[Laser] RE: Cheap laser pointing hardware

[mike at milisake.com]

I crossposted the question to another list I am on.  Here is the response
I received from one of the experts on that list:

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>Has anyone ever used the actuating coil and head
>arm assembly from a hard drive to position a laser diode with?  Would
>it be possible?

If all you want to do is position a diode in 1-D, sure, but I'm not
clear on what the point would be.  Usually you want a collimating
lens in front of the diode, and moving the diode and lens would be
too heavy; moving the diode in the focal plane of the lens will steer
the beam but you need to move it fairly precisely in the correct
plane to maintain collimation.  Most beam-steering applications are
done with a pivoting mirror; you almost certainly could come up with
a way to use a disk head positioner as a mirror drive more easily
than as a way to move a laser diode.
>
>The person asking estimates up to 20 deg. worth of
>adjustment and guesses with the use of a variable current source,
>should be able to get it to position with fair accuracy.

The trouble is that you need a closed-loop system; disk positioners
have no "restoring spring" or damping the way analog meters or
loudspeakers do, so the current (=force) controls the acceleration of
the head, not its position.  All modern disk drives do the head servo
control using embedded servo information on the disk platters.  Old
disk drives (the first couple generations after the switch from
ST506-style stepper motors to voice-coil drives) sometimes had
optical or magnetic position sensors to help keep track of the head
position; AFAIK current ones use embedded servo information on the
disk platters exclusively.

I dunno what you would use to close the loop for your application -
the things I can think of offhand would either be too coarse (optical
interruptor disk) or (probably) too expensive/complex (photopot).

>Would there
>be too much vibration in the arm (current source noise) to allow for
>analog or digital communication methods?

There shouldn't be any significant vibration from the positioning
mechanism once you have a stable control loop -- after all, they're
designed to position disk heads to something better than 1/1000 inch.

Jordin (mirror, mirror, on the move...) Kare
--
Jordin Kare
908 15th Ave. East
Seattle, WA  98112
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