BACKGROUND OF THE INVENTION 
     The present invention relates to utilizing a beam of light to detect the presence of objects, and more particularly to optical sensing equipment which employ a plurality of photodetectors to receive the light which has been reflected by an object to be detected. 
     In manufacturing operations, it is desirable to detect the presence of an object moving down a conveyor. This enables material handling equipment to direct the object safely between conveyor sections or to a work station. It also is desirable to detect when objects become jammed along the conveyor. 
     An optical detector system often is used for these purposes. One type of detection apparatus, referred to as a retro-reflective system places an emitter-detector assembly on one side of the conveyor and a reflector on the opposite side. A beam of light is sent from the emitter across the conveyor to the reflector and then returns back across the conveyor to the detector. An object, moving along the conveyor, interrupts the beam of light, thereby providing an indication of the presence of the object. Retro-reflective systems have the disadvantage of requiring installation of a reflector on the opposite of the conveyor. Installation of the reflector in many situations is difficult or interferes with other operations being performed along the conveyor. Therefore, it is desirable to utilize an object detector apparatus that does not require devices on both side of the conveyor. 
     In response, sensing systems have been developed which detect the reflection of the light beam from objects moving along the conveyor. However, such systems must address several potential problems. First, the reflectivity of the objects vary greatly from very specular in nature to ones that are very diffuse. In addition, black objects naturally absorb more light than white objects. The circuitry that processes the signal from the light detector can be designed with a relatively high sensitivity to detect low reflectivity objects. However, that high sensitivity often results in the signal processing circuitry being saturated in response to light from highly reflective objects. 
     In addition, high sensitivity sensing circuits can falsely respond to highly reflective objects on the opposite side of the conveyor. For example, a shiny metal object being transported on a cart next to the conveyor system can reflect enough light back to the photodetectors to be erroneously interpreted as an object moving down the conveyor. Therefore, it is desirable to have a detector system that has a relatively high sensitivity and a sharp cutoff at a distance equal to the far side of the conveyor. 
     FIG. 1 depicts a prior detection system of this type. In this system, an emitter  10  transmits a beam of light across the conveyor. One ray  12  of that light beam is illustrated passing through an output lens  14 . An object  16  reflects the ray  12  through another lens  18  onto a detector assembly  20 . The detector assembly  20  has an near detector  22  that receives light from objects which are relatively close to the detector assembly and has a far detector  24  that receives light from objects which farther away from the detector assembly. Note that the objects usually move in a direction that is orthogonal to the plane of the drawing. The farther an object is from the emitter  10  the smaller the angle of the reflected ray  12 . For example, the reflected beam from object  16  strikes the near detector  22 , whereas the reflected ray from a more distant object  26 , beyond a given cutoff distance  28  from the emitter, strikes the far detector  24  and not the near detector  22 . 
     It should be understood that the emitter  10  produces a beam of radiation comprising numerous rays. Thus, when the entire beam is reflected from an object, some of the rays may strike the near detector  22  and other reflected rays may strike the far detector  24 . If the object is within the cutoff distance  28  from the emitter, a greater amount of reflected light will strike the near detector  22  than the far detector  24 . Conversely, when the object is beyond the cutoff distance  28 , a greater amount of light is reflected onto the far detector  24  than onto the near detector  22 . The output signal produced by a detector corresponds to the amount of light which impinges that detector. Thus, by comparing the two detector signals, the object detection apparatus is able to distinguish an object moving along the conveyor from objects beyond the conveyor. 
     This dual detector system does well when a diffuse object fully blocks the beam of light from the emitter. However, when an out of range object blocks only a portion of the emitted beam or is specular, light from that out of range object can be falsely interpreted as being from an object on the conveyor because the near detector  22  may receive more light than the far detector  24 . 
     SUMMARY OF THE INVENTION 
     An object detection system according to the present invention has an automatic sensitivity control mechanism that adjusts the photodetector signal processing to account for objects of vastly different reflectivities. 
