US3652955A

US3652955A - Electromechanical oscillator using electret coupling

Info

Publication number: US3652955A
Application number: US59653A
Authority: US
Inventors: Richard E Cruger; Wilmer C Anderson; David Earls; Robert W Windebank
Original assignee: General Time Corp
Current assignee: General Time Corp
Priority date: 1970-07-30
Filing date: 1970-07-30
Publication date: 1972-03-28
Anticipated expiration: 1989-03-28

Abstract

An electrically driven oscillator device is provided with one or more electrets for establishing electrostatic coupling between a mechanical oscillating element and an electrical drive. The mechanical oscillating element can comprise a reed, a tuning fork, a pendulum, or a hair-spring balance wheel. The electrical drive can be an AC voltage source or an electronic feedback circuit. In electrically driven devices, electrets can be used in both the pickup and driving circuits to eliminate hysteresis. Alternatively, in order to isolate the pickup circuit from the driving circuit, electrets can be used in only one of these circuits and magnetic arrangements can be used in the other.

Description

States ateni Cruger et a1.

[ 51 Mar. 28, N72

[54] ELECTROMECHANICAL OSCILLATOR USING ELECTRET COUPLING I [73] Assignee: General Time Corporation, Phoenix, Ariz.

[22] Filed: July 30, 1970 [21] Appl. No.: 59,653

[52] U.S.Cl .331/1 16M,58/23AO,58/23TF,

[51] Int. Cl. ..H03b 5/30 [58] Field of Search ..331/ll6M,154,156;58/23,

58/23 A, 23 A0, 23 TF, 2s A; 84/409; 318/128, 1. 3 12 3 2 52 17 13, ZL 193112211 1 E [56] References Cited UNITED STATES PATENTS 3,243,951 4/1966 Kawakami 58/23 v 3,118,022 1/1964 Sessler et a1. ..179/181 X 3,515,914 6/1970 Steinemann ..58/23 TF 3,350,667 10/1967 Shreve ..333/71 2,292,790 8/1942 Millar ..331/156 Primary Examiner-Roy Lake Assistant Examiner-Siegfried H. Grimm Attorney-Pennie, Edmonds, Morton, Taylor and Adams [5 7] ABSTRACT An electrically driven oscillator device is provided with one or more electrets for establishing electrostatic coupling between a mechanical oscillating element and an electrical drive. The mechanical oscillating element can comprise a reed, a tuning fork, a pendulum, or a hair-spring balance wheel. The electrical drive can be an AC voltage source or an electronic feedback circuit. 1n electrically driven devices, electrets can be used in both the pickup and driving circuits to eliminate hysteresis. Alternatively, in order to isolate the pickup circuit from the driving circuit, electrets can be used in only one of these circuits and magnetic arrangements can be used in the other.

14 Claims, 12 Drawing Figures PATENTED MR 2 8 1912 SHEET 1 OF 3 FIG 2 22| zoz $5 2 219 I 2|o 220 208 206 am 5.5. 55- 207 0 rl 0 I 0 L IMF-4% w-aoo 222% ms 302 2n AMPLIFIER RI 2|4 INVENTORS RICHARD CRUGER BY WILMER C. ANDERSON ROBERT W. WINDEBANK DAVID EARLS ATTORNFYS PATENTED RZBIHIZ 3,652,955

'SHEET 2 BF 3 2" AMPLIFIER DRAIN SUBSTRATE SOURCE .JI- T 400 FIG. 4C

507 INVIZNTORS 0 RICHARD CRUGER 8 ARR? II IIIIISA IPIIK B B 508 Y DAVID RLS M, I 2 2 g; I

503 50! 509B ATTORNEYS PATENTEDHAR28 r972 3,652,955

sum 3 OF 3 v AMPLIFIER VOLTAGE 7 AMPLIFIER THRESHOLD A VipTAGE A ANGLE OF CLOCKWISE ROTATION VOLTAGE AMPLIFIER THRESHOLD F G. VOLTAGE ANGLE OF COUNTER- CLOCKWISE ROTATION INVENTORS RICHARD CRUGER WILMER C. ANDERSON BY ROBERT W. WINDEBANK DAVID EARLS ATTORNEYS ELECTROMECIIANICAL OSCILLATOR USING ELECTRET COUPLING BACKGROUND OF THE INVENTION The present invention relates to an electrically driven oscillatory device which utilizes one or more electrets to establish electrostatic coupling between a mechanical oscillating element and an electrical drive.

