High torque micro slice motor using a multipole ring magnet

Abstract

For cell phone cameras, it is desirable to utilize electromagnetic micromotors with a thin profile and high torque to drive a variable aperture. However, thinning these down to submillimeter dimensions can severely decrease the torque. In this paper, we propose a slice motor featuring both a thin profile (about 10 mm in diameter and less than 1 mm in thickness) and a high torque (above 20 μN m). The proposed micromotor comprises an 80-pole NdFeB ring magnet and a meander micro coil. The 80-pole ring magnet (o9.6 × i7 × t0.3 mm) has a pitch of 0.3 mm and is employed as a powerful micro flux source. To minimize the self-demagnetization effect, we magnetized this by single-sided laser assisted heating. However, the magnetization achieved was insufficient due to the small temperature rise on the opposite side to where the laser irradiation was applied. To improve the magnetization, we tested double-sided laser assisted heating. As a result, the magnetization of the ring magnet was improved from 53.6% to 75.9% of the design value. The meander micro coil (line/space: 0.1/0.2 mm, thickness: 0.05 mm) was fabricated by Si wet etching and silver-paste screen printing. The micromotor was successfully rotated at speeds of 0.16–1.22 rad/s by supplying a current of 250 mA AC at frequencies from 1 to 10 Hz. To evaluate the static driving torque of the motor, which is on the micronewton-meter scale, a new measurement sensor utilizing a tiny hollow cylindrical PDMS beam is proposed. The measurement principle is based on the rotation of the PDMS torsion bar under the driving torque applied by the micromotor. The results indicate that this method is feasible for measuring torque at the micronewton-meter scale. The maximum driving torque generated by the proposed micromotor was 21 μN m at 200 mA DC. This torque exceeds that of existing micro slice motors.