Method for making a holographic tone wheel for use in a sync-off-tach tape drive system

A holographic recording is made on a tone wheel which is arranged to be driven by a capstan motor and capstan assembly. The holographic recording is a series of radial holograms on the tone wheel which recording is matched to the mechanical system driving the tone wheel by incorporating mechanical drive errors in the holographic recordings.

CROSS-REFERENCE TO A RELATED APPLICATION 
Subject matter shown but not claimed herein is shown and claimed in a 
copending application of Michael F. Braitberg et al. entitled "Holographic 
Memory With Moving Memory Medium", filed on Dec. 14, 1973 and bearing Ser. 
No. 424,991 now U.S. Pat. No. 3,976,354 and a copending application of 
Michael F. Braitberg entitled "Error Cancelling Sync-Off-Tach Tape Drive 
System" filed on Mar. 12, 1975 and bearing Ser. No. 557,792 now U.S. Pat. 
No. 4,103,214. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a magnetic tape drive system. More 
specifically, the present invention is directed to a motor, capstan and 
tone wheel assembly having means for cancelling intrinsic errors of the 
tape drive system. 
2. Description of the Prior Art 
Conventional magnetic tape drive systems for driving a magnetic recording 
tape have used various means for controlling the speed of tape in order to 
maintain the speed at a constant predetermined level, e.g., to control a 
tape playback speed to be identical with a tape recording speed. For 
example, the tape can be prerecorded with a clock, or sync, track at the 
time the data is being concurrently recorded in the adjacent recording 
track. When the tape is used in a data-reproducing system to playback the 
data recorded thereon, the sync track is separately read by a control 
system arrangement commonly referred to as a "sync-off-tape" type of tape 
playback speed control. In such a prior art system, the signal from the 
sync track is used in a feedback control to control the speed of the motor 
driving the tape during the playback operation and, hence, to maintain the 
playback speed of the tape at the same speed used during the recording 
process. Thus, the intrinsic tape drive system errors created by capstan 
eccentricity, bearing noise, etc. are minimized since the control signal 
comes from the tape itself and is affected by the same tape drive system 
errors. Since the mechanical errors are included in the aforesaid feedback 
controls, the servo system, if it has sufficient gain and band width, is 
able to cancel all of these intrinsic drive system errors. An example of 
such a prior art tape drive control system is shown in U.S. Pat. No. 
3,789,379 of Ivars P. Breikss, which was issued on Jan. 29, 1974. 
On the other hand, in another prior art arrangement, i.e., in a so-called 
"sync-off-tach" tape drive system, a tone wheel is fastened to the shaft 
of the drive motor used to drive the tape capstan, and a sync signal is 
derived from a sensor operatively associated with the tone wheel in a 
so-called tachometer assembly. In this case, the tape drive errors due to 
capstan eccentricity, bearing noise, mechanical inperfections in the motor 
and tone wheel, etc. are outside the feedback control loop and, hence, 
introduce errors in the playback speed which are reflected as errors in 
the playback data. Since the "sync-off-tape" type servo control system is 
often not practical inasmuch as it involves the use of a data-recording 
track and requires a custom prerecording of a clock track, the 
"sync-off-tach" type control system is more commonly used and is, as 
mentioned as above, subject to the intrinsic errors in the speed of the 
driven tape. An example of a prior art tape drive system for cancelling 
intrinsic errors in a sync-off-tach mode of operation is shown in U.S. 
Pat. No. 3,648,141 of David W. Scheer which issued on Mar. 7, 1972. In 
this prior art tape drive system, the intrinsic errors of the drive system 
for incremental tone wheel positions are recorded in a memory during a 
recording operation and are utilized during the playback operation to 
compensate for the same intrinsic errors at each corresponding incremental 
tone wheel position. However, this prior art compensation technique 
involves the use of a separate data storage memory, digital-to-analog 
converters and associated electronics which greatly increases the cost and 
complexity of a tape transport utilizing such a technique. 
SUMMARY OF THE INVENTION 
Accordingly it is an object of the present invention to provide an improved 
"sync-off-tach" type tape drive system having a simplified control system 
for cancelling intrinsic drive system errors of the tape drive system. 
Another object of the present invention is to provide an improved method 
for making a tone wheel for use in a "sync-off-tach" type tape drive 
system. 
