US 3651278 A
A magnetic transducer defining a substantially closed magnetic circuit including a pair of core halves separated by a front non-magnetic gap and defining an internal aperture having a diverging side wall portion extending from the rear of the front gap. A head winding is disposed about the core half and through the aperture and encompasses the core half so that there is no magnetic shunt across the winding. The turns of the winding closer to the front gap are disposed adjacent the front gap on the diverging side wall portion and in the path of the principal concentration of flux at the back of the front gap so as to significantly reduce the amount of flux which does not link the winding.
Description (OCR text may contain errors)
United States Patent Chupity et al.
[ 1 Mar. 21, 1972  MAGNETIC TRANSDUCER ASSEMBLY WITH WINDINGS POSITIONED SO AS TO PICK UP LEAKAGE FLUX Joseph Chupity, Mt. View; Burnet M. Poole, Los Altos, both of Calif.
Ampex Corporation, Redwood City, Calif.
Filed: May 6, 1970 Appl. No.: 37,362
Related U.S. Application Data Continuation of Ser. No. 822,745, May 7, 1969, which is a continuation of Ser. No. 462,272, June 8, 1965, abandoned.
U.S.CI. ....l79/100.2 C,340/l74.1 F Int. Cl. ..Gllb 5/20 3,325,795 6/1967 Mos ..340/l74.1F
T0 RECORD AND PIAYBACK CIRCUITS M FOREIGN PATENTS OR APPLICATIONS 988,272 7/1965 Great Britain ..340/l74.l F
Primary Examiner-Bemard Konick Assistant Examiner-Vincent P. Canney Attorney-Robert G. Clay  ABSTRACT A magnetic transducer defining a substantially closed magnetic circuit including a pair of core halves separated by a front non-magnetic gap anddefining an internal aperture having a diverging side wall portion extending from the rear of the front gap. A head winding is disposed about the core half and through the aperture and encompasses the core half so that there is no magnetic shunt across the winding. The turns of the winding closer to the front gap are disposed adjacent the front gap on the diverging side wall portion and in the path of the principal concentration of flux at the back of the front gap so as to significantly reduce the amount of flux which does not link the winding.
9 Claims, 12 Drawing Figures PATENTEDMARZI 1972 v 3.651.278
SHEET 1 OF 3 RELUCTANCE DISTANCE raou moi? GAP 5 FIG. I mg 4 Elli llll
INVENTORS Josm cnumv L BURNET n. POOLE FIG. 2
A TTORNEY PATENTEDMARZI I972 3.651.278
' sum 2 UF- 3 F I G. 5 TAPE TRANSPORT PRIOR ART SYSTEM VIIDEBAND sum RECORD PROCESSING AMPUHERS SYSTEM 2s? as 34 V swncumc RELAY TRANSFORMER cmcuns PREAMPL'F'ERS umr COUPLINGS INVENTORS J03 CHUPITY BUR I. POOLE BY wma,
SATTORNEY PATENTEDMAR21 1972 I 3.651.278
SHEET 3 UF 3 INVENTORS JOSEPH CHUPITY BURNET I. P 0L ATTORNEY MAGNETIC TRANSDUCER ASSEMBLY WITI-I WINDINGS POSITIONED S AS TO PICK UP LEAKAGE FLUX This application is a continuation of copending application Ser. No. 822,745, filed May 7, 1969, now abandoned, which was a continuation of copending application Ser. No. 462,272, filed June 8, l965, now abandoned.
This invention relates to magnetic recording and playback systems, and more particularly to magnetic transducers for converting from electrical signals to magnetic recorded patterns, and vice versa.
The art of magnetic recording now encompasses a wide range of electronic systems, including those used for video, digital, instrumentation and audio applications. Record and playback units for these applications take a great many forms, ranging from simple and inexpensive mechanisms to extremely complex precision units required to operate at high speeds and handle extremely wide frequency bandwidths. The magnetic tape systems which cover the broadest bandwidths today utilize the transverse track recording technique, in which the transducer mechanism is caused to scan transversely at high speed across a longitudinally moving tape, thus advancing the tape relatively slowly but providing the high head-to-tape speeds needed for wideband recording.
