US2999135A - Flux gate transducer - Google Patents

Description

Sept. 5, 1961 D. E. wlEGAND 2,999,135

FLUX GATE TRANSDUCER Sept. 5, 1961 D. E. WIEGAND 2,999,135

FLUX GATE TRANSDUCER Filed March 5. 1955 5 Sheets-Sheet 2 /001 /a/KL /04 5 Sheets-Sheet 3 IIIIIIIIIIIIIIII Sept. 5, 1961 Filed March 3. 1955 .Zh VE n fr" .PAW/ f h//fwv 4 a a f g\ a I. z M

/94 W Ma-m, @M- l Sept. 5, 1961 D. E. wlEGAND 2,999,135

FLUX GATE TRANSDUCER Filed March 5. 1955 5 Sheets-Sheet 4 Erg. /Z

Sept. 5, 1961 D. E. wlEGAND 2,999,135

FLUX GATE TRANSDUCER Uur-Pur 5.

United States Patent 12,999,155 g g FLUX GATE TRANSDUCER -David E. Wiegand, Villa Pai-k, Ill., assignor to Armour 'Research Foundation of Illinois .Institute of Technology, Chicago, Ill., n corporation of Illinois Filed Mar. 3, 1955, Ser. No. 492,013 4 Claims. (Cl. 179-1001) The present invention is concerned with an improved 'electromagnetic transducing head Tand method, and is'par- `ticularly directed to an 'electromagnetic reproducing head capable of response directly "to magnetic flux rather'than to rate of change of llux.

An object of the'present invention is to provide an improvedma'gnetic transducin'g assembly which is sensitive directly to magnetic 'flux rathefthan'to 'the rate of change thereof.

Another object of thepresent 'invention'is to provide an improved magnetic reproducing head capable of detecting 4recorded signals of variable frequency, .including zero fre- 'quency Another object of the present invention is to provide an improved magnetic reproducing head ,and method pro- "vid-ing va substantially vhigher signal output than conventional magnetic recording "heads A'further object of thepresent'invention'is to provide 'a .novel magnetic modulator head and method usable for *the reproduction of music tand 'other signals requiring a A still further object of the :present invention is to provide a magnetic modulator head of novel and improved construction.

The novel Vfeatureswhich f-I 'b'elieve 1 to be characteristic of my'invention are 'set forth with particularity in the appended claims. My invention itself, :'however, as to its organization, manner of construction'and 'method of operation, .together with'further objects and advantages thereof may be best understood by reference to the following description, taken in connection with 'the accompanying drawings, in which:

FIGURE l is a somewhat 'diagrammatic top plan view of Va first electromagnetic transducer head according to the present'invention;

FIGURE 2 isa longitudinal sectional view of the head of FIGURE 1;

FIGURE 3 is 'an end elevational view of the head of FIGURE 1;

FIGURE 4 is a diagrammatic view illustrating the energizingand output circuits for the head of FIGURE l;

'FIGURE 5 is va'diagrammatic lsectional 'view of a second form of electromagnetic transducer head according to the present invention;

FIGURE Sais a diagrammatic-cross sectional view Ataken along the line Va-Va'of FIGURES;

FIGURE 5b is a diagrammatic view similar to FIG- URE 5a, but showing a modified energizing circuit.

.FIGURE 6 is a diagrammaticview illustrating a third form of head construction;

FIGURE 7 is a diagrammatic side elevationalview of a further head construction Aaccording to 'the present in- `vention;

FIGURE 8 is a top plan View 'of the head of FIG- 'URE 7;

FIGURE 9 Yis 'a diagrammatic side elevational View of 'a vstill further 'head constructed in accordance with the principles of the present invention;

FIGURE 10 is a diagrammatic view illustrating the energizing and output circuits for the head of FIGURE 9;

FIGURE l1 is a diagrammatic side elevational view of la further head construction;

FIGURE 12 is 1a diagrammatic Side elevational view of a still further modification;

rice

FIGURE 13 is a diagrammatic side elevational'fview of a further head construction utilizing a unidirectional high frequency winding on a saturating strip;

FIGURE 14 illustrates a modification utilizing fhigh frequency windings connected in aiding relation with-frespect to the gap and operating on the incremental permeability characteristics of the record member;

FIGURE 15 illustrates a magnetic head excited hya high frequency pulse generator;

FIGURES 16 and l7 illustrate -a manner vof applyi'gl winding to a staturating strip without wrappinglthe winding completely around the saturating strip;

FIGURES 18 and 19 illustrate a further manner :fdr exciting a saturating strip withoutthe use of a completely encircling the strip;

FIGURE 2O represents graphically the signal'and "no'ise output from a head according to the present linvention as a function of the applied intensity of magnetization from the oscillator; and

FIGURE 21 is a diagrammatic illustration of ama'gnetic head having laterally offset poles.

