CA1061457A - Dynamic skew correction for rotating head magnetic recorder - Google Patents

Abstract

DYNAMIC SKEW CORRECTION FOR
ROTATING HEAD MAGNETIC RECORDER
ABSTRACT OF THE DISCLOSURE
Parallelism between the track on magnetic tape and the path of the rotating head is dynamically adjusted and maintained by adjusting one or both of the entry or exit guides guiding the tape as it helically wraps a mandrel. The mandrel has two halves that flank the rotating head so that as the tape wraps the mandrel, the rotating head traverses the tape at an oblique angle. Skew error is represented by a lack of parallelism between the track on the tape and the path of the rotating head across the tape.
The skew error may be corrected by laterally shifting either the entry or exit tape edge guide without distorting the track over a limited lateral range of movement by the guides. The entry and exit edge guides are contin-uous-compliant air-bearing guides. Similarly, the mandrel that supports the tape adjacent the rotating head is also air bearing.
CROSS REFERENCE TO RELATED APPLICATION
Detection of head to track alignment to provide a skew error signal to the present skew error correction invention can be accomplished a number of ways, but is preferably accomplished as described in the commonly assigned U.S. Patent 3,845,500 which issued, filed October 29, 1974, entitled "Head to Track Alignment in a Rotating Head Magnetic Tape Unit" by G.A. Hart.

Description

10~1~57

2 Field of the Invention

3 This invention relates to a rotating head

4 magnetic tape drive. More particularly, the invention relates to method and apparatus for dynamically correct-6 ing skew error between the transverse track on the 7 magnetic tape and the path of the rotating head.
8 Discussion of Prior Art 9 The problem of skew error correction in cross tape magnetic recording is quite old. It com-11 monly arises with regard to two environments. First, 12 static adjustments have been made to bring the tape 13 into proper alignment with the rotating head and the 14 mandrel that supports the tape along the path of the rotating head. An example of such a teaching is U. S.
16 Patent 3,697,676, entitled "Head to Tape Alignment 17 Apparatus and Method", invented by Arthur I. Protas.
18 The Protas patent teaches that parallelism 19 between the tracks on the tape and the path of the rotating head may be adjusted in a static manner by 21 moving either an entry or exit post guide that guides 22 the tape onto or off of the rotating-head mandrel.
23 Once the parallelism between the path of the magnetic 24 head and track is statically adjusted, moving the paral-lel track into alignment with the head is accomplished 26 by moving both guides laterally relative to the 27 rotating-head mandrel. Static adjustment of the guide 28 as herein referred to means that the guide is moved 10f~1457 1 once over a long period of time manually by an 2 operator. This is in contxast to a dynamic skew 3 error correction which would be accomplished by an 4 automatic system moving edge guides continuously or repeatedly over a short time interval to maintain 6 parallelism between the track and the path of the 7 rotating head.
8 Another rotating-head recording environment 9 in which it is desirable to change the angle of the track on a tape to bring it into parallelism with 11 the path of the rotating head is in converting from 12 a stop mode to a slew mode operation. In stop mode 13 operation in a rotating head magnetic recorder the 14 tape is held stationary as the rotating head sweeps a single time or repeatedly over a single track. In 16 slew mode the tape moves continuously and is synchro-17 nized in speed with the rotational speed of the head 18 so as to control the angle that the path of the head 19 makes with the longitudinal dimension of the tape.
Clearly, to switch from a stop mode to a slew mode 21 of operation would require a change in the angle of 22 the track on the tape to maintain parallelism with the 23 path of the rotating head.
24 Apparatus for adjusting tape path to handle this type of track angle realignment is taught in 26 U. S. Patent 3,376,395, issued to W. L. Rumple. The 27 Rumple patent shows crowned pulleys to guide the tape 28 onto the rotating-head mandrel. The mandrel itself 1 has fixed guides mounted thereon and is adjustable 2 laterally and statically to change the separation of 3 the fixed guides. When the Rumple magnetic recorder 4 is switched to a stop mode (tape stationary during read or write) operation, the fixed mandrel guides 6 are moved farther apart. The crown pulley entry and 7 exit guides then cause a shift in tape angle across 8 the mandrel as the tape moves between the fixed guides 9 spaced farther apart. The shift in tape anyle across mandrel changes the track angle relative to the path 11 of the rotating head. The adjustment in the Rumple 12 patent is a manual adjustment by n operator at the 13 time the conversion between stop and slew mode is 14 made.
Dynamic adjustment of entry or exit guides 16 for tape helically wrapping a mandrel has not been 17 done in the past because it was well accepted in the 18 art that to do so would distort the recorded track.
19 The reason for the distortion is the geometry o~ the tape path relative to the rotating-head mandrel. This 21 geometry controls whether there is equal or unequal 22 tension across the width of the tape as the tape wraps 23 the mandrel. For example, if the tape helically wraps 24 the mandrel 360, the angle at which the mandrel should be positioned relative to the longitudinal dimension 26 of the tape is given by the expression sin ~ = Gs/C
27 where Gs is the guide spacing (distance between the 28 same edge on the entry and exit guides) and C is the ~061457 1 circumference of the mandrel. This angle also corres-2 ponds to the acute angle of a track written by the 3 rotating head relative to the edge of the magnetic 4 tape.
S It was well accepted in the art that if the 6 guide spacing were to change without changing the 7 angle ~ between the mandrel and the tape, an unequal tension distribution would exist across the width of 9 the tape. This unequal tension in the tape would cause the tape to distort and consequently, a written 11 track on the tape would distort relative to the path 12 of the rotating head. This expected distortion will 13 be discussed further hereinafter.
14 The problem of distortion is heightened by the use of wide magnetic tape. As the ratio of L to W
16 twhere L is the length of tape between the entry and 17 exit guides and W is the width of the tape) goes down, 18 the problem of nonuniform tape tension and associated 19 track distortion with changes in guide spacing becomes acute. Typically, the prior art L to W ratios have 21 been in the order of 20 or even 30 to 1. A high L to 22 W ratio helps to eliminate the sensitivity of the track 23 distortion to guide spacing at the penalty of giving 24 up information storage capacity on the tape. The storage capacity of the tape goes down as the width 26 of the tape decreases. When the length to width ratio 27 goes below 10 to 1, the track distortion problem 28 becomes serious and can cause a rotating head to 10~1457 1 misread data from distorted adjacent tracks crossing 2 the path of the head.
3 The need for dynamically adjusting guide 4 spacing is due to skew error. Skew error is caused by tendency of magnetic tape to change its physical 6 characteristics under different environmental condi-7 tions. It is well known that humidity and temperature 8 affec~ the magnetic tape to the extent it may change 9 its length and width. These changes are small; how-ever, when dealing with high density tracks written 11 across a wide magnetic tape, they can cause a signi-12 ficant skew error, i.e., lack of parallelism between 13 the tracks on the tape and the path of the rotating 14 head. Therefore, there is a primary need to handle the skew error correction problem by dynamically adjusting 16 the edge guiding of magnetic tape as it helically wraps 17 the mandrel without, at the same time, introducing 18 distortion into the tracks on the tape.

