Zero reference generating method and apparatus for optical encoders

An encoder for providing positional information about a moveable member comprising code member means couplable to the moveable member for movement with the moveable member, reference means positioned to be stationary relative to the code member, an index sequence positioned on the code member, a mask sequence positioned on the reference member means, and means for detecting the degree of registration between the index sequence and the mask sequence, wherein the index sequence and the mask sequence include a plurality of building block patterns that are separated by predetermined spacings.

TECHNICAL FIELD 
The present invention is directed to a method and apparatus for generating 
a reference signal from a rotating body and, in particular, to a method 
and apparatus for generating an index mark or zero reference signal in an 
optical encoder. 
BACKGROUND ART 
Optical encoders are typically used to provide positional information for 
rotating shafts. To do this, the optical encoder provides a code disc 
which is attached for rotation to the shaft of interest. The code disc 
includes code elements which are positioned circumferentially on the disc. 
Detector means are provided which are fixed in position with respect to 
the rotating code disc and which are positioned in registration with the 
path of the code elements on the disc. As the code disc is rotated by the 
rotating shaft, the code elements are translated with respect to and 
through various degrees of registration with the detector means. In 
absolute encoders, there is a unique set of code elements assigned to each 
angular position of the disc. Thus each angular position is uniquely 
identified, and the detector means outputs a signal representative of the 
code elements currently in registration with the detector means. 
With incremental encoders, the code elements for each angular position are 
the same; however, the detector means count the number of code elements 
which are caused to pass in registration with the detector means when the 
code disc is rotated, so that the total number of elements counted for a 
given rotation of the disc is proportional to the number of angular 
positions through which the code disc was rotated. 
Due to the lack of a unique code for each angular position of incremental 
encoder code discs, an index signal is often required to be generated to 
indicate some reference point on the disc so that counting means in the 
detector can have a starting point to which the count can be referenced. 
This index signal is typically generated by a set of code elements which 
are separate from the code elements in the main track of the code disc. 
The typical detector means include a mask which bears the same index 
sequence of code elements as found on the code disc, as well as means for 
detecting the degree of registration between the code disc index sequence 
and the mask index sequence of code elements. Occasionally, a single code 
element is used as the index mark. However, it has been found that a 
multiplicity of code elements which have been arranged in special 
sequences are required to provide an index signal of sufficient resolution 
and amplitude to be satisfactory for a majority of optical encoder 
applications. 
In order to detect the registration of the sequences, the code disc and 
mask are positioned between a light source and a photodetector. The index 
sequence on the code disc and the mask index sequence collectively modify 
the light which impinges upon the photodetector from the light source. 
Typically, no light is permitted to pass to the photodetector through the 
code disc and the mask when the respective sequences are totally out of 
registration with one another. Similarly, a maximum amount of light passes 
to the photodetector when there is maximum registration between the code 
disc index sequence and the mask index sequence. 
Preferably, for all other degrees of registration the amount of light which 
is permitted to pass through to the photodetector is kept small. This, 
coupled with the requirement that the angular resolution provided by the 
index or reference signal be at least as precise as the angular position 
resolution provided by the code elements in the main track of the code 
disc, results in an index signal which is usually the weakest signal from 
the optical encoder. Thus, special techniques have been employed to 
enhance the signal characteristics of this index signal. Illustrative of 
these is U.S. Pat. No. 3,187,187 to Wingate. In the patent to Wingate, a 
special index sequence of slits is positioned transversely on the code 
disc and apart from the main track of the code disc. The sequence bridges 
several consecutive angular positions of the code disc. A mask, having a 
pattern identical to the index sequence is employed in conjunction with a 
detector. The detector detects the degree of registration between the mask 
and the index sequence as the disc rotates. 
The physical distribution of the code elements in the index sequence and 
mask sequence is selected so that the degree of registration between 
individual elements is at a maximum when the code disc is at the index or 
zero reference position. For all other positions of the code disc, the 
degree of registration is below a designated level, typically one element. 
In order to accomplish this, the code elements in the index sequence are 
arranged to have a selected spacing therebetween determined by a specific 
mathmetical relationship. In U.S. Pat. No. 3,187,187, this mathmetical 
relationship is defined in terms of numerical series; namely, 2, 3, 4, 6, 
8, 12, 16, 24, etc. or 2, 3, 6, 8, 11, 16, 17, 20, 22, 24, etc., in which 
the numbers represent the spacing (in terms of code element widths) 
between the leading edges of successive code elements. Under this 
arrangement, no spacing between any two successive code elements is equal 
to any other spacing or to the sum of any group of immediately successive 
spacing. 
