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United States Patent [ 19 ] 

Sordello et al. 


[54] TRACK FOLLOWING SERVO SYSTEM 

[75] Inventors: Frank J. Sordello, Los Gatos; John 
Cuda, San Jose, both of Calif. 

[73] Assignee: Information Storage Systems, Inc., 
Cupertino, Calif. 

[22] Filed: Nov. 5, 1973 

[21] Appl. No.: 412,675 

[52] U.S. Cl 360/77,360/27 

[51] Int. Cl Glib 21/10, Glib 5/02 

[58] Field of Search 360/77, 72, 22, 26, 70, 

360/27; 318/560 

[56] References Cited 

UNITED STATES PATENTS 
3,404,392 10/1968 Sordello 360/77 


[ii] 3,864,740 
[ 45 ] Feb. 4, 1975 


3,474,432 10/1969 Sevilla 360/77 

Primary Examiner— James W. Moffitt 
Assistant Examiner— Jay P. Lucas 
Attorney, Agent, or Firm — Gerald L. Moore 

[57] ABSTRACT 

A servo system for accurately positioning a transducer 
relative to a recording medium by detecting equal am- 
plitude signals from two adjacent servo tracks. The 
servo tracks are prerecorded on the medium and have 
a frequency difference which is small in comparison to 
the frequency of the signals themselves and by multi- 
plying the resultant servo signal with signals at the fre- 
quencies contained in the servo signal, a resultant sig- 
nal is generated which is suitable for controlling the 
servo system. 

5 Claims, 1 Drawing Figure 


SERVO F, . i 



II 

26 

I 

l 

A r 

— f— 

i 

man,, \ ' 

PHASE 

f l 

*"°/// ' 

LOCKED 

h_ 

V ,o ~ft 

OSCILLATOR 



-C- 


26 


LOW 

PASS 

FILTER 




28 


HIGH 

PASS 

FILTER 


PEAK 

RECTIFIER 




27 


LOW 

PASS 

FILTER 




29 


HIGH 

PASS 

FILTER 


PEAK 

RECTIFIER 


CHANNEL 2 







I i uuw i mun 

AMPLIFIER > 1— ► MODULATOR PASS |-W PASS 


PATENTED FEB 41975 


3 , 864,740 



ACTUATOR 













3 , 864,740 


1 

TRACK FOLLOWING SERVO SYSTEM 

BACKGROUND OF THE INVENTION 

In random access recording systems, it is necessary to 5 
position a transducer at a desired data track on a re- 
cording medium with great precision since the present 
technology calls for the data track to be 5 milli-inches 
or less in thickness and the data tracks themselves to be 
no more than 5 milli-inches in width. In such data sys- 10 
terns, as for instance in disc-type random access mag- 
netic recorders, data is recorded in concentric circular 
tracks on the surfaces of discs with the transducer 
being translated radially for positioning above the data 
tracks. Prior to the time when the data tracks were 15 
made smaller such that more data could be recorded in 
a given medium, the transducer was usually positioned 
by use of some type of mechanical controlling means 
tied in with the transducer positioner. With the advent 
of the closer recorded data tracks, it has been found 20 
that the mechanical controlling means is no longer sat- 
isfactory due to tolerances within the system. For in- 
stance, in disc-type recording, the disc packs are re- 
movable and the mechanical tolerances within the sys- 
tem have required improvements in the servo system 25 
for accurate positioning of the transducer. 

Accordingly there has developed the “track follow- 
ing” method of detecting the transducer position. In 
such a system prerecorded tracks are positioned on a 
recording medium which enables the direct detection 30 
of the position of the transducer relative to the me- 
dium. For instance U.S. Pat. No. 3,404,392, Magnetic 
Track Following Servo System, F. J. Sordello, issued on 
Oct. 1, 1968, discloses one example of a track follow- 
ing servo system. In this example, two frequencies are 3 
chosen to form servo tracks to either side of data tracks 
and the frequencies are separated by electronic filter- 
ing to generate a servo signal. It is imperative in this ex- 
ample that the frequency difference between the two 
servo tracks be sufficiently great to permit effective 
electronic filtering for separating the signals since the 
transducer reads both tracking signals simultaneously. 
Additionally, one must assure that one frequency is not 
the harmonic of the other or other obvious problems 
arise in separating the signals. 

