CA1259695A

CA1259695A - Tape guide mechanism for dynamic tracking control

Info

Publication number: CA1259695A
Application number: CA000504201A
Authority: CA
Inventors: Frederic F. Grant
Original assignee: Datatape Inc
Current assignee: Datatape Inc
Priority date: 1985-03-28
Filing date: 1986-03-14
Publication date: 1989-09-19
Also published as: JPS63500210A; WO1986005765A1; US4573619A

Abstract

TAPE GUIDE MECHANISM FOR DYNAMIC TRACKING CONTROL
ABSTRACT OF THE DISCLOSURE

A tape guide mechanism includes an edge-guide element extending radially from a central axis. A tape guide roller, rotatable about the central axis, is arranged for urging an advancing magnetic tape laterally in the direction of the edge-guide element, to maintain an edge of the tape perpendicular to and in continuous contact with the edge-guide element during tape transport movement.
A piezoelectric actuator, preferably comprising a stack of flexing type bimorph elements responsive to a voltage signal relating to the direction and magnitude of a tape-tracking error, causes movement of the edge-guide element in a single composite direction having a first component which is perpendicular to the edge of the tape, and a second component, which is parallel to the tape edge.
Thus, the tape is caused to move laterally in either direction for tracking control, in one direction under the influence of perpendicular movement of the edge-guide element toward the tape, and in the opposite direction under the urging of the guide roller, to follow perpendicular movement of the edge-guide element away from the tape.

Description

L2~)695 TAPE GUIDE MECHANISM FOR DYNAMIC TRACKI~& CONTROL
BACKGROU~D OF THE INVE~TION
Field Of The Invention The present lnventlon relates to app~ratus 5 for guiding magnetic tape. In particular, the invention relates to ~ tape guide mechanism for dyn~mic tracking control in the presence of transient tracking errors.
Description Relative lo The Prior Art In the following description of ths relevant prior art, reference is made to Fig. 1, of the ~ccomp~nying drawing~, ~hich i~ an elev~t~on view p~rtially in section, of a t~pe guide mech~ism known in the ~rt.
With reference to the magnetic t~pe recordlng art, tr~cking ~ the proceæ~ of keeping a playback head on the path of a track already recorded on magnetic tape. The purpose of tr~cking control i6 to ad~ust the position o the pl~yback 20 heAd relative to the record tr~ck or vice versa, so tha~ the playback he~d i8 aligned with ~he record track for maximum sign~l-to-noise output.
To facilitate tracking, tape guides, which serve to keep the tapè laterRlly ~raight, are 25 normally ~trategically placed, for example, at the entrance ~nd exit point of the record/playback heads. One particularly preferred type of tape guide includes means for produclng a slight lateral orce in one directionJ for mAintQlning ~ given edge 30 of an advanclng tape in continuous contact with ~
fixed reference ~urf~ce, without damaging the tape edge or buckling the tape.
U.S. Patent No. 4,403,720, which is ussigned to the ~sslgnee of the present invention, ; 35 discloses ~ ~ape guide of thls general type`. The ' ' :, ! ': ' `' , ' : . j , ~ S ~ 69~

basic component6 of such a tape guide 10, which is shown in Fig. 1 of the drawings, are as follows.
The ~ape guide 10 has ~ post 12 for supporting a hollow tape gu~de roller 14. The po~t 12 is shown 5 in Flg. 1 as att~ched to and extending vertically from a baseplate 16. The post 12 may extend at any other angle and may, if desired, have the form of, or be replaced by, a rotatable shaft or similar device.
The tape guide 10 includes a sleeve 1~
surrounding the post 12 inside the guide roller 14.
The sleeve 18 has an inside diameter that i6 larger than the outside diameter of the po8t 12, for rece~ving ~n O-ring 24 between the po~t and the 15 ~leeve. The O-ring 24, which ~ s seated ln a per~pheral groove 26 on the poæt 12 and a : . corresponding groove on ~he sle~ve 18, serves as a : fulcrum to provide tilting motion of the sleeve relative to the plane 28 of the 0-ring.
Ball bearingæ 30, which ~re arr~nged in r~ces 32 on opposing æides of the plane 2~ of the O-ring 24, couple the guide roller 14 to the sleeve 18 so that the guide roller is rotatable about the sleeve. Thu8, the spaced bearings 30 mount the tape 25 guide roller 14 for rotation about the post 12 and tilting motion with the ~leeve 1~.
A tape guide flange 34, which has a reference surAce 36 extending radially from the po8t 12, i8 precisely positioned on the po~t 12, 30 ~dJ~cen~ one end of the tape guide roller 14, to control the lateral po~ition of t~pe (not shown).
In practice, the reference surface 36 is a l~yer or stratum of cer~mic or other h~rd m~teri~l, and may be rotatable or non-rotatsble.
Whlle the tape guide 10 ~8 generally ~596~