     This object detection system has an emitter which produces a light beam that will be reflected by objects to be detected. The emitter is connected to an output terminal of and control circuit which produces an electric current that varies in magnitude in response to a first control signal. 
     A photodetector receives light that has been reflected by an object and produces a signal indicating the amount of light which strikes the photodetector. A variable attenuator is connected to the photodetector to attenuate the signal by an amount that varies in response to a second control signal. 
     The amounts of emitter current and attenuation are determined by a controller. The controller receives the signal produced by the photodetector and compares that signal to one or more threshold levels to determine amounts of emitter current and attenuation. The first and second control signals are generated by the controller in response to that determination. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an object detector according to the prior art; 
     FIG. 2 illustrates a conveyor on which the present object detection system is located; 
     FIG. 3 is a schematic circuit diagram of the object detection system; 
     FIG. 4 is a view taken along line  4 — 4  of FIG. 2 illustrating the fields of view for each of the three photodetectors used in the object detection system; 
     FIG. 5 is a schematic diagram of a second embodiment of the present invention which utilizes four photodetectors; 
     FIG. 6 depicts the fields of view for each of the four photodetectors in the circuits of FIG. 5; and 
     FIGS. 7A and 7B form a flowchart of the operation of the second embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to FIG. 2, a conveyor  30  travels in the direction indicated by arrow  32  and carries a plurality of objects  34 . An optical object detection system  36  is located on one side of the conveyor  30  and has a horizontal field of view with boundaries indicated by dashed lines  38 . As will be described, the object detection system  36  emits a beam of light across the conveyor  30  which is reflected as each object  34  passes within the field of view. The reflected light strikes one or more of three photodetectors within the system  36 . 
     FIG. 3 illustrates the details of the object detection system  36  which includes a light emitter  38 , such as a light emitting diode. The intensity of the light from the emitter  38  is a function of the magnitude of the electric current flowing through the device. Thus emitter  38  is connected to a current control circuit  40  which varies the magnitude of that electric current in response to a command signal received from a microcomputer  42 . The microcomputer  42  is a conventional device having digital and analog inputs and outputs. An internal memory stores the program which is executed to provide the object detection function. 
     As will be described, the light from the emitter  38  is reflected by the objects  34  and strikes one or more of three photodetectors  44 ,  46 , and  48 . The first and second photodetectors  44  and  46  are connected in inverse parallel manner between the circuit ground and a first input node  50 . The photodetectors  44 - 48  emit an electric current upon being illuminated. 
     Current produced by the first and second photodetectors  44  and  46  is conducted through a current sensing resistor  52  and a first field effect transistor (FET)  54  which are connected in parallel between a first input node  50  and circuit ground. A voltage is produced at the first input node  50  which corresponds to the magnitude of that current. Because the first and second photodetectors  44  and  46  are poled in opposite directions in the circuit, the voltage level at first input node  50  corresponds to the difference in the amount of the light striking the first and second photodetectors  44  and  46 . The polarity of that voltage indicates whether more light is striking the first detector  44  or the second detector  46 . Specifically, the voltage at node  50  is positive with respect to ground when more light strikes the second photodetector  46 , whereas that voltage is negative with respect to circuit ground when more light strikes the first photodetector  44 . 
     The gate of the first FET  54  is connected to by an analog output of the microcomputer  42 . As will be described, the first FET  54  acts as a variable resistor which the alters the total resistance through the which the photodetector current flows. For example, the FET  54  in the off-state can be characterized as a one megaohm resistor, while in the on-state FET has a resistance of approximately two ohms or less. Thus, the microcomputer  42  by varying the voltage applied to gate of the FET  54  can vary the attenuation of the photodetector signal and the sensitivity of the system. 
     The resultant signal produced at first input node  50  is coupled by a capacitor  56  to a first pre-amplifier  58  having an output that is coupled by a second capacitor  60  to a summing node  62  at the input of a high gain amplifier  64 . The analog output from amplifier  64  is applied to an analog input  65  of the microcomputer  42  and to one input of a comparator  66  having another input connected to a reference voltage V REF . The output of the comparator  66  is applied to a digital input  67  of the microcomputer. 
