Light beam apparatus
United States Patent [1 1 [111 3,875,403 Svensson Apr. 1, 1975 [5 LIGHT BEAM APPARATUS 3.746.863 7/1973 Pronovost 250/208 Inventor Lars Erik Svensson Ombergsvagen 3,805,061 4/1974 Des Rioux dc Missimy et al.250/209 5, 146 00 Tullinge. Sweden Primary E.\&#39;aminerEli Lieberman [22] filed: 1974 Assistant E.\&#39;aminerT. N. Grigsby [21] Appl. No.: 446,423  
  {57] ABSTRACT [30] Foreign Application Priority Data I Feb 1973 Sweden WSW/73 A hght curtain for supervlslng or controlling the operation of machines comprising a series of electrical [52] U S Cl 250/209 250/221 317/177 light sources arranged to give light pulses which are 40/258 short in relation to the repetition frequency. After the [51] Int CL&#34;. G06m 7/8 60% 13/00 Holh 47/24 light pulses have spanned the supervlsed region they [58] Field of Search 250/2521 208 are detected by one or more pulse detectors for each &#34;340/258 317/1273 light source. When the detectors associated with each light source receive simultaneous pulses on their in- [56] References Cited puts a starting pulse for the light source next in sequence in the series is released. UNITED STATES PATENTS 3.742.222 6/1973 Endl 250/209 7 Claims, 1 Drawing Figure LIGHT BEAM APPARATUS The present invention refers to a light beam apparatus.  
  The present invention relates to apparatuses in connection with so-called light curtains or light beam apparatus. Such apparatus is especially used for guarding dangerous machines or installations from unintentional access. This is arranged by the opening which allows access being monitored by light beams placed side by side or sweeping light beams which, when they are interrupted. cause the immediate stoppage of the machine which is being guarded.  
  Apparatus which has the task of preventing accidents in work must be extremely reliable. When a supervising system is being designed, different measures are therefore taken to ensure that faults arising in the supervisory devices themselves should not be able to cause loss of the safety function.  
  Special sensing devices are therefore connected to a light curtain apparatus, for stopping the machine both when an object is introduced into the opening of the light curtain apparatus and when some predictable fault arises in the supervising equipment. A problem here has been to make the sensing devices with the right degree of sensitivity. If there is great sensitivity there is the risk of emergency stoppages even for completely irrelevant disturbances. Such disturbances can be occasional variations in the electric power supply voltage, natural alterations or reasonable wear of the participating components and details in the sensing devices. Light or smoke can also be disturbing and cause interruptions in operation. Too low sensitivity, on the other hand, brings with it the risk of malfunction when function is absolutely required.  
  Previously known light curtains are preferably arranged according to two main principles. The one works with a row of combined lamp and photo-electric cells, the light from the lamps spanning the opening in the light curtain in parallel beams, either being reflected back again by a mirror or directly transferred to the photocells which, in their turn, are connected to electrical amplifiers and relays by the help of which the arresting function is carried out, when one or several light beams are interrupted or weakened by a previ ously determined amount. The other principle utilizes a single lamp and photocell instead of several lamps and photocells, the light from the lamp being given a sweeping movement over the light curtain opening by means of a rotating mirror. The sweeping movement, which has a parallel course over the opening, is provided by a mirror arrangement containing a plane and a parabolic mirror in the center of which the rotating mirror is placed.  
