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wireties
02-12-2012, 12:18 AM
FIRST Team 1296 is trying to get good data from a ADXL345 connected via I2C. We wrote some 50Hz sample data to a file and the device is working but sure seems noisy and the zero G offsets drifts (more than I would expect). I looking for helpful advice from some CDers that have had success with these sensors.

1 - Did you hard mount them? Or attempt to use some sort of vibration mount?
2 - Did you put bypass caps and/or power caps on the connectors?
3 - How much data did you average to get the zero G offset?
4 - How often did you reset the velocity and/or distance accumulators?
5 - Did you low pass the data? If so, what were the filter parameters?
6 - How reliable were you able to get them?
7 - Anyone tried using the FIFO or offset registers?

TIA

wireties
02-12-2012, 11:18 PM
Bueller? Beuller? Beuller?

nileshp87
02-15-2012, 10:10 PM
I have been working on this problem for a few weeks, what seems to be approaching the best idea is first taking the average.

Then get the standard deviation.

Then the range of values that should be zero is:
mean +- 2*standard deviation

If we do this for every axis that seems to work best.


Someone else on the forums also suggested (if you are trying to get distance from acceleration) using simpson's rule.

Here's some wiki pages to get you started:
Z-Score (http://en.wikipedia.org/wiki/Standard_score)
Standard Deviation (http://en.wikipedia.org/wiki/Standard_deviation)
Simpson's Rule (http://en.wikipedia.org/wiki/Simpson's_rule)

by the way we call a calibration method on disabledInit() and it completes in about 30 seconds.
Edit: Hard mounted
Does anyone know of a better way?

EricVanWyk
02-16-2012, 01:08 AM
If I recall correctly, the zero-g drifts while it is coming up to temperature. However, I thought that had more of an impact on the gyro than the accelerometer. Does it stabilize eventually?

How does the measured noise compare with what the data sheet (http://www.analog.com/static/imported-files/data_sheets/ADXL345.pdf) indicate?

wireties
02-16-2012, 02:39 AM
If I recall correctly, the zero-g drifts while it is coming up to temperature. However, I thought that had more of an impact on the gyro than the accelerometer. Does it stabilize eventually?

How does the measured noise compare with what the data sheet (http://www.analog.com/static/imported-files/data_sheets/ADXL345.pdf) indicate?


Great question - I'm not sure how long it takes to come up to temperature. We'll try to determine that - thanks for the input. Maybe we can read the temperate sensor on the gyro and compensate. The noise is much more than the data sheets indicates but we are pretty sure most of it is real. The sensor is fine, the robot is just a nasty mechanical environment.