However, when the frequencies are sufficiently dif- 
ferent to permit electronic filter separation, there exists 
the probability that the signals will be attenuated by 
factors other than the proximity to the data track. In 5Q 
other words, the transducer normally reads a signal 
which has a magnitude directly proportional to the lat- 
eral distance between the transducer and the servo 
track which signal enables the servo system to deter- 
mine and position the transducer at the null point be- J5 
tween the tracks. If other factors attenuate the servo 
frequency amplitude and a resulting erroneous signal is 
fed to the servo system, an erroneous positioning of the 
transducer follows which can cause movement of the 
transducer further from the null point. 60 

There are a number of reasons why attenuation of the 
signal can result when the frequency of the servo tracks 
varies considerably. The causes involve such factors as 
( l ) the recording characteristics of the head changes 
with frequency and therefore the magnitude of the sig- 65 
nal from the recorded trac ks varies with frequency; ( 2 ) 
the flying height of the head varies and as a result the 
two signals are attenuated at rates varying with fre- 


2 

quency; (3) the readback characteristics of the read/- 
write head are different for the two frequencies; (4) the 
magnetic characteristics of the recording medium 
change with frequency; (5) the electronic characteris- 
tics of the circuit change with frequency; and (6) 
changes in the relative speed between the recording 
medium and the head can alter the frequency of the 
readout signal sufficiently to detune the electronic fil- 
ters which are frequency dependent. 

Thus it has been found that by using servo signals 
which differ by a wide frequency range, the servo sys- 
tem may not function at a high efficiency. It is therefore 
the object of this invention to provide a track following 
servo system which minimizes such problems encoun- 
tered with previous systems. 

Summary of the Invention 

A servo system including means for positioning a 
transducer on a storage medium for reading informa- 
tion from the medium, first and second servo tracks for 
marking the position of each data track with the servo 
tracks being of different but similar frequencies, and a 
pair of circuits for receiving the transducer signals re- 
sponsive to a read-back head detecting said servo 
tracks including means to modulate or multiply the 
transducer signal in each circuit, respectively, with 
modulating signals equal in frequency to that of the 
first and second servo track waveforms to generate a 
signal equal to the difference in frequency between the 
original servo track signal and the modulating signal for 
each circuit, and means to detect the magnitude of the 
resultant difference signal to generate a pair of servo 
signals for regulating the servo positioning means. 

Brief Description of the Drawing 

FIG. 1 is a circuit diagram and schematic showing the 
circuit of the present invention. 

Description of the Invention 

In FIG. 1 is shown a transducer 10 which can be 
moved radially across the surface of a recording me- 
dium or rotating disc 1 1 for the purpose of recording 
and reading data on a magnetic surface or coating (now 
shown) on the disc. The transducer is fixed to an arm 
12 which is positioned by an actuator 13. Thus by 
proper energization of the actuator the arm is moved 
in a direction parallel to the surface or laterally of the 
disc 11 for positioning the transducer in the general 
area of preselected recording positions. Energization of 
the actuator 13 is provided by a summing amplifier 14 
acting in response to a signal received from a servo sys- 
tem 15. 

Usually there are a plurality of rotating discs similar 
to the disc 11 fixed to rotate in unison about the same 
axis. For each disc surface there is a transducer 
adapted to record and read information. In such appa- 
ratus all the transducers are mounted for movement to- 
gether by the actuator 13. The one transducer 10 and 
the surface of disc 11 are dedicated to providing servo 
signals for positioning all the transducers relative to 
their cooperating disc surfaces. While the apparatus 
just described (but not shown in the drawing) is not 
necessary for the operation of the present invention, it 
is the preferred embodiments in today’s technology. 

