Capstanless helical drive system

A capstanless helical scan recording system (14) comprises a supply reel (24); a take-up reel (26); and a media transport path (20) extending from the supply reel to the take-up reel. A drum (30) is positioned along the media transport path and has a read head (R1,R2) and a write head (W1,W2) mounted thereon for recording and reading information in helical stripes on the media. The take-up reel comprises a rotor assembly (60), a rotating hub assembly (70), and a gearing system. The rotor assembly includes a sun gear (80) which rotates at a first rotational speed. The rotating hub assembly rotates carries three planetary gears (160) and imparts a velocity to media transported in the media transport path. The gearing system meshes with the rotor assembly and with the hub assembly for causing the hub assembly to have a greater rotational speed than the rotor assembly. The gearing system preferably has a gear ratio on the order of about 8:1. A tension arm (44) protrudes into the media transport path and decouples the supply reel to compensate for reel runnout during high speed operation. The system has no further means other than the take-up reel and the supply reel for imparting impetus to the media along the media transport path.

BACKGROUND 
1. Field of Invention 
This invention pertains to method and apparatus for recording information 
on storage media, and particularly to method and apparatus for 
transporting and reeling storage media in a helical scan recording system. 
2. Related Art and Other Considerations 
Numerous prior art patents and publications teach recording and reading of 
information stored in helical stripes (or "tracks") on magnetic tape. 
Examples of helical scan tape drives are shown, inter alia, in the 
following U.S. patents (all of which are incorporated herein by 
reference): 
U.S. Pat. No. 4,835,628 to Hinz et al. 
U.S. Pat. No. 4,843,495 to Georgis et al. 
U.S. Pat. No. 5,065,261 to Hughes et al. 
U.S. Pat. No. 5,068,757 to Hughes et al. 
U.S. Pat. No. 5,142,422 to Zook et al. 
In a helical scan arrangement, travelling magnetic tape is at least 
partially wrapped around a rotating drum so that heads (both write heads 
and read heads) positioned on the drum are contiguous to the drum as the 
drum is rotated. One or more write heads on the drum physically record 
data on the tape in a series of discrete stripes oriented at an angle with 
respect to the direction of tape travel. The data is formatted, prior to 
recording on the tape, to provide sufficient referencing information to 
enable later recovery during readout by one or more read heads. 
In many prior art helical scan systems, a fixed-radius, capstan is provided 
to control the linear motion to the tape as the tape travels past the 
drum. The capstan is driven by a dedicated capstan motor. In capstan 
systems, a tachometer is typically provided on the capstan to provide 
feedback information for ensuring constant linear velocity of the tape. 
Other helical scan systems (known as "reel-to-reel) do not employ a 
capstan. Once example of a "reel-to-reel system is disclosed in U.S. Pat. 
No. 4,125,881 to Eige et al. (incorporated herein by reference). One reel 
of such a system is typically known as a "supply" reel, the other reel is 
typically called a "take-up" reel. The supply reel and take-up reels 
generally each have dedicated motors. Various one-half inch magnetic tape 
products (such as IBM model 3480) are reel-to-reel streamers with 
stationary heads or linear tracking. 
Generally, prior art capstanless systems involved high speed and high 
tension operating conditions and a stationary head. Prior art capstanless 
systems accordingly are inapplicable to helical scan products such as 
those using 4 mm or 8 mm magnetic tape or other low speed drives 
generally. 
SUMMARY 
A capstanless helical scan recording system comprises a supply reel; a 
take-up reel; and a media transport path extending from the supply reel to 
the take-up reel. A drum is positioned along the media transport path and 
has a read head and a write head mounted thereon for recording and reading 
information in helical stripes on the media. The take-up reel comprises a 
rotor assembly, a rotating hub assembly, and a gearing system. The rotor 
assembly includes a sun gear which rotates at a first rotational speed. 
The rotating hub assembly rotates carries three planetary gears and 
imparts a velocity to media transported in the media transport path. The 
gearing system meshes with the rotor assembly and with the hub assembly 
for causing the hub assembly to have a greater rotational speed than the 
rotor assembly. The gearing system preferably has a gear ratio on the 
order of about 8:1. A tension arm (44) protrudes into the media transport 
path and decouples the supply reel to compenstate for reel runnout during 
high speed operation. The system has no further means other than the 
take-up reel and the supply reel for imparting impetus to the media along 
the media transport path.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows a capstanless a helical scan recording system or drive 14. 
Helical scan drive 14 includes a drive frame 16 and a deck floor 18. FIG. 
2 illustrates generally with reference numeral 20 a tape path for drive 
14. In particular, FIG. 2 shows a magnetic tape 22 (such as an 8 mm 
magnetic tape, for example) having a first end wound around a supply reel 
24 and a second end wound around a take-up reel 26. The path traversed by 
tape 22 is defined at least in part by a series of tape guides 28A-28G and 
a rotating scanner or drum 30. Tape guides 28 and drum 30 are ultimately 
mounted on deck floor 18. In all operations excepting a rewind operation, 
tape 22 travels from supply reel 24 to take-up reel 26 in the direction 
depicted by arrow 31. 
As shown in FIG. 1 and FIG. 2, drum 30 has read heads R1 and R2 as well as 
write heads W1 and W2 mounted on the circumference thereof (the exact 
positioning of which will be described below). Drum 30 rotates in the 
direction depicted by arrow 32. In addition, drum 30 has a servo head S 
mounted circumferentially thereon. As drum 30 rotates, at any moment a 
portion of its circumference is in contact with travelling tape. During a 
recording or write operation, write heads W1 and W2 are periodically 
positioned to record "stripes" or "tracks" as heads W1 and W2 move in a 
direction of head travel across tape 22. 
