JPH02225803A - Electric current-pressure convertor and automatic regulation nozzle therefor - Google Patents

Abstract

PURPOSE: To achieve reduction of cost, mass production and high efficiency of the operation by attracting magnetic fluid in chambers sealed by flexible diaphragms by an electromagnetic means, deforming the flexible diaphragms, and adjusting air pressure in an air supplying means adjacent to the diaphragm. CONSTITUTION: Magnetic members 20, 21 are attached to a base 19 made of low-magnetic materials, and magnetic fluid 30 is housed in chambers 14, 16 connected to each other by a capillary tube 15 and having lower opening ends sealed by diaphragms 13, 17. A magnetic pole piece 11 on the upper end, a nozzle 10 and an air feeding line 24 are provided on a cylindrical magnetic member 22 arranged in contact with the magnetic member 21, and a coil 23 is arranged. Accordingly, the magnetic fluid 30 is attracted to the magnetic pole piece 11 according to a change of the current of the coil 23 to deform the diaphragm 13, and air pressure in the air line 24 can be adjusted. Therefore, this device has low cost, is suitable for mass production, has no individual adjustment, and can be efficiently operated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to the regulation of air pressure in response to electrical signals, and more particularly to a converter for converting an electric current into a corresponding pressure in a system using compressed air. It is. Also,
The present invention relates to nozzles useful for current-to-pressure transducers, particularly self-adjusting nozzle structures.

(Conventional technology) Compressed air is immune to electrical interference.

It is used in many systems to control machinery because it is safe even in explosive environments. Compressed air is widely used, for example, to control control valves and other mechanical devices in industrial systems. If compressed air is used in the system, a sensor is provided that generates a -1R small current, for example in the range 4-20 mA. This electrical current is used to generate compressed air at a corresponding pressure and provide a sufficient amount of pressurized air to accomplish the desired mechanical work. In some systems,
Conversion from current to corresponding pressure is achieved using a current-to-pressure transducer, which can regulate the pressure of a small volume of air, the volume of which is controlled by the action of a standard air amplifier. is amplified by In conventional current-to-pressure converters, a nozzle is used that directs compressed air to the atmosphere to an extent determined by the proximity of the flapper valve to the nozzle orifice. Flapper valves are typically mounted on a rotating suspension member and rotated by magnetic force generated by an electromagnet. As the flapper rotates toward the nozzle, less air escapes to the atmosphere.

A nozzle is a tapered or medium-thin tube attached to a tube, hose or outlet of a pressure chamber. The purpose of the nozzle is to efficiently convert the pressure present within the fluid into velocity. The nozzle directs pressure into a pipe or hose adjacent to it.

Currently known nozzles used to control air flow typically have an end with an outer diameter slightly larger than the inner diameter. 0.035 inch, with an inner diameter of 0°026 inch.

Current-to-pressure transducers using this type of nozzle usually include a flapper, which is a pivotable paddle-like part;
or incorporates a diaphragm that alters the flow of air through the nozzle. In any case, it is necessary that a good seal be provided at the end of the transducer from which there is a strong flow of air or fluid. In order to achieve the desired good seal, the flapper or diaphragm must be precisely aligned with the plane perpendicular to the axis of the nozzle; if this alignment is not correct, the flapper or diaphragm will first will hit the edges and not proceed further to achieve an effective complete seal.

U.S. Pat. No. 4,579,137 discloses a magnetic button 22
There is a description of a transducer comprising a membrane 22 on which A is located. As the magnetic button is attracted to the column 12B, the size of the air gap between the button 22A and the magnetic portion 14 in which the aperture 24 is formed increases. The total gap between the button 22A and the magnetic part 14 and between the button 22A and the column 12B is kept constant. Button 22A and magnetic part 1
An undesired air gap between the two ends of the air gap 4 will have a detrimental effect on the efficiency of the magnetic circuit and, as a result, will impair the normal control of air pressure in the device of the invention.

US Pat. No. 4,053,952 describes the use of magnetic fluids to occlude tubes placed within the human body. The device of the invention does not use any moving parts in the mechanism that selectively serve as valves or pumps.