The servo track or preselected position 16 is defined 
by the adjacent recorded cyclic waveforms Fi and Fj 
represented in schematic form in the drawing and re- 



3,864,740 


3 

corded and read back from the disc surface by the 
transducer 10. The transducer includes a coil 17 con- 
nected to a wide band amplifier 19 and a center tap 
connection 18. By magnetic interaction between the 
coil and the magnetically recorded waveforms on the 5 
disc surface, a servo signal is generated in the coil re- 
sponsive to the waveforms on the disc magnetic mate- 
rial. These signals include the cyclic signals produced 
by the sum of the signals F, and F 2 locating the track. 
The servo tracks are recorded in concentric circles on 10 
the disc surface along paths approximately 5 milli- 
inches or less in width. Because of the small size of the 
servo tracks and the small distance between adjacent 
servo tracks the transducer must be positioned with 
precision for the proper recording and reading of infor- 1 5 
mation data recorded on the other disc surface (not 
shown). It is for this purpose that the subject invention 
is provided. 

In accordance with the present invention there is pro- 
vided a servo system for accurately detecting the rela- 20 
tive position between a transducer and a recording me- 
dium and in which a servo track is marked by prere- 
corded waveforms, one to each side of the track, such 
waveforms being of similar but different frequencies 
and spaced so that when the transducer is positioned 25 
near the track a servo signal responsive to each wave- 
form and indicative of the transducer position will be 
read. Circuit means are provided to modulate the signal 
to generate a difference signal resulting when the mod- 
ulating signal and each servo waveform signal are mul- 30 
tiplied and to detect the amplitude of each difference 
signal thereby sensing the relative position of the trans- 
ducer and the data track. By transmitting this differ- 
ence signal to the summing amplifier 14 the actuator 
can be properly energized to move the head to a posi- 35 
tion over the data track. 

Accordingly, the waveforms F l and F 2 are of different 
but similar frequencies and the modulators 24 and 25 
of standard design are utilized to provide signal inputs 
to first and second channels or circuits respectively by 4 
receiving the transducer signal from the amplifier 19 
resulting from the addition of waveforms Fi and F 2 . A 
modulating signal from a phase locked oscillator 26 is 
provided to generate first and second signals fi and f 2 ^ 
of equal magnitude and preferably near but not neces- 
sarily exactly corresponding to the frequencies of the 
servo signals F, and F 2 originally recorded to mark the 
data track. In the preferred embodiment the phase 
locked oscillator is locked to the rotation of the disc 11, 5Q 
therefore changes in speed in the disc 1 1 which other- 
wise might result in a frequency change in the servo sig- 
nals fi and f 2 will also cause a corresponding change in 
the signals Fi and F 2 to cancel out any effect on the 
servo system. Thus there remains the function of sepa- 
rating the servo signals F, and F 2 thereafter detecting 
the amplitude of each signal for proper control of the 
servo system. The relative amplitude of each servo sig- 
nal F, and F 2 indicates the direction by which the pres- 
ent transducer position differs from the desired posi- ^ 
tion at the null point between the servo tracks, i.e., the 
signal F, or F 2 that is of the greatest amplitude denotes 
that the offset of the transducer from the null point is 
in the direction of that prerecorded waveform. Thus by 
feeding the total transducer signal to the modulator 24 
along with a frequency signal fl, the transducer signal 
is modulated such that there results from the modula- 
tor, resultant signals equal to twice the servo frequency 


4 

Fi or (Fi + /,), a signal /, plus F 2 , a signal f minus F 2 , 
and a signal of a DC level (or F, —ft)- The mathemati- 
cal derivation of these signals is included later. 

In accordance with the invention, the difference be- 
tween the two signals /j — F 2 is used as the servo signal. 
This choice is made since the signals resulting as the 
sums of the signals f (ft + Fi ) ] and [ (/i + F 2 , ) ] are very 
close together in frequency and therefore difficult to 
separate by filtering. The DC component of the signal 
is not predictable because the amplitude may change 
greatly as the frequency of the oscillator changes and 
there is no predicting the phase relationship between 
the signals fed to the modulator, which phase relation- 
ship can also affect the level of the DC signal. 

