Double-bellows vacuum variable capacitor

A vacuum variable capacitor includes a vacuum bellows for allowing a pressure differential between two volumes inside the capacitor, wherein one of the volumes may be a vacuum volume. The vacuum variable capacitor further includes a conductive bellows disposed within the vacuum volume. In such an arrangement, the materials selected for the vacuum bellows and the conductive bellows may be selected to optimize the function of each bellows.

BACKGROUND OF THE INVENTION
 The present invention is directed to variable capacitors and, more
 particularly, to double-bellows vacuum variable capacitors.
 A known water-cooled vacuum variable capacitor 10 previously marketed by
 Jennings Technology, the owner of this patent, having a double-bellows
 configuration is shown partially in section in FIG. 1. The capacitor 10
 generally included a variable end assembly 12 and a fixed end assembly 14
 connected together by a body assembly 16. The end assemblies 12, 14 were
 typically fabricated from steel and, in some instances, were partially
 silver plated. The body assembly 16 was an insulator such as, for example,
 ceramic that mechanically coupled the end assemblies 12, 14 while keeping
 the end assemblies 12, 14 electrically insulated from one another.
 Inside the capacitor 10 was a fixed can structure 20 that formed the first
 half of the capacitor 10. The second half of the capacitor 10 was formed
 by a variable can structure 22, which was mounted to a variable can plate
 24. To change the capacitance of the capacitor 10, the variable can
 structure 22 and the can plate 24 were moved with respect to the fixed can
 structure 20 through the use of an adjustment mechanism 30.
 A vacuum bellows 36 was used to seal the adjustment mechanism 30 from the
 rest of the capacitor 10. The vacuum bellows 36 was sealed to both the
 variable end assembly 12 and the variable can plate 24 so that any volume
 outside the vacuum bellows 36, shown generally as reference numeral 38 in
 FIG. 1, could be evacuated by attaching a vacuum source to one or both cap
 seals 40, 42.
 To facilitate cooling, the capacitor 10 included a water jacket bellows 44.
 The water jacket bellows 44 was disposed between the vacuum bellows 36 and
 the adjustment mechanism 30 and was sealed between the variable can plate
 24 and the variable end assembly 12. To cool the capacitor 10, water was
 circulated through the volume between the vacuum and water jacket bellows
 36, 44 (shown generally as reference numeral 46), via inlet/outlet ports
 50, 52.
 Typically, the vacuum and water jacket bellows 36, 44 were fabricated from
 C510 phosphor bronze and had no perforations or holes therein because
 holes or perforations would either make it impossible to establish the
 vacuum or would allow water to escape from between the bellows 36, 44. As
 shown in FIG. 1, the bellows 36, 44 were convoluted, or corrugated, to
 allow the bellows 36, 44 to flex as the variable can plate 24 was moved.
 The force required to move the can plate 24 was proportional to the product
 of the cross sectional area of vacuum bellows 36 and the pressure
 differential across the vacuum bellows 36. Additionally, the current
 carrying capacity of the capacitor 10 was directly proportional to the
 diameter of the vacuum bellows 36, because the vacuum bellows 36 carried
 the current in the capacitor 10. Accordingly, the more current that the
 capacitor 10 needed to carry, the more force it took to move the can plate
 24 of the capacitor 10.
 During operation, the variable end and fixed end assemblies 12, 14 were
 connected into a circuit requiring capacitance. Current would flow between
 the variable end assembly 12 and the fixed end assembly 14 through the
 bellows 36, 44, which connected the variable end assembly 12 to the
 variable can plate 24. The variable can plate 24 was, in turn,
 capacitively coupled to the fixed end assembly 14, via the fixed and
 variable can structures 20, 22. As the capacitor 10 was operated, water
 was circulated through the volume 46 between the bellows 36, 44, via the
 inlet/outlet ports 50, 52. Additionally, a motor was usually coupled to
 the adjustment mechanism 30 to tune the capacitor 10 by moving the
 variable can plate 24.
 SUMMARY OF THE INVENTION
 The present invention is directed to variable capacitors, and more
 particularly to double-bellows vacuum variable capacitors.
 According to a first aspect, the present invention may include a first
 electrical terminal structure, a second electrical terminal structure, a
 housing and a vacuum bellows disposed in the housing, the vacuum bellows
 having a first diameter, the vacuum bellows and at least a portion of the
 housing defining an interior vacuum chamber having a pressure disposed
 therein that is less than atmospheric pressure. Additionally the present
 invention may include a current-carrying bellows disposed in the housing,
 the current carrying bellows having a second diameter larger than the
 first diameter, the current-carrying bellows comprising a conductive
 material and surrounding the vacuum bellows, the current-carrying bellows
 being disposed in the interior vacuum chamber, the current-carrying
 bellows being conductively coupled to the second electrical terminal
 structure, a fixed-position capacitor structure conductively coupled to
 the first electrical terminal structure and a variable-position capacitor
 structure conductively coupled to the second electrical terminal
 structure, the variable-position capacitor structure being movable
 relative to the fixed-position capacitor structure to generate a variable
 capacitance between the capacitor structures.
