US3783314A

US3783314A - Signal generating mechanism

Info

Publication number: US3783314A
Application number: US00316945A
Authority: US
Inventors: C Kostan
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1972-12-20
Filing date: 1972-12-20
Publication date: 1974-01-01
Anticipated expiration: 1991-01-01
Also published as: DE2361095C2; GB1418239A; JPS4989021A; FR2325293A5; CA985380A; DE2361095A1

Abstract

A signal generating mechanism is disclosed for producing an electrical signal having a frequency proportional to the angular velocity of a rotating shaft. The signal generating mechanism may be used in the distributor for a breakerless ignition system for a multi-cylinder internal combustion engine, in which case the frequency of the alternating signal is equal to the rate at which ignition sparks are to be generated. The signal generating mechanism includes a lower plate and bushing through which the rotating shaft passes. A non-adjustable stator assembly is formed by a hub positioned around the bushing, the hub having an upper plate attached to it on which is affixed a permanent magnet. A stator is positioned on the permanent magnet and has an extending pole-piece that passes through an electrical pickup coil. An armature, or rotor, is formed by a toothed-wheel that is affixed to the rotating shaft. The teeth of the toothed-wheel come into and go out of alignment with the portion of the stator pole-piece that extends through the electrical coil.

Description

United States Patent [191 Kostan Jan. 1, 1974 1 SIGNAL GENERATING MECHANISM Charles C. Kostan, Detroit, Mich.

[73] Assignee: The Ford Motor Company,

Dearborn, Mich.

[22] Filed: Dec. 20, 1972 [21] Appl. No.: 316,945

[75] Inventor:

[52] US. Cl. 310/70, 123/148 [51] Int. Cl. H02k 11/00 [58] Field of Search 310/68 R, 68 C, 69,

[5 6] References Cited UNITED STATES PATENTS 8/1964 Race 310/70 R 2/1966 Tyzack 310/70 R 5/1969 Falge 310/70 R Primary Examiner-J. D. Miller Assistant Examiner-Robert J. Hickey Attorney-Keith L. Zerschling et a1.

[ 5 7 ABSTRACT A signal generating mechanism is disclosed for producing an electrical signal having a frequency proportional to the angular velocity of :a rotating shaft. The signal generating mechanism may be used in the distributor for a breakerless ignition system for a multicylinder internal combustion engine, in which case the frequency of the alternating signal is equal to the rate at which ignition sparks are to be generated. The signal generating mechanism includes a lower plate and bushing through which the rotating shaft passes. A non-adjustable stator assembly is formed by a hub positioned around the bushing, the hub having an upper plate attached to it on which is affixed a permanent magnet. A stator is positioned on the permanent magnet and has an extending pole-piece that passes through an electrical pickup coil. An armature, or rotor, is formed by a toothed-wheel that is affixed to the rotating shaft. The teeth of the toothed-wheel come into and go out of alignment with the portion of the stator pole-piece that extends through the electrical coil.

5 Claims, 7 Drawing Figures PATENTED JAN 1 I974 SHEET 10F 4 PATENTEUJAH nan sum 30; 4 3'783'314 PAIENTEDJAK H974 3.783.314

SHEET HIP 4 1 SIGNAL GENERATING MECHANISM BACKGROUND OF THE INVENTION This invention relates to a signal generating mechanism for producing an alternating electrical signal having a frequency proportional to the angular velocity of a rotating shaft. More particularly, the invention relates to a signal generating mechanism particularly suitable for use in a distributor for a breakerless ignition system for a multi-cylinder internal combustion engine.

Common past and present practice in ignition systems for multi-cylinder internal combustion engines has been to employ a set of breaker points in a distributor to generate sparks as required by the engine. Electronic ignition systems eliminating these breaker points recently have been developed. These electronic breakerless ignition systems require a signal generating mechanism capable of indicating the times or instants at which it is desired to generate sparks in the various engine combustion chambers. A common feature of such signal generating devices is that they produce an electrical signal having a frequency proportional to the.

angular velocity of a rotating shaft, the shaft usually being the shaft in a distributor which rotates at an angular velocity equal to that of the engines cam shaft.

