Abstract:
A method of installing a multi-beam photoelectric safeguard system for making a light curtain of closely aligned light beams closely to a pressing machine first positions main light emitting and detecting devices ( 11, 12 ). The method next mounts a projecting portion ( 21 ) to the pressing machine and identifies optical axes interrupted by the projecting portion ( 21 ). Subsequently, after setting a sub light detecting device ( 13 ) adjacent to one side of the projecting portion ( 21 ), the method adjusts optical axes between the sub light detecting device ( 13 ) and the main light emitting device ( 11 ). Finally, after setting a sub light emitting device ( 14 ) adjacent to the other side of the projecting portion ( 21 ), the methods adjusts optical axes between the sub light emitting device ( 14 ) and the main light detecting device ( 12 ).

Description:
FIELD OF THE INVENTION 
     This invention relates to a multi-beam photoelectric safeguard system and, more particularly, to a method of installing a safeguard system including main light emitting and detecting devices and sub light emitting and detecting devices and a method of adjusting their optical axes. 
     BACKGROUND OF THE INVENTION 
     Multi-beam photoelectric safeguard systems, comprising a light emitting device including a plurality of aligned light emitting elements and a light detecting device including a plurality of corresponding photodetectors as one unit, are commonly employed to detect the intrusion of an optical obstacle in a wide detection area. Multi-beam photoelectric safeguard systems are typically used to make protective fences, i.e. light curtains, along boundaries of prohibited areas where machine tools, punching machines, pressing machines, casting machines, automatic controllers and the like are installed, so that, if a part of the body of an operator, for example, intrudes into such a prohibited area, the system detects the intrusion and immediately stops the machine and/or gives a warning signal. 
     Regarding relative placement between the light emitting device and the light detecting device of a multi-beam photoelectric safeguard system, in case a machinery  1  such as a press as shown in FIG. 1 includes a projecting portion  2  projecting toward the operator, one of solutions is to place the safeguard system  3  in a position beyond the proximal end of the projecting portion  2  where the safeguard system does not interfere the projection  2  at all. 
     This placement, however, increases the horizontal distance X 1  from the work center O of the machinery  1  to the safeguard system  3  (light curtain), hence increases the total area for installment of the press, for example, including the area for its safeguard system, and therefore decreases the working efficiency of the press. 
     In case the machinery  1  includes the projecting portion  2  that projects toward the operator, another solution is to place the safeguard system  3  as shown in FIGS. 2 and 3. In the conventional example shown here, the safeguard system  3  (light curtain) is positioned close to the machinery  1 , and rearranged beforehand to exclude from effective detection the zone  4  encountering the projecting portion  2 , i.e. the zone  4  where some of optical axes  5  forming the light curtain are optically blocked by the projecting portion  2 . That is, a blanking function, which excludes the zone  4  encountering the projection  2  as a non-detection area beforehand, permits the safeguard system  3  (light curtain) to be placed even at a position where it interferes the projecting portion  2 . 
     In this configuration, since the protective fence, i.e. light curtain, can be positioned closely to the machinery  1  (X 2 &lt;X 1 ) so as to keep a safety distance as small as possible with respect to the machinery  1 , the working efficiency can be improved. 
     However, this approach relying on invalidating some of the optical axes  5  in the zone  4  excludes the full extension of the zone  4  from detection, including a section or sections at one or both sides of the projecting portion, although there is equally the possibility that an optical obstacle intrudes into the prohibited are through that section. To compensate this defect, another safeguard measure has to be employed, such as, for example, covering each such section of the zone  4  with a physical fence  6  such as a metal plate or net as shown in FIG.  4 . 
     Japanese Patent Laid-Open Publication No. S63-43099 proposes a multi-beam photoelectric safeguard system contemplating the existence of a projecting portion as discussed above. The safeguard system disclosed in this publication is comprised of a pair of light emitting and detecting devices including a plurality of light emitting elements and complementary photodetectors, respectively, and a pair of reflection mirrors disposed adjacent to the projecting portion so that, in the zone encountering the projecting portion, a light curtain is made at one or opposite sides of the projecting portion by reflecting light beams from the light emitting and detecting devices at the reflection mirrors and receiving the reflected light beams at the same light emitting and detecting devices. 
     With the safeguard system taught by that publication, however, it is difficult to adjust the optical axes between the light emitting and detecting devices and the optical alignment of respective light emitting elements and photodetectors with associated reflection mirrors. Especially when the optical axes are arrayed closely, the difficulty becomes greater. Furthermore, since each of the light emitting and detecting devices has to include light emitting elements or photodetectors for emitting or detecting light beams to and from the reflection mirrors, the light emitting and detecting devices inevitably become bulky. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a method of installation and a method of adjusting optical axes of a multi-beam photoelectric safeguard system capable of positioning a light curtain made of closely arrayed optical axes very closely to a machinery or equipment such as a press, which requires the safeguard system. 
     A further object of the invention is to provide a method of installation and a method of adjusting optical axes of a multi-beam photoelectric safeguard system which is suitable for use with a machinery or equipment such as a press, which requires the safeguard system and includes a portion projecting toward the operator, and can make a light curtain closely to the press with no invalidated zone. 
     A still further object of the invention is to provide a method of installation of a multi-beam photoelectric safeguard system for making a light curtain without non-detection zones around an interfering object by using a main light emitting device and a sub light detecting device operable according to a basic operation sequence to sequentially emit light beams from the main light emitting device at predetermined timings, which can simultaneously generate a new operation sequence incorporating sub light emitting and detecting devices as well. 
     Those objects of the invention can be accomplished by various aspects of the invention. 
     According to an aspect of the invention, there is provided a method of installing a multi-beam photoelectric safeguard system for making a light curtain with a number of light beams around an interfering object, the multi-beam photoelectric safeguard system including: 
     a main light emitting device having a plurality of light emitting elements aligned in an array at equal intervals; 
     a main light detecting device disposed in an opposed relationship with the main light emitting device and having a plurality of photodetectors equal in number to the light emitting elements and arranged in an array at regular intervals; 
     a sub light detecting device disposed adjacent to one side of the interfering object interrupting a light beam of at least one optical axis of the light curtain, and including at least one photodetector capable of detecting a light beam from the main light emitting device; 
     a sub light emitting device disposed adjacent to the other side of the interfering object and capable of emitting a light beam toward the main light detecting device; and 
     the light curtain including a main detection area defined between the main light emitting device and the main light detecting device, a first sub detection area defined between the main light emitting device and the sub light detecting device, and a second sub detection area defined between the sub light emitting device and the main light detecting device, 
     the method comprising: 
     (a) positioning the main light emitting device and the main light detecting device relative to each other and identifying a blanking optical axis interrupted by the interfering object among the light beams between the main light emitting device and the main light detecting device; 
     (b) placing the sub light detecting device adjacent to one side of the interfering object and thereafter positioning same relative to the main light emitting device by moving the sub light detecting device; and 
     (c) placing the sub light emitting device adjacent to the other side of the interfering object and thereafter positioning same relative to the main light detecting device by moving the sub light emitting device. 