applicnble to guiding magnetic tnpe through a recorder, It is particularly well adapted for controlling the lateral position of the tape ~s it advances pas~ a head stack consisting of a plurality of closely spaced recordtplayback heads. In this configuration, tape guide 10 is posit~oned immediately before the head stack eo that an advancing ~pe wraps partially around the tape guide roller 14. The tape guide 10 is positioned so that the reerence surface 36 i8 parallel to the tape transport direct~on, with the centerline, denoted 38, o~ the advancing t~pe being off~et ~ dist~nce, denoted d, with respect to the pl~ne 28 of the O-ring 2S, in the direction of the guide flan~e 34.
The tape approaches the tape guide 10 ; tangential to t~e surface of the roller 14. ~f the edge of the tape i8 not in contact w~th the guide flange 34, the roller 14 ~ilts w~th the sleeve 18, because of ~he offset of the centerline 3~ of the tape from the O-ring 26. In response to this tilting movement, the tape moves axially in the direction of the flange 34. When the edge of the tape engage~ the reference ~urface 36, the tape transport direction becomes p~rallel to the surface 36 again.
In practice, the tape guide 10 of Fig. 1 maintains an edge of the tape generally perpendicular to And in continuous contact with the reference surface 36 during t~pe tr~nsport 30 movemen~. The Eorce~ actlng on the tape from the fulcrum effect of the O-ring 2~ and its off~et ~re balanced with the force~ scting on the tQpe from the guide flange 34.
U.S. P~tent No. 4,150,773 disclose~ an altern~tive ~rrangement for guiding A tape with an :
, 1~5~695 edge thereo~ in continuous contact wlth ~ reference surface. To decrease forces acting on the edge of the tape, and thus prevent d~mage to the tape, a rotat~ble tape-contacting sur~ce has a plurality of 5 spuced, parallel rings of rubber, the rings being inclined tow~rd a tape guide flange. In response to tape advancing movement, the rings in contact with the t~pe deflect. Because of the coefficient of frictlon of tape on the rings, the tape moves with 10 the rings toward the fl~nge, whereby an edge of the tape i8 maintained generally perpendicular to and in continuou~ contact with a stationary reference surface of the flange. The rlngs of the rot~table surface return to their nominal radial position when 15 not engaged by the advanc~ng tape.
There are ~ variety of standard magnetic tape record~ng formats currently used commerc~lly.
For ex~mple, one stand~rd tape recordlng format pre~ently used in the United States 7 lncludes 14 20 parAllel record tracks. W~th a l-~nch wide tape, each tr~ck has a track pitch of 70 mils (S0 mils record track width and 20 mils guardband width).
Tape guides of the type disclosed in the above-mentioned patents provide acceptable trecking 25 control for thi~ recording format and other form~ts in which the track pitch is of comp~rable size.
A general problem arises however when prior art t~pe guides ~re used for tracking control of information that i8 recorded on even more closely 30 spaced record tracks. For example, other magnetic ~pe recording systems use one or more recording heads which sweep transversely across a m~gnetic tape at high speed, a8 the tape i8 moving. With some of these sy~tems, a track pitch a~ small as 1.2 35 mils (1.0 mil record track width ~nd 0.2 mil 12~6~5 guardband wldth) may be used. Thus, even ~ small devlation in alignment of the ~ape as it passes over ~ pl~yb~ck hesd, compared to p~ssing over a record head, may either result in ~ignificnnt loss of playback signal s~reng~h, or playback of the wrong track.
A further problem arises when playback of narrow-width record ~rack~ occurs on apparatus that ia different than the appnratus that was used for recording. In thi~ ~ituation, precislon tracking control has been found to be even more neces~ary because of both ~tAtiC and dyn~mic ~ources of tracking error including mechanical tolernnce build-up ~nherent ~n differen~ playb~ck nppar~tu~, : 15 v~riation~ in the ~c~nning surface and the scanning plane <~f ~he playback head, var~ations ~n trnck centerline locatio~s, and other ~econdary consider~tions such a~ changes in tape tension because of temper~ture and humid~ty variations.
With the varlous tape guides known in the prior art~ trncking control of nsrrow-w~dth record tracks may be ~chieved effectively, at lenst in : theory, when a tracking error source is static, or when the error 60urce ls only slowly varying with ~ime, ~uch as when the dimensions of the tape chnnge : with humidity and temperature. This ~ 8 because a gtat~c or bias error CAn be compensated during plAyback by operator Ad~ustment of the playback ~pparatus.
Howe~er, when a tracking error source exlsts whlch is chang~ng more rnpidly nnd i8 there~y transient, such ns when one phase of ~ scnnning surf~ce during playback i~ different from the corresponding phase of the scnnniQg surf~ce during recording, mistracking results thnt can not be ~25~69~ii corrected effectively by operator ad~ustmen~. When the record tracks are of A relatively narrow width, signlficant loss of playback signal may occur.
Accordingly, prior art tape guides suffer from a disadvantage in that they are not suitable for tracking control of narrow-width record tracks in the presence of transient tracking errors.
SUMMARY OF THE INVENTION
In v~ew of the foregoing, nn ~b~ect of the invention is to provide for dynnmic ad~ustment of a tape gulde so that a record track of n magnetic tap~e i~ autom&tlcally aligned with a corresponding playback head for maximum s~gnal-to-noi~e output.
Thl8 ob~ect i8 achieved w~th a tape guide of ~ known type includ~ng ~n edge-gu~de element extending radially from a centr~l axis, and ~ roller, responslve to the wrapping of an advancing tape partially around the ro~ler, for applying an axial force to the advanclng tape to move the tape later~lly in the direction of the edge-guide element, to malntain a predeterm~ned edge of the tape in continuous contact with the edge-guide element during tnpe transport movement. In accordance wlth the invention, an improved tape guide of this type is char~cterized ~n that the edge-guide element is moun~ed for axial movement relative to the roller. An energizable element havin~ an output arranged for movement in response to a tracking-error voltage signal, causes axial movement of the edge-guide element rel~tive ~o the roller in nccordance with the movement of the output o the energiznble element. Thus, the tape i~
cnu~ed to move lnterally in either dlrection for tracking control, in one direction, under the lnfluence of axinl movement of the edge-guide ..

