Summary
Western Digital announced that their new 2 TB 7,200 RPM Caviar HDDs use dual actuators. Dual (actually dual-stage) actuators provide a coarser movement of the recording head to tracks using head arms moved by a conventional voice coil motor while a finer motion is provided using a piezoelectric actuator closer to the head. Dual stage actuators add some cost to the drive and increase servo control complexity but allow higher track density recording. Dual stage actuators had only appeared in some enterprise (high performance) HDDs from Seagate and other companies in that market to this point
Analysis
Conventional hard disk drive actuators (single stage actuators) position the heads over the recording tracks using a voice coil motor. Voice coil motors use a copper coil with current running through it combined with high energy rare earth magnets to position the head over the disk tracks for writing and reading. By changing the characteristics of the electrical current through the copper coil the actuator moves back and forth across the disk surface. Conventional actuators have limits to the accuracy of their movements since the source of motion, the voice coil, is at one end of the actuator while the magnetic heads are on the other end.
Dual stage actuators place a fine position actuator very close to the recording heads in order to complement the coarse motion of the voice coil with much smaller motion closer to the recording head. The secondary actuator typically uses piezoelectric devices that move the heads across a narrow range in order to provide finer motion control. For a disk drive using a dual stage actuator technology the servo control system that keeps the head positioned over the target disk track during reading and writing must work with two levels of motion. This leads to greater complexity in the servo control but offers a higher track density than is achievable by a single stage actuator. Adding a secondary actuator to the heads also increases the costs for building the hard disk drive.
Increasing the areal density of hard disk drives (the storage capacity per usable area of the disk surface) requires ever evolving technology development in heads, disks and electronics. Dual-stage servos are one way to increase the disk drive areal density and thus overall storage capacity. Another way is to increase the storage capacity that can be stored on individual disk tracks. Technologies such as enhanced perpendicular recording and patterned media should make their way into disk drive products over the next few years. It is by using such technologies that HDD companies will continue to achieve 40+% growth in storage capacity offered annually in their products and remain cost competitive vs. other storage technologies such as flash memory.
Thomas Coughlin consults with leading institutions through GLG
Analyses are solely the work of the authors and have not been edited or endorsed by GLG.
