Fluid rotating apparatus

A fluid rotating apparatus of a positive displacement type pump includes a plurality of rotors accommodated in a housing, a bearing for rotatably supporting the rotors, a suction port and a discharge port formed in the housing, a plurality of motors for individually rotating the rotors, a detector for detecting rotating angles and rotating speeds of the motors, a synchronous controller for controlling rotation of the plurality of motors by a signal from the detector, and a transporting member coaxially provided on one of the rotors and on the upstream side thereof. The transporting member includes a rotary disk rotatable together with the rotor and a fixed disk opposed to the rotary disk fixed to the housing so as to maintain a gap between the rotary disk and the fixed disk. A spiral groove is formed on one of a surface of the rotary disk and an opposing surface of the fixed surface so as to transport one of fluid and gas molecules in a radial direction of the rotor between the rotary disk and the fixed disk. In this manner, fluid (i.e., liquid or gas) molecules are sucked and discharged due to a capacity change of a space defined by the rotors and the housing through synchronous control by the synchronous controller.

BACKGROUND OF THE INVENTION 
The present invention relates to a fluid rotating apparatus such as a 
vacuum pump, a compressor, or the like. 
FIG. 13 shows an example of a conventional sliding vane vacuum pump 
provided with only one rotor. In the vacuum pump with one rotor, when the 
rotor 101 rotates, two blades 102 inserted in the rotor 101 in the 
diametrical direction of the rotor 101 are driven and rotated inside a 
cylindrical fixed wall 103 (stator). At this time, the leading ends of the 
blades 102 are kept in contact with the fixed wall since the blades 102 
are always urged in the radical direction of the rotor 101 by the action 
of a spring 104. Subsequent to the rotation, the capacity of each of the 
spaces 105 partitioned by the blades 102 in the fixed wall is changed, and 
a gas entering from a suction port 106 formed at the fixed wall is 
eventually sucked and compressed and flows out through a discharge port 
107 having a discharge valve In the vacuum pump of this type, in order to 
prevent internal leakage, it is necessary to seal the side surface and the 
leading ends of the blades 102, the side surface of the fixed wall 103, 
and the side surface of the rotor 101 with oil membranes, respectively. 
However, when this kind of vacuum pump is used in the manufacturing 
process of semiconductors, e.g., CVD or dry etching, etc. using a highly 
corrosive reactive gas such as chlorine gas, the gas reacts with the 
sealing oil to thereby generate a reaction product in the pump. In this 
situation, it becomes necessary to perform maintenance work frequently so 
as to remove the reaction product, and moreover, the pump should be 
cleaned and the oil should be exchanged every time maintenance work is 
performed, thus bringing the manufacturing process to a halt. The activity 
rate is hence decreased. So long as the sealing oil is used in the vacuum 
pump, the oil is scattered from the downstream side to the upstream side, 
polluting the vacuum chamber and deteriorating the manufacturing 
efficiency. 
In view of the above-described inconveniences, a positive displacement type 
screw vacuum pump has been developed and put into practical use as a dry 
pump which does not require the sealing oil. FIG. 14 is a side sectional 
view of an example of such screw vacuum pump. Within a housing 111 are 
provided two rotors 112, the rotary shafts of which are made parallel to 
each other. Spiral grooves are formed on the peripheral surfaces of the 
rotors 112. A space is defined when a recess portion (groove) 113a of one 
rotor and a projection 113b of the other rotor are meshed with each other. 
Thus, as the rotors 112 rotate, the capacity of the space changes, to 
cause sucking and discharging of the fluid. 
In addition to the positive displacement type vacuum pump, a turbo type 
vacuum pump as shown in FIG. 15 has been developed. 
The turbo type vacuum pump comprises a rotary shaft 150, a motor 15, ball 
bearings 152a and 152b, and a housing 153. A plurality of rotary disks 154 
arranged in multiple stages is provided on the rotary shaft 150 and a 
spiral groove is formed on each of the surfaces of the rotary disks 154. 
An opposed surface 155 is formed on the fixed side of the pump, with a 
small gap provided therebetween to cause suction and discharge of the gas 
due to molecular drag operation of the spiral groove caused by the high 
speed rotation of the rotary shaft 150. 
