Vacuum variable capacitor

Abstract

A vacuum variable capacitor includes a pre-vacuum enclosure for reducing a pressure differential across the bellows. The vacuum force load on the drive system can thereby be reduced, allowing faster movement of the movable electrode, faster capacitance adjustment of the vacuum variable capacitor and longer lifetimes of the device.

Claims

1. A vacuum variable capacitor adjustable between a minimum capacitance value and a maximum capacitance value, the vacuum variable capacitor comprising: a first vacuum enclosure containing capacitor electrodes separated by a vacuum dielectric, the wall of the first vacuum enclosure comprising a first deformable region, hereafter referred to as first bellows, for transferring mechanical movement between a drive means disposed outside the first vacuum enclosure and a mobile one of the capacitor electrodes inside the first vacuum enclosure, and a second enclosure, referred to hereafter as the pre-vacuum enclosure, containing a gas at a predetermined pressure, lower than atmospheric pressure, the pre-vacuum enclosure being arranged such that the first bellows separates the gas in the pre-vacuum enclosure from the vacuum dielectric in the first vacuum enclosure, wherein the drive means, the electrodes, and the pre-determined pressure in the pre-vacuum enclosure are configured so that the minimum adjustment time between the minimum capacitance value and the maximum capacitance value is less than 0.1s.

2. A vacuum variable capacitor according to claim 1, wherein the drive means comprises a motor disposed within the pre-vacuum enclosure.

3. A vacuum variable capacitor according to claim 1, wherein the drive means comprises a DC motor, an AC servo motor or a linear motor.

4. A vacuum variable capacitor according to claim 1, wherein the maximum capacitance value is at least 10 times greater than the minimum capacitance value.

5. A vacuum variable capacitor according to claim 1, wherein the bellows is configured to sustain 10 million cycles, where one cycle comprises a first capacitance adjustment from a first capacitance value to a second capacitance value, where the second capacitance value is ten times the first capacitance value, and a second capacitance adjustment from the second capacitance value to the first capacitance value.

6. A vacuum variable capacitor according to claim 5, wherein the motor, the electrodes and the drive means are configured so that the minimum adjustment time for one of said cycles is less than 0.05s.

7. A vacuum variable capacitor according to claim 1, comprising control means for controlling the motor, wherein the control means, the motor, and the drive means are configured such that the capacitance is adjustable in increments smaller than 1/5000th of the difference between the maximum and the minimum capacitance values.

8. A vacuum variable capacitor according to claim 1, comprising an insulation element for electrically insulating the drive means from a variable mounting plate of the first vacuum enclosure.

9. A vacuum variable capacitor according to claim 8, wherein the first vacuum enclosure comprises two or more sets of ganged electrodes arranged such that the vacuum variable capacitor is operable without high voltage between the variable mounting plate and the drive measn.

10. A vacuum variable capacitor according to claim 1, comprising a second vacuum enclosure comprising a second deformable wall region, referred to hereafter as second bellows, separating the second vacuum enclosure from the pre-vacuum enclosure, wherein the first bellows is mechanically linked to the second bellows.

11. A vacuum variable capacitor according to, claim 10, wherein the second bellows is substantially identical to the first bellows.

12. A vacuum variable capacitor according to claim 1, wherein the drive means comprise a voice coil or other linear drive.

13. A vacuum variable capacitor according to claim 2, wherein the drive means are configured such that a motor force supplied by the motor and transmitted to the mobile electrode is not transmitted through a threaded connection.

14. A vacuum variable capacitor according to claim 2, wherein the drive means comprise a lead screw and a nut, and wherein the screw and/or the nut comprise a ceramic material.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The invention will now be described in detail, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows, in schematic cross-sectional view, a prior art vacuum variable capacitor.

(3) FIG. 2 shows, in schematic cross-sectional view, an example of a vacuum variable capacitor according to a first embodiment of the invention.

(4) FIG. 3 shows, in schematic cross-sectional view, an example of a vacuum variable capacitor according to a second embodiment of the invention.

(5) FIG. 4 shows, in schematic cross-sectional view, an example of a vacuum variable capacitor according to a third embodiment of the present invention.

(6) The figures are provided for illustrative purposes only, and should not be construed as limiting the scope of the claimed patent protection.

(7) Where the same references have been used in different drawings, they are intended to refer to similar or corresponding features. However, the use of different references does not necessarily indicate a difference between the features to which they refer.

