Your error (difference) log would make a database of tidal /
current info, I should think.
"Analogue" or digital? A magnet passing a coil produces
sinusoidal output. It's a guitar string, folks! Fancy ones use a
threshhold detector (a schmidt trigger) and produce a pulse
(range adjustable by setting a 555 pulse circuit to vary duty
cycle) integrate output into an rc network to drive a d'Arsonval
movement, or just switch capacitors. (I can provide basic
d'Asonvals for about 10 bucks plus shipping, maybe a little less)
You can't just rectify the ac output, because you would lose the
bottom end of the speed output, because the low frequencies are
also a lower level output, and you need a rectifier to feed the
meter, lest it null. A forward biased (by about 0.7vdc)
rectifier would help. The speed scale would not be linear, so a
dc compensation amplifier might be needed, and that would need
readjustment occasionally. Ain't analogue grand?
Dragging a fishing weight on 50 feet of line over the taffrail
pulling against a calibrated spring scale is cheap and dirty. A
fishing scale might work, if you usually catch fish under a
pound. If you ask to buy a gram scale, black helicopters will
follow you silently home.
A trolling or other small dc magnet style motor and some
alligator clips might actually recharge a battery through an
ammeter calibrated in knots. You could drive a clockwork dial to
indicate distance, not considering current or wind drift. Leave
your tranny in drive, pull the spark plugs, and let her rip, eh?
If you changed the cam timing on the exhaust valves, could you
pump air with the engine? Would venting the air under the hull
reduce water friction, even extend hull speed restrictions if
vented at the front and rear of the hull?
Russian torpedo makers seem to think it works.
Could you test this concept using the engine in neutral just for
it's exhaust gas? Even a genny?
How about a car alternator, chimney rod and propellor rig hanging
off the transom taffrail? Epoxy a small magnet (and a counter
weight?) inside the rotor to ensure the alternator always starts
up. An ammeter would reflect your speed. Free electricity? You
would have to maintain a steady load on the output, if you want a
speed indication. Perhaps electrolysing water to make hydrogen
fuel for you engine or a standby bag of H2 for your genny or fuel
cell? A bridle tethered tube bearing rig might extend the life of
the alt. bearings.
'Digital' outputs come from hall effect 'magnets', which do not
make an output until a certain magnetic field is sensed, then
they put out a pulse. The difference in the sending units is the
Hall effect ones have a magnet mounted near a coil pickup on the
housing. The wheel contains a parasitic-active transducer, a
quantum re-radiator that puts out a consistant pulse. Analogues
lack the magnet on the housing.
Either style can drive either type of display, if the right
electrics are hitched.
Fully digital systems are more complicated, using phase locked
loop i.c. clocks to help generate segmented displays using driver
logic designed by boolean principals.
Terry K
Lloyd Sumpter wrote:
On Wed, 21 Jan 2004 19:10:31 +0000, Glen "Wiley" Wilson wrote:
On Wed, 21 Jan 2004 09:18:16 -0800, "Lloyd Sumpter" wrote:
Can I get a new sender, with proper documentation so I know how it
works, and hook it into my laptop?
I'm afraid I don't have anything like an answer to your question, though I would
have thought that the sender wouuld output pulses rather than a variable
voltage, leaving the instrument to count the pulses. You can see some readymade
data acquisition solutions at B&B electronics http://www.bb-elec.com/ but I
suspect the cost would qiuickly approach that of a new instrument, if cost is
your major objective.
If it is pulse data, you might be able to attach (properly buffered/amped) it to
something like the RTS line of the serial port. You can definitely capture RTS
events if you take direct control of the port, and probably even if you use the
standard MSComm activeX object. Since Windows is kind of crappy when it comes
to doing anything in real time, you'd likely lose pulses occasionally though. I
think you'd be better off to count them with a circuit and just pass along the
count. Don't know nuthin bout no parallel ports, though.
I'm actually more interested in your thought process. I have something of a
professional interest in displaying this sort of thing on PCs. To date, I've
worked with NMEA data, but I've been asked to look into direct acquisition of
certain data, so I'm wondering what leads you to want to do this. Of course, if
it's just cost, that's pretty self-explanatory.
Thanks
Doing a bit more research, I've found the OLD Signet meters sent out an analog
voltage (they were basically small generators), but the newer ones, notably
Airmar (www.airmar.com) send out pulses.
Airmar even has some samples of how to interface with CMOS and TTL. I thought of
using one of the RS232 signals, but think I'll use the parallel port instead.
I'm using Linux, so I'll attach the pulse signal to the interrupt and use Unix
signals to minimize CPU time. (Airmar says they output pulses about 4.8/sec at 1
knot, so max is around 100pps, or 10ms between rising edges).
I think I'll be interfacing the GPS as well, so I can compare readings. I may
get energetic and write a "calibration program" that would average the two
readings and adjust the "fudge factor" for the knotmeter (assuming you push the
"calibrate" button, then run a specified distance, then back)
Only questions now a 1. will an Airmar sender fit in my existing thru-hull,
or do I have to buy a new one (apparently they're not expensive), and 2. sample
programs in C showing how to access parallel port in Linux.
Lloyd Sumpter
"Far Cove" Catalina 36
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