Light deflector

A light deflector for use in a laser printer, for example, is composed of a motor having a drive shaft and a light reflecting member such as a rotating multiple-surface mirror or a hologram. The motor is subjected to magnetic field ripples the number of which is equal to a multiple by an integer of the number of light reflecting surfaces of the light reflecting member.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a light deflector such as a rotating 
multiple-surface mirror or a hologram disk for use in various devices 
employing a laser beam such as a laser printer, a display, a flaw detector 
or a POS scanner. 
2. Description of the Prior Art 
Prior light deflectors such as rotating multiple-surface mirrors have been 
subjected to various problems that adversely affect optical 
characteristics of the deflectors. One of such difficulties is caused by 
jitter due to both a phase jitter resulting in cyclic rotational 
fluctuations of the motor which rotates the rotating miltiple-surface 
mirrors and a velocity instability resulting in time-dependent rotational 
fluctuations of the motor. The phase jitter is disadvantageous in that any 
line which would have to be drawn by the optical deflector perpendicularly 
to a main scanning direction or parallel to an auxiliary scanning 
direction tends to suffer from small vibrations. Any velocity instability, 
on the other hand, causes such a line to fluctuate in a greater cycle. In 
any case, these motor fluctuations impair the quality of an image printed 
or otherwise displayed. 
It is known that, in general, as the speed of rotation of the rotating 
multiple-surface mirror or the motor is reduced, the fluctuations 
attributable to the phase jitter and the velocity instability are 
increased. Since the phase jitter and the velocity instability are 
correlated with each other, any reduction of the phase jitter can result 
in a reduction in fluctuations caused by the velocity instability. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a light deflector 
capable of deflecting a light beam in a manner to produce images of 
improved quality through a reduction in phase jitter. 
According to the present invention, a light deflector comprises a light 
reflecting member such as a rotating multiple-surface mirror or a hologram 
disk coupled to the drive shaft of a motor and having a plurality of light 
reflecting surfaces for successively reflecting an incident light beam in 
response to rotation of the drive shaft, the motor being subjected to 
generated magnetic field ripples the number of which is a multiple by an 
integer of the number of the light reflecting surfaces. With the light 
deflector of the above arrangement, magnetic field ripples which are 
chiefly responsible for phase jitter can be cancelled out by being 
equalized in their frequency to the a multiple by an integer of the number 
of reflecting surfaces of the light reflecting member, so that the phase 
jitter and any fluctuations due to a velocity instability of the motor can 
simultaneously be reduced. The light deflector can effectively suppress 
unwanted jitter or fluctuations which would be increased and impair the 
quality of reproduced images particularly at a motor speed of 10,000 rpm 
or below. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which a preferred 
embodiment of the present invention is shown by way of illustrative 
example.

DETAILED DESCRIPTION 
The present invention is particularly useful when embodied in a laser 
printer having a rotating multiple-surface mirror as shown in FIG. 1. The 
laser printer comprises a light source 1 such as a semiconductor laser for 
emitting a laser beam, and a coupling lens 2 in the form of a compound 
lens for directing a parallel laser beam toward a rotating 
multiple-surface mirror 4 rotated by a motor 3 coupled therewith. The 
laser printer also has an f lens 5 for focusing the laser beam as 
deflected by the rotating multiple-surface mirror 4 onto a photosensitive 
drum 6 as a scanning spot thereon. The laser beam emitted from the 
semiconductor laser 1 is modulated with a signal to be recorded or 
printed. Main scanning is effected axially of the photosensitive drum 6 by 
the rotation of the rotating multiple-surface mirror 4, and auxiliary 
scanning is effected in a direction normal to the axis of the 
photosensitive drum 6 by the rotation of the latter. The photosensitive 
drum 6 has its cylindrical surface charged uniformly in advance at a 
prescribed polarity. The charged cylindrical drum surface is selectively 
hit and discharged by the scanning beam spot to form an electrostatic 
latent image in a pattern equal to an image to be printed. Colored minute 
particles, known as "toner", are then electrostatically applied to the 
cylindrical drum surface, thereby developing or visualizing the image, 
which is transferred to an image carrier such as a sheet of paper and then 
fixed thereto. The scanning beam is detected by a light detector 7 
positioned in the vicinity of a point where each main scanning line starts 
for thereby synchronizing the main scanning with the signal to be 
recorded. 
The rotating multiple-surface mirror 4 is liable to undergo various 
problems that adversely affect optical characteristics of the mirror. One 
of such difficulties is caused by jitter due to both a phase jitter 
resulting in cyclic rotational fluctuations of the motor 3 and a velocity 
instability resulting in time-dependent rotational fluctuations of the 
motor 3. The phase jitter is disadvantageous in that any line which would 
have to be drawn by the mirror 4 perpendicularly to a main scanning 
direction or parallel to an auxiliary scanning direction tends to suffer 
from small vibrations as shown in FIG. 2. Any velocity instability, on the 
other hand, causes such a line to fluctuate in a greater cycle as 
illustrated in FIG. 3. In any case, these motor fluctuations impair the 
quality of an image printed or otherwise displayed. 
It is known that, in general, as the speed of rotation of the rotating 
multiple-surface mirror 4 or the motor 3 is reduced, the fluctuations 
attributable to the phase jitter and the velocity instability are 
increased, as shown in FIG. 4. The phase jitter and the velocity 
instability have the following relationship: 
EQU Velocity instability.perspectiveto.Phase jitter+k (1) 
where k is a constant, and thus any reduction of the phase jitter can 
result in a reduction in fluctuations caused by the velocity instability. 
The phase jitter is considered to be caused by various irregularities 
associated with the rotating multiple-surface mirror 4 and the motor 3. 
These irregularities include a mirror surface inaccuracy of the rotating 
multiple-surface mirror 4, a magnetic field ripple in the motor 3, and a 
mechanical inaccuracy of the motor 3. 
The foregoing irregularities affect the phase jitter as shown in FIG. 5. 
More specifically, the magnetic field ripple in the motor 3 has a greatest 
adverse effect on the phase jitter, and the surface inaccuracy of the 
mirror 4 and the mechanical inaccuracy of the motor 3 are less conducive 
to the phase jitter. 
The magnetic field ripple in the motor 3 is correlated to the number of 
stator poles and the number of phases of the motor 3. For example, a 
two-pole two-phase motor is subjected to magnetic field ripples which 
appear four times while the rotor and stator make a single relative 
revolution, as shown in FIG. 6. This indicates that the frequency of 
magnetic field ripples produced during one revolution of the motor 3 is 
given by the following equation: 
EQU Frequency of magnetic field ripples=the number of stator poles.times.the 
number of phases (2) 
From the foregoing analysis, it is concluded that where the frequency of 
magnetic field ripples is equalized to a multiple by an integral number of 
the number of reflecting surfaces of the rotating multiple-surface mirror 
4, the magnetic field ripples can be cancelled out by rotation of the 
mirror 4, resulting in a reduction in the phase jitter. With two-pole 
four-phase motors, four-pole two-phase motors, and four-pole four-phase 
motors, for example, the number of reflecting surfaces of the mirror 4 is 
selected to be eight. The number of reflecting surfaces of the mirror 4 is 
selected to be twelve for four-pole three-phase motors, eight-pole 
three-phase motors, six-pole two phase motors, and six-pole four-phase 
motors, for example. Such a relationship is effective for both AC 
synchronous motors and DC motors. 
Although a certain preferred embodiment has been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims. For 
example, the present invention is applicable to other types of light 
deflectors, such as a hologram disk having a plurality of holograms 
serving as reflecting surfaces.