Note: Descriptions are shown in the official language in which they were submitted.
CROSS REFERENCE TO RELATED PATENT
This application is related to U.S. Patent No.
4,137,451, issued January 30~ 1979, for "A Detecting Circuit
~or a Photocell Pattern Sensing Assembly".
BACKGROUND OF l~E INVEN~ION
Field of the Inventi : .
This invention relates to photocell detecting
circuits for optoelectronîc encoders and more particularly
to an im~roved photocell detecting circuit including a
reference photocell arrangement ha~ing a variable output
that is controlled ~y an ad~ustable shutter assembly
Descri~tion of the-Prior Art:
. _ . . _ . . . ~
In prior optoelectronic encoders~ photocell code
pattern sensing assemblies typically include an arra~ o~
photocell sensors that is arranged so as to have sensing
positions corresponding to positions of binary coded segments
of a code pattern. A specified or varying quantity of
information is represented by the light transmitting or
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~ '`;
~ 3'~ 47,688
blocking states of the code pattern segments. The varying
states of the segments are converted to binary coded elec-
trical slgnals responsive to the outputs of the photocells.
Photocell signal output values are produced by known photo-
electric effects of electron emission, generation of a
voltage or by changes in electrical resistance depending
upon the type of photocell. AGcordingly, increases occur in
the photocell current or voltage or decreases occur in the
photocell resistance with increases in the light transmitted
to the photocell. If light of a given high intensity and
light of a substantially lower intensity are represented by
two different binary states of the photocell outputs, at
least one referenced photocell signal output magnitude must
be established and detected as the photocell passes between
the lighted and unlighted conditions.
It has been found that photocells of a given
specified type may include photoelectric characteristics
that vary between photocells of the common kype. Accord-
ingly, the photocell output values will vary for a given
intensity of light. ~lso, where large numbers of photocell
sensors are used in an array for sensing code pattern seg-
ments, there may be differences in the light intensities
transmitted to different photocel:ls when the several code
pattern segments are in a given light passing or blocking
condition. Therefore, another cause of variations in the
photocell output values is incident light variations. The
above undesired variations are sometimes cumulative and
substantially increased when the photocell sensor arrays
have large numbers of such photocells compactly mounted in
close relationship within a small predetermined space. The
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small mounting spaces require that the photocell sensors
have an extremely small size and close relationships so that
there is some difficulty in shielding and isolating light
inkended to be transm:1tted to one photocell and blocked from
an ad~acent photocell by the segments of the code pattern.
Also~ mass production of photocell arrays by integrated and
printed circuit techniques makes close tolerances of the
photocell positions and the precise deposition of the photo-
cell composition somewhat difficult. In printed circuit
photocell arrays, it has been observed that variations in
photocell outputs also occur because of undesired current
leakages between the printed circuik conductors and between
these conductors and the circuit ground. Some of the above-
mentioned variable conditions are found in photocell detecting
circuits included in optoelectronic utility meter register
encoder assemblies in which the present invention is utilized
in one preferred embodiment.
In U.S. Patent Nos. 3~484,780; 3,609,727; and Re
27,723, meter shaft position encoders of the optoelectronic
type have holes or apertures associated with each photo-
sensor for masking undesired light radiations. No light
adJusting feature is included nor is a reference photocell
array used for detecting the sensor outputc
The aforementioned U.S. Patent No. 3,484,780 also
discloses a photosensor detecting circuit wherein a clipper
and amplifier means or Schmitt trigger is descrlbed directly
connected to cell outpuks to produce binary signals. It has
been found that when such a bistable circuit is made for
very low level signal operation that the bistable threshold
level may vary during operation over an undesirable wide
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~ 3~ 2 47,68~
range, often as much as thirty to seventy percent. These
wide threshold variations make it di~ficult to accommodate
the above-mentioned variations in the photocell character-
istics ancl light :lntensity variatlons 50 as to lead to
inaccurate sensing of` the code pattern. ~he inaccuracies
further include deviations in the photocell signal output
magnitude about the bistable ci.rcuit threshold level for a
constant light transmitting state of a code segment so that
the binary output does not stay stable or at a constant
output during a given sampling time for a photocell.
