Autopilot Installation - Rate Gyro - Fluxgate Placement
Akama posted:
"..So, we conclude that optimally, both the black box (because it contains
the rate gyro) and the fluxgate compass should be mounted low, on the
center line and around the centre of pitch of the boat. And, that if
this cannot be done the location of the fluxgate compass is the less
important element.....I suppose y'all already knew this?.."
Comment: Not this listoid!!!
I'd be interested to hear a bit more about how the fluxgate RATE compass and
separate rate gyro work together. In other words, what's the function of each
component
and how do those functions relate.
We've previously discussed that RATE compass's give better autopilot control
in waves but it's not
clear to me how that function relates to a separate rate gyro....
Happy Holidays,
Rob Brueckner
Hatteras Yachtfish
"Robin" gymkidd405@netzero.com writes:
I'd be interested to hear a bit more about how the fluxgate RATE compass and
separate rate gyro work together. In other words, what's the function of each
component
and how do those functions relate.
We've previously discussed that RATE compass's give better autopilot control
in waves but it's not
clear to me how that function relates to a separate rate gyro....
OK, first note that I am neither an electrical engineer nor a compass
designer. But I am a mathematician, and I just installed a rate-stabilized
ComNav autopilot on Island Eagle, and during the course of debugging it I had
occasion to have a long and enlightening conversation with one of the
engineers at ComNav. I will pass it on here:
First, recall how a regular magnetic compass works: it senses the variation
in the earth's magnetic field, and in the case of a mechanical compass the
floating magnetic card will always point towards the magnetic north pole.
Well, that's the theory anyhow. The problem is that many things can affect
this, such as local magnetic interference (electric motors, the engine
block), as well as the mechanical action of the vessel pitching and yawing
through the water. Still, a magnetic compass has a high degree of absolute
accuracy -- that is to say that if you stand still in a certain spot, N will
indeed point repeatable to the same place. (for the purpose of this
discussion we'll ignore variation)
Now, gyros are sort of the opposite. Since they are inertial, not magnetic,
they are unaffected by interference, magnets, etc. This means that they are
ideal as the vessel pitches and rolls. However, the problem is that they have
no absolute accuracy at all -- when you start up a gyro you have to manually
tell it where North is. And over time a gyro will drift and loose "North".
So, here's the clever thing: Magnetic compasses are excellent at absolute
accuracy, while gyros are good at relative accuracy. Therefor, suppose we
were to mount a gyro and an mag compass in the same vessel, and then make an
arbitrary course change of 30 degrees. (say from a heading of 90 to a heading
of 120). We would expect that that both compasses would respond with the new
heading. However, that's not quite right. The magnetic would be affected by
local interference, and so might be out by a few degrees -- say it settles on
117. The gyro, on the other hand, has no absolute knowledge at all and so
will increase by exactly 30 degrees from whatever it was at -- say from 63 to
93.
But hold on: since we know that the gyro is highly relatively accurate, we
can use the fact that there was a 30 course change indicated by the gyro to
correct the magnetic compass. In other words, the gyro is applied as inverse
feedback to the magnetic signal, to completely compensate for any
locally-induced errors in the magnetic compass. If we keep a table of every
difference between the magnetic and the gyro compass, we'll have an
instantaneous correction table.
Now the cool part: we are also correcting for errors caused by mechanical
factors (pitch, roll & yaw), since the magnetic compass is affected by that
but the gyro is not. This means that we will have a much more stable signal
in heavy weather. We can also be much more flexible with the location of the
magnetic compass. For example (and I know of a boat where this has been done
very successfully) on a steel boat we can put the magnetic compass at the top
of the mast, put the rate stabilizer in the wheelhouse, and have a rock-solid
compass signal free of interference from the hull.
So, as I said, don't take this as the absolute word on the subject, but it
makes sense to me. By the way on Island Eagle I mounted the both the rate
stabilizer and the flux-gate compass in the pilothouse (which is about 10
feet above the waterline) and they seem to work perfectly.
Scott Welch
www.islandeagle.net
"The person who makes no mistakes usually doesn't make anything"
Scott,
This is in accordance with what the Raymarine people told me, but with a
clear explanation why. Thanks for that.
One small point, in the same vein as bumblebees can't fly, perhaps.
According to Raymarine, the rate gyro will work best if put at or near
the centre of motion of the boat. Since this term is apparently not
understood by most people, they dumb it down to a few pictures, and if
asked state "slightly above the waterline, on the centerline and 50 to
70% aft of the bow cutwater".
They made a clear point that they did not want me to put the gyro in the
pilothouse, which like yours is about 10 feet AWL. So, if you can
easily move yours and report what difference it would make, I'd
certainly be interested, as, I'm sure, the other listees would be. I
thought about doing it, but unfortunately my rate gyro is built right
into the black box, making it rather burdensome to move here and there
to do the experiment.
I'd also like to hear from others who may have their rate gyro in the
pilothouse. I've not installed the new A/P yet and it would be way
easier to mount the course computer "black box" in the pilothouse.
Cheers and Seasons Greetings,
Maurice
-----Original Message-----
From: Scott H.E. Welch [mailto:scott@firstclass.com]
Sent: Saturday, 24 December 2005 06:05
To: trawlers-and-trawlering@lists.samurai.com
Subject: Re: T&T: Autopilot Installation - Rate Gyro - Fluxgate
Placement
"Robin" gymkidd405@netzero.com writes:
I'd be interested to hear a bit more about how the fluxgate RATE
compass and
separate rate gyro work together. In other words, what's the function
of each
component
and how do those functions relate.
We've previously discussed that RATE compass's give better autopilot
control
in waves but it's not
clear to me how that function relates to a separate rate gyro....
OK, first note that I am neither an electrical engineer nor a compass
designer. But I am a mathematician, and I just installed a
rate-stabilized
ComNav autopilot on Island Eagle, and during the course of debugging it
I had
occasion to have a long and enlightening conversation with one of the
engineers at ComNav. I will pass it on here:
Maurice & Louise-Ann on AKAMA
Sent: Friday, December 23, 2005 2:25 PM
snip>
I'd also like to hear from others who may have their rate gyro in the
pilothouse. I've not installed the new A/P yet and it would be way
easier to mount the course computer "black box" in the pilothouse.
Cheers and Seasons Greetings,
Maurice
I installed the electronic hockey puck sized Raymarine compass center line,
amidships and on the water line. However, it was in the machinery room and
did not work. I moved it forward just behind the Vee cabin, two foot off
center, at the water line.
I installed the gyro and black box just forward of the stb engine, couple
feet off center line at the water line.
I too called Raymarine because the manual said not to install the black box
in the machinery space. They and several other knowledgeable folks said to
do it anyway. Skooch has limited space for these contraptions.
It works just fine.
BTW: I thought the new rate gyros told the AP not only that the boat was
turning, but how fast. So that the computer could look up how far to turn
the rudder based on previous stored observations of the boats movements.
This to me would greatly improves the APs response. Super accurate compass
readings when your getting thrown around 30 degrees in either direction does
not seem too be of much help.
Skooch Hatteras LRC 42
New Bern NC