(1) Field of the Invention
The present invention relates to an imaging apparatus, such as a digital copier, a laser printer, an optical scanning type display device, a laser radar device mounted on an automobile, and the like, including an optical scanning device with a deflecting mirror module.
(2) Discussion of the Related Art
A conventional optical scanning device uses a deflector to deflect a light beam emitted from a laser emission source, and scans the deflected beam to a scanned surface, such as a photosensitive body, to record an image on the scanned surface. The conventional optical scanning device uses a polygon mirror or a galvano mirror as a deflector.
An image forming apparatus, such as a copier, may be required to form an image at a relatively high resolution or speed. To achieve this result, it may be necessary to rotate the deflector at a relatively high speed. Rotation of the deflector at the high speed may result in disadvantages including bearing damage, heat generation, or wind noise. Thus, these disadvantages limit the scanning speed.
In an attempt to overcome these disadvantages, it is known to use a deflector formed from micromachining silicon. Such a deflector is provided by integrally forming from a single silicon substrate torsion bars that support an oscillating mirror, as disclosed in Japanese patent laid open no. 04-211218 or U.S. Pat. No. 5,959,760. The deflector disclosed in these patents, which oscillates back and forth by resonance, is operable at a relatively high speed as compared to the conventional polygon mirror. Further, as compared to the conventional polygon mirror, use of the resonance oscillated deflector results in advantages including extremely small power consumption and reduced noise.
However, the oscillating mirror described above may scan a relatively narrow area as compared to the conventional polygon mirror. Therefore, United States publication no. 2002/122217 discloses plural optical scanning devices, which use oscillating mirrors as deflectors, to divide an image recording area in a primary scanning direction for recording images in a single line. In an image recording apparatus using the plural oscillating mirrors, driving of the mirrors at different scanning frequencies may not result in recording good quality images.
Conversely, a swing angle of the oscillating mirror can become extremely small when the scanning frequency is outside of the resonance frequency of the mirror. Therefore, the mirrors are usually driven such that the scanning frequency is the same as the resonance frequency. This results in disadvantages, in that each of the oscillating mirrors in the image forming apparatus should have exactly the same resonance frequency as one another. To eliminate difference between resonance frequencies of the oscillating mirrors, Japanese patent laid open no. 8-75475 or no. 2002-40355 discloses that weights are added to certain oscillating mirrors, for example. The additional weight adjusts the resonance frequency of the oscillating mirror. It is also known to choose the oscillating mirrors that have resonance frequencies near a specified frequency. Unfortunately, these measures increase production times for assembly of the image forming apparatuses. Additionally, changes in temperature can vary the resonance frequency of an oscillating mirror.