Other detecting circuits for optoelectronic
encoders are disclosed in U.S. Patent Nos. 3,573,773;
3,609,726 and 3,815,126. The aforementioned U.S. Patents do
not disclose the use of a reference photocell circuit array
having an ad~ustable optical arrangement for establishing a
variable predetermined reference photocell output.
SUMMARY OF THE INVENTION
In accordance with the present invention, a
photocell detecting circuit for a photocell pattern sensing
assembly includes a reference photocell circuit arrangement
including an array of reference photocells producing ~
variable reference output. An array of encoding photocells
is arranged for response to the opaque and transparent coded
segments of a code pattern. Separate radiations are pro-
vided for each of the encoding photocells and variable light
radiations are provided for the reference photocell array.
A sequential sampling control circuit connects each of the
encoding photocell outputs between a voltage source and a
common conductor for sampling the coded values of the
photocell outputs. The common photocell output conductor is
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47,688
connected to the reference photocell circuit output. The
connection of the encoding photocell output to the reference
photocell circuit output provides a voltage divider sensing
network having the Junction of the coding and reference
cells prov:lding an output signal to be sensed. The voltage
divider sensing network output provides predetermined ratios
of an encoding cell output value, in the lighted and un-
lighted conditions thereof, relative to the lighted re~-
erence photocell circuit output value. The signal to be
sensed is applied to a first input of an analog voltage
comparator. A second input to the comparator includes a
fixed voltage reference which establishes a predetermined
threshold to switch the cornparator output between first and
second binary signal levels in response to the mag~itude o~
the signal to be sensed. The comparator further accommo-
dates a feedback connection between the comparator output
and the fixed reference voltage input to provide hysteresis
in the turn-on and turn-off thresholds of the comparator and
stabilize the binary signal output ~hen variations occur in
the signal to be sensed.
It is a general feature of the present invention
to provide an improved adjustably lighted reference photo-
cell circuit arrangement for a photocell detecting circuit
receiving sequentially sampled photocell signal outputs from
photocells having varying photoelectric characteristics and
being lighted by varying light transmission conditions. A
further feature of the present invention is to provide
compensation for variations in the resistance characteristics
of photoconductive types of encoding photocells by providing
~0 adjustable light radiations to a reference photoconductive
--5--
6 ~ %
47,~8
photocell circuit to produce a predetermlned reference
output. A still further feature of the invention is to
provide a photocell pattern sensing assembly with a reference
photocell circuit arrangemellt having a light baffle plate
that includes light regulating apertures which mask undesired
radiations and adjustably control desired radiations to a
reference photocell array. A still further feature of this
invention is to provide shutter members that are threadably
ad~ustable in each of the regulating apertures so as to
calibrate the reference photocell circuit after it is
assembled for connection in the photocell detecting circuit.
These and other f'eatures and advantages of the
present invention will become apparent from the detailed
description of the drawings whlch are briefly described
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a front elevational view wlth parts
broken away illustrating an electric utility meter register
having an optoelectronic encoder including an adJustably
lighted reference photocell circuit arrangement made i~
accordance with the present invention,
Fig. 2 is a sectional view of the encoder taken
aIong the axis II-II in Fig. 1 and looking in the direction
of the arrows;
Fig. 3 is an enlarged fragmentary view of Fig. 1
illustrating an adjustable shutter assembly included in the
reference photocell circuit arrangement;
Fig. 4 is a front elevation view of an integrated
circuit board included in the register shown in Fig. 1 and
illustrating an encoding photocell array and a reference
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~ 3~ ~ .
1~7,6
photocell array included therein; and
Fig. 5 is an electrical schematic diagram of a
photocell detecting circuit including the re~erence photo-
cell circuit arrangemenk Or the present invention.