The arithmetic derivation of the signals is as follows: 
The two servo waveforms being f and / 2 , i.e., equal to 
the modulating frequency, the transducer readback sig- 
nal equals: 

1. [A, Cos (2 7r ft) + A 2 Cos (2tt ft] (K Cos 27 t ft) - 

Channel 1 

2. [A, Cos (27 t ft) + A 2 Cos (27 t ft] (K Cos 2tr ft) - 

Channel 2 

Where 

A, and A 2 are constants representing the amplitude 
of the servo signal components of f and f respec- 

/ tively. 

K is constant representing the amplitude of the multi- 
plying or modulating signal 
By assigning W, = 2nf and W 2 = 2v / 2 the above state- 
ments reduce to: 

1. (KAj/2) Cos 2W,r + (KA,/2) + (KA 2 /2) Cos (W 2 
+ W,)( + (KA 2 /2) Cos (W 2 - W,)( 

2. (KA,/2) Cos ( W 2 + W,)i + KA,/2 Cos ( W, - W ft 
+ KA 2 /2 Cos 2W 2 r + KAJ2 

For each channel there results signals with frequencies 
equal to: 

1. 2 f,f —f ,/i +f, & a DC or low frequency term 
equal to f — F, if f ^ Fj 

2. 2 f,f —f,f+f, & a DC or low frequency term 
equal to/ 2 — F 2 if/ 2 # F 2 

Thus to separate out the difference signal in the first 
circuit responsive to f minus f the lowpass filter 26 is 
utilized to filter from the signal the higher frequency 
signals responsive to the sum of the frequencies and the 
highpass filter 28 is utilized to filter the DC component 
from the signal. 

In the same manner the second circuit including the 
modulator 25 includes a lowpass filter 27 and a high- 
pass filter 29 for filtering out the DC and the sum of the 
frequencies components of the signal leaving the differ- 
ence signal. 

Thereafter by use of the standard peak detectors 30 
and 31 in the first and second circuits respectively, the 
amplitude of the difference signals representing/, — f 
and f — f, respectively, for each circuit can be de- 
tected which signals are indicative of the amplitude of 
the original modulated servo signals F, and F 2 as de- 
tected by the transducer. The amplitude of the signals 
at the output of the summing amplifier 19 as was 
pointed out heretofore, represents not only the direc- 
tion in which the transducer is offset from the null point 
or center of the data track but also indicates the degree 
to which the transducer position varies from the null 
point because of the relative amplitudes of the signals 
are modulated primarily with the lateral displacement 
of the transducer from each servo signal. Thus by peak 



3 , 864,740 



detecting each of the difference signals a signal level 
can be detected which is fed to the summing amplifier 
14 and which signal further is responsive to the position 
of the transducer. By use of these signals the actuator 
13 can be energized in a manner to move the trans- 
ducer to a position centered over the data track. 

We claim: 

1, A servo system for positioning a transducer rela- 
tive to a storage medium to read information stored on 
the medium, said system comprising: 

positioning means energizeable for moving the trans- 
ducer laterally to preselected recording positions 
relative to the storage medium, 
means to effect relative movement between the stor- 
age medium and the transducer thereby to enable 
the transfer of information between the medium 
and transducer, 

a plurality of side-by-side closely-spaced first and 
second servo waveforms recorded on the medium 
such that a pair of first and second servo wave- 
forms identifies a preselected position on the me- 
dium and wherein the first and second waveforms 
are at different but similar cyclic frequencies 
whereby movement of the transducer to a position 
near said preselected position will enable the trans- 
ducer to generate a position signal resulting from 
reading both servo waveform signals with the mag- 
nitude of such position signal being indicative of 
the lateral position of the transducer relative to 
that servo waveform, 

first and second channels receiving the position sig- 
nal from the transducer and each including modu- 
lators capable of mixing cyclic signals, 
means to supply to the first and second channel mod- 
ulators, first and second modulating signals respec- 
tively having frequencies similar to the frequencies 
of the first and second servo waveforms respec- 
tively to generate a difference signal resulting by 
the subtraction of the position signal and the modu- 
lating signal in each channel with each difference 
signal having a magnitude indicative to the relative 
position of the transducer to one servo waveform, 
means to detect the difference signal in each channel, 
and means to energize the positioning means respon- 
sive to the magnitudes of the difference signals 
thereby to move the transducer towards the prese- 
lected position between the servo waveforms. 

