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?