The positive displacement type vacuum pump and the turbo type vacuum pump 
have the following disadvantages: 
In the conventional positive displacement type screw vacuum pump referred 
to above and shown in FIG. 14, the synchronous rotation of the rotors 112 
is achieved by timing gears. That is, the rotation of a motor 115 is 
transmitted from a driving gear 116a to an intermediate gear 116b and 
further to one of the meshed timing gears 116c of the rotors 112. The 
phase of the rotating angles of both rotors 112 is adjusted by the 
engagement between the two timing gears 116c. Therefore, since the screw 
vacuum pump uses the gears both for transmission of the motor power and 
for synchronous rotation of the rotors as described hereinabove, a 
lubricating oil filled in a machine chamber 117 which houses the gears 
must be supplied to the gears. Moreover, a mechanical seal 119 should be 
provided between the machine chamber 117 and a fluid chamber 118 so as to 
prevent the lubricating oil from entering the chamber 118 where the rotors 
are accommodated. 
The vacuum pump with two rotors in the above-described construction has 
disadvantages yet to be solved, in that (1) many gears are required for 
the power transmission and the synchronous rotation, i.e., many parts are 
required, resulting in a complicated structure of the apparatus, (2) a 
high speed operation cannot be expected and the apparatus is bulky in size 
since the rotors are synchronously rotated due to the contact maintained 
between the gears, (3) a mechanical seal must be regularly exchanged due 
to the abrasion thereof, such that a completely maintenance-free pump is 
not realized, (4) a large sliding torque due to the mechanical seal 
induces large mechanical losses, and so on. 
Unlike the screw vacuum pump having two rotors, the turbo type vacuum pump 
has one rotor, namely, one rotary shaft. Accordingly, the rotary shaft can 
be driven at a high speed because the turbo type vacuum pump has no 
sliding mechanism allowing the two shafts to synchronously rotate. A clean 
dry pump can constituted by supplying lubricating oil to only the bearing 
section and providing a sealing section for preventing the penetration of 
the oil into the pump section. 
Since the drag operation of the spiral groove allows the discharge 
performance of the pump to range from a viscous flow region to a molecular 
flow region, a vacuum can be generated to a degree of 10.sup.-5 torr. 
As apparent from the graph of FIG. 4 showing, by a conventional example 
(1), characteristic data of the relationship between discharge speed and 
inlet pressure, in this kind of pump, i.e., the pump in FIG. 14, utilizing 
the molecular drag operation, the discharge speed is reduced to a great 
extent when the inlet pressure is in the range between atmospheric 
pressure and an intermediate degree of vacuum (10-3 to 10.sup.0 torr). 
The generation of heat which occurs in the pump section in the 
above-described range of the inlet pressure makes it difficult to achieve 
continuous operation of the pump. As a result, the discharge period of 
time is long, which deteriorates the operational efficiency of the pump 
used in a semiconductor plant. 
SUMMARY OF THE INVENTION 
In view of the above-described situation, there has been provided a fluid 
rotating apparatus (as disclosed in Ser. No. 07/738,902, filed on Aug. 1, 
1991, in the name of Teruo MARUYAMA et al.) which includes plural rotors 
driven by independent motors so that the rotation of the motors is 
synchronously controlled by the synchronous rotation of the rotors without 
any contact therebetween by using rotary encoders to detect the rotary 
angles and number of rotations of the rotors. The apparatus can be 
operated with high speed rotation of the rotors, eliminates the need for 
maintenance, and can be easily cleaned and miniaturized. 
An object of the present invention is to provide a fluid rotating apparatus 
which enables high speed rotation of the rotors, eliminates the need for 
maintenance, can be easily cleaned and miniaturized, can be shortened in 
size and which improves the above proposed apparatus so as to obtain a 
lower vacuum pressure by preventing its discharge capacity from decreasing 
over a wider inlet pressure range. 
