Figure 2.11: Experimental setup for frequency response measurement.
reflecting sheet on the head slider without causing any significant change in
the dynamics of the actuator mechanism.
Frequency response shown in Figure 2.10 is measured using an experimental
setup involving an LDV, a Dynamic Signal Analyzer, a VCM driver and an
HDD actuator. The schematic diagram of the setup is shown in Figure 2.11.
The Signal Analyzer generates a swept-sine signal, which is used to excite the
VCM actuator. The excitation signal, which is the input to the VCM driver,
and the displacement measurement from the LDV are fed to two channels of
the signal analyzer. The signal analyzer computes the gain and phase at each of
the frequencies of swept-sine signal. It should be noted that the measured gain
is in the unit of V/V as the output of LDV is a voltage signal. However, one
can easily change the unit of gain by taking into consideration the resolution
of LDV (V/µm).
Care must be taken while setting the amplitude of the swept sine signal.
Small amplitude of input signal results in small displacement of the slider and,
therefore, low signal-to-noise ratio (SNR) in the output of LDV. Too large an
amplitude, on the other hand, may cause the actuator to move beyond the
range of LDV. Since the head slider moves on an arc, the reflected beam is not
in line with the incident beam. If the angle between the two beams is large, the
reflected beam is not received well by the measurement electronics. The gain of
the actuator is higher in the lower frequencies and is expected to decrease with
increasing frequency. That means the amplitude of the input excitation should
be lower for in the low frequency range and should be increased as the frequency
of the input goes higher. For the result shown above, input amplitude is kept
reasonably small (below 100 mV) for the lowest range of frequency, and was
increased to almost 1 V for high frequency. Dynamic signal analyzers available
in the market these days come with the capability of automatic adjustment of
the input amplitude.