Patent Publication Number: US-6657762-B2

Title: Optical barrier device

Description:
TECHNICAL FIELD 
     The present invention relates to an optical barrier apparatus used in safety equipment and the like for industrial machinery. In particular the invention relates to an optical barrier apparatus (also referred to as an optical barrier sensor) for scanning a detection area with an optical beam, and judging the absence of object when a reflection beam of the optical beam is received, and judging the presence of object when not received. 
     BACKGROUND ART 
     As such an optical beam scanning type optical barrier apparatus, there is the apparatus disclosed for example in PCT International Publication No. WO97/33186, and this will be briefly described. 
     In FIG. 2 of PCT International Publication No. WO97/33186, a laser beam generating means and a laser scanning means are arranged on one side of a detection area, and an array light receiving elements is arranged on the other side. In this apparatus, a laser beam generated by the laser beam generating device is projected onto the laser scanning device, and the laser scanning device reflects the laser beam so as to scan an area including the detection area. If an object is not present inside the detection area, the laser beam is received by the light receiving element array. If an object is present inside the detection area, the laser beam is blocked by the object so that the light receiving element positioned in the shadow of the object within the light receiving element array does not receive the laser beam. The deficiency of light reception output signal from the light receiving element array, which occurs at this time, is detected by a signal deficiency detecting means, thus notifying of the presence of object. 
     Furthermore, in FIG. 3 and FIG. 6 of PCT International Publication No. WO97/33186, there is disclosed a construction which uses a reflecting mirror. 
     In FIG. 3, the construction is such that a laser beam generating device, a laser scanning device and a light receiving element array are arranged on the same side, and a concave reflecting mirror is arranged on the other side. A laser beam generated by the laser beam generating device is scanned at a predetermined spread angle by the laser scanning device, and projected onto the concave reflecting mirror arranged on the other side. The laser beams reflected by the concave reflecting mirror are passed through a detection area as mutually parallel beams to be directed towards the light receiving element array. Furthermore, in FIG. 6, the construction is such that the light receiving element array of FIG. 3 is replaced with a single light receiving element, and the position of the laser scanning device and the shape and position of the concave reflecting mirror are adjusted so that the reflected light of the concave reflecting mirror is focused onto the single light receiving element. 
     However, with the abovementioned optical beam scanning type optical barrier apparatus, in the constructions of FIG. 2 and FIG. 3, since a light receiving element array is used, it is necessary to adjust light reception directional characteristics of the light receiving element array with respect to each of the elements. Furthermore, a light receiving circuit is needed for each of the respective light receiving elements, and hence there is a problem in that cost reduction is difficult. 
     Furthermore, with the construction of FIG. 6, the light receiving element is only one, and hence the cost can be reduced compared to FIG. 2 and FIG. 3. However there is a problem in that there exists an area where the object detection is not possible, and the detection area thus becomes narrow. 
     The present invention addresses the abovementioned problems with the object of providing an optical barrier apparatus enabling of cost reduction without narrowing the detection area. 
     DISCLOSURE OF THE INVENTION 
     In order to achieve the aforementioned object, an optical barrier apparatus according to the present invention comprises a first and second units facing each other with a detection area therebetween, each of the first and second units comprising: optical beam generating means, optical beam scanning means for reflecting an optical beam generated by the optical beam generating means so as to scan an area containing the detection area, optical beam reflecting means for reflecting a scanning beam incident from the optical beam scanning means via the detection area by turning back at approximately 180 degrees, light receiving means arranged in the vicinity of the optical beam scanning means for receiving a reflection beam from the optical beam reflecting means, and signal deficiency detecting means for detecting the presence/absence of a deficiency of output signal of the light receiving means and generating a notification output for object absence at the time of no deficiency, wherein the optical beam scanning means and the light receiving means of the first unit and the optical beam scanning means and the light receiving means of the second unit are arranged on either side of the detection area at approximately diagonal positions. 
     With such a construction, the optical beam generated from the optical beam generating means is reflected and scanned by the optical beam scanning means. If an object is present in the detection area, the scanning beam does not reach the optical beam reflecting means so that an optical beam at a predetermined level or above is not received by the light receiving means. If an object is not present in the detection area, the scanning beam is reflected by the optical beam reflecting means and the light receiving means receives a reflection beam at a predetermined level or above. The signal deficiency detecting means, if an output level of the light receiving means is at the predetermined level or above, generates a notification output for object absence. This type of object detection is respectively performed in the first unit and second unit. Moreover, since the optical beam scanning means and light receiving means of the first unit, and the optical beam scanning means and light receiving means of the second unit are arranged at diagonal positions on either side of the detection area, the area where object detection is possible becomes a rectangular shape. As a result, the number of light receiving means can be reduced, costs can be reduced, and the detection area becomes rectangular so that the detection area can be widened. 
     The construction may be such that there is provided synchronous drive means for synchronizing the two optical beam scanning means of the first and second units with respect to each other so that when a scanning beam direction on the first unit side is a diagonal direction, a scanning beam direction on the second unit side is also a diagonal direction. Then, when a scanning beam direction of one unit is a diagonal direction where it is easy for an optical beam from the other unit to be erroneously received, if an object is present on an optical axis of an optical beam from the one unit, the scanning beam is blocked by the object so that erroneous notification attributable to reception of the scanning beam of the other unit can be prevented. 
     Moreover, the construction may be such that there is provided selection drive means for selectively driving the first and second units so that object detection operations of the first unit and second unit are not performed at the same time. Since when one unit is being driven the other unit is stopped, erroneous notification attributable to reception of the scanning beam of the other unit can be prevented. 
     Furthermore, the construction may be such the emission wavelengths of optical beams respectively generated from the respective optical beam generating means of the first unit and second unit are made different from each other. Moreover, the construction may be such that blinking frequencies of reflection beams respectively reflected from each optical beam reflecting means of the first unit and second unit are made different from each other. In this case also, since the optical beam of the own unit and the optical beam of the other unit can be distinguished, erroneous notification attributable to reception of the scanning beam of the other unit can be prevented. 
     Moreover, the construction may be such that each signal deficiency detecting means verifies that a light reception output from the light receiving means is one based on a reflection beam from the optical beam reflecting means, to generate a notification output for object absence. 
     With such a construction, since it becomes possible to distinguish between the reflection light from the optical beam reflecting means and the light reflected by the object, then even in the case where the reflectance of the object is high so that the light reception level of reflection light from the object is equal to or above a predetermined level, or the case where the object is near the light receiving means so that the light reception level of irregularly reflected light from the object is equal to or above a predetermined level, erroneous notification can be prevented. 
     Furthermore, the construction may be such that scanning verification means for verifying that the scanning beam is scanned within a range of the area including the detection area is provided in each unit. 
     With such a construction, it becomes possible to verify with the scanning verification means, that the scanning beam is normally scanning the detection area. Therefore, in the case where this construction is used as a safety ensuring facility for a machine, reliability for the optical barrier apparatus can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic configuration diagram of a first embodiment of an optical barrier apparatus according to the present invention. 
     FIG. 2 is a configuration diagram of a unit of the first embodiment. 
