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<== Date ==> <== Thread ==>

Subject: Re: Motor record .RDBD field
From: "Pearson, Matthew R." <[email protected]>
To: Tim Mooney <[email protected]>
Cc: "[email protected]" <[email protected]>
Date: Tue, 28 May 2013 08:47:06 -0400
Hi,

Would it make sense to use MRES to limit the smallest step size (was it originally like that)? On Diamond beamlines they tend to set RDBD to MRES anyway, as retires are not often used. And I suspect where retries are used this change in behavior wouldn't have any consequences, apart from perhaps some step scanning scripts behaving differently. However, it may affect the insertion device systems, which do use retries extensively.

As an aside, I think if a client attempts to do a move smaller than MRES nothing will happen apart from a change in VAL and a flicker of DMOV (1->0->1).

Cheers,
Matt



Pearson, Matthew R.
Data Acquisition and Control Engineer
Spallation Neutron Source
Oak Ridge National Lab


On May 27, 2013, at 11:46 PM, Tim Mooney <[email protected]<mailto:[email protected]>> wrote:

Hi Emma,

I agree.  The original purpose of RDBD was specifically about retries,
and not also about whether or not to make an initial move.  I would
like to explore the consequences of going back to that purpose.

Tim

----- Original Message -----
From: "Emma Shepherd" <[email protected]<mailto:[email protected]>>
To: [email protected]<mailto:[email protected]>
Sent: Monday, May 27, 2013 5:57:25 PM
Subject: Motor record .RDBD field

Hi all,

I've stumbled into this issue a couple of times now so I thought it
was worth a message to tech-talk.

The .RDBD field of the motor record limits the smallest allowed move
- quite often we want to set this very low so that we can do small
step moves.  However, in a few cases we also want to use the motor
record's retry mechanism by setting .UEIP to 'Yes' and feeding the
encoder into .REP (I'm using the tpmac driver which allows you to do
this when the encoder is on a different channel).  Because .RDBD is
now used as the retry deadband, we need to make it larger since it
is not capable of attaining such an accurate position.

It would be better if we could somehow decouple the use of .RDBD in
the retry sense from the smallest allowed move.  Have other people
run into this and found a solution?  In the meantime I need to keep
switching between modes (toggling .UEIP and changing .RDBD
appropriately) to allow me to do scans in very small steps followed
by a wind-back which uses retries to ensure I'm not losing steps.
It's a bit messy and not ideal.

Of course if we had perfect mechanics we wouldn't need to be doing
retries at all, but c'est la vie!

Thanks,
Emma

P.S. I hope I haven't missed a thread on this already, the APS
website seems to be down at the moment so I can't check

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References:
Re: Motor record .RDBD field Tim Mooney

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