On Mon, 15 Dec 1997, William Chops Westfield wrote:

> The read/write head of CDROM drive has a pretty impressive set of degrees of
> fine motion that I image are used for focus and "fine" movement.  Has anyone
> experiemented with these?

On the two that I've taken apart there are 2 degrees of freedom in the
fine motion system.  There is a little lens with a focal length of 1 or 2
millimeters.  One degree of freedom is up and down, the other is side to
side.  One drive did side to side with a pivot point, the other drive used
a 4 bar linkage.  Both systems used coils of fine copper wire.

I mounted a mirror on the mechanism that pivoted.  Since I didn't have an
active feedback system, I could move the mirror rapidly, but it wasn't
very controlled.  I could move it slowly enough that it didn't oscillate.
I just used a potentiometer, a PIC and PWM would have been much spiffier.

Unfortunately only one of the drives had a pivoting movement.  With 2
pivoting movements, I could make a rather small X-Y system.

The motions might be small enough that open loop control would be usefull
if I were projecting a small image on a wall several feet away.  I think
the currents involved are small enough for the PIC to drive the coils
directly through a current limiting resistor.  The coils need bipolar
drive, so it would take 2 pins for each degree of freedom.

Would capacitors help the PWM, and if so, how do I figure out the best
size?  This is a job for my trusty old 20 Mhz oscilliscope (just trying to
bring in another thread).  Further experiments will wait for another day
or another experimenter.  I don't see much point with only one degree of
freedom.  I'll have to wait until I get matching broken CD-ROM drives.

--
paulh@hamjudo.com  http://www.hamjudo.com
The April 97 WebSight magazine describes me as "(presumably) normal".