As shown on the drawings:

Referring to FIGURES 1 through 4, there is illustrated a shielded transducer head construction according 'to present invention.

The head comprises a pair of pole shoes '10 and t1 which are illustrated as being of the confronting type-5to define therebetween a non-magnetic gap 12. Th'e A"ple pieces are shaped to provide tape contacting surfaces for receiving a tape magnetic record member 14 in proxirrty to the non-magnetic gap 12. The pole shoes'dU-and ll are secured in any suitable manner within a housing whdh includes a pair of Mumetal shield plates 16 and 17, n'o'nmagnetic spacers 19 and 20 insulating the pole `shoes the shield plates. The shield plates 16 and x17 lmay be provided from an integral strip of magnetic mateil formed into a generally U-shape with abase :portion 21 of the U being formed with upstanding lugs '22 and 2B, the lugs being formed from the material of slots Y2'4f`atld 25 in the base 21.

Windings 29 and 30 are mounted on saturatng'mernbers 31 and 32 which bridge between the pole shoes .1'0 and 11 and the lugs 22 and 23 to complete a single loop magnetic circuit including the base portion 211 `and Cthe gap 12.

Referring to FIGURE 4, winding portions 29a and 30a of windings 29 and 30, respectively, are connected in 'series opposing relation between leads 40 and 41 from asuitb'le oscillator 42 which is capable of generating a signal fof frequency substantially above the frequencies to be reproduced from the magnetic impulse record member 14, the output of the high frequency source being, for example, on the order of 228 kilocycles per second. While there are a number of suitable ways for obtaining a signal from the head construction of FIGURES 1 4, FIGURE A4 illustrates winding portions 29b and 30b of windings 29 30, respectively, providing pickup windings connected in series aiding relation with respect 'to output leads '43 44. The output signal from leads 43 and 44 may delivered through a condenser 45, an amplifier 46, `and a detector 47 to an output device such as loudspeaker "48. The high frequency excitation windings 29a and 30g are preferably connected in series opposing relation with spect to the magnetic circuit as illustrated so asf'to -a'vid the generation of a net high frequency leld at the "n'o'nmagnetic gap.

A D.C. polarizing flux maybe introduced into the-inagnetic circuit by means of resistor 49 and batteryS Lrto unbalance the high frequency uxes in the magnetic cuit. With such a polarizing ux, the -polarity 'o'f the jiltput from the detector 47 will vary 'in-accordance recorded signal. The level of D.C. polarizing -'x is .ux density in the saturating range between l/a and -medium plus the polarizing flux equals 1/a to 3 .preferably higher than the maximum signal flux in the saturating strips to avoid distortion. In the absence of a D.C. bias liux, the output signal can be passed through a Iphase demodulator or phase sensitive rectifier so as to obtain an output the amplitude of which varies in accordance with the polarity of the recorded signal. The phase ldemodulator may comprise a full wave rectifier network with the amplified output from the pickup windings 29h, 30b transformer coupled to one pair of terminals and a double frequency reference signal transformer coupled to the other pair of terminals of the rectifier network, the output being taken between center taps on the secondaiies -of the two transformers. 'I'he rectifier network may comprise four rectifiers connected in aiding relation abouta closed circuit with the transformer secondaiies each connected between one pair of opposite terminals of the network. A suitable phase demodulator is shown in my copending application Serial No. 294,684 of which the present application is a continuation-impart.

`As illustrated by the arrows 51 and 52 in FIGURE 4, the excitation windings 29a and 30a produce oppositely directed fluxes in the magnetic circuit including the saturating strips 31, 32 and pole pieces 10 and y11 so that the high frequency fluxes may balance out at the gap 12. As diagrammatically indicated in FIGURE 4, the high frequency ux paths 51 :and 52 may include portions of the l. magnetic shield members, here indicated as a single flat plate 55 with a rectangular central s'lot at 56. In the construction of FIGURES l-3, it will be apparent that the leakage high frequency tlux paths for each winding includes portions of each of the shield plates 16 and 17.

It has been discovered that the cross section of the saturating strips 31 and 32 is critical to a satisfactory signalto-noise ratio for certain applications, and that such a satisfactory signal-to-noise ratio is obtained by substantially reducing the cross-sectional area of the saturating parts. Further, it has been found that, within limits, the strength of the useful signal from the head is essentially unaffected by this reduction in cross-section.

More specifically, I have found that the optimum signalto-noise ratio and linear operation are obtained when the saturating parts 31 and 32 are narrowed down so that a maximum signal on the tape 14 will produce a members 31 and 32, which when added to any polarizing ux density is within the 1% of the value of intrinsic saturation induction for the material of the saturating members. In other words, the maximum flux capacity of the record of the product of the value of the saturation induction for the material of the saturating members and the cross section thereof.

