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Publication numberUSRE23664 E
Publication typeGrant
Publication dateJun 2, 1953
Filing dateMar 14, 1944
Publication numberUS RE23664 E, US RE23664E, US-E-RE23664, USRE23664 E, USRE23664E
InventorsFrederick William Meredith
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Electric motor control system
US RE23664 E
Abstract  available in
Images(2)
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Claims  available in
Description  (OCR text may contain errors)

June 2, 1953 F. w. MEREDITH 23,654

ELECTRIC MOTOR CONTROL SYSTEM Original Filed March 14, 1944 2 Sheets-Sheet 1 39 57,2 4] RATE OF 33 I 515. TURN l -37 31 I H 36 I CONSTANT VOLTAGE 33/ l2HY-STERESIJ MOTOR FIG.6

TORQUE SPEED SPEED O SIGNAL I T- Mimi-n4 'BY: f

A wrancx's June 2, 1953 F. w. MEREDITH I ELECTRIC MOTOR CONTROL SYSTEM Original Filed March 14, 1944 2 Sheets-Sheet 2' INvzN-rom I BY:

UNITED sures PA r Nrfor-FlcE p 23,664 ELECTRIC MOTOR 'ooN'raor. -sYs'rEM Frederick William Meredith, ohatentam, England, assignor 'to'S. Smith & Sons (England) Limited, aBrit ish company- Original No. 2,512,665, dated June 27, 1950, Se-

rial No. 526,434, March 14,1944. Application for reissue June 26,1951, Serial No. 233,510. In Great Britain October 18,1943

27 Claims: (01:318-1-18) Matter enclosed in heavy brackets I: appears in original patent but forms no part of this.

reissue specification; matter printed in italics indicates the-additions made by reissue.

This invention consists of lmprovements in'dr 400 cycle supply. To meet the above requirerelating to electric motor control .systems in ments only commutatorless motors were deemedwhich a variable quantity controls an electric acceptable and it was decided to use induction motor. The invention is particularly applicable type signal generators, except for a few power to automatic control systems for maintaininga d-rivenpotentiometers which could employ rela-'-' physical condition at a datum value in which the 'tively-heavy-brush pressures.

variable quantity is the variation of the physical In the invention of a device of such complex-- condition from the datum value and the electric ity as an auto pilot one forms-known elements'in motor operates to restore the condition to the, newcombinations and combines them with newdatum value. Thus a platform on an aircraft 9 components to produce the final successful re'-' or on a ship may be stabilised about a horizontal sult. The characteristics of the new elementsaxis by arranging that any movement of the here permeate thecombination and are vital to' platform from a datum position shall operate the the present invention "which would not exist motor to restore the platform to its initial posiwithout them.

tlon. [and broadly speaking the physical condi- Fortunately greatly improved relays in miniation herein referred to may be the position of a ture form and hermetically sealed have recentlybody in relation to a datum position, the speed me v il bl lving part of the above DI of linear or t ti movement f body i lem without risk of contact'failures. The p'osirelation to a datum speed, the temperature of a; tion'on the motor side was not favorable as no body in re1ation t a datum t t t 0 small low inertia motors of the induction type of pressure in a fluid body in relation to a datum reasonable efficiency e available.

pressure or any other physical condition which e etical possibilities of the hysteresis can be controlled directly or indirectly by the motor Seemed attractive and it s P ap rtorque f an electric, t a tunate that in turning from the development of [Although capable of the above application] fluid p wer y em to le t Systems pp t The present invention was primarily developed to s lacking in the conventional training of an meet the requirements of an aircraft autopilot electrical engineer and so was unaware that the' designed to meet the followingspecifications hysteresis motor was little more than a laboraamong others. I

-- (a) Capable of rapid correctly coordinated eo w w limited wp nt.

tory freak orlof the Steinmetz proof thatits maneuven. Technical Paper 47-218 of the American Insti- (b) Able to maintain a compass course 'indefie of Electrical Engineers, August 1947 states nitem 1 3 r I in the opening lines of'the'introduction, The