Electrically driven oscillatory devices are useful in a variety of applications such as timekeeping devices and high-Q oscillators. Such devices typically comprise a mechanical element capable of periodic oscillation which is magnetically coupled to an electrical drive by interacting electromagnets and permanent magnets. The oscillating element can comprise any one of a number of known mechanical elements, such as a reed (U.S. Pat. No. 2,036,917 issued to M. P. Favre-Bull, Mar. 8, 1935), a tuning fork (British Pat. No. 155,854 issued to W. H. Eccles et al., Jan. 6, 1921), a pendulum (French Pat. No. 1,092,411 published Apr. 21, 1955), or a hairspring balance wheel (U.S. Pat. No. 2,769,946 issued to H. D. Brailsford, Nov. 6, 1956). The electrical drive can comprise an AC source, a transistor amplifier, or vacuum-tube amplifier.

A typical AC driven oscillatory motor is illustrated by the F avre-Bull device which comprises, in essence, a spring reed fixed at one end and a permanent magnet attached at the other end. The reed is mounted so that one pole of the magnet is disposed between the two poles of a curved electromagnet. When an AC current is applied to the electromagnet, the reed is driven back and forth as the polarity of the electromagnet alternates.

A typical electronic oscillatory motor is illustrated by the device disclosed in U.S. Pat. No. 2,971,323 issued to M. Hetzel, Feb. 14, 1961. The Hetzel device comprises a tuning fork having a pair of permanent magnets attached to its tines for interacting with separate pickup and driving coils. The pickup coil is connected to the input terminals of a transistor amplifier, and the driving coils are connected to the amplifier output terminals. In operation, the vibrations of the fork move one of the magnets into the pickup coil and induce a voltage which is applied to the input terminals. When the input voltage becomes sufficiently large, it triggers a current pulse which passes through the drive coil and augments the vibration of the fork.

A number of problems arise because of the use of magnetic coupling between the mechanical oscillating element and the electrical drive. One such problem is hysteresis and eddy currents which can introduce loss and frequency distortion into the system. 1n order to reduce loss, it is often necessary to use laminated steel cores, such as are shown in Favre-Bull and Brailsford; in order to avoid undesirable distortion, such as phase shifting between the pickup and the driving coils in an electronically driven motor, the coupling arrangements and the circuit elements must be very carefully designed and put together. (These problems are discussed in greater detail in U.S. Pat. Nos. 2,950,447 and 2,034,787 issued to C. H. McShan and A. J. Williams, Jr., respectively.) Another difficulty which arises in such electronically driven oscillatory devices is that of spurious self-induced electrical oscillations which occur at the frequency determined by the inductance and the distributed capacity of the coils rather than at the natural frequency of the mechanical oscillatory element. This problem is discussed in detail in the previously cited McShan reference.

While attempts have been previously made to eliminate the use of magnetic coupling by providing an oscillatory motor with electrostatic coupling between the oscillating element and the electrical drive, these attempts have generally proved impractical or unsatisfactory for many applications. For example, U.S. Pat. No. 2,934,887 issued to R. Keller on May 3, 1960, describes a hairspring balance wheel oscillatory motor in which the oscillating balance wheel is electrically charged due to the emission of alpha or beta radiation from a coating of radioactive isotope, such as Co, C, Sr, Pu One difficulty with such a motor, however, is the potential health hazard that the radioactive isotopes may present. For example, the fabrication of such motors is difficult because of the elaborate safety precautions which must be taken in the bandling of radioactive isotopes. Moreover, the motors may present health hazards to nearby persons as, for example, the wearer of a watch employing such a motor. Another example of an electrostatic motor purporting to eliminate magnetic losses is described by J. Favey in U.S. Pat. No. 2,835,105 issued May 20, 1958. To achieve the high voltage required for this motor, however, a transformer is employed in the drive circuit; and, as is well known, transformers typically produce loss and distortion due to hysteresis and eddy currents.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS The nature, features, and advantages of the invention will appear more fully upon consideration of the various illustrative embodiments now to be described in detail in connection with the accompanying drawings.

