Oscillating-piston drive for a vacuum pump and an operating method for said drive

Abstract

An oscillating-piston drive for a vacuum pump (1) with a piston (2), which presents has two piston sections (3,4) and an intermediate zone provided with a drive magnet (11). Cylinder sections (8, 9) slidingly receive the piston sections (3, 4). An annular recess (12) is defined between the cylinder sections (8, 9) at a central yoke (19). The recess provides space for movement of the drive magnet (11). An electromagnetic drive which surrounds the piston (2) includes yoke components (17, 18, 19) and coils (15, 16) situated to the sides of said central yoke. Negative influences on the delivery rate of the pump are reduced by a can (34) which delimits the recess (12) peripherally or by controlling the current supply to the coils, such that only one coil conducts current at a time.

Description

The invention relates to an oscillating-piston drive for a vacuum pump with a piston, which presents two piston sections and an intermediate zone provided with a drive magnet, cylinder sections associated to said piston sections, an annular recess arranged between the cylinder sections at a central yoke, said recess forming the space for movement of said drive magnet and an electromagnetic drive surrounding the piston, which comprises yoke components and coils situated to the sides of said central yoke. Moreover, the present invention relates to an operating method for the drive.

Generally it is the aim of the here affected developers and designers to improve the delivery rate or effect (pumping capacity, compression) of a vacuum pump while simultaneously maintaining or even reducing, if possible, the construction volume and/or preferably even reducing energy consumption. This aim is equivalent in that in the course of the further development, respectively design of a pump of the affected type, measures which become necessary must not be associated with impairments affecting the delivery rate.

It is the task of the present invention to propose an oscillating piston drive for a vacuum pump in which delivery rate impairments are reduced.

This task is solved through the present invention through the characterising features of the patent claims.

An oscillating piston drive of the aforementioned kind is known from WO 00/63 556, drawing FIG. 8. It exhibits a number of components (coils, pole components, cylinders etc.), adjacent with respect to the space for movement of the drive magnet and which have an influence on the delivery rate. There exists the risk that the space for movement be linked by means of slots between the components or current feedthroughs to the outer surroundings. Through such slots, air enters into the space for movement and increases the low pressure forming during operation of the pump within the space for movement. Measures for sealing these slots (for example, adhesive or sealant layers) may impair the efficiency of the electromagnetic drive, since these will increase the distance between the individual components.

By means of a first solution for the posed task, it is proposed that a can delimits peripherally the outer recess. A pipe section peripherally delimiting the space for movement of the drive magnet reduces the number of slots opening out into the space for movement so that the risk of unwanted pressure increases in this volume is substantially removed. The wall thickness of the pipe may be very small, below 1 mm, for example, so that impairments in the efficiency of the electromagnetic drive are negligible.

Expedient materials for the can are those which offer good sliding properties, like plastic, aluminium, stainless steel¹⁾ or alike (not—or only weakly ferromagnetic)²⁾. ¹⁾ Translator's note: The German text states “Edelstahlt” here whereas “Edelstahl” would be appropriate. Therefore the latter has been assumed for the translation. ²⁾ Translator's note: In the German text the right bracket is missing. It has been added to the translation.

Alternatively the can may consist of a more strongly ferromagnetic material, and its wall thickness selected at least in the area of the sections outside the central yoke such that the drive magnet magnetises the respective section to the saturation point when it is located in the zones outside of the central yoke. This embodiment of the can has the effect that it at least partly becomes part of the drive. The in each instance saturated section is practically no longer existent for the magnetic field of the related coil. This has an effect equivalent to an enlargement of the air gap for this coil and results in a reduction in the inductance of specifically this coil. The current in a coil is built up when the drive magnet is located in the area of this coil, i.e. the corresponding can section is saturated. Lower inductance means faster current build-up at a given voltage. With the magnetic field of this current, now the drive magnet of this coil is repelled towards the axially opposing coil. The saturation effect of the drive magnet on the can disappears. But since the current now has the required level, the increase in inductance is not disturbing.

This operating principle requires that the magnetic field of the drive magnet be stronger compared to that of the coil. If this were not the case, then the field of the coil would practically “overwrite” the field of the drive magnet in the can (the directions of the fields oppose each other) thereby cancelling the saturation immediately. In the instance of this drive, the necessary forces may, however, only be implemented by sufficiently strong rare earth magnets, for example. In the instance of these magnets this requirement is always fulfilled.

From the above descriptions it is apparent that it is expedient to control the current through the coils such that a current is allowed to flow only through one coil at a time. In this manner it is achieved that the current in one coil is built up precisely when the drive magnet is located in the area of this coil.