:~259695 element toward the roller, and in the Opposite direction, under ~he urging of the roller, to follow movement of the edge-gulde element away from the roller.
In a preferred e~bodiment of the in~ention, the energizable member comprises a piezoelectric bimorph element arranged for bidirectlonal bending movement in respon~e to the tracking-error slgnal.
The bimorph element i8 coupled to the edge-guide 10 element 80 that bending movement of the bimorph element c~uses corresponding axi~l movement o the sdge-guide element. The magnitude of the bending movement of the piezoelectric element ~ rel~ted to the amplitude of the tr~cking-error volt~ge signal 15 and the direction o the bending movement i~
determiDed hy the polarity of the ~gnal, the amplltude and polarl~y corre~ponding re~pect~vely to the magnitude and the d~rectlon of the deviat~on of a magnetic head from a desired track centerline 20 pOSition.
Preferably, the edge-guide element is mounted to maintain normal contact with the tape, independently of the voltage Applied to the bimorph element. A particul~rly preferred embodlment 25 includes pnrallel cantilevered flexure beams, the free ends of which are attached respectively to top ~nd bottom surfaces of an edge-guide mounting block, to constraln the free ends of the be~ms to parallel pl~nes. The mounting block moves axially in reponse 30 to bending movement of the bimorph element. The parallel beams flex, in response to movement of the block, in a configur~tion that constr~ins the mountlng block to A fixed, upright position. This llmlts movement of the block to A single composite 35 direction having A first axial component, whlch ~ 95 maintain6 the edge-guide element perpendicular to the edge of the tape, and a second radial component, parallel to the tape edge.
The roller and the edge-guide element are 5 arranged wlth respect to each other to maintain the edge of the tape against the edge-guide element, reg~rdle~s of the direction in which the bimorph element i6 bent. When the bimorph element causes the edge-guide element to move away from the tape, 10 the roller causes the tape to move laterally to follow the edge-guide element. When the edge-guide element i6 moved in the opposite directlon toward the tape, the rigidity of the tape and the yieldable nature of the roller permit the t~pe to move 15 laterally under the urging of the edge-guide element, wlthout buckl~ng or otherwise damaging the tape.
The invention; and i~6 other adv~ntages, will become more apparent in the detailed 20 description of a preferred embodiment presented below.
DESCRIPTION OF THE ~RAWI~GS
In the detAlled description of preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
Fig. 2 i8 an elevation view, partially in section, of a tape guide mechanism according to the present inventioQ;
Fig. 3 18 a perspective view of a 1exure hinge for mounting an edge-guide element of the tape guide mechanism of the pre~ent inventlon;
Fig. 4 lnclude~ an alternate preferred embodiment for movably positioning the edge-guide 35 element of Fig. 2;

1i~5~6~5 g Fig. 5 is ~ perspective view of an ~lternate embodiment for movably mounting the edge-guide element;
Fig. 6 is a diagrammatic illustration of three tracking conditions of tracking control 3pparatus for use with the tRpe guide mechanism of the present invention;
Fi~. 7 illustrates the envelopes of playb~ck signals corresponding to the three tracking conditions of Fig. 6;
Fig. 8 illustrfltes the use of tRpe guide apparatus of the present invention with rotary transducer apparatus; and F~g. 9 is ~ schematic block diagram of : circuitry associated with the tr~cking control app~r~tus of Fig. 6, for controlling the tape guide mech~n~sm of the present invention.
I DETAIL~D DESCRIPTION OF PREFERRED
- EMBODIMENTS OF THE INVENTION
._ The invention is most effective ~hen used in combination with a tape guide of the aforementioned type that produces ~n axial force for urgin8 fl magnetic tape laterally ~n one direction, to malntain one edge of the t~pe in continuous cont~ct with an edge-guide element during tape transport movement. Prefer~bly the desired force is ~ust enough to prevent substanti~l engagement forces between the ed8e of the t~pe and the edge-guide element, to avoid damaging the tape edge or buckling the tape during tRpe adv~ncing movement. However, unllke the tApe guide flppara~us of the ~rior ~rt, the present invention provides for dynamic tracking control by electrically controlling the axial position of the edge-guide element, whereby the t~pe is caused to move laterally in either dIrection in .

~5~ 35 accordance wlth the axial position of the edge-guide element.
Fig. 2 illustrates a preferred embodiment of a tape guide 40 according to the present 5 invention. Elements of Fig. 2 whlch are common or substantially similar to tape guide elements of F~g.
1 are ident~fled by like numerals.
To move a magnetic tape lnO laterally in either direction, the invention requlres that the 10 tape guide flange 34 of Flg. 1 be replaced by an edge-guide element 42 mounted for axial m~vement in either directlon. For that purpose, the element 42 i8 ~ecured to a flexure hinge 44, al~o shown in Fig.
3, which is ~tt~ched to the basepl~te 16 by 6uitable 15 means such as screws or cement. A no~inal poæition of the element 42 prov~des ~ clear~nce, denoted 46, to allow for the element to move toward the roller 14. Bearing in m~nd th~t tracking control correc~ions usu~lly are less than o`ne track pitch, 20 the clearance 46 may be on the order o~ only a mil or so for tracking control of closely spaced record tracks.
; The invention rurther requires that means are provided for moving the element 42 automatically 25 in elther axial direction. Preferably, the means are responsible to an electrical signal that is functionally rel~ted to the magnitude and the ~ direction of a playback head tracking error. For ; that purpose, a preferred embodiment of the 30 invention includes a piezoelectric actuator Sl ~n ; the form o~ a flexing type bimorph element 52, connected to the end of the flexure hinge 44 carrying the element 42. The element 52 i8 arrQnged for bending movement in response to a tracking error 35 sign~l, for c~us~ng axial movement of the edge-guide . - ,. . :' ~S~ i;95 element 42 in accordance with the bending movement of the blmorph element 52. The voltage error signal is npplied by control circuitry 54, an embodiment which ~s described in det~il with respect to Fig. 9.
PLezoelectricity is a well known phenomenon exhibited by certain cryst~ls and ceramic elements.
When a piezoelectric element iB compressed or extended in a particulnr direction, electric charges of opposite slgns are produced at opposite ends of 10 the element. Not only is ~n electric moment induced in a piezoelectric element ~y ~n applicAtion of mech~nical stress or Btrain~ there is also an inverse effect. Th~t iB, on applying an electric field, the elemen~ ch~nges ~h~pe by exp~nsion in one 15 direction and contraction in ~nother. A fuller discussion of ~he direct ~nd inverse ef~ects may be found in Encyclopaedic ~irectory of Physics, Pergamon Press, 196Z7 pages 503-505.
The flexing-type bimorph element 52 of Fig.