Details regarding the positioning of the heads W1, W2, R1, R2 and S, as 
well as the particular track recording scheme achieved by drum 30, are 
disclosed in U.S. patent application Ser. No. 08/150,726 (filed Nov. 12, 
1993) now abandoned, of Georgis and Zweighaft entitled "Method And 
Apparatus For Controlling Media Linear Speed In A Helical Scan Recorder" 
(incorporated herein by reference). 
FIG. 1 also shows that deck floor 18 has a flexible circuit/return plate 
assembly 40 positioned thereon as well a tensioning arm 44. As shown in 
FIG. 4, flexible circuit/return plate assembly 40 has mounted thereon a 
high resolution quadrature encoder 50 and a brake release 52. In addition, 
assembly 40 has mounted thereon upstanding stationary shafts 54 and 56 for 
supply reel 24 and take-up reel 26, respectively. Shaft 56 has an shaft 
axis 57 which, as illustrated in a horizontally-lying drive 14, extends in 
a vertical direction (see FIG. 5 and FIG. 6). 
Positioned in a circular pattern around each shaft 54, 56 are six motor 
coils 58. Each motor coil has a Hall sensor 59 centrally positioned 
therein on assembly 40. Utilization of motor coils 58 and Hall sensors 59 
are described in greater detail in U.S. Pat. No. 5,426,355 (filed Nov. 12, 
1993) of James Zweighaft entitled "Power-Off Motor Deceleration Control 
System"as well as in U.S. patent application Ser. No. 08/150,731 (filed 
Nov. 12, 1993), now abandoned, of James Zweighaft et al. entitled "High 
Performance Power Amplifier", both of which are incorporated herein by 
reference. 
Take-up reel 26 includes a geared take-up motor illustrated in FIG. 3-FIG. 
7. Take-up reel 26 includes a rotor assembly 60 (illustrated in FIG. 6) 
and a planetary hub assembly 70 (illustrated in FIG. 5). 
ROTOR ASSEMBLY 
As shown in FIG. 6, rotor assembly 60 has ball bearing 72, bearing/gear 
support housing 74, and insert molded bushing 76 for fitting over shaft 
56. Concentrically positioned about a center spindle of bearing/gear 
support housing 74, and sitting on a highest horizontal shoulder thereof, 
is insert molded sun gear 80. 
On a lower horizontal shoulder thereof, bearing/gear support bushing 74 
carries a disk-shaped steel return plate 84. An outer peripheral edge 86 
of return plate 84 is bent vertically and carries a tachometer ring 88. On 
its underside, between bushing 72 and tach ring 88, return plate has 
mounted thereon a disk-shaped magnet plate 90. 
FIG. 3 shows how rotor assembly 60 is mounted on flexible circuit/return 
plate assembly 40. In particular, shaft 56 is supported on plate assembly 
40 by an annular anchor 92. Near its base, anchor 92 has an annular seal 
94 extending therearound (see also FIG. 4), upon which coils 58 are 
mounted. At its top, anchor 92 carries ball bearing 72. 
As also shown in FIG. 3, flexible circuit/return plate assembly 40 also 
carries, at the outer periphery of seal 94, a stationary internal gear 
100. Internal gear 100 has the shape of a ring, and is mounted on plate 
assembly 40 by three pins or fasteners 102. As shown in FIG. 3 and FIG. 7, 
on its upper, interior peripheral surface internal gear 100 has teeth 104. 
Flexible circuit/return plate assembly 40 also has mounted thereon a 
tachometer 110 (see FIG. 3). Tachometer 110 is positioned intermediate 
plate assembly 40 and internal gear 100 at a selected point on the 
circumference of take-up reel 26. Tachometer 110 is thus positioned to 
interface with tachometer ring 88 for providing signals relative to the 
rotational speed of rotor assembly 60. 
From the foregoing discussion, it should be understood that the entire 
rotor assembly 60 rotates in unison as selected coils 58 are energized for 
affecting magnet plate 90. The ensuing discussion treats planetary hub 
assembly 70, the components of which also rotate in unison with one 
another but at a different speed from rotor assembly 60 in view of a 
gearing arrangement about-to-be-described. 
PLANETARY HUB ASSEMBLY 
Planetary hub assembly 70 (shown isolated in FIG. 5) includes an annular 
retaining plate 120. Retaining plate 120 has a radially extending plate 
flange 122 as well as a cylindrical, vertically (i.e., axially) extending 
spindle 124. Retaining plate 120 is provided with a central interior 
aperture which two bushings 126 mounted therein. Bushings 126 permit 
retaining plate 120 to rotate about shaft 56. 
Spindle 124 is axially slotted as illustrated in FIG. 4. A hub ring 130 is 
axially positioned atop spindle 124 and concentrically mounted thereabout. 
Hub ring 130 is also axially slotted. As shown in FIG. 5, a helical spring 
132 is concentrically positioned about the outer periphery of spindle 124. 
A first end of spring 132 rests on plate flange 122; a second end of 
spring 132 abuts an internal radial flange on hub ring 130. An annular hub 
cap 134 has a lower surface thereof abutting spindle 124. A retaining 
washer 136 is mounted to the top of shaft 56 by fastener 138. 