DETAILED DESCRIPTION

(8) FIG. 1 shows a highly simplified, diagrammatical cross-section of an example of a prior art vacuum variable capacitor. It comprises a pumped and sealed vacuum enclosure (2) formed with two metallic collars (3, 4) electrically insulated from each other by a cylindrical ceramic piece (5) joined in a vacuum tight manner to the collars (3, 4). Inside the enclosure (2) and conductively attached to each metallic collar (3, 4) are a static electrode (6) and a movable electrode (7) whose function, together with the vacuum dielectric (12), is to generate electric capacitance. The static electrode (6) is mechanically fixed to one collar (3) and the movable electrode (7) can be moved by means of a drive system comprising a lead screw (9) and nut (14).

(9) An expansion joint or bellows (11) separates the vacuum dielectric (12) from the atmospheric pressure outside the vacuum enclosure (2). Note that there is a force due to the pressure differential (P1 bar) that acts on the bellows (11) and the contact surface between the nut (14) and the lead screw (9). To change the capacitance value of the vacuum variable capacitor, the overlap of the electrodes (6) and (7) may be adjusted by turning the screw (9) an appropriate number of turns or fraction of turns. This is done by typically using a motor (15). The vacuum force, which can be as much as 300N or more, acts on the bellows (11) to pull the bellows and the nut towards the vacuum (ie downwards in FIG. 1). The magnitude of the vacuum force depends on the geometry of the bellows (11), which form the interface between the vacuum (12) and the surrounding atmosphere. This leads to a high torque requirement for the motor (15), which in turn limits its speed, as discussed above.

(10) FIG. 2 shows, in similarly simplified form, an example of a vacuum variable capacitor (1) according to the present invention. It comprises a first vacuum-tight enclosure (2), electrodes (6, 7), motor (15), lead-screw (9), nut (14) and bellows (11) as already described in relation to FIG. 1. In addition, a low-pressure enclosure (21), also referred to as a partial vacuum or pre-vacuum enclosure, is sealed to the first vacuum enclosure (2). The pre-vacuum enclosure (21) contains a gas (20) at a pressure lower than atmospheric pressure, for example 0.1 bar.

(11) Instead of separating the vacuum (12) from the atmosphere, as in FIG. 1, the bellows (11) of FIG. 2 now separate the vacuum (12) from a low-pressure gas (20) contained within the sealed pre-vacuum enclosure.

(12) If the pressure in the pre-vacuum enclosure is 0.1 bar, then the vacuum force acting on the bellows (11) and the nut (14) will be approximately one tenth of the corresponding vacuum force in the vacuum variable capacitor illustrated in FIG. 1.

(13) Because the vacuum force is reduced, the torque required by the motor (15) is also smaller than for the vacuum variable capacitor of FIG. 1. As a consequence, the same motor (15) as the one used in FIG. 1 can operate at higher speeds.

(14) It can be noticed that in this embodiment, the motor (15), being in the pre-vacuum enclosure (21) is electrically insulated from the collar (4) which carries high electric power when the vacuum variable capacitor (i) is in RF operation. This is illustrated symbolically in FIG. 2 by an insulating material (8).

(15) This collar (4) on the variable side of the vacuum variable capacitor (1) is often refered to as the variable mounting plate because it is used to mount the vacuum variable capacitor into an impedance matching network or other system. A different electrode arrangement inside the first vacuum tight enclosure (2) allows to simplify the mounting of the motor (15), as will be explained in relation to the second embodiment of the invention.

(16) Coming back to the present embodiment (FIG. 2), let us assume that the pressure in the pre-vacuum enclosure (21) is 0.1 bar for the following discussion about the increase of the lifetime of the vacuum variable capacitor.

(17) Firstly, the bellows (11) lifetime improves because the pressure differential (P) across the bellows (11) is now reduced by 90%, and this reduction will produce lower membrane stress and lower bending stress of the bellows (11) in extension or compression, thus leading to an extended lifetime. Secondly, the lifetime of the screw (9) and nut (14) is also improved, because the PV value is reduced thanks to the lower pressure value. PV is the product of pressure and velocity, where the pressure and velocity here are those at the contact surfaces of the mating threads of the screw (9) and nut (14). The PV value is a common engineering value that may be used to predict mechanical wear and the time to failure of two sliding surfaces in contact such as those of screws and nuts. A decreased pressure difference across the bellows (11) results in a lower contact pressure between the mating thread surfaces of the screw (9) and the nut (14). With the vacuum variable capacitor (1) illustrated in FIG. 2, the reduction in contact pressure between screw (9) and nut (14) gives rise to one or more of the following beneficial properties:

(18) For a given screw/nut pairing, less wear and longer lifetimes;

(19) For a given screw/nut system and the same lifetime requirements, it allows the screw/nut drive system to operate at faster speeds without reducing lifetime;

(20) Choosing a less expensive combination of screw/nut materials and still reaching the same lifetimes at the same speeds;

(21) Choosing smaller screws and nuts (and therefore contributing to the miniaturization of the vacuum capacitor) without reducing lifetime.

(22) The motor (15) may be a stepper motor, for example. Alternatively, one may use other types of DC motors or AC servo motors. It is also possible to use linear motors without any rotating part in the drive, thereby achieving even higher speeds with a given size motor.

(23) FIG. 3 shows an example of a vacuum variable capacitor according to a second embodiment of the present invention. In this example, the arrangement of two ganged sets of electrodes (24, 25) inside the first vacuum enclosure (2) and the use of a second ceramic insulator (32) as part of the vacuum enclosure (2) makes it possible to connect the motor (15), located in the pre-vacuum enclosure (21) such that the pre-vacuum enclosure does not require an extra insulating piece to electrically insulate the motor from the high voltages applied during operations of the vacuum variable capacitor (1). This allows a more compact layout of the motor in the second vacuum enclosure.

(24) In both FIGS. 2 and 3, the motor (15) has been shown as being located inside the pre-vacuum enclosure (21). However, the motor (15) may alternatively be arranged wholly or partially outside the pre-vacuum enclosure (21). The pre-vacuum enclosure (21) serves as a pressure vessel, for reducing the pressure differential across the bellows (11), and its use for housing the motor (15) is secondary.

(25) FIG. 4 shows an example of a vacuum variable capacitor (1) according to a third embodiment of the present invention, which comprises, as in the first and second embodiments, a first vacuum enclosure (2) containing electrodes (6, 7) in a vacuum (12), and bellows (11), which separate the vacuum (12) from a pre-vacuum enclosure (21) containing a gas (20) at low pressure, as described in relation to the first and second embodiments.

(26) The vacuum variable capacitor of FIG. 4 also comprises a second vacuum enclosure (22) and second deformable wall region, or bellows (27), and a pre-vacuum enclosure (21), which are constructed such that the net vacuum force of the second bellows (27) due to the pressure differential between the second vacuum (13) and the pre-vacuum gas (20), and the bellows spring force of the second bellows (27), are substantially the same as, but acting in the opposite direction to, the corresponding net vacuum force and bellows spring force on the first bellows (11).

(27) As shown in FIG. 4, the first and second bellows are connected by a mechanical linking means (in this case a common shaft, 9), which ensures that a movement of the first bellows (11) is countered by a similar, but opposite movement of the second bellows (27), and vice versa. In other words, if the first bellows (11) moves against its vacuum force (upwards in the FIG. 4), the second bellows (27) moves with its vacuum force (also upwards in the FIG. 4).

(28) In this way, the vacuum and spring force on the bellows (11) can be substantially (or even completely) compensated by the second, similar (but counteracting) bellows (27) and vacuum enclosure (22) arrangement.

(29) Various possible mechanical linkages can be envisaged for linking the two bellows (11 and 27), but a straight-through shaft (28), fixed at either end to the respective end portions of the first (11) and second (27) bellows has the advantage that it requires no threaded joint or other moving parts.

(30) FIG. 4 shows an arrangement in which the first (2) and second (22) vacuum enclosures share a common pre-vacuum enclosure (21) for reducing the pressure differential across the respective bellows (11, 27). However, it would be possible to use two separate pre-vacuum enclosures to achieve the same result.

(31) With this arrangement, it is particularly advantageous to use a linear drive or any other moving means which do not contain a screw and nut. Furthermore with this embodiment, the force necessary to adjust the vacuum variable capacitor is reduced even more than in the previously discussed embodiments, and even higher speeds can be achieved. A linear motor (29, 34), such as a linear induction or voice-coil type motor can for example be used to adjust the vacuum variable capacitor of FIG. 4. Furthermore, because the nett vacuum and spring forces on the bellows are effectively reduced to zero, the capacitance adjustment speed does not depend on the pressure in the pre-vacuum enclosure (21). The pressure in the pre-vacuum enclosure (21) could thus be any value, including atmospheric pressure, or a higher-than-atmospheric pressure. Indeed, the vacuum variable capacitor of the third embodiment may dispense with the pre-vacuum enclosure (21) altogether. The vacuum/spring forces transmitted by the bellows (11, 27) to the mechanical linkage (28) would still be cancelled out.

(32) Note that it would be possible in all three embodiments of the invention to locate the motor (15) or voice coil (29) in the vacuum (12) inside the first vacuum enclosure (2), or, in the third embodiment, in the vacuum (13) inside the second vacuum enclosure (22). However, while some motors are known to work in outer space and are therefore vacuum compatible, it is not feasible to integrate directly an electric motor into the vacuum enclosure containing the electrodes. The reason is that even such motors outgas and degrade the vacuum required for dielectric purposes: vacuum pressures better (lower) than 10-3 mbar are necessary to be maintained, but those were found to be incompatible with long term outgassing rates of motor parts.