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simple analog / digital -> USB needed

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Sep 30, 2013
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Hi All;
I need to build or purchase (pre-built preferred) a device that can measure the physical location of the rudder on my boat and make this information available via a USB connection on my laptop. The purpose here is to enhance a home built auto helm.
I can build the mechanics no problem but need assistance with the rest:
Option 1) Be able to measure distance (~1 foot hopefully with up to 1+ mm accuracy) via whatever means (laser? Transducer?) and translate the measurement into any digital form.
Option 2) I can set up a long slide POT (i.e. 100k Ohm) via mechanical means (less accurate but will do) and somehow translate this into a digital measurement read via USB
Any recommendations here? I’d like to keep my costs down if possible. I’m hoping there is a pre-built simple solution such as the magnetometer I used for compass heading.
Any assistance is appreciated.

The thread below had a similar topic.

Option 1) Be able to measure distance (~1 foot hopefully with up to 1+ mm accuracy

That is asking a lot. Does the rudder pivot about an axis? It might be easier to read markings on a pivot, to indicate angle, right or left of center.

I would avoid using a slider pot for the simple reason that their slot is usually open to the environment and you can guess what that means at the rudder of a boat!

some options-

1. if you have an accessible pivot point, anywhere in the mechanism but locked to the rudder position, an optical encoder is the best solution. They can be bought or quite easily made at home with a laser printer and OHP sheet. The idea is you use a transparent disc with concentric striped rings, an LED and photosensor for each ring and some simple electronics. The printed pattern blocks the light from the LED so for each different angle a different pattern of lit or shaded LEDs produces a value corresponding to the absolute angular position. It tells you you the rudder angle directly and can be accurate to a fraction of a degree.

2. use a rotary potentiometer in the same way, if it turns in sympathy with the rudder and you apply a voltage from one end of it's track to the other, the wiper will pick off a proportion of that voltage according to the angle it is turned. This is less accurate because it relies on the physical properties of the potentiometer and is prone to interference upsetting the readings but you should still be good to about 5% accuracy with good components.

3. The 'high tech' method of fitting a magnetic compass sensor directly to the rudder mechanism. This is very accurate but you need to sample lots of readings to find the average or other movements of the boat will skew the readings.

4. If your rudder is moved by cables, it might be possible to detect the cable position by displacement from a 'home' position. For this you need a method to recalibrate it, for example to move the rudder fully to one extreme then see how far it has moved in the other direction. A rotary encoder turned by the cable can then measure it's position and hence rudder angle. Some math is needed but the method is accurate.


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