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bluesky / ophyd

hardware abstraction in Python with an emphasis on EPICS
https://blueskyproject.io/ophyd
BSD 3-Clause "New" or "Revised" License
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Fall back to KILL if STOP fails? #475

Open danielballan opened 6 years ago

danielballan commented 6 years ago

See https://github.com/NSLS-II/ophyd/blob/master/ophyd/epics_motor.py#L115

danielballan commented 6 years ago

I created this issue hurriedly in the middle of a tutorial. Let me now provide a little more context.

The EPICS motor record exposes a STOP field and a KILL field. According to the controls engineers in my tutorial, which of these is more certain to effectively stop the motor can depend on the kind of motor it is. Sometimes a powered abort is safer; other times just killing it is more sure to work.

It might be nice to make this configurable through an argument to __init__, such as stop_mode='kill'. Or, alternatively, to generically STOP, try to detect whether that seems to be working, and then possibly KILL if needed.

Of course, it is currently possible to customize this by subclassing EpicsMotor and overriding the stop() method. Perhaps the KILL case is rare enough that subclassing is a reasonable approach and no new argument is needed. I'll be the first the say I have no idea what I'm talking about here but I wanted to kick off a conversation.

dchabot commented 6 years ago

Small, but perhaps significant correction: the Motor Record does not have a KILL field. There is a supplementary support record included for the Delta Tau controllers that does have a kill command ($(SYS)$(DEV):Cmd:Kill-Cmd).

IIRC, for those controllers, the STOP field issues a command to "decelerate to a controlled stop", and the kill record means "kill output signal to motor" (i.e. coast to a stop).

klauer commented 6 years ago

The closest analog is the .CNEN field, which could open-loop the device. It's built into the motor record, but support for it is spotty on a controller-by-controller basis.

It again would have to be optional, as open-looping motors may be problematic (depending on their mechanical design).