SPECIFICATIONS FOR HWP CONTROL FOR SHARC-II POLARIMETRY MODULE:

MAY 2004


(1) general specifications:

size of half-wave plate:  3" - 4" diameter.  about 0.5 cm thick.  Rough
dimension TBD by June.  Exact dimension TBD by July.

space constraints:  No difficult space constraints expected.  Rough idea
TBD by June, exact specs by July.

motion control specifications: need to control position to 0.5 degree.  
Need to be able to accomplish 360 degree moves in about 2 seconds, and 45
degree moves in about 0.5 seconds.

switching between 450 and 350 micron HWP's: we would like to be able to
easily switch between the two wavelengths in just a few minutes.


(2) devices:

(a) stepper motor.  we have some left over from SPARO project.

(b) absolute digital rotary encoder.  the encoder should couple to hwp,
not to motor.

(c) "HWP computer".  accepts commands from SHARC II acquisition computer
via "tcp/ip socket" interface.  controls motor and reads encoder.  to
control the stepping motor, a stepper motor controller ('indexer') is
needed.  it may be that this could be part of the "HWP computer".  Or it
may be that the "HWP computer" will talk to a separate "indexer".  For
example, we could use the indexers left over from SPARO (see below).  For
HWP computer, reliability at 14,000 ft. is an important consideration.  
we prefer a system with no hard disk.  a system with a watchdog timer is
preferred.  We should have Darren check with Troy Ames before buying any
particular "HWP computer", just to make sure that Troy has no objections
to it.

options include:

An "Industrial PC" (similar to what Industrial Computer Source used to
sell and now available from Chassis Plans at
http://www.chassis-plans.com/single_board_computers.html)
                                                                                
An "EDAS", e.g. from "Intelligent Instrumentation"
(http://www.edasce.com/index.asp)
                                                                                
A littler version of the "Industrial PC" called the PC-104.


(3) SPARO indexers:

We have a Panther indexer from intelligent motion systems plus three
spares.  These have been used reliably for 5 years at South Pole, and for
a few days in 1998 at Mauna Kea (SPARO engineering run).  They cost $1000
each.  They can control a stepper motor via commands sent over a serial
line.  These devices are described at:
 
http://www.imshome.com/pan_systems.html


(4) communications protocol:  

The control computer sends the order to move to a desired angle to the HWP
computer.  The HWP computer replies in some way when the move is done,
reporting any failures to the control computer.