DESCRI~PrlON OF rh- P~EFERRED EM~O~D,IMENT
Re~erring now to the drawings and more parti-
cularl~ ~o Fig. 1 therein i9 shown an optoelectronic meter
regis~er encoder 10 ror encoding dial readings o~ an elec-
tric utility meter as described and claim~d in U~S~ Patent
No, 4,037,219 issued Jul~ 199 1971, assigned to the assignee
of this invention~
For purposes of understanding the present in-
vention, the general arrangement Or the meter reg~s~er
encoder 10 is briefly described hereinafter with reference
to Figs. 1 and 2. Mounted in ~ronk is a re~ister dial plate
12 carrying the forward ends of rive pointer shafts 13 each
having a pointer indicator 14. A photocell p~tern sensin~
assembly 15 includes a light guide plate 16 and a li~ht
source lB providlng plural radiations 17 for the encoderO
The assembly 15 ~urther includes an optical code pattern
arrangement and photocell sensors described herein~elow~
The Iight gulde plate 16 i~cludes recesses defi~in~ point
light sources 22 each producing ~ sepa~ate one of the plural
radlations 17. I~ is to be underskood that each of the
sh~ts 13 has associated ~herewith a:separate group of ~ive
circumferentially spaced point light sources 22 as described
in the U.S. Pa~en~ No. 4jO37,219~
Five discs 26 are carried separately b~ each Qf
the pointer shafts 13 and 1nclude predetermined shaft angle
code patterns fo~med b~ opaque and ~ransparent code segments
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~ 47,688
in which transparent segments 28 are formed by shorter
radius peripheral disc portions 30 and the opaque code
segments 32 of the discs are formed by longer radius peri-
pheral portions 34. The ci.rcularly arcuate transparent
segments 28 are defined by open spaces between the ends of
the arcuate opaque segments 32 of the code pattern dlscs 26.
The code segments 28 and 32 are disposed in a circular
orientation for light transmission and blocking alignment
with the circular disposed point light sources 22 so as to
transmit or block the rearwardly projectlng radiations 17 to
five pattern sensing positions associated with each shaft as
described further in ~he aforementioned U.S. Patent No.
4~037,219.
An integrated circuit board 38 carries an encoding
phokocell array 39 having five groups of five encoding
photocells each, sho~n in Figs. 1 and 4, with each group
aligned with the code pattern of one of the discs 26 and a
circular group of five of the point light sources 22. The
encoding photocells are each located at the predetermined
sensing positions having one of the point light radiations
dlrected thereto. The angular positions of the discs 26
correspond to dial indicating positions of the pointers 14
for corresponding encoding by each group of encoding photo-
cells. The photocell array 39 is provided by integrated
circuit photocells on the board 38 that are of a common
photoelectric type. In the preferred embodiment disclosed
herein, they are a photoconductive type formed by known
circuit disposition techniques utili~ing photosensitive
materials.
A first group of five encoding photocells 40, 42
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44, 46 and 48 is shown in ~igs. 1 and 4 as it is associated
with the lowest order and most right-hand one of pointers
14. Printed circuit conductors shown in Fig. 4 connect the
encodin~ photocells 40~ 42, 44~ 46 and L~8 between a common
terMinal 49 and separate terminals 50, 52, 5LI, 56 and 58,
respectively. ~our other identical groups of photocells
corresponding to the photocells Llo, L~2, l~4, 46 and 48 are
shown in Figs. 1 and 4 as they are provided for each of the
other four pointer shafts which is included in the register
encoder 10. The remaining four groups of twenty encoding
photocells are similarly connected between the common
terminal 49 and separate terminals of the photocell array
circuit board 38. Accordingly, the last two encoding
photocells 62 and 6L~ of the most left-hand and highest order
of the dial pointers lL~ are connected between the common
terminal 49 and terminals 66 and 68 which are the last two
of the twenty-five encoding photocell terminals.
The resistances of the photoconductive encoding
photocells change to a lower value when sub~ected to a
change from a darkened condition to an illuminated condition
when the transparent segments 28 of the code pattern pass
the light radiations 17 from the point light sources 22 to
the photocells associated therewith. The resistance of the
encoding photocells increases substantially when returned to
the unlighted condition by the opaque segments 32, as noted
further hereinbelow. A baffle plate 69 aids in isolating
the separate radiations 17 between one of the polnt light
sources 22 and an associated encoding cell. Apertures 73a
in the plate transmit separate ones of the radiations 17
through the plate 69.
