1. Field of the Invention
The present invention relates generally to a printer, and more particularly to a printer which makes a print by scanning a sheet of photo-sensitive material with a beam of light emitted from a light source of light emitting diodes, or the like.
2. Description of Related Art
There is a conventional printer which makes a print by scanning a sheet on which a photo-sensitive material is applied (hereinafter referred to as a photo-sensitive sheet) with a beam of light emitted from a light source of light emitting diodes (LED), or the like. In the conventional printer, for main scanning, a light beam from the LED light source is deflected by a scanning mirror which is oscillated by a cam mechanism so that the light beam can traverse the photo-sensitive sheet in the form of a line of light (hereinafter referred to as a line beam) in a main scanning direction. For sub-scanning, a unit (a stage) on which the photo-sensitive sheet is set (hereinafter referred to as a sub-scanning unit) is horizontally moved in a sub-scanning direction which is perpendicular to the main scanning direction so that the photo-sensitive sheet can be scanned with the line beam from the LED light source in the sub-scanning direction. Thus, the photo-sensitive sheet can be scanned with the light beam from the LED light source in both the main scanning direction and the sub-scanning direction, and thereby an image is printed on the photo-sensitive sheet.
In the conventional printer, a face cam is used in the cam mechanism for oscillating the scanning mirror, and a cam follower is pressed against a cam face of the face cam by a biasing spring, so that the cam follower can follow the cam face of the face cam, which is rotated by a motor. This cam mechanism, however, has a problem in that a contact friction between the face cam and the cam follower and a load torque of the drive motor are great. Moreover, the load torque fluctuates according to the movement of the cam follower which moves up and down on the cam face of the face cam, and the fluctuation of the load torque has bad effects on the print. If the conventional printer using the cam mechanism makes a print at higher speed, the force of the biasing spring to press the cam follower must be increased in proportion to the square of the rate of revolution of the face cam, and the driving force of the diving motor must be increased in proportion to the cube of the rate of revolution of the face cam.
In the conventional printer, the scanning mirror is oscillated by the cam mechanism shown in FIG. 19, and a light beam in the form of a line in the main scanning direction is generated. As shown in FIG. 19, a swing pin 304 is attached to the end of a swing lever 302, and the swing pin 304 is biased against a face cam 300, which is rotated by a motor (not shown), by a biasing spring (not shown). A scanning mirror 306 is fixed to the swing lever 302. A light beam enters from the right side in FIG. 19, and is reflected downward by the scanning mirror 306. When the motor rotates the face cam 300, the swing lever 302 swings up and down, and the light beam changes its reflecting directions to the right and left to thereby generate a line beam.
In the conventional printer, the line beam in the main scanning direction falls on the photo-sensitive material while the sub-scanning unit, on which the photo-sensitive material is set, is moving in the sub-scanning direction as described above. Thus, the line beam which is reflected by the mirror 306 as shown in FIG. 20(A) describes forward (left to right) scanning lines and backward (right to left) scanning lines on the photo-sensitive sheet as shown in FIG. 20(B), and both scanning lines are inclined because the photo-sensitive sheet moves (upward in the drawing). For this reason, the forward and backward scanning lines cannot be parallel to one another, and the resolution of the print can be deteriorated. Hence, the conventional printer makes a print using only the forward scanning lines, and then the number of scanning lines is reduced by half. In this case, the quality of the print is lowered, and the quantity of light is poor. To solve these problems, there is a printer which uses a fast-forwarding mechanism to oscillate the scanning mirror, but there is a disadvantage that the printer shakes and makes noise.
In the conventional printer, if one motor drives the cam mechanism which oscillates the scanning mirror and the sub-scanning unit on which the photo-sensitive material is set, the cam mechanism oscillates the scanning mirror when the sub-scanning unit returns to the start printing position at a high speed after moving to make a print (for example, because the swing pin, which swings the scanning mirror, abuts on a cam member such as the face cam). Thus, a large load is applied to the motor, and the cam mechanism makes loud noise while rotating at a high speed.