(c) Capableof. being coupled to radio devicesysteresis motor, as a'p l ype (if D to seek and follow a radio track and to-follow 35 I moton'is' virtually unknown]? The efficiency aglide path beam for blind approach. limits range from a high of 0.5% to a low ((1) Simple and fool-proof to operate. of 0.16%. (e') Safe in the event of any foreseeable fail- Accordingly, as p'artof the present invention ure of t pilot, applicant was compelled by necessity to develop (f) Low maintenance requirement. 40 a small motor'of reasonably high efficlency which (g) Capable of rapid replacement by units in would have a straight line torque characteristic the event of failure so that the device will funcindependent of speed or armature" current, tion without delay and dislocation of flight schedwould accelerate from standstill to 12,000 or ules 24,000 R. P. M. in a few milliseconds and would These requirements are believed best met by deliver full load torque continuously when stalled an electrical signal system which is deemed es-" without over heating. A practical motor of these sential to provide the n e sary flexibility t characteristics was not available at the time satisfy present and continually developing needs; applicant made his present invention. Such a of radio guidance and the stability peculiarities motor was successfully invented by applicant and of new designs of aeroplanes. The use of"A. C. 0- ap ears in Figs. 2 and 3 of the drawings and i for both signal and po-werfsystems not only helps described in detail in copending application S. N.

avoid some of the contact troubles of electric 526,436'filed the same day as this application, systems, but is in line with the trend of electric This new motor is an important element of supply in aircraft. The invention has been the auto pilot and is an integral [and vital] partfound to operate to advantage on 115 volt B-phase- 0f the electric motor control system of the pres-' 3 ant invention as will appear below in the discussion of such-systems.

In such systems it is desirable that the motor should have a high acceleration since any lag in the response of the motor to changes of control voltage may lead to hunting. An [Onlobject of the invention is to provide a control system I having these characteristics.

According to this invention an electric motor control system in which a variable quantity controls an electric motor is characterised by' the use of a hysteresis motor. By a hysteresis motor is meant one in which the rotor has a Rich hysteretic constant, is subjected to a rotating field, [and] is not provided with any winding (squirrelcage or otherwise) and preferably is not provided with any salient poles. Since 'the rotor carries no copper it can'be made exceedingly light and at the same time a large torque can be obtained. "In such systems it is alsosometimes desirable that the torque of the motor shall be determined by'the control voltage applied to the motor and notby the other variables such as the speed of the motor, and a further object o! the invention" is to provide a controlsystem. having thatcharacterl'stic. According to one form of this inventionan electric 'motor control system in which a variable quantity regulates the torque of an electric motor is characterized by the use of a hysteresis "signal isfed through leads 40 and phase-changmotor to ensure that the torque of the motor shall be determined by the control voltage applied to the motor and shall be substantiallyindependent of the speed of the motor. Preferably the hysteresis torque is made as large as possible by suitably proportionin'g and selecting the iferro-magneti'c material.

"Up to synchronous speed the torque of a 'hysteresis motor is substantially independent of speed and is determined by the control voltage applied to the motor.

the accompanying drawings";

Figure 1 is a diagram in plan indicating an arrangement used for stabilizing in azimuth a I turntable rotatably mounted on an aircraft, theuse of a hysteresis motor;

. Figure .2 is a longitudinal sectional elevation of one form of hysteresis 'motorrsuitable m use in this invention; and

Figure 3 is a transverse sectionon thelin'e 1-4 qf'FiguI 'Zi;

.Fig. 4. is a schematic representation of a two-1 phase motor of the induction type.

5 is a family of curves, representing: thev torque-speed characteristics of a typical. wellknown Ferraris motor.

Fig. 6 is a family of curves representing the torque-speed characteristics ofv the hysteresis motor forming an integral part of present invention.

Fig. 7 is a-family of curves derived iroml ig'.

ins devicei! to the phase winding 38 of a hysteresis motor shown in Figs. 2 and 3. The rate of turn signal. modulated carrier of the same frequency is fed through leads 4],. to the. second phase'w-inding 31 of the motor which is shown in quadrature with windings 38, representingthe action of phase'changing device 39 and the ninety degree shift in phase of the constant amplitude A. C. control signal in winding 38.

