Data storage advances have made it possible to store vast libraries of data on smaller and smaller hard-disk drives. Application developers are keeping pace by creating fantastic portable applications, like the iPod and cell phones which use these drives. These new and exciting applications–often holding irreplaceable photos, movies, and music–have greatly changed and improved lives.
Unfortunately, these high-tech marvels, which operate by flying a magnetic head just nanometers above fast-spinning disks, are a bit like a glass library perched on the edge of a shelf: they are one slip, nudge, or fall away from data disaster. Should one of these drives fall during operation, the magnetic head will crash into the spinning platter, potentially damaging both the head and the disk, resulting in an irretrievable loss of data.
It doesn't have to be that way. Drives and mobile systems can be designed with an effective, air-bag-like capability that detects free fall and protects the drive and its data. This article explores how an accelerometer can be used to detect when a system is falling, and should that happen, put the portable system and drive into a safe mode to minimize damage.
Head Slap is a bad thing
Today's modern mobile drives are built to withstand some level of abuse. This level is specified by two specific parameters: operating shock, and non-operating shock. Operating shock is the g-force a drive can withstand while the disk drive is powered up, and reading and writing data. Non-operating shock is the g-force a drive can withstand without data loss when powered down. Typically the operating shock will be on the order of 300 g, while the non-operating shock is on the order of 900 g.
Why the difference?
Look at a mobile disk drive's innards (Figure 1 ).
Figure 1: Mechanical structure of a hard-disk drive
(Click to Enlarge Image)
When the drive is reading and writing information, the magnetic read-write head is flying just nanometers over the magnetic disk. It is attached to the end of a suspension/actuator that moves the head across the spinning disk to access the data.
If the hard drive is dropped while reading data, the resulting shock of hitting the ground could cause the read-write head to bounce onto the disk. This is called “head slap”, and it is a bad thing to happen. When it does, the head can be damaged or that spot on the disk can be magnetically erased, resulting in data loss.
Hard-drive manufacturers will guarantee certain shock survivability while the drive is reading and writing data. This is the “operating shock” of around 300 g. To increase the survivable, “non-operating shock” to 900 g's, the manufacturer builds a ramp into the drive. When not running, the suspension/actuator with the sensitive read-write head is moved up the ramp and “parked” in a detent position on the ramp.
How can we take advantage of higher non-op shock in a mobile drive?
If there were a way to determine that a shock was about to occur, the drive in our mobile system could be commanded to move the head up the ramp and put the drive into a non-operating position. In effect, it could “deploy the airbags”.
The most common “shock inducer” to a mobile device is a drop. Typical scenario is this: a favorite mobile device sits on a desk minding its own business. A careless arm, reaching for a cup of coffee cooling nearby, knocks the innocent device into the air. It executes a graceful double back flip before crashing to the floor, losing all its data.
Now, for a short flashback to high school physics. When an object is at rest on a desk, the earth's gravity is exerting one g of force downward on the object. If the table is removed, the object will accelerate toward the floor at 9.8 meters/second. In this “free fall” case (from the object's point of view) each axis (up/down, left/right, front/back–X/Y/Z) is at 0 g, until the point of impact, at which time the g's the object will experience get quite large, Figure 2 .
Figure 2: The typical g-forces on a disk drive due to shock of hitting floor after free fall, versus time.
(Click to Enlarge Image)
This information can be used to anticipate that something bad is going to happen. “If I experience 0 g's in X, Y, and Z axis, there is a very good chance I am falling, and I am about to hit the ground with some significant force.” An accelerometer detects this acceleration and its magnitude, where “one g” is the nominal acceleration due to gravity at the Earth's surface.
Several types of accelerometer are available in the market today. The most familiar ones probably are air bag sensors. These accelerometers detect when a specific threshold of g-force is reached, and then alert the car to deploy the appropriate airbags. In the case of the acrobatic mobile device, it must anticipate the impending crash and put itself into a safe mode. This can be done with an accelerometer that can detect values near 0 g in three axes, which is the “free-fall” condition. One kind of three-axis accelerometer is a MEMS device.
There are several manufacturers of MEMS accelerometers. These parts are manufactured in a silicon process, and typically consist of a mechanical element that will sense the acceleration, and an electrical interface component. Hence, these are Micro Electro-Mechanical Systems (MEMS). The mechanical elements can be built to sense either two or three axes of acceleration, Figure 3 .
Figure 3: Sensor structure of a MEMS device.
(Click to Enlarge Image)
The electrical interface component can provide an analog output that is proportional to the acceleration sensed on a particular axis, or a digital output that can be conditioned with internal logic to generate an interrupt once the sensed g-level falls below a certain value.
How to integrate free-fall detection into a system
A notebook computer is a good example of a system that can benefit from free-fall detection. Several notebook manufacturers already offer various free-fall protection schemes based on MEMS accelerometers. The implementations are similar and can be extended to other systems.
In the notebook computer, the MEMS device interfaces with a housekeeping microprocessor. Typically this is the keyboard controller. A designer can use either an analog-output accelerometer (if there is a free analog to digital converter in the microprocessor), or a digital output accelerometer.
In either case, the microprocessor will continuously measure the acceleration in the X, Y, and Z axes. These measurements are compared to see how close to zero-g each axis is. Once a predefined threshold is crossed, the microprocessor interrupts the system, and forces an immediate retract on the hard-disk drive to get the head up the ramp and locked in a protected position, away from the disk.
Some accelerometers on the market have the ability to digitally set thresholds internally to the part, and will generate a single interrupt when the threshold is crossed, thus eliminating a lot of work for the housekeeping microprocessor.
Other system-level features can also be added once an accelerometer is integrated into the system. When an object is at rest, 1 g is always “down.” As such, the direction of “down” can be calculated by reading the X, Y, and Z values. With this information, tilt angles can be determined and other features added, such as automatic Landscape/Portrait display-mode switching, based on how the device is being held.
MEMS-based accelerometers on the market today provide a method to sense free fall when integrated into a system. By anticipating a free-fall incident, you can design into your systems a “safe” condition that will improve the likelihood of saving precious data. Once the accelerometer is integrated into a portable device, it can enable other possible applications that may need to know which way is “down”.
About the author
Bill Raasch is the Vice President of Market Development for the Computer and Peripherals Business Unit of STMicroelectronics. During his 18 years at ST, he has supported design and development efforts in the Aerospace, Data Storage, and Computer Peripheral areas. Bill has a BS degree in Engineering from California State University (Fullerton), and is a classically trained chef. He can be reached at firstname.lastname@example.org