Intermediate spindle 124 of retaining plate 120 and an outer peripheral 
edge of plate 120 are provided three apertures, equally angularly spaced 
about shaft 56. Each aperture is sized to received a respective gear 
support pin 150. Gear support pins 150 depend parallel to axis 57 below a 
lower surface of retaining plate 120. At a distal end of each gear support 
pin 150 is a respective planetary gear 160 centrally mounted and retained 
thereon by retaining washer 162. Each planetary gear 160 has gear teeth 
166 provided on its outer periphery. 
GEARING SYSTEM 
The gearing system of take-up reel 26 thus includes sun gear 80 (of the 
rotor assembly 60); the three planetary gears 160 (of the planetary hub 
assembly 70); and the stationary internal gear 100. 
Table 1 shows the number of gear teeth provided on each gear of the 
illustrated embodiment. Table 2 shows gear ratios for gears included in 
the gearing system of the illustrated embodiment. 
TABLE 1 
______________________________________ 
Gear Number of Teeth 
______________________________________ 
Sun gear 14 
Planet gear 42 
Internal gear 100 
______________________________________ 
TABLE 2 
______________________________________ 
Mesh Ratio 
______________________________________ 
Sun/Planet 3 
Sun/Internal 7.14 
Planet/Internal 2.38 
Overall (Sun/Internal +1) 
8.14 
______________________________________ 
Thus, in the illustrated embodiment, as rotor assembly 60 carrying sun gear 
80 rotates, planetary hub assembly 70 rotates about shaft 56 on the order 
of 1/8 the speed of rotor assembly 60. 
TENSIONING ARM 
FIG. 8A-FIG. 8C show tensioning arm 44, also known as a "swingarm". The 
tension arm 44 is positioned along the media path between the supply reel 
24 and the first tape guide 28A on the media path. Tensioning arm 44 
includes a ribbed horizontal frame member 200. Tensioning arm frame member 
200 has a roller end 202 and a tail end 204. Approximately mid-way between 
ends 202 and 204 a bushing collar 206 depends from an underside of frame 
member 200. Bushing collar 206 centrally accommodates an unillustrated 
mounting shaft which extends vertically upward from deck floor 18. By 
virtue of bushing collar 206, tensioning arm 44 pivots as necessary about 
the unillustrated mounting shaft. 
At its roller end 202, frame member 200 carries a cylindrical spindle mount 
210, which in turn centrally carries a vertically up-standing spindle 212. 
Rotatably mounted about spindle 212 is a roller 214. Roller 214 is axially 
retained on spindle 212 by a retaining washer 216. The outer 
circumferential surface of roller 214 selectively forms part of tape path 
20 as necessary. 
At its tail end 204, frame member 200 carries a magnet assembly 220 on its 
underside. A counterweight 224 is also provided to counter the presence of 
roller 214 at the roller end 202. Deck floor 18 is provided with an 
unillustrated sensor beneath magnet 220 for detecting the position of 
tensioning arm 44, and thus tension in tape 22. 
A range of tensioning arm 44 positions are shown in broken lines in FIG. 2. 
Tensioning arm 44 is normally spring-biased into the tape transport path 
20. 
STRUCTURE: SERVO ZONES 
Servo zones recorded on at least selected tracks are utilized by the 
present invention for controlling the linear velocity of tape 22. In the 
illustrated embodiment, servo zones are recorded on tracks written by 
write head W2. Tracks written by write head W2 are formatted to have servo 
zones written at one or more predetermined locations along the track. The 
choice of particular locations along the track are not material to the 
present invention, although in the preferred embodiment servo zones are 
recorded near the beginning, near the center, and near the end of tracks 
recorded by head W2. 
FIG. 12 shows a servo zone SZ.sub.B2 recorded on track B2 by write head W2, 
as well as a servo zone SZ.sub.A2 recorded on track A2 (one drum 
revolution later) by write head W2. Servo zones SZ.sub.B2 and SZ.sub.A2 
are recorded at essentially the same distances from the beginning of the 
tracks in which they lie. 
FIG. 12 also shows that tracks recorded by write head W1 are formatted to 
include a plurality of servo search fields (SSF). In particular, FIG. 12 
shows a servo search field SSF.sub.A1-1 on track A1 in a location just 
prior to the beginning of the servo zone to-be-recorded on an upstream 
neighboring track by write head W2 (i.e., servo zone SZ.sub.A2 on track 
A2), as well as a servo search field SSF.sub.B1-1 on track A1 in a 
location just after the end of the servo zone recorded on a downstream 
neighboring track by write head W2 (i.e., servo zone SZ.sub.B2 on track 
B2). Similarly, track B1 has SSF.sub.B1 recorded thereon just before the 
beginning of SZ.sub.B2 on pair track B2 (other servo search fields on 
track B1 not being show). 
The width of read head R1 is sufficiently large to provide overlap of the 
two tracks adjacent to the track followed by head R1. The overlap 
facilitates off-azimuth pick up of signals recorded on the two adjacent 
tracks. 
From FIG. 12 it can further be seen that read head R1, following a track 
(such as track A1) recorded by head W1, picks up signals from servo zones 
recorded on adjacent tracks (i.e., tracks recorded by head W2). In view of 
the helical scan arrangement, as shown in FIG. 12 head R1 will first 
encounter servo zone SZ.sub.A2 and then, a predetermined time later, 
encounter servo zone SZ.sub.B2 (since the beginning of the respective 
tracks A2 and B2 are offset with respect to one another along the 
direction of head travel as depicted by arrow 34). As explained 
hereinafter, the servo search fields serve as a synchronizing field to 
alert read head R1 to expect to encounter a servo zone on a neighboring 
track. 
