The Sudden Motion Sensor (SMS) is a patent-pending data protection feature introduced by Apple Computer into its refreshed PowerBook and iBook lines on January 1, 2005 and July 26, 2005 respectively. The feature monitors the orientation of the laptop and detects sudden accelerations (changes in velocity). Laptops are designed to utilize this feature to prevent damage to the hard disk caused by strong vibrations or movement, such as a drop or fall. It has also been included in the MacBook and MacBook Pro.

When the sensor is triggered, the disk heads are disengaged from the disk platters, thereby preventing or minimizing any data loss which may occur as a result of disk head crash. When the feature detects that the laptop's orientation and movement has stabilised, it then unlocks the disk heads.

As the feature can be used to determine the angle at which the laptop has been tilted along with the speed of tilt, various hacks have been developed to use the feature as an input device, and harness the output of the feature for use in "tilt-sensitive" applications. Examples include orienting the laptop window to always remain upright relative to the user, even when he or she is upside down. Apple has not documented the API for the device and it's characteristics vary from model to model.

Another interesting application would be to use Macs as seismic sensors. Practical use of Mac-produced ground motions for emergency response or scientific use is problematic for several reasons. First, it would be difficult to discriminate between earthquake ground motions and the routine motions of a laptop under daily use. To recover actual ground motion (acceleration) the sensors must be calibrated and many readings would need to be gathered and correlated at a central site. Also, to be useful the precise location of each sensor must be known. Finally, the SMS has a very limited dynamic range; the MacBook and MacBook Pro SMSs have only 12 bits of precision, while older SMSs have only 8-bits of precision. This severely limits its use for seismic applications.

IBM has implemented an analogous system known as the Active Protection System on some ThinkPads starting as far back as 2003.

See also

• Data recovery

External links

• SeisMac a scrolling display of the three components of acceleration produced by the SMS
• MacSaber 1.0, create lightsaber sound effects while moving a Mac notebook in different directions and with varying forces
• "Bubblegym, the tilt-controlled computer game for the PowerBook"
• kernelthread.com article, "The Sudden Motion Sensor"
• kernelthread.com follow up article, "The Apple Motion Sensor As A Human Interface Device"
• Wired News article March 22, 2005, "Hackers Tilt PowerBook for Tricks"

Computer data

Sudden Motion Sensor | Sudden Motion Sensor | Sudden Motion Sensor