Electromagnetically Driven Fluid Compressing Apparatus

Abstract

A pneumatic supply includes a permanent magnet mounted to the upper end of a driven lever. An encircling rectangular member has an opening through which the lever extends to locate the permanent magnet generally centrally of the encircling magnetic frame. Coils are wound and oriented on the second frame to establish a pair of alternating current fields within the frame with corresponding polarity with respect to the permanent magnet to create oppositely directed driving forces on the permanent magnet in synchronism with the alternate polarities, thereby causing the permanent magnet to oscillate. Pole shoes are provided on opposite sides of the permanent magnet to concentrate and direct the flux. A pair of diaphragm type air compressors, each of which includes an operating piston aligned with and located to the opposite sides of the lever. The piston members are connected to each other and to the lever for simultaneous opposite corresponding directional movement. The reciprocating motion of the electromagnetic drive unit results in the opposite actuation of the compressors; the outputs of which are interconnected to a common pressure load.

Description

BACKGROUND OF THE INVENTION

This invention relates to an electromagnetically driven fluid compressing apparatus and particularly to such an apparatus which is especially adapted to form a small air compressing unit for incorporation in commercial air conditioning and process control systems.

Conditioning and process control systems may be of an all pneumatic, all electric or a combination pneumatic and electric variety; depending upon the particular design requirements. Purely electrical systems have certain distinct advantages from the design of suitable sensors for detecting variables such as temperature, pressure, humidity and the like. Further, electrical signals can be conveniently transmitted to operating and actuating control devices. However, pneumatic systems have been widely employed because of the high power characteristic of pneumatic operators at relatively low cost and because the overall control systems are generally somewhat simpler, more reliable and less costly than a comparable electrical design. This is particularly true because in electrical systems, it is difficult to modulate accurately the significant electrical power levels required to produce the necessary mechanical output. Thus, the electrical output will normally drive a motor device which, in turn, is converted into a mechanical output through a motor driven gear train or a motor driven hydraulic pump operator.

Although pneumatic systems have generally predominated in the commercial control field particularly for institutional and commercial air conditioning systems and the like, all electric systems have more recently found increasing applicability, particularly in relatively smaller systems where the additional expense associated with the electrical operators is only slightly greater than the cost associated with the necessity of a relatively large single air compressor. Thus, the compressors must be capable of producing pressures of the order of 20 pounds per square inch (psi). The same advantage does not apply in larger overall systems where a generally similar compressors cost is relatively a much smaller percentage of the total cost.

As pointed out in applicant's issued U.S. Pat. No. 3,411,704, a very substantial need exists for a small, compact and efficient electromagnetic fluid compressing apparatus which can be constructed at a minimum cost such as to permit application in relatively small environmental conditioning and process control systems. The above patent discloses a small, electromagnetically driven compressor. An aquarium air pump, manufactured and sold by Metaframe of Maywood, New Jersey, includes a reed mounted permanent magnet aligned with and interacting with the pole ends defined by an alternating-current powered E-shaped magnetic unit, with the reed mounted permanent magnet coupled to actuate an air compressor. Although this latter pump is relatively quiet and also compact, the output is of the order of four psi which is not sufficient for the usual industrial and commercial control system application which needs at least twenty psi.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a relatively compact and highly efficient fluid compressing apparatus which is relatively simple and will operate without undue noise such that the apparatus can be applied with presently known pneumatic operators for environmental and process control systems to produce a pneumatically controlled actuator in response to an essentially all electric control.

Generally, in accordance with one aspect of the present invention a pair of fluid compressing means each include a reciprocating drive element which is mounted for reciprocal movement and is operative to establish a fluid output pressure signal in accordance with the stroke and frequency of the reciprocation of the movable input means. An electromagnetic drive means includes a pair of magnetic units, one of which is coupled to the pair of reciprocating drive elements to establish opposite operative movement. One magnetic unit establishes a unidirectional field interacting with the alternating current magnetic field of the other unit to establish the reciprocal movement of the one unit. The pair of fluid compressor means coupled to the common drive structure for opposite movement provides increased compressor efficiency. Thus, some amount of the compressed fluid is retained at the end of each compression stroke and expands as the inlet stroke is established. This provides an energy feedback to drive the opposite compressor.