In accordance with this invention, tape ?l wrapping a rotating-head mandrel is edge guided for 22 skew error correction without distorting the track on 23 the tape by dynamically edge guiding the tape immediately 24 before and immediately after the length of tape that wraps the path of the rotating head. Further, support 26 of the tape in the region where the tape wraps the 27 path of the rotating head is air bearing, whereby 28 unequal tension distributions across the width of the 10~1~57 1 tape tend to redistribute themselves. In an environ-2 ment where the ratio of length of tape between guides 3 to width of tape is less than 10 to 1, the guide 4 spacing has been changed over a range of 20 mils without distorting a track on the tape. Instead, the 6 track has rotated without distortion so that the 7 track is brought into a parallel relationship with the 8 path of the rotating head.
9 As an additional feature of the invention, the edge guides may be implemented by utilizing long, 11 continuous compliant edge guides usually having a 12 length at least equal to the width of the tape. These 13 guides also have an air bearing surface to support the 14 tape as it is edge guided. The function of the continu-ous guide is to prevent concentration of discrete edge 16 forces. ~iscrete edge forces would tend to distort 17 the tape when edge guiding forces are applied to change 18 the angle of tape and thus the angle of the track.
19 Also, a characteristic of a long, continuous compliant edge guide is that it assists uniform tension distri-21 bution across the width of a guided tape.
22 As a further feature of the invention, the 23 dynamic tape guidance is implemented by detecting the 24 parallelism between a track on the tape and the path of the rotating head, and thereafter control circuitry 26 activates electro-mechanical devices to move one or 27 both of the edge guides to bring the track back into 28 parallelism with the path of the rotating head. In 1 addition to corrections for skew error, preset cor-2 rection values may be gated into the control circuitry 3 to selectively position the guides for slew mode or 4 stop or incremental mode operation. During stop or incremental mode the tape is stationary during read-6 ing or writing operations.
7 The great advantage of our invention is that 8 it permits a rotating-head magnetic tape drive to 9 dynamically adjust itself to different track angles on the tape. By so doing, the temperature, humidity 11 environment that the tape is stored in need not be 12 as strict, and the tolerances on tape drives need not 13 be as tight. Stated another way, changes in dimen-14 sions of the tape caused by temperature and humidity can be accommodated by dynamic skew correction of the 16 invention. Also, tapes may easily be interchanged 17 between tape drives without tight tolerances to assure 18 that each tape drive is identical. Further, because 19 of the ability of the invention to assure accurate parallelism between track and the path of the rotating 21 head, the track widths can be smaller. With smaller 22 track widths the track density, and thus the data 23 density on the tape, can be increased.
24 The foregoing and other features and advan-tages of the invention will be apparent from the 26 following more particular description of preferred 27 embodiments of the invention, as illustrated in the 28 accompanying drawings.

~0614S7 2 FIGURE 1 shows one preferred embodiment 3 of the invention wherein the magnetic tape helically 4 wraps a mandrel 360, and the entry guide is adjusted in position dynamically in response to head-track 6 alignment detection by monitoring the signal from the 7 rotating head.
8 FIGURE 2 shows an example of a long, con-9 tinuous compliant guide with a guide adjust motor which can be used to provide the adjustable entry 11 guide in FIGURE 1.
12 FIGURES 3a and 3b show distortion normally 13 induced in a track where there is a low L to W ratio, 14 and the spacing between entry and exit guides is changed from the normal spacing for helical wrap. To 16 more clearly indicate the position of the track on 17 the tape, the tape has been unwrapped from a rotating-18 head mandrel and laid flat.
19 FIGURE 4 is a graph showing that with the apparatus and method of this invention, a track on a 21 magnetic tape may be rotated approximately + 5 mils 22 about a zero skew line without distorting the track~
23 FIGURE 5 is a diagram of pressures and 24 forces involved in providing air bearing support to the tape as it passes over the rotating-head mandrel.
26 FIGURE 6 shows one preferred embodiment for 27 implementing the skew error correction network of 28 FIGURE 1, including a guide adjust control network 29 utilizing a D.C. motor to move the guide.