The magnetic transducer or head assembly is of course of vital importance to all of these systems, and special adaptations have been devised to meet particular requirements. The permeability and physical properties of the head material, and the width and the depth of the nonmagnetic gap are important design considerations. The circuit parameters, including inductance, figure of merit (Q) and the number of ampere-turns in the winding must also be adjusted to achieve optimum results for a particular application. Extremely wideband recording requires a small gap width and a material having suitable high frequency characteristics. Thus these heads are usually constructed of special metallic magnetic materials such as Alfesil, or of a magnetic semiconductor such as ferrite, or of an assembly of parts using these materials in combination. The necessary small gap width is established by making the transducer in separate halves, planning the opposed surfaces of the two halves to extreme flatness, coating the flat surfaces to a desired thickness with selected nonmagnetic gap material, and then assembling the two halves to form a substantially complete magnetic path. The pole faces of the transducer are positioned to pass across or along the magnetic record member, and the windings for the transducer are usually disposed about the sides of the halves or about the rear part of the transducer.
Although, as will be described hereinafter, the invention is applicable to all forms of magnetic transducers, it is of greatest benefit to the types of transducers which must meet the most stringent performance requirements, in particular those employed in extremely high frequency recording, and the examples given relate specifically to these types. In a television recorder of the transverse type, for example, the tape scanning mechanism comprises a rotary head drum typically having 1, 2 or 4 heads positioned on its periphery and scanning the tape in an oblique or transverse direction. In order to obtain the necessary frequency response, the gaps in the individual heads must not only be extremely short but the head must also be in direct contact with the tape while moving at high speed relative to the tape. The magnetic oxide particles in the tape, however, rapidly abrade the magnetic head under these conditions, and therefore head life is limited in accordance with the hardness of the head material and the depth of the recording gap. Head life is limited in wideband systems particularly because a relatively shallow gap depth is generally needed for both recording and playback. As gap depth is increased, the fringing field, or that portion of the field which emanates from the pole tip region and penetrates the record member, is correspondingly and substantially reduced relative to the total field because of leakage flux across the gap. Thus as gap depth is increased, the intensity of the magnetizing source must be increased to provide the same magnetization of the record member. The required increase of signal is almost directly related to gap depth over the useful range of depths. Because the power required increases as the square of the signal, substantial power may be demanded for a given wideband recording application. The usual compromise for recording is to sacrifice some head life by choosing a gap depth which is suitably small.
During playback, the amplitude of the reproduced signal is also dependent upon gap depth, as it is evident that the very shallow gap more effectively intercepts the fields immediately above the surface of the record member. Obviously, an increase in playback signal is highly useful, particularly where an adequate signal-tomoise ratio cannot otherwise be obtained. Even where the signal-to-noise ratio is adequate, a higher playback level is advantageous in terms of system design. The inductance and figure of merit of the transducer assembly are other important considerations, because of the limitations which these factors impose upon frequency response characteristics. Given substantial improvements in some or all of these factors, profound system changes, such as increased bandwidth or noncontract recording, may be achieved for given purposes.
Similar problems are encountered in other magnetic tape systems. Digital recording systems, for example, are required to operate with increasingly higher data transfer rates. These systems usually employ saturation recording with compact multichannel transducer assemblies, in which 7 or 9 closely packed magnetic heads may be disposed along a 1/2 inch computer tape, and operated at a kc. data transfer rate. The amount of power needed under these circumstances sometimes is high enough to require external or internal cooling to avoid softening the oxide binder or introducing temperature demagnetization of the tape. The amplitude of the reproduced signal is also of particular significance to these systems because it has a direct bearing on reliability and achievable data transfer rate.
A completely different example is provided by modern incremental or low speed playback systems for magnetic media. With such systems, the tape is moved in small increments to reproduce recorded digital signals asynchronously, or the tape is driven at very slow speeds. In an incrementing playback system of the modern type, it is not feasible to reach a substantial velocity while stepping because the individual steps are too minute, so that the playback head must generate adequate signals or a flux responsive head must be used. Extremely slow speed playback devices are used to conserve tape for narrow band and compact systems. These devices also operate at the lower permissible limit of the signal-to-noise ratio, so that a further increase in playback signal would provide a corresponding system improvement. As is understood by those skilled in the art, the dependence of signal amplitude on relative head-to-tape speed derives from the fact that most magnetic transducers in present use are responsive to rate of change of flux, and not to the magnitude of the flux itself.