Although various signal generating mechanisms have been proposed for use with multi-cylinder internal combustion engines, the present invention is directed to the type which employs a toothed-wheel affixed to the rotating shaft. The toothed-wheel forms the annature, or rotor, of an alternating signal generator. The stator assembly of the alternating signal generator includes an electrical pickup coil through which a stator pole-piece passes. The teeth of the toothed-wheel come into and go out of alignment with the stator pole-piece to vary the flux produced by a permanent magnet having a magnetic circuit including the stator and its polepiece and the rotating toothed-wheel. This produces an alternating electrical signal across the terminals of the electrical coil. This alternating signal, in turn, may be coupled to a breakerless ignition system capable of producing sparks in relation to the alternating signal, for example, at one or more selected points in the signal waveform.

SUMMARY OF THE INVENTION In accordance with the invention, a signal generating mechanism for producing an alternating electrical signal having a frequency proportional to the angular velocity of a rotating shaft comprises a lower plate that is fixed relative to the rotating shaft. The lower plate has an annular opening therein and a bushing is affixed in the annular opening. The shaft passes through the bushing. A stator assembly is positioned above the lower plate. The stator assembly includes a hub which is positioned around a portion of the bushing affixed to the lower plate. Attached to the hub is an upper plate, parallel with the lower plate, on which is positioned a permanent magnet having a magnetic field that is perpendicular to the upper plate. The stator assembly fur ther includes the stator itself, which is positioned in contact with the permanent magnet, the permanent magnet being located intermediate the stator and the upper plate. The stator has a pole-piece which passes through and supports an electrical coil the axis of which is perpendicular to the shaft axis. An armature, or rotor, is formed by a toothed-wheel that is affixed to the shaft for rotation therewith. The toothed-wheel is concentric with the shaft axis and the teeth of the toothed-wheel are positioned to come into and to go out of alignment with the pole-piece of the stator.

Rotation of the shaft and the toothed-wheel attached to it produces a voltage across the terminals of the electrical coil. This voltage is determined in accordance with the well known equation E= -N dda/dt where E is the electromotive force produced across the terminals of the electrical coil, N is the number of turns in the electrical coil and dldt is the time rate of change of magnetic flux.

The entire stator assembly is non-adjustably secured together, thus, eliminating screw adjustments or the like. Prior art devices have required adjustments of this type to obtain voltage signals of a desired level. The elimination of such adjustments simplifies the man ufacture of the signal generating mechanism and eliminates the possibility of maladjustment during use of the device The invention will be better understood by reference to the detailed description which follows and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view, the section being taken along the line 1-1 of FIG. 2, of an ignition distributor for a multi-cylinder internal combustion engine, the distributor including a signal generating mechanism for producing an alternating electrical signal having a frequency proportional to the angular velocity of the rotatable distributor shaft;

FIG. 2 is a plan view of the distributor of FIG. 1;

FIG. 3 is a sectional view of the distributor, the section being taken along the line 3-3 in FIG. 1, and illustrates a centrifugal weight mechanism for timing advance;

FIG. 4 is an enlarged plan view of the stator assembly used in the signal generating mechanism of the distributor of FIG. 1; 7

FIG. 5 is an enlarged elevational view, partially in section, of the stator assembly of FIG. 4;

FIG. 6 is an enlarged pictorial view of the stator assembly of FIGS. 4 and 5; and

FIG. 7 is an illustration of the alternating signal waveform that may be produced by the signal generating mechanism in the ignition system distributor.

DETAILED DESCRIPTION With particular reference now to FIGS. 1 through 3, there is shown an ignition system distributor 10 for supplying sparks to a multi-cylinder internal combustion engine. The distributor 10 includes a housing 12 having a cylindrical bearing 14 positioned therein. A shaft 16 is rotatably joumalled within the bearing 14. The shaft 16 is driven by a gear 20 that, in use, meshes with another gear (not shown), driven by the internal combustion engine. The shaft 16 has a reduced-diameter portion 18, and both the larger-diameter and reduced diameter portions of the shaft contain grooves for lubrication purposes.

A ferromagnetic sleeve 22 fits over the reduceddiameter portion 18 of the shaft 16. The sleeve 22 is retained 0n the shaft 16 with a wire retainer 24. The sleeve 22 is rotatably mounted on the reduceddiameter portion 18 of the shaft 16, and this rotation relative to the shaft is controlled by a centrifugal advance mechanism of the usual design.

