Abstract:
A vacuum pump assembly is provided with a vacuum pump and external unit coupled by a vibration damper. The vibration damper comprises a plurality of piezoelectric actuators and a plurality of sensors. Actuators attenuate vibration propagated from the pump to the external unit to which the pump is connected and/or vice versa, while the sensors are capable of providing a measure of the vibrations to controlling said actuators.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a vacuum pump, which is provided with a vibration damper.  
         [0002]     More particularly, the present invention relates to a turbomolecular vacuum pump provided with a damper attenuating the propagation of vibrations induced by the rotation of the pump rotor to an external unit to which the pump is connected. The external unit may be a chamber in which it is desired to create vacuum conditions.  
       BACKGROUND OF THE INVENTION  
       [0003]     Several conventional applications utilising the vacuum chamber with a vacuum pump attached thereto are particularly sensible to the mechanical vibrations inevitably generated by the rotation of the pump&#39;s rotor. The electron microscopes and the systems for measuring and repairing the masks for manufacturing integrated electronic circuits may serve as examples of such applications.  
         [0004]     In order to reduce the transmission of mechanical stresses from the pump to the vacuum chamber, some manufacturers have replaced the conventional mechanical bearings with magnetic bearings or suspensions. However, the use of magnetic suspensions does not always allow for damping the pump-generated vibrations down to the desirable levels.  
         [0005]     Moreover, turbomolecular pumps customary used in the applications demanding high degrees of vacuum, do not discharge directly to the external environment, but they are connected to a forepump. Thus, it is necessary to consider the vibrations generated by forepump and transmitted to the vacuum pump and from the latter to the vacuum chamber.  
         [0006]     For reasons mentioned above, the vacuum pump is often equipped with a vibration damper disposed between the pump and the vacuum chamber.  
         [0007]     According to the prior art, and referring to  FIG. 1 , a vacuum pump  100  has an inlet port  110 , a discharge port  120  and gas pumping means  130  that, in case of turbomolecular pumps, consists of a set of pumping stages, each comprising a rotor disc co-operating with a corresponding stator ring. An example of a turbomolecular pump is disclosed in the U.S. Pat. No. 5,387,079. A flange  115  is provided in correspondence with the inlet port  110  for coupling with flange  210  of chamber  200  where vacuum conditions are to be created. A similar flange  125  is provided in correspondence with discharge port  120  for coupling with a forepump  300 , generally through a flanged bellows  400 . According to the prior art, vibration dampers  140  are, for instance, connected between the pump  100  and the chamber  200 . They essentially comprise a first flange  150  coupled with flange  115  of pump  100 , a second flange  160  coupled with flange  210  of vacuum chamber  200 , a flexible steel bellows  170  ensuring vacuum tightness, and a plurality of rubber members  180  (three in the embodiment shown in  FIG. 1 ), uniformly spaced around bellows  170  along the circumferences of flanges  150 ,  160  and ensuring damping of the mechanical vibrations transmitted by pump  100 .  
         [0008]     Rings  190 ,  290  are provided between the flanges to allow centring O-rings  195 ,  295  intended to ensure vacuum tightness between the flanges.  
         [0009]     A similar damper could also be used downstream vacuum pump  100  and be connected between flange  125  of the discharge port of the pump and flange  310  of the inlet port of forepump  300 .  
         [0010]     It is clear that rubber members  180  used according to the prior art form a passive damper, which attenuates vibration propagation from the pump to the vacuum chamber only in part and in small frequency ranges.  
         [0011]     It is a main object of the present invention to provide a vacuum pump equipped with a mechanical vibration damper having improved characteristics.  
         [0012]     It is another object of the present invention to provide a vacuum pump equipped with a small-size, reliable and inexpensive damper.  
         [0013]     The above and other objects are achieved by a vacuum pump as claimed in the appended claims.  
       SUMMARY OF THE INVENTION  
       [0014]     Advantageously, the vacuum pump according to the invention comprises a damper utilising piezoelectric actuators.  
         [0015]     Piezoelectric devices are devices that, when fed with an appropriate voltage, are capable of generating a force which intensity depends on the applied voltage and therefore is controllable. Conversely, these devices can be used to generate a voltage signal proportional to a possible applied force.  
         [0016]     By using piezoelectric actuators it is therefore possible to control the actuators so that they impart a vibration of substantially the same amplitude as that measured onboard the vacuum pump but in phase opposite thereto, whereby a substantially null resulting vibration is obtained.  
         [0017]     According to an embodiment of the present invention, said piezoelectric actuators are arranged around the metal bellows, in place of the conventional rubber members.  