When the above numerical series are employed, all angular positions of the 
code disc, except the index position, will exhibit a degree of 
registration between the mask sequence and the index sequence below some 
background registration level. At the index position of the code disc, the 
degree of registration is at a maximum and the detector output is large. 
However, the degree of registration is always zero for positions to either 
side of the index position. Although this further maximizes the change in 
detector output magnitude between the index or zero reference position and 
the positions adjacent thereto, there are certain disadvantages to this 
configuration. 
It has been found that, despite this large output magnitude signal the 
signal-to-noise ratio therefor is often unsatisfactory. The relative 
magnitudes of the degree of registration for the index position and for 
the background registration provide a signal-to-noise ratio which is 
indicative of the quality of the index signal being generated. 
In practice, a "safety factor" is applied in using the zero reference 
signal from the detector in the generation of an index signal at the 
encoder output. The safety factor takes into consideration variation of 
the signal over temperature, component aging, and different operating 
conditions of the encoder. For example, due to frequency response 
limitations in the detector means, the higher the speed at which the 
optical encoder is operated, the lower the output of the detector means 
will be. Additionally, the period-width of the index signal is related to 
the resolution of the encoder, such that for higher resolution encoders 
smaller period-width index signals are required. 
Typically, the "safety factor" is implemented in an optical encoder by 
selecting a threshold level on the index signal waveform where the 
waveform is one code element wide. It is to be understood that the 
threshold levels selected are a matter of design choice which involves the 
trade-off of performance of the encoder in other respects. Thus, it is not 
a requirement that the threshhold level be set at the one-code-element 
wide point in order to properly practice the present invention. 
The above can be better understood by considering that the registration 
between a code element in the index sequence and one of the elements in 
the mask sequence proceeds from a state where the elements are initially 
out of registration with one another, through a state where registration 
increases until full registration is achieved, and then through a state of 
decreasing registration. Finally the elements fall out of registration 
with one another. The waveform for such a progression takes the shape of a 
triangle. The left side of the triangle occurs as the two elements first 
begin to come into registration with one another. The peak of the triangle 
occurs as the two elements are in full registration with one another, and 
the right side of the triangle occurs as the two elements are falling out 
of registration with one another. The one-code-element-wide point is 
selected symmetrically about the peak of the triangle to correspond to the 
points on the triangle where the triangle is as wide as one of the code 
elements, i.e. where movement along the curve between the points 
corresponds to the angular displacement of the code disc through a 
distance equal to the width of a code element. 
It is to be understood that the height of the triangle is a function of the 
number of code elements in the index sequence. It is also to be understood 
that the width of the triangle is related to the width of the code 
elements. 
In practice, the index sequences which are actually used to generate the 
index signal are limited in the number of elements which practicably can 
be used. This is because, as the number of elements in the index sequence 
increases, which corresponds to an increase in the physical area which 
must be monitored, there is a rapid decrease in the optical efficiency of 
the detector. A large number of elements in the index sequence requires 
large area sensors. In turn, large area sensors exhibit greater 
capacitance effects than detectors for smaller areas, hence a limited 
frequency response. This capacitance effect increases at a faster rate 
than the increase in signal-to-noise ratio due to more code elements. 
A further limitation has been found regarding the minimum number of code 
elements which can be used. When the safety factor, as discussed above, is 
applied to selecting a threshold level on the registration waveform from 
the detector, it has been found that the signal-to-noise levels for a one 
or two element sequence configured according to the teachings of the 
patent to Wingate are unacceptable, and that the level for a three-element 
sequence is barely adequate. 