The centrifugal advance mechanism, generally designated by the numeral 26, comprises a plate 28 affixed to the shaft 16 and a plate 30 affixed to the sleeve 22. In the usual manner, the plates 28 and 30 are coupled together by means of springs 32 and 34. In order for the plate 30 and sleeve 22 to rotate about the plate 26 and shaft 16, the force of the springs must be overcome. When the shaft 16 rotates, a pair of weights 36 and 38, pivotally connected to the plate 28, exert a force that acts against that of the springs 32 and 34 and tends to rotate the plate 30 and sleeve 22 with respect to the shaft 16. The magnitude of this force is proportional to the shaft angular velocity. This provides a centrifugal advance in the ignition timing. For the purpose of the present invention, the sleeve 22 may be regarded as a part of the shaft 16 with which it rotates.

The ignition system distributor is shown without the usual cap and high-voltage distribution rotor. It should be understood that these elements would be present in a complete distributor installation. The distributor cap would be of the usual configuration in which a plurality of electrical contacts would be connected by high-voltage leads to spark plugs for the multi-cylinder internal combustion engine. The highvoltage distribution rotor would be secured to the sleeve 22 and would rotate with it to distribute voltage from the high-voltage side of an ignition coil to the electrical leads to the various spark plugs.

The ignition system distributor 10 includes a signal generating mechanism of the invention, generally designated by the numeral 40, for producing an alternating electrical signal having a frequency proportional to the angular velocity of the rotating shaft 16 and sleeve 22. Preferably, the alternating signal generating mechanism has a frequency equal to the rate at which sparks are to be generated by the ignition system. Typically, this frequency is equal to the speed of the shaft in revolutions per second times the number of cylinders in the internal combustion engine.

The signal generating mechanism 40 illustrated in the drawings is intended for an S-cylinder engine. It comprises a toothed-wheel 42, hereinafter referred to as a rotor, and a stator assembly 44. The rotor 42 is attached to the sleeve 22 for rotation therewith by means of a roll pin 46. Also, the position of the rotor 42 on the axis of the sleeve 22 is determined by a rotor ring retainer 48. The rotor 42 may be made from a sintered high-permeability material and has eight radially extending teeth 50, these corresponding in number to the number of cylinders in the engine. Preferably, to achieve a desirable signal, the teeth extend radially outward a distance of from about five to times the dimension of the air-gap 51 between the rotor and stator. Also, it is preferred that the air-gap 51 have a dimension ranging from a maximum of 0.062 in. to a minimum of 0.023 in., the midpoint of this range being most satisfactory. In this connection, it should be noted that similar signal generating mechanisms of the prior art were limited to air-gap dimensions ranging from about 0.006 in. to 0.001 in. and required adjustment means to maintain this dimension.

The stator assembly 44 is located and positioned by means of a lower plate 52 that has an annular opening therein in which an annular bushing 54, made from a ferromagnetic material, is fixed. The shaft 16 and as sociated sleeve 22 pass through the bushing 54 and rotate freely within it. The lower plate 52 is positioned perpendicularly to the axis of the shaft and is secured to the distributor housing 12 by a plurality of screws 56. The bushing 54 has a portion which extends above the lower plate 52. This upwardly extending portion of bushing 54 has lubrication grooves 53 on its radially outward exterior that are filled with grease during the assembly of the mechanism.

The stator assembly 44 may best be seen in the enlarged views of FIGS. 4 through 6. It includes an annular hub 58, also made from a ferromagnetic material, that is positioned around the bushing 54 so that the stator assembly may rotate about the bushing. A wire ring retainer 60 (FIG. 1) is used to hold the stator assembly 44 in position on the bushing 54. The stator assembly 44 comprises the hub 58, an irregularly shaped, ferromagnetic upper plate 62 positioned parallel to the lower plate 52, a permanent magnet 64, a stator 66 and an electrical pickup coil 68.

The upper plate 62 of the stator assembly is securely staked in locations 70 to the hub 58. A pin 72 extends upwardly from the upper plate 62. In FIG. 2, it may be seen that a rod 74 of a vacuum motor 76, used in addition to the centrifugal timing mechanism 26 for igniti0n-timing advance and retard, is pivotally connected to the pin 72 on the upper plate 62. The rod 74 controls rotation of the hub 58, and the remainder of the stator assembly 44, about the bushing 54.