2 is comprised of two aligned strip~ of p~ezoelectric materi~l 56 and 5~> with the direction of expansion of the strip 56 aligned with the direction of contraction of the strip 5~ and vlce versa.~ Suitable adhesive means (not shown), such as 25 epoxy, ~oin the strips 56 and 58 together at their interface.
Bending movement of the bimorph element 52 is tr~nsl~ted into axial movement of the edge-guide element 42, a8 iollows. When a voltage signnl is 30 ~pplied of a polnrity th~t causes expansion of the 8tr~p 56 and thereby contraction of the 8trip 5~, the free or movable end of the element 52 moves the flexure hinge 44 upw~rdly, thereby urging the element 42 ~xiAlly in the dlrection of the gui~e 35 roller 14. Conversely, when an ~pplied voltage is 1~5~ 35 of the opposite polarity, the element 52 flexes the hinge 44 downwardly, thereby pulling the element 42 axlally In a dlrection away from the guide roller 14. Until ei~her volt~ge is removed, the bimorph element 52 remains bent.
Fig. 4 illustrates an alternate preferred embodiment of the invention for movably positioning the edge-guide element 42. A particular advantage of the piezoelectric actuator 51 of Fig. 4 is thst it provides bending movement that is amplified in response to a given level of applied voltage, for providing a correspondingly greater ~xial movement to the edge-guide element 42. For that purpose, the actuator 51 includes a st~ck of bimorph elements 72, three of which are shown electrically connected in parallel to the output of ~he ~forementioned circuitry 54.
Each element 72 in the stack consists of a mating palr of curved strips 74 ~nd 76 joined together at their ends so that the dlrectlon of expansion of the strip 74 is aligned with the direction of contraction of the strlp 76 and vice vers~. Central portions of ad~acent elements 72 are securely ~oined to each other, by suitable means, so that the elements 72 are mechanically connected in series, to the basepl~te 16.
Bending movement of the actuator 51 in response to an applled voltage occurs as follow6.
When a voltage i8 applied of a polarlty that causes the contracting o~ the 6trips 76, and thereby the expanding of the strips 74, each of the strips 76 flexes inwardly. This inward movement increases the curvature of the corresponding strip 74~ and thereby pu6hes it6 central portion upwardly. This upward , :

5~ ~5 movement is further enh~nced with the contempornneous expnnsLon of the strlp 74.
Accordingly, the strip 74 of the element 72 nttnched to the basepl~te 16 flexes upwardly n given 5 Rmountl in accordnnce with the magnitude of the applied voltage. Because the other elements are serinlly conne~ted, each of the other elements 72 in the stack moves upwardly by the given amount multiplied by its position in the stack. That ls, 10 the uppermost element 72 in the st~ck moves three times the distRnce of the first of the elements 72 in the stack, etc.
When ~n appl~ed volt~ge is of the opposite pol~rity, each of the ~trip~ 76 expand, and thereby 15 flex outwardly. This outward movement reduces the curvature of the strips 743 and thereby pulls the strips 74 downwardly. With the contemporaneous contract~ng of the strips 74, thls downward move~ent is nmplified. Accordingly, the expanding of the 20 strips 76 and the contr~cting of the strlps 74 cnuse each of the elements 72 to flex downwnrdly, wlth the total amount of downw~rd movement of each of the elements 72 be~ng related to its position in the : 6 tack.
A cantilevered rlgid beam 80, nttached to the output of the piezoelectric actuator 51 by means ~ of ~ link 82, cnuses axinl move~ent of an edge-guide ; mounting block 84 ln response to deflecting movement of the piezoelectric nctuntor. P~r~llel 30 cnntilevered flexure benms 86 and 88, coupling the block 84 to the post 12, prevent l~ter~l movement of the edge-gu~de element 42 under the influence of ~dvnncing movement of the tnpe.
,The beams 86 nnd 88 are further nrrnnged to ~25~695 ~ 14-prevent tilting of the block 84 when it i~ moved axially, to maintain normal contact between the tape-engaging surface of the element 42 and the tape, independently of the voltage applied to the 5 piezoelectric actuator 51. For that purpose, the free ends of the beams ~6 and 88 are ~tached to and across respectively the top and bottom surfaces of the mounting block 84, to constrain the free ends of the flexure beams to planes that are parallel to 10 each other.
When the mounting block 84 moves axially under the influence of bending movement of the bimorph element 72; the beams 86 and 88 flex in a configuration ~hat constrainæ the mounting block to 15 an upright position, parallel with the post 12.
When the bimorph elements 72 are actu~ted, either up or down, the block 84 moves in a composite direction hav~ng a fir~t axi~l component perpendicular to the edge of the tape, and a second radi~l component 20 parallel to the t~pe edge, in the direction of the post 12. The radinl displacement of the block 84 is illustrated ln Fig. 4 when the edge-guide element 42 ~s moved upwardly. Of course, the width of the element 42 is selected so that the edge-guide 25 element is not moved radlally inwardly beyond the surface of the gulde roller 14.
: Flg. 5 illustrates a wheel-like element 9 for ~xially moving the edge-guide element 42, wlthout any tilting component. An inner ring 94, 30 which is attached to the post 12, serves as a hub, and an outer ring 98, serves ~s ~ rim, supporting the edge-guide element 42. R~di~lly extending, relatively narrow spoke-like strips 96 are attached to corresponding upper and lower surfaces of the 1~5'~5 inner ring 94 and the outer ring 98, to prevent the outer ring from tilting relative to the inner ring.
The ring 98 may be moved ~xially, relative to the ring 94, by actuating one side of the rim or the other with the piezoelec~ric actuator 51. A
further advantage of the element 90 is that the ring 98 ensbles the edge-guide element 42 to extend circumferentially with respect to the guide roller 14.
To provide for dyn~mically ~d~usting the lateral position of a magnet~c tape, the tape guide 40 of the present invention should be used w~th trscXing control system that can provide a tracking-error voltage signal which is a me~sure of the deviation of a plsyback he~d from ~ track ceneerllne position ~nd which indicates the direction of the deviation.
There are various tracking control systems known in the magnetic recording art that provide tr~cking-control error signal corresponding to ~he magnitude and the direction of misalignment of ~
magnetic head with respect to a reference line of an associated information-bearing record track. The systems disclosed in U.S. Patents Nos. 4,044,388, 4,120,00~, 4,184,181 and 4,297,931 are exemplary of tracking control ~ppar~tus for use with ~ video recorder~
Figs. 6 and 7 illustr~te tracking control apparatus of the type that is not limited to tracking of predetermined sign&ls, such ~s video slgnals, ~s is the ca~e with ~2596~35 the tracking control apparatus of the nbove-identi~ied pa~en~s, ~nd therefore offers particular advantages for use with the tape guide 40 of the present lnvention.
Fig. 6 illu6trates the m~gnetic tape 100, ha~ing signals recorded thereon ln parallel, oblique tracks. In this figure, three ad~acent signal tracks 112, 114 and 116 are aligned in the direction of tape movement, denoted by the arrow 118. As 10 illu6trated by the phAntom lines, ad~acent edges of ad~acent tracks coincide so thnt guard bands on the tape 100 sre elimin~ted. It will be understood that this side-by-side arrangement of the record tracks of Fig. 6 is for illustration only. As made 15 apparent hereinbelow, ~d~aeent tracks may actually overlap to some degree, or may be separated by a guard band.
To limit cros~ tal~ between adjacent record tracks, alternating tracks h~ve information recorded 20 at mutu~ different azimuth nngles. These different azimuth angles are illustrated in Fig. 6a ;n that recorded ~normation ls represented by groupB of closely spaced par~llel llnes. Ihe information on one track, i.e. track 112, is 25 recorded with a record head, the gap of whlch is orlented in azlmuth at an angle of ~u and the next tr~ck, i.e. track 114, the informatlon ls recorded at ~n azimuth angle of -u, then on track 116 an angle o ~u, etc~
The magnetic heads used in playbnck must, of course~ al80 have gnps oriented in nzimuth corresponding to the ~zlmuth angle of the recorded inform~tion. Accordingly, Fig. 6 illustrAtes a pair of pl~yback heAds 122 and 124. The playback head 35 122 hns nn nzimuth angle of ~u, Q8 diagrammntically :1~50695 illustrated 9 corresponding to the az~muth angle of signals recorded on tr~ck 112 and trQck 116~ and other alternating tr~cks (not shown).- Llkewise, the playback he~d 124 has an azimuth angle of -u, 5 corresponding to sign~ls recorded on track 114 and ~lternating tr~cks (~lso not shown).
The tracking control apparatus of Flg. 6 requires that a playback head may stray from a reference line, such as track centerline 125, a 10 limited amount, without the attenuat~ng of its playback signal To prevent signal loss when limited track misalignment occurs, the playback head 122 has a track-w~se width, X, that i6 llArrOW
relative ~o the width, W, of the record tracks. The 15 nsrrow trac~-wise width of the head 122 permits l~mited displacement from tr~ck centerllne 125, without the pl~yback head 122 extending beyond the borders of its respective record tracks, which would cause signal attenuation.
Prefer~bly, both the playback head 122 and the playback he~d 124 h~ve a track-w~se width X
wh~ch is one-half of the track width W. Thus, both playback heads 122 and 124 may stray from tr~ck centerline 125 one-fourth of the width W without 25 appreciably attenuating their playback signals.
The tr~cking control apparatus of Fig. 6 further produces a tr~cking control signal that is directly dependent on the position of the playback head 122 relative to the centerline 125 of its

