1. Field of the Invention
The present invention relates to a focus controller and an optical apparatus using the same.
2. Description of the Related Art
Conventionally, Japanese Patent Laid-Open Nos. 05-150154 and 05-064056 disclose mountain climbing driving as an autofocus (“AF”) for an optical apparatus, such as a video camera or a TV camera. The mountain climbing driving is a method that detects a definition of a photography screen from a high frequency component in a video signal obtained from an image pickup lens, and controls a position of a focus lens in an optical-axis direction so that the definition can become maximum. The conventional position control finely reciprocates (wobbles) the focus lens in the optical-axis direction at a constant period through an open loop control using a stepping motor.
However, the stepping motor loses synchronization between a control pulse and a motor's rotation in the high-speed rotation or with an excessively large load, (stepping out). In order to solve this problem, Japanese Patent Laid-Open No. 10-150798 proposes use of a blushless DC motor instead of the stepping motor. Since the blushless DC motor detects a rotor's position by using a sensor, it has an advantage in maintaining synchronization between the instruction signal and the rotor's position and in preventing stepping out.
Nevertheless, the open loop control using the blushless DC motor cannot control a lens's position, and has a problem in that the wobbling amplitude becomes inaccurate. A closed loop is needed for accurate positioning control with the blushless DC motor, but thereby causes a problem in which a circuit becomes complex.