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bias stability
Posted:
Mon May 17, 2010 12:38 pm
by pizza
Hi tom, have you tried allan variance for estimation of non tuned noise/measurement covariances?
Re: bias stability
Posted:
Sat May 22, 2010 2:48 pm
by Tom
This is something completely new for me.
Wikipedia:
http://en.wikipedia.org/wiki/Allan_varianceA google search on "allan variance gyro" returns a lot of links, most interesting ones:
http://www.xbow.com/support/Support_pdf ... rement.pdfand
http://docs.google.com/viewer?a=v&q=cac ... YPfEUiNJRgAs I understand it is a way to model the noise and bias instability of a gyro, and not a way to compensate for it?
Re: bias stability
Posted:
Wed May 26, 2010 8:10 am
by pizza
i have read a few papers on how people use allan variance analysis on inertial sensors to identify the noise sources namely bias instability, ARW, RRW, quantization noise, flicker noise and ramp..then these parameters are plugged into calibration model to correct the sensor data, reducing/eliminating effects from drift and improving accuracy. ARW and RRW coefficients are used to compute the Q and R covariance matrices for Kalman filter in their implementation.
well im not sure how well it works, still trying out.
i realised the less number of bins used, the less significant those allan variances are(error increases), so i use as many bins as i can for each averaging period.
ran the av tests on sparkfun 5dof(idg500,adxl335) ..the idg500 gives really unstable data(i don't know why) while the ADXL335 gives very smooth, consistent AV curve.
tom, what has happened to your quad project from the old website? just curious why you've decided to make a jet instead
Re: bias stability
Posted:
Thu May 27, 2010 1:57 am
by pizza
Hi all,
if it interests you, attached are my allan variance results ran at 100Hz for 24 hours each(total 86400*100samples), on IDG500 of sparkfun's 5DOF(gyrox, gyroy), STmicron's LY530alh gyroscope(gyroz), and Freescale's MMA3761L accelerometer(accx, accy, accz).
apparently the results are rather similar except for gyro X and gyro Y. Can anyone teach me how to identify the problem on the hardware?
thanks & best regards,
pizza
Re: bias stability
Posted:
Thu May 27, 2010 8:39 am
by Tom
Hi Pizza!
It seems logical that the accelerometer is the most stable one.
Taking into account the algorithmic scale, the IDG-500 seems to have a big variance indeed. I never measured the variance over a day, but it can drift quite a lot. It is weird that the STmicro one seems so much better, while I always heared it was worse.
I could do some shorter measurements on the sensor if that would be usefull for you. What exactly is tau/s ?
PS: quadrocopter is ready and is undergoing tests this week
Re: bias stability
Posted:
Sun May 30, 2010 12:01 pm
by pizza
Hi Tom,
the taus that I use are
float taus[12]= {0.1, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024}; //note: unit = seconds
as these are equal intervals on the allanvariance log-log graph.
Tau is the averaging time used on the bias.
A 24 hr sample will have 86400*100 samples on a 100Hz sampling rate.
If tau = 1s, the samples are divided into 86400bins(i.e no. bins = total no. samples/samples in each tau). The samples in each bin are averaged. Then the allan variance is calculated. this is repeated for every tau. for tau=2s, there will be 43200bin etc. the less number of bins will cause higher error rate i.e ε(%)=100/√(2(bins-1)). A 2.4 hr sample will have less number of bins etc.