     In a preferred embodiment of the invention, relative positioning of the main light emitting and detecting devices and adjustment of their optical axes may be carried out either without any interfering object or under the existence of such object. 
     In an embodiment of the invention, an optical axis adjustment display or optical axis adjustment display lamp is typically provided on the main light emitting device and/or main light detecting device. The operator can confirm completion of relative positioning of the main light emitting and detecting devices and adjustment of their optical axes by watching the optical axis adjustment display. Similarly for sub light detecting and emitting devices, an optical axis adjustment display or display lamp is preferably provided on the sub light detecting device and or sub light emitting device. 
     In another preferred embodiment, a controller for substantially controlling light emitting and detecting devices of the safeguard system may be provided, and an optical axis adjustment display or display lamp may be provided on the controller such that adjustment of optical axes of all light emitting and detecting devices contained in the safeguard system can be confirmed totally on the optical axis adjustment display of the controller. 
     In another preferred embodiment, the main light emitting and detecting devices forms a basic unit of the safeguard system, and sub light emitting and detecting devices may be added as an optional unit if a user requests. The main light emitting and detecting devices as the basic unit operate according to a preset basic operation sequence. In the basic sequence, light emitting elements contained in the main light emitting device are sequentially activated at predetermined timings for a predetermined length of time, individually. 
     When the sub light detecting and emitting devices are added to the main light emitting and detecting device activated by the basic operation sequence, by identifying the blanking optical axis, it is possible to automatically generate a modified operation sequence for additionally determining operations of the sub light emitting device on the basis of the blanking optical axis. 
     When optical axes are again adjusted upon maintenance after installation of the multi-beam photoelectric safeguard system suitable for use of the present invention, it is advantageous to first adjust optical axes between the main light emitting and detecting devices by moving them relative to each other, next adjust the optical axes between the main light emitting device and the sub light detecting device by moving the latter, and finally adjust optical axes between the sub light emitting device and the main light detecting device by moving the former. 
     As another method of optical axis adjustment, it is possible to first adjust optical axes between the main light emitting and detecting devices by moving them relative to each other, next adjust optical axes between the sub light emitting device and the main light detecting device by moving the former, and finally adjust the optical axes between the main light emitting device and the sub light detecting device by moving the latter. 
     These and other objects and advantages of the invention will appear from the following description of preferred embodiments mainly in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating a conventional multi-beam photoelectric safeguard system from its side angle to explain a way of installation thereof; 
     FIG. 2 is a diagram of a conventional multi-beam photoelectric safeguard system from its side angle to explain another way of installation; 
     FIG. 3 is a diagram illustrating the conventional multi-beam photoelectric safeguard system of FIG. 2 from its front angle to explain the same way of installation; 
     FIG. 4 is a diagram illustrating a conventional system covering a non-detection area with a metal net or the like; 
     FIG. 5 is a diagram schematically showing the entire configuration of a multi-beam photoelectric safeguard system taken as an example for use of the present invention; 
     FIG. 6 is a diagram illustrating the safeguard system according to the embodiment of FIG. 5 from its side angle to show a way of installation thereof; 
     FIG. 7 is a diagram for explaining a main detection area defined between a main light emitting device and a main light detecting device, a first sub detection area defined between the main light emitting device and a sub light detecting device, and a second sub area defined between a sub light emitting device and the main light detecting device in the safeguard system shown in FIG. 5; 
     FIG. 8 is a diagram schematically showing the entire configuration of the multi-beam photoelectric safeguard system shown in FIG. 5; 
     FIG. 9 is a diagram for explaining an example of optical axis adjustment display lamp or optical axis adjustment display provided in a multi-beam photoelectric safeguard system related to the present invention; 
     FIG. 10 is a diagram for explaining another example of optical axis adjustment display lamp or optical axis adjustment display provided in a multi-beam photoelectric safeguard system related to the present invention; 
     FIG. 11 is a block diagram of the main light emitting device and the main light detecting device constituting the basic units of the multi-beam photoelectric safeguard system shown in FIG. 5; 
     FIG. 12 is a block diagram of the sub light detecting device involved in the safeguard system shown in FIG. 5; 
     FIG. 13 is a block diagram of the sub light emitting device involved in the safeguard system shown in FIG. 5; 
     FIG. 14 is a diagram for explaining a basic operation sequence of the main light emitting and detecting devices as the basic unit of the main light detecting device shown in FIG. 5; 
     FIG. 15 is a diagram for explaining a multi-detection operation sequence or modified operation sequence of the safeguard system shown in FIG. 5; 
     FIG. 16 is a diagram for explaining a multi-detection operation sequence or modified operation sequence as another example related to FIG. 5; 
     FIG. 17 is a diagram for explaining the situation of intrusion of an optical obstacle in the main detection area made by the safeguard system shown in FIG. 5; 
     FIG. 18 is a diagram for explaining the situation of intrusion of an optical obstacle in the first sub detection area made by the safeguard system shown in FIG. 5; 
     FIG. 19 is a diagram for explaining the situation of intrusion of an optical obstacle in the second sub detection area made by the safeguard system shown in FIG. 5; 
     FIG. 20 is a diagram illustrating the entire configuration of the multi-beam photoelectric safeguard system shown in FIG. 5; 
     FIG. 21 is a flowchart of procedures in a teaching mode for automatically generating the modified operation sequence; 
     FIG. 22 is a diagram for explaining the first step of procedures according to the invention for optical axis adjustment of the light emitting and detecting devices of the multi-beam photoelectric safeguard system; 
     FIG. 23 is a diagram for explaining the second step of procedures according to the invention for optical axis adjustment of the light emitting and detecting devices of the multi-beam photoelectric safeguard system; 
     FIG. 24 is a diagram for explaining the fourth step of procedures according to the invention for optical axis adjustment of the light emitting and detecting devices of the multi-beam photoelectric safeguard system; 
     FIG. 25 is a diagram for explaining the fifth step of procedures according to the invention for optical axis adjustment of the light emitting and detecting devices of the multi-beam photoelectric safeguard system; 
     FIG. 26 is a diagram for explaining the sixth step of procedures according to the invention for optical axis adjustment of the light emitting and detecting devices of the multi-beam photoelectric safeguard system; 
     FIG. 27 is a diagram for explaining a spacer preferably used in the fourth step of procedures according to the invention for optical axis adjustment of the light emitting and detecting devices of the multi-beam photoelectric safeguard system; and 
     FIG. 28 is a diagram for explaining the first step of alternative procedures according to the invention for optical axis adjustment of light emitting and detecting devices of a multi-axis photoelectric safeguard system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the invention will now be explained below with reference to the drawings. 
