Posted by anaximander on January 5, 2010 at 4:29am
I've noticed a lot of people are talking about autoland - and no wonder; landing can be tricky, and is generally the part of the flight where you're most likely to break something. I plan to get my UAV to autoland, because I'm not a great RC pilot, and I don't want to spend money and time on it to then have it crash and break. So far, however, there's one thing that has consistently stumped most autoland projects: altimeters. GPS is not accurate enough, laser is expensive, ultrasound is fuzzy and doesn't like soft surfaces like grass, IR has calibration issues with differences in colour and reflectiveness, and barometric is less precise and changes with airspeed, weather, and a bunch of other things.So obviously, a cheap yet accurate solution must be found. So here's an idea:- Take a cheap camera (a keychain digital camera, a basic webcam, whatever) and mount it staring straight down underneath your aircraft.- Take a cheap laser pointer and mount it pointing down and at an angle (either forward or back) underneath your aircraft.- Note how the laser dot moves across the camera's field of vision as the aircraft's altitude changes, and calibrate accordingly.This could be implemented a number of ways. Those with any kind of video streaming could watch the camera feed directly and just mark the altitudes on. Or, you could write a simple image processing algorithm that finds the dot and measures how far across the field of vision it is and calculates the altitude from there. If you want, you could add a second laser pointer such that the dots overlap at the moment the wheels touch ground, and use the distance between the dots to gauge altitude, and then, measuring whether the dots are symmetrical about the direction of flight would also tell you whether the aircraft is roughly level relative to the ground.Obviously, to get the most accuracy you'd have to mount the laser at quite an angle so that the laser dot moves a long way across the field of vision per centimetre of altitude. I'd suggest setting it such that the dot is at the furthest extent possible at the GPS's accuracy limit, and almost at the furthest opposite extent at touchdown. In other words, if your GPS gives altitude to +/- 15m, then the laser should be turned on at 20m, and enter the camera's field of view at 15m AGL. Then, the dot moves across the screen as the aircraft descends, and reaches the other side of the screen about 10cm BELOW the point where the aircraft's wheels hit the ground (you don't want the dot to go off the screen by accident, because that'd confuse it and possibly cause a crash, so have a safety margin).I don't know how well this would work, but it's the best way I can think of making a simple, affordable altimeter that might give acceptable accuracy. What do you guys think?
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Following are two excerpts from a historical application of using light for low altitude measurement;
"...the use of fixed beams of light to determine the altitude of a plane had been developed during World War I" (See also THE DAM BUSTERS (1954))
"Two spotlights were mounted under the nose and under the fuselage such that their beams would intersect 60 feet (18 m) from the underside of the plane. At the correct height, the two spots of light would merge into one on the surface of the water. The crews practised over the Eyebrook Reservoir in Leicestershire (built in 1940 to supply Corby steelworks), and the Derwent Reservoir, Derbyshire." (See also THE DAMBUSTES RAID)
AND YOU CALL YOURSELF A SCIENTIST! - The Dam Busters (1954)
It won't help accuracy, but it will help make the laser easier for the camera to pick up (they can be tricky in bright light, or on varying terrain). A laser bandpass filter is a filter for a camera that only allows one wavelength (color) of light (namely, the one of your laser) to pass through. This will make most everything in the image black (or at least very dark) and make the laser stick out really well. You can find them at optics supply houses (edmund optics, thor labs, melles griot, etc.) if you search for it. You just place it in front of your camera, and you're good to go (if you plan on using a webcam, you'll have to mount it yourself. Good luck!
I would suggest using a laser bandpass filter on the camera so you can clearly pick out the laser (you might have issues distinguishing it from the background otherwise).
Sorry, Your idea similar with Sharp IR sensors. This sensors including 1 line CCD and sending Infrared spot to the front, it reflecting to 1 line CCD and it mesuring the cells for finding the reflected spot. Simple, cheap and useful method. If you want to using laser for this, just buy a IR laser module and design a laser repeater with an IR filtered optotransistor and your laser. Optotransistor reads sharp's output and sends same signal over IR laser, then you can read it with Sharp's receiver.
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"...the use of fixed beams of light to determine the altitude of a plane had been developed during World War I" (See also THE DAM BUSTERS (1954))
"Two spotlights were mounted under the nose and under the fuselage such that their beams would intersect 60 feet (18 m) from the underside of the plane. At the correct height, the two spots of light would merge into one on the surface of the water. The crews practised over the Eyebrook Reservoir in Leicestershire (built in 1940 to supply Corby steelworks), and the Derwent Reservoir, Derbyshire." (See also
THE DAMBUSTES RAID)
Sorry, Your idea similar with Sharp IR sensors. This sensors including 1 line CCD and sending Infrared spot to the front, it reflecting to 1 line CCD and it mesuring the cells for finding the reflected spot. Simple, cheap and useful method. If you want to using laser for this, just buy a IR laser module and design a laser repeater with an IR filtered optotransistor and your laser. Optotransistor reads sharp's output and sends same signal over IR laser, then you can read it with Sharp's receiver.