Hardware Reference
In-Depth Information
the feedforward compensation discussed in this section.
3.6 Emergence of Dual-Stage Actuator
The design methods discussed so far in this chapter attempts to improve the
performance of the closed loop head positioning servomechanism by working
around the limitations and shortcomings that come with the system built using
conventional VCM actuator. The torque producing VCM is located at one end
of the actuator arm while the read-write head slider sits at the other end.
The mechanical structure lying between the point of actuation (VCM) and the
point to be controlled (head) is not perfectly rigid and gives rise to different
modes of vibration that affect the positioning accuracy. The suspension arm
and the gimbal used to attach slider to the suspension contribute to these
vibrations. These vibrations as well as the nonlinearity and stiffness of pivot
bearing can not be neglected when it is desired to have the servo bandwidth
as high as possible to suppress the effects of disturbances and noise.
Removing the bottleneck factors at the source is always a better approach
to accomplish the task of achieving high servo bandwidth. Over the years,
continuous efforts have been made to improve the performance of the VCM
actuator in various ways such as improved mechanical design, use of new ma-
terials to make actuator and suspension arms, using high bandwidth voice coil
motor (VCM) [9], alternative design of arm and suspension, application of ac-
tive damping etc. The bandwidth of the conventional single-stage actuator has
been above 1 kHz since 2000.
Another approach, investigated in parallel to improve the performance,
adds a second actuator placed conveniently between the pivot and slider in a
piggy back fashion. Such actuator, known as the dual-stage actuator, still uses
the conventional VCM actuator for long-range seek while the secondary stage
located closer to the slider can improve tracking performance during track
following. The actuation mechanism for the secondary stage can be classified
as electro static, electromagnetic, or piezo electric. Depending on where the
secondary stage actuator is located, the dual-stage actuator can be one of the
following,
1. Actuated suspension [50] [149] [50],
2. Actuated slider [173] [179] [51] [148] and
3. Actuated head [134] [57] [136].
When the actuator is located nearer to the read/write head, i.e., either
actuating the suspension or actuating the slider or actuating the read/write
head, the load to be moved by the secondary actuator is reduced. The plant
model for the secondary actuator has less low frequency flexible modes, and
higher servo control bandwidth can be achieved.