By way of example, a specic head construction according to the embodiment of FIGURES 1-4 may comprise a pair of .006 inch thick by .060 inch wide Mumetal pole shoes, .010 inch thick insulating spacers, .020 inch thick Mumetal shield plates, and saturator strips of .001 inch thick by .010 inch wide by .380 inch long Molypermalloy. Mumetal may comprise an alloy of copper, 2% chromium, 77% nickel, and the remainder iron Iand minor constituents, while Molypermalloy or Molybdenum Permalloy may comprise 4% molybdenum, 79% nickel and the remainder iron and minor constituents.

FIGURES 5 and 5a illustrate a head construction including a magne 'c core portion formed from a magnetic lamination 80 of block I configuration folded about a conductor strip 81 to provide enlarged cross-section pole portions 80a and 80h having upper edges 80e and 80d for receiving -a tape magnetic record member successively thereacross. The connecting portion of the core 80e (the vertical part of the I) is of reduced cross-section such that a maximum signal on 'the tape will produce a tlux density in the saturating portion 80e which when added to the polarizing ux density therein produced by magnet 81' of the order of B/s the value of saturation induction for the material of the lamination 80.

The lamination may have an exciting winding 83 thereon connected with an oscillator 84 for establishing bucking magnetic fields at the gap between poles 80a and 80b. FIGURE 5b illustrates a modification wherein a high frequency conductor 83a links the magnetic circuit provided by lamination 80 to establish a circulating high frequency ux in the lamination and across the gap between poles 80a and 80b. The value of circulating ux is selected to be too low to erase the signal on the record member 85. Such a circulating iiux will still provide the desired magnetic modulator action. As indicated, the exciting conductor 83a is preferably energized through an inductance 86 and condenser 87 tuned to the frequency of oscillator 88. The second harmonic output is then obtained from pickup conductor 81 in the same manner as in FIGURE 5a to be now described. Polarizing flux'may be supplied by means of battery 89 and resistor 89'. The oscillator 88 and battery may be connected to conductor 81 if desired, and conductor 83a omitted.

I-t will be observed that the conducting strip 81 forms a portion of a single turn primary winding 90 .which is coupled to a high impedance secondary winding 91 on a transformer core 9 2'. by means of a U-strap 90a. The transformer construction may be similar to that shown in my Patent 2,585,065. 'Ihe secondary winding 91 of the transformer is tuned by means of a variable capacitance 93 to Ythe second harmonic of the oscillator frequency from 84 in FIGURE 5a (or from oscillator 88 in FIGURE 5b), and the output is delivered to a tuned amplifier 94 and detector 95 for driving a suitable output device such as aloud speaker 96. As previously, if no D.C. polarizing uxis introduced into the core 80, the detector 95 must take the form of a phase demodulator for obtaining an output with polarity varying in accordance with the recorded signal on the tape.

FIGURE 6 illustrates a further form of modulator head wherein the magnetic circuit has a gap therein between a pair of pole shoes and 101. The upper edges 100a and 101a have a non-magnetic gap 102 therebetween and receive a record member 104 thereacross as in the embodiment of FIGURES 1 4. Leads 105 and 106 connected to series opposing coils 107 and 198 are preferably excited with a high frequency signal, while leads 111 and 112 are connected with series aiding coils 113 and 114 whichy are utilized to pick up the output signal. Bias is applied by means of battery 115, and resistor 116, while the output is taken through condenser117, tuned amplifier 118, detector 119 and loudspeaker 120.

The high frequency ux path in FIGURE 6 is partly in air. The signal flux path preferably includes saturating strip 121 between the lower ends of the core pieces. The thin saturating strip is preferably secured to a nonmagnetic backing member 122.

FIGURES 7 and 8 illustrate a head which may also involve high frequency flux paths through air wherein pole shoes and 131 are mounted on brass backbone pieces 132 and 133 which may be clamped together with a suitable gap spacer 134 therebetween to define the non-magnetic gap. The pieces 132 and 133 may be secured, for example, by inserting cement in the space 136 therebetween. The pieces 132 and 133 are recessed to receive the coils 140 and 141 which may be arranged and excited as indicated in FIGURES 4 and 6. The saturating strips 143 and 144 may be secured in .low reluctance relation to pole shoes 130 and 131 by means of plates 146 and 147 which in turn maybe cemented in place. At the lower end, the magnetic circuit may be completed by a pair of yokes 150 and 151 which may also be cemented in place on the backbone 132, 133.