In accomplishing these and other objects, according to one aspect of the 
present invention, there is provided a fluid rotating apparatus of a 
positive displacement type which comprises: a plurality of rotors 
accommodated in a housing; a bearing for rotatably supporting the rotors; 
a suction port and a discharge port formed in the housing; a plurality of 
motors for individually rotating the rotors; a detecting means for 
detecting rotating angles and numbers of rotations per minute (i.e., 
rotating speed) of the motors; a synchronous control means for controlling 
rotation of the plurality of motors on the basis of a signal from the 
detecting means; and a transporting means coaxially provided on one of the 
rotors and on the upstream side thereof. The transporting means includes a 
rotary disk rotatable together with the rotor and a fixed disk opposed to 
the rotary disk fixed to the housing to maintain a gap between the rotary 
disk and the fixed disk. A spiral groove is formed on one of a surface of 
the rotary disk and an opposing surface of the fixed surface so as to 
transport fluid (i.e., liquid or gas) molecules in a radial direction of 
the rotor between the rotary disk and the fixed disk. In this manner, the 
fluid or gas molecules are sucked and discharged due to a capacity change 
of a space defined by the rotors and the housing through synchronous 
control by the synchronous control means. 
The rotors are driven by independent motors and the control of the 
synchronous rotation of the rotors is carried out by the noncontact type 
rotation based on the synchronous control means. Thus, it is unnecessary 
to use gears used for power transmission and lubricating oil, and thus the 
high-speed operation of the apparatus can be achieved. The transport means 
serving as a centrifugal element type vacuum pump is provided coaxially 
with at least one of the rotors of the displacement type vacuum pump and 
on the upstream side of the rotor. As a result, both the centrifugal 
element type vacuum pump and the displacement type vacuum pump can be 
miniaturized. 
In addition, the pump can be operated in a region of a high degree of 
vacuum by using a drag pump having a spiral groove formed therein as the 
centrifugal element type vacuum pump. 
Since the displacement type vacuum pump can be of a screw type, fluid or 
gas molecules flow continuously, the influence of the internal leakage of 
fluid or gas molecules is small, and a large space can be formed in the 
rotor. The space can be utilized as a space for accommodating the bearing 
or the motor, which contributes to the miniaturization of the apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before the description of the present invention proceeds, it is to be noted 
that like parts are designated by like reference numerals throughout the 
accompanying drawings. 
FIG. 1 illustrates a positive displacement type vacuum pump as a first 
embodiment of a fluid rotating apparatus according to the present 
invention. The vacuum pump has a first bearing chamber accommodating a 
first rotary shaft 2 in a vertical direction within a housing and a second 
bearing chamber 12 accommodating a second rotary shaft 3 in the same 
vertical direction. Cylindrical rotors 4 and 5 are fitted from outside at 
the upper ends of the rotary shafts 2 and 3. Spiral grooves 42 and 52 are 
formed at the outer peripheral surfaces of the rotors 4 and 5 in a manner 
to be meshed with each other. The section defined when the spiral grooves 
are meshed constitutes a structural part of the positive displacement type 
vacuum pump. That is, a space between a recessed portion (groove) and a 
projecting portion of the engaged spiral grooves 42 and 52 and the housing 
1 periodically changes its capacity in accordance with the rotation of the 
rotary shafts 2 and 3, thereby acting to suck/discharge the fluid. 
Rotary disks 57 and 58 are installed on the first rotary shaft 2 at an 
upper portion thereof through a bushing 56. A spacer 56a is installed 
between the rotary disks 57 and 58 through the bushing 56 to maintain a 
space between the rotary disks 57 and 58. Fixed disks 59, 60, and 61 are 
mounted on the housing 1 on the fixed side thereof in opposition to the 
rotary disk 58, the bushing 56, and the rotary disk 57, respectively, with 
small gaps provided therebetween. The small gaps are continuously 
connected from the suction port 14 to a space 63 in the housing 1 to allow 
the fluid to flow therethrough. 
A spiral groove 62 is formed on each surface of each of the rotary disks 57 
and 58 as shown in FIG. 3A. The fixed disks 59, 60, and 61, the rotary 
disks 57 and 58, the bushing 56, and the spacer 56a constitute a 
structural part of a centrifugal element type vacuum pump so that fluid or 
gas molecules can be sucked and discharged through the small gaps due to a 
molecular drag operation between the spiral grooves 62 of the upper and 
lower surfaces of the rotary disks 57 and 58 and the opposing surfaces of 
the fixed disks 59, 60, and 61 caused by the high speed rotation of the 
rotary shaft 2. The centrifugal element type vacuum pump is a vacuum pump 
which functions to transport fluid or gas molecules in a radial direction 
of the rotary shaft 2, i.e., a direction from the outside towards the 
center of the rotary shaft 2 or vice versa in the radial direction, 
between the surfaces of the rotary and fixed disks. The drag operation of 
the spiral grooves 62 causes the gas which has flowed from the suction 
port 14 into the housing 1 to be discharged to the space 63 accommodating 
the structural portion of the positive displacement type screw vacuum 
pump. Then, the gas is discharged through the discharge port 15. 
