Hello mark.

15 Jul 04 14:14, you wrote to me:

 ml> not to mention dealing with other forces like natural wind ;)

 JB>> finally I fitted it on a long arm and swept it in a circle at
 JB>> a known rate (indoors)

 ml> by hand?

yes, I rigged a slip-ring setup using a headphone jack and plug  and a
cable hanging from the ceiling  and used a camer tripod as the base
attaching the arm to the handle and arranging a contact at one point in the
circle
wrote some software to log pulses from the detector and from the perimeter
contact and sat beneath the tripod and spun it as smoothly as I could.

Ideally I guess a motor driven method (record player?) coul;d have given a
more uniform speed.

 JB>>>> it worked pretty well until the ball-bearing went rusty
 JB>>>> and then the cups (cut from ping-pong balls fell off)

 ml>>> i hear that...

 ml> actually, the ones that i've seen have a triangle on them that acts as
 ml> a vane to keep it turned the proper way... i was thinking of a
 ml> resistive wiper riding on that vane and setting the measurements at
 ml> the specific reading for that point... the arm that the wind raises
 ml> has a pointer to this vane where the speed marking are located...

using a potentiometer as the axle for the speed-sensing flap might be better..

a 1M Ohm linear one would be about right for interfacing with a joystick port.
(IIRC the specs say 0-270K end, but you's be only using 1/3 (at most) of
the 270 degree movement of the potentiometer.

getting a potentiometer with a low turning resistance could be tricky as
the engineer them to stay put once you let them go. but I gues all you
really want is the wiper and the track.

keeping it dry could be tricky, but oil shouldn't degrade its performance.

the rotating electric coupling for the wind direction axle will be another challenge.

hmm.  if you could arrange it for a fixed sensor on the top of the pole
(possibly a coil that' effected by a metal strip (shaped something like a
rhino horn) so that as the flap movees back it gets gradually further from
the coil (or thinner)

personally I prefer something optical or magnetic just to reduce friction
and as theyre less effected by dampness.

here's another way to measure angle optically
mont a polariser on the input shaft and put behind it two leds with
oppisitely angled polarisers in fron of them. feed them with AC signals 90
degrees out of phase with DC offset. in fron of the disc put a light
sensor, the phase of the peak in the signal with reguard to the AC will
indicate the angle of the shaft bearing the polariser (only good for one
quadrant of the circle)

Jasen

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