Either the unidirectional field or the alternating field can of course be controlled to thereby control the operation of the fluid compressor means.

In a preferred and a particularly novel construction of the present invention, the one magnetic unit includes a permanent magnet mounted to the upper end of the driven element, the lower end of such element being connected to a resilient means to permit pivotal movement. A highly satisfactory support is a leaf spring member. The second magnetic unit includes an encircling magnetic frame which may conveniently be formed as a rectangular member having an opening through which the driven element extends to locate the permanent magnet generally centrally of the encircling magnetic frame. The magnetic frame and the magnet are generally coplanar. The magnetic frame is preferably formed with a slightly inwardly projecting portion centrally of the base of the frame which is closely spaced and aligned with the one end of the permanent magnet. The portions of the magnetic frame to the opposite sides of the permanent magnet are provided with coils which are interconnected to an alternating current power supply means. The coil means are wound and oriented on the frame to establish a pair of alternating current fields within the frame with corresponding polarity with respect to the permanent magnet. Pole shoes are located on opposite pole faces of the permanent magnet to concentrate and direct the flux. Thus, the coil means will establish alternate polarity fields across the permanent magnet means which extend normal to such field. This will result in oppositely directed driving forces on the permanent magnet in synchronism with the alternate polarities, thereby causing the permanent magnet to oscillate.

The driven element is coupled to the pair of air or fluid compressors, each of which includes an operating element, such as a piston member, aligned with and located to the opposite sides of the driven element and in the path of the driven element. Each piston furthermore forms a part of an individual fluid compressing means of a diaphragm type having an inlet valve assembly within the piston unit for transferring air or other fluid into or through the piston unit and the diaphragm unit into a compression chamber during a suction stroke. The piston members are connected to each other or directly to the driven element for simultaneous opposite corresponding directional movement. During the opposite or compression stroke the compressed fluid is transferred through an outlet valve assembly through a valved discharge passageway means. The reciprocating motion of the electromagnetic drive unit results in the opposite actuation of the compressors; the outputs of which may be interconnected to a common output connection means to maintain a continuous output signal proportional to the frequency and stroke of the electromagnetic drive unit.

The mechanical resilient system is preferably constructed to have a resonant movement corresponding to the frequency of the exciting power to the coils with the compressor output at approximately the center of the pressure range in order to produce the maximum work output. Thus, the vibrating mass in combination with the spring effect due to the resilient mounting and the expansion or suction stroke of the compressor establishes a natural frequency for the compressor. Although the weight and resilient mounting effect can be relatively constant, the compressor spring action introduces a variable rate spring related to the output pressure level of the compressor. This results in a variable natural frequency which should be considered in the design of the fluid compressing apparatus in accordance with the present invention.

Applicant has found that the driven element is preferably coupled to the pistons by a coupling member which extends through the driven element and is secured to the pistons. Resilient means are disposed between the driven element and the facing portions of the piston unit such that the driving force is transmitted to the piston units through the resilient means. This produces a minimal amount of noise as the result of the movement of the driven element with respect to and with the pistons.

The compressing units are preferably adjustable mounted to allow accurate centering of the two compressors with respect to the driven elements such that the oscillation of the latter establishes a corresponding actuation of the compressor piston elements and thereby establishes the length of stroke for operation of the corresponding compressors.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing furnished herewith illustrates a preferred construction of the present invention in which the above advantages and features are clearly disclosed as well as others that will readily be understood from the following description.

In the drawing:

FIG. 1 is a front elevational view of the electromagnetically driven fluid compressing apparatus constructed in accordance with the present invention, with parts broken away and sectioned to more clearly disclose details of the construction;

FIG. 2 is a side elevational view of FIGS. 1;

FIG. 3 is an enlarged front view with parts broken away and sectioned to show details of the construction;

FIG. 4 is a diagrammatic view of the vibrating assembly shown in FIGS. 1 - 3; and

FIG. 5 is a graphical illustration of the pressure volume characteristics of a fluid compressing apparatus.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring to the drawing and particularly to FIG. 1, an electromagnetically driven fluid compressing apparatus constructed in accordance with the present invention is illustrated including a pair of corresponding air compressors 1 and 1' interconnected to actuate a suitable pneumatic actuator 2. The compressors 1 and 1' are coupled to a driven element 3 forming a coupling element between the compressors and an electromagnetic drive means 4. A pair of synchronized alternating current supplies 5 and 6 are connected to actuate the drive means 4 as more fully developed hereinafter to produce oscillation of the element 3 with a corresponding continuous opposite actuation of the compressors 1 and 1' to maintain a predetermined and related output to the pneumatic actuator 2. A common support 7 is provided upon which the compressors 1 and 1' are mounted immediately below the electromagnetically driven means 4.