Controlling the drive by means of semiconductor switches allows the avoidance of further losses. To explain this improvement, the existence of a linear drive in accordance with drawing FIG. 8 of WO 00/63 556 is again assumed. In the instance of this linear drive, the magnetic field of one of the two coils will only generate a force on the piston provided the corresponding piston section is located in the area of the respective coil. The other—current carrying—coil is ineffective during this time. When commonly letting a current flow through both coils simultaneously thus higher losses in the coils are created as would be necessary for producing the forces. Moreover, letting a current flow simultaneously through both coils implies that the drive electronics must be capable of switching on and off both polarities of the current. This not only increases the power loss but also the complexity for the drive electronics.

In a second solution for the task of the present invention it is proposed that only one current polarity be assigned to each of the coils, i.e. the positive current polarity is assigned to one coil and the negative current polarity to the other. For example, the two polarities of the 50 Hertz mains AC can be “distributed” to both coils.

This may be implemented with a simple thyristor regulator. The current amplitude of each half-wave may be adjusted by means of a simple, cost-effective phase angle regulator, as is known from electric drilling machines, for example.

The input signal for the phase angle regulator may for example be

• • defined at a fixed value for a pump application (pressure, mains voltage, number of stages)
  • made to depend on the mains voltage,
  • defined by a sensor for detecting the position of the piston or
  • defined by a sensor for observing the valve movement.

The frequency of the piston's stoke will in all cases result from the frequency of the supplied alternating current.

The advantages of these measures are on the one hand that losses in the coils are reduced, since the current is allowed to flow only through one coil at a time. Also the implementation of the control electronics is more simple, since it is no longer required for the currents to flow simultaneously through both coils.

Further advantages and details of the present invention shall be explained with reference to the examples of embodiments depicted in the drawing FIGS. 1 to 3.

Depicted is in

• • drawing FIG. 1, a sectional view through a piston vacuum pump equipped with a drive in accordance with the present invention,
  • drawing FIG. 2, a partial sectional view at the level of the can and
  • drawing FIG. 3, a schematic representation of a pump in accordance with the present invention with means for supplying the drive coils with current.

The drawing figures each depict a piston vacuum pump 1 with a piston 2. This exhibits piston sections 3 and 4, to the unoccupied face sides of which each a cylindrical pump chamber 5, respectively 6 is assigned. The piston 2 and the pump chambers 5, 6 are located in a housing 7 with cylinder sections 8, 9 for the piston sections 3, 4. The materials for the sliding cylinder surfaces and the corresponding piston surfaces are selected in a basically known manner such that the pump may be operated dry, i.e. without lubricant.

A linear drive is assigned to the piston 2. Said linear drive comprises on the side of the piston a permanent magnet ring 11, encompassing the piston 2 at its central zone. The permanent magnet ring 11 moves in the annular volume (recess 12) encompassing the piston 2. On the stator side, further permanent magnet rings 13, 14 are assigned to the permanent magnet 12 on the side of the piston, said further permanent rings axially delimiting the annular recess 12. At the level of these permanent magnetic rings, also cylinder sections 8, 9 terminate.

Moreover, the coils 15 and 16 are components of the linear drive on the stator side. These are partly encompassed by yoke components 17, 18 and jointly with these yoke components said coils encompass the cylinder sections 8, respectively 9. Located between the coils 15, 16 and the yoke components 17, 18 is an annular center yoke 19, the inner surfaces of which face the annular chamber 12. Currents are made to flow through coils 15, 16 such that the magnetic field produced by the coils and guided by the yoke components 17 to 19 interact with the magnetic fields of the permanent magnet rings 11, 13 and 14 in the desired manner. The piston 2 shall oscillate about its centre position such that during this movement the face sides of the piston may fulfil their pumping function.

For the purpose of fulfilling the desired pumping effect, the compression chambers 5, 6 are each equipped with an inlet valve and an outlet valve (only depicted in drawing FIG. 1). To each of the inlet valves there is associated an inlet aperture 21, respectively 22 which is each located between an outer inlet chamber 23, respectively 24 and the corresponding pump chamber 5, respectively 6. The inlet apertures 21, 22 are designed by way of slot-like radially extending openings in the respective cylinder wall 8, respectively 9. The piston sections 3 and 4 release the respective inlet aperture when assuming one of their two dead centres (each in the retracted position in the cylinder section). The outlet valves 26, 27 are located at the respective face sides. There closure components 28, 29 separate the respective compression chamber 5, respectively 6 from an outlet chamber 31, 32 so long until they are opened by the respective piston section 3, respectively 4—at high pressure differences also by the generated pressure. The closure components 28, 29 are designed by way of flexible discs extending over the entire cross-section of the cylinder sections 3, 4, said disks being centrally affixed at the housing 7 and which are peripherally actuated by the produced pressure or the face sides of the piston 2. To this end, the piston face sides have been designed to have a concave contour. The face sides of the components forming the cylinder sections 8, 9 have the function of the valve seats.

In all, two compression stages are present. They may be operated in series or in parallel. Details on this are not presented.