In like manner, any basic improvement in magnetic transducers would extend to other magnetic tape systems, such as those using contact or noncontract recording and operating with drums, disks or magnetic sheets, all of which systems are in widespread use today.
It is therefore an object of the present invention to provide an improved magnetic transducer.
Another object of the present invention is to provide improved magnetic transducer devices without attendant increases in cost or complexity.
Yet another object of the present invention is to provide improved transducer means for high performance magnetic recording and playback systems.
A further object of the present invention is to provide improved high frequency wideband magnetic transducers having characteristics substantially superior in a number of basic respects to those of existing devices.
These and other objects are achieved for magnetic recording and playback applications by transducer devices which are characterized by the effective use of nonuniformity in the reluctance of different parts of the transducer and the elimination or reduction of available leakage flux paths in transducer structures in accordance with the invention. The nonuniform reluctance characteristic is used by disposing the windings about a high reluctance region of the transducer adjacent to the nonmagnetic gap. This placement provides highly effective interaction with the gap region and consequently with the record medium but without affecting the record medium directly during recording or playback. It serves to minimize leakage flux, thus developing increased levels of transducer output during both recording and playback and also provides an effective reduction in the transducer inductance, thus permitting the transducer to be operated at higher frequencies.
In a specific example of a transducer in accordance with the invention, intended for use in a transverse track video recording and playback system, separate halves of an Alfesil transducer include abutting arms defining the recording gap and the pole tip surfaces. The head windings are disposed about these arms, and are wound in a direction substantially parallel to the pole tip surfaces at a selected small spacing from the surfaces and the record member adjacent thereto during operation of the transducer. This spacing is of the order of 25 mils (.025 inch) or less. With a structure in accord with this configuration, it has been found that during recording a given magnetomotive force is converted with high efficiency to a fringing flux at the gap. On playback the coupling of the magnetic field to the windings is also improved. The interaction between the energizing current at the windings and the field at the recording gap is so much more efficient that reductions in the amount of the recording power substantially in excess of a factor of 10 can be achieved. Alternatively, the gap depth, and consequently the head life, may be increased by a factor of4 or more. On playback, the same wear considerations apply, and head inductance is reached by a comparable factor. Thus for the same inductance, a comparably greater number of turns may be used and the playback signal is increased, both because of the increased turns which are possible and because of the improved coupling between the field and the coil. The inductance of this transducer may be readily matched to an existing system through the use of a special transformer, and consequently material system advantages may be derived without any change except in the transducer.
Other specific examples of transducers in accordance with the invention may comprise modifications of existing wideband transducers, transducers for extremely high frequency applications and transducers for longitudinal recording, ineluding digital recording.
A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:
FIG. I is a perspective view, partially broken away, of one particular magnetic transducer in accordance with the invention;
FIG. 2 is an enlarged side view of the transducer of FIG. 1;
FIG. 3 is a front edge view corresponding to FIG. 2 of the transducer ofFlG. 1;
FIG. 4 is a graphical representation of the reluctance variations in transducers in accordance with the invention;
FIG. 5 is a generalized block diagram of a wideband recording and playback system of the type wherein transducers in accordance with the invention may be used.
FIG. 6 is a perspective view of a different form of transducer in accordance with the invention;
FIG. 7 is a front view of the transducer of FIG. 6;
FIG. 8 is a side view of the transducer of FIG. 7;
FIG. 9 is a perspective view, partially broken away, of a multichannel digital transducer in accordance with the invention;
FIG. 10 is an enlarged front view of the transducer of FIG. 9; and
FIGS. HA and 11B are diagrammatic views presented to illustrate the improved flux paths provided by arrangements in accordance with the invention.
