Vacuum pump

Abstract: The invention concerns a vacuum pump (1) for achieving high vacuum conditions comprising a pump casing (5), a first plurality of molecular pumping stages (2) formed by rotor disks (8a,8b) cooperating with stator rings (9a,9b) and housed in a first portion (4a) of said pump casing, and a second plurality of pumping diaphragm stages (3) housed in a second portion (4b) of said pump casing, a first electric motor (13) for driving said rotor disks of the molecular stages, a second electric motor (30) for actuating the diaphragms (40) of the diaphragm stages, and at least a common passage (7) for the passage of the gas discharged from said molecular pumping stages (2) and sucked by said diaphragm pumping stages (3).

Description: The present invention relates to a vacuum pump. More particularly the invention relates to a vacuum pump for producing high vacuum and ultrahigh vacuum. In the field of vacuum pumps, in order to obtain pressure values lower than 10 -2 Pa it is known the use of mechanical pumps named turbomolecular pumps. An example of a vacuum pump of the turbomolecular type is disclosed in EP-A-0 445 855 in the name of the present applicant. However turbomolecular pumps are not capable of discharging the sucked gas directly to the atmospheric pressure, typically 105 Pa. In order to obtain high vacuum or ultrahigh vacuum conditions when using turbomolecular mechanical pumps complicated pumping systems are to be devised in which there are simultaneously present both a primary pump of the turbomolecular type and a secondary pump or pre-vacuum pump of a mechanical type, such as a rotating or diaphragm pump. The two teamed-up pumps are connected in series by a manifold that communicates the exhaust port of the primary turbomolecular pump with the suction port of the secondary or pre-vacuum pump, while the exhaust port of the secondary pump communicates directly with con the environment surrounding the pumping system for discharging to the atmospheric pressure. During the operation of the pumping system, the secondary or pre-vacuum pump is operated in advance with respect to the primary pump so as to establish the proper pressure conditions within the manifold coupling the two pumps, and only after reaching such conditions the primary turbomolecular pump is started. Thereafter both pumps are operated together until they achieve a compression ratio that could not be obtained by separately operating each pump. A pumping system comprising a primary pump and a secondary pump as discussed above is disclosed, for example, in EP-A-0 256 234 and DE-A-4 320 963. The system disclosed in the above EP application provides for teaming up two vacuum pumps coupled to each other by a curved duct that is external to both pumps and communicating the exhaust port of the primary pump with the suction port of the secondary pump. Such a system has nevertheless the drawback to be cumbersome and complex since it requires a frame for supporting the two pumps coupled in series, and external connections to the pumps for the gas passage. A first object of the present invention is therefore to provide a vacuum pumping system capable of discharging to the atmospheric pressure and that is both compact and easy to be manufactured. This first object of the invention is accomplished through a vacuum pump as recited in claim 1. Presently dry diaphragm pumps are employed as secondary or pre-vacuum pumps. Advantages of the diaphragm pumps reside in that they are highly compact, do not require lubricant and are quite effective, but on the other hand their average operating life is rather short because of the diaphragm wear that requires a frequent replacement of the diaphragm. A second object of the present invention is therefore to provide a simpler diaphragm replacement in diaphragm pumps used as pre-vacuum pumps of pumping systems for high vacuum and ultrahigh vacuum. This second object of the invention is accomplished through a vacuum pump as claimed in claim 5. A common drawback of the pumping systems further resides in that each pump is driven by a dedicated electric motor and therefore each motor requires a separate power supply which increases the power comsumption. Therefore the known systems are not suitable for use in portable units where the electric power supply is provided by batteries. To overcome this drawback it has been suggested to periodically shut off the secondary or pre-vacuum pump when some predetermined conditions are reached. A pumping system adapted for achieving high vacuum conditions and providing for the periodically stopping of the secondary pump is disclosed in EP-A-0 373 975. Such system comprises a gas reservoir chamber disposed between the primary pump and the secondary pump, and a