In accomplishing this and other objects, there has been provided, in 
accordance with the present invention, a tape drive control system 
utilizing a tone wheel having a prerecorded hologram thereon. The hologram 
is recorded during an initial operation of a motor/capstan/tone wheel 
assembly by a holographic recording technique on the tone wheel which tone 
wheel includes a transparent substrate coated with an erasable 
thermoplastic film. The holographic recording on the tone wheel is read 
during subsequent operation of the motor/capstan/tone wheel assembly by a 
laser playback system to provide control signals for a motor drive 
energizing means to correct intrinsic drive system errors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Detailed Description 
Referring to FIGS. 1 and 2 in more detail, there is shown a pictorial 
diagram of an embodiment of the present invention used to prepare a tone 
wheel/motor/capstan assembly. The tone wheel 2 is mounted on a drive shaft 
rotateably driven by a motor 4. A tape drive capstan 6 is also mounted on 
the motor driven drive shaft on the other side of the motor 4 from the 
tone wheel 2 a source of an energizing signal (not shown) for driving the 
motor 4 at a predetermined speed. The tone wheel 2 includes a transparent 
substrate 10 having a thermoplastic layer 12 covering a face of the 
transparent substrate 10. A laser source 14 is arranged to supply a 
coherent light beam through a light modulator 16. A modulated output beam 
from the modulator 16 is applied to a prism beam splitter 18 to provide a 
reference beam 20 and a signal, or object, beam 22. The recording beam 22 
is deflected by a mirror 24 through a Fourier transform optical system 26. 
The Fourier transform optical system 26 includes cylindrical Fourier 
transform lens, not shown, to converge the signal beam 22 along an X axis 
which is the radial axis of the tone wheel 2 and a subsequent cylindrical 
lens, not shown, to converge the signal beam 22 along a Y axis which is in 
the direction of motion of the tone wheel 2. Further discussion of this 
structure and operation is found in the aforesaid copending Braitberg et 
al. patent application. A summary of holographic recording techniques is 
found in "Holographic Information Storage" by E. G. Ramberg in the "RCA 
Review", Vol. 22, March, 1972, pages 5 to 53. Briefly, the orientation of 
the reference beam 20 and the object beam 22 is such that two requirements 
are met. First, the hologram to be stored on the tone wheel 2 is a 
one-dimensional Fourier transform hologram having its essentially single 
dimension substantially normal to the direction of motion of the recording 
medium to yield a high hologram packing density. Secondly, the 
interference fringes run parallel to the motion of the tone wheel 2 
whereby the exposure time during recording of the hologram may vary 
without adversely affecting the stored hologram. 
An output beam from the Fourier transform optics 26 is directed through a 
radially oriented slit 30 onto the thermoplastic surface 12 of the tone 
wheel 2. The reference beam 20 and the signal beam 22 interfere at the 
thermoplastic surface 12 to produce a one-dimensional Fourier transform 
hologram. The recording of a hologram on the surface of a thermoplastic 
surface is well-known in the art as shown in U.S. Pat. No. 3,821,722 and 
is further discussed in the aforesaid copending patent application. An 
electrical charging means 32 is located adjacent to the thermoplastic 
surface 12 to provide an electrical charge on the tone wheel 2 prior to 
the holographic recording operation. The charging means 32 is provided 
with an input terminal 34 for connection to a suitable energizing source 
(not shown). 
MODE OF OPERATION 
The holographic recording on the tone wheel 2 is a holographic memory which 
makes use of a thermoplastic memory medium 12 upon which many individual 
holograms are sequentially stored during the rotation of the tone wheel 2. 
Information is stored by directing a coherent signal beam 22 and a 
coherent reference beam 20 derived from a laser 14 to a desired location 
on the memory medium. The signal beam 22, which contains a pattern formed 
by the Fourier transform optics 26 and the slit 30, interferes with the 
reference beam to form the holographic recording on the thermoplastic 
medium 12 since the beams 20 and 22 are modulated by the modulator 16, at 
a high frequency, e.g., one MHz, the holographic recording is a succession 
of one-dimensional Fourier transform patterns. Since the holographic 
recording process is performed on the capstan/motor/tone wheel 
combination, the recordings are each affected by any corresponding 
intrinsic defects of this rotating structure which occur as each recording 
is made. Thus, each recording is unique to the rotating structure upon 
which it is made and is related to the rotational speed of the rotating 
assembly, i.e., the capstan speed. The use of the holographic tone wheel 2 
in a tape drive system in which the holographic recordings are read from 
the tone wheel 2 is effective to provide a means for eliminating cyclical 
meachanical imperfections in the rotating assembly since the mechanical 
errors will similarly affect the tape recording and playback operations 
whereby identical tape speeds can be maintained. 
