Optical deflector provided with scanning mirror rotatable around shaft by dynamic air pressure

An optical deflector is provided with a scanning mirror which is rotatable about a shaft and accompanied by a sleeve and hub. The pneumatic pressure around the mirror which is located within a cavity formed by a motor case of the deflector is maintained lower than an inside pneumatic pressure of a pneumatic pressure creation arrangement. The arrangement has the shaft provided with herringbone grooves engraved on the periphery thereof and the sleeve rotatable about the shaft together with the hub and the scanning mirror. When the scanning mirror rotates about the shaft, windage loss may be reduced, and when the inside pneumatic pressure between the shaft and the sleeve is kept higher than the atmospheric pressure, the rigidity of the shaft may be maintained during the rotation of scanning mirror about the shaft.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical deflector provided with a 
scanning mirror rotatable about a shaft by pneumatic pressure. This sort 
of mirror forms a part of a scanner system adaptable for an image or 
information processing device and a measurement apparatus. One prior art 
scanning mirror rotatable about a shaft by pneumatic pressure will be 
explained hereinafter with reference to the attached drawings, including 
FIG. 8, FIG. 9, FIG. 10 and FIG. 11. 
FIG. 8 indicates a sectional view of a scanning mirror 5 rotatably 
supported by a vertical shaft la accommodated in a cavity V of a motor 
case 3 having a cover 4. The lower end of the shaft is fixedly held by the 
motor case. The shaft 1a is provided with a pair of herringbone grooves Ha 
and Hb formed around the periphery of the shaft 1a, as shown in FIG. 9. 
the lower end of the shaft is shrinkage fitted to the motor case 3 while 
the upper end thereof is fitted to the cover 4. A cylindrical sleeve 2 is 
rotatably arranged about the shaft 1a, leaving a clearance therebetween. A 
cylindrical magnet 22 is fixedly disposed around the lower portion of the 
sleeve 2. On the vertical inner wall of the motor case 3, a yoke 16 having 
coil 15 mounted thereon is fixed. Between the cover 4 and the motor case 
3, there exists an O-ring 21, while another O-ring 20 is located between 
the shaft la and the cover 4, whereby the cover 4 and the motor case 3 are 
hermetically sealed. Numeral 17 is a Hall element holder to which a Hall 
element 18 is fitted. A thrust magnet 12 located on an inner base of the 
motor case 3 is arranged around the shaft 1a. Numeral 13 is another 
magnet, facing the thrust magnet 12 and having an opposite polarity, while 
numeral 14 is a spacer located underneath the magnet 22 and fitted to the 
magnet 13. 
A hub 6 fixed around the sleeve 2 is provided with a polygon mirror 5, 
which is disposed horizontally on the hub 6 by means of a screw 9. Thus 
the sleeve 2, the hub 5 and the polygon mirror 5 are simultaneously 
rotatably arranged about the shaft 1a. A pair of magnets 10 and 11, each 
having an opposite pole N and S, are disposed face-to-face on the top end 
of the shaft 1a and on the under surface of the cover 4, respectively. It 
should be noted that instead of a polygon mirror 5, any type of flat 
mirror may also be adaptable. 
With the energization of the magnet 22 by electrical current supplied via 
the coil 15, a motor driving means which, in this embodiment, is composed 
of a magnet 22 and the sleeve 2, begins to rotate about the shaft 1a with 
the sleeve 2 being supported by the spacer 14 and the magnet 13. 
Simultaneously with the rotation of the sleeves, a mirror rotating means 
which, in this case, is composed of the hub 6, the magnet 22 and the 
sleeve 2, is rotated about the shaft 1a accompanied by the mirror 5. 
When the sleeve 2 is thus rotated, a clearance is maintained between the 
lower surface of the case 4 and the upper end of the sleeve 2 by means of 
a repulsion force created by the pair of magnets 10 and 11 and the pair of 
magnets 12 and 13, respectively positioned face-to-face and provided with 
opposite poles. 
Corresponding to the rotation of the mirror rotating means and the motor 
driving means, as explained heretofore, air is sucked from suction intakes 
s, as show in FIG. 10, into the clearance existing between the shaft 1a 
and the sleeve 2 (FIG. 10), and a dynamic pneumatic pressure is created 
therein with the aid of the herringbone grooves Ha and Hb, whereby a 
constant rigidity of the shaft 1a is maintained. 
