REFLECTOR: Where to put your AHRS

[Hiroo Umeno]

It seems that judging from much of the responses I got, many of you have units that has separate AHRS and magnetometer.  As it turns out, mine is an integral unit.  One black box that has inertial sensor, magnetometer, fiber-optic gyro, all built in.  I currently have the unit on top of the sump tank and I see the attitude reading drops off as I get above 40Kts or so.  During the flight, usually in turns, the reading would come back for a second or so but for the most part, they remain off.

The manufacturer suggested it may be caused by magnetic interferance.  They told me that under the seat would be tricky since movement of the seats will affect the magnetic field.  They also told me to keep the unit close to the aircraft's rotational axis and away from major high-current wires.  Well, on a Velocity, that pretty much rules out the whole plane...

Hiroo

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of michalk
Sent: Tuesday, September 26, 2006 6:42 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Where to put your AHRS

My magnetometer is almost under the pilots seat.
The electronic gyros are mounted in the keel.

As far as I can tell with my ground tests, the magnetometer will be okay here.  I have oil lines and starter cables running through the left duct.  All other wires go through the right duct.

The panel mount compass does not appear to be affected except when toggling the propeller pitch motor, and starting.  I can live with those.  I specifically wired the panel to keep high current wires away from these devices.

Magnetic lines are perpendicular to the current carrying wires.  Mount your sensor such that the plane (base of instrument) is co-planar with nearby wires.

Magnetic lines cancel each other out as well.  Keep power and return lines paired, and as close to each other as possible.  Twisted pair wiring is even better.  Sheilding doesn't do any good in this case, because that's for capacitive coupling ... but may be required for other non-magnetic reasons.

As a final attempt to get a clean compass reading, it *is* possible to magnetically shield your power cables.  Use Mu-metal.  It's ungodly expensive, and it's a band-aid solution anyway.  It as well as other ferrous materials will distort the magnetic fields by redirecting them.
  You can compensate for this somewhat with adjusting the magnetics supplied in the base of your compass.

Al Gietzen wrote:
> For those who are flying with AHRS / Magnetometer...
>
>
>
> Where did you put yours?
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