In FIG. 3, there is shown a playback system for reading the stored 
holograms from the tone wheel 2 during the use of the capstan/motor/tone 
wheel assembly to drive a magnetic recording tape 38. To read out the 
information stored in each of the recorded holograms, a coherent readout 
beam 40 from a coherent light source is arranged to successively 
illuminate each of the holograms stored on the tone wheel 2 to produce a 
reconstructed image thereof. The readout beam 40 is directed onto the 
hologram recordings on the tone wheel 2 at the same angle that the 
reference beam 20 made during the recording of the holograms. An array of 
photo detectors 44 is positioned on the other side of the tone wheel 2 
from the source 42 to detect the reconstructed pattern. The readout beam 
40 is preferably the complex conjugate of the reference beam whereby a 
real image readout is produced which eliminates the need for a readout 
lens. The output signal from the photo detector array 44 is applied to a 
suitable conventional control system 46 for controlling the speed of the 
motor in accordance with the detected signal from the tone wheel 2. 
The holographic memory technique can increase the number of stored tone 
wheel "lines" per revolution from ten to a hundred times above the present 
tone wheel "line" recording technology to improve motor control system 
operation at low rotational tone wheel speeds. Because the number of 
"lines" per revolution can be greatly increased, the diameter of the one 
wheel disc can be reduced to reduce the moment of inertia of the rotating 
tape drive system and further increase the performance of the tape drive 
system. Another advantage of the holographic memory is that the 
information stored in the hologram is stored uniformly throughout the 
hologram rather than in discrete areas. The hologram is, thus, relatively 
insensitive to blemishs or dirt on the code wheel, i.e., a small blemish 
or dust particle on the tone wheel cannot obscure the recorded data 
inasmuch as the same data is found distributed uniformly across the 
hologram. Further, the improved holographic system of the present 
invention overcomes some of the limtations caused by components of prior 
art holographic memory by not requiring a light beam system capable of a 
large number of resolvable spots. 
While the holographic recording technique used in the present invention is 
generally shown in the aforesaid copending U.S. Patent application of 
Michael F. Braitberg et al., that patent application discloses a system 
wherein the recording surface is a thermoplastic medium supported by a 
photo-conductor in a four layer structure including a transparent film 
base and a transparent electrical conductor underlying the 
photo-conductive layer. In the present novel recording technique it has 
been found that a simplified direct recording technique is possible 
wherein the exposing laser is used in a direct thermal development on a 
thermoplastic layer located directly on a transparent substrate. This 
simplified technique only requires that the thermoplastic layer is charged 
with a corona charger or similar device prior to the laser heating the 
thermoplastic film in the desired area. The charge on the thermoplastic 
layer creates internal stresses which deform the thermoplastic surface 
that has been made liquid, or plastic, due to the absorbed energy from the 
laser beam. The stored image, or surface deformation, on the thermoplastic 
layer is made permanent as the thermoplastic layer is subsequently cooled. 
A holographic recording on the thermoplastic surface of the tone wheel 2 
structure has several advantages. First, the resulting hologram is a phase 
hologram having a high readout efficiency. Secondly, the hologram recorded 
in the thermoplastic medium can be erased by subsequent application of 
heat to the thermoplastic medium for a duration which is longer than that 
used for development of the holographic recording. The ability to erase 
recorded holograms allows the tone wheel 2 to be reused with subsequent 
capstan/motor combinations. Thus, the memory medium is easily 
transportable, replaceable, alterable and interchangeable. Finally, the 
optical system for recording and reading the hologram achieves a 
non-contact operation of the tone wheel which eliminates any mechanical 
wear problem such as those found in magnetically recorded tone wheels. 
Accordingly, it may be seen that there has been provided, in accordance 
with the present invention, a sync-off-tach tape drive control system 
having integral means for correcting intrinsic errors on the tape drive 
system.