The permanent magnet 64 has an arcuate shape. Preferably, it is made from a ferrite material, such as sintered ceramic strontium ferrite. It is preferred that the permanent magnet 64 be magnetized at assembly of the signal generating mechanism. Its magnetic field should be oriented such that its direction through the magnet is perpendicular to the upper plate 62. Accordingly, the upper surface 78 of the permanent magnet 64 may be made a south pole and the lower surface, that is, the surface in contact with the upper plate 62, may be made a north pole. In such case, an imaginary line interconnecting these poles is perpendicular to the upper plate 62. The permanent magnet 64 preferably is magnetized to an induction of about 3600 gauss.

The stator 66 and the permanent magnet 64 are nonadjustably secured to the upper plate 62 of the stator assembly with rivets 80 and 82. To avoid impact damage to the permanent magnet, a spin-type riveting process may be used. The stator 66 is made from a ferrogmagnetic material and preferably is made from a sintered-iron, highpermeability, low-carbon material hav ing a minimum density of about 6.8 grams per cubic centimeter. The stator 66 has a generally arcuate shape but also has a pole-piece 86 of rectangular crosssection extending through the central region of the electrical coil 68 and radially toward the axis of the shaft 16. The teeth 50 of the rotor 42 come into and go out of alignment with the pole-piece 86 of stator 66 as the shaft 16 and its sleeve 22 rotate. It should be noted that the axis of rotation of the stator assembly 44 about the bushing 54 is the same as the axis of shaft 16, and, thus, the air gap between the rotor teeth and pole-piece does not change during ignition timing changes.

The electrical pickup coil 68 is supported by and surrounds the stator pole-piece 86 and preferably is formed from a helical winding of from about 4,000 to 4,075 turns of gauge number 39 (American Wire Gauge) copper wire and may have a resistance of from 600 to 700 ohms. Preferably, the electrical pickup coil 68 is encapsulated by molding in a suitable insulating material. The pickup coil 68 has leads 9t) and 92 connected to the electrical winding terminals that extend out of the coil encapsulation material. Also, the pickup coil has a hook 94 attached to it. A spring clip 96 is connected to the hook 94 and to the stator pole-piece 86 to hold the electrical pickup coil on the pole-piece 86 in a secure manner. The axis of the coil 68 is perpendicular to the axis of the shaft 16..

In FIG. 2, it may be seen that the wires 90 and 92 enter and pass through a grommet 98, preferably made from a molded polyvinyl chloride material, to a connector 100. The grommet 98 contains a third wire 102 that is connected by one of the screws 56 to the lower plate 52 to establish an electrical ground. The Wire 102 extends out of the grommet 98 and into the connector 100. The wire 102 is a ground lead.

In operation, the rotor 42 preferably turns in the counterclockwise direction indicated in FIG. 2 by the arrow on one of its teeth. As the rotor 42 rotates, the magnetic flux changes to produce a time varying voltage across the electrical leads 90 and 92 of the electrical coil 68. This voltage or EMF is produced in accordance with the aforementioned equation E= -N dqb/dt.

The magnetic circuit is illustrated by the dotted line 104 in FIG. 6. This magnetic circuit includes the stator 66 and its pole-piece 86, the toothed rotor 42 and shaft 16 (not shown in FIG. 6), and the upper plate 62 that is in contact with the bottom surface of the permanent magnet 64.

With reference in particular to FIGS. 2 and 7, wherein one of the teeth 50 of the rotor 42 is in alignment with the pole-piece 86 of the stator 66, let it be assumed that the rotor is turning in a counterclockwise direction. At this instant with the tooth in alignment with the pole-piece 86, there is no change in flux with respect to time and the generated voltage is zero. This corresponds to the time t 0 of FIG. '7. As the tooth moves out of alignment with the pole-piece 86 of the electrical coil, a voltage is generated across the coil leads 90 and 92 due to the change in magnetic flux. This flux change, and consequently the generated voltage, reaches a maximum and then decreases to zero at a time t= T/2 corresponding to the time when the polepiece 86 is midway between two of the teeth 50 of the rotor 42. Continued rotation of the rotor 42 produces a change in magnetic flux that generates a voltage of opposite polarity. This voltage increases until it once again becomes zero at time t= T when the next tooth 50 is in alignment with the pole-piece 86. Thus, an alternating signal is generated having a frequency proportional to the angular velocity of the rotating shaft of the distributor.