1 FIGURE 7 shows an alternative guide adjust 2 control network for FIGURE 6 implemented with digital 3 circuits and using a stepping motor to move the guide.
4 FIGURE 8 shows an alternative preferred embodiment wherein the tape wraps the mandrel 540, 6 and the adjustable entry guide and the fixed exit guide 7 are mounted on the mandrel.

9 In the pr ferred embodiment shown in FIGURE
1, tape 10 is moving from left to right as it crosses 11 the adjustable guide 12, helically wraps the mandrel 12 14, and exits across guide 16. Rotating head sweeps 13 obliquely across the tape as the tape wraps the mandrel 14 14 360. The angle theta of the track on the tape is given by the expression sin ~ = the guide spacing Gs 16 divided by effective circumference of the mandrel 14.
17 Guide spacing Gs, as shown in FIGURE 1 is the spacing 18 between the fixed edge of guide 12 and the fixed edge 19 of guide 16.
When the tape is wrapped 360, the expression 21 sin ~ = GS/CE also gives the angle of the mandrel 22 axis relative to the axes of the cylindrical guides 23 12 and 16. At this mandrel angle, the tape will heli-24 cally wrap the mandrel, and at the same time have a uniform tension distribution across the width of the 26 tape. If the guide spacing Gs is changed without 27 changing the mandrel axis angle relative to the axes 28 of the guides, then wrap of the tape about the mandrel 106~457 1 will not be a natural helix and will induce non-2 uniform tension distribution across the width of 3 the tape. It is this non-uniform tension distribu-4 tion which would cause a distortion in the shape of the track written by the rotating head 18. This 6 problem is discussed hereinafter in more detail with 7 reference to FIGURES 3a and 3b.
8 In FIGURE 1 the problem of track distortion 9 when the guide spacing is changed does not exist, probably for a number of reasons which are not now 11 completely understood. It is likely that a major 12 reason is the fact that the mandrel 14 is air-bearing 13 and aids the tape in redistributing the tension 14 across the width of the tape when the guide spacing 15 - changes.
16 The air bearing mandrel might be achieved 17 a number of ways as, for example, by making the man-18 drel from a porous ceramic through which air may be 19 forced from an inner plenum chamber in the mandrel.
Alternatively, the mandrel might have a pattern of 21 holes spaced about the mandrel along the path of the 22 tape.
23 In addition, the rotor 20 which carries the 24 magnetic head 18 would also be air-bearing. The rotor would be air-bearing either by a hydrodynamic bear-26 ing created by its own motion, or by an additional 27 hydrostatic bearing. In other words, the rotor 28 motion could create a hydrodynamic air bearing, but lQ6~457 1 alternatively, air could be forced through the sur-2 face of the rotor to provide a hydrostatic air 3 bearing.
4 A schematic representation of electronic controls to adjust the guide 12 is also shown in 6 FIGURE 1. In the preferred embodiment, the rotating 7 head 18 picks up track servo information from servo 8 tracks located at each edge of the magnetic tape 10.
9 The servo tracks and the head/track alignment detect-ing networks 22 and 24 are fully described in 11 United States Patent No. 3,845,500 issued 12 October 29, 1974.

14 Briefly, these detecting networks 22 and 24 detect whether the head 18 is properly aligned 16 with an oblique track at each end of the track. The 17 output from each alignment detecting network is a 18 plus or minus count indicating the departure of the 19 head from alignment with the center of the oblique data track at each end of the track. With the posi-21 tion of each end of the data track known, it is 22 possible then to calculate the skew error or lack of 23 parallelism between the track and the path of the 24 rotating head by merely taking the difference between the count from alignment detecting network 22 and 26 from alignment detecting network 24.
27 The alignment detecting networks are 28 initialized or enabled in accordance with the position ", i, , 106~4S7 1 of the magnetic head along its rotary path. Detecting 2 network 22 is enabled while the head moves across the 3 servo track adjacent the entry edge of tape 10. The 4 entry edge is the edge crossed by the magnetic head as it starts to scan across the tape. Alignment de-6 tecting network 24 is enabled while the head moves 7 across the servo track adjacent the exit edge of tape 8 10.
9 - To determine the skew error, the count from alignment datector 22 would be subtracted from the 11 count from alignment detector 24. A positive differ-12 ence count for the skew error would indicate the track 13 is skewed counterclockwise relative to the path of the 14 rotating head. A negative difference count for the skew error would indicate the track is skewed clock-16 wise relative to the path of the rotating head.
17 The skew error correction networ]c 26 performs 18 the subtraction operation to arrive at the skew error.
19 The skew error network 26 then generates a drive signal for the guide adjust motor 28. The motion by the motor, 21 and thus the guide 12, is monitored by the skew error 22 correction network until the correct adjustment has been 23 made to eliminate the skew error.
24 Skew error correction network 26 may also be used to compensate for preset skew errors. This 26 would be desirable in a situation where the tape drive 27 was being switched between a stop mode of operation 28 and a slew mode of operation. Also, the introduction 106~57 1 of a preset skew error might be useful to compensate for a slight permanent angular misalignment between 3 mandrel 14 and guides 12 and 16.
4 In FIGURÆ 1 the counts from the alignment detecting networks 22 and 24 may also be used to con-6 trol the tape mover 30 to move the tape to a point 7 where a track is centered over the path of the rotating 8 head. Position error or pitch error correction is 9 made by the pitch error correction network 32 which averages the two counts from alignment detecting 11 networks 22 and 24. The skew error correction net-12 work provides a correction signal to make the track 13 parallel to the path of the rotating head, while the 14 pitch error correction network provides an error signal which can be used to center the track on the 16 path of the rotating head. The pitch error correction 17 network 32 and the tape mover 30 are not a part of the 18 present invention. Pitch error correction is shown to 19 complete an understanding of the environment in which the skew error correction network might be used.
21 The adjustable guide 12 and its associated 22 guide adjust motor 28 are shown in detail in FIGURE 2.
23 Guides 12 and 16 of FIGURE 1 are identical except 24 that guide 12 is mounted so that it may be laterally adjusted by the guide adjust motor 28. Both of the 26 guides 12 and 16 take the form of a cylindrical air-27 bearing support surface 34 with a fixed edge guide 36 28 at one edge and a compliant edge guide 38 at the other 106~457 1 edge. The air bearing surface for the guides is 2 provided by holes 40 in the support surface 34.
3 Channels 42 under each row of holes provide air under 4 pressure to the holes. The air passes through the holes and provides air pressure between the tape and 6 the surface of the support 34 so as to support the 7 tape with an air bearing above the surface 34 of the 8 guides 12 and 16.
- 9 Tape moves across the guides 12 and 16 by being wrapped approximately 90 to 180 about the air 11 bearing surface 34 before it enters or exits from the 12 mandrel in FIGURE 1. The fixed edge 36 of the guides 13 provides a reference service for guiding, while the 14 compliant edge 38 biases the tape uniformly against the fixed edge 36. The function of a continuous com~
16 pliant guide is discussed in more detail in copending 17 application Serial No. 335,609, filed February 26, 18 1973, now U. S. Patent 3,850,358, entitled "Continuous 19 Compliant Guide for Moving Web."
To adjust the guide 12 laterally so as to 21 change the guide spacing Gs in FIGURE 1, motor 28 is 22 driven by an error correction signal from the skew 23 error correction network 26 in FIGURE 1. The shaft 44 24 of the motor 28 is threaded and engages threads inside the guide 12. Accordingly, since the motor is fixed 26 in position, a rotation of the shaft 44 will cause the 27 guide 12 to move linearly and thus permit adjustment 28 of guide spacing Gs.

