Hardware Reference
In-Depth Information
Another method of obtaining informationonthepositionofthereadhead
scanning the data block was proposed in the patent [208]. The method pro-
posedinthispatentmodifies the process of writing by assigning different bit
intervals on adjacent tracks. As a result, the readback waveform from the data
block of one track differs in frequency or phase from the readback waveform
of the adjacent track. Samples from the read waveform are processed by a
discrete Fourier transform type algorithm to determine the magnitude of the
frequency component associated with the track being scanned. The output of
this process provides an indicator to the position of the head. This result is
further smoothened using a simple first order filter.
One drawback of these methods of estimating head position from data
block is that it can be used only during read operation. The readback wave-
form is available when a data block is being read and, therefore, can be further
processed to estimate PES at high sampling rate. However, during a write
operation, the write head is enabled and read sensor is disabled. The read-
back signal is not available for any kind of processing and the data track is
continuously being modified with the new data overwriting the old pattern of
magnetization. Realization of either of the two methods would require major
change in the head-slider configuration by inserting an additional read sensor
for servo only.
2.4 High Frequency Dynamics
The rigid body model of equation 2.7 represents the dynamics in the mid-range
frequencies, from about 50-60 Hz to approximately 1.5 kHz, for most actuators
used in modern hard disk drives. However, the frequency response of a practical
VCM actuator shown in Figure 2.10 suggests that the structure of actuator
is anything but rigid. The frequency response measured in frequencies above
1.5 kHz shows large gain and phase changes in a narrow range of frequencies
indicating presence of dynamic modes with low damping coefficients. These
flexible modes of the actuator are contributions from various bending modes,
torsional modes and sway modes of the suspension, the VCM coil, and the
gimbal with which the slider is attached to the suspension. The torsional mode
of the suspension twists it along the center line of the load beam causing a small
amount of in-plane head motion. The first torsional mode of commercially
available suspensions lies typically around 3 kHz. There is a second torsional
mode in the frequency range between 5 kHz and 8 kHz. The sway mode is
caused by the in-plane deformation of the load beam; it is the result of in-plane
bending of the suspension. It was explained earlier that part of the load beam
is left without edge so that the suspension arm has necessary compliance to
accommodate vertical disk runout. This section of the load beam is the weakest
part of the suspension. The sway mode produces large amount of radial motion
and contribute to the off-track error. Typical frequency of the sway mode lies in
the range of 8 kHz - 12 kHz. The sway mode is a greater problem in the rotary