A high frequency transducer 10 in accordance with the invention is shown in FIGS. 1-3, as it is adapted to be applied to a wideband recording and reproducing system 'of 'the transverse track type (illustrated in FIG. 5). The transducer design of FIGS. 1-3 offers advantages for either of two high frequency purposes, the term high frequency" as used herein relating generally to the frequencies required for wideband magnetic recording systems. As one example, the transducer is applicable to a television tape recorder of the four head, transverse track type, for recording television signals of approximately 5 mes.
As shown in FIG. 5, a tape head drum 12 is caused to scan a relatively wide tape 14 in a transverse direction at high speed. A tape transport system 16 is utilized which cups the tape 14 about the'head drum 12, so that intimate head-to-tape contact is assured. Only the principal elements of this type of system are illustrated in FIG. 5, because elements will be recognized as those found in widely used videotape recorders. Thus more detailed features have not been illustrated and the system will not be described in detail. The rotating head drum 12, containing four heads 20-23 symmetrically disposed about the periphery, is controlled by an associated servo 25 to maintain a head-to-tape speed of approximately 1,500 inches per second (IPS) and proper timing relation during both record and playback. Signals to be recorded are provided from a wideband signal processing system 30 through record amplifiers 31, a relay unit 34 and transformer couplings 35 to slip ring assembly 32, and then to the four heads 20-23 concurrently. If desired, rotary transformers may be employed in place of the slip ring assembly 32. Reproduced signals are generally passed from the slip ring assembly 32 through separate transformer couplings 35 in each of the four channels to the relay unit 34 which determines the direction of the signals (record or reproduced), then through pre-amplifiers 36 to switching circuits 37 operated in synchronism with the head drum 12. The signals are thereafter passed through equalizing and amplifying circuits (not shown) into the signal processing system 30.
This type of wideband recording and reproducing system is described in general form, although not necessary to an understanding of the invention, because of the significance of the individual transducer 10, such as is illustrated in FIGS. 1-3, to the successful operation of the system. The conflicting requirements as to gap width G gap depth G and the types of material needed for high frequency operation result in relatively rapid wearing of the pole tip portion of the transducer. The signal ultimately becomes degraded to a point of unacceptable quality for recording and reproduction when the gap depth has been completely worn through. A relatively high power driving signal must be utilized during recording, and the problems of modulating and controlling this driving signal are substantial and contribute materiallyto the cost of the system. In addition, the playback voltage generally required substantial preamplification, although the noise characteristics of the tape comprise the principal limitation on the signal-to-noise ratio of the system. The magnetic tape 14 is of course not a perfect recording medium and has an inherent noise factor which cannot be eliminated by the system and which is a substantial proportion, typically half, of the noise present in the reproduced signal. Nevertheless, any improvement in the signal-to-noise ratio enhances system performance, even though not large in numerical terms.
A second type of system with which the transducer of FIGS. 1-3 a specifically useful is in extremely wideband systems, in terms of the present state of the art. Thus, heat characteristics may be the principal limiting factors on extension of the operation of the system of FIG. 5 to a 10 megacycle or 20 megacycle bandwidth. Such frequencies can otherwise be achieved by a 5 me system by increasing the relative head-totape speed by a multiple of 2 or 4 respectively. Heretofore, however, wear considerations and head inductance have prevented effective use in this frequency range. As the frequency is increased, the inductance of the head becomes a more significant limiting factor on both efficiency and response.
The novel transducer illustrated in FIGS. 1-3 provides the desired improvement in a number of characteristics without sacrificing other performance aspects. The transducer 10 is illustrated as it is employed in conjunction with the rotary head drum and tape of FIG. 5, only a fragment of the tape 14 having been shown for reference. A magnetic circuit is provided in conventional fashion by a pair of thin magnetic core halves 40, 41, the opposing edge surfaces of which are made precisely flat, and to which a shallow nonmagnetic spacer is added by a plating, coating, or deposition technique. A nonmagnetic gap 11 of approximately 2 microns in width is utilized for recording the 5 megacycle signal bandwidth of a television signal. Because the gap length imposes a definite limitation of the minimum wave length which can be recorded and reproduced, the gap width must be precisely defined. Each core half of the transducer is here preferably made of a magnetic high frequency material such as Alfesil. When assembled, the two halves 40, 41 form a principal central aperture which is filled with brazing material to rigidly affix the halves together, and each comprises a pair of arms disposed in a somewhat distorted U" shape. Each arm from each core half abuts the like arm of the other core half in complementary fashion, to define the front and rear of the transducer. The front arm portions form the gap poles and include inner and outer notches close to the pole faces. The inner notches define a second internal aperture within the magnetic transducer, adjacent the front gap and encompassed by a loop of magnetic material. Windings 13 are disposed substantially parallel to the pole faces, and seated in the notches of each pole arm.