~0973025 -15-1(~61457 1 Motor 28 is attached to mounting wall 46 2 via the annular bracket 48. Bushings 50 guide the 3 shaft 44 of the motor, and at the same time permit it 4 to rotate relative to the mounting wall 46. Mounting wall 46 also contains pins 52 (one shownj to steady 6 the guide 12 as it is moved linearly by the threaded 7 shaft 44~
- 8 Lateral movement of the guide 12 may be 9 monitored by a two-phase tachometer. In FIGURE 2 the position sensing for the tachometer is accomplished 11 by a disk 54 which is monitored by transducers 55 in 12 mount 56. The dlsk 54 might contain magnetic marks, 13 in which case the transducers 55 would consist of two 14 magnetic read heads. Alternatively, disk 54 might have transparent and opaque areas r in which case the 16 transducers 55 would be a photocell. A light source 17 would be placed on the other side of the disk 54 18 opposite the photocells. For a two-phase tachometer, 19 two tracks on disk 54 are used, with each track being monitored by a read head in the transducer assembly 56.
21 The electronics of the two-phase tachometer are in the 22 skew error correction network 26 of FIGURE 1, which 23 will be described hereinafter in more detail. The 24 two-phase tachometer is used herein as it is able to detect changes in direction as well as distance moved.
26 At this point, before proceeding to the 27 electronics of the skew error correction network, a 28 discussion of prior art problems when changing guide ~061457 1 spacing to correct skew error, and the actual operative 2 result obtained by the present invention, will be rP-3 viewed. In addition, a possible explanation for why 4 this invention is able to correct for skew error with-out track distortion will be reviewed. It should be 6 emphasized, however, that high-density recording with - 7 a rotating head using air bearing support and flying 8 heads at the present state-of-the-art is not completely 9 understood. It may ~e that factors other than those discussed herein significantly contribute to the 11 successful operation making up this inventive combina~
12 tion.
13 Theoretical Background for Distortion-Free skew Error 14 Correction As stated earlier under discussion of prior 16 art, it is known that when tape is wrapped 360 heli-17 cally about a stiff cylindrical bearing, there is one 18 natural angle-at which the tape will wrap the cylindrical 19 bearing or mandrel without distorting its shape or the shape of any track the tape carries. Any departure 21 from this angle when wrapping a stiff bearing mandrel 22 caused the tape to distort due to unequal tension 23 distributions across its width. This in turn causes 24 track distortion for tracks on the tape. Since the natural angle is related to the spacing between the 26 entry and exit guides for the tape as it enters and 27 exits the mandrel, it was well accepted in the art 28 that guide spacing could not be changed from the natural 1()6~457 1 angle to correct skew error without also causing 2 distortion of tracks previously written on the tape.
3 The track distortion caused by a change in 4 guide spacing in the environment of helically wrap-ping tape about a stiff bearing mandrel is depicted 6 in FIGU~ES 3a and 3b. To more clearly indicate the i path of the track on the tape, the tape has been un-8 wrapped from the mandrel and laid flat. The positions 9 of the entry and exit guides are shown relative to the tape.
11 Also in FIGURE 3a the dimensions L and W in 12 calculating the L to W ratio discussed herein are shown.
13 L is the length between the entry and exit guides, and 14 W is the width of the tape. As stated earlier, typical L to W ratios in the rotating-head magnetic recording 16 art are 20:1 or even 30:1. With such a high ratio, 17 changes in guide spacing do not materially affect track 18 distortion in the tape. However, in the environment of 19 the present invention where the L to W ratio is more in the range of 5:1 to 10:1, a change in guide spacing 21 introduces non-uniform tape tension and thus track 22 distortion along the path of the rotating head when 23 the tape is wrapped about a stiff bearing mandrel.
24 In FIGURE 3a, tape 10 is moving right to left from an entry guide 53 onto the mandrel where 26 the oblique tracks are written or read and then to an 27 exit guide 60. Solid line 62 represents the actual 28 position of the track while dashed line 64 represents 1 the path of the rotating head. The angle between 2 solid line 62 and dashed line 64 represents the skew 3 error. The skew error might be caused by changes in 4 the dimensions of the tape, or by slight differences in the geometry of the tape drives whereby the ~rack 6 written by one tape drive is not parallel to the path 7 of the rotating head in another tape drive.
8 To correct the skew angle entry guide 58 9 would be moved up, which would have the effect of decreasing the guide spacing Gs (see FIGURE 1). How-11 ever, when a stiff bearing mandrel is used instead of 12 simply rotating track 62 to a position overlaying 13 track 64, the movement of the guide causes track 62 14 to distort. The actual shape of track 62 as a result of moving entry guide 58 would be that of dashed line 16 62'. Distorted track 62' makes an S curve that criss-17 crosses the path 64 of the rotating head.
18 The offset skew error condition and track 19 distortion when using a stiff bearing mandrel is shown in FIGURE 3b. In this case the track 66 is 21 positioned above the path of the rotating head indi-22 cated by dashed line 68. Accordingly, to correct this 23 skew error between track 66 and the path 68, entry 24 guide 58 would be moved down which would increase the guide spacing Gs. However, because of the low length 26 to width ratio, and because of the stiff bearing 27 mandrel, track 66 instead of simply rotating also 28 distorts. Distorted track 66 is represented by dashed 1 line 66'. Distorted track 66' makes an S curve again 2 criss-crossing the path of the rotating head 68.
3 For clarity of illustration, the amount of 4 skew error and the amplitude of the track distortion in FIGURES 3a and 3b has been exaggerated. For high 6 track densities, wherein the track width is in the 7 order of 15 mils and the tracks are immediately 8 adjacent each other with no spacing between tracks, 9 the actual problems in reading previously written tracks are well represented by the exaggerated dis-11 tortions shown in FIGURES 3a and 3b.
12 In a typical environment with 15 mil wide 13 tracks, a read head might be 8 mils wide. This would 14 allow 3-1/2 mils on either side of the path of the read head as a margin for skew error or pitch/position 16 error. With the present invention, a six inch track 17 has been rotated about one of its ends to the extent 18 that the other end of the track moves approximately 19 + 5 mils about a zero skew position. The shape of the track for different guide spacings Gs is graphed 21 in FIGURE 4.
22 In the graph of FIGURE 4, the horizontal axis 23 is the distance along the track relative to the exit 24 tape guide that does not move. Accordingly, the left end of the track is substantially unmoved as the track 26 rotates due to the entry guide being moved up or down.
27 The vertical axis of the graph in FIGURE 4 is the 28 change in mils of a point on the track. The track 1 is graphed for different positions of the entry guide.
2 As can be clearly seen from FIGURE 4, as 3 the entry guide is moved from i2 mils up to 8 mils 4 below the natural or æero skew guide position, the track rotates without distorting~ Of further interest 6 in graph 4 is the fact that the zero skew line or path 7 of the rotating head does not align with track when 8 the entry guide is at the zero skew position. In 9 other words, the drive upon which the data was taken had a slight amount of built-in skew error. In this 11 drive the negative 2 mil guide down position would be 12 preset into the entry guide position to bring the 13 track into parallelism with the path of the rotating 14 head.
Assuming for example that as discussed imme-16 diately above, the track width is 15 mils and the read 17 head width is 8 mils, the advantages of a dynamic skew 18 error correction entry guide are apparent. Without 19 such a guide, the read head may wander only 3-1/2 mils off center line position before erroneous data will be 21 picked up. With a dynamic skew error correction guide 22 that could move the track 5 mils either direction about 23 the path of the rotating head, the read head would 24 have to move 8-1/2 mils laterally as it scanned down the length of the track before it would read erroneous 26 data. Accordingly, because of the dynamic skew error 27 correction, the lateral skew error tolerance in six 28 inches of track length has been increased from 3-1/2 29 mils to 8-1/2 mils. In other words, almost 2-1/2 times ~061457 1 greater skew error can be tolerated with dynamic 2 skew error correction than can be tolerated without 3 skew error correction.
4 The dynamics of the inventive combination whereby a change in guide spacing may be used to 6 correct for a skew error without causing track distor-7 tion is not completely understood. However, one 8 possible explanation is diagrammed in FIGURE 5 which 9 is a representation of a cutaway of the tape being supported over the air bearing mandrel. The mandrel's 11 radius is Rm. Tape 10 is supported on an air bearing 12 above the mandrel surface 70 by an air bearing. The 13 air hearing is provided by air pressure supply Ps 14 inside the mandrel forcing air through the holes 72 in the surface 70 of the mandrel. Pressure PFl between 16 the tape 10 and the surface of the mandrel is given by 17 the expression PF=T/R, where T is the tape tension and 18 R is the radius of curvature of the tape as it wraps 19 the mandrel. Thus, PFl is equal to Tl divided by R
in FIGURE 5.
21 If tansion were to increase in the tape, 22 tape 10 would then seek a new equilibrium position as 23 represented by dashed line 10' in FIGURE 1. The 24 pressure between the tape 10' and the surface of the mandrel would be PF2 and would be equal to T2 divided 26 by R2 as shown in FIGURE 5. Since T2 is greater than 27 Tl and R2 is less than Rl, the pressure PF2 would be 28 higher than a pressure PFl when tape 10 is riding 29 further above the surface of the mandrel.

