Hardware Reference
In-Depth Information
One possible reason for the discrepancy is the assumption that the velocity
and position feedback are decoupled.
2.5.3 Flex Cable Bias
A flat sheet of plastic, known as flex cable, is used to support thin conducting
wires carrying signals between the PCB and the heads. The flex cable is
attached to the side of the actuator arm. This flat sheet gets deformed when
the actuator moves in one direction or the other; it is bent for one direction of
movement and is relaxed when the actuator moves in the opposite direction.
The deformation in the structure of the flex cable exerts a reaction force on the
actuator. This reaction depends on the amount of deformation of the flat sheet
and, therefore, is dependent on the actuator's position. However, during the
track following mode, the variation in flex cable reaction force is very negligible
as the movement of the actuator arm is in the scales of nanometers, and hence,
the reaction force can be taken constant during track following.
2.5.4 External Shock and Vibration
These are the disturbances contributed by factors external to HDD. Operating
heavy machinery in the proximity of the computer, accidental hit by user, car-
rying the laptop etc are factors that cause this kind of disturbances. As drives
become smaller with lighter actuators and suspensions, these disturbances turn
to be prominent issues. The HDD servomechanism must have the capability
to reject such disturbances or, at least, to detect the occurrence and suspend
reading/writing operation. Typical solution includes the use of an accelerom-
eter to sense the disturbance and move the actuator accordingly. The use of
accelerometer feedforward was found to improve the disturbance rejection ca-
pability of HDD. But these methods have not been widely practiced because
of cost factor and reliability of the accelerometers.
2.5.5 Other Sources of Noise
Besides the noise and disturbances discussed so far, many other sources of
noise affect the performance of the servo loop of HDD. Airflow at the interface
of disk and head, interaction of airflow with the suspension and actuator arm,
noise associated with the PES generation process, quantization noise of the
ADC, noise introduced by truncation of words in the processor, DAC, and
power amplifier - all of these have detrimental effect on the performance of the
servomechanism. Airflow induced disturbances are hard to model but several
attempts has been made to obtain them. These models, mostly based on
finite element analysis, are good for understanding the process better and to
identify the factors contributing to the process. Disk diameter, mechanical
properties of the disk, aerodynamics of the slider-disk interface, aerodynamics
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