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Fly Height Monitor Improves Hard Drive Reliability New Feature Significantly Reduces Hard Error Rates O V E R V I E W / E X E C U T I V E S U M M A R Y Western Digital's Fly Height Monitor protects write operations by detecting when a recording head is flying outside its normal operating range. If an unsafe fly height condition is encountered, the write process is stopped, and the information is rewritten or reallocated to a safe region of the hard drive. This constant monitoring process increases the reliability of write operations and reduces the probability of read errors. The new Fly Height Monitor is being implemented in Western Digital's drives, beginning with the WD Enterprise WDE18300 and WDE9180 Ultra2 SCSI hard drives, and will be included on all future WD Enterprise products. B A C K G R O U N D The amount of data that can be stored on a disk is a measure of its areal recording density, i.e., the tracks per inch and the data bits per square inch. The product of these two is areal density in bits per square inch. Therefore, to increase areal density you must increase the number of bits on a track or the number of tracks per inch. Unfortunately, as the number of bits increases, the read signals generated by the bits weaken. This situation, in turn, makes it more difficult for the hard drive's read channel electronics to identify the bit patterns that re-create the user data. The best way for read/write heads to maximize areal density is to reduce the distance from the head to the disk--the head's "fly height". A reduction in fly height makes the bit pattern's output signal stronger and easier to detect. The objective of reducing fly height is to improve the signal quality, and not create a condition that might damage stored data. The fly height is controlled by the suspension attached to the slider and the airbearing of the slider. (See Figure 1.)This aerodynamic system controls the variation in fly height as the slider is positioned over the surface of the media. Traversing the head between the inner and outer radius of the disk causes a two-to-one change in velocity. Prior to current technology in airbearing designs, this change in velocity would have created a two-to-one change in nominal fly height. With new innovative airbearing designs, this variation can be reduced to a fraction of the nominal value. For magnetic purposes, the fly height is measured as a distance between the read/write elements and the magnetic surface. Current technology requires that the fly height be greater than 1.0 micro-inch. Figure 1. Flying Height of Hard Drive Heads There are several conditions that create disturbances between the airbearing and the disk surface that can change the fly height. These conditions are altitude, temperature, and contamination. An extreme in any of the