         [0018]     According to another embodiment, said piezoelectric actuators may be directly mounted on the flange of the inlet and/or discharge port of the vacuum pump, around the centring ring and the O-ring, so that the metal bellows and the related flanges can be dispensed with, thereby reducing the axial size of the pump-damper assembly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Preferred embodiments of the vacuum pump according to the invention, given by way of non limiting example, will be described in more detail hereinafter, with reference to the accompanying drawings, in which:  
         [0020]      FIG. 1  is a longitudinal sectional view of a vacuum pump equipped a damper according to the prior art.  
         [0021]      FIG. 2  is a longitudinal sectional view of a damper according to a first embodiment of the invention;  
         [0022]      FIG. 3   a  is a block diagram of a first embodiment of a control logic arrangement for the damper;  
         [0023]      FIG. 3   b  is a block diagram of a second embodiment of a control logic arrangement for the damper;  
         [0024]      FIG. 4   a  is a block diagram of a third embodiment of a control logic arrangement for the damper;  
         [0025]      FIG. 4   b  is a block diagram of a fourth embodiment of a control logic arrangement for the damper;  
         [0026]      FIG. 5   a  is a partial cross-sectional view of the damper according to a second embodiment of the invention;  
         [0027]      FIG. 5   b  is a partial cross-sectional view of a detail of the damper of  FIG. 5   a;    
         [0028]      FIG. 6   a  is a plan view of the damper according to a third embodiment of the invention;  
         [0029]      FIG. 6   b  is a cross-sectional view, taken along line B-B, of the damper of  FIG. 6   a;    
         [0030]      FIG. 6   c  is a cross-sectional view, taken along line C-C, of the damper of  FIG. 6   a;    
         [0031]      FIG. 6   d  is a cross-sectional view, taken along line B-B, of the damper of  FIG. 6   a;    
         [0032]      FIG. 7   a  is a plan view of the damper according to a fourth embodiment of the invention;  
         [0033]      FIG. 7   b  is a cross-sectional view, taken along line B-B, of the damper of  FIG. 7   a;    
         [0034]      FIG. 8   a  is a plan view of the damper according to a fifth embodiment of the invention;  
         [0035]      FIG. 8   b  is a cross-sectional view taken along line B-B, of the damper of  FIG. 8   a.    
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     Referring to  FIG. 2 , there is shown a vibration damper  14  according to the present invention, which is mounted, by means of corresponding flanges  150 ,  160 , between a vacuum pump  100  and a chamber  200  where vacuum is to be created.  
         [0037]     Damper  14  further comprises a vacuum-tight steel bellows  170  arranged between flanges  150 ,  160 . Piezoelectric actuators A i , which in this embodiment made of parallelepiped or cylindrical blocks  18 , are arranged around bellows. Preferably the piezoelectric actuators A i  are uniformly arranged around bellows  170 : for instance, three actuators, spaced apart by 120°, are provided.  
         [0038]     The actuators A i  are actively controlled through a driving signal capable of generating vibrations substantially equal and opposite to the vibrations, which are produced onboard the vacuum pump and are measured by corresponding sensors, and which are not to be transmitted to vacuum chamber  200 .  
         [0039]     Referring to  FIG. 3   a,  a first embodiment of the control logic circuitry of actuators A i  is disclosed in which an independent closed-loop control system is provided for each sensor-actuator pair. Each control system includes a single-variable regulator R i , implemented in analogue or digital technology, which receives from a corresponding sensor S i , for instance an accelerometer, the value of the corresponding acceleration measured at the pump. Depending on such value, regulator R i  determines the suitable signal to be sent to driver D i  acting upon the corresponding piezoelectric actuator A i . It is possible that the control signals from regulator R i  may also depend on external quantities E i  different from those measured by sensors S i .  
         [0040]     The external quantities E i  may represent the external disturbances acting on the system, and measurement thereof may serve to implement an open-loop feed-forward control. A corresponding implementing diagram of the control logic of actuators A i , shown in  FIG. 3   b,  allows for compensating the external disturbances before they affect the vibrations. Such a result can be obtained by implementing inside the regulator an accurate mathematical model capable of predicting the effects of the disturbances on the mechanical system.  
         [0041]     Generally the same piezoelectric actuators A i  are capable of acting as sensors for detecting an acceleration: thus, other piezoelectric members, with the same structure as the actuators but acting as vibration sensors, can be used in place of the usual accelerometers. By uniformly distributing a sufficient number of piezoelectric members A i  along the circumferences of flanges  150 ,  160 , the even-position members could for instance be used as actuators and the odd-position members as drivers.  
         [0042]     Of course, also in case when actual accelerometers are used, it will be convenient to uniformly arrange a sufficient number of said accelerometers along the circumferences of the flanges  150 ,  160 , by alternating the accelerometers with the piezoelectric actuators A i .  