The present invention provides that in the presence of the magnetic field of the permanent magnet, the pressure of the ferrofluid forces the membrane against the pole piece, thus blocking the flow path (column 5, lines 34-37). The device acts as an on-off valve or pump,
It does not act to change the size of the air gap in response to an incoming current that changes the strength of the magnetic field in selected regions of the ferrofluid. In the present invention, a backup magnetic field is required to counteract the magnetic field from the permanent magnet (line 41!I, lines 11-17), and this permanent magnet is It surrounds the part of the tube that is to be affected by the pressure generated within. This patent does not teach how to move thin films through the use of ferrofluid, nor does an essentially uniform magnetic field acting on the entire volume of ferrofluid (as shown in Figure 2) create pressure within the fluid. 6 The device of the present invention, which is supposed to cause the movement of a thin film, requires the compression or expansion of a fluid, or the creation of a vacuum region within a chamber containing the magnetic fluid, thereby enabling the device to operate. do. As shown in FIG. 2 of this patent, for the membrane 38 to move, either the volume of the ferrofluid 28 would have to increase or a vacuum region would have to be created within the chamber containing the fluid 28. The compressibility of the fluid is such that volume changes are negligible, and the vacuum region will be interrupted by the ambient or atmospheric pressure through which the contents of the reservoir are vented.

The atmospheric pressure imparted to the magnetic fluid through the membrane 38 is always greater than any pressure that can be generated by magnetic means, so the membranes described in this patent do not move.

(Problems to be Solved by the Invention) However, devices according to the prior art as described above are formed as delicate mechanical assemblies, require various adjustments, and are relatively expensive to manufacture.

Particularly in planar positioning relative to the axis of the nozzle, it is relatively difficult to assemble the flapper or diaphragm in the desired orthogonal configuration.

There is therefore a great desire to use simple current-to-pressure transducers that do not require separate mechanical adjustments and are easy to manufacture at low cost.

That is, a first object of the invention is to increase the air pressure in the air supply line such that the pressure difference between the air pressure in the air supply line and the ambient air pressure varies substantially linearly with the supply current. 6 below, the term "pressure" will be used to mean the pressure relative to the surrounding environment.

The object of the present invention is to provide a converter that is relatively efficient and has similar characteristics, so that no individual adjustment is required.

Another object of the present invention is to provide a current-to-pressure transducer that can be mass-produced at low cost.

It is a further object of the invention to provide a current-to-pressure transducer that is immune to electrical interference and safe even in explosive environments.

Another object of the invention is to provide a nozzle, pole piece structure that provides an effective and secure seal at the end of the nozzle where the fluid or air flow exits into the surrounding environment.

Yet another object of the present invention is to provide an integral nozzle, pole piece construction that is easier to manufacture than previously known nozzles of this type.

SUMMARY OF THE INVENTION In accordance with the present invention, a current-to-pressure transducer is coupled to a magnetic fluid in contact with a flexible membrane or diaphragm. The diaphragm moves toward the nozzle in response to forces acting on the magnetic fluid, thereby narrowing the space through which air exiting the nozzle passes toward the surrounding environment. The diaphragm moves according to the incoming current. Electrical current is supplied to a coil wrapped around a magnetic circuit that moves the flexible membrane toward the nozzle with the magnetic fluid. Movement of the membrane toward the nozzle reduces the air flow from the nozzle and increases the pressure of the air in the air supply line. In one embodiment, pressure sensing means and electronic feedback are used to achieve the desired linearity between the pressure in the line supplying air to the nozzle and the incoming current.

Another feature of the invention is that the self-adjusting nozzle and pole piece structures useful in current-to-pressure transducers are formed from an integral bead of magnetic material.

The nozzle, pole piece structure is formed such that the respective ends of the nozzle and pole piece facing the sealing element, in this case a flexible diaphragm, are coplanar. Also, a slot is provided at the end of the integral bead facing the diaphragm to allow excess air to escape.

OPERATIONS AND EFFECTS OF THE INVENTION The novel current-to-pressure transducer disclosed herein uses a magnetic fluid that cooperates with a flexible diaphragm juxtaposed in close proximity to the nozzle of an air line. The converter is suitable for mass production at low cost, is efficient in operation, and does not require individual adjustments. In addition, the nozzle structure according to the present invention has the nozzle and the magnetic pole piece integrally laid out, making it easy to manufacture and process, and also has an automatic adjustment characteristic that compensates for any inclination or misalignment of the diaphragm. We are prepared.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. The current-pressure transducer includes an air supply line 24 with a nozzle 10 at one end;
It is connected to an air amplifier 32 at the other end.

