Experimental Physics and
Industrial Control System

"Gillingham, IJ (Ian)" <[email protected]> · Thu, 14 Aug 2008 09:50:57 +0100

As most of my EPICS experience is with the OMS VME58 and MaxV
controllers, I am familiar with the concept of the 'Home' signal meaning
that the axis is 'At Home'. These controllers are level triggered. The
concept of a persistent 'Homed' state seems a bit arbitrary, as my
understanding (and please correct me if I'm off track) is it simply
states that at some time in the past, the controller detected the 'Home'
signal and reset the raw motor and encoder steps to zero. In the ideal
world and with the implementation of autosave/restore, the controller
would then never need to consider itself as 'not homed'. Are there any
events which would cause a state transition to 'not homed'?  Of course,
we do not have an ideal world and even autosave/restore sometimes
appears to get it wrong (albeit very rarely) - but even this would not
cause a transition to a 'not homed' state.
It maybe that my understanding is a bit simplistic, as the OMS and Delta
Tau controllers are quite different animals.

With regard to Nick's suggestion to disable soft limit checks whilst
homing, I agree completely. I know of several users have been caught out
on this one. I always manually widen the soft limits prior to homing and
it would be good to remove the need to do this.

Ian Gillingham

Senior Electronic Systems Engineer
Diamond Light Source

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Rees, NP (Nick)
Sent: 14 August 2008 09:16
To: [email protected]
Subject: RE: Motor home status

All,

I agree with Jim, which is why I coded the Delta Tau that way. The way I
see it, most home indicators are edge triggered, not level triggered and
so an "At home" signal makes no sense. However, an indicator that the
system has been homed - which is recovered even after the IOC is
rebooted, makes a lot of sense since it is basically telling you "you
can probably trust the position I'm reporting".

As usual, however, the reason why things are the way they are is
historical, and we need to respect that, and since there is still
clearly a need for an at home signal, I think it is best if Matt adds a
separate, optional, "Homed" flag that will gradually get implemented by
all the drivers.

On a separate note, for some time I have been thinking whether the motor
record should disable soft limit checks whilst homing. The argument
being that soft limits are meaningless when you don't know your absolute
position, and usually the reason you home is that you don't know your
absolute position. This has the added advantage that when you home
against a limit switch, you don't have the motor record interfering with
the home procedure. How do people handle this and what do other users
think of the suggestion?

Cheers,

Nick Rees
Principal Software Engineer           Phone: +44 (0)1235-778430
Diamond Light Source                  Fax:   +44 (0)1235-446713 
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System