. EMC2 has the ability to move to the home switch,
back off, then move onto the home switch again, but slower, then use an
encoder index to mark the actual home position.

> Home would be defined as "the first zero index pulse after the home
> switch closes." just as the zero index pulse on some encoders is not
> just the outer track, but the outer track plus one channel state or both
> channel states (e.g. ZIP is true AND "A" is high AND "B" is high, which
> is far narrower than the zero index pulse is on the encoder.) Index
> pulses are often not one slit, but a large number of them set up so only
> when they are properly aligned does the pulse go true. Similar to a
> vernier in concept.
>

Sure, you can configure homing to work that way. And of course the home
switch is ignored when not homing.

- Steve







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