     FIG. 3 is an operation time chart of the first embodiment. 
     FIG. 4 is a diagram of a retroreflector. 
     FIG. 5 is an explanatory diagram of problems of an optical barrier apparatus of the present invention, FIG. 5A being a diagram showing a condition where one scanning beam is blocked by an object while another scanning beam is received, and FIG. 5B being a time chart showing a light reception output condition for the case of FIG.  5 A. 
     FIG. 6 is a configuration diagram of the main parts of a second embodiment of the present invention. 
     FIG. 7 is a diagram for explaining an operation of the second embodiment. 
     FIG. 8 is a configuration diagram of the main parts of a third embodiment of the present invention. 
     FIG. 9 is a configuration diagram of the main parts of a fourth embodiment of the present invention, FIG. 9A being a diagram of the scanning conditions of a scanning beam, and FIG. 9B being a configuration diagram of a signal deficiency detection circuit. 
     FIG. 10 is an operation time chart of the fourth embodiment. 
     FIG. 11 is a configuration diagram of the main parts of a fifth embodiment of the present invention, FIG. 11A being a configuration diagram of a reflector array, and FIG. 11B being a time chart of a light reception output of each unit. 
     FIG. 12 is a configuration diagram of the main parts of a sixth embodiment of the present invention. 
     FIG. 13 is a diagram showing an example of another method for preventing direct reception of a beam from another unit, FIG. 13A being a top view of the optical barrier apparatus, and FIG. 13B being a front view of the optical barrier apparatus. 
     FIG. 14 is a configuration diagram of the main parts of a seventh embodiment of the present invention. 
     FIG. 15 is an operation time chart of the seventh embodiment. 
     FIG. 16 is a configuration diagram of the main parts of an eighth embodiment of the present invention. 
     FIG. 17 is a configuration diagram of the main parts of a ninth embodiment of the present invention, FIG. 17A being a configuration example for receiving a scanning beam except for its own, and FIG. 17B being a configuration diagram of a circuit of a unit. 
     FIG. 18 is an operation time chart of the ninth embodiment. 
     FIG. 19 is a configuration diagram of a scanning verification section. 
     FIG. 20 is a block diagram of the scanning verification section. 
     FIG. 21 is an operation time chart of the scanning verification section. 
     FIG. 22 is a configuration diagram of the main parts for the case where the scanning verification section is applied to the optical barrier apparatus of the present invention. 
     FIG. 23 is a perspective view of a semiconductor galvano-mirror. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereunder is a description of embodiments of an optical barrier apparatus according to the present invention based on the appended drawings. 
     FIG. 1 shows a schematic configuration of a first embodiment of the optical barrier apparatus according to the present invention. 
     In FIG. 1, the optical barrier apparatus according to this embodiment comprises first and second units  10  and  20  facing each other on either side of a detection area  1 . 
     Each unit  10  and  20  comprises; an optical beam generating circuit  11 ,  21  (shown in FIG. 2) serving as optical beam generating means, a scan mirror  12 ,  22  for reflecting an optical beam generated from the optical beam generating circuit  11 ,  21  so as to scan the detection area  1  at a predetermined spread angle and generate scanning beams BM 1 , BM 2 , a reflector array  13 ,  23  serving as optical beam reflecting means, having multiple reflectors  13 - 1  to  13 - n ,  23 - 1  to  23 - n  arranged in the vertical direction of the detecting area  1 , for reflecting the scanning beams BM 1 , BM 2  incident via the detection area  1  by turning back at approximately 180 degrees, a light receiving element  14 ,  24  serving as light receiving means arranged in the vicinity of the scan mirror  12 ,  22  for receiving reflection beams reflected by the reflector array  13 ,  23  and incident via the detection area  1 , and a signal deficiency detection circuit  15 ,  25  (shown in FIG. 2) serving as signal deficiency detecting means for detecting the presence/absence of a deficiency of an output signal of the light receiving element  14 ,  24  to generate a notification output for object absence when there is no deficiency. 
     A detection area by one unit of the optical barrier apparatus of this embodiment is of approximate triangular shape with the scan mirror  12  (scan mirror  22 ) on the first unit  10  (second unit  20 ) side and opposite end reflectors  23 - 1  and  23 - n  (reflectors  13 - 1  and  13 - n ) on the second unit  20  (first unit  10 ) as apexes. Furthermore, with the optical barrier apparatus of this embodiment, the scan mirror  12  and the light receiving element  14  of the first unit  10 , and the scan mirror  22  and the light receiving element  24  of the second unit  20  are arranged diagonally as shown in FIG. 1 on either side of the detection area  1 . Therefore with the optical barrier apparatus of this embodiment, an approximate rectangular shape detection area  1  can be obtained as in FIG.  1 . 
     Here, the optical beam to be used may be of narrow directionality, for example, a laser beam is desirable. However, an optical beam where light rays generated using a LED or the like as a light emission element are given narrow directionality with a lens optical system or the like may also be used. 
     Referring to FIG. 2, the configuration of the first unit  10  will be described in detail. 
     In FIG. 2, the optical beam generating circuit  11  generates an optical beam from a light emitting element  11 B by means of a light emitting element drive circuit  11 A. By making the optical beam as a high frequency pulse emission, the influence of disturbance light such as sunshine can be effectively suppressed. For example, the light emitting element drive circuit  11 A is made an oscillating circuit which uses for example an astable multivibrator, to blink the light emitting element  11 B with an oscillation output from the astable multivibrator. 
     The scan mirror  12  reflects the optical beam incident from the light emitting element  11 B to scan the same in a range of the predetermined spread angle. The scan mirror  12  is rotated at a predetermined period about a rotation axis  12   a  as shown by the arrow in the figure by means of a scan mirror drive circuit  16 , so that the scanning beam BM 1  reaches to each reflector  23 - 1  to  23 - n  of the other unit  20 . The scan mirror  12  and the scan mirror drive circuit  16  constitutes optical beam scanning means. 
     The signal deficiency detection circuit  15  comprises a light reception circuit  17 , a comparator  18 , and a pulse deficiency detection circuit  19 . The light reception circuit  17  amplifies the light reception signal from the light receiving element  14 . The comparator  18  comprises a rectifying circuit  18 A for performing an envelope detection on an output signal Sa from the light reception circuit  17 , and a threshold value computing circuit  18 B for computing a threshold value for a rectified output S 1  from the rectifying circuit  18 A, and when the level of the rectified output S 1  of the signal Sa is equal to or above a threshold value Vt 1 , outputs Sb=1 (logic value 1), while when the level of the rectified output S 1  is less than the threshold value Vt 1 , outputs Sb=0 (logic value 0). The pulse deficiency detection circuit  19  comprises an off-delay circuit  19 A having a capacitor C, a diode D, and a threshold value computing circuit  19   a  having a threshold value Vt 2 , for delaying the falling of the signal Sb by a predetermined off-delay time Tof, and an on-delay circuit  19 B for delaying the rising of the output S 2  from the off-delay circuit  19 A by a predetermined on-delay time Ton. 
     The second unit  20  also has the same construction as that of the first unit  10 . 
