Laser targeting with photo diodes

I'm a junior in Electrical Engineering and concurrent with my senior year I need to do a project to be presented at the end of the year.
what I was thinking is to make a laser designation circuit like you see in the movies, ie I could aim a laser diode at a surface, and then I'd have a photodiode array to "see" the laser spot.

the first idea was a robot to shoot ping-pong balls at a target, and eventually I started thinking it would be cool to have it use laser designated targeting. well now I'm concentrating on the laser designation portion, and once I get a handle on that I will determine how I use it.

one idea that refuses to leave my head is to make a glider, to which I would attach my circuit, so that the circuitry could guide the glider to the target. I figured it shouldn't be too much harder to move rudders and fins than it would be to move legs (although wheels might be easier if I choose a land-bound vehicle)

I was wondering about laser power, I have some cheapo-Ebay laser pointers that are <1mw, and 630-680nm wavelength. I was wondering if I'll need a more powerful laser...

a second question would be about filtering. I've found some photodiode arrays on digikey that filter to a certain wavelength, would it be as simple as getting one of these and a matched laser diode?
if this is the case I'm definitely going to need to do some more work for it to be a suitable project, (which makes me want to start researching gliders )

Sorry to be so long-winded, I'm just getting a little excited :wink:

Before researching gliders, you need a transmitter and receiver that works. If you modulate the laser transmitter it will make the detection and amplification of the reflected signal much easier. I think you will need a lens that will focus the spot on the array. The Sidwinder missle used a parabolic reflector to do that. The reflector and detector array was mounted on gimbols with servo motors so it looked like a giant eye which would follow a heat source.

with a gimbol mounted lens, then I could basically make the lens/array track the laser spot, and then actually navigate the vehicle based on the gimbol positions. basically just make it try to keep the "eye" centered.
am I on the right track?

how about the laser? my advisor seemed to laugh at the suggestion I buy a powerful one because the light is so focused that the array would be able to detect it from very far off, but he may have been saying that a person would be able to see it from far away...which is quite obvious... he's pretty eccentric and kind of hard to talk to, but he's the resident optics prof, so I'll have to get used to him.
I think I'll need to be in the market for a laser module anyways, if only because the ones I have now list a pretty wide wavelength range on the case.

Thanks!

yes on the gimbol question.
The output from the gimbol should be a signal that controls the attitude of the delivery vehicle. You might want to think "bomb" instead of "plane" There are more control surfaces in a plane and the math on the fly is more complex, course I'm just lazy


You can get laser diodes in power ranges from 5mW to 1000mW, visible or infrared I.E 35 mW @685 nM <$42. here
http://www.optima-optics.com/ld.htm#Panasonic

I think most weapons systems use IR because it's no as apt to filtering and scattering as visible light and works better in occluded weather ( cloud , smoke etc.) . A higher output will be visible farther then a low output. The usability of laser for targeting over distance is also a factor of "beam spread", given as a width over the length of the beam, which is also affected by optical and thermal turbulence.

well, as far as bomb vs plane, I'm basically trying to make a smart-bomb (minus the bomb) but in order for it to be demonstrated it needs to have more control than a bomb type creation would have, I'm not dropping it from a mile above the target so it would have time to track in using small fin adjustments. I was hoping for a basic glider that I could just worry about a rudder on the back, and wing pitch, nothing all that complicated.(hopefully)
then for a demo I was thinking toss the glider in the general direction of the target and have it turn itself if needed until it strikes the target with the laser on it. (followed by applause... :wink: )

also with the demonstration in mind I'm leaning towards a visible laser.

looking online I've found some pretty nifty "RC sailplanes" that are remote controlled, I was thinking I could trade the prop and motor on them for the detector, then just hijack the control system so that insteadof being guided by me with a remote it would be guided by the detector.
hoping of course that a 75 or so inch wingspan would give enough lift for a respectable flight distance when the engine is removed.

but I'm getting ahead of myself, without a detector I'm just wasting my money on a sailplane (although I could still have some fun with one of those... )

how large would the detector have to be? ideally it could be somewhat small, I guess it depends on the photodiode array. I'll do some research in that department.

Thanks for the info, now it's off to google to learn some stuff!

Well the dV/dT is a factor of a smart bomb.."sidewinder" etc., it's just not dropped , it's a damn rocket.

I think you will find solar or sidereal tracking of 'scopes a good place to start ( caveat , their both sloooo) as You possess enough knowledge to apply the concepts to Your projects. The info is prolly a hell of a lot easier to obtain VS a targeting schema of a super cooled refractometer array Ala stingers. Dig deep sir.

Did some reading on the systems and the preferred method seems now to be targeting using a CCD camera. Instead of mechanical manipulation of the sensor grid -photo diode array- the CCD can be considered on say a 2 megapixel device an array of 1,632 (H) x 1,224 (V) discrete elements.

The way to use it is that the proccessor takes the brightest pixel, interpolates the data from surrounding pixels and seeks to center the brightest signal at the middle if the CCD . That will "center" the vehicle on the target. The processors are of course proprietary DSP engines but with a RISC core ( or even a 486 computer on a board ) you should be able to acquire and analyze the data in real time with relative ease as your vehicle will not be traveling at the Mach 2.5 + speeds of air force weapons.

that definitely sounds like a better method. I think I'll look into that, Sadly it would call for more Digital Signal Processing than I was hoping for, and may bring up costs. (although I believe I get some funding from the university )
I'm looking at some ccd cameras now, there are some "covert" ones that are pretty small and look relatively lightweight.
also some really small wireless ones, that would keep weight down(seems to have a decent range to them, suitable for my needs), I could do the processing off the vehicle, but would then need to have wireless control to the vehicle, which if in an rc based vehicle would not be too tough.

the slow speed I'm going to be dealing with will definitely make things easier.

well I gotta say thanks for the info, I wasn't too sure where to start, and now I've got a couple different options and directions to head in.

Well now I hate to be long winded, but as I said I find this project to be fascinating. The effort to produce the science for particular, single case areas of study is inversely proportionate to to the various disciplines involved. The more specialized you become ( in any science ) the larger the scope of information You must have at your disposal.

My forth year was spent on finite size limits of operation of semiconductor structures due to quantum noise, good ole kT=q , sexy no? I used data from Arecibo's bird poop findings ( saw you had seti as a sig line ).This is one of the reasons that I know anything about this subject.

Anyway I say that because I had to document the effect of collectors under various thermal considerations ( room temp, super cooled, bla bla ) Stingers are supercooled so ar a few other smart bombs, less thermal noise.

One design consideration you must factor is the illumination of the target. As far as I can discern there are three basic methods. 1) acquire and drop, which means that the target is lased by the delivery vehicle and continuously lased until detonation , 2) acquire, sync, drop -n - scoot, the target is lased by the plane and telemetry is taken over by the device after it has acquired the target and lases by an internal source, 3) the target is lased by another source ( AWACS) dragged and dropped by the plane .

Number two requires much more sophisticated electronics and processor power.The alternates offer a smaller footprint and less on-board processing power (and circuit power ). I'm not sure of Your design constraints ( I at least hope you can point the delivery vehicle in the direction of the target).
Let Us know how the project goes.

Well it looks like I have some research to do. I should probably also give my advisor some updates, I'll see if he'd go for the ccd camera idea or if he'd prefer I stick with Photodiodes (he's into optics, and my initial proposal called for photodiodes, but I think it's pretty flexible at this point. to be honest I doubt 50% of my classmates have even decided on a project)

this week and most of next week will be finals, so I'm not sure how much time I'll be able to spend on this pursuit, but I'll definitely keep you updated.