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Forum index » Science and Technology » Physics » Electromagnetics
EMI effects on Inertial Navigational Unit in a car.
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Lorenzo
science forum beginner


Joined: 04 Jun 2005
Posts: 4

PostPosted: Tue Jul 05, 2005 12:20 pm    Post subject: Re: EMI effects on Inertial Navigational Unit in a car. Reply with quote

"bowse" <bozo@buzzoff.net> wrote in message
news:remjc1dr944vr794v4k969sj4rlfm1m50s@4ax.com...
Quote:
On 3 Jul 2005 13:27:38 -0700, "None really" <serpent17@gmail.com
wrote:

Hello,

I am using an IMU made of MEMS that has the particularity to provide
not only acceleration, rate of rotation but also heading.
The heading is a mangetic one. When I use this unit in my car, I have
noise in readings at two levels. First, it looks like I have DC
component that, I guess, could be taken care of through the use of a
calibration scale (like they do in small airplanes). Second, I get a
high frequency component that makes it very annoying to even think I
can use this for a good heading. The second part can somehow be
attenuated through a gyro compensated input, and it works well when the
car is running (not driving). However, when the car is driven, even the
gyro compensated heading is not accurate (high frequency noise). Moving
the IMU to the back of the car helps when the car is not moving (but
the engine is on), however it does not help when the car is driven.
Even when you put the IMU on top of the car outside, the same noise is
still there, making it difficult for the IMU to be used for the heading
information.

So I am thinking I would like to use a faraday cage, is this a good
idea ? Should I put the cage around the engine or the IMU (they are of
different sizes) ? What type of material and grid should I use ? any
pointers or redirection in my thinking would be much appreciated.
Thanks in advance,

Jake.

It is not likely that a "Faraday cage" will work here, since the
coupling is likely to be magnetic, rather than electric. Any
automobile wiring carrying in excess of 10 amps or so may within a
meter or so cause a measureable deflection.

For example, the magnetic flux density at a distance of 15 cm from a
straight wire carrying a current of 20 amperes is about 0.26 Gauss, a
little over half the "natural" magnetic field of the earth.

H = I/(2*pi*r), about 21 ampere/meters in this case
B = 26 * 10^-6 tesla (1 tesla = 10000 gauss]
thus B = 0.26 gauss

So it stands to reason that any current-carrying conductors that are
near the sensor are very likely to upset it, creating a heading error.
Of course a lot depends on the orientation of the wires with respect
to the sensor, whether they are twisted with a return lead and other
factors that might tend to reduce the problem.

-kBob
Are you sure that the noise is not mechanical? I am not sure what MEMS are,

but since they detect motion and rotation they must be susceptible to
mechanical noise or vibrations. Can you associate the frequency components
seen in the IMU with speed? This could be electrical noise as motor speed
increases or mechanical. You should be able to separate these effects with
some simple tests.
Lorenzo
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bowse
science forum beginner


Joined: 25 Jun 2005
Posts: 5

PostPosted: Mon Jul 04, 2005 11:24 pm    Post subject: Re: EMI effects on Inertial Navigational Unit in a car. Reply with quote

On 3 Jul 2005 13:27:38 -0700, "None really" <serpent17@gmail.com>
wrote:

Quote:
Hello,

I am using an IMU made of MEMS that has the particularity to provide
not only acceleration, rate of rotation but also heading.
The heading is a mangetic one. When I use this unit in my car, I have
noise in readings at two levels. First, it looks like I have DC
component that, I guess, could be taken care of through the use of a
calibration scale (like they do in small airplanes). Second, I get a
high frequency component that makes it very annoying to even think I
can use this for a good heading. The second part can somehow be
attenuated through a gyro compensated input, and it works well when the
car is running (not driving). However, when the car is driven, even the
gyro compensated heading is not accurate (high frequency noise). Moving
the IMU to the back of the car helps when the car is not moving (but
the engine is on), however it does not help when the car is driven.
Even when you put the IMU on top of the car outside, the same noise is
still there, making it difficult for the IMU to be used for the heading
information.

So I am thinking I would like to use a faraday cage, is this a good
idea ? Should I put the cage around the engine or the IMU (they are of
different sizes) ? What type of material and grid should I use ? any
pointers or redirection in my thinking would be much appreciated.
Thanks in advance,

Jake.

It is not likely that a "Faraday cage" will work here, since the
coupling is likely to be magnetic, rather than electric. Any
automobile wiring carrying in excess of 10 amps or so may within a
meter or so cause a measureable deflection.

For example, the magnetic flux density at a distance of 15 cm from a
straight wire carrying a current of 20 amperes is about 0.26 Gauss, a
little over half the "natural" magnetic field of the earth.

H = I/(2*pi*r), about 21 ampere/meters in this case
B = 26 * 10^-6 tesla (1 tesla = 10000 gauss]
thus B = 0.26 gauss

So it stands to reason that any current-carrying conductors that are
near the sensor are very likely to upset it, creating a heading error.
Of course a lot depends on the orientation of the wires with respect
to the sensor, whether they are twisted with a return lead and other
factors that might tend to reduce the problem.

-kBob
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None really
science forum beginner


Joined: 03 Jul 2005
Posts: 2

PostPosted: Sun Jul 03, 2005 6:27 pm    Post subject: EMI effects on Inertial Navigational Unit in a car. Reply with quote

Hello,

I am using an IMU made of MEMS that has the particularity to provide
not only acceleration, rate of rotation but also heading.
The heading is a mangetic one. When I use this unit in my car, I have
noise in readings at two levels. First, it looks like I have DC
component that, I guess, could be taken care of through the use of a
calibration scale (like they do in small airplanes). Second, I get a
high frequency component that makes it very annoying to even think I
can use this for a good heading. The second part can somehow be
attenuated through a gyro compensated input, and it works well when the
car is running (not driving). However, when the car is driven, even the
gyro compensated heading is not accurate (high frequency noise). Moving
the IMU to the back of the car helps when the car is not moving (but
the engine is on), however it does not help when the car is driven.
Even when you put the IMU on top of the car outside, the same noise is
still there, making it difficult for the IMU to be used for the heading
information.

So I am thinking I would like to use a faraday cage, is this a good
idea ? Should I put the cage around the engine or the IMU (they are of
different sizes) ? What type of material and grid should I use ? any
pointers or redirection in my thinking would be much appreciated.
Thanks in advance,

Jake.
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