     Next is a description of an object detection operation of the present embodiment, based on a time chart of FIG.  3 . 
     A high frequency optical beam is generated from the light emitting element  11 B by the oscillating output from the light emitting drive circuit  11 A. The optical beam is reflected by the scan mirror  12  so as to cross the detection area  1 , and is incident on the second unit  20  as the scanning beam BM 1 . The scan mirror  12  is rotated at the predetermined period by the scan mirror drive circuit  16 , so that the light projection direction of the scanning beam BM 1  for scanning the detection area  1  changes by every moment, to thereby scan the detection area  1  with the scanning beam BM 1 . The scanning beam BM 1  passes through the detection area  1  if an object  30  (shown in FIG. 2) is not present, and is successively incident on the reflectors  23 - 1  to  23 - n  of the second unit  20 , and is then reflected so as to turn back at approximately 180 degrees (for convenience of explanation, in the figure, the scan mirror  12  and the light receiving element  14  are shown apart from each other with an angle between the scanning beam BM 1  and scanning beam BM 1 ′), and the scanning beam BM 1 ′ having passed through approximately the same path as the scanning beam BM 1  is received by the light receiving element  14 . 
     A light reception output from the light receiving element  14  is amplified by the light reception circuit  17  and input to the comparator  18  as the signal Sa. The signal Sa input to the comparator  18  is envelope detected by the rectifying circuit  18 A and input to the threshold value computing circuit  18 B as the signal S 1 . The threshold value computing circuit  18 B compares the level of the signal S 1  with the input threshold value Vt 1 , and if the signal S 1  is equal to or above Vt 1 , generates Sb=1 (logic value 1), while if the signal S 1  is a lower level than Vt 1 , generates Sb=0 (logic value 0). The output signal Sb from the comparator  18  is input to the off-delay circuit  19 A inside the pulse deficiency detection circuit  19 . The off-delay circuit  19 A outputs a signal S 2 =1 coping with the rising (0→1) of the signal Sb, but continues the signal S 2 =1 for an off-delay time Tof without coping with the falling (1→0) of the signal Sb. Since the off-delay time Tof is set to be longer than the period where there is no light reception of the reflection beam BM 1 ′ produced at normal times, then if at normal times the object  30  is not present, then as shown in FIG. 3, the signal S 2 =1 is continued. If this continuous time is equal to or more than the on-delay time Ton of the on-delay circuit  19 B, then Z 1 =1 is generated from the on-delay circuit  19 B to notify of the absence of object  30 . 
     As shown in FIG. 2, in the case where the object  30  is present in the detection area  1 , since the scanning beam BM 1  is blocked by the object  30 , the reflection beam BM 1 ′ from the reflector positioned in the shadow of the object  30  does not appear. In this case, even if the light irregularly reflected by the object  30  is received by the light receiving element  14 , the light reception level thereof is small. Consequently, the level of the signal S 1  is a lower level than the threshold value Vt 1  of the threshold value computing circuit  18 B, so that the output from the comparator  18  becomes Sb=0. If this Sb=0 condition is continued for the off-delay time T of or more, signal S 2 =0 results, and the output from the on-delay circuit  19 B becomes Z 1 =0, thus notifying of the presence of object  30 . 
     Since the on-delay time Ton of the on-delay circuit  19 B is set to be longer than one scanning period of the scanning beam BM 1 , Z 1 =0 generated once from the signal deficiency detection circuit  15  is held thereafter for at least more than one scanning period of the scanning beam BM 1 , and Z 1 =0 is continued provided S 2 =0 is continued in the next and subsequent scanning periods. 
     Also in the second unit  20 , an operation similar to the above is performed. The notification output from the first unit  10  is made Z 1 , and a notification output from the second unit  20  is made  79 , and both outputs Z 1  and Z 2  from the two units  10  and  20  are input to logical product computation circuits, and the logical product computation results becomes the final notification output Z for the optical barrier apparatus. As a result, the first unit  10  monitors the lower side triangular area of the detection area  1  of FIG. 1, while the second unit  20  monitors the upper side triangular area of the detection area  1 , so that in total, a rectangular shape detection area  1  can be monitored. 
     With the abovementioned embodiment, the construction is such that the respective reflectors  13 - 1  to  13 - n , and  23 - 1  to  23 - n  are arranged with a gap provided. Therefore, the reflection beam BM 1 ′ becomes intermittent so that the output signal Sb from the comparator  18  becomes a pulse signal. In the case where the reflectors  13 - 1  to  13 - n  and  23 - 1  to  23 - n  are arranged in succession without a gap, the signal Sb is not a pulse signal but becomes a continuous signal. In this case also, the presence/absence of object can be monitored by adopting the signal deficiency detection circuit of FIG.  2 . Moreover, also if DC light is used for the optical beam rather than the high frequency pulse, only the rectifying circuit  18 A of the comparator  18  becomes unnecessary, while the rest can be applied as is. 
     With the optical barrier apparatus of this construction, only one light receiving element need be provided for each unit  10  and  20 , so that the number of light receiving elements and light receiving circuits can be significantly reduced, enabling a reduction in cost. Furthermore, the area between the units  10  and  20  can be monitored as a rectangular shape. 
     In the case where each reflector  13 - 1  to  13 - n  and  23 - 1  to  23 - n  is a flat plate, if the spacing between the units  10  and  20  (that is the spacing between the scan mirrors  12 ,  22  and the reflector array  23 ,  13 ) is changed, then a deviation such as in the end point positions of the reflection beams occurs. Therefore in the case where the distance between the units  10  and  20  is changed, then each time, angle adjustment of each reflector  23 - 1  to  23 - n  and  13 - 1  to  13 - n  is necessary. However, if for the reflectors of each reflector array  13  and  23  a retroreflector as shown in FIG. 4 is used, this has the advantage that the scanning beam can be reflected back at 180 degrees, so that even if the spacing between the units  10  and  20  is changed, angle adjustment of the reflectors becomes unnecessary. 
     Incidentally, in the case of the optical barrier apparatus of the present invention where, as shown in FIG. 1, two units  10  and  20  are combined, the scanning range of the scanning beams is previously adjusted so that the scanning beam from one unit is not received by the light receiving element of the other unit. However, such a situation may arise where due to a change in the environment or the like, another scanning beam is received rather than the beam which should be received. For example, there is the case such as where the scanning beam BM 2  from the scan mirror  22  of the second unit  20  is erroneously directly received by the light receiving element  14  of the first unit  10 . 
     In such a case the following problem arises. 
     That is to say, there is the object  30  as shown in FIG.  5 A. In this case, as mentioned before, the scanning beam BM 1  on the first unit  10  side is blocked, so that the reflection beam from the reflector positioned in the shadow of the object  30  is not received by the light receiving element  14 , and as shown by the dotted line in FIG. 5B, a pulse deficiency occurs in the light reception output for the scanning beam BM 1 . If at a timing where this pulse deficiency is covered up, the scanning beam BM 2  from the scan mirror  22  on the second unit  20  side is directly received by the light receiving element  14  so that a light reception pulse is generated, the actual light reception output from the light receiving element  14 , as shown in FIG. 5B becomes a signal without a pulse deficiency, similar to that at normal times. As a result, although the object  30  is present, absence of object  30  is notified. 