  The known devices mentioned have a number of weaknesses. Continuously burning filament lamps having a limited lifespan are used throughout. Light intensity demands are comparatively high, which also contributes to lessen lifespan. The light striking the photocell or cells is weak and therefore large amplification must be resorted to for driving the associated supervising devices. In this case amplification is done in DC amplifiers, which are difficult to make with a stable degree of amplification. The use of pulsating light has been precluded because of the sluggishness of the element and the requirement of a certain longest reaction time for the light curtain. The rotating mirror system is moreover mechanically complicated and expensive, and in practice it gives considerable variations in light intensity and thereby the sensitivity of the light curtain opening. The scanning speed is limited by the sluggishness of the mechanical system. Furthermore, special controlling means in the shape of extra photocells are required to indicate when the light beam leaves and reenters the light curtain opening. Both the devices mentioned suffer from sensitivity to outside light. The spread of light can further cause light to strike the photocell associated with a certain beam although the beam has been blocked, thus triggering the protecting mechanism. Lately, the use of so-called light diodes or light-emitting semiconductors, as light sources has been proposed and these can be fed with voltages pulses. It has been found that operational reliability has thereby been able to be increased. However, the requirement of double safety is put on such installations these days, i.e., at least two operating means independent of each other shall be switched on for the machine in question to be started. This has naturally had repercussions on the construction of the light curtain circuit.  
  The object of the present invention is to put right the weaknesses accounted for above and to provide light curtains having large tolerance against variations of sensitivity and high functional reliability and which work solely with cheap and operationally reliable electronic components.  
  The special distinguishing features of the subject invention may be apprehended from the claims which follow.  
  The subject invention will now be described in connection with a pair of embodiment examples while referring to FIG. 1, which is a principle diagram over the electronic circuiting of a suitable embodiment of the invention.  
  With reference to FIG. 1, where the function of the light curtain or light beam apparatus can be followed, a pulsing source 1 for electrical pulses is arranged. The source 1, which can be a pulse generator known per se, works with a suitably selected pulse repetition frequency, e.g., lkl-Iz, and feeds a rapid light supply 3 via a pulse amplifier 2, the light supply being a light diode or a gas discharge lamp. The light pulse obtained in this way is concentrated by suitable optical means on a mirror 4 arranged on the far side of the area to be supervised, and which reflects the light pulse back again in the present case to two light pulse detectors 5 of photodiode or phototransistor type, mounted on each side of the light source in the plane of the light curtain. The output signal obtained in the light pulse detectors 5 is an electrical pulse representing light pulses and is taken to a special pulse control circuit 6, which is in its turn arranged to give a further electrical impulse only on the condition that the reflected light pulse has the right amplitude and arrives within a time period determined by pulse source 1, or the feeding pulse to light diode 3. In the present example, this is done so that a portion of the feeding pulse is portioned off and used as a control pulse. In this connection it should be pointed out that the propagation time of the pulses is negligable, and for this reason the control pulse and the output pulse from the light pulse detectors 5 occur in the pulse control device 6 practically simultaneously. If the light pulse has now not met any obstacle, the pulse control device 6 gives an output pulse as mentioned, which in principle is a copy of the one obtained from the pulse source 1.  
 The output pulse now feeds a second combination of amplifier 7, light source 8, mirror 9, photodetector and pulse control device 11, completely in agreement with the arrangement described before, and having the same working principle and a special supervisory circuit 12, the task of which is to ensure that the output pulse from the control circuit 6 is actually present.  
  From the figure it may be further seen that the supervisory circuit 12 is designed with two inputs and that in the present case two further combinations of light source/detectors and pulse controlling devices, 13-16 and 17-20 resp. are connected in series. The inputs of the supervisory circuit 12 are connected to alternate links in the chain, i.e., to 26 and 13-16. A further supervisory circuit 21 is arranged in conjunction with units 7-11 and 17-20. An operating relay 22,23 is respectively connected to each supervisory circuit. Said relays are suitably arranged in the operating circuit of a machine, e.g., the operating circuit for a press, whereat the connection is arranged to take place between terminals 24,25. Terminals 26,27 can be connected to a signal circuit. As may be seen from the diagram both the relays 22,25 must be in circuit for the line between 24 and 25 to be closed, i.e., for the operating circuit of the machine to be closed. In its turn, this means that all the pulse sequences have gone undisturbed from unit &#39;to unit, and that the supervisory circuits 12,21 have been regularly actuated for putting in circuit their respective relays 22,23. If any disturbance should occur in any of the light beam paths, e.g., the one starting from the light diode 3, no light pulse is received in the appropriate manner by any of the detectors 5, whereon neither is there a pulse the next step 7-11 in sequence or to the supervisory circuit 12. Both relays 22,23 will then cut out, making it impossible to operate the machine. Safety is thus complete.  
