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.