The hard disk drive or HDD plays an important role in the modern era of digital technology. The HDD industry began its journey in 1956, and since then, it has traveled through a history of extra-ordinary achievements which is rivaled only by the semiconductor revolution. Storage capacity of the HDD has grown from mere 5 MB (Mega Bytes) in 1956 on fifty 24-inch disks to more than 100 GB (Giga Bytes) stored on one disk of 31 2 inch diameter. During this relatively short period, the HDD industry has fostered excellent innovations in various scientific and technological disciplines related to the design and manufacturing of HDD. Mechatronics and control played a vital role in this path of achieving rapid growth in the capacity of HDD and continuously decreasing cost. The term Mechatronics, originated in Japan in late 1970s, describes a branch of engineering that is firmly established now. According to the Mechatronics Forum (UK), Mechatronics is the synergistic integration of mechanical engineering with electronics and control in the design and manufacturing of product process [138]. A mechatronic system is neither just a marriage of electrical and mechanical systems nor just a control system; it is a complete integration of all of them. Everyday we find systems and devices that involve mechatronics, e.g., a camera with auto-focus and auto-exposure, an automatic cash machine, a printer, a robot, and an automatic production line etc.
Many mechatronic systems demand for ultra-high precision in controlling the output of the system. The HDD is one such system where the tolerance limit for position error is only few nanometers. The HDD includes several subsystems some of which are mechatronic systems, and the integration of all these subsystems to realize a practical product is a challenging task. The mechatronic parts of HDD include the servomechanism that controls the position of the read-write heads of the HDD and the spindle motor system that spins the disks at precisely regulated speed. The challenging task of the HDD servo engineers can be visualized using the following analogy, which helps one to comprehend the difficulties faced in making an HDD and thus to appreciate the achievements of researchers and engineers from a variety of disciplines [81]. Imagine an airplane flying at 5M miles per hour but only 1 16 inch above the ground on a highway with 100,000 lanes where the width of each lane is only fraction of an inch. The challenge of the problem is further intensified by the fact that the airplane is expected to switch lanes frequently and then follow the new lane with the same precision. A scaled down version of this scenario is what one finds in the head positioning servomechanism of an HDD. We live in an era of information technology where every aspect of our life is affected by some kind of information processing or information storage. Modern computing systems use different technologies to store information, either temporarily or permanently. These are semiconductor memories such as ROM (Read Only Memory), RAM (Random Access Memory) etc, magnetic storage such as hard disk, floppy disk, tape etc, and optical storage such as CDROM (Compact Disk - ROM), DVD (Digital Versatile Disk) etc. Important attributes of a storage device considered by users include cost, rate of data transfer, access time, and reliability. If low cost is the main consideration while selecting the storage device for a specific application then one must accept less desirable features such as slower response, lower transfer rate and poorer reliability.