_g_
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47,688
The photocell array circuit board 38 further
includes an array 71 of reference photocells 70, 72, 74, 76
and 78. The array is lncluded ln the ad~ustably lighted
referellce photocell c:lrcuit arrangement in accordance wlth
this invention. ~he array 71 is generally equally spaced
and distributed across the board 38 ad~acent each one of the
five groups of encoding photocells. The outputs o~ the
reference photocells are variable by adJustable shutter
members 79 made in accordance with this invention. Five
movable shutter members 79 ln the plate 69 are shown in Fig.
1 and two of the members 79 are shown in Fig. 2.
An enlarged fragmentary vie~ in Fig. 3 illustrates
one of the members 79 as it formed by a machine screw threaded
into the plate 69 to extend within one of five light regu- -
lating apertures 80. Each regula~ing aperture is aligned
with the optical path between one of the point light sources
22 and one of the five reference photocells 70, 72, 74, 76
and 78.
The reference photocells are connected in series
between the terminals 82 and 84 of the board 38 as shown in
Fig. 4. The reference photocells are constantly illuminated
through thè regulating apertures 80 from associated ones of
the point light sources 22 provided in the light guide plate
16. The le~els of light radiation intensity are controlled
by the positions of the shutter members 79. The reference
photocells 70, 72, 74~ 76 and 78 are manufactured to have
substantially identical characteristics to those of the
twenty-five encoding photocells of the board 38 for pro-
ducing a reference output in response to controlled light
intensities for reasons that will become apparent after the
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descrlption of Fig. 4 hereinbelow.
In Fig. 3, there is shown one of the ad~ustable
shutter members 79 formed by the screw threaded partially
into one of the apertures 80 aligned with the reference
photocel]. 70. The apertures 80 are derined by cylindrical
light tunnels slightly longer than tunnels form:Lng the aper-
tures 73, both in khe baffle plate 69. ~he screw shutter
members are threadable mounted in bores 81 extending ver-
tical and at right angles into the apertures 80. Accord-
ingly~ turning the slotted head portion 7ga movably advancesor retracts the threaded end portion 79b so that it masks
more and less of the light radiations through an associated
aperture 80. The light variations correspondingly produce
changes in the photoconductances of the reference photocells
to establish the desired outputs thereof. Thus, the ad-
justably lighted reference photocell circuit arrangement of
this invention easily ad~usts the output of the reference
photocell circuit array 71 since the screw head portions 79a
are conveniently accessible at the bottom of the meter
register 10.
A photocell detecting circuit 90 is shown in Fig.
5 and is carried on another circuit board 92 shown in Fig 2.
The twenty-five encoding photocells included in the array
39, represented by the photocells 40, 42, 44, 62 and 64, and
the reference photocells 70, 72, 74, 76 and 78 of the
circuit board 38 and the light source 18 are illustrated as
they are connected to the circuit 90. The encoding photo-
cells 40, 42, Ll4, 62 and 64 shown in Fig. 4 are representa-
tive of the twenty-five encoding photocells which are
included in the circuit board 38 shown in Fig. 4 and the
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1~7, 6
remaining encoding photocells are connected between their
associated separate terminals and the common terminal 49 in
the same manner ~hat the representative photocells are
connected as described hereinbelow. The broken lines 17
represent ~he radiation pa~hs ~rom the light source 1~
and the point light sources 22 shown in Fig. 2c Accord-
ingly, the light transmission paths 17 are alterna~ively
opened or blocked to produce light variations which corre-
~pondingl~ vary the photoelectric resistance characteristics
of the encoding photocells o~ the arra~ 39.
The broken lines 93 in Fig. 5 represent the con-
trolled continuous radiations illuminating the array 71 from
the light source 1~ associa~ed point l~ght sources 22q The
ring~ 94 represent a radiation con~rol means formed by the
shutter members 79 and regulating apertures ~0 ~or directing
the radiations 93 of the light sources 22 to ~the re~erence
photocells 70, 72, 74, 76 and 7~ so long as the light source
1~ is energized from conductors 96 and 97 connecked to the
circuit board 92.
Referring in further detail to the detectin~
circuit 90 in Fig. 5, which is also described and claimed in
the above cross re~erenced U. S~ Patent No~ 49137,451,
an electr~cal power supply source 95 supplies a nominal plus
twelve volts d~co to a supply conductor 9~ The other
terminal of the remote supply source is connected in common
wlth the grounded conductor 99. A sequential sampling
control circuit 100 can be provided by a multiplexer circuit
or, as described in the a~orementioned U~S. Patent No.
