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+--- Thread: z-accelerometer offset? (/showthread.php?tid=1012)
z-accelerometer offset? - jeglin - 08-28-2020
Hello, I'm a new user to phyphox and I have question/observation. I'm looking at the accelerometer data without g with the phone at rest on a table (this is an iPhone 6s). Since an accelerometer naturally measures the local gravitational field, I'm assuming that the "accelerometer without g" applies a correction to the z-data to ostensibly get zero.
However, when I do the experiment, I see a small positive offset in the z-data. The x- and y-data are very close to zero (variable due to noise and no more than ±0.02 m/s²) but the z-reading is consistently between +0.05 and +0.10, more or less (~5x larger than x or y). And indeed, when I switch to the "with g" mode, the z-data is consistently about 9.9, not 9.8.
I'm trying to understand why this might be – should I assume this is merely a systematic offset in the accelerometer on my particular phone? Is the non-zero value an error on the part of other sensors that are used to determine the correction (e.g., magnetometer to determine "down")? Is the applied correction simply not any more accurate than to the 0.1 level? Is this common to all smartphone accelerometers? Any thoughts or suggestions?
RE: z-accelerometer offset? - Jens Noriʇzsɔɥ - 08-28-2020
Have you checked against x- and y-direction if “g” is at the same level then? There could be multiple ways that the values are off: shifted, scaled, individual axes…
You could have a look into our phyphox.org/sensordb, select then the line of your 6s, and click through the “(i)nfo circles” below the table. A value of 9.9 does not seem to be that unlikely as a result:
[edit: moved inline picture into an attachment]
There are also graphs for standard deviations…
Acceleration without g is essentially a blackbox provided by the smartphone OS. Gyroscope and magnetometer are likely to be taken into account – in iDevices at least. I have no idea if an error in these sensors could account for such a systematic.
RE: z-accelerometer offset? - jeglin - 08-28-2020
Jens, thanks for the tip. Agreed those plots are very helpful. I've also compared an iPad Pro 10.5 and a friend's iPhone 11 Pro. All give slightly different results on different axes, even side by side on the table. This provides me with the information about the limits of a generic device that I was looking for!
RE: z-accelerometer offset? - Jens Noriʇzsɔɥ - 08-28-2020
(08-28-2020, 09:57 PM)jeglin Wrote: I've also compared an iPad Pro 10.5 and a friend's iPhone 11 Pro.
With newer iDevices (being there) I am always wondering what to do with the bumps of camera(s) and buttons, screen side down the only position being “stable”…
RE: z-accelerometer offset? - jeglin - 08-28-2020
(08-28-2020, 10:09 PM)Jens Noritzsch Wrote:(08-28-2020, 09:57 PM)jeglin Wrote: I've also compared an iPad Pro 10.5 and a friend's iPhone 11 Pro.
With newer iDevices (being there) I am always wondering what to do with the bumps of camera(s) and buttons, screen side down the only position being “stable”…
Indeed – in exactly the position where I can't casually look at the result!
RE: z-accelerometer offset? - Jens Noriʇzsɔɥ - 08-31-2020
… unless you utilize remote access (see three vertical dots menu) – and a web browser on a second device.