valve for closing the gas passage from said chamber to the secondary pump. When the pressure in the reservoir chamber is lower than a predetermined value, the valve is closed and the secondary pump is stopped thus reducing the electric power consumption. This system is however complicated by the presence of a reservoir chamber for the gas, and is more subject to breaking because of the presence of a stop valve. A third object of the present invention is therefore to solve the problem of easily obtaining a reduction of the power consumption in high vacuum pumping systems. This third object of the invention is achieved through a vacuum pump as claimed in claims 17 to 20. A further advantage of the solution claimed in claims 17 to 20 is that the average life of the diaphragms in the diaphragm pumping stages is considerably increased since these pumping stages are operated at a speed considerably lower than that usually employed in the pumping systems. A detailed description of some preferred embodiments of the invention will follow with particular reference to the attached drawings in which: Fig. 1 is a sectional side view of a first embodiment of a vacuum pump according to the invention; Fig. 2 is a sectional view along line II-II of Fig. 1; Fig. 3 is a sectional side view of a second embodiment of a vacuum pump according to the invention; Fig. 4 is a sectional view along line IV-IV of Fig. 3; Fig. 5 shows a curve illustrating the minimum pressure level that can be obtained through a conventional diaphragm pump, as a function of the rotational speed; Fig. 6 is a schematic diagram of a system for controlling the rotational speed of a motor driving the diaphragm stages according to a first embodiment of the invention; Fig. 7 is a curve showing the current drawn by the motor of the molecular pumping stages as a function of the evacuation pressure of said stages; Fig. 8 is a schematic diagram of a system for controlling the rotational speed of the motor of the diaphragm stages according to a second embodiment of the invention. With reference to Fig. 1 there is shown a vacuum pump 1 for achieving high vacuum and ultrahigh vacuum conditions according to the invention, and comprising a plurality of pumping stages 2 of molecular type and a plurality of pumping stages 3 of diaphragm type. The molecular pumping stages 2 are housed in a first portion 4a of a pump cylindric casing 5, and the diaphragm pumping stages 3 are housed in a second portion 4b of said casing 5. Advantageously between the first portion 4a and the second portion 4b there is provided a common passage 7, formed in the wall 28 of the cylindric casing 5, through which the gases discharged from the molecular pumping stages 2 are sucked by the diaphragm pumping stages 3. Channel 7 is axially directed with respect to the pump body, and is completely housed inside the wall of the pump casing. In the example illustrated in Fig. 1 the molecular pumping stages 2 are made up through the cooperation of rotor disks 8a provided with blades and smooth rotor disks 8b with respective stator rings 9a and 9b. Between such rotor disks and stator rings there are present channels 6a and 6b for pumping the gased sucked through the suction inlet 10 of the pump 1 and discharging them into channel 7. The rotor disks 8a and 8b are integer with a shaft 11 supported by a pair of ball bearings 12a and 12b. Between the ball bearings 12a and 12b there is located a first electric motor 13 that drives into rotation the shaft 11, typically at a speed comprised between 20,000 and 100,000 RPM. The electric motor 13 is fed through an electric supply line 14 from a first electronic control unit 15 of the motor 13, which unit is housed in a space 16 at the opposed end of the pump 1 with respect to the suction inlet 10. Near the opposed end of the pump 1 with respect to the suction inlet 10, in a substantially cylindric space 20 of the second casing portion 4b there is housed a plurality of diaphragm pumping stages 3 formed by three elastic diaphragm 40, preferably of Viton™, radially disposed about a crankshaft 22 and connected to such crankshaft 22 by respective connecting rods 23. A first end 24 of each connecting rod 23 is secured to the center of the corresponding diaphragm 40, while the opposed end is fitted with a head member 25 provided with a transverse hole for rotatably coupling the rod to the central tubular portion of the crankshaft 22. The crankshaft 22 further comprises a first end section 26 located towards the suction side of the pump 1 and a second end portion 27 located towards the other end of the pump 1. Fans 29a and 29b provided with blades are fitted to said first and second crankshaft end portions 26 and 27, respectively, forming a forced air cooling unit for