Resonant reluctance actuator for large range scanning mirrors

Abstract

Scanning mirror systems are used in various different applications, in which a scanning motion of a laser beam is necessary. Especially resonant Micro Electro Mechanical Systems (MEMS) receive much attention in high precision scanning and projection systems. They can reach high scanning frequencies and large deflection angles due to the driving of the reflecting mirror in its mechanical resonance. However, they have in common that the mirror aperture size is limited to a few millimeters, depending on the scanning frequency, which is too small for applications, in which a laser beam with larger diameter has to be manipulated, e.g. to reach a sufficiently low beam divergence.In this thesis a novel reluctance force based resonant scanning mirror system is developed and characterized, aiming to make the favorable properties of the smaller MEMS systems, like the high scanning range and scanning frequency, also accessible for systems with larger aperture sizes. In contrast to proposed non-resonant reluctance force based scanning mirror systems, the ferromagnetic yoke, which is needed to guide the magnetic flux in the system, is located on the side of the moving mirror and not underneath it. This has the advantage, that the scanning motion is not restricted by the air gap between the yoke and the moving mirror. For the designed system a multi-domain mathematical model is developed, which includes the dynamics of theentire scanning mirror system. To stabilize the oscillation amplitude of the highly non-linear system in a desired operation point, a tailored control strategy is developed. For this purpose an angular position sensor system based on optical proximity sensors is integrated into the system design. On the manufactured prototype system a closed-loop system identification was carried out, confirming the stability of the developed control strategy in the entire operation range. A closed-loop bandwidth of at least 10 Hz is achieved in every operation point. The achieved performance of the system are a scanning frequency of 500 Hz, with a maximum mechanical scanning amplitude of 7.8 deg, corresponding to an optical field of view of 31.2 deg. Together with the mirror aperture size of 15 x 15 mm2, a θd-product of 117 deg*mm is achieved, which outperforms almost the entire state of the art in this performance metric. The achieved system performance shows the potential of the proposed design, of extending the aperture size of resonant scanning mirror systems, while still similar scanning range and scanning frequency as state of the art systems.