4,037,219, by a counter circuit for sequentially swi~ching
and sæmpling each indi~idual encoding photocell o~ the array
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6 3~ Z
l~7,688
circuit board 38. In one preferred embodiment the circuit
100 includes four interconnected type CD4051 COS/MOS analog
multlplexers described in the RCA Integrated Circuits Databook
published ~pril, 1976 by RCA Sol~d State, Somerville, N.J.
o8876, at pages 5liO-5ll5. The sequentlal sampling control
circu.lt 100 includes a common input 102 connected to the
supply conductor 98 and twenty-..ive of the eight channel
outputs of the four multiplexers (Cl through C25) are separ-
ately connected to the separate encoding photocell terminals
of the circuit board 38. All of the channel outputs of the
circuit 100 are not shown, it being understood they will be
at least the same number of` outputs as there are encoding
photocells in the array circuit board 38. ~ccordingly, each
of the channel outputs C]., C2, C3, C24 and C25 of the circuit
100 are connected to one terminal, 50, 52, 54, 66 and 68,
respectively, of each of` the encoding photocells 40, 42, 44,
62 and 64, respectively. Four chip-select conductors 106
are connected to the four inhibit (INH 1-4) inputs of the
four multiplexers and three channel-select conductors 110
are connected to the A, B~ C inputs of the ~our multiplexers.
Twenty-five different binary logic signals on the conductors
106 and 110 switch the common input 10Z to one of the channel
outputs Cl through C25. The circuit 100 connects the encoding
photocells shown in Fig. 4, and correspondingly all of the
encoding photocells of the array 39 one at a time between
the voltage source on conductor 98 and the common encoding
photocell terminal 49O This produces sequential sampling of
the encoding photocells in the detecting circuit 90.
The encoding photocells have the other common
terminals thereof each connected through the terminal 49 to
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~ . ~
~ 3~ % 47,688
a conductor 112 at a Junction termlnal 11~. The array 71 of
reference photocells including the series connec~ion of the
re~erence photocells 70, 72, 7LI, 76 and 78 is connected at
terminals 84 and ~2 across the ~unct:~on 114 and ~he circult
ground conductor 99. The ~unction 114 provldes the output
of a voltage d:Lvider sensing networ~ 116 formed by the
sample switching of one of the encoding photocells to the
junction 114 and the reference photocells 70~ 72, 74, 76 and
7~
The ~unction 114 produces a signal to be sensed
118 from the divider sensing network 116. The re:~erence
photocell circuit array 71 provides a predetermined ref-
erence output at the junction 114 due to a predetermined
combined output value of the reference photocells 70, 72~ 74
76 and 78. The reference output is determined by the series
photoconductivity of the array 71 when lighted by the con-
trolled radiation intensities in accordance with the present
invention. Although five reference photocells are shown in
the circuit array 71, a single or preselected numbers o~
reference photocells having a predetermined resistance
output value can be used advantageously with the present
invention so as to produce a predetermined reference signal
output at the ~unction 114 with respect to the output produced
across each of the sampled encoding photocells. By way of
example and not limitation, each encoding photocell resistance
when not illuminated is determined to be greater than approxi-
mately five times the resistance of the re~erence pho~ocell
circuit 71 when illuminated. The minimum ratio of the
resistance of a non-illuminated photocell to the resistance
of an illuminated photocell is selected to be preferably in
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the order of twenty-five to one. The lighted resistance
variakions of the encodin~ photocells, in one working embodi-
ment, averages around 50K to 60K o~mls. It has been found
that a preferred maxin~um varlation in illuminated resistances
between photocells when the encoding photocells are separately
illuminated should be at an optimum ratio of rnaxlmum to
minimum lighted resistance of five to one or slightly less.
This ratio is to guarantee that each lighted encoding photo-
cell resistance is always less than the total series resis-
tance of the reference photocells. A smaller than five toone ratio accommodates variations in the circuit operation.
The effective operation of the detecting circuit 90 compen-
sates for these resistance variations and other ~ariations
in the illuminating conditions on the photocells and varia-
tions due to temperature and humidity ambient conditions.