1~14S7 1 It appears from this physical analysis of equilibrium position for the tape that if there were a non-uniform tension distribution across the width of the tape as, for example, caused by a change in the guide spacing Gs, then the tape would tend to seek different equilibrium positions across the width of the tape by adjusting its radius of curvature. In other words, instead of the tape carrying internal forces that would tend to stretch and distort it, the tape tends to merely change its flying height above the surface of the mandrel.
Very slight changes in flying height above the mandrel do not harm the flying height characteristics relative to the head because the head itself and the rotor assembly that carries the head are designed to penetrate into the tape and generate their own controlled air bearings to control flying height between head and tape.
Description of Skew Error Correction Network Referring now to FIGURE 6, one preferred embodiment for implementing the skew error correction network is shown. The skew error correction network 26 of FIGURE 1, and as detailed in FIGURE 6, monitors skew error by monitoring the counts received from the head track alignment detectors 22 and 24. Subtractor 74 takes the difference between the two counts produced 1()61457 1 by the alignment detectors and generates a positive 2 or negative correction count representative of the 3 skew error. The sign of the correction count from 4 the subtractor 74 will be defined such that a posi-tive difference means the entry guide should be moved 6 up while a negative difference count means the entry 7 guide should be moved down (see FIGURE 4).
8 The correction count for the skew error is 9 passed by adder 76 to a gate 78 to be loaded into a register 80 until the guide adjust control network is 11 ready to use the correction. Register 80 is updated 12 periodically by gating in the correction with a 13 position correction command that enables gate 78.
14 Adders 76 and 82 are provided to add in preset counts if desired. Binary Preset Number 1 may 16 be used to compensate for built-in mechanical skew 17 error caused by the mounting hardware that positions 18 the guides relative to the rotating-head mandrel.
19 Gate 83 is enabled once each time the rotating-head tape drive is initialized. The initialize pulse 21 enables the gate 83 to pass the Binary Preset Number 1 22 count to register 80 via adders 76 and 82.
23 Binary Preset Number 2 may be used to feed 24 in a count to operate the drive in a slew mode. In other words, a fixed skew error would be defined by 26 the Binary Preset Number 2 which would change the angle 27 of the tape and thus the angle of the track when the 28 tape drive switched from a stop mode to a slew mode 29 operation.

