Insertion Devices Control and EPICS

[Rok Sabjan <rok.sabjan@cosylab.com>]

Hello everyone!

I'm trying to prepare a proposal for ID control system at Diamond. 
During that time I have been asking some people some questions how this 
is currently managed in EPICS. To my surprise I found out that there was 
no clear way to solve this problem. I had a discussion with Tim Mooney 
and we were thinking of some possible solutions for the biggest problem 
- synchronous motor movement under EPICS. We have developed some 
suggestions, but I think it is time to broaden the discussion to 
tech-talk. There may be more people interested in such solutions and 
there are probably some people with some experience on the matter.

First let me remind everyone on the problem. We have two (or more) 
motors which we would like to move synchronously. This means to 
guarantee the movement will start and end at the same time. This is 
extremely important when changing the gap of an insertion device.

One may suggest that we should just wire the signal lines together 
(hardware solution) and use one signal line to control multiple motors, 
but we also want to move these motors independently. We may implement 
some logic (using FPGA or some other programmable logic) to multiplex 
this one signal line in one operation mode and use multiple signals for 
different motors in some other mode.

The OMS VME58 card, which is normally used in EPICS for motion control, 
supports such synchronized moves, but this is (to my knowledge and 
experience) not supported via EPICS motor record and device support. We 
should also provide some kind of sync record, which would take control 
of correspondent motor records (when instructed to do so) and would then 
perform the synchronized moves. Needless to say, there are a lot of 
design choices to identify and make on the behaviour of this record.

This mail is intended to identify people who have common interest for 
such a development in EPICS community. Definitely, we will have to make 
some modifications to the existing motor software if we decide to go 
through with any of these ideas. The idea is to make a general EPICS 
solution for such situations.

Cheers,
Rok