The number of reference photocells optimizes the specifica-
tions for the array 39 of encoding photocells. For example,
if the unlighted to lighted photocell resistance varies in a
ratio of sixteen to one, the effective resistance of four
reference photocells is used and a required optimum ratio of
lighted photocell resistance is four to one or slightly less
so that the resistance of the lighted reference photocell
circuit array is always more than the resistance of a lighted
encoding cell.
From the previous description, it is seen that the
reference photocell circuit output at junction 114 is critical
for proper operation. Variations in the circuit so~etimes
occurs because of differences due to manufacture or because
of circuit defects including low resistance leaka~e paths
between the printed circuit conductors and between the
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conductors and ground. Lighting variations also cause
variati.ons. The baffle plate aids in Masking reflected or
e~traneolls light by having the apertures, such as the
apertures 73 for the encod:l.ng photocells and regulating
apertures 80 for the reference array 71. The shutter members
79 of the rad~ation control means 94 permit changes in the
light through to apertures 80 to achieve to the predetermined
lighted output response for the reference photocell circuit
array 71. When light vari.ati.ons produce undesired outputs
or when the reference photocell output characteristics vary
for a given level of radiation, the desired output is obtained
by movement of the shutter member 79 to effect the required
light through the aperture 80. ~or example, the optimum
ratio of five to one for the reference photocell circuit
output relative to a lighted encoding cell output is achieved,
if not found in the manufactured arrays 39 and 71, b~ adjust-
ing the light radiations 93 with the ad~ustable shutter
member 79. Accordingly, it should be apparent that a single
or fewer than five reference photocells can be used in the
ad~ustably lighted reference photocell circuit arrangement
of this invention having a radiation control means 94.
Having described the circuit arrangement and
connections for sampling and sensing encoding photocell
outputs, an analog voltage comparator 120 and the associated
circuitry is now described for detecting the signal to be
sensed 118. The comparator 120 in one pre~erred embodiment
is a voltage comparator type LM211 available from the National
Semiconductor Corp., Santa Clara, California 95051, and
described in the National Linear ~ata Book dated June, 1976.
The analog comparator 120 is connected at a first input 122
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to the voltage divider sensing network 116 at ~unction 114
~or receiving the slgnal to be sensed 11.8. A second compar-
ator input 124 is connec-ted to the Junction 126 of two
voltage reference reslstors 128 and 130 connecked across the
supply conductor 98 and grounded conductor 99. The resistors
128 and 130 provide a fixed reference voltage to precisely
control the analog signal threshold val.ue 119 of the analog
com,parator 120. The values o~ resistors 128 and 130 are
established with respect to the changes in magnitude of the
signal to be sensed 118 which, as described hereinabove, is
established by the voltage divider sensing network 116.
The output 134 o~ the comparator 120 is connected
through a resistor 136, being a current limiting and pro~
tecting resistor, to the detecting circuit output terminal
138. A detecting circuit binary Olltput signal 140 is
produced between the output terminal 138 and grounded,
provided by the conductor 99. A pull-up resistance 142 is
connected between the supply conductor 98 and the comparator
output 134. The voltage supply inputs to the comparator
120, not shown, are connected in a known manner bet~een:the
supply conductor 98 and ground conductor 99.
A feedback resistor 144 is connected between the
output 134 of the comparator 120 and the second input 124
which is also connected to the ~unction 126. Hyst~eresis is
provided by the feedback resistor 144 so that the transfer
function characteristic of the comparator 120 prevents
oscillations in the ou,tput signal 140 from occurring during
variations in the signal to be sensed 118 when a sampled
encoding cell is in a given lighted or unlighted coding
30 condition. Such outut signal variations can be produced by
- 17 -
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variations in the arnbient llght intensity or point light
source transmissions impingi.ng on an encoding photocell
being sampled.