         [0043]     The regulators R i  can possibly act more effectively if vibration detection is carried out at the point where the actuator force is applied: in such case, sensors and actuators may be located as close as possible to one another, as it is disclosed in more details below.  
         [0044]     Referring to  FIG. 4   a,  a third embodiment of the control logic circuitry of actuators A i  is disclosed. According to this embodiment a plurality of vibration sensors S 1  . . . S n  mounted onboard pump  100 , a plurality of drivers D 1  . . . D n  capable of controlling piezoelectric actuators A i  . . . A n , and a multi-variable regulator R are provided.  
         [0045]     The regulator R, implemented in analogue or digital technology, receives the signals representative of the vibrations from the vacuum pump, through sensors S 1  . . . S n . Depending on such signals, regulator R determines the control signals to be fed to drivers D 1  . . . D n  acting on piezoelectric actuators A i  . . . A n . The actuators generate a vibration that depends on the signal sent by regulator R, the signal being chosen so that the vibration produced is substantially equal and opposite to that measured by the sensors S 1  . . . S n .  
         [0046]     Also in this case, the control logic is a closed-loop logic. Moreover, it is possible to make such control signals depend also on other quantities E measured at the pump.  
         [0047]     Similarly to what disclosed above in connection with the single-variable regulators R i , an implementing diagram of the control logic of actuators A i  providing for an open-loop feed-forward control, as shown in  FIG. 4   b,  may be envisaged also when a multi-variable regulator R is used.  
         [0048]     Regulator R is a multi-variable regulator, in which the control law for drivers D i  is the same for all actuators A i  and depends on the signals coming from all sensors S i .  
         [0049]     In the alternative, regulator R might be implemented as a cascade of as many single-variable regulators R i  as the sensor-actuator pairs are, and of a final multi-variable synthesis block.  
         [0050]     It is to be appreciated that number of sensors S i  may not be equal to the number of piezoelectric actuators A i , even though it is convenient to use the same number of sensors S i  and actuators A i  for constructive reasons.  
         [0051]     Due to the optimum performance attainable by the control systems described above, piezoelectric actuators of a small size (i.e. much smaller than those of the conventional rubber members) can be used to dampen the vibrations measured on the vacuum pump. Thus, it is possible to have embodiments according to the present invention having a reduced axial size of the vacuum pump and its damper. Moreover, these embodiments could allow for improving the pumping characteristics of the pump-damper assembly, by reducing the flow resistance.  
         [0052]      FIG. 5   a  shows part of a damper  24  of a vacuum pump according to a second embodiment of the invention.  
         [0053]     In this embodiment, flange  115  of the vacuum pump inlet port is directly coupled with counterflange  210  of a vacuum chamber through securing screws  20  uniformly distributed along the circumference of said flange  115 , around centring ring  190  and the corresponding O-ring  195 , and through corresponding securing nuts  21 .  
         [0054]     Piezoelectric actuators A i  are formed by cylindrical washers  28  mounted around stems  20   a  of securing screws  20 , in contact with flange  115  on the one side and with counterflange  210  on the other side. Thus, the axial thrust (shown by arrows F 2 ) of actuators  28  can be effective on the one side on the pump and on the other side on the vacuum chamber, thereby compensating for the axial vibrations measured onboard the pump and resulting in a reduction of the transmitted vibration.  
         [0055]     In this second embodiment metal bellows  170  and the corresponding flanges  150 ,  160  can therefore be dispensed with, a consequent reduction of the axial size of the pump-damper assembly.  
         [0056]     Also in this second embodiment the vibrations can be measured by accelerometers mounted onboard the pump. Similarly to the preceding embodiment, damper  24  may comprise a plurality of piezoelectric members A i  used as sensors. Also these sensors preferably consist of washers arranged around stems  20   a  of securing screws and alternating with the piezoelectric actuators along the circumference of damper  24 .  
         [0057]      FIGS. 6   a  to  6   c  show a third embodiment of the invention. According to the third embodiment, piezoelectric actuators A i  are formed by parallelepiped or cylindrical blocks  38 . They are mounted between a pair of circular supports  116 ,  211 , directly located between flange  115  of the vacuum pump inlet port and counterflange  210  of a vacuum chamber, around centring ring  190  and the corresponding O-ring  195  ensuring vacuum tightness, similarly to the embodiment shown in  FIG. 5   a.    
         [0058]     Preferably, support  116  comprises suitable seats  116  receiving said actuators  38 . As shown by arrow F 3  in  FIG. 6   b,  due to such an arrangement, the axial thrust of piezoelectric actuators  38  can be directly transmitted to the vacuum pump and the vacuum chamber through respective flanges  115 ,  210 , whereby a substantially null resulting vibration is obtained.  