By way of example each coil may comprise 20 turns of No. 30 HF wound biilar in two layers on a 4&2 inch diameter form. The gap spacer may be a .0002 inch is preferably acont-1.9.15

nicwthe 1pole's`hUe's=may?be :006 .'by 3&2 inch*Mumetal umd the annulationl strips :may bel-.001 'by 5&2 by 'J1/' -inch "fMlypermalloy" vFIGURES 9 .and l0 'illustrate a .magnetic modulator dread utilizing a single satnratng strip v180 completing 'nt magnetic circuit vwi-thipole 'shoes 1'81 and `1182 iand non- :magnetic gap 183. The pole fshoes'may be vsoldered to- 4igether 'as indicated lat :184 vand :the fsaturating :strip 180 may be clamped with turned-n portions 181a and '182e iof tthepole'shoes by :means of'p'lates 186 and 187. An iinner -winding .i190 .is connected to an oscillator =191 by means of leads..192 and A193 "and provides thigh 'frequency rti'elds -duc to the -oppositely wound `portions 190a Vand `19017 thereof as indicated in FIGURE l0. The .signal H p'ickup winding 194 has leads 195 and 196 connected fto .amplifier 197 tuned vto the 'second .harmonic of vthe :oscillator frequency, detector 198 which may be a con- *ventional amplitude modulation rdetector such 'as .found :in ra'dio circuits, power amplifier v198 'and loudspeaker y199. 'I'he detectors kof FIGURES 5 vand 6 aswell as :FIGURES `1`2 to l5 and 17 may, of course, include :power-amplification stages if necessary. Bias flux 'greater than the maximum signal linx Imay be provided by bat- -tery 200. This .head construction :has the advantage of .Providing oppositely directed high .frequency fluxes at :the gap v183 while requiring .only -a single :saturating strip :.for .ease 'of assembly. By vway 'of example the inner high frequency winding 190 may comprise 30 turns of No. 33 HF, 15 right hand fturns and .15 left hand turns in onelayer and have an .inside `diameter `of g2 'of an inch, while the signal coil ".may comprise .30 .turns No. 33 IHF all-1in the :same direction intwolayers with an-inside diameter of 52 of an inch. The oscillator coil may be 9&2 of 4Van inch long while Athe 'signal coil may be '%4 of aninch'long. The single .modulatorfstnp 4may :advantageously be .001 by -lz'in'ch .FIGURE ll illustrates `a head construction similar :in operation to that'of FIGURES '9 and 'l0 wherein a signal 'coil 194is `connected in a circuit as shown in .iiIGURE l10 and vis wound around a high frequency coil (not shown) which is wound part in one direction and part .in the opposite direction as the winding 190 in .FIGURES 9 `and 10 -and is energized as the winding "190 in FIGURE 10. The rsatura-ting member 201 is of :reduced cross-section as in the lprevious embodiments. . fllhepolc shoes 203 and 204 define a non-magnetic gap #205 and are in magnetic circuit with the saturating strip S201 by means of core portions 207-2l0.

FIGURE .12 illustrates a head .construction utilizing n single 'winding 220 all wound in the same direction 'on a fsaturating strip 221 bridging between va pair of pole pieces 222 and 223 defining a non-magnetic gap 224. .The 'winding 220 is excited by means of an loscillator .225 'through capacitance y226 and inductance 227 'tuned ato the fundamental :frequency of oscillator 225. The :output .is Ptaken 'from 'the winding V220 through an amapliler i229'tuned to twice the frequency of the oscillator 225, a suitable detector 230 and output device 231. Polarizing ilux 'is introduced by means of battery 233 andtresistor '234. PIt will be observed that the saturating istrip 221 may have enlarged portions 221a engaging flat- Wisewith the lower faces of the poles 222 and '223 to `4provide .a complete loop magnetic circuit. This head construction has the disadvantage that the fundamental *frequency component 'fromthe oscillator is not balanced 'out satrthe gap, -so that -thehigh frequency excitation 'must beadjusted -to a value too .low .to cause erasure of .the rsignal'uon record .member 1235. In practice, with a thin v4saturating strip portion 22lb, .it has been .found substanttially impossible to produce an appreciable erase vfield `at gap224. dtamaybe .noted that in .magnetic -circuitswhere the :xamrating .strips are '.nfseries, the stri-ps maybe .twice csovi'defas where the strips `ar'e'in parallel,- providing .a

Vof the ux from the record member .passeslthrough each of the saturating strips. It `may also be noted that with the embodiment of FIGURES 9 and .10,'there is the advantage that the two winding :portions a and 1901) may be formed from the same electrical conductor thereby avoiding the necessity for interconnecting `two windings in series or parallel.

While the windings have been described as being energized by alternating electrical energy, other suitable excitation energy may be employed with Acorresponding modifications inthe electrical output 'means for deriving an electrical signal from the resultant llux variation in the head.

