I'm looking for a solution which will allow me to determine the distance between two points, one of which is static and the other not. It's part of a project I'm working on which will involve the creation of a sailing app for my boat. Based on a wireless network of sensors, I want to be able to determine and display on a tablet, information which shows how far the sails have been hoisted up on the mast. I need to put some sensors on the mast and in the sails themselves. I worked on RFID tags but got poor results due to high power consumption. I tried ZigBee distance measurements and accelerometers but ended up with a lot of noisy data. Any ideas on the matter would be much appreciated. Please remember that there are environmental matters to consider (weather) and the fact that the boat is in constant movement.
Just brainstorming here.
You could put barometric pressure sensors on the head and tack of the sails. If you measure the pressure difference you can calculate height. Might not be super accurate, but it would probably work to some extent. The short term change in pressure in the head sensor would be somewhat accurate while the sail is going up, even without reference to another sensor.
You could also put one sensor on the base of the mast to be your zero reference, and in that case, each sail would have just one sensor at the head. This would work better for conventional spinnakers which have considerable variation in tack height, depending on conditions.
Another way is to use something like an optical encoder on the mast. The detector would be on the head of the mainsail (this would only work for the main, probably not for headsails).
None of these things are going to be super reliable. In other words, I don't think this holds promise as an industrial type of system. It would be more like a teaching aid that could be kind of interesting (basically a gimmick). If you need rock-solid reliability, I think you will need to consider special halyards or something like that.
I kinda like the idea of Ultrasonic measurement here. There would be two transmitters: one is battery-powered and mounted on the sail. It would emit a chip periodically. The other transmitter is with one of the receivers.
You would have TWO receivers: one at the top of the mast, the other at the bottom of the mast. If I am thinking correctly, the ratio between the received times would give you the position of the sail on the mast.
Note that you do NOT need to know exactly when the transmit chirp occurred. That is: you don't need any connection (wired or wireless) between the transmitter on the sail and your electronics. Instead, you simply see which signal arrived first: the base or the top of the mast.
You calibrate the system by firing the reference transmitter (located with one of the receivers) and measuring the time for that signal to be received by the other receiver.
You could also encode a signal into the sail transmit chirp if you have multiple sails. This allows you to ignore signals from a sail that is not on that particular mast.
You could try it with a ultrasonic time measurement. You place a ultra sonic speaker on one side of the measurement and a receiver on the other. By measuring the time of flight you can get the distance in between.
If discrete measurements are OK you could install magnets on the mast and a hall effect sensor on the sail (or vice versa).