John,
That is about the clearest most logical response I could have asked
for. Quite honestly I am unsure if my machine has three switches per axis
as you described. I will have to check. It is definitely a PRO CLASS
commercial machine as you say but I am unaware of the actual limit design
of the original control. Lets assume for a minute tho that it does not
have a third switch for homing here. How would you say would be the best
limit setup in that case? I only saw two physical switches on the x axis
the last time I looked at it. It is entirely possible that the switch
contains two actual switches in one or something like that I will have to
check. This discussion is quite enlightening for me here and I am looking
forward to getting the machine running soon. I am waiting on a call back
right now from Machmotion about my AC servo drives and motors order. They
should probably be here by the end of the week. Peace
Pete
On Tuesday, February 12, 2013, John Kasunich <jmkasun...@fastmail.fm> wrote:
>
>
> On Tue, Feb 12, 2013, at 11:44 AM, Pete Matos wrote:
>> Sam,
>> okay now we are getting somewhere, you bring up a valid point here. I
>> have been in that situation before with a runaway servo and buddy it is
no
>> fun. I can see what you mean about getting the drive depowered in the
event
>> of a runaway.
>
> Yep, that is what the hard limit switches are for.
>
>> I guess I was more thinking about a limit trip during a
>> program run accidentally when I screwed up the program placement or
>> overestimated the size of my travels ( I NEVER DO THAT REALLY!! LOL)
>
> That is what the soft limits are for. You set the soft limits a bit
inside the
> hard limits, and once homed EMC will refuse to execute a move that
> would exceed them. It also refuses to let you jog past them. As a
result,
> you should never be able to hit the hard limits (no matter how bad a
> programmer you are ;-) unless there is some kind of serious malfunction
> like encoder failure or drive runaway.
>
> Note that you must be homed for soft limits to work right, and if your
> hard limits kill power, you must have a separate home switch that is
> inside the hard limits (and probably just inside the soft limits) for
> homing. All pro-class machines have separate home and limit
> switches. Using a limit as home is an acceptable hack for a smaller
> machine, but prevents you from having the truly safe "power kill"
> hard limits.
>
>> yeah I can see your point and I wonder if there is a happy medium somehow
>> with a limit setup tied to the estop circuit and still being able to
power
>> up and jog off the damn switch so I can get back to work without major
>> machine disassembly...
>
> Soft limits prevent nuisance trips. Hard limits save expensive smoke
> when something breaks, but since something DID break, you have some
> real fixing to do before you can get on with your work. Getting the
> machine off the limit is a part of that fixing, but finding and fixing the
> initial failure will be most of the work. Should ideally be a very rare
> case.
>
> John Kasunich
>
>
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