It should be appreciated that this head assembly is extremely small, being of the order of one-eighth inch in size. The distance along each pole face is approximately .02 inch and the gap depth G, is approximately of the order of A to 4 mils (typically about 1.5 mils) for a video head. The internal aperture adjacent the nonmagnetic gap has an internal diagonal dimension of approximately .010 inch in this example. The two windings 13 are series-coupled, and arranged in opposite sense so that the induced fields or currents combine additively during record and playback. The gap depth dimension is approximately l.5 mils in comparison to prior art heads having a gap depth of 0.6 mils with a comparable signal-to-noise ratio. An insulating and sealing epoxy is used in the notches and winding region, and the windings, on the average, are about 10 mils from a record medium with which the head is in contact.
In manufacturing this assembly, the gap depth is originally made substantially larger, of the order of 10 mils, and then is machined down to a smaller size for a particular application. When installed, the entire rear portion may be potted in epoxy if desired. The rear portion is used in this arrangement primarily as a support for the active regions of the transducer.
It is evident that a transducer of FIGS. l-3 approaches the ultimate in simplicity inasmuch as only two magnetic parts are employed. Nevertheless, truly radical improvements over prior art heads using similar materials, ferrite alone, or a combination of materials, are achieved by this structure. During recording, saturation is achieved in the record medium utilizing voltage only one-fourth or one-fifth of the prior voltages, and recording power of as little as one-thirtieth of that previously employed for the same gap depth. Again, for the same gap depth, playback voltage is increased by a factor of several times, given the same number of turns in the windings. The particular structure shown, of course, utilizes substantially greater gap depth in order to provide substantially longer wear. Under these conditions, and using an optimum number of turns for playback, the recording power is approximately the same as previously, the playback voltage is substantially higher and head life is increased by a factor of approximately 5. The inductance of the transducer is substantially lower than prior art devices; therefore, a greater number of turns may be utilized for a given inductance, and the increased multiple of turns substantially increases the playback signal although the inductance does not exceed that of the previous device. With the increased number of turns and an increased gap depth for longer head life, the recording power approximates that previously used.
Alternatively, the transducer may be designed to realize the permissible reduction in driving power, thus utilizing a lesser gap depth but achieving comparable recording and playback signal levels with a power reduction of between 1 and 2 orders of magnitude. Suitable signal levels may be achieved with a driving power of less than 1 watt, thus materially reducing the requirements imposed on the driving circuitry, eliminating the need for special cooling arrangements and the like. As a con sequence, the required circuitry is reduced, the recorder may be driven from a standard power supply, and substantial savings in cost, weight and space are effected which accrue to particular advantage in units designed for airborne or other portable use.
These operative advantages and flexibility of transducer and system design are achieved by virtue of the use of nonuniform reluctances existing in the transducer and the provision of a configuration which minimizes flux leakage. The windings are placed in the region of maximum reluctance to effect a close magnetic coupling to the front gap and to achieve minimum inductance. As shown in FIG. 4, the reluctance increases as the transducing gap is approached. As a consequence, as seen by the arrows representing magnetic flux in FIG. 2, a flux distribution tends to follow a small enclosed path about the secondary internal aperture within the transducer.