Reading of servo zones by read head R1 on adjacent tracks is facilitated by 
the manner in which the servo zones are formatted. FIG. 11 illustrates 
geometry involved in the format of a servo zone. FIG. 11 shows track pitch 
P and written track angle .alpha.. For an embodiment having a track pitch 
of 15.5 .mu.m, .alpha.=4.8991 degrees, whereas for an embodiment having a 
track pitch of 10.75 .mu.m, .alpha.=4.8954 degrees. FIG. 11 also shows the 
distance d that tape 22 must move to create track pitch p, as well as 
track skew s. From the geometry it is readily apparent that s=d.div.(cos 
.alpha.)=p+(tan .alpha.). 
In accordance with the present invention, the length of the servo zones SZ 
are formatted to be equal to track skew s (described above). In addition, 
a servo data tone recorded in the servo zone is recorded by head W2 at a 
low frequency 530 kHz so that it can be read by the off azimuth read head 
(i.e., read head R1). The servo search fields (SSFs) on the neighboring 
tracks recorded by write head W1 comprise a string of all "1"s recorded at 
a frequency approximately fifty times higher than the frequency of the 
servo zones. The servo zone frequency of the present invention is thus low 
enough to read off azimuth, yet high enough to record and overwrite 
easily. 
Thus, as read head R1 traverses stripe A1, read head R1 first picks up the 
off azimuth servo signals from servo zone SZ.sub.A2 and, a predetermined 
time later, picks up the off azimuth servo signals from servo zone 
SZ.sub.B2. By comparing the amplitude differences between the signals 
obtained from servo zone SZ.sub.A2 and servo zone SZ.sub.B2, in a manner 
described in more detail hereinafter, the tape drive system 20 determines 
an amplitude difference which is used to derive a value head.sub.-- 
position.sub.-- error. 
STRUCTURE: DRIVE ELECTRONICS 
FIG. 10 shows electronics of the tape drive system 20 of the embodiment of 
FIG. 1, including reel motor 1050 for rotating supply reel 24 and reel 
motor 1052 for rotating take-up reel 26 and a reel motor control circuit 
1054. Reel motor 1052 for take-up reel 26 is provided with a tachometer 
1056 which generates a signal motor.sub.-- speed.sub.actual. In addition, 
FIG. 10 shows read signal processing circuitry 1060 involved in processing 
signals obtained from read heads R1 and R2; write signal preparatory 
circuitry 1062; and servo signal processing circuitry 1064; all under 
control of control microprocessor 1066. 
Read signal processing circuitry 1060 includes signal conditioning circuits 
1070, 1072 for conditioning signals read by read heads R1, R2, 
respectively. The man skilled in the art will understand that signal 
conditioning circuits 1070, 1072 include amplifiers and filters. Signal 
conditioning circuits 1070, 1072 are connected along respective channels 
to conversion, modulation & deformatting circuits 1074. Intermediate 
circuits 1074 and signal conditioning circuit 1072 is a FIFO register 
1076. 
Details of the read signal processing circuitry 60 are understood from the 
above-referenced and incorporated U.S. Pat. No. 5,068,757 to Hughes et 
al., entitled "Servo Tracking For Helical Scan Recorder", as well as U.S. 
Pat. No. 5,065,261 to Hughes et al. entitled "Method and Apparatus for 
Synchronizing Timing Signals for Helical Scan Recorder", also incorporated 
herein by reference. For purposes of the present invention, it is 
sufficient to know that data output from the conversion, modulation, & 
formatting circuits 1074 is applied on line 1077 to servo signal 
processing circuitry 1064 and that read signal processing circuitry 1060 
produces a timing signal (HEAD.sub.-- SYNC on line 1078) during each 
rotation of drum 30. In particular, signal HEAD.sub.-- SYNC is high at a 
time at which read head R1 travels over a track written by head W1. In 
addition, the signals obtained from read head R1 are applied on line 1079 
to servo signal processing circuitry 1064. 
On the write side, write signal preparatory circuitry 1062 includes 
formatting, modulation, and serializer circuits 1080. Circuits 1080 are 
connected to output signal conditioning circuit 1082 for write head W1 and 
through FIFO register 1084 to output signal conditioning circuit 1086 for 
write head W2. Details of the circuits 1080 are likewise understood from 
the above-incorporated patents. In connection with the present invention, 
the man skilled in the art will understand how write signal preparatory 
circuitry 1062 prepares servo zones SZ and servo search fields SSF in 
accordance with the criteria discussed above in connection with FIG. 11 
and FIG. 12. 
Servo signal processing circuitry 1064 includes a bandpass filter 1094; a 
servo search field detector 1096; and, a delay 1098. Bandpass filter 1094 
receives on line 1079 the conditioned signal obtained by read head R1. 
Servo search field detector 1096 receives the HEAD.sub.-- SYNC signal on 
line 1078. As explained hereinafter, upon detection of a search field SSF, 
detector 1096 outputs a signal to delay 1098, with delay 1098 thereafter 
providing a SERVO.sub.-- SYNC signal at the time read head R1 is 
anticipated to be beginning travel over a servo zone. 
As further shown in FIG. 10, servo signal processing circuitry 1064 also 
includes a servo zone read controller 1100 which receives the SERVO.sub.-- 
SYNC signal from delay 1098 and thereafter sequences operations involved 
in reading and processing the servo signals from servo zones SZ. In this 
respect, servo zone read controller 1100 selectively issues a RESET and 
NEXT.sub.-- SAMPLE command to a peak detect & sample and hold circuit 
1102; a CONVERT command to an analog-to-digital converter (ADC) 1104; a 
SELECT command to a MUX 1106. 