Referring now to FIGURE 13, a further embodiment 'has been illustrated wherein a magnetized record memlber 250 travels across pole shoes 251 and 252 at the nonmagnetic gap 253 therebetween, and the pole shoes have a thin saturating st-rip 256 wit-h enlarged end portions 25611 .in atwise engagement with the inturned ends of 'the pole shoes and a reduced cross section .central portion 256b of similar outline to that shown at 221 in FIGURElZ. The thin saturating strip may -be carried by a suitable nonm-agnetic support 257 of corresponding configuration and may be secured thereto as Vby a suitable cement .for ease in handling of the saturating strip. Polarizing ux may `:be introduced by a very weak magnet 'such fas indicatedaat 259 which in practice may comprise a .magnetized piece of magnetic recording tape less than a 'quarter :inchin width. The required value of polarizing fuxfhas been found insuiiicient to affect the tape `250 at the fgap `253.

As previously described, the polarizing tlux in the 'saturatin'g strip is preferably greater than'the 'maximum signal flux produced in the saturating strip by the record member 250. This same relationship holds true for the .embodiments of FIGURES 1 to l2. f It will fbe observed in FIGURE 1.3 'that'the hig'h sfrequency excitation is supplied -from an oscillator i260 through condenser 261 and inductance 262 tuned'to the fundamental frequency of the oscillator 260. The'win'dings on the saturating strip are bilar wound with one winding portion 264 connected in series with an inductance 265 and the high frequency 'oscillator 260,while'the 'other bilar winding portion 267 is connected 'in series with -an inductance 268 and the input terminals of ran lamplifier 269 preferably tuned to an even order harmonic 'of-"the fundamental of oscillator 260. The voutput of thetuned amplifier 269 is fed through detector '270 to the output device 271 which may be a loud speaker. ' Theinductances 265 and 253 are coupled in such a way as to balance'o'ut the fundamental component induced-in the output `wind- 'in'g 267, so that with no signal introduced at the 'gap 253, there will be no fundamental frequency component in'the input circuit of tuned amplifier .269 including coils 267 and 268. It will be understood that as asgnal "tluxfiis introduced into the saturating strip 256, the "fundamental component in the input circuit to tuned amplier "269will be unbalanced and there will be a fundamental `frequency perniIc having a composition of 50% .nickel and the .remainder iron and minor constitutents, or Perminvar having a composition of 25 cobalt, 45% the embodiment, the

remainder iron and minor constituents. Further, in this high frequency windings 304 and 305 are preferably connected in series aiding relation with respect to the gap 301 so as to actually excite the magnetic material of the record member 302. The high frequency magnetic intensity at the gap 301 is preferably of a small amplitude in comparison with the coercive force of the record member. With such excitation, the magnetic material of the record member operates on a minor hysteresis loop, the incremental permeability of which depends on the residual magnetization of the portion of the record member at the gap 301.

In this embodiment it will be apparent that the head responds not to the external leakage ux from the record member, but to the actual internal magnetization of the record member. The signal flux acting on t-he head is thus independent of the recorded wave length of the signal so that a fundamental defect of heads relying on leakage ux from a record member is overcome. Signal pickup windings 310 and 311 may be bitilar wound with windings 304 and 305 respectively, and also in series aiding relation with respect to the gap 301. The oscillator circuit is provided with an inductance 312 and the pickup circuit is provided with an inductance 313 coupled therewith, so that the coupling between inductances 312 and 313 may be varied to balance out the fundamental component in the pickup circuit in the absence of a signal flux from the tape 302. Polarizng ux may be introduced by means of a magnet 315. An amplifier 316 has its input connect- -ed to the pickup circuit and is preferably tuned to an even 1 :order harmonic of the frequency of the oscillator 317.

The output of the tuned amplifier 316 lis connected through a conventional amplitude modulation detector and power amplifier unit 318 to an output device 319 such as a loud speaker.

In FIGURE 15, a magnetic head is illustrated having a pair of pole shoes 320 and 321 defining a non-magnetic gap 322 receiving a record member 323 thereacross. A relatively thin flat member 325 of magnetic material bridges across the pole shoes 320 and 321 and is disposed in 'llatwise engagement with the ends of the pole shoes 320 and 321. The member 325 may have a window 325a therein to denne a pair of thin saturating strips 325b and 325e. A magnet 327 is illustrated as applying polarizing flux to the strips 325b and 325e, the magnet being too weak to affect the record member 323. In this embodiment, windings 330 and 331 are excited by means of fluctuating electrical energy in the form of a lseries of unidirectional rectangular pulses such as indicated schematically at 334 from a pulse generator 335. The windings 330 and 331 are preferably connected in series opposing relation with respect to the gap 32.2 so that there will be no net exciting iiux at the gap. Pickup winding 338 and 339 may be connected in aiding relation with respect to the gap 322 and be connected through an integrating circuit such as resistance 340 and capacitance 341 to a detector 342 and output device such as a meter 343. It will be understood that pulse operation of the head makes possible multiplex operation wherein a multiplicity of channels may be successively scanned by means of successive pulses to successive heads so as to reproduce a signal distributed across a multiplicity of channels. In this instance, each head would be pulsed at intervals as shown in FIGURE 1S. In the illustrated embodiment, the output at 343 would consist of a series of pulses at the frequency of pulses 334 and varying in amplitude and polarity in accordance with the signal on .the record member 323.