  To make the light curtain insensitive to random light, general lighting or light pulses striking a photodetector other than the one intended, said pulse control circuit 6 is so designed that its output pulse is delayed in relation to the feeding pulse which controls the respective light diode. Scanning is hereby obtained along the whole of the light curtain ramp, line for line. The pulse length and the pulse repetition frequency of pulse source 1 is therefore adjusted so that the whole of the light ramp can be scanned line for line between two consequent pulses from said pulse source. If, for example, the light curtain shown, which includes four light diodes 2,7,13,17, is supplied with a pulse repetition frequency of lkl-lz the pulse length is arranged to be less than 0.25 milliseconds. Further, embodiments are possible, where the number of light detectors may be varied as well as their placing. Instead of the mirrors which are normally used, the detectors can be put in the place of the mirrors.  
  With the subject invention a light curtain or light beam apparatus is obtained, built from simple reliable electronic components which may preferably be built together in a single solid mechanical unit. Thanks to the rapid development of techniques in the fields of optical-electrical semiconductors and integrated circuits, an apparatus according to the invention can be made very small and efficient. As scanning of the light curtain opening takes place in steps, resonance between the circuits is eliminated completely. It is further possible arbitrarily to graduate sensitivity by arranging more photodetectors for each light diode. Certain narrow objects can thereby be passed through the light curtain without the machine stopping. Thanks to the use of AC. amplifiers sensitivity can be further kept very constant and very high. It is thus possible to span large distances, the insensitivity to general lighting contributing to this possibility. As is usual in connection with light curtains, it is also possible to connect means for single, double or triple sequences, whereby intentional interruption of the light beams is used to start the machine, after the introduction of one or more objects during its working cycle.  
 What I claim is:  
  1. A light curtain apparatus for supervising and/or operation of machines, comprising a series of electrical light sources (3) arranged to give, through a periodic pulse source (1) light pulses which are short in relation to the repetition frequency of the pulse source, said light pulses after having spanned the opening in the light curtain being detected by one or more light pulse detectors (5) arranged for each light source (3), wherein the light detectors (5) for each light source control a pulse control circuit (6) which, on receiving essentially simultaneous pulses on its inputs, releases a starting pulse for the light source (7) next in sequence in the series, and simultaneously gives a supervisory circuit (12) an acknowledgement pulse.  
  2. A light curtain apparatus according to claim 1, wherein the starting pulse for each light source is also arranged to feed an input of the respective pulse control circuits (6,11,16,20).  
  3. A light curtain apparatus according to claim 1 wherein at least two supervisory circuits (12,21) are arranged, each having a number of inputs corresponding to half the number of light source circuits (26, 13-16, etc.), each input being connected to an appropriate pulse control circuit (6,16).  
  4. A light curtain apparatus according to claim 3 wherein alternate pulse control circuits (6,16 and 11,20 respectively) are connected to one and the same supervisory circuit (12 and 21 respectively).  
  5. A light curtain apparatus according claim 1 wherein each of the supervisory circuits (12 and 21 respectively) controls its own operating circuit (22 and 23 respectively) for the machine which is to be supervised and/or operated.  
  6. A light curtain apparatus according to claim 5 wherein the operating circuits comprise relays (22,23) the contacts of which are co-actively arranged to control the controlling circuits of the machine in question.  
  7. An apparatus according claim 1 wherein each pulse control circuit (6) contains a time delay means, a pulse being taken off said time delay means to feed the appropriate supervisory circuit (12).