3~ as~ociated record tr~cks. For that purpose, there is provided a tracking coatrol head 126, which has an azimuth angle of ~u. The head 126 i8 mounted with respect to the playback heads 122 and 124 so as to overlap a given portion of each record track when 35 the plnyback heads 122 and 124 ~re allgned with the 125~69S

centerlines 125 of their associated record tracks.
Similarly, when ~he playback hends are misaligned relative to their respective track centerlines 125, the control head 126 overlaps one of the record 5 tr~cks by a greater amount, ~nd over~aps the ad~acent record track by a corre~pondingly l~sser amount. The direction that the playback heads 122 and 124 are mis~ligned determines which track is overlapped by more of the control head 126 and which 10 track is averlapped by less of the control head.
The tracking control head 126 has a track-wise width, Y~ that i8 wlde relative to the track-wise width X of the playb~ck he~ds.
Preferably, the width Y of the control he~d 126 is 15 equ~l to the width W of the record tracks.
Accordingly, when the pl~yb~c~ he~ds 12~ and 124 are tracking their centerlines Accurately and the edges of adjacent tracks coincide, the control head 126 overlaps each tr~ck by the amount X, the ~idth of 20 the playback heads.
Although the control he~d 126 is mounted to overlap adjacent ~u and -u record tracks, signal flux from the -u record track is negligible because the output of the head 126 is influenced essentially 25only by flux from the ~u record track. Thus, the amplitude of the tracking control signal is directly proportion~l to the amount that the head 126 overlap~ a ~u record track, for a head whose output varies linearly with width.
FiB. 6 al80 lllustrates three tracking alignment conditions relating to the playback o~
recorded information, and Fig. 7 illustrates the corresponding efect each o~ these conditions h~s on the signal~ produced by the playb~ck heads 122 and 35124 and the tr~cking control head 126. As shown in 5 ~ ~S

