I. Introduction
The Head assembly in HDDs with bonded piezo film on one side of the suspension beam. Flying height variation suppression mechanism. need for ever-greater storage capacity has resulted in the data-storage recording densities of magnetic hard disk drives (HDDs) growing at a rate of about 60% every year in the past decade. In order to support these developments, work has been undertaken in parallel to improve the heads, media, channel and electronics. However, one of the most critical and effective parameters in increasing areal density is the flying height or spacing between the read-write head and the recording disk medium. Since 1990, flying height has been reduced from above 140 nm to under 20 nm for the leading-edge products of the magnetic recording industry. It is expected that the head-disk spacing will reduce from the current value of 2030 nm to a level of 510 nm, as the technology moves from the current 10 Gb/ areal density to beyond 40 Gb/ areal density of commercial hard disk drives by the year 2004 [1]. In such a low flying height condition, it becomes even more important to keep the head-disk spacing variation as small as possible during disk operation. This is because the flying height variation will cause modulation of the readback signal due to the spacing loss [2], thus increasing the bit-error-rate of the read channel. Especially in shock and vibration conditions, there is a high possibility of undesirable head-disk contact, which will deteriorate the tribological performance of the head-disk interface and its reliability. Therefore, active flying height control is highly desirable for suppression of this spacing variation. The reason why there are so few results and publications in this area is probably because of the difficulty of real-time detection of the flying height, or flying height variation in a real operating hard disk drive. Gao et al. investigated the active actuation and control of a miniaturized suspension structure in HDDs to reduce head-disk friction/wear and suppress induced vibrations [3]. However, since the displacement of the head suspension arm is used for the feedback control signal, their investigation falls into the active vibration control of the suspension arm itself, not the control of head-disk spacing variation.