Accelerometers protect hard drives

The explosive growth of portable equipment such as laptop computers, media players and PDAs has fueled an increasing demand for hard-disk drives and a pressing need to protect them from severe impact when a product that contains one is accidentally dropped.

To make hard-disk drives more robust, their impact resistance must be enhanced, either actively or passively. Passive approaches simply cushion the device with impact-absorbing materials, usually rubber or gels. These cannot protect devices from falls of more than one meter, however, precluding their use in portable equipment. Active approaches prevent a collision between the head and the platter by parking the read/write head when accelerometers detect a drop. The approach was first used commercially in a notebook PC released by IBM in October 2003.

When an object suddenly drops, the acceleration detected by accelerometers oriented along each of the three axes becomes zero because the accelerometers are accelerating toward the earth at the same rate as the falling body. Angular acceleration may also be imparted to the object.

The traditional hard-drive protection algorithm is based on free-fall modeling. The outputs from the accelerometers can be divided into four consecutive intervals: "static," "rollover," "free-fall drop" and "impact." When the object is pushed off the table, the accelerations are small during the static, rollover and free-fall drop intervals, but increase rapidly upon impact.

If a fall could be distinguished earlier, there would be much more time available for protective action. The sensor outputs do vary during the rollover interval, but the deviations are not sufficiently pronounced to directly trigger the hard-drive protection process. If, however, a function equal to the sum of the squares of the time derivatives of the X- and Y-axis accelerometer outputs is formed, a large signal can be detected during rollover (see Fig. 1). The plot is calculated based on the output of a 12-bit analog-to-digital converter. The samples are in 5-millisecond increments.

As expected, the sum of the squares of the time derivatives is quite large during the rollover time interval, but becomes quite small during the free-fall drop. This sequence of events can be employed to provide a reliable indication that a fall has occurred.

We can now establish a new test algorithm, called the differential acceleration algorithm:

(dX/dt)2 + (dY/dt)2 > Threshold

The principal components of a system to implement the differential acceleration algorithm are the ADXL320 dual-axis accelerometer, AD8542 dual rail-to-rail amplifier and ADuC832 smart transducer front end, as shown in the simplified schematic of Fig. 2. The accelerometer contains a sensor and signal-conditioning circuitry to implement open-loop acceleration measurement. The analog output voltages are proportional to the orthogonal accelerations. The surface-micromachined polysilicon sensor is suspended over the surface of the wafer by polysilicon springs. Its deflection is measured using a differential capacitor formed by fixed fingers on the silicon and moving fingers attached to the sensor. The fixed plates are driven by square waves that are 180 degrees out of phase. When the device is subjected to an acceleration force, the beams deflect, unbalancing the differential capacitor-which results in an output square wave with amplitude proportional to acceleration. Phase-sensitive demodulation circuitry rectifies the signal and determines whether the acceleration is positive or negative.

One might ask whether a three-axis sensor is essential for hard-drive protection. The answer is no. For, as demonstrated above, the use of an ADL320 dual-axis accelerometer, when employed in a protection system that implements the differential acceleration algorithm discussed above, performs the task quite ably. In addition to lower cost, the dual-axis sensor approach saves space and reduces power dissipation.

In the hard-drive protection system that we built, the response time between the start of free fall and when the alert signal is generated is 40 milliseconds, with a sampling rate of 300 samples per second per channel and a sensor bandwidth of 100 Hz. The time required to park a hard-disk drive head should not exceed 150 ms, so the total time from a detected free fall to the completion of the parking is no more than 190 ms. This is far less than the 432 ms required by a portable product to fall 3 feet.

Read more about protecting hard-disk drives in Analog Dialogue (www.analog.com/analogdialogue). Naoki Asakawa's "Tech Analysis: HDD for Mobile Phones Withstand 1.5-Meter Drop," Nikkei Electronics Asia, January 2005, was used as a reference for this article.

By Wenshuai Liao (wenshuai.liao@analog.com), senior field applications engineer, and Yiming Zhao (yiming.zhao@analog.com), field applications engineer, Analog Devices Inc. (Beijing)