     Referring to FIG. 5, the multi-beam photoelectric safeguard system  100  includes a main light emitting device  11  and a complementary main light detecting device  12  as the basic units thereof. Both the main light emitting device  11  and the main light detecting device  12  constituting the basic units can be extended by connecting one or more additional such devices in series or in parallel, respectively. The safeguard system  100  further includes a sub light detecting device  13  complementary with an opposed section of the main light emitting device  11 , and a sub light emitting device  14  complementary with an opposed section of the main light detecting device  12 . 
     The main light emitting device  11  has an elongate case  11   a . N (eight in this embodiment) light emitting elements (not shown), which may be light emitting diodes (LEDs), are arranged in the case  11   a  at regular intervals in an array along the lengthwise (longitudinal) direction thereof. The interval of the light emitting elements may be 20 mm, for example, although it may be determined otherwise, either longer or shorter. 
     The main light detecting device  12  also has an elongate case  12   a , and photodetectors (not shown) equal in number to the light emitting elements (eight in this embodiment) are arranged in the case  12   a  at regular intervals. The interval of the adjacent photodetectors is equal to that of the light emitting elements. If the interval of the light emitting elements is 20 mm, then the interval of the photodetectors is also 20 mm. 
     The sub light detecting device  13  has a relatively short case  13   a , and one or more light photodetectors (not shown) less than the light emitting elements or photodetectors of the main light emitting device  11  or main light detecting device  12  are arranged in the case  13   a  in an array. In this embodiment, two photodetectors are provided, and their interval is equal to that of the light emitting elements of the main light emitting device  11 . Thus, if the interval of the light emitting elements of the main light emitting device is 20 mm, the interval of the photodetectors of the sub light detecting device  13  is also 20 mm. 
     The sub light emitting device  14  includes a relatively short case  14   a , and one or more light emitting elements (not shown) equal in number to the photodetector or photodetectors of the sub light detecting device  13  are arranged in the case  14   a  in an array. Here again, LEDs are typically used as the light emitting elements. Two photodetectors are provided in this embodiment, and their interval is equal to that of the photodetectors of the main light detecting device  12 . Thus, if the interval of the photodetectors of the main light detecting device  12  is 20 mm, the interval of the light emitting elements of the sub light emitting device  14  is also 20 mm. 
     The numbers from 1 to 8 shown in FIG. 5 represent the numbers of optical axes between the main light emitting device  11  and the main light detecting device  12 . As seen from FIG. 5, the main light emitting device  11  and the main light detecting device  12  are placed in an opposed relation on a common plane to emit and receive light beams that form a light curtain (FIG.  7 ). The area where the light beams run between the light emitting and detecting devices  11 ,  12  is herein named the main detection area  15 . The sub light detecting device  13  and the sub light emitting device  14  are placed to interrupt one or more optical axes between the main light emitting and detecting devices  11 ,  12  to define the first sub detection area  16  between an opposed section of the main light emitting device  11  and the sub light detecting device  13 , and the second sub detection area  17  between the sub light emitting device  14  and an opposed section of the main light detecting device  12  (FIG.  7 ). 
     More specifically, the sub light detecting device  13  is placed close to one side surface of the a projecting portion  21  that projects toward an operator of a machinery  20 , and opposed to the main light emitting device  11  to define the first sub detection area  16  together with opposed light emitting elements of the main light emitting device  11 . The sub light emitting device  14  is placed close to the opposite side surface of the projecting portion  21 , and opposed to the main light detecting device  12  to define the second sub detection area  17  together with opposed photodetectors of the main light detecting device  12 . 
     As a result, light beams traveling in the main detection area  15  and the sub detection areas  16 ,  17  form a light curtain all around the non-detection area defined between the sub light detecting and emitting devices  13 ,  14  and occupied by the projecting portion  21  of the press  20 . 
     FIGS. 5 and 7 illustrate the sub light detecting device  13  and the sub light emitting device  14  as lying to partly take over one or more of optical axes between the main light emitting and detecting devices  11 ,  12  at opposite sides of the non-detection area defined between the sub light detecting and emitting devices  13 ,  14 . In the drawings, the sub light detecting and emitting devices  13 ,  14  are positioned to partly take over the third and fourth optical axes; however, their position relative to the optical axes is determined, depending on the position of the projecting portion  21 . The number of the pairs of photodetectors and light emitting elements in the sub light detecting and emitting devices  13 ,  14  is determined in accordance with the size of the projecting portion  21  or other obstacle to equally compensate for the number of optical axes between the main light emitting and detecting devices  11 ,  12 , which will be optically blocked by the projecting portion  21 . 
     The main light emitting device  11 , main light detecting device  12 , sub light detecting device  13  and sub light emitting device  14  are connected altogether via a communication line or signal line  22 . 
     Referring to FIG. 8, the main light emitting and detecting devices  11 ,  12  each include an optical axis adjustment display  30  composed of a plurality of light emitting diode (LED) segments vertically aligned side by side. Here are used dichromatic light emitting diodes that can emit, for example, red and green light. Each of the main light emitting device  11  and the main light detecting device  12  also has an output display such as ON/OFF light using LED that normally emits green light, for example, and otherwise emits red light, for example, when any unexpected optical axes are blocked or detected, or when the system itself fails, for example. 
     The optical axis adjustment display, or optical axis adjustment display lamp,  30  composed of a plurality of light emitting diode segments may be used in any appropriate mode of display. Typically, when all beams of all optical axes enter into the main light detecting device  12 , all LED segments may emit green light. Then, if part of the optical axes are blocked, a number of segments proportional to the blocked optical axes, i.e. proportional to the light beams failing to reach the main light emitting device  12 , may emit red light sequentially from the bottom one, and a number of segments corresponding to the number of the blocked optical axes turn off from the top one. That is, the optical axis adjustment display  30  displays a bar type representation in which a red bar extends upward as the ratio of incident beams becomes higher, or in response to the degree of optical axis adjustment, in other words, the ratio between interrupted beams and detected beams, typically for facilitating an operator to confirm accurate alignment between the light emitting elements and photodetectors of the light emitting and detecting devices  11 ,  12  upon installing the safeguard system  100  on site. 
     The sub light emitting device  13  and the sub light detecting device  14  each include a optical axis adjustment display  32  having substantially the same function as the optical axis adjustment display or display lamp  30  already explained. 
     The optical axis adjustment displays  30  of the main light emitting and detecting devices  11 ,  12  and/or the optical axis adjustment displays  30  of the sub light detecting and emitting devices  32  may be any of the below-listed conventional types. 
