Accelerometers keeping hard disks safe from impacts

It's never a great idea to drop your laptop, but these days there's less chance that you'll do it serious damage if you do. The principal of using an accelerometer inside the casing is a development from the idea of fitting air bags into cars. The accelerometer is a sensor that detects proper acceleration – that which is felt by people and objects. It is an acceleration experienced relative to freefall, commonly measured in terms of g-force. Vehicles use Microelectromechanical Systems (MEMS) technology which is typically faster in its response time than larger types of sensors. They are used to detect the rapid negative acceleration of a vehicle, determining when a collision has occurred and how severe it is. The same technology is used in electronic stability control systems, with cornering forces being measured using a lateral accelerometer.

The development of MEMS technology has enabled the construction of micro-detectors which are microscopic in scale, and so fitting such a sensor into a laptop is no longer a challenge. A free-fall sensor (FFS) detects if a computer is falling, and can then apply safety measures to limit the amount of damage it suffers internally. The hard disk can be retracted, to prevent a head crash that could result in data being lost on impact.

Earlier MEMS accelerometers converted force into a measurable displacement using a microminiaturised cantilever-type spring. Later, they were improved by using a heated glass bubble like a tiny spirit level. Thermal sensors detect when the accelerometer, and thus the laptop, is tilted or accelerated. The square in the middle of the chip is a resistor that heats up the glass bubble, and as the device is moved, surrounding thermal couples sense the location of the bubble. Though active hard-drive protection can be used in all computers, they have tended to be introduced in laptops as they are carried around and are more prone to be dropped. Accelerometers alert the system it is subject to excess vibration or acceleration, and the software tells the hard disk to unload its heads to stop them coming in contact with the platter and causing head crash.

You can find more info on sensors online to see which laptops have active hard-drive protection. For example, Apple uses what they call Sudden Motion Sensor, which is their motion-based hardware and data-protection system introduced in 2005. Since 2006, the company have included the patent-pending system in all their portable computers. The technology uses a triaxial accelerometer to prepare the hard disk drive for impact, disengaging the heads from the platters. When the computer is stable, the drive will operate normally again.

Among the laptop vendors to have implemented the technology are Acer, HP, Dell and Toshiba. This technology is only engaged when the hard drive is running at the moment of impact, as modern hard drives are designed to unload their heads when they are not being powered. There are also some companies, such as Western Digital and Seagate Technology, who produce hard-drives that include active protection independently.