Re: Optical pickup for Prop Balancer
- From: jeff <jalowe44INVALID@xxxxxxxxxxxxxxxxxxx>
- Date: Thu, 29 Mar 2007 15:58:42 GMT
Robert Scott wrote:
I'm trying to develop a prop balancer for small planes, but I can't seem to make
a decent optical pickup. Other companies make such balancers, so I know it is
possible. Following the practice of the other prop balancers, I want to place a
piece of reflective tape on the back of the prop so that the optical pickup can
identify this particular blade. The plan is to correlate an accelerometer
signal with the timing of the marked blade to determine where weight needs to be
added or subtracted to achieve better balance and reduce vibration. Here is
what I have done so far:
I used a PIN photodiode that comes with an IR filter centered around 940 nm and
connected it to a transimpedance amplifier, and it works fine. I mounted the
photodiode in a tube with a plastic lens at the other end so that it focuses at
a point about 11 inches from the end of the tube. That is because I hope to
mount this tube about 11 inches behind the prop. But focus is not critical
since resolution only needs to be within 2 degrees of rotation.
The question is how to illuminate the target. I could try to use the ambient
light, given that prop balancing always occurs outside and in the daylight. But
I have no control over the background as seen from the detector. The plane
might be pointing at a reflective surface or a grassy field. There is no way I
can guarantee that my tape on the back of the prop will appear that much
brighter than the background. I thought of active illumination with a matching
940 nm LED. But that appears impossible if the sun happens to be shining. Even
with a focused beam, I can't compete with the sun. For safety reasons, I can't
put anything in front of the prop. And for safety and convenience I don't want
to rely on anything that requires line power, so big heat lamps are out.
Any suggestions on how to optically detect a marked prop blade from 11 inches
away would be greatly appreciated.
Our Sigma Balancer uses a Keyence FS-M1H fiberoptic pickup for once per rev.
--
jeff
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