Sudden Motion Sensor
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The Sudden Motion Sensor (SMS) is Apple's patent-pending motion-based hardware and data-protection system used in their notebook computer systems. Apple introduced the system January 1, 2005 in its refreshed PowerBook line, and included it in the iBook line July 26, 2005. Since that time, Apple has included the system in all of their portable systems (since October 2006), now the MacBook, MacBook Pro and MacBook Air.
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.
The motion-sensor can also be read by programs running on the Apple portables and a number of programs exist that use the motion sensor to make the computer tilt-aware [1]. This way, tilting can be used as a Human Interface Device for instance for scrolling or for controlling games.
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[edit] Aftermarket hardware issues
Among the MacBook and MacBook Pro community there have been several owners who installed aftermarket hard drives already equipped with anti-shock features who reported experiencing Kernel Panic errors whenever their unit was physically moved. This is believed to be due to a confliction between SMS and the new drive's anti-shock function. The Western Digital Scorpio series of notebook hard drives have been the most frequently reported as being susceptible to this problem. In practically all cases, disabling SMS alleviated this problem without any negative performance impact.
[edit] Return Policies
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Apple also uses this system to determine if a breach of warranty has happened. Every time the driveheads are turned off the approximate perceived velocity is stored. If the approximate perceived velocity is too high, Apple will not honor the warranty since the AppleCare Protection Plan and the limited warranty do not cover abuse or neglect.
[edit] Similar systems
IBM, now Lenovo, has an analogous system, the Active Protection System, found in some ThinkPads starting in 2003.
Acer has a similar system, known as GraviSense, found in some TravelMate-series notebooks manufactured in 2006.
HP has two equivalent systems named 3D DriveGuard and HP Mobile Data Protection System 3D.