     (1) A display lamp turned on or off when optical axis adjustment is confirmed by detection of light beams of all optical axes; 
     (2) A display lamp changed in color from red to green, for example, when optical axis adjustment is confirmed by detection of light beams of all optical axes; 
     (3) A display lamp having a plurality of LEDs that are selectively, cumulatively turned on or off in response to the intensity of light detected by the light detecting device; 
     (4) A display lamp changed in flickering speed in response to the ratio between the interrupted optical axes and the other optical axes of detected light beams; 
     (5) A display lamp changed in flickering speed in response to the intensity of light detected by the light detecting device; 
     (6) A set of display lamps individually associated with respective optical axes to individually represent interruption or detection of their own associated optical axes; 
     (7) A set of display lamps, each associated with several divisional blocks of optical axes made by dividing a number of optical axes between the light emitting and detecting devices, to represent the interruption or detection status of its own associated block. 
     FIG. 9 shows an example of the optical axis adjustment display  30 , which is a set of display lamps  33 , associated with individual optical axes, respectively, as listed in (6) above. FIG. 9 illustrates a configuration having display lamps for individual optical axes only on the part of the main light detecting device  12 . However, the display lamps  33  may be provided only on the part of the main light emitting device  11  or in both devices  11   12 . Similarly, display lamps  33  for individual optical axes may be provided as the optical axis adjustment display  32  in the sub light detecting device  13  and/or the subs light emitting device  14 . 
     As one type of one or more above-listed examples, FIG. 10 shows optical axis adjustment displays  30  in form of a liquid crystal display or seven segment LEDs  34  configured to make a numerical representation of the number of optical axes of detected light beams, number of interrupted optical axes or ratio between interrupted optical axes and the other optical axes of detected light beams. FIG. 10 illustrates such numerical displays  34  in both the light emitting and detecting devices  11 ,  12 , but only one of the devices  11   12  may have such a numerical display  34 . Similarly, one or both of the sub light detecting and emitting devices  13 ,  14  may have such a numerical display  34 . 
     In FIG. 8, reference numeral  36  denotes a teaching switch whose functions will be explained later. Although the teaching switch  36  is provided on the main light detecting device  12  in the example of FIG. 8, it may alternatively be positioned on the main light emitting device  11 . 
     Referring to FIG. 11, the main light emitting device  11  includes N (eight, for example) emitter circuits  41  for driving N LEDs  40  used as light emitting elements, an LED switching circuit (optical axis switching circuit)  42  for scanning these light emitting circuits  41  in a time-sharing manner, and an LED control circuit  43  for totally controlling the main light emitting device  11 . The LED control circuit  43  outputs a control signal to the optical axis adjustment display  30  and the output display  31 . 
     The main light emitting device  11  further includes a first emitter communication control circuit  44  for controlling bi-directional signal exchange of the main light emitting device  11  with the main light detecting device  12 , sub light detecting device  13 , etc., and a second emitter communication control circuit  45  for controlling communication between the main light emitting device  11  and a further main light emitting device (not shown) that may be additionally connected in series thereto for making a larger light curtain. 
     On the other hand, the main light detecting device  12  has N (eight, for example) detector circuits  51  for driving N photodetectors  50 , a photodetector switching circuit  52  for scanning these light detecting circuits in a time-sharing manner, an amplifier circuit  53 , and a photodetector control circuit  54  for totally controlling the main light detecting device  12 . The photodetector control circuit  54  outputs a control signal to the optical axis adjustment display  30  and the output display  31 . 
     The main light detecting device  12  further includes a first detector communication control circuit  55  for controlling bi-directional signal exchange of the main light detecting device  12  with the main light emitting device  11 , sub light detecting device  13 , etc., and a second detector communication control circuit  56  for controlling communication between the main light detecting device  12  and a further main light detecting device (not shown) that may be additionally connected in series thereto to make a larger light screen. 
     Furthermore, the main light detecting device  12  includes a signal processing circuit  57 . The circuit  57  is typically configured to be always fed by the photodetector control circuit  54  with signals indicating whether light beams of respective optical axes have been normally detected by respective photodetectors or not, and to process the signals accordingly. When the signal processing circuit detects from those signals that optical blockage has occurred two or three times within a predetermined period of time, it supplies an OFF signal through the output circuit  58  to an external device (not shown) such as a control panel of the press  20  or an alarm lamp associated with the light curtain made by the main light emitting device  11  and the main light detecting device  12  in order to stop the press  20  immediately. 
     The sub light detecting device  13 , best shown in FIG. 12, includes two detector circuits  61  for driving two photodetectors  60 , in case of this embodiment, a photodetector switching circuit  62  for scanning these detector circuits in a time-sharing manner, an amplifier circuit  63 , a photodetector control circuit  64  for totally controlling the sub light detecting device  13 , and a sub detector communication control circuit  65  for controlling bi-directional signal exchange of the sub light detecting device  13  with the main light emitting device  11 , sub light emitting device  14 , etc., so that a control signal is output from photodetector control circuit  64  to the optical axis adjustment display  32 . 
     The sub light emitting device  14 , best shown in FIG. 13, includes N emitter circuits  71  for driving two LEDs  70  used as light emitting elements, an LED switching circuit (optical axis switching circuit)  72  for scanning these emitter circuits  71  in a time-sharing manner, and an LED control circuit  73  for totally controlling the sub light emitting device  14 . The sub light emitting device  14  includes also includes a sub emitter communication control circuit  74  for controlling bi-directional signal exchange of the sub light emitting device  14  with the main light emitting device  12 , sub light detecting device  13 , etc., so that a control signal is output from the LED control circuit to the optical axis adjustment display  32 . 
     The safeguard system  100  is configured to selectively activate LEDs and photodetectors in associated light emitting and detecting devices at predetermined sequential timings, thereby to prevent the photodetectors from receiving light beams of optical axes other than their own optical axes, by exchanging information among the main light emitting device  11 , main light receiving device  12 , sub light receiving device  13  and sub light detecting device  14  via the signal line or communication line  22 . 
     The main light emitting device  11  and the main light detecting device  12  is desirably preset to follow a basic operation sequence shown in FIG.  14 . For example, in case the light curtain is formed without using the sub light detecting and emitting devices  13 ,  14 , that is, in case a light curtain is made solely by the main light emitting and detecting devices  11 ,  12 , the main light emitting and detecting devices  11 ,  12  operate according to the preset basic operation sequence of FIG. 14 (basic operation mode). Although FIG. 14 shows the basic operation sequence of the main light emitting device  11 , individual photodetectors of the main light detecting device  12  are activated synchronously with activation of associated individual LEDs of the main light emitting device  11 . 