In the illustrated embodiment of the invention the compressors 1 and 1' are correspondingly constructed and consequently the air compressor 1 will be described in detail with the corresponding elements of the compressor 1' identified by corresponding primed numbers.

The air compressor 1 includes a base body portion 8 which is secured to the right lower edge or side of the support 7 by a suitable clamping bracket 9. The bracket 9 is bolted or otherwise secured to the support plate 7 and allows lateral positioning of the compressor 1 with respect to the normal standby position of the driven element 3. The compressor 1' is similarly mounted by a bracket 9' to allow corresponding positioning of the two compressors with respect to element 3.

The base portion 8 is provided with an output chamber 10 centrally formed thereof and with an outlet passageway 11 connected in common with the outlet passageway 11' of compressor 1' to the pneumatic actuator 2. The chamber 10 is closed by an intermediate body member or portion 12. An outer clamping ring 13 clamps a diaphragm 14 against the outer face of the body 12. The three body portions 8, 12 and 13 are clamped together in any suitable means; shown as a simple plurality of U-shaped clamping springs 15 which project over the outer periphery of the body portions.

The central body portion 12 is provided with a recess 16 which is closed by the diaphragm 14 and defines a compression chamber. The diaphragm 14 is a suitable flexible member such as a conventional rubber on fabric diaphragm, with the central portion thereof bonded or otherwise affixed to an adjacent end of a piston 17. The movement of the piston 17 results in the expansion and contraction of the compression chamber. The piston 17 includes a plurality of L-shaped inlet passageways 18 which extend from the periphery inwardly and then axially toward the diaphragm. A cup valve 19 is disposed within a valve chamber 20 formed in the face of the piston 17 immediately adjacent the diaphragm 14 and is biased to close the inlet passageway 18. The valve member 19 is formed of a suitable soft rubber such as silastic or "Buna N." Thus the reciprocation of the piston 17 results in the alternate establishment of suction and compression strokes to produce timed spaced output pulses at line 11.

The diaphragm 14, in turn, is provided with openings 20a establishing communication between the compression chamber 16 and the piston chamber 20. Thus when the piston 17 is moved outwardly or to the right, as shown in FIG. 1, the valve member 19 is free to open thereby admitting air into the chamber 20 and the compression chamber 16. When the piston moves during a compression stroke in the opposite direction, however, the reduction of chamber 16 compresses the air within the chamber 16 resulting in a build-up of pressure which is fed back through the chamber 20 to the exterior of the valve 19 and causes it to close. The air within the chamber 16 will, consequently, be compressed during the compression stroke.

A plurality of coaxially arranged outlet passageways 21 are provided in the base of the wall 12 and provide communication between the compression chamber 16 and the outlet chamber 10. An outer valve cap 22 overlies the passageways 21 with chamber 10. During the compression stroke, the air within the chamber 16 will be compressed to a level sufficient to overcome the holding force on the valve member 22, causing it to move outwardly and allowing discharge of the compressed air outwardly through the chamber 10 and passageway 11 to the actuator 2.

The air compressor 1' is similarly located to the opposite side of the element 3 and thus operates in alternate synchronism with the compressor 1, such that its output pulse occurs during the suction stroke of the piston 1. In this manner a continuous output pressure signal is supplied to the pneumatic actuator 2.

The pistons 17 and 17' are coupled to each other and to the driven element 3 in the illustrated embodiment as follows. A pin 23 extends through an opening 24 in the element 3 in coaxial alignment with the pistons 17 and 17'. Suitable recesses or central openings are provided within the ends of the pistons 17 and 17' with the pin 23 secured therein to produce a rigid interconnection between the pistons 17 and 17'. The opposed ends of the pistons 17 and 17' are spaced slightly from the driven element 3 with an O-ring member 25 located between the piston 17 and element 3 and an O-ring member 26 similarly located between the piston 17' and the element 3. The O-ring members 25 and 26 are formed of a suitable relatively soft rubber and establish a resilient contact or engagement of the corresponding piston with the driven element 3 to establish a significantly quiet operation as the lever 3 moves to drive the pistons 17 and 17'.