In all drawing figures the can is designated as 34. It encompasses the annular chamber, respectively the recess 12, and extends into the area of the stator permanent magnet rings 13, 14.

Drawing FIG. 2 depicts that the can 34 exhibits two lateral sections 35, 36 of relatively small wall thickness and a centre section with a greater wall thickness. The wall thickness of the lateral sections 35, 36 is below 1 mm, preferably 0.7 mm. At these wall thicknesses, the desired saturation through the drive magnet 11 occurs, provided the drive magnet is located in the vicinity of the sections 35, 36. The greater wall thickness in the centre zone is only required when the can needs to offer a sufficient degree of mechanical strength.

Drawing FIG. 3 depicts the vacuum pump 1 with its linear drive only in a highly schematic manner. Additionally depicted is an embodiment for the power supply in accordance with the present invention for the coils 15, 16. Through the connection 41 an alternating current, preferably the mains current at 50 Hz is supplied to two thyristor regulators 42, 43, of which each is connected to one coil 15, respectively 16. Regulator 42 allows the passage only of the positive half-wave, regulator 43 allows the passage of only the negative half-wave of the alternating current. Passing of currents through the coils is thus no longer effected simultaneously but alternatingly at only one of the two current polarities. The current/time diagrams 44, 45, 46 presented in each instance the area of the current feed and between the regulators 42, 43 and the coils 15, 16, render apparent the power supply in accordance with the present invention.

Expediently the coils 15, 16 are switched on in the respective supply circuit in such a manner that they effect repelling forces on the drive magnet 11. Thus the piston will oscillate about its central position at the frequency of the supplied alternating current.

The permanent magnets 13, 14 are expediently magnetised such that they will effect on the drive magnet a repelling action. This solution offers the advantage that mechanical springs which move the piston back to its central position can be omitted.

Claims

1. An oscillating piston drive for a vacuum pump comprising: a piston having two piston sections and a central zone; a drive magnet mounted in the central zone; cylinder sections in which the piston sections are slidably received; an annular recess located between the cylinder sections at the level of a central yoke said recess receiving and forming space for movement for the drive magnet; an electromagnetic drive with yoke components encompassing the piston with coils located to the side of the central yoke; and a can peripherally delimiting the recess.

2. The drive according to claim 1, wherein the can is constructed of a material with good sliding properties.

3. The drive according to claim 1, wherein the can is constructed of a ferromagnetic material.

4. The drive according to claim 3, further including: stator permanent magnets which delimit the recess in an axial direction and outside the central yoke zone, the can (34) extending into the area of the stator permanent magnets.

5. The drive according to claim 4, wherein the stator permanent magnets (13, 14) are magnetized such that they exert a repelling force on the drive magnet.

6. The drive according to claim 3, wherein the wall thickness of the can at least outside the central yoke zone is so selected, that the drive magnet magnetizes the respective can section to the saturation point when said magnet is located in the zones outside the central yoke.

7. The drive according to claim 6, wherein the can has in the zone of the central yoke a greater wall thickness.

8. The drive according to claim 1, wherein the drive magnet is a rare earth magnet.

9. An operating method for an oscillating piston drive with the characteristics of claim 1, wherein current flow through the coils is controlled such that current passes through only one coil at a time.

10. An operating method for an oscillating piston drive for a vacuum pump including a piston with two piston sections and a central zone equipped with a drive magnet, cylinder sections associated to the piston sections, an annular recess defined between the cylinder sections adjacent the central yoke, said recess forming space for movement for the drive magnet, and an electromagnetic drive with yoke components encompassing the piston with coils located to the side of the central yoke, the method comprising: supplying an alternating current to the coils with only one current polarity being supplied to each of the coils.

11. The operating method according to claim 10, wherein the current flows through the coils such that they alternately exert repelling forces on the drive magnet.

12. The operating method according to claim 10 further including: adjusting amplitudes of the current flows to each of the coils with a thyristor regulator.

13. The operating method according to claim 12, further including: adjusting current amplitude to the coils by means of a phase angle regulator.

14. The operating method according to claim 13, wherein an input signal for the phase angle regulator is one of: defined at a fixed value for a given pump application, dependent on the mains voltage, defined by a sensor for detecting a position of the piston, or defined by a sensor for observing valve movement.

15. The operating method according to claim 13, wherein the current amplitude is adjusted with thyristor regulators.

16. The oscillating piston drive according to claim 1, further including: thyristor regulators for controlling current flow to the coils.

17. The oscillating piston drive according to claim 1, further including: a means for supplying an alternating current to the coils with only one current polarity being supplied to each of the coils.

18. The oscillating piston drive according to claim 17, wherein the means for supplying alternating current to the coil includes: a phase angle regulator and a means for supplying an input signal to the phase angle regulator which input signal is one of: a fixed value for a given pump application, dependent on a voltage of supply means, defined by a sensor for detecting a position of the piston, or defined by a sensor for observing valve movement.