From: shoppa@almach.caltech.edu (TIM SHOPPA)
Subject: Re: Astro-(disposable)-cam
News-Software: VAX/VMS VNEWS 1.41    
Organization: California Institute of Technology
Date: Mon, 14 Sep 1992 17:13:00 GMT

In article <565@rml.UUCP>, jack@rml.UUCP (jack hagerty) writes...
>In article <1992Sep14.022929.20412@hpcvaac.cv.hp.com> billn@hpcvaac.cv.hp.com (bill nelson) writes:
>>jack@rml.UUCP (jack hagerty) writes:
>>: 
>>: You could try using a segmented gear. 
>>
>>Good idea. That would simplify the mechanism quite a bit. That would also
>>clear up the problem of relieving the spring tension that would be required
>>with a lever. The only problem would be making the segmented gear with the
>>exact number of teeth required. Make it spring loaded so it has a few degrees
>>of leeway?
> 
>Either that, or maybe you could use a friction drive but that's
>a little tricky, i.e. you need enough friction to drive the
>advance, but not so much to do damage once the film is advanced
>and the drive wheel slips; all while enduring gee loads in the
>+/- 20g range.
> 
>Since you're removing teeth from the gear, it would seem that getting
>the right number is the most literal example of "cut and try" I can
>think of! Too much advance? File off another!
> 
>- Jack
> 
> 
>=============================================================================
>||Jack Hagerty, Robotic Midwives, Ltd.                 jack@rml.com        ||
>||Livermore, CA		           NOTE! New, improved address! ^^^        ||
>||(510) 455-1143	            (old ones will still work)             ||
>||-------------------------------------------------------------------------||
>||  "I, like all men, have never grown up. Other men lie and say they      ||
>||    have. I refuse to lie. I try to be a creative child, using my        ||
>||    immaturity in such ways as benefit my society rather than harm it."  ||
>||                           - Ray Bradbury, _Yestermorrow_                ||
>=============================================================================

You're right, Jack, I pretty much do need some sort of friction drive
mechanism.  This is because the angle that the wind lever has to turn through
to advance a frame depends on how much film has been wound onto the takeup
spool!  In 36 frames, the change in angle is quite large, as the diameter
of the (take-up spindle + used film) at least doubles.  Bill's suggestion
of a spring to absorb some of this difference might work OK for a
12-exposure roll.  For anything longer, a friction drive is an absolute
necessity!

Another option might be to electrically sense when the shutter has been
cocked.  Two metal contacts would do the job of controlling the advance
motor.  At some time after the motor stopped, the shutter could be triggered
(with a small solenoid maybe?), causing the contacts would close again
and powering the motor to advance the film to the next frame.

My experiment over the weekend consisted of taking a Kodak panoramic disposable,
reloading it with 36 exposures of Gold 200 film, clamping the shutter button
into "fire", and advancing the film with a DC  gear motor whose shaft was duct-
taped to a dowel epoxied to the take-up spindle.  I took 36 frames of a
parking lot at advance rates from 1/2 to 3 exposures per second.  (This was
all on the ground, by the way.  The DC gear motor I was using weighs 5-7 lbs
and I am not planning on using this particular one in a real launch!)  I have
to go develop the film tonight, and will see how bad the motion streaking
really is!

				Tim (Shoppa@erin.caltech.edu)

P.S.  It seems that the message of mine from the middle of last week somehow
just got reposted, at least on my news-server!  Is this evidence of some
type of binary time warp?