A few weeks ago Gizmodo put the iPhone 5s through some simple tests to show that its motion sensor calibration was significantly off. Now RealityCap CEO Eagle Jones is reporting that this is due to the use of a new accelerometer, as revealed by a teardown of the phone in Chipworks.
Jones indicates that the iPhone 5s uses a Bosch Sensortech accelerometer, as opposed to the STMicroelectronics accelerometer used in previous iPhones. According to Jones, both parts deliver similar consistency in measurement, but a difference in the “zero-g offset” could be at the root of the miscalibration issue. Jones writes:
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.
Apple could potentially fix this issue by manually adjusting the bias in the manufacturing process, but considering it hasn’t yet acknowledged this problem exits, an immediate fix seems unlikely.
And it is unclear why Apple made the switch in chips, since both cost the same amount of money. The Bosch accelerometer uses slightly less power, but according to Jones, the difference is likely miniscule – less than 0.1 percent.
For now, it’s possible for app developers to compensate for the bias of the new accelerometer by incorporating a calibration procedure into their apps, which RealityCap is working on now.