IN THE DRAWINGS:

FIG. 1 is a schematic cross section of an AC driven oscillating reed in accordance with the invention;

FIG. 2 is a schematic cross section of an electronically driven tuning fork in accordance with the invention;

FIG. 3 is a schematic cross section of an electronically driven pendulum;

FIGS. 4A, 4B, and 4C illustrate a first example of a balance wheel oscillator device in accordance with the invention;

FIGS. 5A and 5B illustrate a second balance wheel device in accordance with the invention which is adapted to utilize both electrostatic attraction and repulsion in driving the wheel;

FIGS. 6A and 6B illustrate yet another example of an electronically driven balance wheel;

FIG. 7 is a graphical illustration useful in understanding the embodiment of the invention shown in FIG. 6.

DETAILED DESCRIPTION In reference to the drawings, FIG. 1 illustrates an oscillating reed device comprising an oscillating reed 100, such as a blade of spring steel, having one end fixed to a stationary base 101 and the other end attached to a pair of

electrets

102 and 103. In the reed rest position, the electrets are disposed midway between a pair of

electrodes

104 and 105 which, in turn, are electrically coupled to a source 106 of AC voltage, such as an outlet for volt, 60 cycle per second line voltage. Means, such as the well-known pawl and ratchet wheel arrangement, are provided for converting oscillatory motion of the reed into rotary motion. Specifically, a pawl 107 tipped with a hard stone, such as ruby or sapphire, is attached to reed 100 and disposed to engage the teeth of a ratchet wheel 108. Advantageously, a pair of

stops

109 and 110, such as plastic covered metal stops, are fastened at suitable positions on a stationary frame (not shown) for limiting the amplitude of the reed and preventing the electrets from contacting the electrodes.

Electrets, as is well known in the art, are dielectric materials in which a permanent state of electrostatic polarization has been established by the displacement of the positive charges in constituent atoms with reference to negative charges. They were first described by Heaviside in 1892. Electrets are typically produced by subjecting a dielectric material, such as Mylar, polycarbonate, or polytetrafiuoroethylene to heat in the presence of a high electric field. The dielectric is then permitted'to cool while the field is applied. The properties of and the methods for fabricating electrets are explained in detail in the literature of the art. See, for example, U.S. Pat. No. 3,1 18,022 issued to G. M. Dessler on Jan. 14, 1964.

In operation, the application of an AC voltage across

electrodes

104 and 105 drives the reed back and forth as the electrodes change polarity. This oscillatory motion is converted into rotary motion by the pawl and ratchet wheel arrangement and can be used, for example, to drive the gear train (not shown) of an electric clock.

The primary advantage of this device over typical prior art structures is its simplicity. No curved electromagnets with wound coils of copper wire or heavy laminated steel cores are required. In addition, the losses and distortion associated with hysteresis and eddy currents are avoided.

FIG. 2 shows an example of an electronically driven tuning fork in accordance with the invention comprising, in essence, atuning fork coupled to both the input terminals of an amplifier and the output terminals, wherein at least one of the coupling arrangements includes one or more electrets. In the embodiment of FIG. 2, both the input and the output terminals are coupled to the fork by electret arrangements. More specifically, a tuning fork 201 having

tines

202 and 203 is rigidly attached by its base 204 to a stationary baseplate 205. Tine 202 of the fork is provided with electret means, such as a pair of

electrets

206 and 207 attached to an extension 208 affixed to the tine, for electrostatically coupling the tine to an electronic drive circuit. This is accomplished with a pair of

pickup electrodes

209 and 210 disposed adjacent to the electrets at the fork rest position. A resistor R, is placed in parallel with the pickup electrodes to permit current flow between the electrodes. The pickup voltage signal is applied to the

input terminals

211 and 212 of electronic amplifier 213. The amplifier can comprise an NPN-transistor T, a DC voltage source V and a resistor R in a common base arrangement. As will be appreciated by those skilled in the art, a wide variety of other amplifying arrangements, such as those using vacuum tubes or MOSFET devices can also be used.