The driven element 3 is formed as a flat lever extending downwardly from the pin 23 to the outer portion of the support plate 7. A leaf spring 27 is riveted or otherwise secured to the end of the adjacent lever 3 and extends outwardly in a corresponding plane therefrom. The spring 27 is located between a pair of clamping blocks 29, one of which is secured to the support plate 7 and the other of which is releasably forced against the oposite face of the spring 27 by a suitable clamping screw 30.

The opposite end of the lever 3 projects outwardly in the opposite direction to the drive means 4. A magnet 31, shown as a permanent magnet, is integrally formed with, or may be separately formed and suitably secured to the outer end of the lever 3. Magnet 31 extends outwardly from lever 3 with the poles to the opposite side of the plane through the lever 3.

In the illustrated embodiment of the invention, the magnet 31 is a generally rectangular block-type permanent magnet, with the north pole formed to the right side of the magnet and the south pole to the left side thereof, as viewed in FIGS. 1 and 3. The magnet 31 is located within a generally rectangular magnetic frame 32, the one branch of which is formed with an opening through which the lever 3 extends. The opposite side legs or portions of the frame 32 are provided with coils 33 and 34, respectively, which coils are connected to the alternating current power supplies 5 and 6.

The frame 32 is secured to the support 7 by suitable clamping bolts 35 and 36 with the frame encircling the magnet 31 and furthermore with the magnet 31 located generally centrally of the frame 32. The magnet 31 is provided with pole shoes 37 and 38 on opposite pole faces. The upper end of the magnet 31 and the pole shoes 37 and 38 are spaced inwardly from the base portion of the frame 32 which, in turn, is provided with an inwardly projecting extension 39 terminating in slightly spaced relation to the magnet 31. The width of the pole 39 generally corresponds to the width of the magnet 31, such that the pole shoes 37 and 38 project outwardly or laterally of the pole 39. The opposite side of the magnetic frame 32 is provided with the opening through which lever 3 passes and thus defines a pair of pole arms or ends 40 and 41. The pole arms 40 and 41 are spaced from each other generally in accordance with the total width of the magnet 31 plus the width of the two pole shoes 37 and 38, to locate their ends generally in alignment with the outer faces of the pole shoes 37 and 38, as shown in FIG. 1. Furthermore, the ends are curved as at 42 to extend outwardly and laterally away from the corresponding pole shoe. The sources 5 and 6 are operated with a predetermined phase relationship in accordance with the orientation of the coils 33 and 34 to establish oppositely directed fluxes in the magnetic frame 32. The coils 33 and 34, therefore, provide a corresponding directional flux within the frame 32 at the position of the magnet 31.

During one-half cycle, the pole 39 will be at a relative north polarity with respect to the arms 40 and 41, with a flux as diagrammatically shown by the flux lines 43. A repelling force is established between the pole 39 and the right edge or north pole of the magnet 31. Simultaneously, there is an attractive magnetic force between the pole 39 and the shoe 37 connected to the south pole of the magnet 31. This tends to move the upper or outermost end of the element 31 and lever 3 to the right as viewed in FIG. 1. Simultaneously, the end of arm 40 defines a south magnetic pole which interacts with the south magnet shoe 37 with a repulsive force thereby tending to also move the magnet 31 to the right. The arm 41, which is also a south pole, attracts the north pole shoe 38 of the magnet 31, thereby establishing a further force moving the magnet 31 to the right. With the selected configurations of frame 32 receptive to magnet 31 and its pole shoes 37 and 38 the described widths, maximum drive is obtained from the magnetic field. As a result, the lever 3 will pivot to the right about the leaf spring 27. When the outputs of the alternating current power supplies 5 and 6 are reversed, the magnetic field 43 reverses thereby generating an effective north pole at the end of arms 40 and 41 and a south pole at the element 39. This will reverse the force interaction with the magnet 31 causing the magnet 31 to move in the opposite direction.