It will be appreciated by those skilled in the art that this utilization of the variation in head reluctance substantially modifies existing transducer theory. For example, in an article entitled Structure and Performance of Magnetic Transducer Heads by Otto Kornei, in the Journal of the Audio Engineering Society, Vol. 1, No. 3, page 225, July 1953, electrical equivalents of the magnetic circuit are described for both recording and playback. In these equivalents, the reluctances of the cores are related to the leakage reluctances at the gap, the gap reluctances themselves, the reluctances introduced by the physical relation to the record medium, and the reluctance of the medium itself. Shunt series networks are shown to represent these relationships, and the desirable relationships for playback and record are discussed. It is apparent, therefore, that transducers in accordance with the invention substantially modify this accepted concept, because the reluctance of the core can no longer be considered separately from the reluctance of the gap and the leakage reluctance at the gap.
It is true that the closely confined magnetic path, distributed around the secondary aperture in FIG. 2 as shown, provides a very low volume and has inherently extremely low losses. Further, however, there appears to be a substantially higher flux concentration, partly due to the reduced flux leakage in the magnetic circuit, and partly due perhaps to changes in permeability in the region of the pole tips, such that the efficiency of the transducer becomes extremely high. The transducer shown in FIGS. 1 to 3 may be utilized for both record and playback. It appears that in recording, the given magnetomotive force results in the generation of a large fringing flux for best recording characteristics. On playback, the magnetomotive force presented by the recorded patterns sees a much lower reluctance path in the direction of the pole faces than in the direction of the gap or the leakage reluctances, and high output signals are generated. Thus, during recording, a given magnetomotive force generates a substantially greater flux at the front gap and in the record medium. During playback, substantially all of the flux emanating from the record member intercepts the windings and currents are induced with high efficiency. This device accordingly takes advantage of the nonuniformity in reluctance, by placing the windings directly in the high reluctance region while eliminating certain previously available leakage flux paths, and at the same time maintains the coil an adequate distance from the record member, so that no problems exist with stray fields tending to affect the magnetization of the record member or with head wear tending to actually abrade the windings themselves.
The substantially direct coupling between the nonmagnetic gap and the windings, and the low loss magnetic path, result in an extremely low inductance characteristic for this magnetic transducer. As a result, the transducer is capable of working at substantially higher frequencies than prior art devices. The inductance of the transducer, although inherently low, may be readily matched to existing systems by the use of step-up transformers. In a practical television tape recording system in which associated circuits are designed for use with transducers having a much higher inductance, the novel transducers in accordance with the invention are directly usable, with their improved properties, simply by the inclusion of stepup transformers ofa l-lO ratio in the system of FIG. 5.
In summary, devices in accordance with the invention provide multiple increases in performance characteristics over comparable prior art devices. The improvement can be considered to be a fivefold increase in operating characteristics, which can be concentrated in any one particular variable or distributed over a number of variables, but particularly the playback voltage, record voltage, and power and head life. If it is desired primarily to decrease recording power without affecting playback voltage, for example, the inductance of the head can be kept extremely low by the use of only sufficient turns to generate the needed playback signal, and a reduction in the recording power in the ratio of 50:1 or better can then be achieved. If it is desired, on the other hand, to increase the playback voltage by a factor of or so, the number of turns may be substantially increased in which event an improvement in reduction of recording power in a ratio of approximately 4:1 is still feasible.
A-different device 50 in accordance with the invention is illustrated in FIGS. 68, representing a magnetic head assembly of the type disclosed and claimed in an application for patent, Ser. No. 260,465 now U.S. Pat. No. 3,303,292, filed Feb. 25, 1963 and entitled Magnetic Head Assembly" by Edwin A. Bedell, Jr. and Burnet M. Poole, and assigned to the assignee of the present invention. As described in that application, superior properties for wideband transducers are achieved through the use of a magnetic core 51 of Alfesil to define the pole faces and front gap, and physically associated ferrite slab 52 providing a magnetic shunt across the rear gap region to form a low reluctance path.The winding 54 for the assembly is disposed about the Alfesil core and the ferrite shunt element in the rear gap region. In this manner, the wear properties of the Alfesil and the magnetic properties of the ferrite are both utilized to advantage. The magnetic circuit has inductance and Q characteristics for which associated system elements are specifically designed.
