Re: Lookup table problem

["Steven M. Hartman" <hartmansm@ornl.gov>]

Silver wrote:

>   I need to implement a function for insertion device like this:
>   Read a two-dimensions table file from the disk, which is values for gap positions and Correction coil power supplies (one gap value corresponding to 4 power supply value). If a gap position is set from the operator, four power supplies will be set automatically.
> I want to use SNC language to do this, for reading the data from disk, use an asub record for it. Is it ok? Is there any better ways? Did anyone finish this job before? 
>   Any suggestions are appreciated.


I did something very similar a few years back. I used lookup 
tables (breakpoint tables) as a feed forward for non-linear tune 
compensation for an insertion device (wiggler). The feed forwards, 
based on wiggler 'K', drove 9 quad families (about two dozen power 
supplies including series main coils and trims) to compensate for 
the non-linear betatron tune shifts from the wiggler. (With 
another feed forward implemented in a similar manner to compensate 
for orbit shifts.)

The ai and ao records included in base support a breakpoint table 
facility which may be adequate for your needs. But this conversion 
is to/from RVAL (which is an int) and VAL (which is a double). I 
hacked a bpt record (based on the ai record) which used the break 
point table functions included in base to do double to double 
conversions for my needs. In this way, everything was done in the 
EPICS database (no sequencer, no subroutine records, etc).

Note that in base >= 3.14.9, breakpoint tables must be monotonic 
by default. There is a flag to over-ride this requirement which 
must be used when using the breakpoint table in this type of 
compensation which likely needs a non-monotonic lookup table.

--
Steven Hartman
hartmansm@ornl.gov || 865-466-6473