cooling the spaces 16 and 20. In addition to the already mentioned electronic unit 15 for controlling the motor 13, the space 16 contains a second motor 30 for rotating the crankshaft 22, typically at a speed comprised between 1,000 and 4,000 RPM, and a second elettronic unit 31 for controlling the second electric motor 30. The end portion 26 of crankshaft 22 is further supported by a bearing 21 and is substantially coaxial with the shaft 11 of the molecular pumping stages. The air in space 16 is sucked by the fans 29a and 29b and discharged through a radial exhaust passage 32 in the wall 28 of the casing 5. A wall 33 is disposed between the fan 29b and the space 16 to separate the space 20 from the space 16, and the second motor 30 as well as the electronic control units 15 and 31 are secured to such wall. The space 16 is further enclosed by a substantially cylindric cover 34 provided with slits 35 for the inlet of the air for cooling the electronic components and the motor housed therein. As better shown in Fig. 2, each of said diaphragms 40 is received in a corresponding pumping chamber 41a, 41b and 41c formed in the wall 28 of the casing 5, and is circumferentially retained within the pumping chamber by a metal ring 42 fixed to the outer edge of the pumping chamber 41a, 41b and 41c by a plurality of screws 43. The diaphragm pumping stages 3 are further connected in series to each other by circumferential channels 44a and 44b formed in the wall 28 of the second portion 4b of the casing 5. Channel 44a communicates the gas exhaust hole or port 45a of the pumping chamber 41a with the gas suction hole or port 46b of the adjacent pumping chamber 41b, and channel 44b communicates the exhaust port 45b of the pumping chamber 41b with the suction port 46c of the adjacent pumping chamber 41c. The pumping chamber 41a is further provided with a suction port 46a that directly opens into the common gas passage 7, while the pumping chamber 46c is provided with an exhaust port 45c that in the illustrated example of Figures 1 and 2 is closed by a plug 47. Again with reference to Fig. 1, advantageously the cylindric casing 5 is formed by two halves that are secured to each other by screws 61, the joining line between the two halves being indicated by the arrow 60. This way the pump servicing operations, particularly the access to the motor 13 of the molecular pumping stages, are made both possible and easy. Moreover, in correspondence of the portion 4b containing the diaphragm pumping stages 3, said casing 5 is provided with an open section substantially shaped like a horseshoe, facilitating the replacement of the diaphragms 40 contained inside the diaphragm pumping stages 3. With reference to Fig. 3 a second embodiment of the invention is illustrated in which a vacuum pump 101 for obtaining high vacuum conditions comprises a plurality of pumping stages 102 of molecular type and a plurality of pumping stages 103 of diaphragm type. The molecular pumping stages 102 are housed in a first portion 104a of the pump cylindric casing 105, and the diaphragm pumping stages 103 are housed in a second portion 104b of the pump casing 105, corresponding to the base 150 of the vacuum pump. Advantageously the first portion 104a and the second portion 104b are communicating through a common passage 107 formed in the wall 128 of the cylindric casing 105, and the gases discharged by the molecular pumping stages 102 are sucked by the diaphragm pumping stages 103 through this common passage 107. Similarly to the example illustrated in Figures 1 and 2, the molecular pumping stages 102 consist of rotor disks 108a provided with blades and of smooth rotor disks 108b cooperating with respective stator rings 109a and 109b. Between the rotor disks and the stator rings there are formed gas pumping channels 106a and 106b with the gases that are sucked through the suction inlet 110 of the pump 101 and then discharged into channel 107. Similarly to the embodiment illustrated in Figures 1 and 2, said rotor disks 108a and 108b are integral with a rotatable shaft that is supported by a pair of bearings, and a first electric motor for driving the pump rotatable shaft is located between such bearings. The first electric motor is fed through an electric supply line from a first electronic control unit 115 housed in a space 116 at the opposed end of the pump 101 with respect to the suction port 110, substantially around the base 150. Again on such opposed side of the pump 101 with respect to the suction port 110, in correspondence of the pump base 150, a pair of diaphragm pumping stages 103 each comprising a diaphragm 140 connected to a crankshaft 122 by respective connecting rods 123 are housed. Moreover such diaphragms 140 substantially lie in a same