DESCRIPTION OF THE INVENTION 
These and other problems of the prior index signal generating means are 
overcome by the present invention of an improved optical encoder having a 
code disc supported for rotation on a stationary member wherein a zero 
reference signal is generated by comparing the degree of registration 
between a first set of indicia on the code disc and a second set of 
indicia fixedly positioned on the stationary member, wherein the first set 
of indicia include a first sequence of code elements which are staggered 
in positional relation with one another; and further wherein the second 
set of indicia include a second sequence of code elements which are 
staggered in positional relation to one another; and further wherein the 
degree of registration between the first sequence of code elements and the 
second sequence of code elements is at a maximum for a predetermined zero 
reference position of the code disc, and at or below a background 
registration level for all other angular positions of the code disc; and 
further wherein the code elements are arranged in each set so that the 
degree of registration for the angular position to either side of the zero 
reference position is greater than zero and no greater than the background 
registration level. Preferably, the degree of registration for the 
adjacent positions corresponds to the background registration level. 
A disadvantage of the code element sequence as taught by the patent to 
Wingate or those taught in accordance with the present invention, is that 
the improvement offered by these sequences becomes less pronounced as the 
number of elements in the sequences increases. This is due to the 
requirement of wide area sensors for reading such sequences. With the 
longer sequences, the signal-slit pattern is so sparse that the resulting 
signal level, when viewed in light of the size of the sensor required to 
receive the signal, does not yield a usable result. 
However, it has been found that the shorter sequences discussed above can 
be used as building blocks to form an index sequence from sequences of 
such building blocks. With such sequences, the optical patterns are such 
that large area sensors can be used with greatly improved efficiency over 
the earlier sequences. Thus, it has been found that the sequences of 
individual elements discussed above can themselves be used as building 
blocks for an index sequences. It has been found that a single pattern can 
be repeated at specific spacings to yield an index pattern which affords a 
substantial safety factor. 
Alternatively, it has been found that different index patterns can be used 
as building blocks in a single index sequence to provide satisfactory 
results. 
It has also been found that when a single pattern is used as a building 
block the pattern can be used in its normal order and also in its reversed 
order to provide a useable index sequence. 
In applying these concepts, it is to be understood that a particular index 
pattern comprises two parts, a code disk component, positioned on the code 
disk, and a mask component, positioned on the mask. When an index sequence 
is built by repeating an index pattern (or using different index 
patterns), the corresponding code disk component and mask component are 
repeated(or combined) to form a code disk sequence and a mask sequence 
respectively. The two together form the index sequence. 
It is, therefore, an object of the present invention to provide an improved 
optical encoder wherein the reference signal generating means generates an 
index signal corresponding to a reference point on the code disc and 
further wherein the signal generated for the angular positions of the code 
disc to either side of the reference position are at a level which is 
greater than zero and no greater than the background reference level of 
the reference generating means. 
It is therefore an object of the present invention to provide an index 
sequence which is useable with large area sensors. 
It is another object of the present invention to provide an index sequence 
which is constructed of a number of building block sequences. 
It is a further object of the present invention to provide an index 
sequence in which a selected sequence of elements is repeated at 
designated intervals. 
It is still a further object of the present invention to provide an index 
sequence in which a single pattern in utilized as a building block where 
such building block is used in the normal order as well as in a reverse 
order. 
These and other objectives, features and advantages of the present 
invention will be better understood upon consideration of the following 
detailed description of certain preferred embodiments when taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
U.S. Pat. Nos. 3,187,187, and 3,995,156 are hereby incorporated by 
reference. 
U.S. Pat. No. 3,995,156 is illustrative of a typical configuration for an 
optical encoder. Shown therein is a stationary member, such as a housing, 
a code disc, code elements arranged circumferentially on the code disc to 
form a main track, and photodetector means for detecting individual 
elements within the main track as the code disc is rotated past the 
detector means by the shaft of interest. 
FIG. 1 illustrates a typical arrangement of the code sequence 10 employed 
to generate an index mark in relation to a main track 12, a mask 14, and 
detector means 16. As is apparent from U.S. Pat. No. 3,187,187 optical 
detector means can be employed to measure the degree of registration 
between the index sequence 12 on the code disc 11 and an identical 
sequence on the mask 14. In FIG. 1 the code disc 11 is illuminated by a 
light source 18 such that the degree of registration between the index 
sequence 12 on the code disc 11 and the sequence 18 on the mask 14 
determines the amount of light which is permitted to fall onto detector 
means 16, such as a photodetector. The greater the degree of registration, 
the larger the photodetector output. Conversely, the smaller the degree of 
registration, the lower the photodetector output. 
For purposes of the description provided herein, it is to be understood 
that optical detector means are employed to determine the degree of 
registration between the index sequence 10 and the mask sequence 18. 
