CA1116302A - Adjustably lighted reference photocell circuit arrangement for a photocell sensing assembly - Google Patents

Abstract

47,688 AN ADJUSTABLY LIGHTED REFERENCE
PHOTOCELL CIRCUIT ARRANGEMENT FOR A
PHOTOCELL SENSING ASSEMBLY

ABSTRACT OF THE DISCLOSURE
A photocell sensing assembly for an optoelectronic encoder includes an adjustably lighted reference photocell arrangement for providing a variable predetermined reference output value. An adjustable shutter assembly regulates light entensities in the arrangement. The reference output value is established relative to individual outputs of a plurality of encoding photocells to be sampled by a detecting circuit.

Description

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 ~ '`;
~ 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 ~ 3~ Z 47~688 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 ~ 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 3~ Z
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 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 ~ 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 ~ 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 3~ ~

47,688 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_ 3~ 2 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 ~ ~ Z 47,688 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 L6~Z
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 .~;, 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 ~13-~ . ~
~ 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 ~ 3~ ~ ~7,688 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 3~ ~
47~688 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 3~
1~7,6g8 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 ~ 3~ 2 47,688 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---lg--..~

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-~ 3~Z 47,688 tions and alterations may be made wlthout departing from the spirit and scope o~ this invention.

Claims

47,688 What is claimed is:

1. A photocell sensing circuit comprising:
a plurality of code discs each having a circularyl disposed optically coded pattern and being rotatable by an associated shaft for which the angular position thereof is to be electrically encoded;
a circuit board having disposed thereon an encoding photocell array including plural groups of circularly dis-posed ones of such encoding photocells aligned with said code discs for producing variable outputs responsive to different rotated conditions of the optically coded patterns, and further having disposed thereon a reference photocell array including such reference photocells distributed across said circuit board adjacent said plural groups of encoding photocells and in series connected relationship;
plural sources of light radiations each separately aligned with different ones of the encoding and reference photocells;
a baffle plate extending between said code discs and said plural sources of light radiations, said baffle plate including plural circular recesses receiving said code discs with plural groups of tunnel apertures ex-tending therethrough wherein the groups of apertures are in mutual alignment with said groups of encoding photo-cells and with the associated ones of the optically coded patterns of said code discs and with the associated ones of said plural sources of light radiations, said baffle plate further including light regulating apertures in mutual alignment with said reference photocells and associated ones of said plural sources of light radiations, 47,688 and said baffle plate still further including mutually align-ed threaded openings each extending between a common edge of said baffle plate and a separate one of said regulating apertures and receiving one of plural threaded shutter members in threaded adjusting relationship therewith for selective positioning across said regulating apertures to vary the light radiations so as to produce predetermined outputs of the reference photocells relative to the vari-able-outputs of the encoding photocells; and a photocell detecting circuit including a reference circuit including the outputs of said array of reference photocells, a voltage comparator circuit, and a sampling circuit for consecutively connecting each of said outputs of said encoding photocells with an output of said reference circuit and to the input of said voltage compar-ator circuit so that the output thereof produces binary signals corresponding to said variable outputs of each of said encoding photocells.