 The invention may also include a vacuum bellows fabricated from stainless
 steel and a current-carrying bellows fabricated from a metal having a high
 copper content, such as phosphor bronze. Additionally, the
 current-carrying bellows may be fabricated from a porous material or may
 be perforated.
 According to a second aspect, the present invention may include a first
 electrical terminal structure, a second electrical terminal structure, a
 housing and a substantially air-tight separation member disposed in the
 housing, the substantially air-tight separation member and at least a
 portion of the housing defining an interior vacuum chamber having a
 pressure disposed therein that is less than atmospheric pressure. The
 present invention may also include a perforated current-carrying structure
 disposed in the housing, the current-carrying structure comprising a
 conductive material and being conductively coupled to the second
 electrical terminal structure, a fixed-position capacitor structure
 conductively coupled to the first electrical terminal structure and a
 variable-position capacitor structure conductively coupled to the second
 electrical terminal structure, the variable-position capacitor structure
 being movable relative to the fixed-position capacitor structure to
 generate a variable capacitance between the capacitor structures.
 According to a third aspect, the present invention may include a first
 electrical terminal structure, a second electrical terminal structure, a
 housing and a substantially air-tight separation member disposed in the
 housing, the substantially air-tight separation member having a first
 diameter, the substantially air-tight separation member and at least a
 portion of the housing defining an interior vacuum chamber having a
 pressure disposed therein that is less than atmospheric pressure. The
 present invention may also include a current-carrying structure disposed
 in the housing, the current carrying structure having a second diameter
 larger than the first diameter, the current-carrying structure comprising
 a conductive material and surrounding the substantially air-tight
 separation member, the current-carrying structure being disposed in the
 interior vacuum chamber, the current-carrying structure being conductively
 coupled to the second electrical terminal structure, a fixed-position
 capacitor structure conductively coupled to the first electrical terminal
 structure and a variable-position capacitor structure conductively coupled
 to the second electrical terminal structure, the variable-position
 capacitor structure being movable relative to the fixed-position capacitor
 structure to generate a variable capacitance between the capacitor
 structures.
 The features and advantages of the invention will be apparent to those of
 ordinary skill in the art in view of the detailed description of the
 preferred embodiment, which is made with reference to the drawings, a
 brief description of which is provided below.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS
 FIGS. 2 and 3 illustrate an embodiment of a double-bellows vacuum variable
 capacitor 70, in accordance with the invention, which may be provided with
 a housing including a variable end assembly 72, a fixed end assembly 74
 and a body assembly 76, which may be fabricated from an electrical
 insulator such as ceramic. The variable and fixed end assemblies 72, 74
 may be conductive terminal structures to which other circuitry or
 electrical components may be connected when the capacitor 70 is in
 operation.
 A fixed position can structure 80 and a variable position can structure 82
 are disposed inside the capacitor 70. The can structures 80, 82 may each
 include cylindrical, concentric members arranged such that concentric
 members of the fixed position can structure 80 may mesh or engage with
 concentric members of the variable position can structure 82 to create
 capacitive coupling therebetween. The degree to which the fixed and
 variable position can structures 80, 82 mesh or engage with one another
 dictates the capacitance between the fixed and variable end assemblies 72,
 74. The fixed position can structure 80 may be mounted to the fixed end
 assembly 74, and the variable position can structure 82 may be mounted to
 a variable can plate 84, which may be electrically connected to the
 variable end assembly 72.
 The degree to which the fixed and variable position can structures 80, 82
 engage one another may be controlled by an adjustment mechanism 90, which
 may include a leadscrew 92, an adjust plug 94 and a shaft support plug 96.
 When the leadscrew 92 is turned, by a motor, by hand or by some other
 means, the adjust plug 94 may move axially with respect to the leadscrew
 92 and may, in turn, cause the variable can plate 84 to move with respect
 to the leadscrew 92, thereby changing the degree to which the fixed and
 variable position can structures 80, 82 are spaced or engaged. A cartridge
 bearing 98, which may be retained by a retainer clip 100, may support the
 leadscrew 92 in the variable end assembly 72.
 Referring to FIG. 3, a vacuum bellows 110, or any other substantially
 air-tight separation member may be fabricated from stainless steel or any
 other material able to withstand repeated flexing due to repeated movement
 of the variable can plate 84. The vacuum bellows 110, which may be sealed
 between the variable end assembly 72 and the variable can plate 84, may be
 corrugated to allow the variable can plate 84 to move axially with respect
 to the leadscrew 92, while maintaining the seal between the vacuum bellows
 110 and the variable can plate 84. After the vacuum bellows 110 is
 installed in the capacitor 70, the vacuum bellows 110 may have a pressure
 differential between the inside and the outside thereof.