Rotor l2 comprising a sleeve of term magnetic material of high coercivity is "operatively connected through suitable reduction gearing 3=6'-with' turntable '32 so as to rotate the turntable 32 in the opposite sense 'or'direction to the rate of turn t0.Wh-1Gh the device 33 is subjected. Rotor "i 2 then rotates turntable 32 at the samerate as, but in the opposite sense to, the rate of turn of thesiscraft in azimuth. If the two rates of turn are not equal and opposite thedevice '33 is subjected to a rate of turn and rotor l2 of the hysteresis motor is actuated until the absolute rate of turn.

of; the device-"33 is zero and no signalfl's supplied to'leads M and phase winding 31. i

As the two rates "are always "equal, the angle through-which the. turntable 32 is rotated is equal to. the angle. through which the aircraft turns in azimuth and hence the turntable 32- is "stabilized against azimuthal movements of the aircraft.

The torque exerted by rotor it of the hysteresis motor and "therefore the acceleration oiirotor .I 2 is proportional to the output *throughileads I l and 5 representing the corresponding torque-sig?nal' senting the corresponding torque signal characteristics of the hysteresis motor forming pantof the invention and showing it to be independent of rotor speed.

.In. the drawings-like numerals refer toparts throughout. 1

"Referring to Fig. 1., .a turntable is rotatably',

mounted in azimuth on a fixed-part'31 of an aircraft. A device 33 is mounted on turntable. 32

for. detecting and. measuring rate of turn inaziand :rotor' 1211s a narrow air-gap I8 of tipto the detected rate 0'! turn. It follows that the stabilizing system itself is stable;

"I'heexamples relate only to the stabilization ot a turntable in azimuth. It willbe understood that the invention is equally applicable to the stabilization offanother body about one or more asses such as a pivoted or gimballed platform.

Tmehysteresis motor indicated in Fig. 1 at 1'2,

31 and 38 is illustrated in detail in Figs. 2 andv 3.:

'rhetrotor' i2 consists-of a sleeve of .ferro-magneticmaterial of high coercivity (for example a coercivity of 250). This sleeve Ii: is secured-to the shaft lil:rotatably mounted in anti-friction bearings. Theestator is a. tubular body of ferromagnetic material of high permeability and of comparatively low cocrcivity comprising an .uninterrupted cylindrical wall i3 and a number (say twelve) of equally-spaced radial bars [4 havingbetween them'slots 15 which are open at their outer ends and receive the polyphase windings. These bars I 4- and slots #5 are embraced by-a s'leevezlfi. Thefstator I3, M'and sleeve I6 are laminated toreduce eddy currents. The stator lamination: maybe held together by end-plates:

Between thecylindrical wall l3 ofv the stator proximately 0.005". Although the width of gap 18 is ejactor of considerable importance in the eflicz'encu and the response of the hysteresis motorra'nd J01" :optimum results in automatic pilot each signal level in winding systemsthe gap 18"shonld be kept small, a larger gap" may be used where sacrifice in performance and efiiciency is permissible. In general it has been found that a gap greater than two per cent of rotor diameter exceeds what may be regarded aslthe maximum tolerance for acceptable results with the present invention.

The motor is provided with a casing l9 which houses the sleeve [6 and therefore carries the stator and also carries the bearings for the rotor shaft II. An end cap for the casing l9 encloses a panel 2| for the terminals 22 of the stator windings. Figures 2 and 3 illustrate on a scale twice the actual size a hysteresis motor which has been successfully operated. The motor excitation is arranged to give maximum efilciency as described in United States patent application Serial No. 526,435 corresponding with British tity so that the motor torque is substantially pro portional to the variable quantity over a limited range. v 1 1 As indicated one ofthe basic problems of the present invention was the provision of a small practical motor of the induction type with the characteristics outlined above. The Ferraris motor was well-known in the art and if connected with two phases in quadrature as shown in Fig. 4 has the characteristics of an induction motor with extremely high rotor resistance. With the windings shown in Fig. 4 and a constant amplitude A. C. control voltage applied to winding [38' and a signal modulated voltage of the same carrier frequency applied to winding I31, the family of torque-speed characteristic curves shown in Fig. 5 is obtained. Each curve correspondsto the labeled multiple of a signal voltage '0 applied to winding I31.