~ further important feature of the detecting
circlllt 90 is a malfunctlon :Lnhibi.ting circult connected to
the balance/strobe input 148 of` the comparator 120. A
resistor 150 is connected between the inpuk 148 and a
ju~ctlon 152 connected between the light source conductor 96
and a light energizing supply conductor 154. When a remote
energizing voltage source 155 is being applied f'rom the
conductor 154 to the light source 18, the strobe input ls
enabled through the resistor 150 so that the analog com-
parator 120 is active to output normally. If the energizing
voltage 155 is improperly omitted and not being applied to
the light source 18, the low resistance of the lamp 18
effectively pulls the strobe input 148 to ground and ln-
hibits operation of the comparator 120 so it is inactive. A
resistor 15~ can be connected in parallel with the light
source 18 to protect the strobe input in case the light
burns out or is defective. It is contemplated that other
malfunction conditions may be selected to inhibit the
comparator 120.
The nominal analog signal threshold 119 of the
analog comparator 120 is initially selected to be approx-
imately six volts or about one~half of' the circuit supply
voltage on conductor 98 as established by the reference
voltage resistors 128 and 130. With the general relative
photocell photoelectric resistance parameters noted herein-
above, the signal to be sensed 118 provides an analog input
to the comparator input 122 that is very low approaching the
6 3~ Z 47,68~
ground potential when the encoding photocell being sampled
is not lighted and the adjustably lighted reference photo-
cell circuit arrangement lights the circuit array 71. The
resistance ratio o~ the unlighted encoding photocell to the
lighted reference photocell circuit resistance i~ adjusted
so that there is a substantially higher voltage drop across
the sampled unlighted encoding photocell, connected by
circuit 100 between the conductor 98 and junction 114, than
there is across the lighted reference photocell circuit
array 71. When the encoding photocell being sampled is
illuminated the voltage of the signal to be sensed 118 goes
substantially higher and approaches the voltage o~ the
conductor 98 slnce the ratio of the lighted encoding photo-
cell resistance to the combined resistances of the lighted
reference photocell circuit is low. Since the photocell
resistances vary between photocells, th~range of lighted
photocell resi~tance variations are required to be within
the prescribed limits such that the unllghted maximum
resistance is not more than slightly less than five times
the minimum lighted resistance. The variable ratios pro-
duced by the lighted and unlighted encoding photocell resis-
tances relative to the reference photocell resistances is
such that the threshold o~ the comparator 120 is still
exceeded by the signal to be sensed 118 when any encoding
cell is lighted.
The input-output transfer characteristic o~ the
analog comparator 120 has a turn-on lnput threshold level
119 ~hich is exceeded by the signal to be sensed 118 to
trigger a high-to-low change in the comparator ou~put signal
140. me hysteresls provided by the resistor 144 maintains
the low binary output voltage level as the comparator input
signal 118 decreases below the turn-on value until the turn-
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6 3~ Z 47,688
off threshold is reached which is approximately one volt
below the comparator turn-on threshold voltage. In one
embodiment the turn-on and turn-of~ levels are 6.6 and 5.4
volts, respectively. Accordingly, when there are slight
variations occurring in the slgnal to be sensed 118 during a
sampling period in which one of the encoding photocells is
being detected, the input variations decreasing below the
turn-on threshold will not produce a change in the detecting
circuit output signal 140. Accordingly, when the photocell
is not lighted due to an opaque segment of the code pattern,
the output signal 140 has a high binary level state approach-
ing the level of the circuit supply conductor 98. When a
transparent coded segment o~ the code pattern passes light
transmissions to the encoding photocell~ the turn-on threshold
of khe analog voltage comparator 120 is exceeded and the
output signal 140 drops to approach the ground voltage of
the conductor 99 and change from the high to the low binary
state. 'rhe high binary sta'ce has a signal level of approxi-
mately 9.6 volts at the output 138 in one embodiment.
The strobe input 148 of the comparator 120 ls
enabled by the energization of the lamp 118 so that an
improper or omitted circuit connection of the lamp 18 inhibits
operation of the comparator 120 and no output will be produced
until the malfunction condition is corrected. An outage
because of a defective lamp is readily detectable by the
output signal 140 not changing state with opera'cion of the
register and rotation of the code patterns.
While the description of the present invention has
been made with reference to a specific embodiment it is
apparent to those skilled in the art that other modifica-
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~ 3~Z 47,688
tions and alterations may be made wlthout departing from the
spirit and scope o~ this invention.
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