1()61457 1 Normally the tape would be operating in a 2 stop mode operation where the tape is stopped each 3 time a track is scanned by the rotating head. However, 4 when slew mode operations begin, a slew initialize pulse would enable gate 84 to pass the Binary Preset 6 Number 2. This preset count adds in a slew correc-7 tion when the tape moves continuously as the rotating 8 head scans the tracks on the tape.
9 Adder 82 adds the fixed counts from Binary Preset Number 1 and Binary Preset Number 2 if present, 11 and passes them to adder 76. Adder 76 adds the preset 12 error counts to the dynamic skew error correction from 13 subtractor 74 and passes the total error correction to 14 gate 78. When gate 78 is enabled by the Position Correction Command (PCC), register 80 is loaded with 16 the error correction count. The preset error counts 17 are only added in once during the first PCC. There-18 after the PCC would only gate dynamic skew error 19 corrections from subtractor 74 to register 80.
The guide adjust control network 86 in 21 FIGURE 6 utilizes a D.C. motor 88 for the guide adjust 22 motor 28 in FIGURE 1. When the Position Correction 23 Command (PCC) loads a new correction into register 80, 24 adder 89 adds the corrbction to the present position count from up/down counter 92. The sum gives a new 26 desired position for the entry guide. Subtractor 90 27 takes the difference between the new position for the 28 entry guide from adder 89 and the present position i457 1 of the entry guide as given by the up/down counter 2 and stored in register 91. This difference is con-3 verted by the digital to analog converter 94 to a 4 D.C. signal. The D.C. signal is amplified by ampli-fier 96 and used to drive the D.C. motor to move the 6 entry guide.
7 Two-phase tachometer network 98 monitors 8 signals from the tachometer disk 100 to detect the 9 direction and distance moved by the guide. If the guide is moved up, the two-phase tach has a signal on 11 its up output line causing the up/down counter to in-12 crease its count. If the guide moves down, the two-13 phase tach 98 would have a signal on its down output 14 line causing the up/down counter 92 to count down.
As the count from the counter 92 begins to 16 approach the new position for the guide, the difference 17 produced by subtractor 90 decreases, and the drive to 18 the D.C. motor decreases. When the count from counter 19 92 matches the new position count, subtractor 90 has zero output. The D.C. motor is no longer driven and 21 the guide is properly positioned to compensate for 22 the skew error and binary presets, if any. At this 23 time subtractor 90 also ~enerates a pulse on its com-24 pare equal output line to enable gate 93. Gate 93 loades the new position count from counter 92 into 26 register 91.
27 - To prepare the skew error correction network 28 to begin correction for another block of data (a plur-29 ality of-oblique data tracks), the registers 80 and 91 1 are reset to zero by their respective reset lines.
2 Subtractor 90 would produce a difference count to move 3 the guide to zero skew position. As the guide ap-4 proaches zero position, the up/down counter 92 would count down to zero.
6 The initialize pulses for gates 83 and 84 7 would be coincident with the first Position Correction 8 Command (PCC) during the next read/write operation.
9 One possible timing implementation for the PCC would be during the dead time of the rotation cycle for the 11 head. The dead time is time interval after the rotating 12 head has left t~e edge of tape and before the head 13 enters the tape again to read the next track. If the 14 PCC were timed in this manner, servo data from one track could be used to generate the skew error correc-16 tion count for the next track.
17 In FIGURE 7 an alternative guide adjust 18 control network 86 is shown which uses a stepping 19 motor to move the entry guide. Adder 102 receives the correction count from register 80 of FIGURE 6.
21 To this correction count adder 102 adds the present 22 position of the entry guide as defined by a present 23 position count from register 104. Accordingly, the 24 output of the adder 102 is the new or desired position for the entry guide as specified by a count. This new 26 position count is loaded into register 106 when gate 27 108 is enabled. A signal that might be used to enable 28 gate 108 would be the trailing edge of the Position 10ti1457 1 Correction Command (PCC) previously described with 2 reference to FIGURE 6.
3 The new position count from adder 102 is 4 also passed to subtractor 110 which subtracts the present position count from the new position count.
6 This gives an adjust count which is passed to counter 7 112 when gate 114 is enabled. Gate 114 could be 8 enabled by the trailing edge of the Position Correction 9 Command (PCC) in the same manner that gate 108 was enabled. With the adjust count loaded into counter 11 112, the not-zero line out of counter 112 will come 12 up enabling gate 116 and enabling clock 118. Clock 13 pulses from the clock are then passed by the gate 116 14 to the stepping motor drive to advance the stepping motor one step for each clock pulse. The same clock 16 pulses are used to count down the adjust count in 17 counter 112.
18 The forward or reverse direction for the 19 stepping motor drive 120 is picked up from the posi-tive or negative sign of the adjust count calculated 21 by subtractor 110. If the adjust count is positive, 22 the stepping motor 122 is driven in the direction to 23 move the entry guide up, and thus decrease the guide 24 spacing (see FIGURE 1). If the sign of the adjust count is negative, stepping motor 122 is rotated in 26 a direction to move the entry guide down and thereby 27 increase the guide spacing.