There are contact preventing gears 44 and 54 as shown in FIG. 5 which 
function to prevent contact between the spiral grooves 42 and 52 on the 
outer peripheral surfaces at the lower ends of the rotors 4 and 5, 
respectively. A solid lubricating film is formed on each contact 
preventing gear 44 and 54 so that it can withstand some metallic contact. 
A gap (backlash) .delta..sub.2 formed when the contact preventing gears 44 
and 54 are meshed with each other is set smaller than a gap (backlash) 
.delta..sub.1 between the spiral grooves on the outer peripheral surfaces 
of the rotors 4 and 5. Therefore, when the rotary shafts 2 and 3 are 
rotated smoothly and synchronously, the contact preventing gears 44 and 54 
are never brought in touch with each other. However, if the synchronous 
rotation of the rotary shafts 2 and 3 is broken, the contact preventing 
gears 44 and 54 are turned in contact with each other before the spiral 
grooves 42 and 52 contact with each other, thereby preventing contact and 
collision between the spiral grooves 42 and 52. If the backlashes 
.delta..sub.1 and .delta..sub.2 are minute gaps, it may be difficult to 
process the members of the apparatus accurately at a useful level. 
However, the total amount of the fluid which leaks during one stroke of 
the pump is proportional to the time period of the one stroke, and 
therefore, the performance of the pump (ultimate degree of vacuum) can be 
sufficiently maintained even if the backlash .delta..sub.1 between the 
spiral grooves 42 and 52 is increased a little so long as the rotary 
shafts 2 and 3 are rotated at high speeds. Accordingly, in the vacuum pump 
of the embodiment wherein the rotary shafts 2 and 3 are rotated at high 
speeds, the backlashes .delta..sub.1 and .delta..sub.2 of the size 
required to prevent the collision of the spiral grooves 42 and 52 can be 
readily obtained with normal processing accuracy. 
In the housing 1, the suction port 14 is provided on the upstream side of 
the structural part of the positive displacement type vacuum pump, and the 
discharge port 15 is provided in the downstream side thereof. 
The first rotary shaft 2 and the second rotary shaft 3 are supported by 
non-contact type (contactless) hydrostatic bearings provided in the 
internal spaces 45 and 55 of the cylindrical rotors 4 and 5. More 
specifically, thrust bearings are constituted by supplying a compressed 
gas to the upper and lower surfaces of disk-like parts 21, 31 of the 
rotary shafts 2 and 3 from orifices 16. On the other hand, radial bearings 
are constituted by supplying a compressed gas to the outer peripheral 
surfaces of the rotary shafts 2 and 3 from orifices 17. In this case, when 
clean nitrogen gas generally kept in semiconductor plants is used as the 
compressed gas, the pressure inside the internal spaces 45 and 55 
accommodating the motors 6 and 7 can be made higher than the atmospheric 
pressure, whereby a reactive gas which is corrosive and liable to produce 
deposit is prevented from entering the internal spaces 45 and 55. 
The bearings may be magnetic bearings instead of the hydrostatic bearings 
described above, and since the magnetic bearings, like the hydrostatic 
bearings, are contactless, high speed rotation can be easily achieved and 
a completely oil-free construction can be realized. When a ball bearing is 
used in the bearing and a lubricating oil is used for lubrication of the 
bearing, it is possible to prevent the lubricating oil from entering the 
fluid operation chamber by use of a gas purge mechanism utilizing the 
nitrogen gas. 
The first rotary shaft 2 and the second rotary shaft 3 are rotated at high 
speeds of several tens of thousands of rotations per minute by the AC 
servo-motors 6 and 7 provided in the lower part of the respective shafts. 