The curves of Fig. 5 readily convert to the torque-signal curves of Fig. '7, showing that with I31 a different torque-speed relation obtains.

The hysteresis motor of Figs. 2 and 3 yields the torque-speed characteristic shown by the family of curves in Fig. 6. Fig. 8 shows the torque-signal characteristic derived from the curves of Fig. 6 and graphically demonstrates that below synchronism the torque-signal characteristic is linear and independent of rotor speed. This is attested by the top curve in each of graphs XVIII through XXIII of the Technical Papero is the initial velocity of displacement. As the platform returns to its original position in an exponential manner and is stabilized in its datum position, (Q) is the first derivative of 6 with respect to time and o is theinitial velocity of the original displacement 9c.

6 in the same manner isthe second derivative of B-With respect to time, that is the acceleration 6 of the platform or the rate of its rate of dis-e placement.

I is a constant determined by the inertia of the system. T is the restoring torque, opposite in direction to displacement 9 of the platform as well and 5, generated by the rotor l2 of the motor.

If it be assumed that an aircraft applies an acceleration force 0 to a platform with an inertia constant -I then the platform can be restored to datum by an equal and opposite force or its kinematic equivalent torque T acting in an opposite direction to e. We may then equate I:-T I+T:0

Where, as in the present invention, a detector 33 is employed giving a signal proportional to the rate of turn or angular acceleration of the platform and this signal is supplied to a motor having the torque-speed, torque-signal characteristic of Fig. 8 the above equation of motion becomes With a control system using a motor of.- the characteristics of Fig. 5, where T is a function of both 9 and e as determined by combining the motor and signal pick-off characteristics, the solution of the differential equation if possible at all, will in general involve long and intricate mathematical computation for each set of initial conditions in which a complete series of rate signals in winding I31 must be evaluated for each setting of control amplitude in winding I38 and interpolated curves plotted. It is believed that any results obtained would be of highly uncertain accuracy and unreliable.

The hysteresis motor of Figs. 2 and 3 is an integral part of the solution of the control problem of the present invention. It eliminated any necessity of the laborious and doubtful computations just referred to and provided a power motor over the entire speed range plus the required control behavior with certainty and reliability.

Such an hysteresis motor has been constructedin a twelve and a half watt size and successfully operated as an element in the present invention as part of an auto pilot. With a thirty-five percent cobalt magnet steel rotorl2 it accelerates from rest to 24,000 R. P. M. in 1/10 second.

Truly spectacular accelerations are possible with such materials as Alcomax III which has about five times the hysteretic power of thirtyfive percent cobalt steel. tion while continuously delivering full load torque at stand still is a limiting factor in rotor design in the auto-pilot of the present invention.

The above disclosure sets forth What are at present considered to be the preferred embodiments of the invention. The invention is set forth in generic terms in the appended claims which are intended to cover all forms which fall within the true spirit and scope of the invention.

I claim:

1. [In] An automatic pilot control system for maintaining a physical condition at a datum value in a movable craft comprising in com- I-Iowever, heat dissipa:

case;

7 bination 'a member the position of which in relation to a datum position represents the'variation of the physical condition from the datum value, means whereby the movement of said member generates an A. C. control voltage, a motor the field of which is energised by said voltage and means by which the motor operates to restore the physical condition to the datum said motor being a self-starting asynchronous hysteresis motor including, cooperating stator and rotor members, one of said members comprising'a magnetic core structure having a low conductivity to eddy currents and having a series of radiallyextending, angularly spaced winding slots and magnetic bridges for the ends thereof adjacent the other member and a polyphase distributed energizing winding disposed in said slots, the other of said members comprising an annular magnetic armature constructed at least in part of .a magnetic material having a high hysteretic constant said members having complementary cylindrical adjacent surfaces forming an air gap therebetween and having such relative diameters that said air gap is substantially the minimum permitted by manufacturing tolerances, and means for mounting said members for relative rotation.