FIG. 11 is a chart of the pneumatic pressure distribution existing in the 
clearance between the sleeve 2 and the shaft 1a when the sleeve 2 is 
rotated accompanied by the mirror 5. FIG. 10 indicates a sectional view of 
the sleeve 2 and a flat view of the shaft 1a. The structure shown in FIG. 
10 may hereinafter be called a dynamic pneumatic pressure creation means 
wherein, corresponding to the rotation of the sleeve 2 as explained, air 
is sucked via suction intakes s into the dynamic pneumatic pressure 
creation means, with the result that pneumatic pressure inside the dynamic 
pneumatic pressure creation means is maintained higher than the 
circumferential air pressure outside thereof. Pneumatic pressure around 
the outside of the dynamic pneumatic pressure creation means and in the 
cavity may be maintained equal to atmospheric pressure. 
The air pressure distribution inside the dynamic pneumatic creation means 
shown in FIG. 10 is illustrated in FIG. 11. In FIG. 11, the ordinate 
indicates dimensionless air pressure P, which represents an atmospheric 
pressure, while Z is the abscissa. The atmospheric pressure around the 
outside of both ends of the dynamic pneumatic pressure creation means is 
indicated by P=1, while on the inside of the means the pressure is 
indicated by P&gt;1. Therefore, the outside or surrounding air pressure of 
the dynamic pneumatic creation means is equal to atmospheric pressure. 
Windage loss W due to the rotation of the mirror is usually indicated as 
follows: 
W=Ps.times.Nr.sup.3 .times.Km, where 
Ps: circumferential pressure, 
Nr: number of rotation, and 
Km: coefficient of mirror configuration. 
In this prior art, windage loss Wo may be indicated as follows: 
Wo=1.0.times.Nr.sup.3 .times.Km (watt). 
As is explained heretofore, windage loss W due to the rotation of the 
mirror is usually shown as: W=Ps.times.Nr.sup.3 .times.Km (watt), while in 
the prior art, as the circumferential air pressure around the pneumatic 
pressure creation means and in the cavity is already set as Ps=1, and the 
windage loss may be indicated as Wo=Nr.sup.3 .times.Km (Watt). Therefore 
it is impossible to reduce the windage loss less than the value shown 
above. 
It is natural in the case of the prior art that in proportion to the 
increase of the number of rotations Nr, or due to the coefficient of the 
configuration of the mirror, the windage loss becomes larger. If the motor 
torque becomes smaller than the windage loss, a motor of a large size, 
which requires the supply of more electric current, is required to resolve 
the situation, which results in bringing forth the generation of more heat 
by the increasing supply of electric current. In order to prevent the 
generation of heat, a type of radiation system must be installed, with a 
resultant increase in the production costs, coupled with the difficulty of 
the miniaturization of the device at a low cost. 
SUMMARY OF THE INVENTION 
An objective of the present invention is to present a scanning mirror which 
is arranged rotatably about a shaft accompanied by a sleeve and a hub, 
wherein pneumatic pressure around a mirror and a hub located within a 
cavity formed by a motor case with a cover is maintained lower than an 
inside air pressure of a pneumatic pressure creation means comprising a 
shaft provided with herringbone grooves engraved in the periphery thereof 
and a sleeve which is rotatable about the shaft together with the hub and 
the mirror, whereby windage loss due to the rotation of the mirror around 
the shaft is decreased. 
Another object of this invention is to provide a scanning mirror arranged 
rotatably about a shaft accompanied by a sleeve and a hub wherein 
pneumatic pressure inside a dynamic pneumatic creation means is kept 
higher than atmospheric pressure, while a circumferential pressure around 
the hub and the mirror accommodated in a cavity formed by a motor case and 
a cover is maintained lower than atmospheric pressure, whereby a constant 
rigidity of the shaft is maintained during rotation of the mirror, and at 
the same time windage loss due to the rotation of the mirror is kept small 
.

DETAILED DESCRIPTION OF THE INVENTION 
Now referring to the drawings, preferred embodiments of this invention will 
be explained hereinafter. 
FIG. 1 illustrates an embodiment of this invention. The same reference 
numerals shown in FIG. 8 of the prior art shall refer to like parts of 
this embodiment. Therefore, a detailed description of the same numerals 
will not be repeated. 