FIG. 10 further illustrates that output from bandpass filter 1094 is 
applied to an input terminal of peak detect & sample and hold circuit 
1102. Output from peak detect & sample and hold circuit 1102 is applied to 
an input terminal of ADC 1104. A digital output terminal of ADC 1104 is 
connected to MUX 1106 which multiplexes a converted value either to 
register 1108 or register 1110. Upon request, registers 1108 and 1110 are 
connected to apply values stored therein to a data input port of servo 
zone read controller 1100. 
Controller 1100 is connected to receive the signal 
motor.sub.--speed.sub.actual from take-up reel motor tachometer 1056. 
Further, controller 1100 receives a signal drum.sub.-- speed from a 
tachometer 1120 which is used to monitor revolutions of drum 30. In 
addition, controller 1100 has access to non-volatile memory 1122 in which 
are stored various values and constants, including K1, motor.sub.-- 
speed.sub.ref (the specification speed for take-up reel 26 during a record 
operation), and K3 (axial offset variance). 
An output terminal of servo read zone controller 1100 applies a signal 
tape.sub.13 speed.sub.13 correction to reel motor control circuit 1054. 
Examples of structural details of reel motor control circuit 1054 are 
provided in simultaneously-filed U.S. patent application Ser. No. 
08/150,727, now U.S. Pat. No. 5,426,355 of James Zweighaft entitled 
"Power-Off Motor Deceleration Control System" as well as in 
simultaneously-filed U.S. patent application Ser. No. 08/150,731 of James 
Zweighaft et al. entitled "High Performance Power Amplifier", now 
abandoned, both of which are incorporated herein by reference. 
OPERATION 
Take-up reel 26 as described above provides the necessary low torque ripple 
(less than 1%) for achieving low speed velocity control of tape 22 in 
drive 14. FIG. 9 is a graph showing tape speed variation verses tape speed 
for various torque ripple levels for drive 14 (assuming the supply reel 
motor to be held constant at 5%). 
Drive 14 utilizes various features in order to obtain enhanced performance. 
These features include the above-described gearing of take-up reel 26; the 
usage of quality gears; reduced torque ripple; decoupling of supply reel 
24 using tensioning arm 44; and the particular write tracking technique 
disclosed in U.S. patent application Ser. No. 08/150,726 (filed Nov. 12, 
1993) of Georgis and Zweighaft entitled "Method And Apparatus For 
Controlling Media Linear Speed In A Helical Scan Recorder", now abandoned, 
(incorporated herein by reference). 
Thus, the drive 14 of the present invention offers tape speed control 
without the use of a capstan by using a precision highly geared take-up 
reel motor, an in-path speed detection system, and a tension control 
system. Elimination of a capstan entails elimination of a capstan motor 
and related components (e.g., bearings, encoder, and associated 
electronics). Accordingly, problems of capstan alignment are not 
encountered (e.g., problems of tape edge damage, embedding of particles, 
distortion of thin media, etc.). 
Nor does drive 14 require a conventional pinch roller and its related 
components, as well as pinch roller alignment issues (including pinch 
roller misalignment and attendant tape damage). Thus, drive 14 achieves a 
fewer part count with fewer moving parts, and thus enhanced reliability, 
less cost, and less tape path space. 
By gearing take-up reel 26 with the above-described gearing arrangement, 
numerous benefits are realized. The approximately 8:1 gear ratio allows 
take-up reel 26 to stay above 80 RPM with a full reel of tape, thereby 
keeping the motor away from stick/slip problems (which are typical of 
capstan motors) in rotating mechanisms at slow speed. 
Advantageously, in drive 14 speed variation is reduced due to an increase 
in takeup motor angular momentum. Due to the gearing, the speed and 
therefore the angular momentum of the rotor assembly 60 is increased (a 
flywheel effect). 
Speed variation is reduced due to a significant increase in reflected 
inertia of the take-up reel 26 due to the 8:1 gear ratio. The reflected 
inertia is the square of the gear ratio and therefore tape (22) sees an 
inertia of 64 times the actual rotor inertia. This desensitizes the takeup 
system from outside disturbances. 
Speed variation is reduced due to an increase in the torque ripple 
smoothing. By gearing and therefore running the rotor at higher speeds, 
the torque ripple frequency increases. This allows the takeup motor/gear 
system to essentially filter the torque ripple and reduce its effect on 
speed variation. 
Speed variation is reduced due to an increase in sample rate for one 
feedback loop in the reel to reel speed control system. By gearing the 
takeup system 8:1, the rotor spins 8 times faster than the tape reel, and 
since the tach ring is attached to the rotor, the system sees 8 times more 
tachometer samples than without gearing. This allows for an increase in 
frequency response in the servo system, and also increased sample 
averaging which reduces tach jitter effects. 
The planetary gear system design has numerous features to minimize speed 
variation and noise. For example, a very high quality (low runout) plastic 
sun gear (80) which is insert molded to the rotor of the takeup motor-gear 
system. Also provide are very high quality plastic planet gears (160), as 
well as a high quality support plate (122) for the planet gears (160) 
which has insert molded bearings and insert molded shafts (for the planet 
gears). Using the insert molding process tolerances which contribute to 
speed variation and noise are minimized. Further, drive 14 has a high 
quality metal internal gear (100) which minimizes 1x runout and adds 
stiffness to the system. 
Drive 14 features a high quality takeup motor design which uses a special 
magnetization pattern for the rotor magnet which significantly reduces 
motor torque ripple resulting in greatly reduced speed variation. 
