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Output format from Autohelm ST50 masthead unit

Discussion in 'Boat Electronics' started by J, Oct 9, 2003.

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  1. J

    J Guest


    Does anyone know the format of the data returned from the masthead unit of a Autohelm

  2. Ian Malcolm

    Ian Malcolm Guest

    The Autohelm ST50 masthead transducer is DUMB and analog with a five wire
    connector. The smarts are in the display unit which outputs Seatalk.
    Last Autumn I wrote a program that captures boat heading, wind speed and
    relative direction off the seatalk bus and plots a scattergram wind rose.

    I didnt get round to subtracting out the spead and COG from the GPS as I
    wrote it purely for my own entertainment when alongside the berth to let
    me check that the wind transducer was working properly after a thourough
    clean and service without having to stay on deck to watch the readout.

    While I was in the transducer, I noticed two apparently identical
    quadrature sensing elements for the direction vane and a single sensor for
    the speed cups.

    From some troubleshooting info on the Rayithon site, I belive the
    transducer runs on a regulated 8V feed from the display and outputs two
    analog voltages for the direction, proportional to the Sine and Cosine of
    the vane position. AFAIK the speed is a pulse waveform either 1 or 2
    pulses per turn, although it might be a sinewave as well (The reference
    was fairly obscure and I didnt have access to a scope on the boat.)

    The sensors appear to be magnetic in nature and I belive uses hall effect
    pickups. After removing the grub screws in the vane and cups , they can
    be removed and you will then find that the two sensors are push fits in
    the aluminium body with O ring seals and can be prized out carefully to
    let you wash out he bearings. If you do dismantle it, the pickups appear
    to be extremely fragile and unless you are experienced with delicate
    mechanisms it would be extremely easy to break. Reassembling it is pretty
    obvious, provided you hook up the display to setup the correct vane
    direction before finally tightening its grub screw. Small adjustments may
    be made by rotating the wind sensor slightly before pushing it fully home
    in the Aluminium body, which you can do at the top of the mast. A
    worthwile improvement in accuracy when heeled can be achived by balancing
    the vane properly using a little metal foil of a wine bottle capsule or
    some solder secured with a drop of superglue. I was surprised how much
    was needed to balance it with its axis held horizontal.

    ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at][email protected], [dash]=- &
    *Warning* SPAM TRAP set in header, Use email address in sig. if you must.
    'Stingo' Albacore #1554 - 15' Uffa Fox designed, All varnished hot moulded
    wooden racing dinghy circa. 1961
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