Based upon the foregoing description of the invention, what is claimed is:

l. A signal generating mechanism for producing an alternating electrical signal having a frequency proportional to the angular velocity of a rotating shaft, said signal generating mechanism comprising, in combination: a lower plate fixed relative to said rotating shaft, said lower plate having an opening therein; an annular bushing affixed in said lower plate opening, said bushing extending above said lower plate, said shaft passing through said bushing; a stator assembly, said stator assembly including an annular hub positioned around the portion of said bushing extending above said lower plate, said hub being rotatable about said bushing, an upper plate parallel to said lower plate and affixed to said hub, a permanent magnet attached to said upper plate, said magnet being polarized to produce a mag netic field having a direction through said magnet that is perpendicular to said upper plate, a stator positioned on said permanent magnet, said stator having a polepiece, and an electrical coil positioned on said stator pole-piece; and a rotor attached to said shaft for rotation therewith, said rotor having at least one tooth positioned to come into and go out of alignment with said stator pole-piece; whereby rotation of said rotor produces an alternating electrical signal across the terminals of said electrical coil having a frequency proportional to the angular velocity of said rotating shaft.

2. A signal generating mechanism for producing an alternating electrical signal having a frequency proportional to the angular velocity of a rotating shaft, said signal generating mechanism comprising, in combina tion: a lower plate fixed relative to said rotating shaft, said lower plate having an opening therein; an annular ferromagnetic bushing affixed in said lower plate opening, said bushing extending above said lower plate, said shaft passing through said bushing; a stator assembly, said stator assembly including an annular ferromagnetic hub positioned around the portion of said bushing extending above said lower plate, said hub being rotatable about said bushing, a ferromagnetic upper plate parallel to said lower plate and affixed to said hub, a permanent magnet polarized to produce a magnetic field having a direction through said magnet that is perpendicular to said upper plate, a ferromagnetic stator positioned on said permanent magnet, said stator and said permanent magnet being non-adjustably secured to said upper plate, said stator having a pole-piece extending radially towards said shaft axis, and an electrical coil positioned on said pole-piece, said pole-piece extending through said electrical coil; and a ferromag netic rotor affixed to said shaft for rotation therewith, said rotor having a plurality of teeth positioned to come into and to go out of alignment with said pole-piece of said stator during rotation of said shaft; whereby rotation of said shaft produces an alternating signal across the terminals of said electrical coil.

3. The signal generating mechanism of claim 2 wherein a gap exists between a tooth of said rotor that is in alignment with said pole-piece, said gap being within the dimensional range from 0.062 of an inch to 0.023 of an inch.

4. A signal generating mechanism for producing an alternating electrical signal, which comprises, in combination: a housing; a shaft rotatably journalled in said housing; a lower plate secured to said housing, said lower plate having an opening therein and being positioned perpendicular to said shaft; a ferromagnetic bushing affixed in said lower plate opening, said bushing extending above said lower plate and having lubrication grooves on the radially exterior portion thereof, said shaft passing through said bushing; a ferromagnetic sleeve on the portion of said shaft passing through said bushing, said sleeve being rotatable about said shaft; a centrifugal mechanism interconnecting said sleeve and said shaft for controlling rotation of said sleeve about said shaft; a toothed ferromagnetic rotor secured to said sleeve for rotation therewith, the teeth of said rotor extending radially outward from the axis of said shaft; a stator assembly, said stator assembly ineluding an annular ferromagnetic hub positioned around the portion of said bushing extending above said lower plate, said hub being rotatable about said bushing, a ferromagnetic upper plate parallel to said lower plate and affixed to said hub for rotation therewith, an arcuately-shaped permanent magnet positioned on said upper plate, said permanent magnet having a north pole and a south pole, an imaginary line interconnecting said north and south poles being perpendicular to said upper plate, said permanent magnet being made from a ceramic ferrite material, a ferromagnetic stator positioned on said permanent magnet, said stator and permanent magnet being non-adjustably secured to said upper plate, said stator having a polepiece positioned to permit said rotor teeth to come into and to go out of alignment with it, and an electrical coil surrounding said pole-piece of said stator, said electrical coil being helically wound and having an axis perpendicular to the axis of said shaft; and means connected to said upper plate of said stator assembly for controlling the rotation of said stator assembly about said bushing.

