Method and apparatus for rotating one or more mirrors in a beam scanning system

#The present invention relates to a beam scanning system including one or more rotating mirrors and, more particularly to an apparatus and method in which the mirrors are rotated with reduced risk of contamination of system optics by oil leaking from the mirror support bearings.

#In many prior art beam scanning systems it is common to mount one or more mirrors on a mirror support wheel and to rotate the wheel in the path of the beam. This effects a repeated sweeping of the reflected beam through a desired path. Such scanning arrangements may be utilized, for example, in laser beam scanning systems of the type which are used to read bar codes of various types on product labels.

#The scanning wheel is mounted on a drive spindle which is rotated at a relatively high speed, on the order of 6000 RPM, to effect scanning of the beam. The drive spindle is supported by one or more bearings and is driven by an electric motor. In the past is has been found that some of the lubricant oil from the spindle bearing may leak from the bearing over a period of time. When such leakage occurs, the oil may find its way onto the rapidly rotating mirror support wheel or other parts of the rotating structure and then be thrown from the wheel by the effects of centripetal force. If the oil is deposited on any of the mirrors or lenses which commonly make up a beam scanning system, the oil will result in significant degradation of the performance of the scanning system.

#FIG. 1 is a side view of the apparatus according to the present invention for rotating one or more mirrors in a beam scanning system; and

#FIG. 2 is a partial sectional view, taken generally along line 2--2 in FIG. 1.

#Reference is made to FIGS. 1 and 2 which illustrate apparatus for rotating a plurality of mirrors 10 according to the present invention. Such a mirror rotation arrangement is intended to be incorporated in a beam scanning system in which the beam strikes each mirror in turn as the mirrors are rotated into the beam path, such that the beam is caused to repetitively sweep through a desired arc. Toward this end, the mirrors 10 are attached to a support wheel 12 which is mounted on a drive spindle 14. Four such mirrors are illustrated in a pyramid configuration, but the present invention is equally applicable to the rotation of any number of mirrors in a beam scanning system. Mirrors 10 may be adhesively mounted on wheel 12 and spindle 14 may be adhesively secured to wheel 12 in bore 16. A drive motor 18, preferably a small, electrical servo motor, is operatively connected to the drive spindle 14, either directly or by an intermediary drive gear arrangement, to rotate the spindle 14 and the wheel 12.

#A spindle support arrangement 20 includes a generally cylindrical spindle housing 22 and a bearing 24 mounted within the housing 22 at a point which is a predetermined distance from the open end 26 of the spindle housing 22. The spindle housing 22 may be sized with respect to the bearing 24 such that the bearing 24 may simply be pressed into position within the housing 22 and maintained in this position by frictional forces. The spindle 14 extends through the spindle housing 22 and the bearing 24, and out of the open end 26 of the spindle housing 22. In order to protect the surrounding optical components as much possible from the adverse effects of any oil leakage from the bearing 24, the mirror support wheel 12 defines a generally annular counter bore 28 into which the open end of the cylindrical spindle housing 22 extends, thereby shielding the bearing 24 to a degree.

#The mirror support wheel further defines a central spindle engaging portion 30 which is radially inward of the annular counter bore 28. The central spindle engaging portion 30 has an inner surface 32 facing the bearing 24 and spaced therefrom by a distance A. It has been found that if the surface 32 is close to the bearing 24, the rotation of the wheel 12 results in a Bernoulli effect reduction in air pressure in the region of the bearing. As a consequence, oil in the bearing 24 will tend to be sucked from the bearing. The oil may then migrate to other portions of the rotating wheel 12 and be spattered onto the system optics as the wheel rotates at relatively high speeds. To overcome this problem, the distance A is made to be sufficient to substantially reduce this Bernoulli effect pressure drop in the region within said spindle housing 22 adjacent the side of the bearing facing the inner surface 32. Preferably, the inner surface 32 of the central spindle engaging portion 30 of the mirror support wheel 12 is spaced from the bearing 24 by a distance of approximately 0.200 inches.