A substrate 19 of magnetically soft material is joined to the magnetic member 20 by five screws 37 or other suitable means. Magnetic member 21 is attached to member 20 by screws 38 or other attachment means. The cylindrical magnetic member 22 is in contact with the magnetic member 21, and a portion thereof is in contact with the magnetic member 21.
located within the opening. As shown in FIG. 4, a magnetic pole piece 11 is provided at the upper end of the tubular member 22. As shown in FIG.

A coil 23 is wound around a portion of the magnetic member 22. The substrate 19, the magnetic members 20, 21, 22, the gap between the pole piece 11 and the magnetic fluid 30 in the chamber 14, and the magnetic fluid 30 in the chamber 14 form a magnetic circuit. When a current is passed through the coil 23, the magnitude of the magnetic flux in the magnetic pole piece 11 changes according to the magnitude of the electrical signal.

Two chambers 14, 16 are formed in the substrate 19, which are connected by a capillary tube 15. In keeping with the invention, a ferrofluid 30, which is a colloidal suspension of magnetic particles in a non-magnetic carrier, for example a ferrofluid. Nashua, New Hampshire (Trademark of Ferrofluidex Corporation) in chamber 14.16 and capillary 15.
It can be put inside. The ferrofluid 30 can be any synthetic, non-colloidal material that is incapable of supporting shear forces and exhibits magnetic properties.
4.16 with magnetic fluid 30.

According to the invention, a flexible membrane or diaphragm 13.1
7 are located at the lower open ends of the chambers 14 and 16, respectively, and seal the lower ends of the chambers to accommodate the magnetic fluid 30 therein. The visible membranes 13,17 are held by non-magnetic holding members or rings 25 that abut them. The member 25 is secured to the substrate 19 at its exposed surface by screws or other suitable means.

In operation, air amplifier 32 supplies compressed air to nozzle 10 through supply line 24 .

This air passes through the gap between the diaphragm 13 and the surface of the nozzle 10. Relief holes 12 or other suitable means are provided in the upper part of the magnetic material 22, as shown in FIG. 4, to prevent undesired pressure build-up between the pole piece 11 and the diaphragm 13.

An air pressure sensor 26 senses the pressure of compressed air passing through the air supply line 24 and provides a signal indicative of the pressure value. This signal is provided to electronic feedback circuit 27, which also receives an incoming current through conductor 34. The incoming current and the pressure-indicating signal are compared in circuit 27, and a current representative of this comparison is applied to coil 23. Electronic feedback circuit 27 regulates the actual current flowing through coil 23 so that the pressure within air line 24 is substantially linearly related to the incoming current. During operation, the incoming current keeps the coil in an excited state, so that the pole piece 11 distributes magnetic flux in the area near the diaphragm 13 as a result. The magnitude of the magnetic flux emitted from the magnetic pole piece 11 changes according to changes in the current supplied to the coil 23. Magnetic fluid 3 in chamber 14
0 is attracted towards the pole piece 11 and the diaphragm 13 deforms to an extent directly related to the magnitude of the current supplied to the coil 23. As the diaphragm 13 deforms and partially moves towards the pole piece 11, the space between the diaphragm 13 and the nozzle 10 is reduced. As a result, the pressure of the air within the air line 24 supplying air to the nozzle 10 increases. During the deformation of diaphragm 13 caused by the movement of ferrofluid 30 toward pole piece 11 , the volume of ferrofluid displaced within chamber 14 associated with the movement of diaphragm 13 is transferred from chamber 16 to chamber 14 . Supplied. Diaphragm 1
7 moves inwardly within chamber 16 in an equal but opposite direction to diaphragm 13.

In FIG. 2, air supply line 24 containing nozzle 10
is located below chamber 16. The coil 23 and the associated magnetic members 20, 21, 31, substrate 19 and pole piece 11 remain associated with the chamber 14 for cooperative action with the diaphragm 13. coil 2
3 causes the diaphragm 13 to deform towards the pole piece 11 and the volume of ferrofluid transferred from the chamber 16 to the chamber 14 causes the diaphragm 17 to move towards the nozzle 1
Move it away from 0. As a result, the air pressure in line 24 decreases. In this embodiment shown in FIG. 2, the air line 24 is made of a non-magnetic material such as aluminum, or alternatively is magnetically insulated from the magnetic circuit including the coil 23, particularly the magnetic member 21.