Tensioning arm 44 located next to supply reel (24) provides numerous 
advantages, including the following: (1) isolation of the supply reel (24) 
from the tape path (20); (2) isolation of supply reel torque ripple, and 
torque drag variation which would otherwise adversely effect speed and 
tension control; (3) isolation of supply side (low tension) tape slip; (4) 
isolation of the supply side system mass from the tape path (20) which 
minimizes tension variation which would otherwise effect speed control; 
(5) isolation of supply side runout; and, (6) replacement of tape 
stretching (tension transients) with swing arm motion. 
OPERATION: TAPE LINEAR SPEED CORRECTION 
The present invention uses the value track.sub.-- pitch.sub.-- error to 
correct the linear velocity of tape 22. As shown in the following 
expressions, track.sub.-- pitch.sub.-- error is a function of only the 
actual tape speed (i.e., the variable tape.sub.-- speed.sub.actual): 
Expression 1 
track.sub.-- pitch.sub.ref =tape.sub.-- speed.sub.ref .div.drum.sub.-- 
speed.times.sin (.alpha.) 
Expression 2 
track.sub.-- pitch.sub.actual =tape.sub.-- speed.sub.actual 
.div.drum.sub.-- speed.times.sin (.alpha.) 
Expression 3 
read.sub.-- head.sub.-- delay.sub.-- factor=.theta./360.degree. 
Expression 4 
track.sub.-- pitch.sub.-- error=(track.sub.-- pitch.sub.ref -track.sub.-- 
pitch.sub.actual).times.(read.sub.13 head.sub.-- delay.sub.-- factor) 
Expression 5 
track.sub.-- pitch.sub.-- error=(tape.sub.-- speed.sub.ref -tape.sub.-- 
speed.sub.actual).times.(sin(.alpha.).times..theta..div.drum.sub.-- 
speed.div.360.degree.) 
where tape.sub.-- speed.sub.ref, sin (.alpha.), drum.sub.-- speed, and 
.theta. are all system constants. 
Thus, in particular, a value for the variable tape.sub.-- speed.sub.-- 
correction is determined from the following Expression 6: 
Expression 6 
tape.sub.-- speed.sub.-- correction=K1.times.(tape.sub.-- 
radius.times.(motor.sub.-- speed.sub.ref -motor.sub.-- 
speed.sub.actual))!+(K2.times.(track.sub.-- pitch.sub.-- error+K3)) 
whose terms are defined by Table 3. In Table 3, it will be understood that 
the values .theta., drum.sub.-- speed, sin (.alpha.), and tape.sub.-- 
speed.sub.ref are constants for a given recording format, and .alpha. is 
the recording (track) angle. 
TABLE 3 
______________________________________ 
Term Definition 
______________________________________ 
K1 a system damping constant for 
(constant) stablizing the control system 
depending upon such system- 
specific parameters as motor 
torque constant, amplifier 
gain, motor resistance, motor 
inductance, motor damping, for 
example! (stored in memory 
1122) 
motor.sub.-- speed.sub.ref 
the required speed of the 
take-up reel during a write 
operation calculated as 
(tape.sub.-- speed.sub.ref /tape.sub.-- radius) 
motor.sub.-- speed.sub.actual 
the actual speed of take-up 
reel as sensed by tachometer 
1056 
K2 (drum.sub.-- speed * 360) .div. .theta. .multidot. sin 
(.alpha.), 
wherein .theta. is the angular 
distance between W2 and R2; 
drum.sub.-- speed is obtained from 
drum tachometer 1120; and .alpha. is 
recorded track angle 
track.sub.-- pitch.sub.-- error 
computed by controller 1100 
K3 axial offset variance: a 
(constant) constant reflecting any 
deviation of vertical 
displacement of heads W2 and 
R2 from specification (stored 
in memory 1122) 
tape.sub.-- radius 
the latest calculated or 
measured value of the radius 
of the reel tape pack 
______________________________________ 
The value of the parameter tape.sub.-- radius can be determined by any of 
several known techniques. In the illustrated embodiment, both reel motors 
are equipped with angular position sensors known as tachometers. If the 
tape is loaded and rewound to its beginning ("BOT"), the radius of both 
reels can easily be found by moving the tape forward and comparing the 
output of the tachometers. By definition, the takeup reel has minimal tape 
wound on it at BOT, so its radius is equal to the known radius of its hub. 
The supply reel radius is determined by multiplying the known takeup reel 
radius by the ratio of the output of the two tachometers (i.e., the ratio 
of the output of the takeup reel tachometer and the supply reel 
tachometer), as in Expression 7: 
Expression 7 
SUPPLY.sub.radius =TAKEUP.sub.radius * (Ttach/Stach) 
where: 
SUPPLY.sub.radius =radius of the supply reel (unknown) 
TAKEUP.sub.radius =radius of the tapeup reel (known at BOT) 
Ttack=output of the takeup reel tachometer (measured) 
Stack=output of the supply reel tachometer (measured) 
Since the total amount of tape is fixed for a given reel, so is the total 
surface area as indicated by Expression 8: 
Expression 8 
PI*SUPPLY.sub.radius.sup.2 +PI*TAKEUP.sub.radius.sup.2 =K4 
where: 
K4 is a known constant 
PI=3.14159. The value of K4 does not change as tape is transferred from one 
reel to the other. The constant K4 can be used along with the ratio of the 
angular motion from the tachometers at any time to determine the radius of 
either reel by solving the system of two equations with two unknowns 
(Expressions 7 and 8). 
