Ultrastar 7K2
HGST Hard Disk Drive
Technical Reference Manual
22
Rotary Acceleration Feed Forward (RAFF)™
Rotary Acceleration Feed Forward (RAFF) helps to overcome the effects of rotational vibration (RV) on a
hard drive by generating an additional control effort to counter the RV disturbances, thereby keeping the
drive head(s) within the safe operating region during reading and writing operations.
Figure 4 Dual Linear Sensor Rotational Acceleration Feed Forward (RAFF)
The RAFF implementation has three major components: RV sensing, RV control effort feed- forwarding
and adaptation to environmental conditions.
RV sensing in the RAFF implementation is accomplished by using two relatively inexpensive linear
accelerometers placed on the printed circuit board assembly (PCBA). The sensor locations are
optimized for separation distance and PCB mounting conditions. Since the difference signal from
two similar linear accelerometers placed in a parallel orientation and separated by some distance is
indicative of RV, the signals are subtracted from each other to generate a Differential Sensor Signal
(DSS).
RV control effort feed-forwarding is achieved by digitizing the DSS, then, sending it to the
microprocessor of the drive. Using a control algorithm, the microprocessor generates a control effort
signal based on the DSS. This feed forward control effort is in addition to the conventional servo
control approach in hard drive operations.
Adaptation to environmental conditions is crucial to the successful deployment of RAFF. This
design intelligently applies RAFF selectively and adapts to with individual drive parameters to
maintain maximum performance in the hard drive.
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