     Second to sixth embodiments for solving this problem are shown hereunder. 
     The second embodiment shown in FIG. 6 is constructed such that the scan mirror drive circuits  16  and  26  of the first and second units  10  and  20  are synchronized by a synchronous signal from a synchronous signal generating circuit  40  being synchronous drive means. That is, as shown in FIG. 7, the construction is such that the rotating operations of the scan mirrors  12  and  22  are synchronized with each other so that when the direction of the scanning beam BM 1  on the first unit  10  side is a diagonal direction, the direction of the scanning beam BM 2  on the second unit  20  side is also a diagonal direction. In FIG. 6, other construction is the same as for FIG.  2  and diagrams thereof are omitted. 
     The operation of the second embodiment will now be described. 
     When the scanning beam BM 1  on the first unit  10  side is directed towards the reflector  23 - 1 , the scanning beam BM 2  on the second unit  20  side is directed towards the reflector  13 - n . At this time, if the object  30  is not present on the optical axis, there is a possibility that the scanning beam BM 2  is received by the light receiving element  14  on the first unit  10  side, and the scanning beam BM 1  is received by the light receiving element  24  on the second unit  20  side. However, reception of the scanning beam due to object absence is no problem from the point of safety. 
     On the other hand, if as shown by the shaded portion in the figure, the object  30  is present on the optical axis, the scanning beams BM 1  and BM 2  are both blocked by the object  30  so that these do not reach the light receiving elements  24  and  14 . Moreover, due to the presence of object  30 , the scanning beams BM 1  and BM 2  do not both reach the reflectors  23 - 1  and  13 - n . Therefore there is no light reception of the reflection beam. Consequently, neither of the light receiving elements  14  and  24  generates a light reception output, and the presence of object  30  is thus detected. Therefore, the presence of object  30  can be detected without error. 
     FIG. 8 shows the third embodiment of the present invention being a different construction example. 
     This embodiment is of a construction where the first and second units  10  and  20  are operated alternately by time sharing so as not to perform mutually duplicate object detection operations. In this way, while in one unit the scanning beam is being generated to perform object detection, the scanning beam of the other unit is not generated. Therefore the erroneous light reception described in FIG. 5 does not arise. 
     In FIG. 8, with the present embodiment, the construction is added with a selection circuit  50  serving as selection drive means, for generating selection signals E 1  and E 2  for selecting the unit to be driven, and a signal processing circuit  51  serving as signal selection means, for processing the light reception output. 
     The selection circuit  50  generates the selection signals E 1  and E 2  complementary to each other which do not simultaneously become logic value 1, and respectively supplies the selection signal E 1  to the light emitting element drive circuit  11 A of the optical beam generating circuit  11  of the first unit  10 , and supplies the selection signal E 2  to the light emitting element drive circuit  21 A of the optical beam generating circuit  21  of the second unit  20 . 
     The signal processing circuit  51  is constructed to comprise a first logical product computing circuit  52  for computing a logical product of the output from the signal deficiency detection circuit  15  of the first unit  10  and the selection signal E 1 , a second logical product computing circuit  53  for computing a logical product of the output from the signal deficiency detection circuit  25  of the second unit  20  and the selection signal E 2 , and a logical sum computing circuit  54  for computing a logical sum of the outputs from both logical product computing circuits  52  and  53 , so that the output from the logical sum computing circuit  54  is an object detection output Z. 
     The operation will now be described. 
     When the selection signal E 1  of the selection circuit  50  is logic value 1, an optical beam is generated from the light emitting element  11 B of the first unit  10 . At this time, the selection signal E 2  is logic value 0, and the detection operation of the second unit  20  is stopped. On the other hand, when the selection signal E 2  is logic value 1, an optical beam is generated from the light emitting element  21 B of the second unit  20 . At this time, the selection signal E 1  is logic value 0, and the detection operation of the first unit  10  is stopped. Since the selection signals E 1  and E 2  do not simultaneously become logic value 1, the scanning beams BM 1  and BM 2  are not generated simultaneously. 
     The signal processing circuit  51 , when the selection signal E 1  is logic value 1, that is, only when the scanning beam BM 1  is being generated, transmits the output Z 1  from the signal deficiency detection circuit  15  to the logical sum computing circuit  54  as the output from the logical product computing circuit  52 , to make the output Z 1  effective. Moreover, when the selection signal E 2  is logic value 1, that is, only when the scanning beam BM 2  is being generated, the signal processing circuit  51  transmits the output Z 2  from the signal deficiency detection circuit  25  to the logical sum computing circuit  54  as the output from the logical product computing circuit  53 , to make the output Z 2  effective. 
     With such a construction, even if the scanning beam BM 1  on the first unit  10  side is directly received by the light receiving element  24  on the second unit  20  side, or even if the scanning beam BM 2  on the second unit  20  side is directly received by the light receiving element  14  on the first unit  10  side, the outputs from the logical product computing circuits  52  and  53  do not become logic value 1 so that when an object is present, an erroneous output of object absence due to erroneous reception of a scanning beam on the other unit side is not generated. 
     If the scanning beams BM 1  and BM 2  are processed so as not to be received by the light receiving elements  24  and  14  on the other unit sides, respectively, then in the period where the scanning beam BM 1  being generated, the output from the signal deficiency detection circuit  25  becomes Z 2 =0, while in the period where the scanning beam BM 2  is being generated, the output from the signal deficiency detection circuit  15  becomes Z 1 =0. Therefore, in the signal processing circuit  51 , the two logical product computing circuits  52  and  53  may be omitted, the logical sum computing circuit  54  only being sufficient, so that there is also no need to supply the selection signals E 1  and E 2 . 
     FIG.  9 A and FIG. 9B show the fourth embodiment of the present invention being yet another construction example. 
     The fourth embodiment of the present invention shown in FIG. 9 is of a construction where the light emission frequency of the optical beam generated by the light emitting element  11 B on the first unit  10  side is different from the light emission frequency of the optical beam generated by the light emitting element  21 B on the second unit  20  side. 
     As shown in FIG. 9A, with this embodiment, the light emission frequency of the optical beam from the light emitting element  11 B is made f 1 , and the light emission frequency of the optical beam from the light emitting element  21 B is made f 2  (f 1 ≠f 2 ). 
     FIG. 9B shows the construction of the signal deficiency detection circuit of this embodiment. 
     In the figure, a light reception circuit  17 ′ of the signal deficiency detection circuit  15  of the first unit  10  has an amplifying circuit  61 , and a band pass filter  62  of a central frequency fi serving as signal extraction means for extracting only scanning beam signal components of its own unit. The signal deficiency detection circuit  25  of the second unit  20 , with the exception that the central frequency of the band pass filter inside the light receiving circuit is f 2 , is the same as that of the first unit  10  side, and hence this is omitted from the figure. Other construction is the same as for the first embodiment shown in FIG.  2 . 
     The operation of the first unit  10  side will now be described with reference to a time chart of FIG.  10 . 