         [0059]     In this embodiment also the metal bellows and the corresponding flanges are eliminated, with a substantial reduction of the overall axial size of the pump-damper assembly.  
         [0060]     Even though the vibrations can be detected by accelerometers mounted on the vacuum pump,  FIG. 6   a  shows an alternative solution, already mentioned hereinbefore, in which damper  34  comprises piezoelectric sensors  39 . The sensors consist of piezoelectric parallelepiped or cylindrical plates, of the same kind as used for actuators  38 , and are arranged along the circumference of support  116  alternated with actuators  38 .  
         [0061]     In the embodiments disclosed above, piezoelectric actuators A i  are mounted so as to attenuate transmission of axial vibrations from vacuum pump  100  to vacuum chamber  200 .  
         [0062]      FIGS. 7   a  and  7   b  show a fourth embodiment of the invention, where a damper  44  according to the invention comprises piezoelectric actuators A i , consisting of parallelepiped or cylindrical plates  48 , which can be used to prevent transmission of radial vibrations.  
         [0063]     In that embodiment, piezoelectric actuators  48  are mounted between a pair of circular supports  117 ,  212  located between flange  115  and counterflange  210 , so that they can exert a radial thrust on flanges  115 ,  210  (as shown by arrows F 4  in  FIG. 7   b ).  
         [0064]      FIGS. 8   a,    8   b  show a pump arrangement according to a fifth embodiment of the invention, comprising first and second piezoelectric actuators  581 ,  582  capable of dampening axial vibrations and radial vibrations, respectively (as shown by arrows F 51 , F 52  in  FIG. 8   b ).  
         [0065]     The first and second piezoelectric actuators  581 ,  582  can exert an axial thrust and a radial thrust, respectively, on the vacuum pump and the vacuum chamber connected to the pump. The vacuum chamber is connected through flange  210  to a support  213  shaped so as to have a pair of mutually orthogonal walls facing corresponding orthogonal walls of a corresponding support  118  connected to flange  115  of the vacuum pump.  
         [0066]     Thus, piezoelectric actuators  581 ,  582  can be mounted as follows. The first actuators  581  are in contact at their bottom ends with support  118  connected to the vacuum pump, and their top ends with support  213  are connected to flange  210  of the vacuum chamber. Therefore the first actuators  581  are capable of transmitting an axial thrust. The second actuators  582  are in contact at their inner sides with support  118  connected to flange  115  of the vacuum pump and at their outer sides with support  213  connected to flange  210  of the vacuum chamber, whereby they are capable of transmitting a radial thrust.  
         [0067]     The first and second actuators  581 ,  582  consist of piezoelectric parallelepiped or cylindrical plates uniformly arranged along the circumference of flange  115 .  
         [0068]     In this embodiment also the pump vibrations can be detected by accelerometers mounted on the pump. In the alternative, damper  54  may comprise first and second piezoelectric members A i  used as sensors to detect axial vibrations and radial vibrations, respectively.  
         [0069]     In yet another embodiment of the invention, instead of alternating piezoelectric actuators and sensors along the circumference of vacuum pump flange  115 , integrated pairs of piezoelectric members are used, wherein one member acting as a sensor and the other as an actuator.  
         [0070]     An example of such embodiment is shown in  FIG. 6   d,  with reference to a damper of the kind shown in  FIGS. 6   a  to  6   c.    
         [0071]     A piezoelectric sensor  39 ′ and a piezoelectric actuator  38 ′, separated by a plate  37 , are received in each seat  115   a  formed in flange  115 . Arrows F 3 ′, F 3 ″ denote the operational directions of the sensor and the actuator, respectively, which are therefore coaxially mounted.  
         [0072]      FIG. 5   b  shows an arrangement relevant to the second embodiment of the invention shown in  FIG. 5   a.  A piezoelectric sensor  28 ′ and a piezoelectric actuator  29 ′, both consisting of a washer, are stacked on stem  20   a  of each screw  20  and are separated by a washer  37 . Arrows F 2 ′, F 2 ″ denote the operational directions of said sensor and actuator, respectively, which are therefore coaxially mounted.  
         [0073]     It is clear that this embodiment of the invention provides considerable advantages in terms of accuracy in vibration damping, since the actuator preventing transmission of vibrations is located exactly at the same position where the vibrations are detected.  
         [0074]     Though the above description refers to a vacuum pump equipped with a damper located at the input port, in order to attenuate vibrations transmitted from the vacuum pump to a vacuum chamber, a similar damper could be for instance located also at the discharge port, to attenuate vibration transmission from the forepump to said vacuum pump or between the pump and other external units.