 The volume inside the vacuum bellows 110, in which the adjustment mechanism
 90 may be disposed, and which is generally represented by reference
 numeral 112, may be at atmospheric pressure and is referred to hereinafter
 as the atmospheric pressure volume 112. The volume outside the vacuum
 bellows 110, which may include the fixed and variable position can
 structures 80, 82, is represented by reference numeral 114 and may be
 referred to hereinafter as a vacuum volume 114 or a vacuum chamber. The
 vacuum volume 114 may have a pressure of -8 torr (mmHg) or any other
 suitable pressure that is lower than that of the atmospheric volume. In
 addition to providing a barrier to isolate the vacuum volume 114 from the
 atmospheric pressure volume 112, the vacuum bellows 110 may provide some
 electrical connectivity between the variable end assembly 72 and the
 variable can plate 84.
 Still referring to FIG. 3, a conductive bellows 116 is disposed around and
 encloses the vacuum bellows 110 and is connected between the variable end
 assembly 72 and the variable can plate 84. The conductive bellows 116 may
 be fabricated from C510 phosphor bronze or any other suitable material
 having similar conductive properties. The conductive bellows 116 may
 electrically connect the variable end assembly 72 to the variable can
 plate 84. The variable can plate 84 may be, in turn, capacitively coupled
 to the fixed end assembly 74, via the fixed and variable position can
 structures 80, 82. As with the vacuum bellows 110, the conductive bellows
 116 may be corrugated to allow the variable can plate 84 to move axially
 with respect to the leadscrew 92.
 The torque required to turn the leadscrew 92 to move the variable can plate
 84 of the capacitor 70 may be directly proportional to the cross sectional
 area of the vacuum bellows 110, due to the pressure diffferential across
 the vacuum bellows 110. Because the current carrying conductive bellows
 116 does not have a pressure differential thereacross, the radius of the
 conductive bellows 116 may be increased to accommodate large currents
 without increasing the force required to move the variable can plate 84 or
 the torque required to turn the leadscrew 92. By disposing the conductive
 bellows 116 within the vacuum volume 114, the current carrying capacity of
 the capacitor 70 is not necessarily proportional to the torque required to
 turn the leadscrew 92 and the capacitor 70 may have a relatively high
 current carrying capacity and may be adjusted using a relatively low
 torque.
 The use of both the vacuum bellows 110 and the conductive bellows 116 also
 allows the material used for each bellows 110, 116 to be optimized for its
 function. For example, because stainless steel can withstand repeated
 flexion without cracking or fracturing, stainless steel may be used as a
 vacuum bellows in a variable vacuum capacitor without losing its vacuum
 seal. Conversely, a highly conductive bellows fabricated from, for
 example, C510 phosphor bronze may not withstand repeated flexion while
 maintaining a vacuum seal. By using the two bellows in combination, the
 highly conductive bellows 116 need not be able to sustain a pressure
 differential between the inside and the outside thereof. In fact, as
 disclosed below with respect to FIG. 4, the conductive bellows 116 may be
 perforated or have slots cut therein, thereby assuring there will be no
 pressure differential between the inside and outside of the conductive
 bellows 116.
 The use of two bellows 110, 116 also minimizes the axial force required to
 move the variable can plate 84 by minimizing the cross sectional area
 having a vacuum thereacross. Because the conductive bellows 116 does not
 have a pressure differential thereacross, the vacuum variable capacitor 70
 may accommodate a relatively large current flow while requiring low torque
 to turn the leadscrew 92. By contrast, a vacuum variable capacitor using a
 single bellows fabricated from stainless steel would require more torque
 to turn the leadscrew 72 as the diameter of the bellows is increased to
 accommodate an increased current flow.
 A lower axial force required to move the variable can plate 84 may result
 in a lower torque required to turn the leadscrew 92 to adjust the axial
 position of the variable can plate 84. Reduced torque and axial force may
 also reduce the wear and the tear on the adjustment mechanism 90 of the
 capacitor 70. Additionally, reduced torque and axial force may allow a
 smaller motor to be used to turn the leadscrew 92. Because of the optimal
 selection of material for both the vacuum bellows 110 and the conductive
 bellows 116 and the reduced axial force required to move the variable can
 plate 84, the use of two bellows, as disclosed herein may lengthen the
 life cycle of a vacuum variable capacitor by as much as 50%.
