Servo Signal Data Processing for Flying Height Control in Hard

Introduction In recent years, thermal actuated flying height control has been implemented in Hard Disk Drives (HDD) to reduce the effect of flying height variations due to manufacturing tolerances or write current introduced pole tip protrusion (Fig. 1). A stable low flying height is necessary to achieve low bit error rates (BER) [1]. Thermal flying height control is used in a “static manner” in today‟s disk drives. The principle of thermal flying height control is based on the application of a current to a resistance heater positioned in close proximity to the read write element, thereby causing a thermal deformation of the head which causes a reduction in the static flying height at the read write element. Dynamic flying height variations of a slider over a disk are composed of repeatable and non-repeatable contributions. As the name suggests, repeatable variations of flying height occur at the same angular and radial position of the slider above the disk [2]. Thus, the question arises as to whether a thermal flying height control slider can be used to dynamically control the flying height variations between slider and disk. The objective of this paper is to study whether a thermal actuator could be used to reduce the effect that disk waviness and vibrations have on the flying height modulation of the head disk interface. Our main goal is to investigate active flying height control of a thermal slider up to the kilohertz regime. In-situ flying height estimation Most of the reported algorithms on in-situ flying height estimation are based on a specific data pattern that is written on the disk thereby resulting in certain harmonics in the frequency spectrum of the read signal. Commonly used is the triple harmonics method [2,3,4] that uses the „111100‟ data pattern to create three major harmonics. The logarithm of the ratio of the third over the first harmonic is proportional to the flying height modulation. Ratios of different harmonics other than the third and the first have also used and servo pattern might be taken into account [5]. Approaches have also been implemented based on maximum or average amplitudes [6] rather than harmonics amplitudes. Also, approaches that employ random data instead of a fixed data pattern were proposed [7].