Figure 2.10: Frequency response of a typical VCM actuator.
the two paths results in a phase change of 360 ◦ in the interference signal.
The measuring electronics measures and accumulates the phase and provides
a position output. This method has some drawbacks for application in the
measurement of displacement of HDD actuator. When a retroreflector, which
is usually quite heavy, is attached to the actuator, dynamics of the actuator
arm is significantly modified. Besides, our aim is to measure the displacement
of the head slider, which is too small to carry the load of the retroreflector.
One possibility is to attach the retroreflector on the E-beam of the actuator,
but then the measured displacement does not reflect the dynamics of either
the suspension or the slider-gimbal assembly.
Laser Doppler Vibrometers (LDV) are optical instruments for accurately
measuring velocity and displacement of vibrating structures completely with-
out contact. A rugged laser head is mounted on a large vibration-free platform
with the laser beam pointing to the object whose displacement or velocity is to
be measured. Christian Doppler was the first to describe the frequency shift
that occurs when sound or light is emitted from a moving source. If a laser
beam of precise frequency is incident on a moving reflector then, according to
Doppler effect, the frequency of the reflected beam is different from that of
the incident beam. The velocity of the moving reflector can be measured by
measuring the change in frequency between the incident and reflected beams.
The LDV measures the velocity according to this principle and then integrates
it to provide displacement measurement. Since the measurement with LDV
requires only a reflecting surface on the moving object, we can attach a tiny