Sudden Motion Sensor

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The Sudden Motion Sensor (SMS) is Apple Computer’s patent-pending motion-based hardware- and data-protection system used in their notebook computer systems. Apple introduced the system 1 January 2005 in its refreshed PowerBook line, and included it in the iBook line 26 July 2005. Since that time, Apple has included the system in all of their portable systems, now the MacBook and MacBook Pro (as of October 2006).

With a triaxial accelerometer, the system detects sudden acceleration, such as when the computer is dropped, and prepares the relatively-fragile hard disk drive mechanism for impact. The system disengages the disk drive heads from the hard disk platters, preventing data loss and drive damage from a disk head crash. When the computer is stable, the drive operates normally again.

Broadly speaking, there have been two types of Sudden Motion Sensor. The sensor used in the G4-based laptops resolved approximately 52 counts per gravity, while the sensor used in the current Intel-based laptops resolves about 250 counts per gravity. In at least one model of Intel-based laptop, the MacBook Pro 15", Apple uses the Kionix KXM52-1050 three-axis accelerometer chip, with dynamic range of +/- 2g and bandwidth up to 1.5KHz.

Since the accelerometer can indicate the tilt of the computer, various hacks use the sensor. One example keeps the displayed image upright regardless of the computer’s orientation. As of October 2006, Apple has not documented the API for the device, and its characteristics vary from model to model.