According to the instant embodiment, the two rotary shafts are controlled 
to be synchronously rotated in a manner as depicted by a block diagram 
shown in FIG. 7. In other words, there are provided rotary encoders 8 and 
9 at the lower ends of the rotary shafts 2 and 3, as is clear from FIG. 1. 
The output pulses from the rotary encoders 8 and 9 are compared with 
command pulses (target values) set for a virtual rotor and the deviation 
between the target value and each output value (rotational speed, 
rotational angle) from the shafts 2 and 3 is processed by a phase 
difference counter. In consequence, the rotation of the servo motors 6 and 
7 is controlled to remove this deviation. 
A magnetic encoder or a general optical encoder may be used as the above 
rotary encoder. A laser type encoder having high resolution and high speed 
response utilizing the diffraction/interference of laser beams is used in 
the instant embodiment. FIG. 6 shows an example of the laser type encoder. 
In FIG. 6, reference numeral 291 represents a moving slit plate having 
many slits arranged in the shape of a circle. The moving slit plate 291 is 
rotated by a shaft 292 such as the first rotary shaft 2 or the second 
rotary shaft 3. Reference numeral 293 indicates a fixed slit plate, 
opposed to the moving slit plate 291, where slits are arranged in the 
configuration of a fan. The light emitted from a laser diode 294 passes 
through each slit of the slit plates 29 and 293 through a collimator lens 
295 and received by a light receiving element 296. 
The fluid rotating apparatus embodied by the present invention may be a 
compressor for air conditioning. In this case, a rotor 10 of the rotary 
section of the compressor may be a Roots-type rotor as indicated in FIG. 
8, a gear-type rotor of FIG. 10, a single or double lobe-type rotor of 
FIGS. 9A and 9B, a screw-type of FIG. 11 or an outer peripheral 
piston-type of FIG. 12, etc. 
Since the synchronous rotation of the rotors is electronically controlled 
to be contactless according to the present invention, a timing gear used 
in a conventional screw pump or the like is dispensed with. Moreover, 
since the rotors are driven separately by independent motors, the 
transmission of power via a gear is not required. Meanwhile, it is 
necessary to form a closed space which changes in capacity upon relative 
movement of two or more rotors in a positive displacement type pump or 
compressor. Therefore, in the prior art, the two or more rotors are 
synchronously rotated by a transmission gear, a timing gear, or a 
complicated transmission mechanism employing a link and a cam mechanism. 
Although it is possible to rotate the rotors at some high speeds if a 
lubricating oil is supplied to the timing gear or transmission mechanism, 
the upper limit of the rotating speed is ten thousand rotations per minute 
(rpm's) at most when the vibration, noises, and reliability of the 
apparatus are taken into consideration. In contrast, according to the 
present invention without requiring a complicated mechanism as in the 
prior art, the rotary section of the rotors can be rotated at such high 
speeds as not lower than ten thousands (rpm's) and moreover, the apparatus 
can be simplified since the transmission mechanism, etc. is omitted. At 
the same time, no oil seal is necessary and no loss of torque due to 
mechanical sliding is brought about, thus making it unnecessary to 
regularly replace the oil seal and oil. The power of the vacuum pump is a 
product of the torque and the rotating speed, and therefore the torque can 
be reduced as the rotating speed is increased. Accordingly, the torque can 
be lowered in the present invention since the rotors are rotated at high 
speeds, whereby the motor can be made compact. Besides, the rotors are 
driven by independent motors, and the torque for each motor can be further 
reduced. When each motor is built in the rotor as in the first embodiment, 
the apparatus can be compact in size and light in weight, and requires 
less space as a whole. 
In addition, the pump according to the present invention has the 
centrifugal element type pump disposed on the upstream side of the 
displacement type vacuum pump. Consequently, unlike the conventional 
displacement type vacuum pump or the turbo type vacuum pump, the vacuum 
pump according to the present invention has the following advantages: 
(1) The pump can be operated in a wide range of vacuum, namely, the 
ultimate vacuum is obtained at a degree as high as approximately 10.sup.-5 
torr or more. 
(2) The discharge performance does not deteriorate in a low degree of 
vacuum close to atmospheric pressure, unlike the turbo type pump, and thus 
is as efficient as the conventional displacement type pump. 