lO~l~S7 1 When the counter 112 has been counted down 2 by the clock pulses to a count of zero, the not-zero 3 output line from counter 112 will go down. Thus, 4 gate 116 is inhibited and clock 118 is turned off.
At the same time, single shot 124 converts the tran-6 sition from up level to down level by the zero output 7 line from counter 112 to a single shot pulse. This 8 pulse enables gate 126 to pass the new position count 9 to register 104. Register 104 is thus loaded with the new position and now the present posi~tion of the entry 11 guide.
12 To reset the skew error correction network 13 after a read/write operation of a data hlock and in 14 preparation for the next read/write operation, a reset signal is applied to register 80 (FIGURE 6) and register 16 106 of FIGURE 7. With register 106 reset to zero, and 17 register 104 holding the present position count for the 18 entry guide, subtractor 110 would subtract the present 19 position from zero. This would produce an adjust count that would return the entry guide to the zero 21 skew error position.
22 A signal slightly delayed from the reset 23 signal would then enable gate 114 to load this reset 24 adjust count into counter 112. The guide adjust control network 86 in FIGURE 7 would then operate as just des-26 cribed to count the adjust control count down to zero 27 and move the entry guide a distance corresponding to 28 the adjust count. This would return the entry guide 29 to the zero error position.

` ` 1061457 1 Also, as soon as the not-zero line from the 2 counter 112 transitioned from the up level to the 3 down level at the end of the count down operation, 4 single shot 124 would enable gate 126. Gate 126 would then load the zero count from register 106 into 6 register 104. This would complete the resetting of 7 the skew error correction network.
8 Alternative Entry Guide Embodiment 9 In FIGURE 8 the tape has been wrapped 540 about the rotating-head mandrel. In the additional 11 180 degrees of wrap, an adjustable entry guide has been 12 placed in the first 90 of the 540 wrap, and a fixed 13 exit guide has been placed in the last 90 of the 540 14 wrap. In FIGURE 8 only the entry of the tape onto the mandrel can be seen. The exiting of the tape from the 16 mandrel is hidden on the back side of the mandrel.
17 In the first 90 of wrap on the mandrel 128, 18 the tape 10 is guided by an entry guide. The entry 19 guide consists of adjustable rigid guide 130 and compliant edge guide 132.
21 The tape leaves the adjustable entry guide, 22 wraps the mandrel 360, and enters the fixed exit 23 guide. The fixed exit guide is made up of fixed rigid 24 edge guide 134 and continuous compliant guid0 136.
Only a portion of the guides 134 and 136 are seen, 26 since they wrap about the back side of the mandrel 27 128. Guides 134 and 136 would extend for about 90 28 of wrap. At the end of that wrap, the tape 10 would 29 leave the mandrel 128.

10~1457 1 The continuous compliant guides 132 and 136 2 could be constructed in the manner of those shown in 3 copending commonly assigned Patent Application Serial 4 No. 335,609, filed February 26, 1973, now Patent No. 3,850,358 . In particular, the pneumatic imple-6 mentation of the long continuous compliant guide might 7 be used to implement guides 132 and 136. The function 8 of these guides 132 and 136 is that they should pro-9 vide the same lateral force to the edge of the tape,.
irrespective of deflection of the guides.
11 Adjustability of the entry guide is accom-12 plished by motor 138 which rotates cam 140. As cam 140 13 rotates, it will push against the flange end 142 of 14 . edge guide 130. Edge guide 130 has slots 144 that lie in planes perpendicular to the axis of the mandrel 128.
16 Slots 144 engage pins 146 that are fixed to the mandrel.
17 Accordingly, as cam 140 rotates, guide 130 moves 18 around the mandrel 128 and at the same time the tape 19 edge guiding portion of the guide 130 moves in a direction parallel to the axis of the mandrel 128.
21 Edge guide 130 moves in groove 148 in mandrel 128 as 22 it is pushed by the cam 140. Groove 148 is wide 23 enough to let the guide 130 move parallel to the axis 24 of the mandrel 128. Spring 150 in groove 148 keeps flange 142 of guide 130 in contact with cam 140.
26 Movement of guide 130 is equivalent to moving the entry 27 guide 12 of FIGURE 1 up or down as previously described.