plane. A first end 124 of each connecting rod 123 is secured to the center of the corresponding diaphragm 140, while the opposed end is fitted with a head member 125 provided with a transverse hole for rotatably coupling the rod to the opposed ends 126 and 127 of the crankshaft 122. The crankshaft 122 further comprises a first end section 126 fastened to the rotor of a second electric motor 130 driving the crankshaft 122. Inside the space 116, in addition to the above mentioned crank mechanism for moving the diaphragms 140a and 140b and to the second motor 130 there are located a second electronic control unit 131 for the second motor 130 and a pair of electric fans 129a and 129b. Thanks to such electric fans 129a and 129b the components housed inside the space 116 are exposed to an air flow that maintains under control the temperature inside the space 116. The air flow generated by the electric fans 129a and 129b further cools the pump base 150 that contains the "hot" components such as the motor and the support bearings of the rotating shaft of the molecular stages. The space 116 is further enclosed by a prismatic container 134 provided with slits 135a and 135b, respectively for the inlet and the outlet of the cooling air sucked by the electric fans 129a and 129b. As better shown in Fig. 4, each of the diaphragms 140 is housed in a corresponding pumping chamber 141a and 141b formed in the base 150. The diaphragms 140 are circumferentially retained inside the corresponding pumping chamber 141a and 141b by a metal ring 142 secured to the outer edge of the pumping chamber by a plurality of screws 143. The diaphragm pumping stages 103 are connected in series to each other by a circumferential channel 144 formed in the base 150 of the casing 105. This channel 144 comunicates the exhaust port 145a of the pumping chamber 141a with the suction port 146b of the adjacent pumping chamber 141b. The pumping chamber 141a is further provided with a suction port 146a that directly opens into the gas common passage 107, while the pumping chamber 141b is provided with an exhaust port 145b for evacuating the gases to the outside of the pump through a channel 147 that radiallly extends through the base 150 and terminates with a hole 148. The diaphragm pumping stages are designed and dimensioned so as to meet the proper requirements for the maximum discharge pressure (PMOLmax) of the molecular stages and for the maximum flow (Qmax) needed in the particular use conditions of the vacuum pump. Therefore, in the design of the diaphragm pumping stages both the minimum pressure (PMEMmin) that can be achieved by the diaphragm stages without any gas flow - that must be lower than PMOLmax - and the rated pumping speed (SMEM) of the diaphragm stages - that must be larger than Qmax/PMCLmax - have to be taken into account. The first condition: C1) PMEMmin < PMOLmax ensures that the diaphragm stages are capable of creating in the suction channel the maximum pressure that the molecular stages can reach, at least in the absence of gaseous load. The second condition: C2) SMEM > Qmax/PMOLmax ensures that all the gas pumped by the molecular stages is dealt with by the immediately adjacent diaphragm stages. By using a pair of diaphragm pumping stages disposed in series, the minimum pressure PMEMmin that can be reached without any gas flow is about 1,000 Pa, whereas when using four pumping stages in series it is generally possible to achieve pressures of about 100 Pa. By using turbomolecular pumping stages equipped with smooth rotor disks it was possible to obtain discharge pressures for the molecular stages in the order of 1,000 Pa thus allowing the use of pre-vacuum pumps of the two-stage diaphragm having a low cost. The investigation carried out by the applicant showed thas in many applications where a vacuum pumping system is used, the maximum gas flow condition only occurs for short times of the pump operation, while for most of the operating time the gas flow is practically zero. During the time interval when the flow is zero, only condition C1) has to be met for maintaining optimum operating conditions in the pumping system. Fig. 5 is a curve showing the dependence of the minimum pressure PMEMmin that can be achieved through a conventional diaphragm pump, as a function of the rotational speed. As illustrated, the minimum pressure PMEMmin is substantially constant until about 1/5 of the rated rocational speed. In case the gas flow decreases and all the more when the flow ceases, so that it is no longer required to meet condition C2, the rotational speed of the diaphragm pump can be reduced without this reduction being of prejudice to the working of the primary molecular pump. Thus both the wear of the diaphragms and the noise due