1. Field of the Invention
The present invention relates to a drive system for a rotatable optical element, which can be inserted into and removed from an optical path and which produces a required optical effect by being rotated about an optical axis when positioned in the optical path like a polarizing filter.
2. Description of the Related Art
A drive system for driving such a rotatable optical element has been proposed in Japanese Unexamined Patent Publication No. 2006-337695. Specifically, in this drive system an insertion/removal frame which is pivoted about a rotational shaft parallel to an optical axis of a photographing optical system is supported to be rotatable forwardly and reversibly (swingable) between an inserted position, in which the rotatable optical element is positioned in the optical path, and a removed position, in which the rotatable optical element is removed away from the optical path, and the rotatable optical element is held by an annular holding frame provided as a separate member from the insertion/removal frame. The annular holding frame is supported to be rotatable relative to the insertion/removal frame about a rotational axis parallel to the optical axis. A sun gear of a planetary gear mechanism for transmission of a driving force of a motor is coaxially supported by a shaft support portion of the insertion/removal frame, and a planet gear of the planetary gear mechanism that is in mesh with the sun gear is supported by the insertion/removal frame thereon. A circumferential surface gear is formed on the outer periphery of the annular holding frame, while a gear train capable of transmitting a rotational force between the circumferential surface gear and the planet gear is arranged on the insertion/removal frame. A friction having a predetermined magnitude is applied to rotation of a gear of the gear train. In a state where no restrictions are imposed on the rotation of the insertion/removal frame, rotations of the gears of the gear train are limited by the aforementioned friction, so that rotating the sun gear causes the planet gear to move (revolve) around the sun gear, thus causing the insertion/removal frame to rotate. The insertion/removal frame is prevented from further rotating beyond each of the inserted position and the removed position. If the sun gear is made to keep rotating in the same rotational direction with the insertion/removal frame prevented from rotating, the force resisting (limiting) the rotation of the insertion/removal frame exceeds the aforementioned friction, which causes the planet gear to rotate on its axis without revolving around the sun gear to thereby rotate the gears of the gear train. Consequently, the annular holding frame, to which the driving force is transmitted to the circumferential surface gear thereof, rotates relative to the insertion/removal frame. By making the annular holding frame perform this rotational operation when the insertion/removal frame is in the inserted position, a required optical effect of the rotatable optical element (e.g., a polarizing filter) on the annular holding frame can be obtained.
The polarizing filter drive system disclosed in Japanese Unexamined Patent Publication H03-192231 is provided with a drive ring which rotates about an optical axis of a photographing optical system by a motor and a rotational member which is rotatable relative to this drive ring. In addition, a polarizing filter is pivotally supported by the rotational member thereon, and a sector gear on the polarizing filter is engaged with an internal toothed portion formed on an inner peripheral surface of the drive ring. The polarizing filter is held in an inserted position, in which the polarizing filter is positioned in an optical path, by abutting against one of the two pins (stoppers) formed on the rotational member, while the polarizing filter is held in a removed position, in which the polarizing filter is removed from the optical path, by abutting against the other pin. When the drive ring is driven to rotate in a direction in which the polarizing filter is not prevented from rotating by the aforementioned two pins, this rotational driving force is transmitted to the polarizing filter via gears to thereby rotate the polarizing filter between the inserted position and the removed position without rotating the rotational member. On the other hand, in a state where the polarizing filter is in contact with one of the two pins which determines the inserted position of the polarizing filter, if the drive ring is driven to rotate in a direction to give the polarizing filter a force that further moves the polarizing filter in the filter insertion direction, the polarizing filter which is prevented from further rotating relative to the rotational member is rotated together with the rotational member about the optical axis.
According to the rotatable optical element drive system disclosed in Japanese Unexamined Patent Publication No. 2006-337695, simplification and miniaturization of this drive system can be achieved by making a single motor perform both the insertion/removal operation of the rotatable optical element and the rotational operation of the same optical element on the optical path. On the other hand, there is room for improvement in the rotatable optical element drive system in the following respects. Firstly, limitations exist on miniaturization and weight reduction of the insertion/removal operating parts because the rotatable optical element drive system has a structure in which the gear train and other components which are components of the rotatable optical element drive system are supported by the insertion/removal frame thereon. Additionally, the portion which supports the annular holding frame to be rotatable relative to the insertion/removal frame makes cylindrical surfaces, the axes of which are parallel to an optical axis of the photographing optical system (more precisely, conical surfaces which are slightly tapered with respect to the optical axis), slidingly contact each other, thus being required to have a certain amount of length in the optical axis direction. Therefore, limitations exist also in slimming down the insertion/removal frame and the annular holding frame in the optical axis direction. Secondly, since the gear train supported by the insertion/removal frame thereon serves as a reduction gear train, the torque required to rotate the annular holding frame tends to be great, so that there has been a problem with it being difficult to rotate the annular holding frame at a high speed.
In the rotatable optical element drive system disclosed in Japanese Unexamined Patent Publication H03-192231, the polarizing filter, which also constitutes part of the drive system that performs an insertion/removal operation with respect to an optical path, has a simple structure having no gear train or other moving parts, so that miniaturization, weight reduction and reduction in driving load can be achieved. On the other hand, since the polarizing filter can be inserted into and removed from the optical path from any position in the rotational direction about an optical axis, each of the rotational member and the drive ring needs to be formed as a large-diameter member, which corresponds to any removed position of the polarizing filter over the entire circumference of the drive ring, so that there is a possibility of the drive system being enlarged in the radial direction. In addition, if the polarizing filter reaches the inserted position and the removed position at different positions in the rotational direction of the rotational member, respectively, when the insertion/removal operation of the polarizing filter is performed, the configuration of the position detector becomes complicated and the degree of difficulty in position detection is high.