Fig. 6a, wherein the playback head 122 and the playback head 124 are exactly centered on their respective tracks, the tracking control head 126 is positioned to overlap the tracks 114 and 116 the same amount, which i5 the track-wise width X of the playback heads 122 and 124. When this condition exists, corresponding Fig. 7a shows that the envelope of the signal from the he~d 126 is approximately equal to the envelope of the signal from the head 122.
Figs. 6b and 6c and corresponding Figs. 7b and 7c illustrate respectively two ways in which mistracking occurs and how corresponding pl~yback head and control ~rack head output signals vary.
Fig. 6b illustrates a condition in which the playback heads 122 and 124 stray from a tr~ck centerline position toward ~he left hand edge of their respective record tracks, ~s viewed in the dr~wing. When th~s tracking condition exists, the ampli~udes of the signals produced by the pl~yb~ck heads 122 ~nd 124 would not change because the relatively narrow playback heads still overlap their respective record tracks by the same amount as when they were exactly centered on their tracks.
However, ~s shown in Fig. 7b, ~he ~mplitude of the signal produced by the tracking control he~d lZ6 would increase because of the corresponding increase in the amount that the head 126 overlaps the ~u record track 116.
Fig. 6c illustrates the playback heads 122 and 124 when they stray to the opposite side of their respective record tracks. In this condltion, there is still no change in the strength of the si~nals from the playback heads. However, because the tracking control head 126 now overlaps more of 5 ~ ~95 the -u record tr~ck 114 and less of the ~u record track 116, a corresponding decre~se ~n the amplltude of the slgnAl from the head 126 occurs.
During playback, the t~pe 100 and the he~dwheel of the plAyback he~ds 122 and 124 are driven at ~ppropriate rAtes. When the tape 100 is too advanced, the plAyback heads 122 and 124 stray too far to the left from ~heir desired track centerline positions. Similarly, when the tApe L00 is not Adv~nced fnr enough, the plAyback heAds are to the right of trAck centerline~ One way to correct for thi~ mi~tracking i8 to ad~ust position of the tape relative to the playb~ck heAds USiQg the tape guide 40 of the present invention.
Referr~ng now to Fig. ~, there is shown ~
pair-of t~pe guide~ 40a and 40b ~n ~ccord~nce with the invention, with basic components of sl~nt trAck msgnetic tape tr~nsport ~pparatus. The m~gnetic tape 100 is unwound from a reel (not shown) ~n the 20 direction of the arrow 140. The f~rst tape guide 40a i8 po6itioned to control ~he entrance point of ~he t~pe 100 with respect to two coaxlally arranged drum sections 144 and 146. The magnetic tApe 100 i6 then pAssed along a helical p~th adjacent the surf~ces of the drum sectlons 144 ~nd 146, through an Angular range of approximAtely 180. The second tape guide 40b is positioned to control the exit point of the tApe 100 A8 it leAves the drum sections 144 ~nd 146. The tApe 10~ i6 driven p~st 30 the second tape guide 40b in the direction of an ~rrow 154 by A CAp8tAn 156, which i8 connected to CAp~t~n motor 160. Thereafter, the tApe 100 i8 taken up by A reel (not 6hown) which i~ similar to the AforementLoned supply reel.
In order to effect the desired guidAnce ~25~95 along the hellcal p~th, the axes of the two tape guLdes 40~ and 40b are skewed relative to the axis of the drum assembly, and also relative to the axis of the capstan 156. Also, because of the hellcal 5guid~nce of the tape 100 around the drum assembly, the tape gulde 40a is offset upwardly from the tape guide 40b in the direction of the axis of the drum assembly.
The drum sections 144 and 14fi are separated 10 to form a circumferential gap 162. A motor 164 drives ~ he~dwheel 166, whlch is mounted at the center of the drum~ for rotation in an operational plane 167 in alignment with the gap 162. It is noted that the tape 100, while on the drum body9 is 15transported relative to the plane 167 of the he~dwheel 166, in accordance with the illustrated arrangement of the ~ape guides 40a and 40b.
;~ For its power, the motor 164 i6 connected to the output of a regul~ted power supply (not 20shown), which derives Its power preferably from line voltage. A head tachometer 172, typically present in slant track record/playback apparatus, ig employed 80 that the motor 164 dr~ves the headwheel 166 at the appropriate speed. In particular, the 25 80 called "once around" tachometer i9 suitable in thi6 respect because it controls angular head positLon. Thus, the tachometer 17Z, upon e~ch revolution of the transducer assembly, produces tlming pulses which ~re comp~red wlth a reference 3061gnal for regulat~n8 the motor 164.
The playb~ck heads 122 and lZ4 (not shown in Fig. 8) are mounted on the headwheel 166 80 th~t they ~weep across the tape 100 at different times.
The track head 126 is mounted on the he~dwheel 166 35midway between the two playback heads. All three ~5~6~35 head6 are mounted on the cLrcumferentlal edge of the headwheel 166, so that the tip of each head ls in contacting relatlonship with the tape 100 as the he~dwheel 166 moves each head through 180, past 5 the tape 100 from one edge to the other.
Preferably, the plAyback head 122 starts an operative sweep ~long the p~th of ~ record tr~ck, for example track 116, as the playback head 124 ls ~ust leeving the preceding track 114. At such time, 10 the control head 126 iB h~lfway across the tape, ~nd thereby lAgs the he~d 124 by 90 and leads the head 122 by 90. Similarly, the control head 126 comes into contact with the tape lQ0 when the playb~ck he~d 124 is halfway across ~he tape7 ~nd 15 leaves the tepe when the playback head 122 is only :~ halfw~y across the tape.
There ~ 8 shown in Fig. 9 a block dl~gram of the eforementioned control circuitry 54, for use with the tr~cking control apparatus of Fig. 6. In 20 the circuitry of Fig. 9, means are provided for producing ~ control signal corresponding to when the playback head 122 begins its operative sweep across the t~pe 100. To ~his end, ths a0rem2n.1Oned head tachometer 172 is used for producing an output 25 timing pulse, P, in synchronism with the head 122 as it begins each pass ~cross the t~pe 100.
A logic switch driver 173, which is timed by the output pulse P of the head tachometer 172, is arranged to oper~te a pair of electronic switches 30 174 and 176 ln ~ynchronism with each pa6s of the playback head 122. In doing 80, the driver 173, in response to the output timing pulse of the tachometer 172, causes the ~witches 174 ~nd 176 to close against their respective contacts 174a and 35 176~ while the head 122 is moving across the tape ~5~95 100. Simil~rly, the driver 173 mAintains the switches 174 and 176 closed agsinst their contacts 174b and 176b while the head 122 is off the tape 100 .
In a rotary tr~nsducer appar~tus operable, for exnmple, st 1800 revolutions per minute, i.e. a period of 1/30 of A second, the switch drlver 173 would nccordingly cause the switches 174 ~nd 176 to open and close each 1/60 of a second. Although the switches 174 and 176 are illustrated in the drawings as mech~nicAl in nature, it will of course be clesr to those skilled in the art that because the sw~tches ~re electronic, the switchlng from one condition to the other condition occur~ within a 15neglig~ble intervnl.
A plAybAck signA1 amplifier 180, receives the 8ign~1 S122, produced by the pl~yback head ~ 122. The amplifier 180 applie~ its output to i~ rectifier circuitry 182. The purpose of the 20 circultry 182 is to produce an output that ls ~
; duplicate of the positive-going portion of its lnput A low pass fllter 184 recelves the output of the rectifier circuitry 182. The purpose of the 25filter 184 is to provide a moving average of its lnput. This moving average provides ~ rel~tively smooth output of the positive-going portions of the signAl S122 as applled by the circultry 182.
Reference i~ now to the circuit pAth 30 ~ollowed by the sign~l, S126, which i~ produced by the tracking control head 126. The slgnal S126 i8 ~mplified by a sign~l amplifier 186 ~nd applied to a del~y clrcuit 188. The circuit 188 del~ys the amplified sign~l S126 by an amount, T, and 3sprovides an identicAl but delayed output ~ignnl ~L2~ i9S