The

amplifier output terminals

214 and 215 are electrically connected to a pair of

drive electrodes

216 and 217 disposed adjacent to a second pair of

electrets

210 and 219 which are attached to an extension 220 affixed to tine 203. The polarities of the electrets and the electrodes are chosen and adapted to provide a drive signal of suitable polarity for sustaining oscillations of the fork. As in the device of FIG. 1, a pawl 221 and a ratchet wheel 222 are provided for converting oscillations of the fork into useful rotary motion.

In operation, the vibration of tine 202

moves electrets

206 and 207 back and forth with respect to

electrodes

209 and 210. This motion produces an alternating voltage across resistor R which is applied to the input terminals of amplifier 213. Advantageously, amplifier 213 is chosen or adapted to have a minimum threshold voltage so that the pickup voltage signal triggers either an attracting drive pulse to the drive electrodes just before

electrets

218 and 219 reach the position of alignment with the drive electrodes or a repelling drive pulse just as they pass this position. (In even more sophisticated embodiments the circuitry can be designed to provide both types of driving pulses, as will be shown below.) In the example shown attracting pulses are applied to the drive electrodes. In this manner, electrical energy from the amplifier is used to sustain oscillations in the fork.

The primary advantage of this device is the elimination of the loss and distortion due to hysteresis and eddy currents. A secondary advantage is the savings in cost due to the elimination of the coils used in prior art devices.

FIG. 3 shows a schematic side view of an electronically driven pendulum in accordance with the invention comprising a rod 300 pivotally mounted at one end to a support member 301 and attached to a transverse member 302 at the other end. The transverse member includes electret means comprising, for example, a pair of oppositely oriented

electrets

303 and 304 for coupling to a pair of

pickup electrodes

305 and 306 and another pair of

electrets

307 and 308 for coupling to a pair of

drive electrodes

309 and 310 for electrostatically coupling the pendulum to an electronic drive circuit. The electrets, pickup electrodes, and drive electrodes are advantageously positioned so that the electrodes and electrets overlap at the pendulum rest position. The electronic circuitry for this device is substantially the same as that described in connection with FIG. 2.

In operation, the movement of electrets 303 and 3041 through the

pickup electrodes

305 and 306 induces alternat ing voltage pulse across resistor R, applied to the input terminals of amplifier 213. The output of the amplifier, in turn, is applied to the

drive electrodes

309 and 310 to sustain oscillations in the pendulum. The oscillatory motion of the pendulum is converted into useful rotary motion by the pawl and ratchet wheel arrangement, and stops are provided for maintaining the amplitude of the pendulum within useful limits.

FIGS. 4A and 4B illustrate, respectively, perspective and cross section views of a first example of a balance wheel oscillatory device in accordance with the invention. This device comprises, in essence, a balance wheel oscillator including electret means for coupling the balance wheel to an electronic drive circuit. In particular, the device comprises a balance wheel 400 rigidly attached to a rotatably mounted shaft 401 and elastically attached to at least one of a pair of fixed members (stators) 402 and 403 by a hairspring 404. Stator 102 is provided with a pickup electrode 405, and stator 403 is provided with a drive electrode 406.

Balance wheel 4100 is provided with electret means comprising pickup electret 4107 and drive electret 408 for electrostatically coupling with

electrodes

405 and 406, respectively. It is also provided with conductive means which act as a common ground electrode with respect to the pickup and drive electrodes. In particular, the balance wheel is conveniently made of a conductive material such as a metal and is provided with electrical contact means such as terminal 409 attached to a metal hairspring 404.

Shaft 401 extends through stator 402 and is mechanically coupled to means, such as gear 410, for turning a ratchet wheel.

The electronic drive circuitry is substantially the same as that described previously. It comprises, in essence, an electronic amplifier preferably comprising a metal-oxide-silicon field effect transistor (henceforth MOSFET) having its input terminals electrically connected between balance wheel 400 and pickup electrode 405 (which acts as the pickup electrode). The amplifier output is connected between the balance wheel and electrode 406, and electret 400 is appropriately driven by the output voltage.