     When the light receiving element  14  receives the reflection beam, the light reception circuit  17 ′ amplifies the output signal from the light receiving element  14  in the amplifying circuit  61 , and generates a light reception signal S 3 . The signal S 3  is input to the band pass filter  62  to be frequency detected. The band pass filter  62  outputs the optical beam of light emission frequency f 1  to be received by the light receiving element  14  with practically no attenuation. In this case, the level of the input signal S 3  to the band pass filter  62  and the output signal Sa is Sa≈S 3 . The signal Sa is input to the comparator  18  to be level detected similarly to the case of the first embodiment of FIG.  2 . If the rectified output S 1  level of the signal Sa is equal to or above the threshold value Vt 1 , Sb=1 is generated, while if the signal S 1  level is a lower level than Vt 1 , Sb=0 is generated. 
     The output signal Sb from the comparator  18  is input to the off-delay circuit  19 A inside the pulse deficiency detection circuit  19 , and if the reflection beam BM 1 ′ of frequency f 1  is normally received, the signal S 2 =1 is continued, and when the continuation time becomes the on-delay time Ton of the on-delay circuit  19 B or more, Z 1 =1 is generated from the on-delay circuit  19 B to thus notify of the absence of object. If there is an object inside the detection area  1 , a pulse deficiency as shown by the dotted line in FIG. 10 is produced, and if the signal Sb=0 is continued for the off-delay time Tof or more, S 2 =0 is generated and the output Z 1  from the on-delay circuit  19 B becomes Z 1 =0, thus notifying of the presence of object. 
     On the other hand, even if the optical beam BM 2  of frequency f 2  on the second unit  20  side, which is not to be received by the light receiving element  14 , is received by the light receiving element  14  so that a signal S 3  is generated, this signal S 3  is frequency detected by the band pass filter  62  and removed. That is, the attenuation characteristic of the band pass filter  62  is set so that even if the signal S 3  becomes a maximum level due to the light reception of the optical beam BM 2  of emission frequency f 2 , the signal S 1  level as shown in the figure becomes a level lower than the threshold value Vt 1 . As a result, the signal Sb=1 is not generated, so that the problem due to erroneous light reception of the scanning beam from the other unit can be avoided. 
     Furthermore, as with the fifth embodiment of the present invention shown in FIG. 11A, the construction may be such that the pitches L 1  and L 2  of the reflectors  13 - 1  to  13 - n  and  23 - 1  to  23 - m  in the reflector array  13  of the first unit  10  and the reflector array  23  of the second unit  20  is made different. In this case, as shown in FIG. 11B, the frequencies of the envelope detection signals of the respective light reception signals of the light receiving elements  14  and  24  in one scanning period, that is, the blinking frequencies of the reflection beams, become different. The signal deficiency detection circuit of this embodiment is the same as the circuit of FIG.  9 B. However, the central frequencies f 1  and f 2  of the band pass filters  62  become the blinking frequencies of the reflection beams. 
     Moreover, as with the sixth embodiment of FIG. 12, also if the construction is such that masks  13   a  and  23   a  with widths L 1  and L 2  are attached at predetermined spacing on single plate reflectors  13   a  and  23   a , so that substantially a plurality of reflecting sections (in the figure shown as reflectors  13 - 1  to  13 - n  and  23 - 1  to  23 - m ) are provided to make the reflector arrays  13  and  23 , the operation and effect similar to those of FIG. 11 can be obtained. 
     Here, if reflectors (or reflecting sections) of the two units  10  and  20  are arranged in the same number, and the scanning speeds of the scan mirrors  12  and  22  are made different, the blinking frequency of the light reception output in the one scanning period of the two units  10  and  20  becomes different. Hence the operation and effect similar to those for the cases of FIG.  11  and FIG. 12 can be obtained. In the cases of FIG.  11  and FIG. 12, also if DC light is used in the scan beams BM 1  and BM 2 , the influence from disturbance light such as from sunshine can be suppressed. 
     According to the constructions of the fifth and sixth embodiments in FIG.  11  and FIG. 12, an error as described later where the light reception strength of the irregularly reflected light from the object is large as if this is regarded just as reception light from the reflector (or reflecting section), can be avoided. 
     That is, in the fifth and sixth embodiments, the reflection beams from the reflector arrays  13  and  23  become blinking lights at predetermined frequencies f 1  and f 2  in the respective one scanning periods. On the other hand, when irregularly reflected light is received from the object, this type of blinking light is not generated. Consequently, light from the reflector array and irregularly reflected light can be discriminated, and if irregularly reflected light is present, the output from the signal deficiency detection circuit becomes logic value 0. 
     As a method for discriminating between the scanning beams BM 1  and BM 2 , the wavelength of the scanning beams BM 1  and BM 2  may be made different. In this case, wavelength filters for passing only scanning beams BM 1  and BM 2  of respective wavelengths which should actually be received, and strength attenuating or blocking off scanning beams BM 2  and BM 1  of wavelengths which should not to be received, may be provided on the light receiving surfaces of the respective light receiving elements  14  and  24  of the units  10  and  20 . In this way, erroneous light reception of scanning beams which should not to be received can be prevented. In the signal deficiency detection circuits  15  and  25  on the light receiving side, the construction of FIG. 2 may be used. 
     By devising a geometrical arrangement of detection area of approximately triangular shape in each unit, the problem of erroneous reception of scanning beam from the other unit can be resolved. In general, as shown in FIG.  13 A and FIG. 13B, the respective scan mirrors and light receiving elements of the units  10  and  20  may be arranged so that the face of the triangular detection area  1 A of the first unit  10 , and the face of the triangular detection area  1 B of the second unit  20  are not in the same plane. By arranging in this manner, direct reception of the scanning beam from the other unit can be prevented. FIG. 13A is a plan view and FIG. 13B is a front view. 
     Incidentally, in the case of an object with good reflectance (for example a mirror or the like), it is likely that, depending on the position of the object inside the detection area  1 , the scanning beam is reflected back at 180 degrees by the object without scattering so that light of sufficient strength is received by the light receiving element. Furthermore, in the case where the object is present near the scan mirror and the light receiving element, then even if the scanning beam is scattered and reflected by the object, it is possible for this scattered light to be received by the light receiving element at a strength of the degree to erroneously show object absence. 
     An embodiment to resolve this problem is shown below. 
     In a seventh embodiment shown in FIG. 14, the construction is such that there is provided a function for verifying that a reflection beam received by a light receiving element is one which is reflected by a reflector. The basic construction of FIG. 14 is known for example from Japanese Unexamined Patent Publication No. 10-38194. 