to vibrations of the diaphragm pump are reduced, while also saving electric power which is particularly advantageous in battery fed systems. In accordance with a preferred embodiment of the invention, the rotational speed of the motor driving the diaphragms is adjusted by modifying the armature voltage of a D.C. motor. Of course, the electric motor speed can be changed in any kind of electric motor by using a proper control system. A pressure transducer detects the output pressure of the molecular pumping stages and sets the motor feeding voltage so as to maintain such pressure at a predetermined value lower than the maximum discharge pressure PMOLmax of the molecular stages. Fig. 6 schematically illustrates the system for controlling the rotational speed of the motor in the diaphragm stages of a vacuum pump. With reference to the block diagram of Fig. 6, reference 201 indicates a vacuum pump according to the invention, 202 the molecular pumping stages, 203 the diaphragm pumping stages, 207 the common passage between the two pumps, 211 the rotatable shaft of the molecular stages, 213 the motor of the molecular stages, 215 the electronic control unit for motor 213, 222 the crankshaft of the diaphragm pumping stages, 230 the second pump motor of the diaphragm stages, 231 the electronic control unit for the second motor 230, and 245 the exhaust port for evacuating the gases from the diaphragm pumping stages. Arrows A and B indicate the inlet direction and the outlet direction of the gas flowing into and out from pump 201, respectively. Fig. 6 further shows a pressure transducer 270 connected to the common passage 207 and an operational amplifier 271 to which the signal of the transducer 270 and a signal (input 273) corresponding to the pressure threshold that is to be maintained inside channel 207 are applied. As shown in Fig. 7 in the pressure range comprised between 10 Pa and 104 Pa the power W drawn by the motor of the molecular pumping stages is substantially proportional to the output pressure of the molecular stages. Because of this proportionality, the signal from the pressure transducer employed in the illustrated example of Fig. 6 can be replaced by a signal proportional to the current drawn by the motor of the molecular stages, which signal is available at the electronic control unit of said motor. Fig. 8 schematically illustrates this simplified embodiment of the system for regulating the rotational speed of the diaphragm stages motor in a vacuum pump where the signal applied to amplifier 271 is directly obtained from control unit 215.

Claims: A vacuum pump (1; 101; 201) comprising: a pump casing (5; 105); a first plurality of pumping stages (2; 102; 202) of the molecular type formed by rotor disks (8a, 8b; 108a, 108b) secured to a rotatable shaft (11; 211) and cooperating with stator rings (9a, 9b; 109a, 109b) housed in a first portion (4a; 104a) of said pump casing (5; 105); a second plurality of pumping stages (3; 103; 203) of the diaphragm type, housed in a second portion (4b; 104b) of said pump casing (5; 105); a first electric motor (13; 213) for driving said rotatable shaft (11; 211) of the molecular stages; a second electric motor (30; 130; 230) for driving the diaphragms (40; 140) of said diaphragm stages; characterized by said first casing portion (4a; 104a) and said second casing portion (4b; 104b) being provided with at least a common passage (7; 107; 207) therein for the flow of the gas discharged by said molecular pumping stages (2; 102; 202) and sucked by said diaphragm pumping stages (3; 103; 203). A vacuum pump as claimed in claim 1, characterized in that said common passage (7; 107; 207) is substantially axially directed with respect to the pump body, and is completely housed inside the wall (28; 128) of said pump casing (5; 105). A vacuum pump as claimed in claim 1 or 2, characterized in that a substantially cylindric space (20) is provided in said second casing portion (4b; 104b) for housing said diaphragm pumping stages (3; 103; 203). A vacuum pump as claimed in claim 3, characterized in that said first casing portion (4a; 104a) and said second casing portion (4b; 104b) are formed as two separate parts joined together by a plurality of fastening means (28). A vacuum pump as claimed in claim 4, characterized in that said second casing portion (4b; 104b) housing the diaphragm pumping stages (3; 103; 203) has an open section substantially shaped like a horseshoe and adapted to make easier the replacement of the diaphragms (40; 140) contained in said diaphragm pumping stages (3; 103; 203). A vacuum pump as claimed in claim 3 or 4 or 5, characterized in that said plurality (3; 103; 203) of pumping diaphragm stages is formed by elastic diaphragms (40; 140) radially located in