An on-vehicle radar and a projection-type display device scanning a light flux emitted from a light source, such as a laser, are in practical use. A conventional optical reflecting device includes an outer frame, an inner frame separated from the outer frame across a groove, an outer spindle that is disposed in this groove and supports the inner frame inside the outer frame, a mirror separated from the inner frame via a groove, an inner spindle that is disposed in this groove and supports the mirror inside the inner frame, an outer piezoelectric diaphragm that has one end connected to the outer frame and the other end connected to the outer spindle, and an inner piezoelectric diaphragm that has one end connected to the inner frame and the other end connected to the inner spindle. The mirror vibrating about the outer spindle and the inner spindle causes the reflected light of the light entering the mirror two-to dimensionally scan a screen to display an image on the screen.
An optical reflecting device similar to this optical reflecting device is disclosed in, e.g. Patent Literature 1.
A high-definition image using this scanning optical reflecting device requires a large-size and high-speed mirror. However, the increase in size and speed provides the mirror with a dynamic warp during driving. This dynamic warp causes the mirror to be a concave mirror or a convex mirror. Thus, the laser beam reflected on the mirror spreads and prevents an image from being displayed accurately, hence displaying a defocused image.
The dynamic warp is caused by an inertial force applied to the mirror. Therefore, as the distance from the rotation axis increases, the inertial force increases, accordingly increasing the dynamic warp. The mirror has an axis about which the mirror vibrates. The distribution of the dynamic warp depends on a mechanism for supporting the mirror. In other words, the distribution of the dynamic warp depends on a balance between the inertial force and the supporting mechanism.
Patent Literature 2 discloses a structure in which an aperture is provided between the center and the outer periphery of the mirror for preventing the dynamic warp from transferring from the outer periphery to the center. In the outer periphery of the mirror, a larger dynamic warp occurs due to a stress concentrating at a connecting position between the outer peripheries to a torsion bar, which transmits a drive force for vibration, than in the periphery. In Patent Literature 2, the dynamic warp caused by the stress concentration is prevented from transferring to the center.
However, when a connection part between the outer periphery and the center becomes small in order to inhibit the dynamic warp of the outer periphery from transferring to the center, the torque for drive concentrates in the periphery of the connection part and causes the dynamic warp at the center.
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