     The third photodetector  48  is connected between circuit ground and a second input node  68 . A second current sensing resistor  70  and a second field effect transistor (FET)  72  are connected in parallel with the third photodetector  48 . The gate of the second FET is controlled by a second analog output of the microcomputer  42 . When light strikes the third photodetector  48  a proportional negative voltage is produced at the second input node  68 . The second input node  68  is coupled by capacitor  74  to a second pre-amplifier  76  having an output coupled by another capacitor  78  to the summing node  62 . 
     Important to operation of the object detection system  36  is the aiming the three photodetectors  44 ,  46 , and  48  with respect to the conveyor  30 . That aiming is shown in FIG.  4 . The emitter  38  produces a generally conical beam of radiation with the upper and lower boundaries of that beam being depicted by the solid lines  81  and  82  respectively. The first and second photodetectors  44  and  46  are mounted above the emitter  38  with the first detector  44  having a field of view with upper and lower boundaries  83  and  84 , respectively, are indicated by dotted lines. Note that the lower boundary  84  passes into the emitter&#39;s light beam and the upper boundary  83  extends considerably above the upper boundary  81  of that light beam. The second photodetector  46  has a field of view with upper and lower boundaries  85  and  86  depicted by dashed lines. The upper boundary  85  extends through the emitter&#39;s light beam crossing the midpoint of that beam at approximately a point corresponding to the cutoff distance  90 . The lower boundary  86  extends beneath the beam of radiation across the conveyor. The fields of view for the first and second photodetectors do not insect. The cutoff distance corresponds to the opposite side of the conveyor  30  from the detection system  36 . As will be described, the system does not respond to light reflected by objects that are beyond the cutoff distance  90 . 
     The field of view for the third photodetector  48  is defined by upper and lower boundaries  87  and  88 . It is important that the lower boundary  88  of that field of view does not cross the lower boundary  82  of the light beam from emitter  38 . Otherwise there is an opportunity that the second detector  46  could be the only one receiving light which would result an erroneous output from the object detection system  36 . Note that the upper boundary  87  of the third photodetector&#39;s field of view crosses the upper boundary of the field of view for second photodetector  46 , but does so beyond the distance cutoff line  90 . 
     As shown in FIG. 3, the inverse parallel connection of the first and second photodetectors  44  and  46  results in the output current from the first photodetector  44  being subtracted from the output signal of the second photodetector. The resultant current flows through the sensing resistor  52  and produces a voltage at first input node  50  corresponding to the level of that resultant current. If more light impinges on the first photodetector  44  than on the second photodetector  46 , the first photodetector will produce a greater signal resulting in a negative voltage being produced at first input node  50  with respect to ground. When a greater amount of light strikes the second photodetector, the resultant current produces a positive voltage at first input node  50 . The voltage produces at first input node  50  produces a proportional output signal from the first pre-amplifier  58 , which is applied to the summing node  62  by coupling capacitor  60 . 
     The intensity of light striking the third photodetector  48  produces a corresponding negative voltage at the second input node  68 . In response, the second pre-amplifier  76  produces a proportional negative signal at its output which is coupled to the summing node  62 . As a result, the signal level at the summing node  62  arithmetically equals the output signal from the second photodetector  46  minus the output signal from the first photodetector  44  and minus the output signal from the third photodetector  48 . The arithmetic signal summation at node  62  is amplified by the high gain amplifier  64  and applied to the analog input  65  of the microcomputer  42 . The output signal from the high gain amplifier  64  is compared to the detection threshold V REF  by comparator  66  to determine whether the output signal is above that referenced threshold. The digitized bit from that comparison is then applied to a digital input of the microcomputer  42 . 
     The alignment of the fields of view for the three photodetectors  44 - 48 , as shown in FIG. 4, are such that when an object is between the emitter  38  and the cutoff distance  90 , the second emitter  46  will produce a signal that is significantly greater than the combination of the signals from the first and second detectors  44  and  48 . Therefore, the amplified sum of the three detector signals applied to the comparator  66  exceeds the detection threshold V REF . As a consequence, a true logic level will be applied to the digital input  67  of the microcomputer  42  thereby indicating the presence of an object on the conveyor  30 . 
