Measuring aperture size through Lumen intensity -- Advice.please

[taylaron]

Greetings everyone.
I'm working on a project that requires the very accurate measurement of distance varying between 0.500'' and 0.050''. I need to measure the distance within at most 0.010'' tolerance and give the value digitally. The read-rate is once every second.

I'm on a very tight budget and there's obviously too many ways to do this than are readily countable- notably a micrometer, but i'd like to avoid that for certain reasons. Instead, I'd like to focus on the possibility of using a light sensor to measure the amount of light blocked by a barrier. The light source would be a LED and the barrier's movement would be coupled with the object's movement. I'm envisioning a ~0.625'' diameter, black tube with a LED at one end and a photocell at the other with a slit in the tube for the barrier to move up and down in.

My question is whether or not anybody has had much success with measuring such slight differences in light levels using commercially available, cheap photoresistors or photocells. I'm also curious of what some of the challenges for practical application might be.

Thanks.

 

[skeptic2]

If I understand your description correctly, it sounds like a reasonable was of doing it. Just a few thoughts. Both the LED and the photosensor should run off a very well regulated supply. If the size of the light sensitive area is at least 0.45 inches in diameter and square in shape, and if the barrier is immediately in front of the photosensor I think it might work quite well. If however the photosensor is smaller or circular in shape, I think you will have problems.

 

[sophiecentaur]

I could add that a reference path with a fixed aperture and another sensor could eliminate some drift errors. Instead of just measuring sensor output level, you can compare it with a reference and that could help to eliminate LED output variation and temperature sensitivity of sensors etc..

 

[taylaron]

Thank you for your help.

I like the idea of using a second sensor with a fixed aperture using the same light source to establish a baseline. I'm concerned about a photocell's effective scan rate. I know photocells take a moment to stabilize with a change in intensity. Do you think that might be a problem?

 

[Nidum]

(1) Use a moving vane with a variable width slot in it . Put a fixed slit aperture in front of LED .

(2) If you want an interesting spin off project try developing a position sensing system using a digital camera .

 

[sophiecentaur]

I know photocells take a moment to stabilize with a change in intensity.

You are referring to the frequency response and that can be extremely high (tens of MHz), so not a problem. But, if you want to operate in room lighting, you would, presumably want to eliminate stray Mains Hum effects so you would want to band pass filter your detector output and operate with a shutter frequency well above mains frequency (and away from mains harmonics).

(1) Use a moving vane with a variable width slot in it . Put a fixed slit aperture in front of LED .

Good idea. You can then use synchronous detection at the shutter rate and eliminate low frequency noise. You don't need a shutter: you can just drive the LED with the same AC frequency as your detection circuit (a common 'clock frequency') Synchronous detection is a very powerful system and it can increase your effective signal to noise ratio significantly. Don't be put off from the idea just because it sounds complicated. It's something you could do retrospectively if you have error problems, because it could only involve electronics and not affect your mechanical design.