     In FIG. 14, with the present embodiment, at least one of the reflector arrays  13  is a specific reflector  13 -P (P=1 to n), and the reflector  13 -P has one end thereof rotatably supported by a pivot  71 . An electrostrictive element  72  is attached to the other end as modulation means. The electrostrictive element  72  is AC driven at a frequency f 3  by a drive circuit (not shown in the figure). On the other hand, in the signal deficiency detection circuit  15 , to the construction of FIG. 2 there is newly added an envelope detection circuit  73 , a pulse deficiency detection circuit  74 , and a logical product computing circuit  75 . The envelope detection circuit  73  is a rectifying circuit which envelope detects the output Sb from the comparator  18  and outputs Sc=1 only when the input signal frequency is a high frequency signal equal to or above f 3 . The pulse deficiency detection circuit  74  incorporates an off-delay circuit and an on-delay circuit, and detects a pulse deficiency of the output Sc from the envelope detection circuit  73 . However, the off-delay time Tof 1  of the off-delay circuit of the pulse deficiency detection circuit  74  is longer than this period for the output Sc=0 from the envelope detection circuit  73  which is normally generated and shorter than two periods for where Sc=1 is generated. Furthermore, the on-delay time of the on-delay circuit is set to be at least longer than the Sc=1 generating period. The second unit  20  side is also of the same construction. 
     Hereunder is a description of the operation with reference to a time chart of FIG.  15 . 
     At the time of a monitoring operation, the reflector  13 -P is AC driven by the electrostrictive element  72  at the frequency f 3 . When a voltage is not applied, the reflector  13 -P becomes the condition shown by the solid line in FIG.  14 . In this condition, the scanning beam BM 1  is reflected in the direction of the light receiving element  14  and received thereby. When a voltage is applied, the reflector  13 -P becomes the condition shown by the dotted line in FIG.  14 . In this condition, the scanning beam BM 1  is not reflected in the direction of the light receiving element  14  and is thus not received. Consequently, if the drive frequency of the electrostrictive element  72  is f 3 , the light reception output from the light receiving element  14  based on reception of the reflection beam from the reflector  13 -P, repeats alternately at the frequency f 3  between reception and non reception. As a result, regarding the output signal Sa from the light reception circuit  17  based on reception of the reflection beam from the reflector  13 -P, as shown in FIG. 15, the signal of frequency f 1  becomes a waveform with the amplitude modulated at frequency f 3 . This gives the relationship (reception period of reflection beam from reflector  13 -P)&gt;1/f 3 &gt;1/f 1 . 
     Since the scanning beam BM 1  is scanned at a predetermined period, then at normal times, the modulating signal due to the reflector  13 -P also is periodically generated as shown in FIG.  15 . Of the output signals Sb from the comparator  18 , the signal of frequency f 3  is detected by the envelope detection circuit  73  and output as Sc=1. If Sc=1 is periodically generated, the pulse deficiency detection circuit  74  continues to generate an output Y 1 =1. Moreover, in the case where the reflection beam from the reflector  13 -P is not received, the signal of frequency f 3  is not generated in the signal Sb. Therefore, the output from the envelope detection circuit  73  becomes Sc=0, and a pulse deficiency is produced as shown by the dotted line in FIG.  15 . As a result, the output from the pulse deficiency detection circuit  74  becomes Y 1 =0. This signal Y 1  and the output Z 1  from the pulse deficiency detection circuit  19  are subjected to logical product computation by the logical product computing circuit  75 , and the resultant output ZY becomes a signal indicating the presence/absence of object on the first unit  10  side. 
     Also in the case where the abovementioned modulation means such as an electrostrictive element is attached to a plurality of reflectors of the reflector array, the circuit configuration of FIG. 14 can be used. Moreover, if the construction is such that the modulation means is attached to all of the reflectors to modulate the scanning beam at the same frequency f 3 , then even when the optical beam is direct current light, this becomes alternating current light of frequency f 3  when received. Therefore, it is not necessary to make the emission frequency f 1 . Furthermore, if the modulation means is attached to all the reflectors on the second unit  20  side so that the scanning beam BM 1  is modulated at frequency f 3 , and similarly on the first unit  10  side so that the scanning beam BM 2  is modulated at a frequency different from f 3 , then as with the fourth embodiment of FIG. 9, the problem of erroneous light reception of scanning beam by another unit side can also be resolved. In this case, the circuit of FIG. 9B may be used in the reception circuit of the signal deficiency detection circuit. 
     Next an eighth embodiment of the present invention being another construction example is shown in FIG.  16  and will be described. This embodiment is applied to the case where the optical beam reflecting means is constructed at a plurality of divided reflecting areas, that is, where a reflecting portion and a non reflecting portion are multiply arranged alternately. For example, this embodiment is applied to the case where this is constructed with a plurality of reflectors, or the case where a plurality of masks as shown in FIG.  11  and FIG. 12 are provided. Here, the description is for the case where the reflector array comprises a plurality of reflectors. 
     In FIG. 16, there is provided a counting circuit  90  input with the output Sb from the comparator  18  of the signal deficiency detection circuit  15  of FIG. 2, and the drive signal SD 1  from the scan mirror drive circuit  16 , for counting the number of generations of the signal Sb per one scanning period of the scan mirror  12 . The construction is such that an output Zc from the counting circuit  90  and the output Z 1  from the signal deficiency detection circuit  15  (the output from the pulse deficiency detection circuit  19 ) are computed by a logical product computation circuit  91 , and an output from the logical product computation circuit  91  becomes a detection signal Z 1  for object presence/absence. 
     In the case where as shown in FIG. 1, the reflector arrays  13  and  20  comprise the plurality of divided reflectors  13 - 1  to  13 - n  and  23 - 1  to  23 - n , the number of pulses of the reflection beams received in one scanning period of the scanning beam is equal to the number of reflectors of the reflector arrays  13  and  23 . If an object is present, at least one or more reflection beams is not received, and hence the number of pulses of reflection beams is decreased. Furthermore, in order to prevent the error where due to the large light reception strength of the irregularly reflected light from the object, it is considered that there is reception light, if the spacing of adjacent reflectors is set narrow so that the irregularly reflected light from the smallest object to be detected is continued for the amount of the reflected light from two or more reflectors, the number of pulses of reflection beams is decreased. 
     Regarding the operation, the counting circuit  90  counts the number of signals Sb input per one scanning period of the scan mirror  12  based on the signal SD 1 , and compares this count value with a set value previously set corresponding to the number of reflectors. If the count value coincides with the set value, the counting circuit  90  outputs Zc=1. If the count value does not coincide with the set value, the counting circuit  90  outputs Zc=0. The verification signal Zc of this pulse number and the output Z 1  from the pulse deficiency detection circuit  19  are input to the logical product computation circuit  91 , and an output Z of the computation result thereof becomes a detection signal for object presence/absence. 
     The counting circuit used in this embodiment has a function the same as the pulse deficiency detection circuit  19 , since this counts the pulse number per one scanning period of the scan mirror. Therefore, the pulse deficiency detection circuit  19  in each of the aforementioned embodiments may be replaced by the counting circuit  90 . 
     Furthermore, the method for detecting the frequency of the pulse signal generated as signal Sb may also be adopted. As also mentioned in the description for the aforementioned fifth and sixth embodiments, the reflected pulse light of frequency f 1  to be received due to an object absence, if an object is present, does not occur while the object is being scanned by the scanning beam. That is, due to an object presence, the frequency of the reflected light becomes outside the frequency f 1 . Hence by performing frequency detection of the reception light signal, the presence/absence of object can be detected. The blinking frequency of the reflected light for the units  10  and  20  may be the same, if for example problems due to receiving scanning beam light from other units are not considered. 