5. The signal generating mechanism of claim 4 wherein a gap exists between a tooth of said rotor that is in alignment with said pole-piece, said gap being within the dimensional range from 0.062 of an inch to 0.023 of an inch.

Claims

1. A signal generating mechanism for producing an alternating electrical signal having a frequency proportional to the angular velocity of a rotating shaft, said signal generating mechanism comprising, in combination: a lower plate fixed relative to said rotating shaft, said lower plate having an opening therein; an annular bushing affixed in said lower plate opening, said bushing extending above said lower plate, said shaft passing through said bushing; a stator assembly, said stator assembly including an annular hub positioned around the portion of said bushing extending above said lower plate, said hub being rotatable about said bushing, an upper plate parallel to said lower plate and affixed to said hub, a permanent magnet attached to said upper plate, said magnet being polarized to produce a magnetic field having a direction through said magnet that is perpendicular to said upper plate, a stator positioned on said permanent magnet, said stator having a pole-piece, and an electrical coil positioned on said stator pole-piece; and a rotor attached to said shaft for rotation therewith, said rotor having at least one tooth positioned to come into and go out of alignment with said stator pole-piece; whereby rotation of said rotor produces an alternating electrical signal across the terminals of said electrical coil having a frequency proportional to the angular velocity of said rotating shaft.

2. A signal generating mechanism for producing an alternating electrical signal having a frequency proportional to the angular velocity of a rotating shaft, said signal generating mechanism comprising, in combination: a lower plate fixed relative to said rotatiNg shaft, said lower plate having an opening therein; an annular ferromagnetic bushing affixed in said lower plate opening, said bushing extending above said lower plate, said shaft passing through said bushing; a stator assembly, said stator assembly including an annular ferromagnetic hub positioned around the portion of said bushing extending above said lower plate, said hub being rotatable about said bushing, a ferromagnetic upper plate parallel to said lower plate and affixed to said hub, a permanent magnet polarized to produce a magnetic field having a direction through said magnet that is perpendicular to said upper plate, a ferromagnetic stator positioned on said permanent magnet, said stator and said permanent magnet being non-adjustably secured to said upper plate, said stator having a pole-piece extending radially towards said shaft axis, and an electrical coil positioned on said pole-piece, said pole-piece extending through said electrical coil; and a ferromagnetic rotor affixed to said shaft for rotation therewith, said rotor having a plurality of teeth positioned to come into and to go out of alignment with said pole-piece of said stator during rotation of said shaft; whereby rotation of said shaft produces an alternating signal across the terminals of said electrical coil.

4. A signal generating mechanism for producing an alternating electrical signal, which comprises, in combination: a housing; a shaft rotatably journalled in said housing; a lower plate secured to said housing, said lower plate having an opening therein and being positioned perpendicular to said shaft; a ferromagnetic bushing affixed in said lower plate opening, said bushing extending above said lower plate and having lubrication grooves on the radially exterior portion thereof, said shaft passing through said bushing; a ferromagnetic sleeve on the portion of said shaft passing through said bushing, said sleeve being rotatable about said shaft; a centrifugal mechanism interconnecting said sleeve and said shaft for controlling rotation of said sleeve about said shaft; a toothed ferromagnetic rotor secured to said sleeve for rotation therewith, the teeth of said rotor extending radially outward from the axis of said shaft; a stator assembly, said stator assembly including an annular ferromagnetic hub positioned around the portion of said bushing extending above said lower plate, said hub being rotatable about said bushing, a ferromagnetic upper plate parallel to said lower plate and affixed to said hub for rotation therewith, an arcuately-shaped permanent magnet positioned on said upper plate, said permanent magnet having a north pole and a south pole, an imaginary line interconnecting said north and south poles being perpendicular to said upper plate, said permanent magnet being made from a ceramic ferrite material, a ferromagnetic stator positioned on said permanent magnet, said stator and permanent magnet being non-adjustably secured to said upper plate, said stator having a pole-piece positioned to permit said rotor teeth to come into and to go out of alignment with it, and an electrical coil surrounding said pole-piece of said stator, said electrical coil being helically wound and having an axis perpendicular to the axis of said shaft; and means connected to said upper plate of said stator assembly for controlling the rotation of said stator assembly about said bushing.