     It will be appreciated from FIG. 14 that, in the basic operation sequence of the main light emitting and detecting devices  11 ,  12 , the activated duration of time (T 1 ) of each LED is constant for all LEDs and photodetectors, and the pause time from deactivation of each LED or photodetector to activation of the next LED or photodetector (T 2 ) is also constant. That is, respective sets of associated LEDs and photodetectors are sequentially activated periodically for the same duration of time. The basic operation sequence shown in FIG. 14 can be automatically established, taking account of the periods of time T 1 , T 2  and the number of all optical axes between the main light emitting and detecting devices  11 ,  12 . An operation program or an electric circuit may realize this operation sequence. 
     In contrast, FIG. 15 shows an example of a multi-detection or modified operation sequence for use when operations of the sub light detecting device  13  and the sub light emitting device  14  are incorporated. As shown in FIG. 15, just after activating the LED for the third optical axis of the main light emitting device  11 , the modified operation sequence activates the LED for the third optical axis of the sub light detecting device  14 , while deferring activation of subsequent LEDs for subsequent optical axes. After that, the modified operation sequence activates the LED for the fourth optical axis of the main light emitting device  11  and, just after thereof, activates the associated LED of the sub light emitting device  14 , while here again deferring activation of subsequent LEDs for subsequent optical axes. 
     Instead of the sequence shown in FIG. 15, another sequence is also employable, in which the sub light emitting device  14  starts emission after the main light emitting device  11  completes emission from its all LEDs, and after the sub light emitting device  14  completes emission from its all LEDs, emission from the main light emitting device  11  is resumed (FIG.  16 ). 
     In the safeguard system  100 , as apparent from the foregoing explanation, full extensions of six optical axes, namely, the first, second and fifth to eighth optical axes, between the main light emitting device  11  and the main light detecting device  12  form the main detection area  15 , sectional extensions of the third and fourth optical axes between the main light emitting device  11  and the sub light detecting device  13  form the first sub detection area  16 , and other sectional extensions of the third and fourth optical axes between the sub light emitting device  14  and the main light detecting device  12  form the second sub detection area  17 . Then, light beams traveling in the main and sub detection areas,  15 ,  16 ,  17  make a light curtain extending all around the projecting portion  21 . 
     For example, if an optical obstacle such as a part of the body of personnel blocks the first optical axis in the main detection area  15  formed between the main light emitting device  11  and the main light detecting device  12  as shown in FIG. 17, then the first photodetector of the main light detecting device  12  activated in sync with the first LED of the main light emitting device  11  cannot receive the optical beam. From this fact, it can be immediately acknowledged that optical blockage has occurred. Responsively, an OFF signal is supplied from the output circuit  58  through the signal processing circuit or detection circuit  57  contained in the main light detecting device  12  to an external circuit, and the press  20  is stopped immediately. 
     In another example shown in FIG. 18, if the optical obstacle blocks the third optical axis in the first sub detection area  16  formed between the main light emitting device  11  and the sub light detecting device  13 , the photodetector in the sub light detecting device  13  activated in sync with the third LED of the main light detecting device  11  cannot receive the optical beam. Responsively, the LED in the sub light emitting device  14  for the third optical axis does not emit light, and the associated photodetector in the main light detecting device  12  cannot receive any light beam at the predetermined timing. From this fact, it can be immediately acknowledged that optical blockage has occurred. Responsively, an OFF signal is supplied from the output circuit  58  via the signal processing circuit or detection circuit  57  contained in the main light detecting device  12  to the external device, and the press  20  is stopped immediately. 
     In the example of FIG. 18, the information that the sub light detecting device  13  did not receive any light beam from the main light emitting device  11  at a predetermined timing may be directly delivered from the sub light detecting device  13  to the main light detecting device  12  not through the step of non-emission from the sub light emitting device  14  and non detection by the main light detecting device  12  so that a blockage output is issued based on that information through the signal processing circuit or detection circuit  57  and the output circuit  58  contained in the main light detecting device  12  to the external device to stop the press  20  immediately. 
     In a further example shown in FIG. 19, if the optical obstacle S blocks the third optical axis in the second sub detection area  17  formed between the sub light emitting device  14  and the main light detecting device  12 , the photodetector of the main light detecting device  12  cannot receive the optical beam from the LED in the sub light emitting device  14  associated with the third optical axis. From this fact, it is immediately acknowledged that optical blockage has occurred. Responsively, a blockage signal or OFF signal is output through the signal processing circuit or detection circuit  57  and the output circuit  58  contained in the main light detecting device  12  to the external device, and the press  20  is stopped immediately. 
     Since the main light emitting device  11 , main light detecting device  12 , sub light detecting device  13  and sub light emitting device  14  are connected altogether by the communication line or signal line  22 , the safeguard system  100  can be readily modified to include the signal processing circuit or detection circuit  57  and the output circuit  58  in the main light emitting device  11  so as to output the blockage signal or OFF signal to the external device from the main light emitting device  11 . 
     Although the safeguard system  100  heretofore explained is configured to operate according to the operation sequence incorporated in the main light emitting device  11 , for example, the invention is also usable with another type of safeguard system  200  having a controller  38  as an additional separate controller as shown in FIG.  20 . In the safeguard system  200  shown here, the controller  38  substantially controls the light emitting and detecting devices such as the main light emitting device  11 . Thereby, any blockage signal from the main light detecting device  12  or sub light detecting device  13  is input to the controller  38 , and an ON signal or OFF signal is output from the controller  38  toward an external device. 
     Also in the safeguard system  200 , a modified operation sequence may be generated substantially in the photodetector control circuit  54  of the main light detecting device  12  through procedures explained later in detail. Alternatively, the controller  38  may realize this function of the photodetector control circuit  54  to generate the modified operation sequence. 
     In the safeguard system  20 , the optical axis adjustment display  30  on one or both of the main light emitting/detecting devices  11 ,  12 , and the optical axis adjustment display  32  on one or both of sub light emitting devices  13 ,  14  may be replaced by an optical axis adjustment display  39  provided on the controller  38 , or alternatively, a teaching switch  36  may be provided (FIG.  20 ). The optical axis adjustment display  39  may have the same configuration as that of the optical axis adjustment display  30  or  31  already explained, or may be of any type of representation of optical axes among those listed herein before. 
     Alternatively, the controller  38  may include circuits similar to the signal processing circuit  57  and the output circuit  58  (FIG.  11 ), already explained, to output a blockage signal from the controller  38  to an external device (FIG.  20 ). 
     FIG. 21 et seq. are diagrams related to installation of the safeguard system  100  or  200 . Explanation is made below with reference to these figures about procedures for installing the light emitting and detecting devices and automatic generation of a multi-detection sequence or modified sequence triggered by ON manipulation of the teaching switch  36 . 