The alternating current power supplied to the coils 33 and 34 results in a reciprocation of the lever 3 in synchronism and under the control of the energization of the coils 33 and 34. The magnet 31 will thus oscillate between the positions with the north and south shoes generally aligned with the north and south poles of the magnetic frame 32 as shown in FIG. 3. Applicant has found that the combination of the electromagnetic drive means and the air compressing means, particularly, as shown in the drawings, provides a compact and efficient fluid operator which is responsive to an electrical input.

The force relationships of the compressing apparatus can be diagrammatically illustrated in accordance with FIG. 4, wherein the weight 44 of the several moving parts is shown by the block connected to the pivot point 45 by a weightless lever 46. The flexure of the hinge or pivot connection is shown as a pair of opposite acting springs 47 acting on the connecting element and the "force-rate" on the lever by the compressors 1 and 1' during the respective expansion strokes is shown as a pair of opposite acting springs 48 coupled to the lever. The natural frequency of the system is defined by the known equation:

f = (1/2.pi.) .sqroot.c/J

where c = torsional stiffness of lever due to flexure hinge springs, and the "force-rate" springs on the lever from the compressors during the expansion stroke.

where J = the moment of inertia of the weight of the moving parts with the radius of gyration r about the pivot point as

J = .SIGMA.Wr.sup.2

For any given structure, the spring effect of the illustrated hinge structure is fixed. The spring effect of the compressor, however, varies with the output pressure. Typical pressure (P) versus volume (V) characteristics of a fluid compressor is shown in FIG. 5 for varying output pressures.

If the output or load pressure is equal to the maximum, attainable by the compressor 1 or 1', the full line trace 49 results, with essentially the complete work of compression being returned to the system. The compressor thus functions like a spring.

As the output or load pressure decreases, the compression stroke is reduced and connected by the exhaust portion of the cycle, shown by the dotted line 50, to the expansion portion of the cycle wherein the fluid in the clearance volume expands, shown by the dotted line 51, and joins with the suction part of the stroke to complete the cycle. The area of the curve under the expansion line represents the work returned to the system. As this varies with the output or load pressure, the compressor acts as a variable rate spring in the system. As a result, the natural frequency of the system changes with load.

Since for a given design, the spring effect of the compressor and/or the flexure hinge can be varied, the natural compressor frequency can be predetermined for any output pressure of the compressor.

A highest overall efficiency of a compressor results when the natural frequency of the mechanical system is the same as the frequency of the driving force, which in the United States is generally 60 hz.

Further, the highest efficiency is desirably obtained when the compressor work output is maximum which is approximately at the mid-point between the maximum and minimum output pressure of the compressor, such as point C.

If the design is such as to require maximum pressure output, the various parameters would be changed so that the natural frequency of 60 hz occurs at point D and maximum pressure is obtained. Thus, the system can be designed to produce the desired operating characteristic by proper selection and arrangement of the several components.

The apparatus of the present invention may employ relatively inexpensive components which can be readily mass produced. The diaphragm type construction with the rubber valves and resilient coupling between the compressors and the driving element 3 provide for a relatively quiet compressor operation. The apparatus can, therefore, be employed as a part of a local or small air conditioning control apparatus.

The present invention thus provides a system whereby the electrical power can be accurately modulated to produce a proportional mechanical output which is interconnected through the pneumatic compressors to produce a fluidic operator at a minimum of expense.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims, particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

Claims

I claim:

1. An electromagnetically driven fluid compressing apparatus comprising,

an electromagnetic drive means having a reciprocating drive element including a first magnetic unit having a first pole means and a second pole means, a second magnetic unit including an encircling magnetic frame disposed adjacent one end of said first pole means and said second pole means of the first magnetic unit and having a fourth and a fifth pole means at the opposite ends of the frame aligned with the second end of the said first and second pole means of the first magnetic unit, a first magnetic source establishing a unidirectional magnetic flux and connected to one of said first and second magnetic units to establish a unidirectional magnetic field, a second magnetic source establishing and connected to the other of said first and second magnetic units to establish an alternating magnetic field, said unidirectional magnetic field and said alternating magnetic field having a common plane with the fields in superimposed relationship and with the reciprocating drive element mounted to move in said plane, said alternating magnetic field creating opposite polarity fields interacting with said unidirectional field to produce corresponding opposite movement of said drive element,

a compressor means having a working chamber means and a movable input means mounted for reciprocating movement in said working chamber means and coupled to said drive element.