However, it is also to be understood that the present invention is equally 
applicable to other detection means such as capacitive detection, magnetic 
detection, and the like. 
Referring to FIG. 2, the waveform represented by the dotted line 20 is 
representative of the output signal from the detector means 16 for a 
three-element sequence, which is positioned on the code disc 11 and formed 
according to U.S. Pat. No. 3,187,187, as the sequence falls in and out of 
registration with an identical pattern in the mask. As can be seen from 
the figure, the signal output level is at a maximum, i.e., three units 
when the index position is reached. 
In interpreting the waveforms shown in the figures, it is to be understood 
that the units of the horizontal scale correspond to units of angular 
position of the code disc which are referenced to an index or zero 
reference position of the code disc. The vertical scale represents units 
of registration (parenthesized) or the degree of registration, in percent. 
One unit of registration corresponds to the level out of the detector 
means 16 when only one element in the index sequence 10 and one element in 
the mask sequence 18 are in full registration with one another. For 
example, in FIG. 2, the maximum amplitude occurs at angular position zero 
(i.e. the index position) and has an amplitude which corresponds to three 
units. The bottom right hand portion of each figure illustrates the 
relative positions of the elements in the index sequence and the mask 
sequence when the code disc is in the index position. Thus, in FIG. 2 it 
can be seen that when the three element index sequence, constructed 
according to the patent to Wingate, is at the index position, there will 
be three code elements in full registration with one another and, thus, an 
amplitude of three units from the detector means. 
From waveform 20 in FIG. 2, it can be seen that the background registration 
noise has an amplitude of one unit. In other words, for all angular 
positions of the code disc, except the index position, the registration of 
the index sequence with the mask sequence will have a magnitude which is 
equal to or less than one unit. Thus, from FIG. 2, it can be seen that for 
the non-index-position angular positions the degree of registration varies 
from between no registration at all to full registration between only one 
element in each sequence. 
Preferably, the index sequence on the code disc and the index sequence in 
the mask are selected so that the background registration level is 
minimized. Theoretically, for index sequences in which the code elements 
are spaced according to some multiple of the width of the code elements 
used, there will be a background registration level of at least one unit. 
The index code sequences in the patent to Wingate are selected so that the 
background registration level is no greater than one unit. It is to be 
understood that, preferably, the index code sequences of the present 
invention are similarly selected so that the background registration level 
is no greater than one unit. 
Also illustrated in waveform 20 of FIG. 2 is the threshold level which 
would be selected if a safety factor corresponding to one cycle width were 
desired. This threshold level will be one and one-half units. With a 
background registration level of one unit, this translates to a 
signal-to-noise ratio of one-and-a-half to one. 
It has been discovered that this signal-to-noise ratio can be substantially 
improved by modification of the code sequence used in generating the index 
mark. In FIG. 2, the waveform 22, which is drawn with a solid line, 
illustrates the output waveform from the detector for the registration of 
the code sequences shown in the upper righthand corner of the figure. 
These sequences are formed according to the present invention. Waveform 22 
has the same maximum value, and the same background registration level as 
the waveform for the three element code sequence of U.S. Pat. No. 
3,187,187. However, upon comparison of the threshold levels for a 
one-cycle wide safety factor, it is readily apparent that the 
signal-to-noise ratio for waveform 22 has been improved substantially. 
From FIG. 2, it can be seen that the one-cycle wide level on waveform 22 
has an amplitude of two units. Thus, the threshold of waveform 22 is twice 
as far above the background registration level as the threshold level for 
waveform 20. 
It has been discovered that the reason for the lower signal to noise ratios 
when using the code sequences taught in the patent to Wingate, is that for 
the angular position to either side of the index position the index 
sequences taught always yield a zero registration. Thus, the corresponding 
waveform always returns to zero before increasing to the maximum 
registration level thereof. 
According to the present invention, it has been determined that if the 
waveform does not return all the way to zero for the angular positions to 
either side of the index position, a substantially improved 
signal-to-noise ratio, i.e., a greater one-element-wide threshold level, 
is obtainable. In order to implement this discovery, the code patterns 
shown in the upper righthand corner of each of the figures have been 
invented. Note that, for FIGS. 2 and 3, while one of the code sequences is 
identical to that for the sequence taught in the patent to Wingate, the 
other code sequence includes an additional element which bridges two 
pre-existing elements. The result is that, for positions to either side of 
the maximum registration position, there will be a degree of registration 
corresponding to the background registration level, i.e. one unit, instead 
of zero registration. 
