Real time location sensor for following a kite in the air

Question

In search of a new project I decided I want to build a kite which I can fly using my computer. For this I need a couple of things: a kite, an electro motor to steer the kite, and also a way for the computer to see where the kite is located in the sky at any given moment.

This last thing is what I wonder about. It means I need a sensor on the kite which can register "where" the kite is at any given moment. "Where" is defined here as a 2 dimensional context (I keep the rope length constant). The location should not be seen from the sky, but from the base of the kite its rope (i.e. the person flying it), which rules out GPS. It means I need something like a kinect sensor, but it should be able to register the object (kite) at a distance of up to a 100 meters or so. If needed I can of course attach a small "following device" to the kite.

I searched around a bit and I find technologies like Infra Red, Ultrasound, RFID. Does anybody know what kind of technology I need to use to be able to track a fast moving object in the sky in real time? There is of course a line of sight, so IR or Ultrasound could be possible, but then there is also a rope in the air (the one attached to the kite), which might confuse things.

So does anybody have any experience or thoughts about this? All tips are more than welcome!

Please note: it's a home pet project, so buying industry grade equipment is out of the question. It should be reasonably simple (consumer) hardware. If it can be taken from old home appliances that would be even better.. :)

[EDIT] In the meantime, i've found the OpenCV software (Open source computer vision), with which you can follow differently coloured blobs (a kite). Furthermore, it has python bindings (my favorite programming language). So this would already be a very cheap option (just need a webcam). It would create a problem with cloudy weather though, so if there is an option with a little device I can attach to the kite and which can follow the kite through the clouds, that would be a better option..

Answer 1

There are a few ways you might get feedback from the kite, without using vision.

1) GPS. Don't rule this out simply because the kite's base might be in a different location each time you use it. To counter this, you simply need a GPS receiver on both the kite and the base. Take both readings, and convert them into Cartesian coordinates. The difference between these coordinates is the offset of the kite from the base. Note that while GPS offers fairly low accuracy, its resolution is better than its accuracy. The kite and the base will both have the same position error, and so the kite's location relative to the base can be (reasonably) accurately calculated.

2) IMU. A 9-axis Inertial Measurement Unit, will help. There are some pretty small, lightweight parts available, like the MPU-9150 from Invensense. If you use a Kalman Filter to combine the GPS reading with the IMU reading, you can hugely increase your position resolution.

(Sorry, that's a similar part, but the same size)

3) Load cells. The reason I asked how easily you could fly a kite with your eyes shut, is because this will give you some idea of how useful the information from Load Cells will be. Perhaps you wouldn't be able to fly the kite brilliantly, but I bet you could keep it in the air. Each kite string will need a 3-axis force measurement so that you know the exact force and angle of each string. Arrange the three load cells like this:

Each one has a string coming perpendicularly out of the top, in the direction of the load cell's maximum sensitivity. Tie the three strings together, and to the main kite string. The sum of the three measured force vectors will be the tension and direction of that kite string. Make sure that the angle between the strings is greater than the maximum angle of the kite string, otherwise one of the three strings may go slack, making for a false measurement.

4) Pressure. I'm sure you could learn a lot by measuring the air pressure at several points on the kite's inside surface. There are some tiny lightweight barometric sensors available, like the BMP085.

It's even available from Sparkfun on a breakout board. These sensors will also help you to measure the kite's altitude (if they're out of the wind), and you can even use them to measure the windspeed if you place two inside a pitot tube.

Four sensors and three Pitot tubes, plus one sensor on the ground, will give you wind speed and direction, and altitude.

Answer 2

If the rope length is constant, then how about simple directional and angle sensing? Then with some simple trigonometery you have your kite position.

For example, you attach the kite string to a rotary encoder for the rotation, and on the rotary encoder you have another 180 degree encoder for the angle. You could probably hack this together for less than $10 or spare parts from various consumer items.

For the calculations, say your kite string is 50m long, and the measured angle is 45 degrees. Now we are assuming reasonably flat ground here so the opposite angle is ~90 degrees (however even if the ground is not flat under the kite you still know the height from your base position if you imagine a right angle triangle)

So using the above numbers, calculating for height we get (sin calculated in degrees here):

\$ 50m \cdot sin(45^\circ) = 35.36m \$

For the direction, you simply need a north reference and use the encoder reading to see how far offset it is.

EDIT - about the rope length/bending:

If you are concerned about the rope length and bending you could have a second thin string (e.g. fishing nylon, strong but very light) that could be used on a lightly tensioned bobbin to keep it straight and have yet another encoder on the bobbin to calculate length. I know it's a hack but you say you want to do this cheaply using consumer hardware. I can think of other options but they are liable to be quite expensive and complex for possibly not too much benefit (depending on the accuracy/precision you need)

Answer 3

I am working on a similar project, and was very happy to find this discussion.

I am using SimpleCV which is a python binding to OpenCV to get the position and orientation of the kite relative to the camera of my mobile phone. I use the acceleration and magnetometer of my mobile phone to get the orientation of the camera. I then send the data to my computer over wifi and process them into python.

The project is still in development, but everybody's welcome to help. The source code and documentation is available here http://code.google.com/p/robokite/ I have a blog but it is in french, soon in English?

You can also have a look at zenith wind power. It's a student project. The code was on google code as well, but can't be seen anymore for unknown reason. However, you can still get it with svn checkout http://zenith-wind-power.googlecode.com/svn/trunk I was not able to compile it, but they made a great job. Adrien should start working again on the project in September.