     When the light from emitter  38  is reflected by an object that is farther away from the emitter than the cutoff distance  90 , the combination of the negative signals from the first and second photodetectors  44  and  48  will be greater than the positive signal from the first photodetector  46 . As a consequence, the sum of the detector signals at node  62  when amplified by amplifier  64  will be less than the detection threshold V REF . As a consequence, the output from the comparator  66  will be a fault logic level which gets applied to the digital input  67  of the microcomputer  42 . Thus, the microcomputer will not respond to objects beyond the cutoff distance  90 . 
     FIG. 4 illustrates the robust nature of the present three photodetector system. Assume that a nonuniform object  92  passes on the far side of the conveyor  30  from the sensing assembly  20 . This object  92  does not reflect light into the first photodetector  44 , which field of view is indicated between boundaries  83  and  84 , but, this nonuniform object does reflect some light into the second photodetector having a field of view indicated by boundary lines  85  and  86 . As a consequence, a positive voltage will be produced at the first input node  50 . 
     However, with the present system that incorporates a third photodetector  48 , the nonuniform object  92  reflects light into the third photodetector&#39;s field of view bounded by lines  87  and  88 . Thus a negative voltage is produced at the second input node  68  which causes the second pre-amplifier  76  to apply a negative voltage to summing node  62  in FIG.  3 . The negative voltage is greater than the positive voltage from the first pre-amplifier  58 . Therefore, a negative voltage level is produced at summing node  62 , thereby resulting in a voltage being applied to an input to the comparator  66  which is less than the detection threshold V REF . Thus a false logic level is sent to the digital input  67  of the microcomputer  42  indicating that an object is not present on the conveyor. 
     Although the three detector system is a significant improvement over the detector systems with only a pair of photodetectors, it is possible that a mirror located beyond the cutoff distance  90  could reflect light onto only the second detector  46 . This would provide a false input to the microcomputer  42  as though an object had been detected on the conveyor. Although the angle at which light could be reflected to produce that false detection is very small, there is still the possibility for that occurrence. 
     In applications where a high degree of reliability is required, a fourth photodetector can be provided adjacent to the third photodetector thus providing two photodetectors on each side of the emitter. With reference to FIG. 5, object detection system  100  is similar to the previously described three photodetector system  36 . Specifically, a light emitter  138  is coupled to the output of a current control circuit  140  that varies electric current fed through the emitter in response to the signal from the microcomputer  142 . 
     First and second photodetectors  144  and  146  are connected in inverse parallel fashion between the circuit ground and a first sensing node  150 . A first current sensing resistor  152  and a first FET  154  are connected in parallel with the photodetectors  144  and  146 . The first sensing node  150  is coupled by capacitor  156  to the input of a first pre-amplifier  158  whose output is connected directly to the input of a first amplifier  164 . The output of first amplifier  164  is connected to a first analog input  165  of the microcomputer  152  and to an input of a first comparator  166 . The first comparator  166  has another input connected to a source of the detection threshold V REF . The output of the first comparator  166  is applied to one bit line  167  of a digital input to the microcomputer  142 . 
     The third and fourth detectors  148  and  149  are similarly connected in an inverse parallel manner between the circuit ground and a second sensing node  168 . A second current sensing resistor  170  and second FET  172  are connected in parallel with the third and fourth photodetectors  148  and  149 . The microcomputer has separate analog output lines connected to the gates to the two FETs  154  and  172 . The second current sensing node  168  is coupled by a capacitor  174  to the input of a second pre-amplifier  176  whose output is applied to the input of a second amplifier  178 . The output of the second amplifier  178  is applied to a second analog input  175  of the microcomputer  142  and to an input of a second comparator  177 . The second comparator has another input connected to the detection threshold V REF . The output of the second comparator  177  is applied to another bit line input  179  of the digital input for the microcomputer  142 . 