lO~il~S7 1 Other possible implementations of the 2 adjustable guide to correct skew angle might include 3 moving both an entry and exit guide simultaneously to 4 change skew angle. In such a system the entry and exit guides would probably be moved differentially.
6 Differential movement would amount to the entry and - 7 exit guides being adjusted laterally simultaneously 8 but in opposite directions to make a skew error correc-g tion.
Yet another alternative for lateral displace-11 ment of the entry and exit guides would be to use one 12 guide to correct for positive skew error and the other 13 guide to correct for a negative skew error. In such 14 an implementation, the logic for controlling the motion of the guides would have to monitor the position of 16 both guides. For example, if one guide were at a 17 positive position, and the skew error was a negative 18 correction, the negative correction would be made by 19 negative movement of the positively positioned guide until that guide reached its neutral or zero position.
21 If this negative movement of the positively positioned 22 guide did not fully compensate for the skew error, then 23 negative movement by the guide designed to handle nega-24 tive skew error would commence.
While the invention has been particularly 26 shown and described with reference to preferred embo-27 diments thereof, it will be understood by those skilled 28 in the art that various changes in form and details may lO~i457 1 be made therein without departing from the spirit 2 and scope of the invention. In particular, it will 3 be appreciated that other hardware implementations of 4 the adjustable guides and other logical control circuits for controlling the movement of guides might be used 6 to implement the skew error correction in a rotating-7 head tape drive.
:8What is claimed is:

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for dynamically correcting lack of parallelism between oblique tracks on tape and path of rotating head in a rotating-head magnetic recording device where the unguided length of tape about the head relative to the width of the tape is a ratio less than 10 to 1, said apparatus comprising:
means for edge guiding the tape as the tape enters and leaves the region of the head path whereby the spacing between the reference edge of the entry guide and the reference edge of the exit guide defines the angle of the tracks on the tape;
means for sensing lack of parallelism between the head path and the tracks on the tape;
means connected to said sensing means for generating a skew error correction signal indicative of the angular difference between the tracks on tape and the head path;
means connected to said generating means for adjusting said guiding means in accordance with the skew error correction signal so that the angular difference between the tracks on tape and the head path is reduced substantially to zero.

2. The apparatus of Claim 1 wherein said edge guiding means comprises:
fixed guiding means for edge guiding the tape at one end of the region of the head path, said fixed guiding means having a reference edge fixedly mounted relative to the head path;
adjustable guiding means for guiding the tape at the other end of the region of the head path, said adjustable guiding means having its reference edge movable for lateral adjustment relative to the reference edge of the fixed guiding means.

3. The apparatus of Claim 2 wherein said fixed reference edge guiding means is positioned where the tape exits the region of the head path; and wherein said laterally adjustable reference edge guiding means is positioned where the tape enters the region of the head path.

4. The apparatus of Claim 2 wherein both said fixed reference edge guiding means and said laterally adjustable reference edge guiding means comprises a long continuous compliant guide having a continuous rigid edge guide for the reference edge and a substantially continuous compliant guide at the opposite edge of the tape to urge the tape against the reference edge of said guiding means.

5. The apparatus of Claim 1 wherein said correction signal generating means comprises:
means for subtracting head-to-track misalignment at one edge of tape from head-to-track misalignment at the other edge of tape whereby the difference indicates the lack of parallelism between the track on the tape and the head path.

6. The apparatus of Claim S and in addition:
means for adding a preset value to the skew error correction signal where the preset value corres-ponds to fixed skew error due to the geometry of the rotating-head magnetic recording device whereby each magnetic recording device will record tracks on the tape at the same oblique angle.

7. The apparatus of Claim S and in addition:
means for adding a preset value to the skew error correction signal when the recording device switches between a tape stationary mode operation and a tape slew mode operation.

8. Method for dynamically correcting skew error between the data track on magnetic tape heli-cally wrapped about the path of a rotating head comprising the steps of:
detecting misalignment between the head path and the previous data track;
calculating the skew error from the detected misalignment;
adjusting the entry and exit guides at the entry and exit of the helical wrap in response to the skew error to change the helix angle of the tape about the head path, said adjusting step being per-formed during the portion of the rotation cycle when the magnetic head is between data tracks whereby the present data track is dynamically adjusted for skew error based upon detected misalignment between the head path and the previous data track.

9. The method of Claim 8, said adjusting steps comprising the steps of:
holding the exit guide at a fixed position;
moving the entry guide laterally to change the guide spacing between the entry and exit guide reference edges so that the helix angle of the tape about the head path is changed.

10. The method of Claim 8 wherein said adjusting step comprises:
extending the helical wrap for a predeter-mined distance before and after the head path;
mounting the entry guide in the helical wrap before the head path and mounting the exit guide in the helical wrap after the head path;
moving the entry or exit guide along the axis of the rotating head, effectively changing the axial distance the tape moves as the tape wraps the path of the rotating head, and thus changing the helix angle of the tape about the rotating head.

11. The method of Claim 8 and in addition the steps of:
adding preset counts to the detected mis-alignment where said preset counts correspond to fixed mechanical skew in the tape drive or a change in operation of the tape drive between the tape stationary mode and the tape slew mode.