2. [In] An automatic pilot control system in a movable craft in which a variable quantity regulates the torque of an electric motor characterized by the use of a hysteresis motor to ensure that the torque of the motor shall be determined by the control voltage applied to the motor and shall be substantially independent of the speed of the motor, a motor, means to supply said variable quantity to said motor,-said motor being a self-starting asynchronous hysteresis motor including, cooperating stator and rotor members,

one of said members comprising a magnetic core,

members having complementary cylindrical adja-- cent surfaces forming an air gap therebetween and having such relative diameters .thatsaid air gap is less than 0.0090 an inch in width, [substantially the minimum. permitted by manufacturing tolerances, and] means .for mounting said members for relative rotation, and means operatively connecting said rotor to said system.

3. [In] An automatic pilot control system for maintaining a physical. condition at a datum value in a movable craft comprising in combination a movable member the position of which in relation to a datum position represents the variation of the physical condition from its datum value, means whereby the movement of said member generates an A. C. control voltage, a selfstarting asynchronous hysteresis motor including, cooperating stator and rotor members, one of said members comprising a magnetic core structure having a low conductivity to eddy currents and having a series of radially extending, angularly spaced winding slots and magnetic bridges for the ends thereof adjacent the other member and a polyphase distributed energizing winding disposed in said slots, the other of said members comprising a magnetic armature constructed'at least in part of a magnetic material having a high hysteretic constant, said members having complementary-smooth continuous cylindricalfladjacent surfacesforming an air gap therebetween and having, such relative diameters that said air gap is substantiallythe minimum :permitted by manufacturing tolerances, means :for mounting said members for relative rotation, means for applying the A. C. voltage to said stator'winding and'means operated by thereto! to restore the physical condition to its datum value.

'4. An automatic electric-motor control system asclaimed in claim Binwhich the means whereby the movement of said member generates an A. C. control voltage is a rate-of turn device which generates :an A. C. control voltage proportional to the rate of deviation of said member from its datum position and in which the torque of the motor is proportional to said control volt age. I

5. In an automatic pilot control system in :a. movable craft for stabilizing a first body against the effects of rotation of a second *body on which the first is pivotally mounted on an axis about which the first body is to be stabilized comprising a device for detecting and measuring rate of turn mounted on one of the bodiesand arranged to generate an A. .C. control volt-- age proportional to the rate of turn oi said body, a'motor couplingthe two bodies said motor being a. self-starting asynchronous hysteresis motor including, cooperating stator and rotormembers, one of said members comprising amagnetic core structure having a lowconductivity toeeddycurrents and having a series of radially extending, angularly spaced winding slots and magnetic) bridges for the ends'thereof adjacent the other member and a polyphase distributed energizing winding disposed in said slots, the other of said members comprising 'a magnetic armature constructed at least in part of a magnetic material having a high hysteretic constant, said members having complementary smooth continuouscylindrical adjacent surfaces forming an air gap therebetweenand having such relative diameters that said air gap is substantially the minimum permitted by manufacturing tolerances, means for mounting said members for relative rotation, said rotor geared to rotatethe first body inrelation to the second body and means foriapplying the A. Cpcontrol voltage to the wound stator to cause the first vboriyto turn in the 'opposite direction from the direction in whichthe rate of turn device-turns.

6; An automatic pilot control system 'in'amovable craft for stabilizinga first body'against-the effects of rotation of a second body on whichthe first body is pivotally mounted on an axis about which the first body is to be 'stabiIiZedcomprising a device fordetecting and measuring-rater! turn mounted on one'of said bodies and arranged to generate an A. C. control voltage proportional to the rate of turn, means to generate a carrier wave of the same frequency, a hysteresis motor said motor being a self-starting asynchronous hysteresis motor including, cooperating stator and rotor'members, one of said members comprising a magnetic core structure having "a low conductivity to'eddy currents and havinga-series of radially extending, .angularly spaced winding slots and magnetic bridges for the ends thereof adjacent the other mem'berand a polyphase dis tributed energizing winding disposed in said slots, the other of said members'comprising a magnetic armature constructed at least in part-or 9? a magnetic material having a high hysteretic constant said members having complementary smooth continuous cylindrical adjacent surfaces forming an air gap therebetween and having suchrelative diameters that said air gap is substantially the minimum permitted by manufacturing tolerances, means for mounting said members for relative rotation, reduction gearing, said motor and gearing coupling the two bodies, and means for applying the control voltage to one stator winding and for applying the carrier wave voltage to another stator winding in quadrature to cause the first body to turn in the opposite dimotion from the turn of the rate of turn device.