 Referring now to FIG. 4 the vacuum bellows 110 (or a substantially
 air-tight sealing member) may be constructed from a material such as
 stainless steel having a thickness between approximately 0.006" and 0.008"
 and may include a first end 120 and a second end 122. Each of the first
 and second ends 120, 122 may be silver and/or nickel plated to form good
 seals or contact with the variable end assembly 72 and the variable can
 plate 84. Between the first and second ends 120, 122, the vacuum bellows
 110 may be corrugated and, therefore, may have a plurality of large
 diameter portions 130 and a plurality of small diameter portions 132.
 Although, the vacuum bellows 110 is shown in FIG. 4 as being corrugated,
 the vacuum bellows 110 may not be corrugated in all instances.
 The vacuum bellows 110 may be designed to accommodate variable axial
 distances between the variable end assembly 72 and the variable can plate
 84 during tuning of the capacitor 70. When the variable can plate 84 is
 relatively close to the variable end assembly 74, the small diameter
 portions 132 may axially compress between the large diameter portions 130
 to accommodate the separation between the variable end assembly 72 and the
 variable can plate 84. Conversely, when the variable can plate 84 is
 relatively far from the variable end assembly 72, the vacuum bellows 110
 may axially expand to accommodate the distance. Whether the distance
 between the variable end assembly 72 and the variable can plate 84 is
 large or small, the vacuum bellows 110 may axially expand or contract to
 preserve the pressure differential between its inside and its outside.
 Referring to FIGS. 5 and 6, the conductive bellows 116 (or current-carrying
 structure) may include first and second ends 140, 142, respectively. In a
 similar fashion to the vacuum bellows 110, the conductive bellows 116, may
 include a number of large diameter portions 146 and a number of small
 diameter portions 148. Like their corresponding portions in the vacuum
 bellows 110, these portions may axially expand or compress to accommodate
 the varying axial distance between the variable end assembly 72 and the
 variable can plate 84 as the capacitor 70 is tuned. Although the
 conductive bellows 116 is shown in FIG. 5 as being corrugated, corrugation
 is not necessarily required.
 Unlike the vacuum bellows 110, which may be fabricated from stainless
 steel, the conductive bellows 116 may be fabricated from material between
 approximately 0.005" and 0.007" thick that has a high copper context
 (e.g., C510 phosphor bronze). Suitable materials may also include C102 or
 C103 (oxygen-free copper), C104, C105 or C106 (oxygen-free silver-copper)
 or C150 (zicronium-copper). Such materials may be highly conductive and
 may allow the vacuum variable capacitor 70 to accommodate higher currents
 than otherwise possible without the conductive bellows 116. Additionally,
 as shown in FIGS. 5 and 6, the conductive bellows 110 may include a
 plurality of slots 152. The slots 152 may be equally radially spaced
 around the circumference of the conductive bellows 116 and may be 0.025"
 in depth. The slots 152 ensure that there is no pressure differential
 across the conductive bellows 116. While FIGS. 5 and 6 show slots 152 in
 the conductive bellows 116, it will be readily appreciated by those having
 ordinary skill in the art that any perforations (including slots) in the
 conductive bellows 116 may be used. For example, perforations such as
 holes or punctures may be used in place of, or in addition to the slots
 152. Additionally, a porous material may be selected for use as the
 conductive bellows 116.
 Alternatively, the conductive bellows 116 may not be perforated or porous.
 Rather, the variable end assembly 72 and/or the variable can plate 84 may
 be machined to provide air channels or passages between the inside and the
 outside of the conductive bellows 116. Such air channels or passages
 enable air to pass between the inside and the outside of the conductive
 bellows 116 to ensure that there will be no pressure differential between
 the inside and the outside of the conductive bellows 116. Such passages
 may or may not be used in connection with a perforated conductive bellows
 116. Exemplary passages in the variable end assembly 72 and the variable
 can plate 84 are shown at reference numeral 160 in FIG. 3.
 In operation, the fixed and variable end assemblies 72, 74 of the vacuum
 capacitor 70 may be conductively coupled to circuitry or electrical
 components to provide a variable and adjustable capacitance. After the
 vacuum capacitor 70 is installed, the leadscrew 92 of the adjustment
 mechanism 90 may be turned, via a motor or any other suitable means, to
 adjust the position of the variable can plate 84, which in turn adjusts
 the capacitance of the capacitor 70.
 Numerous additional modifications and alternative embodiments of the
 invention will be apparent to those skilled in the art in view of the
 foregoing description. For example, in addition to stainless steel, the
 vacuum bellows 110 may be fabricated from any other suitable material.
 Further, in addition to C510 phosphor bronze, the conductive bellows 116
 may be fabricated from any other suitable material. This description is to
 be construed as illustrative only, and is for the purpose of teaching
 those skilled in the art the best mode of carrying out the invention. The
 details of the structure and method may be varied substantially without
 departing from the spirit of the invention, and the exclusive use of all
 modifications which come within the scope of the appended claims is
 reserved.