     With reference to FIG. 6, the light emitter  138  and its associated lens  139  are adjusted to produce a beam pattern with the upper and lower boundaries  181  and  182 . The first and second photodetectors  144  and  146  and their common lens  147  are aimed to have fields of view as indicated by the dotted and dashed lines in the drawing. Specifically, the field of view for the first photodetector  144  has an upper boundary  183  extending upwardly across the conveyor  30 . The lower boundary  184  for the field of view of the first photodetector  144  extends downward, but does not cross the center line of the light beam from emitter  138  until well beyond a cutoff distance  190 . The upper boundary  185  for the field of view of the second photodetector  146  extends at an angle slightly downward from the lower boundary  184  of the first photodetector  144  and does not cross that lower boundary. The lower boundary  188  for the second photodetector&#39;s field of view extends downward across the conveyor. The fields of view for the first and second photodetectors do not insect. 
     The field of view for the third photodetector  148  has an upper boundary  187  which extends upward but does not cross the center line of the light beam from emitter  138  until well beyond the cutoff distance  190 . The lower boundary  188  of the third photodetector&#39;s field of view extends downward and does not cross the lower boundary  182  of the emitter&#39;s light beam. The fourth photodetector  149  has a field of view with a lower boundary  192  which extends upward into the light beam from emitter  138 , but does not cross the center line of that beam until well beyond the cutoff distance  190 . The upper boundary  191  of the field of view for the fourth photodetector  149  extends upward and crosses the upper boundary of the first photodetector  144  slightly beyond the cutoff distance  190 . The fields of view for the third and fourth photodetectors do not insect. 
     The configuration of the fields of view for the various photodetectors and their arrangement in the processing circuitry of FIG. 5 are such that when an object passes along the conveyor (i.e. between emitter  138  and cutoff distance  190 ) the microcomputer receives a pair of true signals from the first and second comparators  166  and  177 . Specifically, in this situation the light impinging upon the second photodetector  146  will be greater than the light impinging upon the first photodetector  144 , thus producing a correspondingly greater signal from the second photodetector  146 . This results in a positive voltage being produced at the first sensing node  150  resulting in the first comparator  166  applying a true logic level to digital input line  166  of the microcomputer. That object on the conveyor also reflects a greater amount of light onto the fourth photodetector  149  than onto the third photodetector  148 . This similarly produces a positive voltage at the second sensing node  168  and in turn a true signal from the second comparator  177  on the second digital input line  179  of the microcomputer. Thus, when an object is within the cutoff distance  190  from the emitter  138 , the microcomputer receives a pair of true logic levels on input lines  167  and  179 . 
     When an object beyond the opposite side of the conveyor from the object detection system  36  reflects light onto the photodetectors  144 - 149  at least one of the comparators  166  and  177  produces a false output signal. Thus, if either or both comparator output signals is false, a low logic level, the microcomputer  142  determines that there is not a valid object passes on the conveyor  30 . 
     As noted with respect to the three photodetector system in FIGS. 3 and 4, a specular object on the remote side of the conveyor  30  can reflect a light from the emitter  38  directly back to only the second photodetector  146 . That event generates a false object detection output from the microcomputer  42 . A similar object can reflect light from the emitter directly back to either the second or third photodetectors  146  or  149  in the four detector system in FIGS. 5 and 6. However, because that reflected beam from the out of range specular object has a very narrow return angle, the reflected light beam can not strike both the second and fourth photodetectors  146  and  149 . As a consequence, a positive voltage can occur at only one of the input nodes  150  or  168  and a true logic level is produced by only one of the two comparators  166  or  177 . Since the other comparator has a false output, the microcomputer  142  receives only one true logic level on digital input lines  167  and  179  and will not falsely conclude that there is an object present on the conveyor line. Therefore, the four photodetector version is more robust in guarding against false object detections. 