'7. An automatic pilot control system for stabilizing a. table againstithe effects of rotation of an aircraft on which the table is. rotata-bly mounted on an axis about which the table is to be stabilized comprising a rate-of-turn device mounted onsaid table adapted'to generate an A. C. voltage proportional to the rate of turn of said table a motor connected to said r'ate-of-turn device, said motor being a self-starting :asynchronous hysteresis motor including, cooperating stator and rotor members, one of said members comprising a magnetic core structure having a low conductivity to eddy currents and having .a series of radially extending, angularly spaced winding slotsand magnetic bridges for,

the ends thereof adjacent the other member and apolyphase distributed energizing winding disposedin said slots, the other of said members comprising a magnetic armature constructed at least in part of a magnetic material having a high hysteretic constant, said members having complementary smooth continuous cylindrical adjacent surfaces forming an air gap therebetween,

and having such relative diameters that said air gap is substantially the-minimumpermitted by manufacturing tolerances, means for mounting saidmembers for relative rotation, the winding of, said motor being energised by said voltage andgearing operatively connecting said motor to said table to restore said table to its datum slots and magnetic bridges for the ends thereof adjacent the other member and a polyphase distributed energizing winding disposed in said slots, the other of said last two members comprising a. magnetic armature constructed at least in part of a magnetic material having a high hysteretic constant, said last two members having completmentary cylindrical adjacent surfaces forming an air gap therebetween and having such relative diameters that said air gap is less than 0.009 inch in width, means for mounting said stator and rotor members for relative rotation, one phase of said polyphase winding being connected to receive said control voltage and drive means connecting said rotor member and said movable member.

9. The combination set forth in claim 8, said movable member comprising a surface mounted 10 for rotation said air gap being less than 0.007 inch in width.

10. The combination set forth in claim 9, said means to detect movement comprising a rate of turn device mounted on said surface and. con-.- structed to produce a voltage which is a functionof the rate of turn of said surface said air gap being less than 0.005-inch in width.

11. The combination set forth :in claim 10, said motor being asynchronous and saidother of said last two members having an annular magnetic armature having no path of high permeance within said annulus which shunts magnetic flux therefrom. J

12. The combination. set forth in claim 11, a base member, said movable surface member having a gimballed mounting on said base member.

13. In an automatic pilot control system in a movable craft in which a variable quantity representing acondition of said craft regulates the torque Oran electric motor which is of the hysteresis type to ensure that the torque of the motor shall be determined by th control voltage ap-. plied to the motor and shall be substantially in-. dependent of the speed or the motor, in combination a movable craft, a motor, means to detect said variable quantity and apply it to said motor, said motor being a. self starting asynchronous hysteresis motor having coopemting stator and rotor members, one of said members comprising; a magnetic core structure having a low conduc-. tivity to eddy currents and having a series of radially extending, angularly spaced winding slots and magnetic bridges for the ends thereof.

' adjacent the other member and a polyphase dis-- tributed energizing winding disposed in said slots, the other of saidmembers comprising an annular, magnetic armature constructed at least in partof a magnetic material having .a high hysteretic, constant and having no path of low reluctancedcting to shunt magnetic flux from said annular armature, said members having complementary. cylindrical adjacent surfaces spaced apart a distance of the orderof 0.0075 inch, means mounting said members for relative rotation, means carried by said craft for reflecting a condition thereof according to variations in said variable quantity and means openatively connecting rotor member to said means for nfecting the condition of said craft. 14. The combination set forth in claim 13, said distance. between said surfaces beingat least 0.0025 inch said means to detect said variable quantity comprising a rate of turn device constructed to produce a voltage which is a function of rate of turn representing a note of change of condition of the craft.