as many pumping chambers (41a, 41b, 41c; 141a, 141b) and in that each diaphragm (40; 140) is actuated through a corresponding connecting rod (23; 123) rotatably connected to a crankshaft (22; 122; 222) driven by said second electric motor (30; 130; 230). A vacuum pump as claimed in claim 6, characterized in that said space (20) further houses at least a fan (29a, 29b) for cooling said diaphragm pumping stages (3; 103; 203). A vacuum pump as claimed in claim 7, characterized in that said at least one fan (29a, 29b) is coupled to said crankshaft (22; 122; 222). A vacuum pump as claimed in claim 1 or 2, characterized in that said second casing portion (4b; 104b) substantially corresponds to the base (150) of the vacuum pump housing said first motor and the bearings supporting the shaft of the molecular pumping stages (2; 102; 202). A vacuum pump as claimed in claim 9, characterized in that said plurality of diaphragm pumping stages (3; 103; 203) comprises at least a pair of substantially coplanar elastic diaphragms (40; 140) disposed in as many pumping chambers (41a, 41b, 41c; 141a, 141b, 141c), and in that each diaphragm (40; 140) is actuated by a corresponding connecting rod (23; 123) rotatably coupled to a crankshaft (22; 122; 222) driven by said second electric motor (30; 130). A vacuum pump as claimed in any of claims 6 to 8 or 10, characterized in that said pumping chambers (41a, 41b, 41c; 141a, 141b) of the diaphragm stages are connected to each other in series by gas passage channels (44a, 44b; 144), and in that the first of said pumping chambers, in the direction of the gas flow, has a port (46a; 146a) for the inlet of the gas from said common passage (7; 107) and the last of said chambers has a discharge port (46c; 146b) for evacuating the gas at atmospheric pressure. A vacuum pump as claimed in any of claims 3 to 8, characterized in that said plurality of diaphragm pumping stages (3; 103; 203) comprises three diaphragm pumping stages connected in series to each other. A vacuum pump as claimed in one of claims 9 or 10, characterized in that said plurality of diaphragm pumping stages (3; 103; 203) comprises two diaphragm pumping stages connected in series to each other. A vacuum pump as claimed in any of claims 3 to 13, characterized in that a space (16; 116) is provided in the pump end (1; 101; 201) opposed with respect to the suction inlet (10; 110; 210), such space housing said second motor of the diaphragm pumping stages and the electronic control unit (15, 31; 115, 131; 215, 231) for said first motor (13; 213), and said second motor (30; 130; 230). A vacuum pump as claimed in claim 14, characterized in that the rotation axes of said first motor (13; 213) and said second motor (30; 130; 230) are substantially coaxial. A vacuum pump as claimed in claim 14, characterized in that said space (16; 116) in the pump end (1; 101; 201) opposed with respect to the suction inlet (10; 110; 210) is substantially defined around said base (150). A vacuum pump as claimed in any of the preceding claims, characterized in that there are provided means for reducing the rotational speed of said second motor (30; 130; 230) of the diaphragm pumping stages (3; 103; 203) when the outlet pressure of said molecular pumping stages (2; 102; 202) is lower than a predetermined theshold. A vacuum pump as claimed in claim 17, characterized in that said reducing means comprises a first signal proportional to the pressure in said common channel (7; 107; 207), a second signal proportional to a predetermined pressure theshold lower than the maximum discharge pressure PMOLmax of the molecular stages, and means (271) for comparing said first signal with said second signal to generate a third signal for controlling the rotational speed of said second motor (30; 130; 230). A vacuum pump as claimed in claim 18, characterized in that said comparison means comprises an operational amplifier (271), and in that said third signal is applied to said second electronic control unit (31; 131; 231) for said second electric motor (30; 130; 230). A vacuum pump as claimed in claim 19, characterized in that said first signal is obtained from a pressure transducer (270) disposed in correspondence of said common channel (7; 107; 207). A vacuum pump as claimed in claim 19, characterized in that said first signal corresponds to a signal proportional to the power drawn by the motor (13; 213) of the molecular pumping stages (2; 102), said signal being available from the electronic control unit (15; 115; 215) for said first electric motor (13; 213).

Inventors: DE SIMON MAURO, SPAZIANI VINCENZO, DE SIMON, MAURO, SPAZIANI, VINCENZO
Applicants: VARIAN SPA, VARIAN S.P.A.
Application year: 1998
Main classification: F04D19
IPC codes: F04B45, F04D19, F04B27