New Accelerometer Supplier Could Be Causing Incorrect iPhone 5s Level and Compass Measurements

Wednesday October 16, 2013 11:23 am PDT by Jordan Golson

A new supplier for the accelerometer part in the iPhone 5s could be causing incorrect level and compass measurements in the new device, an issue that was first noticed earlier this month.

RealityCap CEO Eagle Jones read into the Chipworks teardown of the iPhone 5s (via Gizmodo) and noticed that the phone uses the Bosch Sensortech BMA220 as its accelerometer. The iPhone 5, for example, used an accelerometer from STMicroelectronics.

Incorrect iPhone 5s Level

iPhone 5s and level, courtesy Gizmodo

Both devices have similar consistency in measurement, but the Bosch device has a larger measurement 'bias'. The end result, as Jones notes, is that the new accelerometer has a different 'zero-g offset' which would need to be compensated for in software.

The second key spec for accelerometers is the zero-g offset, or bias. This indicates the range for a roughly constant offset that will be added to every output sample of data due to manufacturing variance. This can also change over time due to mechanical stress or temperature variation. This is where we find the problem: the typical bias for the ST part is +/- 20mg, while the Bosch part lists +/-95mg. This almost 5x greater offset range is confirmed by our measurements, and is absolutely consistent with the failures being reported by users and the media. Specifically, a +/- 20mg offset range would translate to around a +/-1 degree accuracy range in tilt detection, and a +/-95mg offset translates to +/-5 degrees in tilt.

He goes on to note that developers can compensate for the increased offset because it is unlikely to change significantly from one measurement to another, though Apple could do a system-wide fix at the OS level to ensure accurate measurements in the future.

Related Forum: iPhone

Top Rated Comments

wxman2003

163 months ago

It's amazing how someone can figure out what the problem is in a few weeks, while all of Apple engineering let this one get by them. Developers should not have to "compensate" their apps because of poor Apple QC.

Score: 15 Votes (Like | Disagree)

baryon

163 months ago

This is exactly why it shouldn't just be compensated by the same amount in every device with an update, but rather a simple "calibrate" button should be added that prompts you to use a real level and calibrate the device. Since, as they said, the bias changes over time, you probably want to recalibrate it from time to time, so it's a useful feature to add.

Every scale, measurement device, etc even the most professional ones all have the ability to be calibrated. No one's expecting the iPhone to be accurate for this type of thing, which is one more reason to add a calibration feature.

Score: 14 Votes (Like | Disagree)