15. An automatic pilot control system for maintaining a physical condition at a datum value in a movable craft comprising in combination a member the position of which in relation to a datum position represents the variation of the physical condition from the datum value, means whereby the movement of said member generates an A. C. control voltage, a motor the field of which is energised by said voltage and means by which the motor operates to restore the physical condition to the datum said motor being o. self-starting asynchronous hysteresis motor including, co -operating stator and rotor members, one of said members comprising a magnetic core structure having a low conductivity to eddy currents and having a series Of radially extendmg, angularly spaced winding slots and magnetic bridges for the ends thereof adjacent the other Sane-.1

lli

member and a polyphase distributed energising winding disposed in said slots, the other of said members comprising an annular magnetic armature constructed at least in part of a magnetic material having a high hysteretic constant said members having complementary cylindrical adiacent surfaces forming an air gap thenebetween and having such relative diameters that said air gap is less than .009 inch in width, and means for mounting said members for relative rotation,

16. The combination set forth in claim 15 wherein the said air gap is less than .007 inch in 17. The combination set forth in claim 15 wherein said air gap is less than .005 inch in width.

'18. An automatic .pilot control system in a movable craft in which a variable quantity regubites the torque of an electric motor characterby the use of a hyteresis motor to ensure that the torque of the motor shall be determined by the control voltage applied to the motor and shall substantially independent of the speed c) the motor, a motor, means to apply said variable quantity to said motor, said motor being a asynchronous hysteresis motor ineluding, co-openating stator and rotor members, one or said members comprising a magnetic core structure having a tow conductivity to eddy currents and having a series of radially extending an'galar ly spaced winding slots and magnetic bridges rm"- ends thereby adjacent the other and a polyphase distributed energizing winding disposed in said slots, the other of said members comprising an annular magnetic armatare constructed at least in 'part of a magnetic having a high hysteret'ic constant said members oomplementary "cylindrical adiacen't surfaces forming an air gap therebetween and having such relative diameters that said air gap is less than .009 inch in 'iu'idth,and means for members :jor relative rotation,

. carried by saidorayt for-'afiectingtheoondition thereof according to variations in said variable quantity and means 'operat'iaely connecting said rotor to saiil m Bans for'iafiecting the-conditidn of sdid cmft.

1 .9. The flombimtwn set forth in clm'm 18 wherein the said air gap is less than $007 inh in width. v

20. The set forth in claim wherein the sdid fiir is 5988 :005'indhin width.

2-1.. mbimttion 'set fdrlh claim 8,

12 air gap being greater than 0.0015 oj'an inchin width.

22. The combination set forth in claim Lsaid air gap being within the range oj-0.009 of an inch maximum and 0.0015 of an inch minimumin width.

23. The combination set forth in claim 3, said air gap being within the range of 0.009 of an inch maximum and 0.0015 of an inch minimum;

24. The combination set ,forth in claim 3, said movable member comprising means sensitive to at least one acceleration force acting on said movablecraft. said air gap being within the'range of 0.009 of an inch maximum and 0.002 of an inch minimum.

2-5. The combination set fiorth in claim 7, said air gap being within the range of 0.009 ofan inch maximum and 0.002 of an inch minimum; i

'26. The combination set forth in claim .3 said control system having at least two of said able members each comprising means respectively sensitive to acceleration forces acting respeetively about a major of :movablecraft and each acting on individual means to produce a respective A. C. control voltage, at least twosaid motors each connected to receive its respective A. C. control voltage and each constructed to operate respective means .to .restore thephysicitl condition about its respective said major units to its datum value, said air gap each said being within the range of 0.009 of an inch mum and 0.002 or an inch minimum.

.27. The combination set forth in claim 8., movable member comprising a surface jorrotation about "a plurality o7 axes, said :means to detect movement of said comprising a plurality of rate :of turn devices each arrangedto detect acceleration .jorces about one of said axes and each said means acting on respective means to produce a respective A. C. control voltage, w. plurality of said motors and drive means con-- nected for control by a respective control voltage.

FREDERICK References in the file of this patent or the original Ipatent UNITED STATES PATIENTS Number Name Date 3,014,825 Watson s on Sept. 13,1835 2,115,086 Riggs s Am-.26, 1988 2,287,876 Height $111.30, .1942- 2.303291 Moss .Nov. '24, m2 2,305,878 Krussmam et a1. Dec. :2-2, 119.12