     The second object detection system  100  also provides automatic sensitivity control which is best described in the context of the flowchart commencing on FIG.  7 A. The software execution commences at step  200  where the microcomputer  142  issues a digital command that instructs the current control circuit  140  to apply current pulse a previously determined level to the emitter  138 . A variable indicating that level of current is stored within the memory of the microcomputer  142 , which also contains a default level to be used upon initial power-up of the system. The current results in the emitter  138  producing a pulse of light which travels across the conveyor system. 
     At step  202 , microcomputer  142  receives data from the first amplifier  164  at the first analog input  165 . The level of that analog signal is digitized and then compared at step  204  to a regulation threshold which corresponds to the desired intensity of the light beam from the emitter  138 . If the light beam has the desired intensity, the program execution bypasses the automatic sensitivity control section by entering branch  205 . 
     If the light beam is not at the desired intensity, a determination is made at steps  206  and  208  whether both the input from the first amplifier  164  is less than the regulation threshold and the emitter is at maximum intensity. If that logical expression is true, the automatic sensitivity control routine also is bypassed by entering branch  205 . If that logical expression is not true, the program execution advances to step  210 . 
     The automatic sensitivity control section commences at step  210  with a determination whether or not the input attenuation is active. Relatively fine control of the sensing circuitry is accomplished by regulating the current applied to the emitter  138 . Coarser control is performed by attenuating the input signals from the photodetectors  144 - 149  via activation of the first and second FETs  154  and  172  in unison. Activation of the FETs may either be binary (off or on), or a variable voltage can be applied to the gates of the FETs to provide a varying amount of attenuation. In the binary mode, the gates of the FET are connected to the most significant bit of the microcomputer&#39;s digital output connected to the current control circuit  140 . The remaining bits, but not the most significant bit, are connected to the current control circuit  140 . 
     The input to the current control circuit  140  is based negative logic wherein the greater the numerical value of the digital input from the microcomputer  142 , the smaller the amount of current applied to the emitter. As a consequence, when the most significant bit of that digital output is set, is a one value (for half the values), both FETs  154  and  172  are turned-on, thereby providing a relatively low resistance path in parallel with the sensing resistors  152  and  170 . This reduces the voltage levels at the sensing nodes  150  and  168 , thereby providing greater attenuation of the signals produced by the photodetectors. Additional fixed resistor (not shown) can be connected in series with each FET  154  and  172  to set the level of the binary attenuation. 
     The default emitter current setting is in the lower half of the digital values for the current control circuit  140 . Therefore, the FETs  154  and  172  will be off, non-conductive. When the automatic sensitivity control process increased that digital value to the midpoint, the most significant bit is set to one and the remaining bits are zeroes. This turns on the FETs  154  and  172  activating photodetector attenuation. Because the remaining bits are zeroes, the level of current to the emitter will be at maximum level. From that point, incrementation of the remaining digital bits reduces the emitter current. 
     As a result of this control strategy, the microcomputer  142  determines at step  210  whether the most significant bit of the data for the current control circuit  140  is set. If that is not the case, the program execution jumps to step  214 . However, when attenuation is active, the program execution branches to step  212  at which the microcomputer  142  inspects its first digital input line  167  for a true output from the first comparator  166 . That input is true when the combined signals from the first and second photodetectors  144  and  146  produce a substantial positive voltage level at the first input node  150 , as occurs when an object is present on the conveyor in front of the object detection system  100 . If the comparator output is false, an OUT OF RANGE flag is reset at step  220  to indicate that a highly reflective object is located out of the range of the sensing system, i.e. beyond the cutoff distance  190 . In that case, the sensitivity of the system can not be adjusted and the programs goes to step  222 . 
     However, if the output of the first comparator  166  is true at step  212 , the OUT OF RANGE flag is reset at step  214 . Next at step  216 , the level of the signal at the first analog input  165  is subtracted from the regulation threshold which indicates the desired intensity for the emitter beam, That calculation produces an ERROR value that then is added to the previous value for the emitter current to produce a new value for the emitter current at step  218 . The program execution then returns to step  200  to produce another pulse of light in order to check the performance of the system at the newly determined value for the emitter current. Eventually a determination is made either at step  204  that the emitter current has been properly set or at step  208  that no further adjustment of the emitter power is possible and the program execution advances to step  222 . 
