Acquired Accelerations

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Augustyniak
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Acquired Accelerations

Postby Augustyniak » Fri Jun 06, 2014 12:14 am

Harry;

The other day I had some free time and was interested in finding out how good the iPhone accelerometer is... So.. Since I work in vehicle dynamics, I grabbed a PCB T356B08 tri-axis accelerometer along with my National Instruments setup running at 3200 Hz to do a little correlation work. I securely affixed the accel to the iPhone via the PCB paste and did some driving. I attempted to capture a sinusoidal lateral acceleration trace with increasing amplitude. Below are my results. FYI.. THe PCB trace has been LPF'ed by a 6th order butterworth filter with an Fc of 5Hz. I can send you the raw data and FFTs I used to ensure I captured most of the vehicle gross motion. However, after looking at the HLT data in the phone as well as the export... I found that the HLT data seems to scaled down by a factor of about 2.2. Is this something that has been done purposely based on your testing? Can you help explain the discrepancy? I am not trying to be a pain, just trying to learn. Thanks!!

Lateral_G_Comparison.JPG
Lateral Direction Comparison
Lateral_G_Comparison.JPG (80.45 KiB) Viewed 2169 times


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Harry
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Re: Acquired Accelerations

Postby Harry » Fri Jun 06, 2014 12:44 pm

LapTimer's approach to join acceleration data with GPS fixes and store this, instead of a raw stream of data, has the disadvantage for very short and fast acceleration peeks like this you generated. gplracerx raised that item several times already for his autocross exercises. I assume the factor you have measured above is due to average calculation where peaks are cut (but it may have different reasons). In case you want to improve the results, you may add a higher update rate GPS (10 Hz) as this will keep 10 times as much acceleration information. In case you want to compare the accelerometer quality, try enabling raw sensor logging using GPS Buddy on iOS or LapTimer on Android. On log creation, please check this: viewtopic.php?f=8&t=1933 The log class you are interested in is Raw Data.

- Harry


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Re: Acquired Accelerations

Postby gplracerx » Fri Jun 06, 2014 4:52 pm

Augustyniak,

If you want the raw data from the iPhone accelerometer, there's an app called Sensor Data that will allow you to log it at up to 100Hz.

In HLT, if you want to see quick changes in acceleration you need an external GPS with at least a 10Hz update rate so you get enough points and you need to go to Expert Settings in HLT (scroll to the bottom of the settings page) then scroll to Accelerator Tweaks and change the Update Rate to 100 Hz. HLT applies a filter to the raw accelerometer data whose time constant depends on the Update Rate, faster update = shorter time constant. The standard 30 Hz setting will distort changes in acceleration as fast as what you were doing.

Your accelerometer curve looks much like what I get going through a slalom on an autocross course, except I'm at a higher maximum g.

Image

Forgot to label the axes. The x axis is seconds and the y axis lateral g. And I was using a Dual XGPS160 external Bluetooth GPS.



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