In the embodiment, a polygon mirror 5 is adopted. However, a flat mirror or 
the like may also be useable instead. Circumferential air around the 
mirror 5 in the cavity v formed by the motor case 3 and the cover 4 is 
usually maintained at the same pressure as the atmosphere and is 
connectable to the atmosphere outside the cavity v by circulating it 
through the clearance between the sleeve 2 and the shaft 1, and through 
radial passages b and c defined in the shaft 1, which are connected to a 
thrust-wise central passage g defined in the center of the shaft 1. The 
passage g has an outlet K and a filter 23 at the lower portion thereof. 
With the rotation of the mirror 5 around the shaft 1, accompanied by the 
hub 6 and the sleeve 2, a dynamic pneumatic pressure is created between 
the sleeve 2 and the shaft 1, on the periphery of which two pairs of 
herringbone grooves d and e are engraved, as will hereinafter be 
explained. 
The air pressure distribution state is now to be described, referring to 
FIG. 2-FIG. 7. FIG. 2 illustrates a flat view of a shaft 1 consisting of a 
pair of dynamic pneumatic pressure creation sections M. The pair of 
sections M are each provided with herringbone grooves d and e having 
opposite intake angles .beta..sub.1, .beta..sub.2, .beta..sub.3 and 
.beta..sub.4 and with non-engraved portions f left therebetween. However, 
another type of section M having herringbone grooves d1 and e1, shown in 
FIG. 4, may be adaptable. Non-engraved portions are not arranged between 
the herringbone grooves d1 and e1. 
In FIG. 3, the central passage g, having the outlet K exposed to the 
atmosphere, is defined in the center of the shaft 1. One end of each of 
the radial passages b and c defined in the shaft 1 are connected to the 
central passage g, and the other open ends are defined in the center of 
the non-engraved portions between the respective herringbone grooves d and 
e. As shown in FIG. 4, radial passages b and c are located in the center 
of the herringbone grooves d1 and e1, which are not provided with 
non-engraved portions. 
The air flow inside the optical deflector according to the present 
invention will now be explained with reference to FIGS. 2 and 6. The 
polygon mirror 5 and the sleeve 2 are attached to the hub 6 around the 
shaft 1 and are rotated by the motor. With this rotation, air existing in 
the cavity v, and existing in the clearance between the shaft 1 and the 
sleeve 2, is sucked from the intake S by the movement of the herringbone 
grooves having the suction angles .beta..sub.1 and .beta..sub.4, 
respectively, and flows into the radial passages b and c and passes 
through to the horizontal passage g in the direction shown by the arrows 
in FIG. 6. The air being sucked through the passages from the cavity v and 
in the clearance between the shaft 1 and the sleeve 2 is then finally 
released through the outlet K into the air. 
During the rotation of the motor, the non-engraved portions f between the 
herringbone grooves d and e act as a shaft support means, as will be 
explained hereinafter, holding the shaft to prevent swaying within the 
sleeve 2 during the rotation of the sleeve 2 around the shaft 1. FIG. 7 
indicates a pneumatic distribution in a state of stable rotation of the 
rotor. The pneumatic pressure at the suction inlet s is equal to the 
pressure in the cavity v, while the pneumatic pressure at the non-engraved 
portion f of the herringbone grooves d and e is equal to atmospheric 
pressure. Therefore, the non-engraved portions f hold the shaft 1 to 
prevent swaying within the sleeve 2, whereby a smooth rotation of the 
sleeve 2 around the shaft may be obtained. Thus FIG. 7 shows that the 
pneumatic pressure inside the cavity v, which is less than the atmospheric 
pressure, is equal to the pneumatic pressure at the suction inlet s, while 
the non-engraved portion f of the herringbone grooves d and e is equal to 
the atmospheric pressure, whereby the non-engraved portions f are able to 
hold the shaft without swaying. 
Therefore, the circumferential condition of the dynamic pneumatic pressure 
creation section M of this invention requires the pressure at each open 
end of the radial passages b and c to be equal to atmospheric pressure. 
And as shown in FIG. 6, in a state of stable rotation, the amount of 
radial air flow at intakes s becomes zero, and the pneumatic pressure 
distribution in the dynamic pneumatic pressure creation section M is as 
shown in FIG. 7. In FIG. 7, P represents dimensionless pressure, and when 
P is equal to 1, it means that the pneumatic pressure is equal to 
atmospheric pressure at the non-engraved portions f. 
