Re: Motor home status

[Maren Purves <m.purves@jach.hawaii.edu>]

Mark Rivers wrote:
> > This assumes that the current position information is somehow still
> valid, 
> > at least approximately. However, tehre is no guarantee taht this is so
>
> > before the motor gets homed. Thus, I agree that soft limits make no
> sense 
> > during home operation.
>
> I disagree.  There are other ways besides home to know that a motor is
> in approximately the correct position.  For example, there may be
> indicators on the motor slides, or one may use a limit switch as a
> fiducial.  

If one has such things on that particular device.
> Save/restore motor positions are another example. If the IOC and
> controller have been rebooted, so the motor has not been "homed" in the
> memory of the controller or IOC, but the positions are almost certainly
> still correct.

Yes, but, *almost*.

You have no way of knowing whether anybody has moved anything while
the IOC was off. Somebody may have taken the whole thing apart and
put it back together in which case you really won't know where anything
is.

Case in point (in a somewhat roundabout way): we had a device where
moving parts would have been able to run into one another if that
hadn't been prevented in software.
One of the mechanisms broke, and I was asked to put that one mechanism
(one of 3) into simulation. The software was written such that it didn't
let me do that without basically rewriting it, which I didn't, so it
was "either you can have all of these in simulation or none". This
included datuming (homing).

Aloha,
Maren