2. A servo system for positioning a transducer rela- 
tive to a storage medium to read information stores in 
the medium, said system comprising: 

positioning means energizeable for moving the trans- 
ducer laterally to preselected positions relative to 
the storage medium, 

means to effect relative movement between the stor- 
age medium and the transducer thereby to enable 
the transfer of information between the medium 
and transducer, 

a plurality of side-by-side closely-spaced servo wave- 
forms recorded on the medium such that a pair of 
waveforms identifies a preselected position in the 
medium and wherein each pair of waveforms are at 
different cyclic frequencies whereby movement of 
the transducer to a position near said preselected 
position will enable the transducer to generate a 
position signal including both servo waveform sig- 
nals with the magnitude of the signals resulting 
from detection of the different servo waveforms 


being indicative of the lateral position of the trans- 
ducer relative to the respective waveform, 
first and second channels adapted to receive the posi- 
tion signal from the transducer and each including 
5 modulators capable of mixing cyclic signals, 

means to supply to the first channel a modulating sig- 
nal similar in frequency to that of one cyclic wave- 
form and to the second channel a second modulat- 
ing signal similar in frequency to that of the other 
10 cyclic waveform of the preselected recording posi- 
tion, whereby the modulator of each channel will 
generate a plurality of resultant frequency signals 
to include a difference signal equal in frequency to 
the difference between the modulating signal sup- 
15 plied to the modulator and the cyclic waveform sig- 
nal of different frequency than the modulating sig- 
nal with each difference signal having an amplitude 
responsive to the relative position of the transducer 
and that servo waveform, 

20 means in each channel to filter the difference signal 
from the other resultant frequency signals thereby 
leaving in each channel a difference signal having 
a magnitude indicative of the magnitude of one of 
the cyclic waveform signals of the preselected posi- 
25 tion near which the transducer is positioned, 

and means to energize the positioning means respon- 
sive to the magnitude of the two difference signals 
thereby to center the transducer relative to the pre- 
selected recording position. 

30 3. A servo system as defined in claim 2 including 

means to vary the frequency of the modulating signal 
responsive to changes in the speed of the relative move- 
ment between the transducer and the medium. 

4. A servo system as defined in claim 2 wherein the 
35 modulating signal is supplied by a phase-locked oscilla- 
tor operable to generate the modulating signal in re- 
sponse to the speed of relative movement between the 
transducer and the storage medium. 

5. A servo system for positioning a transducer on a 
40 storage medium to read information from the medium 

comprising: 

means to effect relative lateral movement between 
the medium and the transducer thereby to enable 
the reading of data from the medium, 

45 positioning means energizeable for moving the trans- 
ducer to preselected positions on the medium, 
said medium including a plurality of servo tracks re- 
corded thereon, said tracks comprising a first and 
second recorded cyclic waveform signal positioned 
50 to each side thereof with the cyclic waveform sig- 
nal on one side being of a similar but different fre- 
quency than the cyclic waveform signal on the 
other side of the data track, 
a transducer capable of generating a signal respon- 
55 sive to the reading simultaneously of the cyclic sig- 
nals of two juxtaposed cyclic waveforms of a prese- 
lected position near which the transducer is posi- 
tioned, 

first and second circuit means for modulating said 
? transducer signal, 

means to supply to the first circuit a modulating sig- 
nal equal to said first cyclic waveform signal and to 
supply to the second circuit a modulating signal 
equal to said second cyclic waveform signal for 
modulating said transducer signal, 
means to detect the difference signal between the 
transducer signal and the first and second cyclic 



3,864,740 


signal in said first and second circuit means respec- 
tively, 

means to compare the magnitude of the difference 
signals from said first and second signals for gener- 
ating a servo signal, and 


means for energizing the positioning means with the 
servo signal for moving the transducer laterally to- 
ward the preselected position. 


65