It is to be understood that the sequences of the present invention can be 
used interchangeably on the code disc or in the mask. It is also to be 
understood that the patterns shown in the figures represent but one 
implementation of the present invention for the number of code elements 
shown therein. 
There is an added benefit in providing an index sequence and mask pattern 
which produce the waveform 22 in FIG. 2. The magnitude rate of change 
(slewing rate), which the detector means is required to follow for 
accurate detection of the waveform, is reduced under the present 
invention. As can be seen from FIG. 2, the magnitude of waveform 22 
changes one unit between the plus one or minus one angular position and 
the threshold level. In contrast, waveform 20 changes one and one-half 
units between the plus one or minus one angular position and the threshold 
for that waveform. Accordingly, the waveform in 22 can be handled more 
easily by the detector means than can waveform 20. This becomes important 
when high rotational velocities cause the frequency of the waveforms, and 
thus the magnitude rate of change thereof, to increase. 
It has been found that implementing the index sequence according to the 
present invention provides the greatest amount of improvement over 
previous index sequences arrangement for lower numbers of elements in the 
index sequence. 
Referring to FIG. 3, it can be seen that when using a two-element pattern 
implemented according to the present invention, a threshold which is 
one-half unit above the background registration level can be achieved. 
Recall that for the two-element pattern according to the patent to 
Wingate, in FIG. 3, waveform 24, the threshold level using the safety 
factor was at the background registration level. The two-element pattern 
implemented according to the patent to Wingate thus provides a signal to 
noise ratio of 1:1 and as such is of little practical value. 
In accordance with the present invention, a threshold level is obtainable 
for a two element sequence which is comparable to that found in the 
three-element pattern of the patent to Wingate. Thus, the present 
invention provides in a two-element pattern a safety margin which is 
comparable to a three-element pattern implemented according to the patent 
to Wingate. See FIG. 3, waveform 26. 
Again, it should be noted, in connection with FIG. 3, that the waveform 26 
generated according to the present invention begins at the background 
registration level as the index or reference zero point is approached. In 
contrast, waveform 24, produced by the pattern according to the patent to 
Wingate, returns to zero for the angular position to either side of the 
index or zero reference position. 
In accordance with the present invention the two-element sequence of the 
patent to Wingate is modified by adding a linking element in one of the 
sequences. See FIG. 3. This provides the no-transition-to-zero 
characteristic of waveform 26. 
Referring to FIG. 4, waveforms for a four-element pattern are shown, along 
with the patterns implemented according to the patent to Wingate and 
according to the present invention. Waveform 28 is illustrative of the 
registration waveform for the code sequences configured according to the 
present invention, while waveform 30 represents the waveforms obtained for 
sequences configured according to the patent to Wingate. As can be seen 
from the figure, an improvement of one-half units in threshold level can 
be obtained under the present invention. 
From an examination of the patterns, at the top of FIG. 4, it can be seen 
that the code sequences according to the present invention differ from 
that taught in the patent to Wingate in two ways. First of all, an 
additional linking element has been added as in the case of the two- and 
three-element patterns. Additionally, the pattern has been modified so 
that the rightmost elements have been moved outward by two positions. One 
disadvantage of the four-element pattern, as constructed according to the 
present invention, is that a larger area detector is required over that in 
the patent to Wingate. However, the one-half unit increase in threshold 
level often outweighs this disadvantage. 
From the waveforms in FIGS. 2 through 4, it can be seen that a substantial 
improvement in the threshold level of an index signal can be obtained by 
providing an index sequence and mask pattern which are constructed in a 
manner similar to that taught in the patent to Wingate, but which have 
been modified so that the degree of registration between the index 
sequence and the mask pattern does not return to zero for positions to 
either side of the index or zero reference position. Preferably, the 
degree of registration for the adjacent angular positions is substantially 
equal to the background registration level. In the preferred embodiment of 
the present invention, the same index sequence and mask sequence as taught 
in the patent to Wingate are used, except that one of the sequences is 
modified to have an added linking element which links the two elements in 
the modified sequence which are closest to one another. 