2. The electromagnetically driven fluid compressing apparatus of claim 1 wherein said first magnetic unit includes a permanent magnet defining said first magnetic source and said pole means includes pole pieces secured to the opposite pole faces of said magnet.

3. The compressing apparatus of claim 1 wherein said drive element includes a leaf spring support unit connected to one end of said first magnetic unit, and

said compressor means includes a pair of compressors each having a working chamber and having said input means in the form of members coupled for opposite working movement to said reciprocating drive element whereby one compressor is working during one movement of the drive element and the second compressor is working during the opposite movement of the drive element.

4. The electromagnetically driven fluid compressing apparatus of claim 1 wherein said magnetic frame includes an opening in one side defining a pair of pole ends with a space therebetween through which said driven element extends, said pole ends defining said pair of second pole means and being generally aligned with the pole means of said first magnetic unit.

5. The electromagnetically driven fluid compressing apparatus of claim 4 wherein said frame opposite from said pole ends includes an inward pole projection aligned with the opposite end of the first magnetic unit.

6. The electromagnetically driven fluid compressing apparatus of claim 5 having coil means wound on said frame to the opposite sides of said pole ends and said pole projection to establish said alternating magnetic field.

7. The electromagnetically driven fluid compressing apparatus of claim 1 wherein said first magnetic unit includes a rectangular permanent magnet defining said first magnetic source and said pole means includes pole pieces secured to the opposite long edges of said magnet, and said driven element includes a leaf spring support unit connected to one end of said magnetic unit and operatively connected to said input element.

8. The electromagnetically driven fluid compressing apparatus of claim 7 wherein said second magnetic unit includes a generally rectangular open frame with an opening in one side defining said pair of second pole means with a space therebetween through which said driven element extends, said frame encircling said permanent magnet with said pole ends adjacent the end of the magnet and the pole pieces of the magnet, an opposite frame portion of the frame opposite said pair of second pole means defining said first pole means of said second magnetic unit and being adjacent and aligned with the opposite end of the permanent magnet, one being substantially of the width of said permanent magnet, and coil means wound on said frame to the opposite sides of said pair of pole means and said opposite frame portion to establish a pair of alternating fields in said frame with said coil means oriented to establish corresponding fields on the opposite end of said permanent magnet.

9. The electromagnetically driven fluid compressing apparatus of claim 1 wherein said air compressor means includes a first and a second correspondingly constructed air compressor, each of said air compressors including a diaphragm unit defining one wall of said working chamber and coupled to a piston, said piston including an inlet valve means to introduce air into the compression chamber in response to a suction stroke of the piston, an outlet valve means actuated in response to the compression stroke of the piston to transfer compressed air from said chamber, mounting means supporting said first and second air compressors to the opposite sides of said driven element with said pistons in the path of said driven element, and said input means interconnecting said pistons to each other and to said drive elements.

10. The electromagnetically driven fluid compressing apparatus of claim 1 wherein said first magnetic unit includes a rectangular permanent magnet defining said first magnetic source and said pole means includes pole pieces secured to the opposite long edges of said magnet, said driven element includes plate-like arm with a leaf spring secured to one end and the opposite end of the arm connected to one end of said magnetic unit,

said second magnetic unit includes a generally rectangular open frame with an opening in one side defining a pair of pole ends with a space therebetween through which said driven element extends, said frame encircling said permanent magnet with said pole ends aligned with the pole pieces of the magnet, the frame opposite said pole ends including an inward projection aligned with the opposite end of the permanent magnet, coil means wound on said frame to the opposite sides of said pole ends and said opposite frame portion to establish said alternating field in said frame, and

said compressor means includes a pair of compressors having corresponding rectilinearly movable input elements disposed one each to the opposite sides of said arm for opposite actuation of said compressors.

11. The electromagnetically driven fluid compressing apparatus of claim 1 wherein

said compressor means is constructed to establish a preselected output pressure range,

said first magnetic unit is a permanent magnet and said second magnetic unit is an electromagnet, a resilient means connected to said permanent magnet and said drive element, said electromagnet being selected to operate with a given power frequency corresponding to the natural frequency of the resilient means at essentially the center of said output pressure range.