A feature of the invention is that it is insensitive to forces due to gravity or acceleration. Since the magnetic fluid 30 is relatively incompressible, the diaphragms 13,17 move evenly in opposite directions. In embodiments where the diaphragms are coplanar, the transducer is insensitive to forces acting perpendicular to the plane of the diaphragms. The transducer is relatively insensitive to forces applied perpendicular to the plane of the drawing and a line through the center of the chamber, regardless of whether the diaphragms are in the same plane. Also, the viscous damping associated with movement of fluid 30 through capillary tube 15 makes the transducer insensitive to shocks in either direction. This attenuation is due to the increase in the viscosity of the magnetic fluid 30 and the inductivity of the capillary tube 15.
tanee) decreases. Damping can also be used to limit the high frequency response of the transducer.

As in another embodiment of the invention shown in FIG. 3, a further increase in sensitivity is achieved by affixing a component 28 of a solid magnetically soft material, such as iron, to the center of the diaphragm 3. This is achieved by Element 28 is disposed within the ferrofluid of chamber 14 and has a higher saturation magnetization than the ferrofluid. Element 28 also stiffens the central portion of the diaphragm. To maintain shock resistance, the component 29 is located in the chamber 16 and attached to the diaphragm 17.

Element 29 can be substantially identical to element 28 or can be a non-magnetic material of suitable size and shape to achieve the desired insensitivity.

FIG. 5 is a curve showing the change in pressure (pounds per square inch) as a function of current (mA). In the examples of the present invention, a 450 Gauss, 400 centivoise (Cρ) ferrofluid was used. Element 28 was a 378 inch diameter, 3/16 inch long ingot of steel that was adhered to diaphragm 13 using RTV silicone sealant. The air supply pressure is I♂bond/square inch. It should be understood that these parameters, materials and dimensions are exemplary and the invention is not limited thereby.

In an alternative approach, the transducer consists of a single chamber and a single flexible diaphragm. In such cases, a space is provided above the level of the ferrofluid to allow movement of the diaphragm.

In other approaches, nozzle 10 is made of magnetic material and also functions as pole piece 11.

In this approach, the external coaxial member 11 and aperture 12 shown in FIG. 4 may be eliminated.

Another feature of the invention is the construction of the nozzle and magnetic pole piece tR, which is made of magnetic material such as Car-penter.
ter) is formed from a rod 40 made of high magnetic permeability "49" alloy. Rod 40, in this particular embodiment,
It is approximately 3716 inches in diameter. As shown in FIG. 6, the magnetic rod 40 has a functional magnetic pole piece 41 formed at its slotted end. The slots 43 are for the escape of excess air and are formed by forming associated deep circumferential grooves between the individual escape openings 12 and the elements 10.11 for communicating with these openings, as shown in FIG. It is relatively easy to machine using a sawtooth or slot cutter. The magnetic pole piece 41 cooperates with the current flowing through the electric coil (not shown) of the electromagnetic circuit to form a magnetic flux. Applying current to the coil causes the magnetic fluid 30 to move, which in turn deforms the flexible diaphragm as described above, thereby controlling the flow of air through the nozzle.

Rod 40 has a threaded portion 42 for engagement with a threaded cap 44 of a housing assembly as shown in FIG. The rod 40 also has a hexagonal section 47 at its end near the threaded section 42 for turning the rod, thereby adjusting the height of the nozzle relative to the diaphragm for proper operation, and locking it by a nut 68. .

FIG. 7 is an end view of the nozzle, pole piece structure 40 from the slotted end. The rod 40 has one or more longitudinal slots 43 that define relatively shallow grooves 4 formed in the end of the rod 40.
6 and extends inwardly until reaching at least the outer diameter of 6. These slots 43 serve the same purpose as the openings 12 shown in FIG. 4 to allow excess air to escape, but are easier to machine and fabricate than the lateral through-holes. The well 46 may be omitted if the slot 43 extends inwardly into the vicinity of the narrowed passage 49.