FIGS. 2, 3 and 4 illustrate the preferred implementation of 3 element, 2 
element and 4 element index sequences in accordance with the present 
invention. Using the terminology of the patent to Wingate, these sequences 
can be expressed as follows: (1) three element sequence--(2, 3) and (1, 1, 
3); (2) two element sequence--(1) and (1, 1); (3) four element 
sequence--(2, 3, 6) and (1, 1, 3, 6), wherein each number represents the 
leading-edge to leading-edge distance between consecutive code elements in 
the sequence in terms of units of code element width. 
It is to be noted that, while the code disc index sequence and the mask 
index sequence in the patent to Wingate are identical, the code disc and 
mask sequences of the present invention are different. In the preferred 
embodiment, an additional code element is added to one of the sequences. 
Thus, in accordance with the present invention, the 3 element sequence 
includes two sequences: a sequence in which the leading edge of the second 
element is two units away from the leading edge of the first element, and 
in which the leading edge of a third element is three units away from the 
leading edge of the second element; and a sequence in which the leading 
edge of the second element is one unit away from the leading edge of the 
first element, in which the leading edge of a third element is one unit 
away from the leading edge of the second element, and in which the leading 
edge of a fourth element is three units away from the leading edge of the 
third element. 
The configuration for the two-element and four-element sequences of the 
present invention can be described in a similar manner. 
It is to be understood that other code sequences which implement the 
teaching of the present invention exist, and that such sequences, so long 
as they provide a degree of registration for positions adjacent to the 
index or zero reference position which do not return to zero will be 
satisfactory. 
It is also to be understood that some improvement can be realized in the 
signal to noise ratio for the index signal of an optical encoder in 
accordance with the present invention where the degree of registration for 
positions adjacent the index position of the disc fall anywhere within the 
range which is greater than zero and no greater than the background 
registration level. 
It should also be recognized that the amount of registration for positions 
adjacent to the zero reference position can be greater than the background 
registration level and still provide an improvement over the prior art. 
This is demonstrated in FIG. 5 wherein the signal to noise ratio is 
plotted for adjacent-position levels which are greater than the background 
registration level. Curves for a two-element, a three-element and a 
four-element code sequence are provided. For all of the curves, the points 
on the vertical axis represent the signal to noise ratio provided when the 
patterns of the patent to Wingate are used. Each of the curves represents 
the signal to noise ratio provided when the registration level for 
adjacent position ranges from first greater than zero to approximately two 
units. The bold face portions correspond to signal to noise ratios which 
are greater than that provided by the patterns of the patent to Wingate. 
The peaks of the curves represent the levels provided by the sequences of 
the preferred embodiment of the present invention in FIGS. 2 through 4. 
While the other bold face portions provide levels which are degraded from 
the preferred embodiment levels, these levels are nonetheless an 
improvement over the prior art. These other levels can be realized by such 
techniques as changing the pitch, i.e., separation between elements 
increasing the vertical dimension of certain of the code elements, or the 
like. 
The improved performance in optical encoders provided by the present 
invention can be seen upon considering the performance of an encoder 
utilizing the three-element sequence of the present invention. When a 
single-element index mark has been found to provide a frequency response 
parameter of approximately 20 KHz. When a 3-element sequence according to 
the present invention is utilized, a 100 KHz parameter is not unrealistic. 
In other words, a five-fold improvement can be realized. 
The larger threshold values available under the present invention also 
permit improved performance from the detector means. It is well-known 
that, for the low-gain amplifiers typically utilized in the detectors, the 
phototransistors therein have lower gain for lower collector currents. The 
low threshold levels normally available from prior index mark generating 
schemes dictate that the detector phototransistors be operated at low 
collector current levels. When the index sequence of the present invention 
is utilized, a higher threshold level is provided. As such, higher 
collector currents will be present and the operating point of the 
phototransistors can be increased to enhance the gain of the 
phototransistors. As such, a substantial increase in the performance of 
the phototransistor can be realized. 
In the preferred embodiment of the present invention, the index sequence 10 
on the code disc 11 is opaque and the code disc 11 is clear. Conversely, 
the mask 14 is opaque and the mask sequence 18 is clear. This is 
illustrated in FIG. 1. This clear-field/opaque-field arrangement has been 
found to enhance the performance of the detector 16 by permitting the 
detector photo-transistors to be normally in an "ON", i.e., active, state 
during substantially all of each revolution of the code disc. Only when 
the mask sequence 18 and index sequence 10 come into registration will the 
phototransistors be placed into an off state. As is well known in the art, 
the response of a phototransistor is fastest when it is already in the 
active condition, as opposed to moving from an "OFF" to a "ON" condition. 