     Regarding the frequency test, for example, the construction may be such that, in FIG. 16, a band pass filter (central frequency f 1 ) is provided and the signal Sb input thereto, and the output therefrom input to a separately provided comparator and subjected to threshold value computation, and the output then input to the pulse deficiency detection circuit  19  (in this case, the counting circuit may naturally be omitted). 
     If the frequency of the signal Sb becomes outside f 1  due to an object presence, then as in FIG. 10, the filter output level drops to become lower than the threshold value of the comparator so that a pulse deficiency occurs and the object presence can be detected. Alternatively, using the circuit of FIG. 9B with the emitted beam as direct current light, the frequency of the reflection beam may be detected. 
     FIG.  17 A and FIG. 17B show a ninth embodiment of the present invention being yet another configuration example. 
     In FIG. 17, with this embodiment, as shown in FIG. 17A, the construction is such that reflectors Tp 1  and Tp 2  are separately provided so that when an optical beam other than that of the own unit, for example the scanning beam BM 2  of the second unit  20  side, is projected in a predetermined direction, this optical beam BM 2  is reflected so as to be received by the light receiving element  14  on the first unit  10  side. To this end, as shown in FIG. 17B, the construction is such that on the light reception side, in addition to the signal deficiency detection circuit  15  of the construction of FIG. 9B, there is provided an other beam reception verification circuit  80  for verifying that the scanning beam BM 2  of frequency f 2  is being received at a predetermined period by the light receiving element  14 . 
     The other beam reception verification circuit  80  comprises a band pass filter  81  with a central frequency f 2 , a comparator  82 , a timing signal generating circuit  83  for outputting a timing signal TM with input of a scan mirror drive signal SD 2  of the second unit  20  side, a logical product computing circuit  84 , and a pulse deficiency detecting circuit  85 , and when the scanning beam BM 2  is normally received by the light receiving element  14  at a predetermined timing, generates an output V 1 =1. 
     The operation of the ninth embodiment will be described with reference to a timing chart of FIG.  18 . 
     When a light reception output is generated from the light receiving element  14  on reception of the reflection beam, the light reception signal S 1  amplified by the amplifying circuit  61  is respectively input to the band pass filters  62  and  81 . As described for the fourth embodiment of the FIG. 9, signals outside the frequency f 1  are attenuated by the band pass filter  62  and output to the comparator  18  as the signal Sa, and level detected by the pulse deficiency detection circuit  19 . On the other hand, the band pass filter  81  of the other beam reception verification circuit  80  attenuates the signals outside the frequency f 2  and outputs to the comparator  82  as a signal Sa′. With this embodiment, the light receiving element  14  and the amplifying circuit  61  are not saturated even if the two scanning beams BM 1  and BM 2  are simultaneously received by the light receiving element  14 . 
     The timing signal generating circuit  83  of the other beam reception verification circuit  80  outputs a timing signal TM with input of a drive signal SD 2  indicating that the scan mirror  22  on the second unit  20  side has been driven to a scanning position which becomes the path of the reflectors Tp 1  and Tp 2  and the light receiving element  14  shown in FIG.  17 A. If, at this timing, the scanning beam BM 2  is normally reflected by the reflectors Tp 1  and Tp 2  and received by the light receiving element  14 , then as shown in FIG. 18, Sa′=1 is generated from the band pass filter  81 , Sb′=1 is generated from the comparator  82 , and Sf=1 is generated from the logical product computing circuit  84 . If the timing signal TM=1 is periodically produced, and at this time Sb′=1 is generated, then Sf=1 is periodically generated from the logical product computing circuit  84 . The pulse deficiency detecting circuit  85 , when Sf=1 is generated at a predetermined period, generates V 1 =1. Here, the off-delay time Tof 2  of the off-delay circuit of the pulse deficiency detecting circuit  85  is longer than the period for the output Sf=0 generated from the logical product computing circuit  84  when normal time and shorter than two periods for Sf=1 generation. Furthermore, the on-delay time of the on-delay circuit is set to be at least longer than the Sf=1 generation period. 
     On the other hand, if the scanning beam BM 2  is not received at the generating time of the timing signal TM=1 in which the scanning beam BM 2  should be received as shown by the dotted line in FIG. 18, Sf=1 is not generated from the logical product computing circuit  84  so that a deficiency occurs in the pulse, and the output from the pulse deficiency detecting circuit  85  becomes V 1 =0. This signal V 1  and the output Z 1  from the signal deficiency detection circuit  15  are subjected to logical product computation by the logical product computing circuit  86 , and the resultant output ZV is made a signal indicating the presence/absence of object on the first unit  10  side. 
     With such a construction, if an object is present on an optical path so that the scanning beam BM 2  is blocked, V 1 =0 results, and the output from the logical product computing circuit  86  becomes ZV=0 indicating the presence of object. Furthermore, with a configuration provided with such an optical path, if the positions of the two units  10  and  20  are displaced from the normal position, the scanning beam BM 2  is not incident exactly onto the light receiving element  14 , and hence is not received. Therefore, this is also applicable to alignment of the two units  10  and  20 . 
     In FIG. 17, the case for one optical path is shown, however, reflectors may be added so that a plurality of optical paths are provided. In this case, the timing signal TM=1 is generated for each timing at which the scanning beam BM 2  should be received. Hence TM=1 is generated several times during one scanning period of the scan mirror  22 . Furthermore, instead of using the scanning beam BM 2 , a dedicated light emitting element for generating an optical beam having a frequency different to those of the scanning beams BM 1  and BM 2  may be provided so as to form an optical path. Moreover, if the construction is such that light is continuously received, then the timing signal generating circuit  83  and the logical product computing circuit  84  become unnecessary. 
     The aforementioned ninth embodiment has been constructed using an optical beam of a frequency different to that of the scanning beam BM 1 . However, it is also possible to make the wavelengths of the optical beams different. In this case, since the light receiving elements cannot be shared, this may be processed with a construction where a separate light receiving element and a separate amplifying circuit are provided and the two beams are separately received. 
     Next is a description of a suitable embodiment for use of the optical barrier apparatus of the present invention as a safety ensuring facility for a machine. 
     In order to use the optical barrier apparatus of the present invention as a safety ensuring facility for a machine, it is desirable to verify that the scanning beams BM 1  and BM 2  are reliably scanning the detection area  1 . 
     FIG.  19  through FIG. 21 show a configuration example of a scanning verification section serving as scanning verification means for performing such verification. This scanning verification section verifies that a scanning mirror is scanning an optical beam within a range of a predetermined spread angle. 
     FIG. 19 shows a configuration example of the scanning verification section, FIG. 20 shows a block diagram of the scanning verification section, and FIG. 21 shows an operation time chart for the scanning verification section. The configuration of this scanning verification section is already known in PCT International Publication No. WO97/33186, and will be described briefly here. 