As shown in FIGS. 8A and 8B, the through-bore 48 is
Formed within the nozzle tube 40 to allow air to pass through and escape. The nozzle tube 40 can be tapered at the end facing the diaphragm to form a narrow passage 49 at the end of the bore 48 . Narrowed portion 49 of inner hole 48 (eighth
Figures A and 8B) reduce the volume of air escaping from the nozzle at a given pressure. narrowed part 4
The flow rate of air from 9 is regulated by position 1 of diaphragm 13, which is controlled by the action of a magnetic fluid in response to the current supplied to the coil of the electromagnetic circuit.

The exploded view of FIG. 9 shows the main housing 50 for the integral nozzle, pole piece structure, which is made of soft iron. The diaphragms 52,54 are separated by soft iron spacers 56 and are separated by a ferrofluid chamber 58.
．． 60 is formed. O-ring seals 62, 64 are provided in each chamber. An aluminum threaded retaining ring 66 is attached to the diaphragm 5 for coupling with the spacer 56.
It is located adjacent to 4. A lock nut 67 is positioned toward the retaining ring 66, and four cap screws 70 are attached to the diaphragms 52, 54, the spacer 56, and the retaining ring 66.
is coupled to the main housing 50. A second nozzle (not shown) is connected to the diaphragm 54 as described in the related application.
It can be screwed onto the retaining ring 66 for cooperative action.

At the other end of the housing 50, a threaded element 44 is provided as a soft iron cap with internal threads for engaging the nozzle and is secured with a lock nut 68 using four Allen socket cap screws 72. It is joined to the main housing 50.

FIG. 10 shows the assembled unit, with a notch 74 provided in the housing 50 to permit electrical connection to the coil of the electromagnetic circuit and escape of excess air.

Because the nozzle is machined from a single magnetic rod and the magnetic pole piece is integrated, the end of the nozzle tube 40 and the magnetic piece 4
It is substantially on the same plane as the terminal of No. 1. When an electromagnetic force is applied to the upper surface of the diaphragm 13 by the magnetic fluid 30, the lower surface of the diaphragm has the same shape as the magnetic pole and the space 41. Since the pole piece 41 and the ends of the nozzle tube 40 are substantially coplanar, the diaphragm will provide a perfect seal against the face of the nozzle.

According to existing designs, the torque applied to the flapper valve acts through a point further from the nozzle than the point of contact between the flapper valve and the nozzle 10, so that if the flapper valve does not contact the nozzle at right angles, Any torque applied would only bend the flapper valve, making the imperfect seal even worse. According to the nozzle and magnetic pole piece structure disclosed herein, the force applied to the diaphragm is applied to the first contact point between the end of the magnetic pole piece 41 having a larger diameter and the diaphragm 13, and the end of the nozzle tube that is in the same plane. so that any inclination of the diaphragm 13 is automatically adjusted. The unitary nozzle, pole piece structure is also easier to fabricate because it has slots 43 formed at the ends of the rod structure instead of the lateral openings used in conventional nozzle assemblies.
and the desired air escape is possible. Such lateral openings require difficult methods to machine deep grooves between the nozzle 10 and the pole piece 11, or the nozzle 1
0 and pole piece 11 would require separate machining, which would make it difficult to assemble these parts to achieve the desired coplanarity.

[Brief explanation of the drawing]

FIG. 1 is a sectional side view of a current-to-pressure transducer according to the invention, FIG. 2 is a side sectional view of an alternative embodiment of the current-to-pressure transducer according to the invention, and FIG. 4 is an enlarged partial isometric view showing the relationship of the pole piece 11 to the nozzle 10 and nozzle air supply line 24 used in the transducer of the present invention; FIG. 5 is a current-pressure Graphs showing representative curves plotting pressure versus current in the absence of electronic feedback to help explain the operation of the transducer; FIG. 6 is a side view of a nozzle, pole piece according to the invention; FIG. An end view of the nozzle shown in FIG. 6, a terminal having a slot with a magnetic pole piece structure, and FIG.
FIG. 8B is an enlarged partial cross-sectional view taken along line A-A' of FIG. 7; FIG. 9 is an enlarged partial cross-sectional view taken along line B-B' of FIG. 7; FIG. Exploded View, FIG. 10 is an isometric view showing an assembled housing surrounding the nozzle, pole piece structure of the present invention. Explanation of symbols 10...Nozzle, 11, 4]...Magnetic pole piece, 1
3.17...Flexible diaphragm, 14.16...
Chamber, 15... Communication means (capillary tube), 22... Magnetic tubular element (cylindrical magnetic member), 23. Electromagnetic means (electric coil), 24... Air supply means (tube), 25... Holding means (ring), 26... Pressure sensing means (air pressure sensor), 27... Electronic feedback circuit, 28.
Magnetic piece, 30... Magnetic fluid, 40... Nozzle,
Magnetic pole piece integral structure (rod), 42... Threaded element (threaded part), 43... Slot, 44... Housing cap, 46... Groove, 47... Rotating means (hexagonal part), 48 ...Inner holes for air (nozzle pipe), 4...constriction, 50...housing, 74...opening.入 υ? = [iE, t E 蓼 O緘 6 ＼ ode be seal period V - section a30 bu size size size the