To further enhance the operation of the present invention, a push-pull 
detection scheme is utilized in the preferred embodiment. As is well known 
in the art, in push-pull detection the presence or absence of light 
through a particular code element of mask sequence 18 is detected by a 
pair of phototransistors, or diodes. The apertures for these devices are 
positioned so that the dark period for one of the devices corresponds to 
the light period for the other device. The difference of the outputs of 
the devices is derived by way of a differential amplifier. Alternatively, 
push-pull or complementary tracks on the code disc can be used. The 
push-pull detection described above cancels out common mode signal 
variations to provide a more stable output signal. 
Referring now to FIG. 6a, a number of index sequences are illustrated. 
These index sequences are formed by using a two element pattern as a 
building block and spacing each of the blocks at a predetermined distance 
from the previous block. 
In the figure, the left-hand column indicates the number of times the 
two-element pattern is used. The second column depicts the actual pattern. 
The third column provides a shorthand representation of the index sequence 
in terms of the repeated pattern and the spacing between the patterns. The 
fourth column indicates the maximum amplitude to background noise ratio as 
well as the total number of incremental units required for the pattern. 
With respect to the actual pattern shown, the upper portion of the sequence 
represents the regions of light and dark which are to be positioned on the 
code disk, for example, while the lower portion of the sequence 
illustrates the light and dark portions which are to be positioned on the 
mask, for example. It is to be understood that this designation is for 
illustration purposes only, and that the patterns indicated as mask or 
disk patterns can be interchanged with satisfactory results. Also provided 
in the "actual sequence" is an indication of the number of incremental 
spaces between the repeated pattern. Thus, in the first sequence shown in 
FIG. 6a; there are two incremental spaces between the two-element 
patterns. 
With respect to the third column "shorthand representation", the numbers 
refer to the incremental spacing between the patterns. The pattern is 
indicated by an alphabet. In FIG. 6a, the alphabet "x" is used to indicate 
the two-element pattern. The two-element pattern is defined at the bottom 
right-hand corner of FIG. 6a. Thus, with respect to the first entry of 
FIG. 6a the shorthand representation for the index sequence indicates that 
the two-element pattern is separated by two incremental spaces from the 
other two-element pattern. 
With respect to the "ratio (area)" column it can be seen that, for the 
first entry of FIG. 6a, the maximum amplitude is four units while the 
background amplitude is two units. The sequence requires eight incremental 
units of space. Similarly, for the index sequence in FIG. 6a which 
utilizes seven two-element patterns, a maximum amplitude of 14 is obtained 
with a background level of 7. Such a pattern occupies 48 incremental 
units. 
FIG. 6b illustrates the degree of registration obtained from the second 
index sequence of FIG. 6a. As indicated above, and as can be seen from 
FIG. 6b, a maximum amplitude of six units is obtained when the patterns 
are in complete registration with one another. A background level of no 
more than three units is obtained for all other degrees of registration. 
It should also be noted that, as is the property described in connection 
with the two-element pattern alone, a registration waveform is produced in 
which, for the positions adjacent to the maximum registration position, 
the waveform is at the maximum background level. 
From FIG. 6a it can be seen that the two-element pattern can be repeated in 
accordance with an incrementaly increasing spacing sequence. Thus, for 
each additional two-element pattern added to a sequence, said element 
pattern is spaced by an additional incremental unit than was the spacing 
for the previous two-element pattern. Thus, for an index sequence having n 
two-element patterns the spacing between patterns is defined by the series 
2, 3, 4, . . . ,n. 
It has also been found that the spacing between each two-element pattern 
need not increase uniformly. To the contrary, it has been found that 
practically any order of the specified spacing can be used to obtain 
substantially the same result. Thus, for example, where the two-element 
pattern is repeated 5 times, the patterns can be spaced according to the 
sequence 5, 2, 3, 4 or the sequence 2, 5, 3, 4, etc. 