S126. The delay T is equal to ~ time corresponding to the amount the tracking control head 126 leads the playback head 122, and ls provided 80 that the delayed signal S126 is 5 contemporaneous with the signal S122.
The switch 176, under the control of ~he switch driver 173, applies its output to rectifier circultry 190. The purpose of the circuitry 190 is to produce an output that is ~ duplicQte of the 10 positive-going portion of its input S126.
A low pass filter 192 receives the output of the rectifier circuitry 190. The fllter 192, like the filter 184, produces a moving average of its input, to provide ~ relat~vely smooth output of 15 the po6itive-going portion of the delayed signal 26 ~s applied by the circuitry 190.
The outputs of the filter 184 ~nd the filter 192 ~re applied respectively to the non-inverting ~nd inverting terminals of a : 20 differential amplifier 194. The amplifier 194 functions ax ~ substr~ctlon circuit, and thereby produces an output th~t is proportional to the output of the filter 184 minus the output of the filter 192. Accordingly, the output of the 25 amplifier 194 ls proportional to the difference between the positive-going portions of the signals 26 and S122.
The output of the amplifier 194 18 integrnted by ~n integr~tor 196. The Otltput voltage 30 from the integrator 196 represents ~ trncking-error ~ignal which i8 proportion~l to the integral of the dlference between the output of the fllter 184 ~nd the output o the filter 192. The magnitude of this error sign~l 18 proportion~l ~o the degree that the 35pl~yback head 122 i8 displAced from track centerllne 1259~95 125. The sign of the error signal, i.e. positive or neg~tive, repre~ent6 the direction of the displacement.
The output of the integrator is applied to 5 the piezoelectric actuators 51 of the tape guides 40. When the error slgnal is negative, the bimorph elements 52 are arr~nged to bend upwardly, thereby shifting the path of the tape 100 upwardly under the lnfluence of their respective edge-guide element~
10 42. Conversely, when the error s~gnal is posltive, the elements 52 bend downwardly, to cause the path of the tape 100 to shift downw~rdly under the influence of the guide roller6 14. In the flrst case, the record tracks of the tape 100 are ~hifted 15 to the left relative to the ~weeps of their respective playb~ck he~ds. In the latter case, the record tracks are shif~ed to the right ~elative to their playback he~ds.
- When the magnetic tape 100 ~s adv~ncing 20 from the ~upply reel to the take-up reel 9 tracking control w~th the t~pe guides 40a ~nd 4nb ls achieved under various tracking conditions as ~ollows.
When the playback head 122 is aligned with track centerline 125 of a ~u record track, the 25 tracklng control head 126 overlaps that record traek by an amount which is equal to the tr~ck w~dth ~ of the playback head 122. In that condition, the amplltude of the delayed signal S~26 is ~pproximately equal to the amplitude of the 30corresponding portion o~ S122. The lnput to the lntegr~tor 196 is therefore approximately zero volts, ~o th~t the tr~cklng-error signal volt~ge ~pplied by the integrator to the tape guides 40 is ; negligible. Thu~, the bimorph elements 52 ~re not 35energized to move thelr edge-guide elements 42, 60 ~Z59~95 tha~ no lateral ~d~ustment to the path of the tape 100 occurs.
When the magnetic tape 100 is mlsaligned so that the playback head 122 strays to the left of 5 tr~ck centerline 125, as viewed in Fig. 6b, the head 126 overlaps the ~u record track to a greater degree, in accord~nce with the tracking error of the he~d 122. This tr~cking condition causes the amplitude of the tracking control signal S126 to 10 increase in ~ccord~nce with the amount th~t the playback head 122 has strayed to the left.
Accordingly, the input to the inverting terminal of the ~ubstraction circuit 194 is greater than the input to the non-i~verting terminal. The integr~tor 15 196 produce~ a negative-going tracking-error signal, which indicates that tape record track i~ too ~ar to the right. The amplitude of the error signal is proportional to the dist~nce th~t the head 122 has strayed from centerline 125, and thereby i~ related 20 to the tracking error.
The piez~electric element~ 52 of the t~pe guides 40~ ~nd 40b, in response to a negative-go~ng error signal, bend upwardly ln accord~nce with the amplitude of the applied voltage. Thi~ bending 25 movement c~uses their respective edge-guide elements 42 to move axially toward the corresponding guide rollers 14. Due to its rigidity and the yieldable n~ture of the roller 14~ the tape 100 moves ~xially under the influence of edge-guide elements 42, 30 thereby ~hifting the entrance and exit points of the path of the tape 100 upwardly relatlve to the drum ~ssembly. BecAuse of the upward movement of the t~pe 100, the length of the tape path between the t~pe guide 40a ~nd the headwheel gap 162 incre~se~
35 ~lightly. This increase in tape p~th length hAs the ., same effect that moment~rily decreasing the inst~nt~neous speed of the tape would hAve. Th~
ls, the record tr~ck is shifted to the left rel~tive to the instantaneous position of the playb~ck head 5 122. By this relative movement, the he~d 122 moves from its posit~on ~o the left to track the centerline 125.
A simil~r situation, but opposte ~n result, occurs when the playback he~ 122 6tr~ys to 10 the right of tr~ck centerline 125, AS shown in Fig.
4C. For this ~racking condition, the tr~cking control head 126 overlaps the -u record track to a greAter degree, ~nd, of course, the -hu record track to ~ lesser degree. Accordingly, tbe amplitude of 15 S126 iæ reduced, and the input to the inverting ; terminal of the substr~ction circuit 194 is less ~han the input to its non-inverting terminal. The ` amplltude of the error ~ignal produced by the integrator lg6 is ~gain proportional to the tracking 20 error, although this time the sign of the error sign~ positive to ~ndic~te thAt the tr~cklng error is ln the opposite direction.
A positive error slgnal causes the piezoelectric elements 52 to bend downwardly, 25 thereby pulling their respective edge-guide elements 42 in the axi~l dlrection ~way from the guide rollers 14. As this axlal movement occurs, the guide rollers 14 c~use the tape to move ~xially, to follow the edge-guide element 42, thereby 30 maintaining the edge of the t~pe ~g~lnst the edge-guide element~.
With the downward movement of the t~pe 100, the length o~ the t~pe path from the tape guide 40~
to the he~dwheel g~p 162 decre~ses, thereby shifting 35 the record tr~ck to the right rel~tive to the ~L~59695 position of the playback head 122. With thi~
rel~tive movement, the head 122 returns to tr~ck centerline.
With respect to ~lant track tape transport 5 apparatus, the invention may of course be used to control the t~pe entrance polnt independently of the tape exit point. For example, the control circuitry 54 may be arranged so that the tape guide 40a responds to the tr~cking error signa:l during the 10 first half of the sweep of a playbaclc head across the tape, and the tape guide 40b responds to the tracking error signal during the second half of the sweep acros 8 the tape.
Furthermore, the tape guide mechanl~m, 15 according to the invention, h~æ general application to tracking control of any of a variety of signal recording formats and recording configurations. For ; example, the tape guide 40 may be used for tr~cking control of sign~l~ recorded on slant tracks, as 20 disclosed, but may also be used for tracking control of transverse tracks or for tracking control of signals recorded on longitudinal tracks.
CUrYfltUre errors may be compensated for in slant track tape transport apparatus by ad~usting 25 the tilt ~ngle of the tape guide mechanism at the t~pe entr~nce point. In the context of the scope of the invention~ tllt angle may be adjusted by means of a piezoelectric actuator, responsive to n curvature error signal, for ad~us~ing the angle of 30 the corresponding tape edge-guide element 42 rel~tive to the central axi8 of the tape guide 40a.
The invention has been disclosed in detall with reference to preferred embodiments thereof, but it will be understood that v~riations and 35 mod~fications can be effected within the spirlt and ~cope of the invention.
.