     First Step: Setting and Positioning of the Main Light Emitting and Detecting Devices  11  and  12  (FIG. 22) 
     The main light emitting device  11  and the main light detecting device  12  are first placed in predetermined positions relative to the press  20 , for example, from which the projecting portion  21  has been removed. 
     After that, relative positions of the main light emitting and detecting devices  11 ,  12  are adjusted precisely (FIG.  22 ). That is, optical axes between the main light emitting and detecting devices  11 ,  12  are adjusted. This optical axis adjustment is carried out by fine adjustment of positions of the main light emitting and/or detecting devices  11 ,  12  so that all of their optical axes coincide. The operator can confirm whether the main light detecting device  12  has detected all optical beams sequentially emitted from LEDs of the main light emitting device  11  are certainly detected, that is, whether the main light emitting and detecting devices  11   12  have been precisely positioned relative to each other, by watching the optical axis adjustment displays or display lamps  30  on the main light emitting and detecting devices  11 ,  12 , or the optical axis adjustment display on the controller  38 . 
     Second Step: Mounting of the Projecting Portion  21  (FIG. 23) 
     After completion of the first step, the projecting portion is mounted to the press  20 . As a result, some of the optical axes between the main light emitting and detecting devices  11 ,  12  are interrupted. 
     Third Step: Generation of the Multi-detection Operation Mode (FIG. 21) 
     The operator or user next turns ON the teaching switch  36  (step S 1  of FIG.  21 ). As a result, the safeguard system  100  or  200  enters in the teaching mode for automatically generating the multi-detection or modified operation sequence that determines emitting/detecting operations not only of the main light emitting and detecting devices  11 ,  12  but also of the sub light detecting and emitting devices  13 ,  14  in the multi-detection mode. The ON signal from the teaching switch  36  is input into the photodetector control circuit  54 . 
     Once the system enters in the teaching mode, the photodetector control circuit  54  having acknowledged the teaching mode transfers the information to the LED control circuit  43  through the communication line or signal line  22 , and the main light emitting device  11  starts emission according to the basic operation sequence shown in FIG. 14 (step S 2  of FIG.  21 ). 
     When all LEDs of the main light emitting device  11  complete emission of light, the photo detector control circuit  54  recognizes that the third and fourth optical axis, in case of the example shown in FIG. 7, are interrupted by the projecting portion  21 . Responsively, in case of generating the multi-detection operation sequence or modified operation sequence, the photodetector control circuit  54  makes a first blank (a length of time totaling the time T 1  and the time T 2 ) necessary for activation of one of LEDs of the sub light emitting device  14  for the third optical axis (illustrated as the optical axis No.  3 ′ in FIG. 15) after the activation timing of one of LEDs of the main light emitting device  11  for the third axis (illustrated as the optical axis No.  3  in FIG. 15) while delaying activation timings of LEDs for subsequent optical axes. Additionally, the photodetector control circuit  54  makes a second blank (a length of time totaling the time T 1  and the time T 2 ) necessary for activation of the other LED of the sub light emitting device  14  for the fourth optical axis (illustrated as the optical axis No.  4 ′ in FIG. 15) after the activation timing of one of LEDs of the main light emitting device  11  for the fourth axis (illustrated as the optical axis No.  4  in FIG. 15) while delaying emission timings of LEDs for subsequent optical axes. Furthermore, the photodetector control circuit  54  incorporates timings for activation of the sub light emitting device  14  in the first and second blanks. In this manner, the photodetector control circuit  54  automatically generates the modified operation sequence shown in FIG. 15 for activating the sub light emitting device  14  as well at the timings corresponding to the first and second blanks. Hereinbelow, optical axes interrupted by the projecting portion  21  are called planking optical axis. 
     Alternatively, if the multi-detection operation sequence or modified operation sequence of FIG. 16 should be generated, the photodetector control circuit  54  may make the first and second blanks necessary for activation of LEDs of the sub light emitting device  14  between activation timings of LEDs of the main light emitting device  11  for the eighth and first optical paths, and may automatically generate the sequence for activating the sub light emitting device  14  at the timings corresponding to the first and second blanks. 
     As a result, the multi-detection operation sequence or modified operation sequence as shown in FIG. 15 or  16  is automatically generated (step S 4  of FIG.  21 ), and the teaching mode ends (step S 5  of FIG.  21 ). 
     In the process explained above, the photodetector control circuit  54  that can be regarded as CPU of the main light emitting device  12  recognizes the ON state of the teaching switch  36 , and the photodetector control circuit  54  automatically generates the modified operation sequence (FIG. 15 or  16 ) in response to detection of interruption of particular optical axes. However, this function may be given to the photodetector control circuit  43  of the main light emitting device  11  so that the modified operation sequence is established on the part of the main light emitting device  11 . Alternatively, it is of course possible that the main light emitting device  11  and the main light detecting device  12  share the function of automatically generating the modified operation sequence. 
     Fourth Step: Setting and Positioning of the Sub Light Detecting Device  13  (FIG. 24) 
     The sub light detecting device  13  is placed adjacent to one side of the projecting portion in an opposed relationship with the main light emitting device  11 . For accurate positioning of the sub light detecting device  13  relative to the main light emitting device  11 , it will be necessary to move the sub light detecting device  13  vertically or change its orientation such that optical axes coincide between the sub light detecting device  13  and the main light emitting device  11 . 
     Since the modified operation sequence is already established in the third step  3  such that the sub light detecting device  13  is activated at given timings for detecting light beams only of the third and fourth optical axes, in case of the example of FIG. 7, from the main light emitting device, the operator can accomplish positioning of the sub light detecting device  13  relative to the main light emitting device  11  by moving the former while confirming the degree of adjustment through the optical axis adjustment display or display lamp  32  on the sub light detecting device  13  or the optical axis adjustment display  39 . 
     Fifth Step: Setting and Positioning of the Sub Light Emitting Device  14  (FIG. 25) 
     The sub light emitting device  14  is next placed adjacent to the opposite side of the projecting portion  21  in an opposed relationship with the main light detecting device  12 . Here again, for accurate positioning of the sub light emitting device  14  relative to the main light detecting device  12 , it will be necessary to slightly move the sub light emitting device  14  vertically or change its orientation such that, in case of the example of FIG. 7, light beams emitted from the sub light emitting device  14  are detected by photodetectors of the main light detecting device for the third and fourth optical axes. The operator can proceed with this adjustment while confirming the degree of adjustment through the optical axis adjustment display on the sub light emitting device  14  or the optical axis adjustment display  39  on the controller  38 . Thus the adjustment of optical axes between the sub light emitting device  14  and the main light detecting device  12  is accomplished. 
