FPGA Central

Hi all,

I have an IMU mounted on my RC helicopter and have interfaced it with a
onboard microcontroller to do autonomous hovering. Now that my vehicle i
stabilized in terms of attitude, I want to stabilize it in terms o
position. I want to demonstrate this indoors and thus do not have acces
to GPS.

Is it possible to use the accelerometer data from the IMU to get a
accurate position estimate? I've tried numeric integration, but of cours
the errors accumulate and I'm more than a meter off in a few seconds. I'
guessing I need some sort of EKF...or perhaps there is a better filter t
use?

Thanks in advance,
-weg22

weg22 wrote:
> Hi all,
>
> I have an IMU mounted on my RC helicopter and have interfaced it with an
> onboard microcontroller to do autonomous hovering. Now that my vehicle is
> stabilized in terms of attitude, I want to stabilize it in terms of
> position. I want to demonstrate this indoors and thus do not have access
> to GPS.
>
> Is it possible to use the accelerometer data from the IMU to get an
> accurate position estimate? I've tried numeric integration, but of course
> the errors accumulate and I'm more than a meter off in a few seconds. I'm
> guessing I need some sort of EKF...or perhaps there is a better filter to
> use?

Forget about it.

First, the commercial MEMS accelerometers have the accuracy about
0.1m/c^2. Even if you can improve the accuracy by an order of the
magnitude by the use of Kalman filter, etc. it is still way below
whatever is required for the inertial navigation.

Second, the INS is much more complex math then integrating the
accelerations. The reason for that is that the Earth is not the inertial
system. Your platform is affected by gravity and tide force as well as
the centrifugal and Coriolis forces.

At the third, a state of the art INS has a runaway about 1km/h. Quite
obviously this will not work indoors.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

weg22 wrote:
> Hi all,
>
> I have an IMU mounted on my RC helicopter and have interfaced it with an
> onboard microcontroller to do autonomous hovering. Now that my vehicle is
> stabilized in terms of attitude, I want to stabilize it in terms of
> position. I want to demonstrate this indoors and thus do not have access
> to GPS.
>
> Is it possible to use the accelerometer data from the IMU to get an
> accurate position estimate? I've tried numeric integration, but of course
> the errors accumulate and I'm more than a meter off in a few seconds. I'm
> guessing I need some sort of EKF...or perhaps there is a better filter to
> use?
>
Not with just the IMU. You could figure this out pretty quick if you
spend some quality time with the accelerometer data sheet and do the
math (remember that it's a _double_ integration).

GPS helps to an amazing degree, but you don't have it available.

You need some sort of positive position reference. It can be pretty
noisy and still be correctable with a nice Kalman filter, but you need
some absolute reference. You could probably use a ultrasonic range
finder as a 'radar altimeter'; that'll get you hovering at a constant
altitude, but you'll still be wandering N-S and E-W.

If you _really_ have time for a science project you could consider some
sort of "ultrasonic loran" that would use the time-of-arrival of a
number of different transmitter's signals to tell you where you are --
but that would probably take more time than building your own engine
from bar stock.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott wrote:

> If you _really_ have time for a science project you could consider some
> sort of "ultrasonic loran" that would use the time-of-arrival of a
> number of different transmitter's signals to tell you where you are --
> but that would probably take more time than building your own engine
> from bar stock.
>


Pseudolite

http://en.wikipedia.org/wiki/Pseudolite

http://gps.snu.ac.kr/research/pseudolite/indoor_eng.htm

Donald wrote:
> Tim Wescott wrote:
>
>> If you _really_ have time for a science project you could consider
>> some sort of "ultrasonic loran" that would use the time-of-arrival of
>> a number of different transmitter's signals to tell you where you are
>> -- but that would probably take more time than building your own
>> engine from bar stock.
>>
>
>
> Pseudolite
>
> http://en.wikipedia.org/wiki/Pseudolite
>
> http://gps.snu.ac.kr/research/pseudolite/indoor_eng.htm
>
Indeed. Following your link, I find that -- it's a science project.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html