Linear actuator has been widely used as vibration motor in mobile phones, and it generates a linear vibration through resonance in which the electromagnetic force with the application of a sinusoidal current excites the fundamental natural vibration mode of the moving mass-spring system. Figure 1 shows a linear actuator for mobile phones whose diameter and height are approximately 10 and 1 mm, respectively. The moving coil and mass are attached to a flat spring, and the radial magnetic field between the plate and the moving yoke is generated through a plate connected to the permanent magnet. The magnetic force in the air-gap between the plate and the yoke is generated according to Ampere’s law once the sinusoidal current is applied to the coil, and it excites the axial vibration mode of the linear actuator that generates the axial vibration. The linear actuators have a fast response time compared with other types of vibration motors. Currently, the response time of the linear actuators is approximately 50 msec, but the users and the application software of mobile phones are demanding increasingly faster response time. However, it is very difficult to reduce the response time while maintaining the vibration magnitude under the given constraints of volume and electric power.

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