Sudden Motion Sensor
|This article relies on references to primary sources. (July 2009)|
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 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. A clicking noise can be heard when the sudden motion sensor kicks in if you move your computer up and down for example.
Broadly speaking, there have been two types of Sudden Motion Sensor. The sensor used in the G4-based laptops resolved shifts of 1/52 g (e.g. the dynamic range was close to 6-bit), while the sensor used in the current Intel-based laptops have an 8-bit resolution (250 scale divisions). 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 1.5 kHz.
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 . This way, tilting can be used as a human interface device for instance for scrolling or for controlling games.
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 conflict 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.
Similar systems 
- IBM, now Lenovo: Active Protection System, found in some ThinkPads starting in 2003.
- Acer: GraviSense, found in some TravelMate-series notebooks manufactured in 2006.
- HP: 3D DriveGuard and HP Mobile Data Protection System 3D.
- Dell: ST Microelectronics DE351DL Motion Sensor is found on XPS L401X, L501X and L701X models.
See also 
- Apple.com: Sudden Motion Sensor disable instructions
- SeisMac: Displays seismic data obtained from your SMS
- AMSTracker by Amit Singh Command line utility for reading data from the Sudden Motion Sensor