Determining distance between wireless sensors - sailing project

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.

• How high is your mast (distance to measure)? What is the precision needed? – Botnic Dec 5 '14 at 10:47
• How about mesuring the length of the rope used to hoist the sail? Could be done either directly by trying to find out how much rope went up/down the mast, or indirectly by figuring out how much of the rope is still on the winch, for example. – JimmyB Dec 5 '14 at 12:09
• The mast varies in length but let's say 30 m. The precision needed is approx. 1 m. We've thought about the rope but it gets a bit complicated. We need to feed data into the app so getting it from the sails instead of the rope seems a little easier. – Maggie Winiarczyk Dec 8 '14 at 11:50
• You could use a magnet and hall sensor on the masthead turning blocks to count revolutions. Need to be able to detect direction of revolution also. Anything on the sail is going to be difficult to power up. Also, sails flog around when you luff or tack and get caught in the rigging and so on. – mkeith Dec 10 '14 at 21:59

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.

• Great idea. Two barometric sensors, protected from the wind. st.com/web/en/catalog/sense_power/FM89/SC1316/PF261381 should do the job nicely – Scott Seidman Feb 3 '15 at 21:39
• I think the optical encoder equivalent is a good one. Sails (or at least the head of the sail) ride on a track through the mast, so you could actually have some reliability there. – kolosy Feb 3 '15 at 21:53

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.

• Wouldn't the constant changing wind speeds around the boat severely affect the accuracy of any kind of ultrasonic measurement? Strong winds may even just blow the signal into oblivion. I would go for something passive with hall sensors or the like. The rope seems like the best place to start. – captcha Feb 3 '15 at 21:57
• Wind is primarily horizontal. Speed of sound is much faster than wind, so horizontal wind shouldn't affect the signal too much. Narrow-band detection will help defeat wind noise. If you want to get really fancy, you can use a complex signal with correlation detection. Processing gain. – mkeith Feb 3 '15 at 22:12
• @captch: That is a possibility. But I think that the idea is worth exploring. It's fairly trivial to make ultrasonic transducers weather-tight (think Fish Finder) and almost anything else that has been suggested just isn't sufficiently robust. – Dwayne Reid Feb 3 '15 at 22:19

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 you do this, you need to control for air temperature. One way to do that would be to have two sensors at a known distance. Measure the time of flight over the known distance to calibrate. – mkeith Dec 5 '14 at 17:52
• Oh, I better add, the speed of sound is controlled by air temperature almost exclusively. Don't worry about air pressure. It might seem like that should be an important factor, but it is not. And in any event. this calibration method will control for all variables. – mkeith Dec 5 '14 at 17:55
• The idea is to keep the system simple and reliable. The boat moves constantly in very different and difficult weather conditions. Calibration may be a problem as you may find yourself in weather conditions which do not allow it for even a few days. I think the system has to be as weather proof as possible. – Maggie Winiarczyk Dec 8 '14 at 11:55
• Sorry for not being clear. The calibration should be automatic and more or less continuous. There could even be a dedicated transmitter and receiver strictly for calibration. I don't think I would worry about air temp differences at different heights. It could happen but will generally not be the case. None of the proposals so far strike me as simple and reliable in the real world. Ultrasonic tends to suffer from "stuff" clogging up the grill. Could be seawater, dust, etc. All shipboard electronics are subject to corrosion. It is an interesting but challenging problem. – mkeith Dec 10 '14 at 22:00

If discrete measurements are OK you could install magnets on the mast and a hall effect sensor on the sail (or vice versa).

• If it has to work for headsails, this probably won't work very well because they don't ride up the mast on a track. It could work well for the main sail, however. It would be best to have one magnet on the top of the main, and multiple detectors on the mast. – mkeith Dec 5 '14 at 18:09
• This seems pretty interesting although I don't really know how reliable this method would be however I'll have a look into it. Ideally I'd need a system that works on all sails and not just the main sail ... – Maggie Winiarczyk Dec 8 '14 at 15:12