     Sixth Step: Confirmation of Detection of the Minimum Object (FIG. 26) 
     Next confirmed is whether the system  100  or  200  can detect a certain minimum object in any of the detection areas defined by the main light emitting and detecting devices  11 ,  12  and the sub light detecting and emitting devices  13 ,  14 . The operator can carry out this confirmation by moving a minimum object (not shown) to be detected along the route shown by arrows in FIG.  26  and confirming that a blockage signal is output from the system  100  or  200  when the object intrudes into the detection areas. 
     For the positioning of the sub light detecting device  13  in the fourth step, it is convenient to removably attach a spacer SP on the top and/or bottom of the sub light detecting device  13  as shown in FIG.  27 . The spacer SP may be a plate member, for example, which does not interrupt light beams of adjacent optical axes (in the example of FIG. 7, second and fifth optical axes) when the sub light detecting device  13  is accurately positioned, but does interrupt the adjacent light beams when the sub light detecting device  13  is offset vertically, even if slightly. 
     In another example, the spacer SP may be a plate having a small through hole, not shown. The operator can accurately position the sub light detecting device  13  by finding its position where the light beam of the second or fifth optical axis passes through the hole of the plate. In other words, when the sub light detecting device is offset vertically or in the front and back direction, even if slightly, the light beam of the second or fifth optical axis will be interrupted by the spacer SP having the through hole. 
     For the above-explained adjustment of optical axes of the light emitting and detecting devices provided in the safeguard system  100  or  200 , the projecting portion  21  is removed from the press  20  in the process of adjusting the optical axes of the main light emitting and detecting devices  11 ,  12 . However, as shown in FIG. 28, relative accurate positioning between the main light emitting and detecting devices  11 ,  12 , namely, adjustment of their optical axes, may be carried out under the existence of the projecting portion  21  on the press  20 . 
     In this case, adjustment of optical axes is carried out by positioning the main light emitting and detecting devices  11 ,  12  to ensure that all light beams other than those of the optical axes interrupted by the projecting portion  21  (the third and fourth optical axes in the foregoing example) enter into the main light detecting device  12 . The operator will confirm through the optical axis adjustment displays or display lamps  30  on the main light emitting device  11  and the main light detecting device  12  or the optical axis adjustment display  39  on the controller  38  whether the adjustment of optical axes has been accomplished or not. However, for easier confirmation, it is advantageous to provide a switch SW shown in FIG. 28 on the main light detecting device  12 , main light emitting device  11  and/or controller  38  such that the operator can confirm the intensities of detected light of individual optical axes through the optical axis adjustment display  30  or  39  by manipulating the switch SW. The optical axis display device  30  may be of the type having display lamps  33  exclusive for individual optical axes (FIG.  28 ), or in form of the numerical display  34  using a liquid crystal or seven segments of LEDs as shown in FIG.  10 . The numerical display  34  may have some different display modes for selectively representing the number of optical axes of detected light beams, number of interrupted optical axes, position of an interrupted optical axis, and so on, such that, for example, the position of the optical axis currently interrupted on the numerical display  34  under the operator&#39;s choice to confirm whether positioning of the main light emitting and detecting devices  11 ,  12  has been accomplished or not. 
     Although the modified example of optical axis adjustment has been roughly explained with reference to FIG. 28, its procedures and automatic generation of the multi-detection sequence or modified operation sequence responsive to the instruction through the teaching switch  36  will follow the following steps. 
     First Step: Setting and Positioning of the Main Light Emitting and Detecting Devices  11 ,  12  Relative to the Press  20  Having the Protecting Portion  21  (FIG. 28) 
     The main light emitting device  11  and the main light detecting device  12  are accurately positioned relative to each other (See FIG.  22 ). More specifically, the main light emitting and detecting devices  11 ,  12  are placed at spaced-apart positions from the projecting portion  21  of the press  20  at opposite sides thereof, and their optical axes are adjusted accurately. This adjustment of optical axes is achieved by fine adjustment of the main light emitting and detecting devices  11 ,  12  so as to accurately align their optical axes. The operator can confirm whether the main light detecting device  12  has detected all optical beams sequentially emitted from LEDs of the main light emitting device  11  are certainly detected, that is, whether the main light emitting and detecting devices  11   12  have been precisely positioned relative to each other, by watching the optical axis adjustment displays or display lamps  30  on the main light emitting and detecting devices  11 ,  12 , or the optical axis adjustment display on the controller  38 . 
     Second Step: Generation of the Multi-detection Operation Sequence (FIG. 21) 
     The operator or user next turns ON the teaching switch  36  (step S 1  of FIG.  21 ). As a result, the safeguard system  100  or  200  enters in the teaching mode for automatically generating the multi-detection or modified operation sequence that determines emitting/detecting operations not only of the main light emitting and detecting devices  11 ,  12  but also of the sub light detecting and emitting devices  13 ,  14  in the multi-detection mode. The ON signal from the teaching switch  36  is input into the photodetector control circuit  54 . 
     Once the system enters in the teaching mode, as already explained, the photodetector control circuit  54  having acknowledged the teaching mode transfers the information to the LED control circuit  43  through the communication line or signal line  22 , and the main light emitting device  11  starts emission according to the basic operation sequence shown in FIG. 14 (step S 2  of FIG.  21 ). 
     When all LEDs of the main light emitting device  11  complete emission of light, the photo detector control circuit  54  recognizes that the third and fourth optical axis, in case of the example shown in FIG. 7, are interrupted by the projecting portion  21 . Responsively, assuming here again that the multi-detection operation sequence should be generated, the photodetector control circuit  54  makes a first necessary for activation of one of LEDs of the sub light emitting device  14  for the third optical) after the activation timing of one of LEDs of the main light emitting device  11  for the third axis while delaying activation timings of LEDs for subsequent optical axes. Additionally, the photodetector control circuit  54  makes a second blank necessary for activation of the other LED of the sub light emitting device  14  for the fourth optical axis after the activation timing of one of LEDs of the main light emitting device  11  for the fourth axis while delaying emission timings of LEDs for subsequent optical axes. Furthermore, the photodetector control circuit  54  incorporates timings for activation of the sub light emitting device  14  in the first and second blanks. In this manner, the photodetector control circuit  54  automatically generates the modified operation sequence shown in FIG. 15 for activating the sub light emitting device  14  as well at the timings corresponding to the first and second blanks. Hereinbelow, optical axes interrupted by the projecting portion  21  are called planking optical axis. Also when the multi-detection operation sequence of FIG. 16 should be made, its procedures are the same as those already explained. 
     As a result, as already explained, the multi-detection operation sequence or modified operation sequence as shown in FIG. 15 or  16  is automatically generated (step S 4  of FIG.  21 ), and the teaching mode ends (step S 5  of FIG.  21 ). 