     In FIG. 19, reference numerals  101  and  102  denote a pair of scanning verification light receiving elements, arranged outside of the detection area, which receive optical beams  104  and  105  reflected to outside of the detection area by a scan mirror  103 . Reference numeral  106  denotes a scanning verification signal processing circuit being scanning verification signal deficiency detecting means, supplied with outputs from the scanning verification light receiving elements  101  and  102  to the input side thereof. The scanning verification signal processing circuit  106 , as shown in FIG. 20, is constructed of a circuit the same as the signal deficiency detection circuit  15  of FIG. 2, comprising light receiving circuits  106 A and  106 B, a comparator  106 C, and a pulse deficiency detection circuit  106 D, to detect an abnormality of the scanning by an optical beam, from a pulse deficiency in the signal from the light receiving elements  101  and  102 . 
     FIG. 21 shows an operation time chart. 
     In the case where there is no reflection of the optical beam by the scan mirror  103 , or the scan mirror  103  does not rotate, or a fault such as reduction of rotation angle of the scan mirror  103  occurs, the output from at least one of the light receiving elements  101  and  102  is lost, and as shown by the dotted line in the figure, a pulse deficiency occurs. Since an off-delay time Tof′ of then off-delay circuit inside the pulse deficiency detection circuit  106 D is set to be slightly longer than the generation period of the light reception output, then when a pulse deficiency occurs, the output from the pulse deficiency detection circuit  106 D changes from H=1 to H=0, and while the fault as described above is continued so that the pulse deficiency periodically occurs, H=0 is maintained, and the optical beam scanning abnormality is notified. In order to prevent automatic normal notification even when the abnormality is no longer detected after the abnormality has once occurred, then for example the signal H may be input to a flip-flop so that H=0 is stored. 
     In the case where such a scanning verification section is applied to the optical barrier apparatus of the present invention, the aforementioned scanning verification sections are respectively provided in each unit  10  and  20 , and as shown in FIG. 22, outputs H 1  and H 2  from each scanning verification section, and the signals Z 1  and Z 2  indicating object presence/absence of each unit  10  and  20  are respectively subjected to logical product computation by logical product computation circuits  111  and  112 , and each output for the computation results is subjected to logical product computation by logical product computation circuit  113 , and made the final output from the optical barrier apparatus. At this time, if the construction is such that the scanning verification light receiving elements  101  and  102  are respectively attached to opposite end portions of the reflector arrays  13  and  23  of the units  10  and  20 , this has the advantage that it is possible to verify that the optical beams are being normally scanned at the positions of the reflector arrays  13  and  23 . 
     For the scan mirrors used in the abovementioned respective embodiments, for example commercial galvano-mirrors may be used. Furthermore, if semiconductor galvano-mirrors are used, the scan mirror can be made small, and consequently miniaturization of the optical barrier apparatus can be achieved. 
     As a semiconductor galvano-mirror, there is, in addition to a later mentioned electromagnetic type galvano-mirror, an electrostatic galvano-mirror or a piezoelectric type galvano-mirror. 
     The electrostatic galvano-mirror is an element manufactured by a semiconductor element manufacturing process for moving a movable plate formed with a mirror by electrostatic force. This is disclosed for example in Japanese Unexamined Patent Publication No. 5-60993. Furthermore, the piezoelectric type galvano-mirror is for moving a movable plate formed with a mirror by piezoelectric resonance, and is disclosed for example in “Reprinted from Miniature and Micro-Optics; Fabrication and System Applications Volume 1554” of the SPIE-The International Society for Optical Engineering, published July 1991. 
     Here is a detailed description of a suitable electromagnetic type galvano-mirror serving as a scan mirror. The electromagnetic type galvano-mirror to be described here is known for example from Japanese Unexamined Patent Publication No. 8-220453 by the present applicant. 
     FIG. 23 is an exploded perspective view of an electromagnetic type semiconductor galvano-mirror. In order to facilitate understanding, this is shown in enlarged size. 
     In FIG. 23, on the inside of a silicon substrate mounted on an insulating substrate, there is provided a torsion bar integrally formed with the silicon substrate and a movable plate supported by the torsion bar. A planar coil is provided on the periphery of the movable plate, and a mirror is provided at the central portion of the movable plate. Permanent magnets are arranged on opposite side faces of the silicon substrate. The polarity of the permanent magnets is such that on one side face of the silicon substrate, the top is N and the bottom is S, while on the other side face, the bottom is N and the top is S. 
     Regarding the operation, when a current flows in the planar coil from electrode terminals, the current flows so as to cross the static magnetic field of the permanent magnets, so that a force acts on the opposite ends of the movable plate according to Fleming&#39;s left-hand rule, and the movable plates is rotated. When an AC current flows in the planar coil, the movable plate is rotated periodically, so that the optical beam incident on the mirror can be reflection scanned. The movable plate resonates at a constant frequency, indicating a peak in the amplitude. Consequently, since at the time of resonance a large displacement angle is obtained with a small input, it is desirable to use the galvano-mirror in the resonant condition. 
     With the respective embodiments of the optical barrier apparatus of the present invention described above, the detection area is monitored with a construction where the optical beams from the light emitting elements are reflected by the scan mirrors. However, the present invention is not limited to this, and for example the construction may be such that there is provided a scanning element of a semiconductor galvano-mirror type with a light emitting element mounted at the position of the scan mirror, and the light emitting element is rotated so that the detection area is scanned by an optical beam from the light emitting element. 
     In order to realize the optical barrier apparatus of the abovementioned respective embodiments with high safety, the signal deficiency detection circuit and the scanning verification section may be of a fail-safe construction. In the case where the threshold value computation circuit and the logical product computation circuit used in the respective circuits are configured so as to be fail-safe, then a fail-safe window-comparator/AND gate such as disclosed in U.S. Pat. No. 5,345,138, U.S. Pat. No. 4,661,880, and U.S. Pat. No. 5,027,114 can be used. These circuits and the operation and fail-safe characteristics have been illustrated in the article such as TRAN. IEE of Japan, Vol. 109-C, No. 9, September 1989 (A Method of Constructing an Interlock System using a Fail-Safe Logic Element having Window Characteristics, or “Application of Window Comparator to Majority Operation” Proc. of 19th International Symp. on Multiple-Valued Logic, IEEE Computer Society (May 1989). As the on-delay circuit, it is possible to use a fail-safe on-delay circuit known for example from PCT International Publication Numbers WO94/23303 and WO94/23496, Japanese Examined Patent Publication No. 1-23006 and Japanese Unexamined Patent Publication No. 9-162714. The fail-safe construction of the rectifying circuit and the amplifying circuit is known for example from PCT/JP93/00411. Furthermore, the construction of a fail-safe band pass filter where the attenuation amount does not drop at the time of a fault, is shown in the article such as Japanese Institute of Electrical Engineers Industrial Measurement Control Seminar documents, IIC-94-23, (94-7). By using these, the optical barrier apparatus may be constructed as a fail-safe safety apparatus which does not erroneously notify of the absence of object at the time of a fault. 
     INDUSTRIAL APPLICABILITY 
     The present invention enables a reduction in cost of an optical beam scanning type optical barrier apparatus for scanning a detection area with an optical beam to monitor for objects, without narrowing the detection area. Therefore industrial applicability is considerable.