In accordance with the present invention, the performance of such a transducer is substantially improved by the disposition of an additional winding 55 having a low number of turns in the high reluctance region of the magnetic circuit adjacent to but spaced apart from the front gap. This winding 55 is coupled in series and in an additive sense with the principal winding 54 of the transducer 50. For a principal winding 54 having turns, a supplementary winding 55 having 8 turns may be used. Again, inner and outer notches may be included in the pole arm and the supplementary winding 55'may be seated in place with epoxy. The additional winding has extremely low inductance and the total inductance of the transducer is increased only a few percent. There is substantially little change in the Q of the transducer, so that it becomes interchangeable with the prior devices. When using the same gap depth, a 50 percent increase in playback voltage, and a 50 percent decrease in recording current and voltage is also achieved so that the power required for recording is reduced to onefourth. For many purposes, it will be preferred to maintain these characteristics substantially the same as previously by increasing the gap depth so as to multiply the head life. It will be noted that these advantages may be achieved without the use of an extremely short magnetic circuit, as previously illustrated in conjunction with the arrangement of FIGS. 1 to 3. However, a large part of advantage derives from the placing of the supplementary winding between the gap and the remainder of the magnetic circuit which, by virtue of the high reluctance adjacent the front gap, increases the overall inductance very little but materially increases the coupling between the winding and the record medium, since the same flux links both with very little leakage.
A different arrangement in accordance with the invention is shown in FIGS. 9 and 10 and represents a high power head systems 60 for digital magnetic tape transports. Heads of this nature are generally arrayed as a plurality (typically 7 or 9) of precisely aligned parallel heads 10, and operated with saturation recording at extremely high data transfer rates (such as kc.). Similar multichannel head assemblies are utilized for magnetic drum, disk, and other systems, and may or may not utilize contact recording. The rate of flux reversal and the close physical proximity of the heads requires substantial power, so much so that extraordinary cooling measures are often required. In accordance with the invention, the two halves 40, 41 forming each transducer have notches on the inside and outside edges of the pole arms which meet at the front gap. Additively coupled windings 13 are disposed about these arms and seated in the notches as previously described, about a region spaced within about 15 mils from the pole faces. This places the winding 13 in the high reluctance region and close to the front gap. The close magnetic coupling between the winding and the gap ensures adequate magnetization even though the head power is greatly reduced. Specific features of transducers in accordance with the present invention may further be of particular advantage to digital data recording systems, which often use separate recording and playback assemblies for other reasons than transducer characteristics. Thus with the vastly more efficient magnetic coupling between the windings and the front gap, a minimum number of windings may be utilized for recording, and a maximum number of windings may be utilized on the separate heads for playback. In addition, the size of these assemblies may be considerably reduced.
The effectiveness of arrangements in accordance with the invention minimizing flux leakage paths which are present in devices known heretofore is illustrated by a comparison of FIGS. 1 1A and 1 1B. FIG. 11A shows a prior art transducer ar rangement 62 comprised of two halves of an apertured core joined together and with a winding 63 positioned at the minimum reluctance point about the rear gap. Flux leakage paths are indicated by the dashed line arrows while the operative flux path which couples the winding 63 to the tape 14 is known as a solid line arrow. On recording, maximum flux is at the winding 63 and this diminishes as the tape 14 is approached because of leakage through paths around the rear of the winding 63, across the transducer aperture, and across the front gap. On playback, the principal flux leakage occurs across the front gap and the aperture with the winding 63 being located at the point of minimum flux from the tape. Thus the configuration is such that the tape 14 receives 'minimum flux from the winding 63 and the winding 63 receives minimum flux from the tape 14.
In FIG. 1113 which represents a transducer of the invention, however, the winding 13 is positioned adjacent the operative gap and the front surfaces of the transducer. By virtue of this configuration, most of the flux leakage paths present in the device of FIG. 1 1A are eliminated. That is, their existence has no effect in limiting the flux coupling the tape 14 and the winding 13. The only leakage path remaining in FIG. 11B is across the front gap and the effect'of this may be minimized by suitable design considerations. Any flux from the tape 14 passing through the apertures in the transducer must link the winding 13, thus accounting for a substantial increase in playback signal over prior art arrangements.