     Third Step: Setting and Positioning of the Sub Light Detecting Device  13  (FIG.  24 ) 
     In the same manner as the embodiment already explained, the sub light detecting device  13  is placed adjacent to one side of the projecting portion in an opposed relationship with the main light emitting device  11 . For accurate positioning of the sub light detecting device  13  relative to the main light emitting device  11 , it will be necessary to move the sub light detecting device  13  vertically or change its orientation such that optical axes coincide between the sub light detecting device  13  and the main light emitting device  11 . 
     Since the modified operation sequence is already established in the third step  3  such that the sub light detecting device  13  is activated at given timings for detecting light beams only of the third and fourth optical axes, in case of the example of FIG. 7, from the main light emitting device, the operator can accomplish positioning of the sub light detecting device  13  relative to the main light emitting device  11  by moving the former while confirming the degree of adjustment through the optical axis adjustment display or display lamp  32  on the sub light detecting device  13  or the optical axis adjustment display  39 . 
     Fourth Step: Setting and Positioning of the Sub Light Emitting Device  14  (FIG. 25) 
     In the same manner as the embodiment already explained with reference to FIG. 25, the sub light emitting device  14  is next placed adjacent to the opposite side of the projecting portion  21  in an opposed relationship with the main light detecting device  12 . Here again, for accurate positioning of the sub light emitting device  14  relative to the main light detecting device  12 , it will be necessary to slightly move the sub light emitting device  14  vertically or change its orientation such that, in case of the example of FIG. 7, light beams emitted from the sub light emitting device  14  are detected by photodetectors of the main light detecting device for the third and fourth optical axes. The operator can proceed with this adjustment while confirming the degree of adjustment through the optical axis adjustment display on the sub light emitting device  14  or the optical axis adjustment display  39  on the controller  38 . Thus the adjustment of optical axes between the sub light emitting device  14  and the main light detecting device  12  is accomplished. 
     Fifth Step: Confirmation of Detection of the Minimum Object (FIG. 26) 
     In the same manner as already explained with reference to FIG. 26, next confirmed is whether the system  100  or  200  can detect a certain minimum object in any of the detection areas defined by the main light emitting and detecting devices  11 ,  12  and the sub light detecting and emitting devices  13 ,  14 . The operator can carry out this confirmation by moving a minimum object (not shown) to be detected along the route shown by arrows in FIG.  26  and confirming that a blockage signal is output from the system  100  or  200  when the object intrudes into the detection areas. 
     In the foregoing explanation, optical axes interrupted by the projecting portion  21  are identified in the positioning step of the main light emitting and detecting devices  11 ,  12 . If, however, the operator can identify the interrupted axes, i.e. the blanking optical axes beforehand, the operator may supply the information to the system  100  or  200  through an external means. Similarly, the multi-detection operation sequence or modified operation sequence (FIG. 15) may be generated outside the system  100  or  200 , and this information may be supplied together with the information about the blanking optical axes to the system  100  or  200  through a communication means using infrared rays or electric waves, USB, Ethernet, or the like. Any skilled person in the art will readily understand that the operator can easily generate the multi-detection sequence or modified operational sequence (FIG. 15) by using a personal computer, for example, and inputting ID numbers of the blanking axes to the computer. 
     Once the positioning (optical axis adjustment) of the light emitting and detecting devices is accomplished, the safeguard system  100  or  200  behaves according to the modified operation sequence shown in FIG. 15 or  16  to sequentially emit and detect light from the first optical axis to the eighth optical axis, and repeats this cycle of optical scan again from the first optical axis. In each cycle of the operation, the sub light detecting device  13  is activated in sync with activation of the third and fourth optical axes of the main light emitting device  11  thereby to selectively change each corresponding photodetector thereof active. Each photodetector of the main light detecting device  12  is selectively activated in sync with operations of the main light emitting device  11  and the sub light emitting devices  14 . As a result, as to the third and fourth optical axes, the sub light detecting device  13  detects light beams from the main light emitting device  11 , and the main light detecting device  12  detects light beams from the sub light emitting device  14 . 
     That is, in the sub light detecting device  13 , photodetectors are selectively activated in synch with activation of LEDs of the corresponding third and fourth optical axis of the main light emitting device  11  according to the modified operation sequence (FIG. 15 or  16 ). When each photodetector of the sub light detecting device  13  detects light beam from the main light emitting device  11 , the sub light detecting device  13  supplies an emission command to the sub light emitting device  14  directly or via the controller  38 . 
     When the sub light emitting device  14  receives the information from the sub light detecting device  13  or controller  38  according to the modified operation sequence (FIG. 15 or  16 ) automatically generated by the initial setting, one of LEDs of the sub light emitting device  14  for the corresponding optical axis is changed active. The sub light emitting device  14  may be controlled otherwise such that it emits a light beam exclusively following to the modified sequence of FIG. 15 or  16  without the emission command from the sub light detecting device  13  or controller  38 , or it emits a light beam exclusively following to the emission command from the sub light detecting device  13  or controller  38 . 
     Although some embodiments of the invention have been explained taking examples in witch the safeguard system  100  or  200  includes a set of main light emitting and detecting devices  11 ,  12 , one sub light detecting device  13  and one sub light emitting device  14 , the safe guard system  100  or  200  may include two or more sets of sub light detecting and emitting devices  13 ,  14 , and in addition to that, the system  100  or  200  may include two or more sets of main light emitting and detecting devices  11 ,  12  that are connected by a communication line or signal line to make a wider light curtain. 
     Although some embodiments have been explained as providing the optical adjustment displays or display lamps  30  on both the main light emitting and detecting devices  11 ,  12 , they may be modified to provide the optical axis adjustment display or display lamp  30  on one of the main light emitting and detecting devices. 
     Also regarding the sub light detecting and emitting devices  13 ,  14 , the optical axis adjustment display or display lamp  32  may be provided on only of the sub light detecting and emitting devices  13 ,  14 . In this case, the optical axis adjustment display or display lamp  32  is preferably provided on the sub light detecting device  13 . Optical axis adjustment of the sub light detecting and emitting devices  13 ,  14  may be confirmed through the optical axis adjustment display  39  of the controller  38 . 
     Furthermore, while the system  100  or  200  actually works after installation and optical axis adjustment of the light emitting and detecting devices  11  through  14  according the method explained heretofore, if the light emitting and detecting devices  11  to  14  again need optical axis adjustment as the maintenance of the system  100  or  200 , the operator may proceed with substantially the same procedures as explained above. In this case, the operator may first adjust optical axes between the main light emitting and detecting devices, or may first adjust optical axes between the sub light detecting and emitting devices prior to adjustment of the main light emitting and detecting devices.