As is illustrated in FIG. 7, in the state of rotation, the pneumatic 
pressure at intakes s of the section M becomes smaller than atmospheric 
pressure and the pneumatic pressure inside the cavity may be indicated by 
P&lt;1, which is smaller than an atmospheric pressure, and may be, for 
example, P=0.7 of atmospheric pressure. The air pressure Ps around the 
mirror 5 may then be indicated by Ps=0.7. The quotation of windage loss W 
due to the rotation of mirror is W=Ps.times.Nr.sup.3 .times.Km. Therefore, 
the windage loss due to the rotation of mirror 5, in this embodiment, may 
be shown as: 
EQU W=0.7.times.Nr.sup.3 .times.Km (watt). 
The same principle of air flow is also applicable to the embodiment of FIG. 
4. 
In case the number of rotations Nr and the coefficient of mirror 
configuration Km remains the same, the windage loss of this invention may 
be reduced by 30%. As a result, a small energy consumption by the motor 
dispenses with any heat radiation device, and miniaturization and low cost 
production of motors may be obtained. 
Another embodiment of this invention may be described hereinafter with 
reference to FIGS. 12, 13, 14, 15, 16, 17 and 18. Like numerals indicated 
in the first embodiment represent like elements adopted in this invention. 
Therefore, detailed description of the common elements is eliminated. 
FIG. 12 corresponds to FIG. 1 of the first embodiment. Except for the 
herringbone groove arrangement and the distance between radial passage b 
and c, the structure of both embodiments is almost identical. 
An air pressure distribution in the clearance between the sleeve 2 and the 
shaft 1 will be explained referring to FIGS. 13-18. 
FIG. 13 is a plan view of the shaft 1, wherein two sets of herringbone 
grooves d and e, having opposite suction angles .beta..sub.1 .beta..sub.4, 
are engraved in the periphery of the shaft 1. Each set of herringbone 
grooves d and e composes an air pressure creation section M. Herringbone 
grooves q and r are also defined in the periphery of the shaft 2 in 
parallel with the grooves d and e, on the outer side thereof. An open end 
of the radial passage b is located in the center of a non-engraved portion 
between the herringbone grooves d and q, and another open end of the 
radial passage c is located in the center of a non-engraved portion 
between the herringbone grooves e and r. 
The circumferential conditions of the pneumatic pressure creation shaft of 
this invention requires the pneumatic pressure at each open end of radial 
passage b and c be equal to the atmosphere (see FIG. 18). As shown in FIG. 
17, in a state of stable rotation, the amount of air flow at intakes s 
becomes zero. Therefore a pneumatic pressure distribution in the dynamic 
pneumatic pressure creation section M is as shown in FIG. 18, wherein P, 
taken as the ordinate, represents dimensionless air pressure. When P is 
equal to numeral 1 (P=1), the pneumatic pressure is equal to the 
atmospheric pressure. 
As is illustrated in FIG. 18, the air pressure around the intakes s becomes 
smaller than the atmospheric pressure, and the pneumatic pressure inside 
the cavity v of this invention as illustrated in FIG. 12, especially 
around the mirror 5, becomes smaller than the atmospheric pressure, while 
the maximum pneumatic pressure between the sleeve 2 and the shaft 1, which 
forms a dynamic pneumatic pressure creation section, is kept higher than 
the atmospheric pressure, especially at the non-engraved portions f, which 
function as a shaft support, with the result that, while reducing the 
windage loss, the rigidity of the shaft is maintained during the rotation 
of the mirror 5. It should be noted that the pneumatic pressure at the 
passage b and c is equal to the atmospheric pressure. 
FIG. 15 illustrates a further embodiment wherein non-engraved portions do 
not exist between the herringbone grooves d and e. The same principle of 
air flow as explained above with reference to FIG. 17 may be applicable. 
However, in this case a middle portion h where herringbone grooves d and e 
meet acts as a shaft support, having a pneumatic pressure that is higher 
than that of the non-engraved portions shown in FIG. 18. The pneumatic 
pressure at the middle portions h of FIG. 15 is illustrated by the dotted 
lines shown in FIG. 18. 
Thus the present invention reduces the pneumatic pressure inside the cavity 
with a resultant reduction of windage loss, and simultaneously maintains 
the maximum pneumatic pressure in the clearance between the sleeve and the 
shaft, especially at the non-engraved portions between the herringbone 
grooves, higher than atmospheric pressure. Thus the rigidity of the 
rotating shaft may be maintained.