Claims

What Is Claimed Is:

1. In a tape guide mechanism Including an edge-guide element extending radially from a central axis of said tape guide mechanism, and a tape guide member, positioned axially from said edge-guide element and responsive to the wrapping of an advancing tape partially around said member, for applying an axial force to the advancing tape to move the tape laterally in the direction of said edge-guide element, to maintain a predetermined edge of the tape in continuous contact with a reference surface of said edge-guide element during tape transport movement, the improvement comprising:
a) said edge-guide element being mounted for axial movement relative to said tape guide member; and b) energizable means having en output, coupled to said edge-guide element and arranged for movement in response to a voltage signal that is functionally related to a tape tracking error, for causing axial movement of said edge-guide element relative to said tape guide member in accordance with the movement of the output of said energizable means, whereby the tape is caused to move laterally in accordance with the movement of said edge guide element.

2. A tape guide mechanism as claimed in Claim 1 wherein the output of said energizable means is arranged for causing movement of said edge-guide element in a first axial directon toward said tape guide member, and in a second axial direction away from said tape guide member, and wherein said tape guide member and said energizable means are arranged with respect to each other to maintain the predetermined edge of the tape in contact with said reference surface during movement of said edge-guide element in both the first and second directions.

3. In a tape guide mechanism including an element having an edge-guide surface extending radially from a central axis of said tape guide mechanism, and a tape guide member, positioned axially from said element and responsive to the wrapping of an advancing tape partially around said member, for applying an axial force to the advancing tape to move the tape laterally in the direction of said element, to maintain a predetermined edge of the tape in continuous contact with said edge-guide surface of said element during tape transport movement, the improvement comprising:
a) said element being mounted for axial movement along said central axis relative to said tape guide member; and b) piezoelectric means, coupled to said element and arranged for bending movement in response to a voltage signal that is functionally related to a tape tracking error, for causing axial movement of said element relative to said member in accordance with the bending movement of said piezoelectric means, whereby the tape is caused to move laterally in accordance with the movement of said edge-guide element.

4. A tape guide mechanism as claimed in Claim 3 wherein said piezoelectric means comprises a bimorph element arranged (1) for bending movement in a first direction in response to a voltage signal of a first polarity, and (2) for bending movement in a second direction in response to a voltage signal of the opposite polarity.

5. A tape guide mechanism as claimed in Claim 4 wherein said bimorph element is connected for causing axial movement of said edge-guide element in a direction in accordance with the direction of the bending movement of said bimorph element.

6. A tape guide mechanism as claimed in Claim 4 including means, coupling said bimorph element to said edge-guide element and responsive to bending movement of said bimorph element, for causing movement of said edge-guide element in a composite direction having a first component which is perpendicular to the predetermined tape edge, and a second component which is parallel to the tape edge.

7. A tape guide mechanism as claimed in Claim 6 wherein said element moving means maintains said edge-guide surface of said element normal to the tape during composite movement of said element.

8. A tape guide mechanism as claimed in Claim 3 wherein said piezoelectric means comprises a stack of serially connected bimorph elements electrically connected in parallel to receive the voltage signal, to provide movement of the output of said piezoelectric means that is amplified in response to a given level of applied voltage.