FIG. 1C illustrates the preferred circuit arrangement for using an N-channel MOSFET however, as is easily seen, a P- channel MOSFET can be used with only minor circuit modifications.

FIG. 5A shows a schematic cross section of a second balance wheel device in accordance with the invention which is especially adapted to utilize both electrostatic attraction and repulsion in driving the wheel. The device is substantially the same as that shown in FIGS. 4A and 413 except that the balance wheel is provided with an electrically separate reference electrode for the pickup and the drive electrets. This is accomplished by using a balance wheel comprising a pair of metal discs 500A and 50013 mechanically connected by an insulator 500C. In addition, separate electrical contacts 509A and 50913 are conveniently provided by use of a pair of hairsprings 5041A and 5048. The remaining components are substantially identical to their counterparts previously described in connection with FIG. -1.

The electronic drive circuit for use with this embodiment of the invention is chosen or adapted to supply an attracting voltage pulse to the drive electrode as the drive electret ap proaches the point of maximum overlap and a repelling voltage as it passes this point. In particular, the pickup electrode is electrically connected to the input terminals of two amplifying means, one such amplifying means for applying an attracting pulse to the drive electrode as the pickup electret approaches the pickup electrode and the other amplifying means for applying a repelling pulse as the pickup electret passed the pickup electrode. (The pickup and the drive electrets are, of course, positioned so that they each approach their respective electrodes at the same time.) A preferred embodiment of such a drive circuit is shown in FIG. 5B. In this circuit the amplifying means for applying the attracting pulse comprises a pair of N-channel MOSFETS T and T and the amplifying means for applying the repelling pulse comprises a pair of P-channel MOSFETS T and T Yet another example of an electronically driven balance wheel is shown in schematic perspective and cross section views in FIGS. 6A and 68, respectively. In this arrangement, an electret pickup arrangement is used in conjunction with a magnetic drive arrangement. In addition, a specially shaped electrode or electret is used to prevent activation of the magnetic driving arrangement against the motion of the oscillator.

ln specific reference to the figures, the device comprises a balance wheel 600 rigidly affixed to a rotatably mounted shaft 601 and elastically attached to a stator 602 by a hairspring 603. The balance wheel is provided with electret means comprising electret 604 for electrostatically interacting with pickup electrode 605 attached to the stator and is also provided with permanent magnet means comprising a pair of

magnets

609 and 607 for magnetically interacting with an electromagnet 608 attached to the stator. The electronic circuitry used with this device is simply an amplifier arrangement adapted to deliver a current pulse to electromagnet 608 for driving the balance wheel through the permanent magnets is response to an above-threshold voltage from the pickup electrode 605.

Since, however, the electret produces a pickup voltage on the electrode in both directions of oscillation, the electromagnet will drive in opposition to the balance wheel motion on a return (counterclockwise rotation) unless some corrective means are provided. In this embodiment of the invention, the electrode (or electret) is shaped so that an above threshold voltage is produced during the forward oscillation but not during the return oscillation. This is accomplished by making the electrode roughly triangular shaped with a vertex pointing in the direction of forward oscillation. Since the voltage induced is a function of the rate of change of capacity with respect to time, a forward (clockwise) oscillation of the wheel produces a relatively large positive voltage pulse due to the abrupt increase in capacity as the electret suddenly begins to interact with the base of the triangular electrode and a negative voltage of low magnitude as the electret moves from the base to vertex. This waveform is graphically shown in FIG. 7A. However, on the return trip, the situation is reversed; the positive voltage remains relatively small because the time rate of change of capacity is relatively small and constant as the electret moves from the vertex to the base, and the negative pulse is relatively large as the electret moves over the base and away from the electrode. This waveform is shown in FIG. 7B. Consequently, the arrangement can be designed to selectively trigger a drive current pulse during only one direction of oscillation.

In all cases, it is understood that the above-described examples are only illustrative of the many possible specific embodiments which can represent applications of the principle of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.

