Design, Fabrication and Control of Micro-Actuators for Dual-Stage Servo Systems in Magnetic Disk Files

Since the first hard disk drive (HDD) was invented in the 1950s by IBM, disk drives’ storage density has been following Moore’s law, doubling roughly every 18 months. The current storage density is 10 million times larger than that of the first HDD (Yamaguchi, 2001). Historically, increases in storage density have been achieved by almost equal increases in track density, the number of tracks encircling the disk, and bit density, the number of bits in each track. However, because of superparamagnetism limitations, it is predicted that future areal storage density increases in HDDs will be achieved mainly through an increase in track density (Howell et al., 1999). Research of the HDD industry is now targeting an areal density of one terabit per square inch. For a predicted bit aspect ratio of 4:1, this translates to a linear bit density of 2M bits per inch (BPI), and a radial track density of 500K tracks per inch (TPI), which in turn implies a track width of 50 nm. A simple rule of thumb for servo design in HDDs is that three times the statistical standard deviation of the position error between the head and the center of the data track should be less than 1/10 of the track width. Thus, nanometer-level precision of the servo system will be required to achieve such a storage density. A disk drive stores data as magnetic patterns, forming bits, on one or more disks. The polarity of each bit is detected (read) or set (written) by an electromagnetic device known as the read/write head. The job of a disk drive’s servo system is to position the read/write head over the bits to be read or written as they spin by on the disk. In a conventional disk drive, this is done by sweeping over the disk a long arm consisting of a voice coil motor (VCM), an E-block, suspensions, and sliders, as shown in Figs. 1 and 2. A read/write head is fabricated on the edge of each slider (one for each disk surface). Each slider is supported by a suspension and flys over the surface of disk on an air-bearing. The VCM actuates the suspensions and sliders about a pivot in the center of the E-block. This operation is described in more detail in the following section.