Claims (18)

[Claims] (1) at least one chamber 14 having an open end, a magnetic fluid 30 contained within the chamber, a flexible diaphragm 13 sealing the open end of the chamber, and disposed in close proximity to the diaphragm; Air supply means 2 for supplying air
4 is disposed in close proximity to the diaphragm and applies an incoming current to attract the magnetic fluid and deform the diaphragm toward the air supply means, thereby increasing the air pressure within the air supply means. a current-pressure transducer comprising electromagnetic means 23 for regulating. 2. A current-to-pressure converter according to claim 1, wherein said air supply means and said electromagnetic means are both located in close proximity to said diaphragm. 3. A current-to-pressure transducer according to claim 1, wherein said electromagnetic means comprises a magnetic tubular element 22 and an electric coil 23 wound around said element. 4. A current according to claim 3, wherein the air supply means comprises a tube coaxially arranged within the magnetic tubular element and a nozzle 10 arranged in close proximity to the diaphragm at one end of the tube. - Pressure transducer. (5) The current-to-pressure transducer according to claim 1, including a magnetic piece (28) located in the chamber adjacent to the diaphragm. (6) The current-pressure transducer according to claim 1, wherein a space is provided within the magnetic fluid. 7. A pressure sensing means (26) and an electronic feedback circuit (27) for limiting current to said electromagnetic means, whereby the pressure is substantially linearly related to the incoming current. The current-pressure transducer described in . (8) The current-pressure converter according to claim 1, further comprising holding means 25 for positioning and holding the diaphragm at the open end of the chamber. (9) A means for supplying air flow, first and second chambers 14 and 16 each having an open end, a means 15 for communicating the two chambers, and a magnetic fluid disposed in the same chamber and in the communication means. 3
0, and first and second flexible diaphragms 13 and 17, respectively located relative to the open end for containing the same fluid in the chamber, are energized to cooperate with the magnetic fluid. electromagnetic means 23 for deforming selected one of said diaphragms and thereby changing the air pressure within said supply means;
A current-pressure transducer consisting of. (10) the electromagnetic means is arranged to cooperate with the magnetic fluid in one of the chambers to deform a selected one of the diaphragms, and the air supply means cooperate with the other of the diaphragms; 10. A current-to-pressure transducer according to claim 9, wherein the current-to-pressure transducer is arranged to move. 11. The current-to-pressure transducer of claim 9, wherein the diaphragms are coplanar. (12) A nozzle used in a current-pressure transducer, an integral structure of a magnetic pole piece, comprising a longitudinal rod 40 made of magnetic material and a central portion of the rod for passing air received from the air supply. a nozzle tube 48 formed in the nozzle tube, one end of the rod forming a magnetic pole piece 41, and an end of the magnetic pole piece being substantially flush with the end of the nozzle tube; Integrated structure. 13. The structure of claim 12, further comprising a longitudinal slot formed in said one end of said rod to allow air to escape. (14) Claim 1 comprising a groove 46 surrounding the nozzle pipe, and the slot communicating with the groove.
The structure described in 3. (15) an element 42 forming a thread located on a portion of the rod proximate to the other end of the rod and a housing cap 44 engaging with the element; 13. A structure as claimed in claim 12, including means integrally formed with the rod for rotating the thread-forming element. (16) The nozzle tube surrounds the air bore 48 and is tapered at one end to form a constricted portion 49 in the air bore, thereby varying the pressure of the air flow. 13. The structure of claim 12, comprising: (17) The structure according to claim 12, including a housing 50 that houses the nozzle and the magnetic pole piece. 18. The structure of claim 17 including an opening 74 formed in the housing to allow access for electrical circuit connections and escape of excess air.