We claim:

I. An oscillatory device comprising:

a mechanical element capable of periodic oscillation;

means for applying electrical energy for sustaining periodic oscillations of said mechanical element; and

electret means for electrostatically coupling said mechanical element and said means for applying electrical energy.

2. A device according to claimv 1 including means for producing rotary motion from oscillatory motion of said mechanical element.

3. A device according to claim 2 wherein:

said means for applying electrical energy comprises a pair of electrodes for receiving an alternating voltage; and

said electret means is attached to said mechanical element and disposed between said electrodes.

4. A device according to claim 3 wherein said mechanical element is a reed.

5. A device according to claim 2 herein:

said means for applying electrical energy for sustaining periodic oscillations comprises a pickup arrangement responsive to the motion of said mechanical element, amplifying means responsive to the signal from said pickup arrangement for producing an electrical driving signal, and a driving arrangement for sustaining oscillation of said mechanical element.

6. A device according to claim 5 wherein:

said pickup arrangement comprises at least one pickup electrode; and

said electret mean for electrostatically coupling said mechanical element and said means for applying electrical energy comprises one or more electrets attached to said mechanical element for electrostatically interacting with said pickup arrangement.

7. A device according to claim 6 wherein said mechanical element is a tuning fork.

8. A device according to claim 6 wherein said mechanical element is a pendulum.

9. A device according to claim 5 wherein:

said driving arrangement comprises at least one driving electrode; and

said electret means for electrostatically coupling said mechanical element and said means for applying electrical energy includes electret means attached to said mechanical element for interacting with said driving arrangement.

10. A device according to claim 5 wherein:

said driving arrangement comprises electromagnet means;

and

said mechanical element includes permanent magnet means for interacting with said electromagnet.

11. A device according to claim 5 wherein said mechanical element is a balance wheel elastically coupled to at least one stator member.

12. A device according to claim 9 wherein said mechanical element is a balance wheel elastically coupled to at least one stator member.

13. A device according to claim 10 wherein said mechanical element is a balance wheel elastically coupled to at least one stator member.

14. A device according to claim 13 wherein:

the amplifying means responsive to the signal from said pickup arrangement has a threshold voltage; and

the pickup arrangement is designed to produce an abovethreshold signal during only one direction of oscillation.

l 0* i i

Claims

1. An oscillatory device comprising: a mechanical element capable of periodic oscillation; means for applying electrical energy for sustaining periodic oscillations of said mechanical element; and electret means for electrostatically coupling said mechanical element and said means for applying electrical energy.

2. A device according to claim 1 including means for producing rotary motion from oscillatory motion of said mechanical element.

3. A device according to claim 2 wherein: said means for applying electrical energy comprises a pair of electrodes for receiving an alternating voltage; and said electret means is attached to said mechanical element and disposed between said electrodes.

4. A device according to claim 3 wherein said mechanical element is a reed.

5. A device according to claim 2 wherein: said means for applying electrical energy for sustaining periodic oscillations comprises a pick-up arrangement responsive to the motion of said mechanical element, amplifying means responsive to the signal from said pick-up arrangement for producing an electrical driving signal, and a driving arrangement for sustaining oscillation of said mechanical element.

6. A device according to claim 5 wherein: said pick-up arrangement comprises at least one pick-up electrode; and said electret means for electrostatically coupling said mechanical element and said means for applying electrical energy comprises one or more electrets attached to said mechanical element for electrostatically interacting with said pick-up arraNgement.

8. A device according to claim 6 wherein said mechanical element is a pendulum.

9. A device according to claim 5 wherein: said driving arrangement comprises at least one driving electrode; and said electret means for electrostatically coupling said mechanical element and said means for applying electrical energy includes electret means attached to said mechanical element for interacting with said driving arrangement.

10. A device according to claim 5 wherein: said driving arrangement comprises electromagnet means; and said mechanical element includes permanent magnet means for interacting with said electromagnet.

14. A device according to claim 13 wherein: the amplifying means responsive to the signal from said pick-up arrangement has a threshold voltage; and the pick-up arrangement is designed to produce an above-threshold signal during only one direction of oscillation.