While there have been described above and illustrated in the drawings various transducer assemblies for magnetic recording and reproducing systems, it will be'appreciated that a number of alternatives, forms and modifications may be made, and the invention is to be considered as encompassing all variations falling within the scope of the appended claims.
What is claimed is:
1. An improved magnetic transducer for reproducing and/or recording high frequency signals with a magnetic medium, the transducer comprising in combination: magnetic element means defining a substantially closed magnetic path, said magnetic element means defining a front nonmagnetic gap, pole tip surfaces on the face of the front gap and an internal aperture having a diverging side wall portion extending from the rear of the front gap, magnetic flux passing through said aperture, when said head is reproducing signals from said medium, with principal flux concentration being close to the rear of the gap; and transducing coil means including a plurality of turns of insulated wire terminating in a pair of output or input leads, said turns being disposed about the magnetic means and through said aperture and encompassing said magnetic path generally transversely of its direction so that there is no magnetic shunt across said turns, the coil means being positioned in said aperture with the turns of said coil means that are closer to the rear of the front gap being disposed adjacent the rear of the front gap on the diverging side wall portion and in said principal flux concentration so as to significantly reduce the amount of said magnetic flux which does not link coil means.
2. The transducer of claim 1 in which the spacing between the pole tip surfaces and the turns of said coil means closer to said pole tip surfaces is 25 mils or less.
3. The transducer of claim 1 in which the magnetic element has an outer notch generally transverse from the internal aperture and the turns of said coil means are seated in the notch and aperture.
4. The transducer of claim 1 in which the magnetic element means includes a pair of complementary magnetic core halves forming a substantially closed magnetic path and having abutting arms defining the front nonmagnetic gap, the pole tip surfaces, a rear gap, and the internal aperture intermediate the front and rear gaps; and in which the coil means includes two multiturn windings, one winding being disposed about each arm and through said aperture, the turns of each winding that J are closer to the rear of the front gap being disposed adjacent the rear of the front gap, said windings being series coupled and arranged in opposite sense such that flux passing through said arms induces currents in said windings which combine additively.
5. The transducer of claim 4 in which the spacing between the turns of each winding that are closer to the rear of the front gap and the pole tip surfaces is approximately 25 mils or less and the dimension of the internal aperture between said gaps is approximately 0.010 inch.
6. The transducer of claim 4 in which each core half has an outer notch generally transverse from the internal aperture and the turns of said coil means are seated in the associated notch and aperture.
7. The transducer of claim 6 in which the front gap depth is in the range of 0.5 to 4 mils, and said aperture has a dimension between 10 and 15 mils along the line of the gap depth.
8. A magnetic head assembly for a television tape recorder comprising a pair of complementary magnetic pole halves of a metallic material; means providing front and rear nonmagnetic gap elements between the pole halves; the poles halves defining an intermediate aperture in the region adjacent the front nonmagnetic gap, a ferrite element mounted adjacent the magnetic pole halves in a region including the rear gap but excluding the front gap and providing a low reluctance magnetic shunt from one side of one pole half to the opposite side of the opposite pole half; a first winding disposed about the rear gap region of the transducer and encompassing the pole halves and the ferrite element; and a second winding disposed about one of the magnetic pole halves through the intermediate aperture and coupled in series in an additive sense with the first winding, the second winding being disposed adjacent to and within 25 mils of the tape engaging surface of said front nonmagnetic gap.
9. A magnetic head for a wideband recording and reproducing sgstem of the transverse track type and suitable for bandwidt sin excess of approximately mcs. comprising a pair of metallic magnetic core sections formed along an axis that defines a nonmagnetic gap, such gap also having a front transducing gap and a rear gap; a ferrite body joined to said core sections and extending from the rear gap to regions short of the front transducing gap, said core sections defining an aperture extending along the axis between said gaps; a first transducing coil disposed about said core sections and ferrite body through the aperture in the region of the rear gap; and a second transducing coil disposed about one of said core sections in the region adjacent the front transducing gap and spaced apart from the ferrite body, said first and second transducing coils being coupled in series in an additive sense.