In view of numerous parameters in Expression 5 being constants or otherwise 
determined as described above, the value track.sub.-- pitch.sub.-- error 
(see Expression 5) is directly proportional to tape.sub.-- 
speed.sub.actual. In other words, deviation in tape.sub.-- 
speed.sub.actual from tape.sub.-- speed.sub.ref can be measured on a 
track-by-track basis via track.sub.-- pitch.sub.-- error. Thus, it is seen 
that the actual linear tape speed can be determined and controlled by 
measuring the track pitch error. As used herein, track pitch error is an 
example of track pitch information. 
The larger the angle .theta., the larger the effect that can be measured by 
track.sub.-- pitch.sub.-- error (e.g., by the tracking error). Since it is 
advantageous to maximize signal-to-noise ratio, it is advantageous to 
maximize .theta.. Angle .theta. can be made larger by further displacing 
head heights vertically on drum 30. Whereas in the prior art, the radial 
separation of heads was 180 degrees apart to obtain a head vertical 
displacement for 1/2 track pitch, the present invention radially separates 
heads by 540 degrees to obtain a head vertical displacement of 1.5 track 
pitch with attendant improvement in the value of track.sub.-- pitch.sub.-- 
error. 
Determination of the value tape.sub.-- speed.sub.-- correction by servo 
zone read controller 1100 will now be described primarily with reference 
to FIG. 10 and FIGS. 13A-13D and the particular example of read head R1 
traversing track A1 of FIG. 12. As it does once per revolution of drum 30, 
read signal processing circuit 1060 causes signal HEAD.sub.-- SYNC to go 
high when read head R1 is over a track recorded by head W1. When signal 
HEAD.sub.-- SYNC goes high on line 1078, servo search field detector 1096 
begins looking for search field SSF.sub.A1-1. The line of FIG. 13 labeled 
"R1" shows signals obtained by head R1 while traversing the servo search 
field SSF.sub.A1-1 (and thereafter the essentially consecutive off-azimuth 
signals obtained from servo zones SZ.sub.A2 and SZ.sub.B2). 
Detector 1096 receives data from read signal processing circuit 1060 on 
line 1077. Upon detection of the string of "1"s comprising search field 
SSF.sub.A1-1, detector 1096 generates a signal to delay 1098 corresponding 
to the rising edge of the pulse shown in the line of FIG. 13A labeled 
"SYNC". After a predetermined interval of time (one microsecond), delay 
1098 generates the signal SERVO.sub.-- SYNC (corresponding to the falling 
edge of the pulse shown in line "SYNC") for application to controller 
1100. 
In parallel with the generation of the signals HEAD.sub.-- SYNC and 
SERVO.sub.-- SYNC, the actual conditioned signal obtained by read head R1 
(from track A1 and off azimuth signals from neighboring tracks) are 
applied on line 1079 to the bandpass filter 1094. The filtered output of 
bandpass filter 1094 is shown in FIG. 13D by the line labeled "Svo Filter 
Out". 
Controller 1100 issues a NEXT.sub.-- SAMPLE signal to peak detect & sample 
and hold circuit 1102. In particular, the NEXT.sub.-- SAMPLE signal is 
issued by controller 1100 a predetermined time interval after controller 
1100 receives signal SERVO.sub.-- SYNC. This predetermined interval, 
corresponding to the reaction time of bandpass filter 1094, is illustrated 
by the first letter "b" in the line of FIG. 13C labeled "Sample". In the 
illustrated embodiment, this predetermined time interval "b" is on the 
order of about eight microseconds. 
Upon receipt of the NEXT.sub.-- SAMPLE command, peak detect & sample and 
hold circuit 1102 samples the filtered signal read by head R1 for the time 
interval shown as the first letter "c" in line "Sample" of FIG. 13C. 
During interval "c", the peak amplitude of the off azimuth signal obtained 
by read head R1 from servo zone SZ.sub.A2 is detected. Interval "c" lasts 
on the order of about five microseconds, including two microseconds of 
peak detection. After a five microsecond timeout, the peak amplitude from 
servo zone SZ.sub.A2 is gated to ADC 1104 and controller 1100 issues a 
CONVERT command to initiate analog-to-digital conversion of the peak 
amplitude value from servo zone SZ.sub.A2. 
The digital output of ADC 1104 is gated to MUX 1106. Microprocessor 66 then 
determines to which of registers 1108, 1110 the digitally converted peak 
amplitude value should be routed for storage, and accordingly applies an 
appropriate value of signal SELECT to MUX 1106 to effect the routing. For 
the sake of the present discussion, peak amplitude values from track A2 
are routed to register 1108 while peak amplitude values from track B2 are 
routed to register 1110. 
After completion of peak detection and sample hold for the signal from 
servo zone SZ.sub.A2, controller 1100 issues a RESET command to peak 
detect & sample and hold circuit 1102. Thereafter, controller 1100 waits a 
predetermined time before issuing a further NEXT.sub.-- SAMPLE command. 
This predetermined waiting time is preset in order to provide head R1 
sufficient time to move along track A1 in direction 34 a sufficient 
distance to be able to pick up off azimuth signals from servo zone 
SZ.sub.B2 recorded on track B2. This predetermined waiting time is 
depicted as the interval denoted by adjacent letters "d" and "b" in line 
"Sample" of FIG. 13C, and is on the order of about thirteen microseconds. 
Thus, this predetermined waiting time includes the reaction time of 
bandpass filter 1094. 
After elapse of the predetermined waiting time, the command NEXT.sub.-- 
SAMPLE is issued to enable peak detect & sample and hold circuit 1102 to 
obtain the peak amplitude for the servo signals recorded in servo zone 
SZ.sub.B2. Peak detection and sample hold again occur for an interval "c", 
followed by digital conversion and storage in the same manner as 
aforedescribed. However, the digitally converted peak amplitude from servo 
zone SZ.sub.B2 is stored in register 1110 rather than register 1108. 