     At this point the signals from the first and second photodetectors  144  and  146  are inspected for an indication that an object is present on the conveyor. Specifically, if the binary output of the first comparator  166  appearing on input line  167  is false, a flag called “CHANNEL 1 ” is reset at step  226  to indicate that an object was not found. However, if the output of the first comparator  166  is true, a determination is made at step  220  for whether the OUT OF RANGE flag is set. If that is the case, the true comparator output resulted from a specular object beyond the cutoff distance  190  and the CHANNEL 1  flag is reset to indicate that an object is not present on the conveyor. Alternatively, when the first comparator output is true and the OUT OF RANGE flag is not set, the CHANNEL 1  flag is set at step  228  to indicate that the first and second photodetectors  144  and  146  may have detected a valid object. 
     The program execution then advances to step  230  on FIG. 7B at which a similar automatic sensitivity control adjustment procedure is performed for second channel having the third and fourth photodetectors  148  and  149 . In summary, the emitter  138  is activated to send a pulse of light and a second input signal is obtained at the microcomputer&#39;s second analog input  175  which input signal represents the third and fourth photodetector signals. Then at steps  234 ,  236  and  238  a determination is made whether to bypass the automatic sensitivity control for the second channel. In which case, the OUT OF RANGE flag may get set at step  246 . 
     When the automatic sensitivity control process proceeds the OUT OF RANGE flag is reset at step  244 . Then an ERROR value is calculated at step  248  by subtracting the analog input from the second amplifier  178  from the desired regulation threshold. That ERROR value is used at step  250  to adjust the emitter current level, which operates the current control circuit  140 . This adjustment of the emitter current continues to be executed until either the emitter current is properly adjusted or can no longer be adjusted because the emitter is at maximum power. When that happens, the program execution advances to step  252 . 
     The output of the second comparator  177  is inspected at this juncture. If that output is false, indicating that the third and fourth photodetectors  148  and  149  did not respond a valid object, a flag designated CHANNEL 2  is reset at step  254 . Otherwise, when the output of the second comparator  177  is true and the OUT OF RANGE flag is found set at step  256 , the CHANNEL 2  flag also is reset. If the OUT OF RANGE flag is not found set at step  256 , a determination is made by the microcomputer  142  that a valid object may have been sensed by the third and fourth photodetectors, in which the case the CHANNEL 2  flag is set at step  258 . 
     At this point, signals from the two channels for the pairs of photodetectors  144 - 149  have been analyzed, the results of which are indicated by the CHANNEL 1  and CHANNEL 2  flags. Therefore, at step  260  the microcomputer checks whether the CHANNEL 1  and CHANNEL 2  flags are both true. If not, the program execution jumps to step  266  where the microcomputer output line  180  is reset to indicate that an object has not been detected. If at step  260  both the CHANNEL 1  and CHANNEL 2  flags are found to be true, the difference between the analog input levels from the first and second amplifiers  164  and  178  is derived at step  262 . If these two analog inputs differ significantly, the validity of their indication of the presence of an object is questionable. Thus, at step  264  the absolute value of this difference is compared to a difference threshold. If that difference threshold is exceeded, the microcomputer  142  resets the object detection output line  180  at step  266 . On the other hand, if the difference in the two input levels is less than the difference threshold, the microcomputer  142  concludes that an object is present on the conveyor and the object detection output line  180  is set true. 
     The automatic sensitivity control for the four photodetector object detection system  100  also is used in the first object detection system  36  in FIGS. 3 and 4 which has only three photodetectors. It is understood that the first object detection system  36  has only one analog input  65  and one digital line input  67  to the microcomputer  42 .
An detection system emits a light beam and has a photodetector that senses when light is reflected by objects to be detected. Because reflectivity of the objects varies greatly, the intensity of the reflected light varies over a large range. To prevent false object detection, an automatic sensitivity control mechanism is provided which controls the intensity of the emitted light beam and attenuation of the signal from the photodetector. That control is in response to the level of the photodetector signal.