The graphs of FIG. 4 show an example of the characteristic data of the 
discharge speed with respect to the inlet pressure according to the pump 
of the first embodiment of the present invention, the conventional pump 
shown in FIG. 14 (displacement type screw pump) shown by the conventional 
example (1), and the conventional pump in FIG. 15 (turbo type) shown by a 
conventional example (2). According to the pump of the first embodiment of 
the present invention, the discharge speed is constant in the range from 
the atmospheric pressure to 10.sup.-4 torr while according to the turbo 
type pump, the discharge speed drops in the range from a low degree of 
vacuum to an intermediate degree of vacuum (10.sup.-3 to 10.sup.0 torr). 
In the centrifugal element type pump according to the first embodiment, a 
spiral groove is formed on each of the flat surfaces of the rotary disks 
so that fluid or gas molecules are transported in a radial direction of 
the rotary shaft 2, i.e., a direction from the outside to the center of 
the rotary shaft 2 or vice versa in the radial direction, under pressure 
between the surfaces of the rotary and fixed disks. The spiral groove can 
be formed on only one of the surfaces of the rotary disk. Alternatively, a 
spiral groove can be formed on either of the surfaces of the rotary disk 
or the opposing surface of the fixed disk. Also, the centrifugal element 
type pump can include only one rotary disk and two fixed disks to hold the 
rotary disk therebetween via small gaps. In addition, a turbo type 
centrifugal vane in which fluid flows in the radial direction of the 
rotary disk, for example, an open impeller having the drag operation can 
obtain the same advantages as the centrifugal element type pump of the 
present invention. As one example, FIG. 3B shows projections 62a of the 
above type vane to form spiral grooves between the projections 62a. 
The screw groove type pump utilizes a drag operation. However, in order to 
perform a high speed rotation, it is necessary to make the total length of 
the conventional screw groove type pump longer, thus increasing its 
natural frequency. As a result, it is impossible to obtain a high speed 
rotation. On the other hand, the pump of the embodiment of the present 
invention employs the centrifugal element type pump, not the screw groove 
type pump which also utilizes the drag operation, because the total length 
L.sub.2 of the entire pump and the total length L.sub.1 of the upper 
portion thereof in FIG. 1 can be shortened by the use of the centrifugal 
element type pump as compared with the use of the screw groove type pump. 
As a result, the highspeed operation of the pump can be achieved and the 
ultimate vacuum can be lowered. The centrifugal element type pump may be 
provided on the shaft of each of the two rotors. In this case, the pump 
has a more favorable performance. 
Preferably, the rotors may be provided with a screw on the periphery 
thereof in the structural portion of the positive displacement type vacuum 
pump because the screw type rotor allows working fluid to flow almost 
continuously. As a result, the fluctuation of torque applied to the motor 
of each shaft becomes small. On the other hand, in the Roots-type vacuum 
pump, the working fluid gives rise to a great pulsating flow in the 
discharge thereof per rotation of the rotor. The fluctuation of torque 
caused by the pulsating flow prevents the shafts from rotating 
synchronously. In the embodiment of the present invention, the adoption of 
the screw type rotor makes it easy to control the synchronous rotation of 
the shafts with high speed and accuracy. In the screw type rotor, since 
the space between the suction side and the discharge side is closed by the 
recess-projection engagement between the fixed disks and the rotary disks, 
the influence of the internal leakage of fluid is small and thus a high 
ultimate vacuum can be obtained. 
In the screw type rotor, the sectional configuration of the rotor at right 
angles with the shaft thereof is similar to a circle, unlike a gear type 
rotor or a Roots-type rotor. Therefore, a cavity can be formed in a large 
space in the rotor in the range from the center thereof toward the 
vicinity of the periphery thereof. The cavity can be utilized as a space 
for accommodating bearing sections as embodied in the embodiment of the 
present invention, which contributes to the miniaturization of the 
apparatus. 
Although the present invention has been fully described in connection with 
the preferred embodiments thereof with reference to the accompanying 
drawings, it is to be noted that various changes and modifications are 
apparent to those skilled in the art. Such changes and modifications are 
to be understood as included within the scope of the present invention as 
defined by the appended claims unless they depart therefrom.