When the peak amplitude of servo zone SZ.sub.A2 is stored in register 1108 
and the peak amplitude of servo zone SZ.sub.B2 is stored in register 1110, 
controller 1100 takes the mathematical difference the two amplitudes to 
obtain the value track.sub.-- pitch.sub.-- error. Further, in accordance 
with the above Expression 6, controller 1100 determines a value for 
tape.sub.-- speed.sub.-- correction using the value track.sub.-- 
pitch.sub.-- error as well as other inputs described above, including 
motor.sub.-- speed.sub.actual from tachometer 1056 on take-up reel motor 
1052; and calculated motor.sub.-- speed.sub.ref, .theta., K1, K2, and K3 
(axial offset variance) stored in memory 1122. 
It should be understood that the foregoing steps are repeated for each 
paired occurrence of servo zones. For example, in the embodiment above 
described, servo zones are read not only near the beginning, but also near 
the center and ends of tracks written by head W2. In this respect, while 
head R1 is traversing a track, servo search field detector 1096 monitors 
the signals on line 1077 for the next servo search field SSF until the end 
of track is encountered. 
Thus, from the foregoing it is seen that the servo search field information 
(e.g., SSF) is recorded at normal (high) data frequencies in the odd 
numbered tracks, just preceding the occurrence of the low frequency servo 
burst (e.g., SZ.sub.A2) in the adjacent even numbered track, for which 
they serve as a marker. After read head R1 for the odd numbered track 
encounters the servo search field (SSF) signal, servo controller (100) 
starts a timer (98) used to determine when read head R1 is beside the 
servo burst (e.g., SZ.sub.A2) written in the adjacent track. Because read 
head R1 is deliberately made somewhat wider than one track (typically 50% 
wider), part of read head R1 will overlap the servo burst (SZ) at this 
time, even if the centerline of read head R1 is directly over the 
centerline of the odd numbered track. On its other side, read head R1 will 
typically also overlap the opposite even numbered track. The tape format 
is arraigned to have a high frequency signal recorded in this area. 
When positioned along side a servo burst (SZ), the output of read head is a 
combination of signals from three tracks: high frequency from the odd 
numbered track with which read head R1 is substantially aligned, plus two 
smaller signals: low frequency from the side of read head R1 which 
overlaps the servo burst, plus high frequency from the opposite side which 
overlaps the third track. Frequency filtering easily separates the low 
frequency servo burst from the high frequency signals. 
As mentioned above, in this dual azimuth system a slight tilt (azimuth) is 
added to both write head W1 and read head R1 for the odd numbered tracks, 
while an opposite tilt is applied to write head W2 and read head R2 for 
the even numbered tracks. This technique facilitates separating the three 
signals because the low frequency servo signal is substantially unaffected 
by azimuth loss, while the high frequency read from the other side of the 
read head is attenuated due the mismatched azimuth. 
The extent to which read head R1 overlaps the servo burst SZ determines the 
amplitude of the low frequency signal. If read head R1 is slightly off 
center of the odd numbered track, biased toward the servo burst SZ on the 
even numbered track, read head R1 will pick up more of the low frequency 
servo signal. If read head R1 is off center in the opposite direction, 
less of head R1 will overlap the servo burst SZ and consequently the low 
frequency signal will be smaller. This overlap constitutes a direct 
measure of track pitch and is the basis of the speed correction factor 
calculated by microprocessor 66 and applied to the reel motors 1050, 1052. 
Because signal amplitude can also vary with many other factors such as the 
quality of the tape, the servo zones SZ are read in pairs and the 
difference between them is used as the error signal. 
The relative placement of the read heads and write heads directly affects 
the "gain" of this track pitch signal, meaning how much the servo signal 
amplitude changes for a given change in linear tape speed. If the read 
heads are placed close to the write heads the effect is nil. The further 
apart on the drum they are placed, the higher the gain. For reasons of 
crosstalk from the write to the read heads, it is common practice to place 
them opposite of each other on the scanner (e.g., drum 30). If writing 
occurs for one-half a turn of drum 30, the read heads can be active when 
the write heads are not operating, thus avoid "crosstalk". 
Heads placed 180 degrees apart on drum 30 must also be shifted slightly in 
a direction parallel to the axis of rotation of the drum to account for 
the distance the tape moves in the time it takes for the read heads to 
catch up to the data being written earlier. This direction parallel to the 
axis of rotation of the drum is referred to as the "down" direction. This 
shift in position can occur in discrete amounts corresponding to the track 
pair pitch (distance between pairs of tracks). Thus read heads placed 180 
degrees apart from the write heads must be shifted n*0.5 track pair 
pitches down, where n can be 1, 2, 3 or more. In a system with the read 
heads displaced 0.5 track pitches down, they will encounter the track just 
180 degrees after it was written. If n=2, the read heads will encounter 
the track 180+360=540 degrees after it was written, and so on. In the 
preferred embodiment, n.gtoreq.2 and preferably n=2. 
The value of K3 can be determined from the methods disclosed in U.S. patent 
application Ser. No. 08/150,733 (filed Nov. 12, 1993) of Hughes et al. 
entitled "Method and Apparatus For Determining And Using Head Parameters 
in a Helical Scan Recorder" (incorporated herein by reference), now 
abandoned,. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various alterations in form and detail may 
be made therein without departing from the spirit and scope of the 
invention. 
The embodiments of the invention in which an exclusive property or 
privilege is claimed are defined as follows: