(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2016/0111218 A1
`(43) Pub. Date:
`Apr. 21, 2016
`MILDNER et al.
`
`US 201601 11218A1
`
`(54)
`(71)
`(72)
`
`(73)
`(21)
`(22)
`(86)
`
`VACUUMIVARIABLE CAPACTOR
`
`Publication Classification
`
`Applicant: COMET AG, Flamatt (CH)
`
`Inventors: Mark Joachim MILDNER, Rizenbach
`(CH); Roland BIERI, Selzach (CH):
`Mike ABRECHT, Thórishaus (CH):
`Walter BIGLER, Heitenried (CH):
`Douglas BEUERMAN, Boulder Creek,
`CA (US); Jack GILMORE, Fort
`Collins, CO (US)
`Assignee: COMET AG, Flamatt (CH)
`Appl. No.:
`14/891,576
`
`PCT Fled:
`
`May 30, 2013
`
`PCT NO.:
`S371 (c)(1),
`(2) Date:
`
`PCT/EP2013/061174.
`
`Nov. 16, 2015
`
`(51) Int. Cl.
`HOIG 5/03
`HOIG 5/04
`HOIG 5/0II
`(52) U.S. Cl.
`CPC ............... H0IG5/013 (2013.01); H0IG5/011
`(2013.01); H0IG5/014 (2013.01)
`
`(2006.01)
`(2006.01)
`(2006.01)
`
`(57)
`
`ABSTRACT
`
`A vacuum variable capacitor includes a pre-vacuum enclo
`sure 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 capaci
`tor and longer lifetimes of the device.
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`RENO EXHIBIT 2028
`Advanced Energy v. Reno, IPR2021-01397
`
`

`

`Patent Application Publication
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`Apr. 21, 2016 Sheet 1 of 4
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`

`Patent Application Publication
`
`Apr. 21, 2016 Sheet 2 of 4
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`US 2016/0111218 A1
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`

`Patent Application Publication
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`Apr. 21, 2016 Sheet 3 of 4
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`

`

`Patent Application Publication
`
`Apr. 21, 2016 Sheet 4 of 4
`
`US 2016/0111218 A1
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`

`

`US 2016/0111218 A1
`
`Apr. 21, 2016
`
`VACUUMIVARIABLE CAPACTOR
`
`BACKGROUND AND SUMMARY
`0001. The present invention relates to the field of vacuum
`variable capacitors.
`0002 Vacuum variable capacitors are useful for example
`in impedance matching networks in which the impedance of
`a time-dependent high-frequency load can be matched with
`that of a generator by adjusting one or more vacuum variable
`capacitors. The capacitance of Such capacitors can be con
`trollably adjusted by moving one electrode, or set of elec
`trodes, with respect to the other, and the use of vacuum as the
`dielectric medium allows their use in high power applica
`tions, for example operating at Voltages in the kV range or
`several tens of kV Voltages, carrying currents up to several
`hundredamps, and at frequencies as low as 200kHZoras high
`as 200 MHz. Such capacitors can be used as the tuning ele
`ment in high-power impedance matching networks and are
`often used for high power radio-frequency (RF) applications
`requiring fast, controllable, reliable capacitance adjustment
`over a large range (typically about 1:50 or more) with high
`resolution (typically more than 10000 setpoints in the range)
`and operating lifetimes of many years.
`0003 Vacuum capacitors typically comprise a pumped
`and sealed enclosure, said enclosure typically comprising
`two metallic collars electrically insulated form each other by
`a cylindrical (tubular) ceramic (or other electrically insulat
`ing) piece joined in a vacuum tight manner to the aforemen
`tioned collars. Inside the enclosure and conductively attached
`to each metallic collar are electrodes whose function (to
`gether with the dielectric) is to generate electric capacitance.
`Typically one electrode is mechanically fixed to one collar
`and the other electrode can be moved by means of a drive
`system comprising shaft and Screw/nut System. Axial move
`ment of the screw/nut guiding system outside the vacuum is
`transferred to axial movement of the movable electrode inside
`the vacuum typically by means of an expandable joint, here
`after generally referred to as a bellows, although other
`expandable joints may be used. The vacuum dielectric
`medium gives the name to such capacitors. The vacuum pres
`sure is typically better (lower) than 10-4 mbar. Using vacuum
`as a capacitor dielectric has the advantages of stable dielectric
`value (in particular no temperature, nor frequency dependen
`cies), and allows stable operations of the capacitor at high
`Voltages and high currents with very low dielectrical losses.
`For example, Publication US2010202094 (A1) describes a
`vacuum variable capacitor. Some specific applications of
`vacuum capacitors include broadcasting (eg in an oscillation
`circuit of a high power transmitter) or plasma controlling
`processes in the semiconductor, Solar and flat panel manufac
`ture, for example during industrial Plasma-Enhanced Chemi
`cal Vapor Deposition (PECVD) processes. In such applica
`tions, adjusting the capacitance of the vacuum variable
`capacitors allows to change (and match) the impedance
`between RF loads (such as those generated by the PECVD
`processes) and the fixed impedance of a high power RF gen
`erator, fixed by industry standard to Z
`
`out Generator = (50+ Oj) Ohm.
`
`0004 Vacuum capacitors are the key tunable element for
`RF power transfer to time-varying loads. Despite being rela
`
`tively bulky, vacuum variable capacitors offer several advan
`tages compared to other tuning mechanisms such as inductive
`tuning, or other forms of capacitive tuning (non-mechanical
`technology or non vacuum-technology). Indeed, vacuum
`variable capacitors allow nearly continuous tuning and have
`an excellent resolution (capacitance range can easily be
`divided into more than 10000 setpoints when the micro-step
`features of a typical stepper motor is used) over a very large
`capacitance range and have very high Voltage capabilities
`thanks to the vacuum dielectric. Moreover, because of the
`extremely low dielectric losses, vacuum capacitors allow
`large currents without generating much heat and conse
`quently are essentially unrivalled for the most demanding
`power applications. The adjustment of the capacitance value
`is achieved by mechanically moving one electrode with
`respect to the other electrode, thereby either modifying the
`distance between the two electrode surfaces or modifying the
`electrode surface overlap (the latter is most common), both of
`which result in a change of the capacitance value.
`0005 Typical vacuum variable capacitors for MHIZ appli
`cations are designed to provide capacitance values in the pF
`range (sometimes extending into the low nF range), whereas
`a single unit will cover a capacitance range of approximately
`1:50 or more; that is, if the minimum setting Cmin is for
`example 10 pF, then a maximum of Cmax=500 pF can typi
`cally be set using the same unit. The time taken to move the
`movable electrode between Cmin and Cmax is typically is or
`more in prior art capacitors. A smaller adjustment requires a
`proportionally smaller amount of time. Recently, the adjust
`ment times during, and in between consecutive plasma pro
`cesses used in chip manufacturing or other semiconductor
`manufacturing processes have shrunk considerably, so that
`vacuum variable capacitors have sometimes become the
`bottleneck element in impedance matching and in the overall
`processes using radio frequency power. While there is
`progress towards more rapid control software, there are
`physical limitations on the speed with which a mechanical
`part (the moving electrode) can be moved using a given
`motor. One limiting factor on the speed is the motor power
`required to counter the significant force due to the pressure
`differential (1 bar) between the inside and the outside of the
`vacuum tight enclosure.
`0006 State of the art vacuum variable capacitors are thus
`limited in speed primarily by the power of the motor and by
`the pressure-velocity limit (so-called PV value) of the screw
`and nut of the drive system used to move the movable elec
`trode of the capacitor. A high PV value leads to a high contact
`pressure between the nut and the screw threads of the drive
`system, negatively affecting the wear of said Screw/nut sys
`tem and resulting in earlier failures (or alternatively requiring
`regular exchange of screw/nut System).
`0007 Prior art capacitors also suffer from significant
`membrane stresses and bending stresses in the bellows. The
`greater these stresses, the Smaller the number of compression/
`expansion cycles (lifecycles) which the bellows can endure
`before they fail.
`0008 Irrespective of the type of motor being used for the
`drive system, a high torque is inevitably required to work
`against the pressure differential of a prior art vacuum variable
`capacitor, as explained below.
`0009 Stepper motors are typically used to drive vacuum
`variable capacitors because of their positioning accuracy
`(resolution), high stiffness (stepper motors develop their
`maximum holding torque at Standstill and typically do not
`
`

`

`US 2016/0111218 A1
`
`Apr. 21, 2016
`
`require any brake), and because they have satisfactory speeds
`for most applications. Typically stepper motors can run at 600
`RPM or 1200 RPM to drive most common vacuum variable
`capacitors and still provide enough torque to work against the
`vacuum force. Unfortunately, however, one property of step
`per motors is that increasing the speed decreases the available
`torque, which, at very high speeds, results in step loss and
`inaccuracy. Other motors (such as servo-motors, or linear
`motors) also have decreasing torque at high speeds. Obtain
`ing a combination of higher torque and speed is only possible
`by drastically increasing the size and cost of the motor. This
`is not an acceptable option for components integrated into
`OEM (original equipment manufacturer) impedance match
`ing networks.
`0010. It is desirable to overcome the above and other dis
`advantages with prior art vacuum variable capacitors. In par
`ticular, it is desirable to provide an improved vacuum variable
`capacitor in which the adjustment speed is increased, but
`preferably without increasing the size of the motor, without
`increasing the size of the device, and/or without reducing the
`adjustment resolution of the device.
`0.011
`Additional advantages may include an increase in
`the lifetime of the device (in particular an increase of the
`number of capacitance adjustment cycles), without compro
`mising on the maximum operating Voltage/power, compact
`ness of the device, or its adjustment resolution.
`0012. According to an aspect of the present invention, a
`vacuum variable capacitor is provided, adjustable between a
`minimum capacitance value and a maximum capacitance
`value, and comprising:
`0013 a first vacuum enclosure containing capacitor elec
`trodes separated by a vacuum dielectric, the wall of the first
`vacuum enclosure comprising a first deformable region (also
`referred to as 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 as the pre-vacuum enclosure, containing a gas at a prede
`termined pressure, lower than atmospheric pressure, the pre
`vacuum enclosure being arranged such that the first deform
`able region separates the pre-vacuum enclosure from the first
`vacuum enclosure.
`0014. The pre-vacuum enclosure (also referred to as a
`secondary vacuum enclosure) contains a gas at a pressure
`below atmospheric pressure, and thereby serves to reduce the
`pressure differential across the bellows. This reduction in
`pressure differential in turn reduces the amount of motor
`torque which is required to move the bellows and/or increases
`the adjustment speed which can be achieved using a given
`motor.
`0015 The presence of the pre-vacuum enclosure means
`that the motor needs less torque in order to drive the nut and
`compress or expand the bellows and move the movable elec
`trode(s) inside the first (also referred to as primary) vacuum
`enclosure. This allows faster speeds using a motor of the same
`size and power. Note that the reduction in torque required is
`not due merely to the reduction in the vacuum force on the
`below. The vacuum force gives rise to an axial force between
`the nut and the screw thread of the shaft. This axial force
`causes significant friction between the nut and the screw. A
`reduction in the pressure differential, and hence in the
`vacuum force, results in a significant decrease in the amount
`of rotational friction between the nut and the screw thread.
`
`This reduced rotational friction also results in a significant
`decrease in the amount of torque required by the motor to
`drive the shaft.
`0016. The secondary vacuum enclosure does not need to
`be pumped down as much as the primary vacuum. Indeed the
`primary vacuum pressure must be many orders of magnitude
`less than the atmospheric pressure in order to perform
`adequately as a dielectric, whereas the pressure in the pre
`vacuum enclosure may merely be one order of magnitude less
`than the atmospheric pressure, for example, which is already
`Sufficient to reduce the axial force acting on the drive system
`(the screw/nut etc) by about a factor of 10. With the reduced
`force acting on the drive system, the required torque of the
`motor is reduced considerably which allows for higher
`speeds.
`0017 Moreover, the arrangement can increase the lifetime
`of the bellows, which separates two volumes under a reduced
`pressure differential and will therefore be subject to less
`membrane stress and less bending stress upon compression/
`expansion. The reduced vacuum force also leads to a reduc
`tion in the wear of the Screw-and-nut drive system, thus
`leading to longer lifetimes of those components.
`0018. The fast vacuum variable capacitor described here
`may for example be configured with the motor located in the
`pre-vacuum enclosure, and with the gas in the pre-vacuum
`enclosure at a pressure of approximately 0.1 bar, for example.
`A pressure of 0.1 bar diminishes the vacuum force on the
`bellows by approximately 90% but still provides enough mol
`ecules to allow convection cooling so that the motor does not
`overheat. A better vacuum (a lower pressure) may not allow
`enough heat to be evacuated towards the outside environment
`leading to overheating of the motor and failure of the system.
`Generally, a pressure of between 0.05 bar and 0.5 bar has been
`found to offer a useful reduction in vacuum force, without the
`need for extra cooling measures. However, any pressure up to
`atmospheric pressure may be used, and still offer an improve
`ment.
`0019. In principle, the vacuum force could be reduced to
`Zero by fully evacuating the pre-vacuum chamber. This would
`reduce the required motor torque for driving the screw/nut to
`a very small value. However, the vacuum force provides a
`useful axial biassing force on the screw-nut drive. This axial
`biassing force significantly reduces the amount of play in the
`screw/nut drive, and thereby contributes to the accuracy
`(resolution) of the capacitor adjustment. The bellows may
`have an inherent spring-like force, which also has the effect of
`biasing the screw-nut mechanical interface. However, the
`bellows may be under compression at one point in its exten
`sion range, and in tension in another part of its extension
`range. So it will exerta positive and a negative biasing force on
`the drive screw/nut, depending on where in its extension
`range the bellows happens to be. Thus, it is advantageous to
`configure the pressure differential across the bellows such
`that the vacuum force is greater than the maximum bellows
`spring force acting in a direction opposite to the vacuum
`force. In other words, the resultant "vacuum force--bellows
`spring force' should not change orientation even when the
`bellows go through their neutral position (from being com
`pressed to being extended). Indeed although the bellows force
`alone would change orientation depending whether it is oper
`ated in compression mode or in extension mode, the adding of
`the (diminished) vacuum force still ensures that the sum of the
`forces does not change orientation. This can be guaranteed by
`having the secondary vacuum pressure high enough to at least
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`US 2016/0111218 A1
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`Apr. 21, 2016
`
`equalize the maximum amplitude of the bellows spring force
`of the capacitor. A change of orientation of the resultant force
`would allow backlash in the screw-nut System resulting in an
`inferior position control of the capacitor (and associated
`capacitance and impedance values). In other words, the
`diminished vacuum force should be determined so that it is
`still just big enough to compensate any bellows force in an
`opposite direction (this will depend on the mechanical prop
`erties of the bellows used).
`0020 For similar reasons, another advantage of a reduced
`but not fully compensated vacuum force, is that the capacitor
`can be positioned and integrated in any orientation into an
`impedance matching network provided that the reduced
`vacuum force still at least also compensates the gravitational
`force which applies on the movable electrode when the bel
`lows axis is not horizontal. A pressure of 0.1 bar was found to
`be appropriate with typical choices of bellows and electrode
`mass. However, in other situations a higher or lower pressure
`may be more effective.
`
`DETAILED DESCRIPTION OF THE INVENTION
`0021. The invention will now be described in detail, with
`reference to the accompanying drawings, in which:
`0022 FIG. 1 shows, in schematic cross-sectional view, a
`prior art vacuum variable capacitor.
`0023 FIG. 2 shows, in schematic cross-sectional view, an
`example of a vacuum variable capacitor according to a first
`embodiment of the invention.
`0024 FIG.3 shows, in schematic cross-sectional view, an
`example of a vacuum variable capacitor according to a second
`embodiment of the invention.
`0.025
`FIG. 4 shows, in schematic cross-sectional view, an
`example of a vacuum variable capacitor according to a third
`embodiment of the present invention.
`0026. The figures are provided for illustrative purposes
`only, and should not be construed as limiting the scope of the
`claimed patent protection.
`0027. Where the same references have been used in dif
`ferent drawings, they are intended to refer to similar or cor
`responding features. However, the use of different references
`does not necessarily indicate a difference between the fea
`tures to which they refer.
`
`DETAILED DESCRIPTION
`0028 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 enclo
`sure (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 col
`lar (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 elec
`trode (7) can be moved by means of a drive system compris
`ing a lead screw (9) and nut (14).
`0029. 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 (APs 1 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.
`0030 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) con
`tains a gas (20) at a pressure lower than atmospheric pressure,
`for example 0.1 bar.
`0031. 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) con
`tained within the sealed pre-vacuum enclosure.
`0032. 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.
`0033. 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.
`0034. 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).
`0035. 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 mount
`ing of the motor (15), as will be explained in relation to the
`second embodiment of the invention.
`0036 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.
`0037 Firstly, the bellows (11) lifetime improves because
`the pressure differential (AP) across the bellows (11) is now
`reduced by 90%, and this reduction will produce lower mem
`brane stress and lower bending stress of the bellows (11) in
`extension or compression, thus leading to an extended life
`time. 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 veloc
`ity, where the pressure and velocity here are those at the
`contact surfaces of themating threads of the screw (9) and nut
`(14). The PV value is a common engineering value that may
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`US 2016/0111218 A1
`
`Apr. 21, 2016
`
`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:
`0038. For a given screw/nut pairing, less wear and longer
`lifetimes;
`0039 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;
`0040 Choosing a less expensive combination of screw/nut
`materials and still reaching the same lifetimes at the same
`speeds;
`0041 Choosing smaller screws and nuts (and therefore
`contributing to the miniaturization of the vacuum capacitor)
`without reducing lifetime.
`0042. 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.
`0043 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.
`0044) 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.
`0045 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 elec
`trodes (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.
`0046. The vacuum variable capacitor of FIG. 4 also com
`prises a second vacuum enclosure (22) and second deform
`able 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 direc
`tion to, the corresponding net vacuum force and bellows
`spring force on the first bellows (11).
`0047. 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 (up
`wards in the FIG. 4), the second bellows (27) moves with its
`vacuum force (also upwards in the FIG. 4).
`0048. In this way, the vacuum and spring force on the
`bellows (11) can be substantially (or even completely) com
`pensated by the second, similar (but counteracting) bellows
`(27) and vacuum enclosure (22) arrangement.
`0049 Various possible mechanical linkages can be envis
`aged 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.
`0050 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.
`0051. With this arrangement, it is particularly advanta
`geous to use a linear drive or any other moving means which
`do not containa screw and nut. Fur