Referring now to FIG. 7a, index sequences in which three-element patterns 
are utilized as building blocks are shown. It has been found that the most 
efficient sequences are formed by the normal three-element pattern in 
combination with the three-element pattern in reverse order. In FIG. 7a, 
the three-element pattern is represented in the shorthand representation 
by the letter "y". The reverse order three-element pattern is indicated by 
the "y bar" designation. The actual patterns are defined at the bottom 
right-hand corner of FIG. 7a. As can be seen from FIG. 7a the spacing 
between each three-element pattern is not as regular as was the case for 
the two-element pattern sequences. Thus, for the index sequence which is 
formed of up to five such three-element patterns, the spacing between 
patterns is defined by the series 3, 5, 8, and 9. 
From the "ratio (area)" column it can be seen that the amplitude versus 
background ratio obtained when three-element patterns are repeated is, in 
fact, higher than that obtained when two-element patterns are repeated. 
However it can also be seen that the index sequences constructed of 
repeated three-element patterns occupy a greater elemental area. For 
example, while the index sequence in which the three-element pattern is 
repeated three times, provides a maximum amplitude of 9 units and a 
maximum background of 3 units, it occupies 26 incremental units of area. 
In contrast, an index sequence in which a two-element pattern is repeated 
4 times occupies only 21 incremental units of area while providing a 
maximum amplitude of 8 and a maximum background of 4 units. 
Referring now to FIG. 8b an index sequence formed of a four-element pattern 
which has been repeated twice is shown. From the figure it can be seen 
that a ratio of 8:2 can be obtained and that the sequence occupies 31 
incremental units of area. From FIG. 8b it can be seen that the maximum 
background level is two units while the maximum level at full registration 
is 8 units. The letter "z" designates the four-element pattern and the 
letter "z bar" designates the reverse four-element pattern. 
Referring now to FIG. 9, sequence (a), number of sequences are shown in 
which a mix of patterns is used to form a particular sequence. In FIG. 9, 
sequence (a), a two-element pattern and a three element pattern are 
combined to provide a 5:2 ratio and to occupy 11 incremental units. 
The sequence in FIG. 9, sequence (b) combines a two-element pattern with a 
reverse three-element pattern. The ratio obtained is the same as in FIG. 
9, sequence (a), but additional area is required for the sequence. 
The sequences of FIGS. 9, sequence (c), and 9, sequence (d), combine 
two-element patterns with a sequence of single-elements. The sequence (c) 
of FIG. 9 is represented by x-2-x-5-w-2-w and provides a ratio of 6:2 
while occupying an area 17 incremental units. The letter "w" designates 
the single-element pattern. 
The sequence (c) of FIG. 9, is expanded in FIG. 9, sequence (d). Here the 
two-element patterns are separated by an additional incremental unit and 
the series of single-element patterns are separated, among themselves, by 
two and three incremental units. The single-element series is separated 
from the two-element patterns by seven spaces. For such a pattern, a ratio 
of 7:2 is obtained with an area requirement of 24. This is to be 
contrasted with the sequence of FIG. 7a in which a three-element pattern 
is repeated three times. There, a ratio of 9:3 is obtained and an area of 
26 incremental units is required. 
From the above it is to be appreciated that index sequences can be formed 
by repeating patterns of elements. Additionally, different patterns, each 
differing in the number of elements contained, can be combined to form an 
index sequence. The advantage of using patterns as building blocks is that 
higher amplitudes can be obtained such that large area sensors can be used 
efficiently. This is clearly in contrast to the index sequences taught by 
Wingate in which the longer the sequence the more sparse the spacing of 
the elements. 
In use, the pattern building blocks can be manipulated by the designer so 
that an acceptable ratio can be obtained in light of the particular 
sequence length desired. For example, the optimum sequence length can be 
determined by the resolution of the code disk upon which it is to be 
positioned as well as the particular sensor which has been chosen for use. 
At times, the ratio of maximum amplitude to background level can be 
sacrificed in order to gain efficiency. That is, a smaller ratio might be 
acceptable in order to obtain a more compact pattern. 
It is further to be understood that the building block patterns used can be 
the conventional Wingate patterns, as well as the modified Wingate 
patterns discussed above. Further, additional efficiencies of the index 
sequences can be obtained by using the push-pull techniques discussed 
above, by using a plurality of optical tracks, and other similar 
optimizing techniques. 
The terms and expressions which have been used herein are terms of 
description and not of limitation, and there is no intention in the use 
thereof in limiting the scope of the claims herein or the embodiments 
shown.