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Absolute Encoders



We are currently using the Daresbury designed VG monochromators which have
a ROD 800 incremental encoder on them.  In general, these work great
except when we lose power on the motor controller.  (As we get closer to
the thunderstrom season, these concerns come more to mind) Then, we really
need a hutch access in order to rotate the mono very close to the datum
position so that we can find the index on the ROD800.  If a staff member
is around, this isn't a problem, but if we aren't here, it is a pain.  A
company that we use for encoders U.S. Digital (http://www.usdigital.com)
has an absolute encoder with a maximum resolution of 65536 steps per
revolution available for less than $300.00. With a $48.00 adaptor and
$10.00 cable, this thing speaks RS232.  They have an inclinometer version
which we are going to mount on the external rotation plate of our mono to
give us a 'rough absolute reading' so that we can automagically have the
controller go find its datum of the ROD800 if the inclinometer and the
mono controller differ by more than a certain delta.  I just wanted to
mention this since we are in general happy with U.S.  Digital's products
and if this is successfull we will probably outfit our Huber circles with
the shaft encoder version of these to provide rough 'dial coordinates' for
the same purpose.  If anyone else is interested in doing something like
this, let me know and I'll keep you posted with our progress.  Another
application that comes to mind are mirror tilt sensors. Also, US Digital
has a hybrid version which has an incremental encoder plus the absolute
encoder so that you can design them onto the back of a motor to give you
absolute motor position (the absolute encoder has 'quasi' multiturn
capabilities) plus your quadrature pulses.  

BTW, these encoders live on a proprietary bus which can be converted to
RS232.  The cabling is 6 conductor 'phone type cord' and you can have up
to 15 encoders on a single bus.  This might make it attractive for the
EPICSy folks since you can have 15 devices off of one RS232.
Just a thought.


- John 
    


---
John Quintana                              Phone:630-252-0223
Northwestern University                    FAX: 630-252-0226      
DND-CAT                                    email: jpq@nwu.edu
APS/ANL Building 432/A003
9700 South Cass Avenue
Argonne, IL 60439