FIGURES 16 and 17 illustrate one manner in which .a saturating strip 360 may be excited without winding a conductor continuously therearound in a helix as illustrated in the preceding embodiments. In this case, as

illustrated in FIGURE 16, a conductor 36'1 is wound in one or more layers as illustrated with successive parallel .portions 361:1, 361b, 361C, 361d and 361e. The winding` aiding relation with respect to gap 383,

361 is then' folded oula 'median plane 'indicated by the dot-dash line3463, and the saturating strip 360 slipped into the folded winding as illustrated in FIGURE 17. 'It will be observed in this embodiment that the conductor portions 361a, 361e yand 361e on each side of the strip 360 will induce tiux in one direction longitudinally of the strip 360, while the conductor portions 361b and 361d will induce tiuxes in the opposite direction on both sides of the strip.

FIGURES 18 and 19 illustrate a Afurther way in which a saturating strip 360 may be excited. Here, a conductor 366 is wound in a sinuous manner with successive parallel portions 366a, 366b, 366e, 366d, 366e and 366f. :The winding is then folded on a medi-an line 368 to receive the saturalting strip 360 as indicated in FIGURE 19. In this embodiment, it will be observed that the conductor portions 366a, 366e and 366eon eachside of the strip 360 induce magnetic fluxes in one direction while the alternate conductor portions 366b, 366d and 366f induce magnetic tluxes'in the opposite direction.

It will be understood that in both FIGURES -17 and 19, the pickup windings can be wound on pole `shoes such as 380 and 381 in FIGURE 17. Specifically, wind$ ings 382a and 382b .may be provided connected in series the output being fed to a timed amplifier and detector unit 384 and then to a loud speaker 385 as in previous embodiments. In this case, the head may also be conveniently utilized as a recording head, the windings 382a and 382b being connected with a suitable input 388 by means of a switch 389 to record on'an unmagnetized vrecord member 390. It will be understood that the poleshoes 380 and 3&1 must be of suticientcross-section to carry the required recording ux.

As previously, the saturating strip 360 is preferably of greatly reduced cross-section in relation to the crosssection of the pole shoes 380 and 381. The totaler-osssection of the saturating strip 360 is required to be a fraction of the cross-section of the record- playback pole pieces 380 and 381. It may be noted that placing the windings 382a and b on the pole shoes rather than on the saturating strip results in an appreciable reduction in output voltage; however, with suitable operating conditions, the output is still superior to that obtained with that of induction type heads responding to the rate of change of signal linx. The record-playback head of FIGURE 17 is responsive directly to the signal flux at the gap 383 to give greatly improved low frequency response characteristics to the head and thus avoiding the need for -low frequency equalization as required with an indue tion type head. Further, the output level is such -as to avoid the need Afor a high gain amplifier which tends to introduce hum and other noise into the output.

FIGURE 20 illustrates the relation of output to the applied intensity of the magnetic eld established in the saturating strips. This relationship is applicable to the embodiments of FIGURES 1 to '13 and 17. Curves 400 and 401 illustrate the signal and noise output in decibels (on different scales) while curve 403 is a plot of signal minus noise in decibles as a function of the intensity of the magnetic tield established by the oscillator in the saturating strip. Point 404 on curve 403 may correspond to a signal minus noise value of 68 decibels. Point 405 may correspond to a signal minus noise value of 76 decibels. 'I'he vertical line 408 may correspond to the intensity of the magnetic field in the saturating strip just producing the saturation value of intrinsic inductionin the strip. It will thus be observed that optimum signal-tonoise ratio is obtained by driving the oscillator to produce values of magnetic intensity inthe region of or preferably above the saturation value of magnetic intensity for the saturation strips. It will be understood from FIG- URE 20 that the maximum value of H due to the oscillator excitation is preferably greater than the maximum. I-I provided by the polarizing source and the signal maglagoon-'frais 'of these embodiments must be less than about where B, is the maximum residual induction ofthe .record member, A, is the cross-sectional area of the magnetized portion of the record member, P is the ypolariri-ng flux in the saturating strip, F is any feed back flux which may be introduced into the saturating strip, and B,l is'the intrinsic saturation induction for the saturatng strip, the valves being taken in consistent units. This limit of about 50(B,Ati-P+F) .Bl is the approximate maximum limit for -usefulsreproduction of music since musical reproduction with cross sections appreciably above this limit produces a-result which is usually unacceptable to the listener .and to this extent -represents the critical limit for usefulreproduction.

