imu position and sampling rate

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imu position and sampling rate

Postby pizza » Wed Apr 07, 2010 4:12 pm

hi guys im still new to these sensors..
couldn't really find the basics on the internet...wanted to ask you how does position of accelerometer/gyroscope/combined on your system affect sensing capability? i just can't vsualize it. for example if you place them in the middle of your system or at the edge. what difference will there be? will angles still be accurate?

secondly, how does ADC sampling rate affect your data, pitch and roll? how do you decide a rate to obtain angles correctly?
does nyquist rate and aliasing matter?

thankssss :mrgreen:
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Re: imu position and sampling rate

Postby Tom » Wed Apr 07, 2010 8:36 pm

Hi pizza,

(1) If you don't position your IMU in the center of gravity of your plane, a rotation of the plane around it's center of gravity (eg a roll from 0° to +45°) will cause an acceleration on your IMU (because it changes position compared to the earth, and position changes can only occur when it has a certain speed/speed change).

(2) A high sampling/processing rate is always better because
(a) quick updates means very small changes in angles which can better be threated as if they would be linear (cos(0.000001) ~ 1). Imagine you update only once a second. During that second, the plane pitches 90° and after that it rolls 45° (use your hand to visualise this :-) ). In this case, you receive the sensor readings, but you don't know whether it pitched first, or it rolled first. This means you can never make a correct estimate.
(b) your ADC sampling rate is better always at a higher frequency than your noise (2 times higher - nyquist - in theory, 5 times higher in practice) so you can average it out, otherwise you could always measure on a noise spike (assuming you get an instant reading). In practice, most ADC's give you an average reading over the sampling time, so this is less of an issue.

Tom
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Re: imu position and sampling rate

Postby pizza » Wed Apr 14, 2010 12:52 pm

thanks tom =D,

how do you measure the noise frequency of each sensor?

i tried runge kutta 4th order on the xy axis gyro, at 200Hz sampling rate, I experienced a drift of 1dps, at 800Hz, I experienced a drift of 0.2-0.3dps. Increasing the sampling rate seems to reduce drifting, do you have any idea why?
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Re: imu position and sampling rate

Postby Tom » Mon Apr 19, 2010 6:56 pm

I can only think of some "bad luck" effects such as when your noise frequency is a multiple of the sampling rate, or you don't sample during the complete 1/200th of a second.
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Re: imu position and sampling rate

Postby pinmikaa » Mon Jan 26, 2015 12:40 pm

Now having explored your newest code, I see you already have my primary concern covered with the manual reversion channel in the configuration utility. Most of the RC craft I've used stability augmentation on in the past could not be flown in manual reversion due to onboard mixing and instabilities - thus if a microprocessor gliched, there was no potential for manual recovery without a quick processor reset. I haven't had a problem yet either!
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