Precision Motion Controls for Hard Disk Drives

Masayoshi Tomizuka
Cheryl and John Neerhout, Jr., Distinguished Professor
Department of Mechanical Engineering
University of California

Magnetic hard disk drives (HDDs) are representative mechatronics devices. The aereal data storage density of commercial drives has been increasing at a rate 60% per year. The number of data tracks per radial length is currently about 15,000 tracks per inch (TPI). The importance of the servo control is easy to recognize. In this talk, I will introduce several approaches for meeting the requirement of increased storage densities. These approaches may collectively be best described as mechatronic approaches; they range from the use of additional sensor(s), new actuator(s) and different control algorithms.

The current disk drives use the so called sector servo method. In this method, the disk is divided into angular sections (sectors) and servo information is written at every sector. The recording head reads the position error signal (PES) once in each sector. A challenge in the design of these systems is to minimize the number of sectors (equivalently the sampling rate), while continuing to meet tracking performance requirements. There has been a number of interesting control ideas introduced to disk file controls. Repetitive control is such an idea In standard digital control, the control input is updated at every instant of output (error) measurement. This standard scheme is called the single rate scheme. The updating rate of control input, however, can be more frequent than the measurement sampling rate. Such control methods belong to multirate control, and several kinds of mutirate control have been proposed for hard disk drives. PES is the major information source in the disk drive servo system. There have been several attempts to improve performance by utilizing information from additional sensors. Accelerometers have shown to improve the track following performance by canceling the effect of external vibration on PES and eliminating the effect of pivot friction. As TPI is kept increased every year, it is expected that the current single actuator technology is expected to reach its fundamental limit. To prepare for this situation, the dual stage actuator is receiving an increased level of attention in the disk drive industry. The majority of dual stage actuators utilizes the conventional voice coil motor primary stage and the piezoelectric secondary stage. Emphasis in this talk will be the design of control algorithms for implementation of these ideas along with experimental results.