Feedback is illustrated .in FIGURE 4 as being taken at the output of a stage of amplification indicated at 420 and prior to a further stage of amplification '420" by means of a capacitance 421 and resistance 422 coupled to signal windings 29b and 30b by .leads 43 and-'44. With one polarity of the battery 50, the feedback will -be negative, while with the other polarity ofthe battery 50, the feedback will be positive. It will be observed from the above formula that with positive feedback, and for a given signal-tonoise ratio, the maximum permissable area of the saturating strips is increased. while withnegative feedback, the maximum permissable cross section is reduced. It -has been found that the use of negative or inverse feedback is an elective way of improving the linearity, reliability, and stability 'of vthe heads of the present invention. With a sutlciently high --feedback factor, all distortion and gain variations, except those in the tape recording itself and effects due to varying degree of contact between the tape and playback head, can be reduced to negligibly small eiects. It was Ifound that with the proper value of the feedback components, the net output signal from a constant level recording on tape was essentially unchanged in spite of artificially produced large changes in the gain of the amplifier.

For each of the embodiments of FIGURES 1 to 13, 1S and 17, the polarizing iiux is preferably of magnitude greater than the maximum signal ux in the saturating strips.

For a useful signal-to-noise ratio for reproduction of music, it has been found that the Iarea of the saturating strips must be less than l5 nimes the cross-sectional area of the magnetizable portion of the record tape where a polarizing flux is present and tape has a maximum ux capacity of approximately 1 maxwell. Ihis limit is reduced by a Ifactor of two where there is no polarizing ilux and is reduced proportionately if the residual flux capacity of the tape is less than one maxwell, or increased proportionately if the flux capacity is greater than one maxwell. For example, for a llux capacity of .5 maxwell, the limit would be 71A times the cross-sectional area of the magnetizable portion of the tape.

It has been Ifound that for fine scanning of the record member, a small gap is required. This in turn tends to reduce the amount of tlux linking the head and to require a relatively small saturating strip. With relative Vof the saturating strips.

ly small signal 4ux linking'the Asaturating strip, it'is advantageous to use a high frequency excitation in'thesaturating strip. However, core losses `are not prohibitive at the high frequencies because ofthe Vsmall dimensions 'I'he vpresent invention vthus makes Ifeasible a very tine scanning gap while providing the required high output level for a good signaleto-no'ise ratio.

YIt may be noted that negative feedback reduces the eiective signal llux in the saturating strips, yand t-.hus allows for a further reduction in the minimum size of the strips for distortionless output. On the other hand, 4"a positive `feedback increases the permissible size of the strips by eEectively adding to the signal flux.

The following modifications in the illustrative embodiments maybe noted by way of example. In "FIG- URE 5a, a single loop conductor may excite the 'core rather than a multiplicity of turns. Further, a conductor carrying current of one phase may extend 'on one side of the core, and a conductor carrying currcnt'of the opposite phasemay extend on the other side of the `core to excite the core with bucking high frequency excitation with 'respect to the gap without actually wrapping a conductor completely around 'the lower saturating strip portions. It will be understood that the windings and 141 "of FIGURE 7 may be excited in vt-he manner illustrated in FIGURE 4, and the 'output taken vby means "of the circuit'showu in VFIGURE 4. In FIGURE 6, 'the saturating strip 1211 may be omitted to leave a ga'p `across the lower portion of the core, and "the 4head 'will operate, although unless the legs 'of the pole 'pieces' 100 and 101 are of relatively small cross-sectiomtherestiltin'g noise level will make the head unusable for the 'playback of music signals. The oscillator coils are 'preferably linked to the pole legs at .regions relativelyremote from the gap, so as rto avoid the' possibility of erasing eifects upon the tape due'to the high frequency excitation ux generated by the coils.

It may be noted that in the embodiments of FIGURES 1 through 14 and 17, the oscillator 'excitation may be tuned to a frequency of 500 kilocycles per second, -for example, and the output of t-he pickup coil may be delivered to a conventional radio set tuned to l megacycle per second. In such a case, polarizing iiux is most conveniently supplied by means of a permanent magnet arrangement such as illustrated in FIGURE 13 or 14. With one arrangement utilizing thin cross section saturating 'strips suc-h `as disclosed herein it'was found that the noise from the head was definitely below that of (the tape itself. It was also Ifound that optimum performance in the playback system corresponded to a considerably higher oscillator input to the head than would be expected, and in fact some heating of the head occurred at the optimum value of oscillator input. However, the required power is obtained from the miniature 12AU7 oscillator tube without ditliculty and no erasure of the t-ape 'was found.

