Figure 2.5: Movement of suspension arm for rotary VCM.
Figure 2.6: Micro-jog in HDD servomechanism.
jog distance must be calibrated as a function of disk radius for each drive
using a built-in function in the initialization firmware of the drive. Linear
VCM actuators were already extinct when the HDD industry adopted MR
head technology. However, had there been any drive with MR head and linear
VCM actuator, the micro-jog distance would be constant for all radial position
and would require simpler calibration algorithm.
If we let a current flow through the coil of the VCM, it experiences an
electromagnetic force as shown in Figure 2.7 and the actuator arm which is
attached to the suspended coil and pivoted is subject to a rotating torque. The
electromagnetic force acting on the coil is produced by the interaction between
the magnetic field of the permanent magnet and the fieldproducedbythe
coil current. If we assume the field from the permanent magnet constant for
the entire range of operation, then the magnitude of the torque depends on
the field produced by the coil current and the actuator geometry. Since the