It will be understood that the heads illustrated in the drawings may be used -for recording transversely of the tape, for example, with the gap at an angle other than a angle to the path of travel of the tape or even parallel to the path of travel of the tape.

Further, the pole pieces may be offset parallel to the long dimension o-f the gap to provide closely abutting parallel gap surfaces which overlap for only a fraction of the total extent of the respective gap surfaces. For example, with gap surfaces 425 and 426 offset laterally of the tape 427 as shown in FIGURE 21, and extending at right angles to the direction of travel of the tape as indicated by arrow 428, a short wavelength recorded signal will be concentrated in a longitudinally recorded eld at the central portion of the tape which travels across the longitudinally spaced overlapping portions 425a and 426a of the gap surfaces of the poles. However, for long wavelength recording, diagonal magnetiza- `of the organ.

tion'of the outer portions of the tape 427 will occur for example between the exposed gap surface 426b of one pole and a side surface 425e of the opposite pole, which side surface may, for examp1e, extend parallel to the direction of the tape travel. 425a and 426a may have a length between zero and several gap widths. This olf-set shoe construction in the head results in response to infinitely long wavelengths as well as good short wavelength resolution.

As indicated in FIGURE 5a, a pair of core pieces 500, 501 of magnetic material may be glued, cemented or otherwise bonded -to the pole portions of the nI lamination so as to enlarge the tape contacting area of the head and improve the long wavelength characteristics thereof. The additional core pieces may be made of a suitable ferrite material which may be a homogeneous crystalline material composed of ferrie-oxide and the oxide of another metal.

The embodiment of FIGURE 15 may be utilized in an electronic organ. In this application the pulsed head acts as a frequency changing device which reproduces the waveform of a recorded repeating signal at any desired frequency. With this system, there is required for each pitch of the organ a pulse generator, and a magnetic drum with recorded wave form is required for each stop Thus, an organ with 73 pitches and 20 stops would have 73 pulse generators and 20 magnetic drums, a total of 93 basic elements replacing the 1460 pipes of an equivalent pipe organ.

With respect to the embodiment of FIGURE 13, the modulator strip 256 is cut to shape with the wide end portions annealed at, and then cemented to a Bakelite stiiening piece such as indicated at 257. The coils are then wound on the strip assembly. With this construction procedure there is a minimum possibility of straining or otherwise damaging the modulator strips.

It will be apparent that many further modifications and variations may be effected -without departing from 4the scope of the novel concepts of the present invention.

I claim as my invention:

l. A magnetic transducer device comprising a magnetic core providing a single magnetic path, the Core being adapted to receive flux from an external source linking said path, means including winding means for generating opposed exciting fluxes in said magnetic path, a strip of reduced cross-section forming at least a portion of said magnetic path, and said winding means being provided by a single conductor having portions opposite- The overlapping portions `ly wound onsaid strip, and signal winding means linkv`ing the portion of said strip on which said conductor is wound in radially oEset relation.

A 2. Magnetic apparatus comprising magnetic core means providing a loop magnetic ilux path, means for introducing a signal ux into said loop path, said path including an elongated stnp of magnetic material of relatively small cross section in comparison to the cross section of other portions of said core means, means comprising exciting windings on said strip for producing opposed exciting uxes in said strip, and output means on said strip and` separate from said exciting windings and coupled to said exciting fluxes for producing an electrical output.

3. Magnetic apparatus comprising magnetic core means providinga loop magnetic ux path, means in series in said loop magnetic flux path for introducing a signal ux into said loop path, said path including an elongated strip of magnetic material of relatively small cross section in comparison to the cross section of other portions of said core means, means comprising exciting windings on said strip for producing opposed exciting fluxes in said strip, and means coupled to said exciting fluxes -for producing an electrical output, said last-mentioned means comprising a pickup winding on said strip.

v4. Magnetic apparatus comprising a magnetic vcore providing a series magnetic path and including means for introducing a signal ux in said path, said core including a portion which saturates at a lower value of iiux than other portions of said core, exciting Winding means on said portion for generating oppositely directed exciting fluxes at sections of said portion offset along said path, and an output winding encircling said portion in overlapping relation to said exciting means.

References Cited in the le of this patent UNITED STATES PATENTS 2,608,621 Peterson Aug. 26, 1952 2,700,703 Nordyke Ian. 25, 1955 2,722,569 Loper Nov. 1, 1955 2,804,506 Schurch et al. Aug. 27, 1957 i FOREIGN PATENTS 270,675 Switzerland Dec. l, 1950 OTHER REFERENCES EDVAC Progress Report #2, June 30, 1946, pages -4-23, py0-l64 and py-0-l65.

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