- Case 2561 - ~L6~33~2 This invention relates to apparatus for sensing the presence of objects in a predetermined area, and in particular it relates to apparatus for sensing the presence of an object in a protected area of a machine to act as a safe~uard. Various sensing arrangements are known which sense the presence of an unwanted object in a protected area, i.e. in an operating area, of a machine to prevent the machine from damaging the object. For example, it is known to provide a plurality of parallel beams of light to form a curtain of 1~ light extending in front of a power press whereby interruption of any one of the light beams prevents or stops operation of the press. This provides a safeguard for the operator. If, for example, the operator's hand is in the protected area, it will interrupt one or more of the light beams and prevent the press from operating. This sensing arrangement works satis- factorily with minimum disruption of operation and minimum inconvenience to the operator for normal operations. United States Patent No. 3 742 222 to Alfons Endl, issued June 26, 1973 is an example of this form of machine safeguard. The ~0 patent describes a sensing system which uses a digital detector means to determine when one light beam is broken. Means are provided to operate the light sources at a frequency higher than the frequency of ambient light sources to make it possible to discriminate between the machine light source and ambient light. Other sensing arrangements are known which use infra- red light sources and opposed photodetectors sensitive to in- frared light to provide a protective curtain of infrared light. The use of infrared light reduces sensitivity to ambient light. In all the prior art arrangements the horizontally extending parallel beams form a curtain in a vertical plane in front of the protected area and the plane terminates at the Case 2561 93~32 working surface or at a predetermined distance above the working surface. In other words if the material on which the machine is to operate is wholly inside the protected area, the protective curtain of light covers the entire access area which is normally defined at the bottom by a table or working surface. However, if material must be inserted through the access area and remain there during the machine operation, then the protective curtain of light terminates a distance above the working surface that is just sufficient to accomo- date the material. If the material is, say, one inch plate, then the lowest beam in the curtain of light might be one and a quarter inches above the working surface. It is customary to provide means (normally not accessible to the operator), for disabling one or more light sources and respective photo- detectors at the bottom of the light curtain in order to adapt the safeguard to the machine operation for different thick- nesses of material. The gap set is, however, always fixed and it is normally at the bottom of the curtain of light. In certain machine operations, for example that of bending sheet material in a press brake, the material may project along the working surface prior to the machine oper- ation and thus extend below the lowest beam,but during the operation the projecting part of the material may angle up- wards. In prior machine safeguards this would interrupt one or more of the beams of light and, if nothing was done, stop the machine operation. To avoid this in the prior art, the safeguard curtain was shut off completely during the last part of travel of the press. Therefore the safeguard was off for the last few inches of stroke. It is a feature of the present invention to provide for a machine a safeguard appar- atus which senses the presence of the projecting material and permits the thickness to occult a predetermined number of .. . ., ; :, Case 2561 3~9~ light beams anywhere in the light curtain but nevertheless will stop the machine if additional thickness occults further light beams. Accordingly, there is provided a presence sensor safeguard apparatus comprising a plurality of light sources, a photodetector for each light source spaced from said light source and positioned to receive light from a respective light source, said light sources and photodectors defining there- between a protective curtain, means receiving a signal from each photodetector and providing a count output when the light path between a light source and respective photodetector is obstructed, and comparator means totalling the count output for a predetermined period and comparing the total with a pre- determined number representing the maximum permissible number of obstructed light paths, said comparator means providing an output signal when said totalled count output exceeds said predetermined number. The invention will be described with reference to the accompanying drawings in which ~0 Figure 1 is a front view in schematic form of a machine having a protective light curtain, Figure 2 is a side view in schematic form of the apparatus of Figure 1, Figure 3 is a side view in schematic form of apparatus similar to that of Figures 1 and 2 set up to form a bend in a plate, Figure 4 is a side view in schematic form of the apparatus in Figure 3 after the bend is formed, Figure 5 is a block schematic diagram of circuitry suitable for one form of the invention, and Figure 6 is another form of circuitry that is used in the invention. Case 2561 Rererring now to Figures 1 and 2, a protected area 10 is shown as defined by a light curtain consisting of sub- stantially parallel beams of light lla, llb, etc. between a multiple light source 12 and a multiple light receiver or detector 14. The light source 12 has a plurality of verti- cally arranged individual light sources 15a, 15b, 15c, etc. which preferably emit light in the infrared region. The light sources 15 are preferably light-emitting diodes (LED). The light receiver 14 has a plurality of vertically arranged photodetectors 16a, 16b, 16c, etc., sensitive to the light emitted by light sources 15. There is a photodetector 16 for each light source 15 positioned opposite thereto. As will be referred to again hereinafter, the light sources or LEDs 15 are preferably energized one at a time in sequence, and the corresponding photodetector means is enabled one at a time in the same sequence and at the same time, to avoid misinterpretation that might occur because a light beam 11 could be received not only by the respective photodetector 16 but by photodetectors adjacent thereto. It is possible to provide additional protection against misinterpretation and this is done in a preferred orm of the invention. The light sources 15 are not only energized in sequence, as discussed above, but each light source as it is energized is pulsed on and off a number of times, for example 64 times. The photodetector means which corresponds to the particular light source being energized, is enabled for a predetermined portion of the time the corresponding light source is energized, and the number of pulses received during that portion of time is counted. A ~0 predetermined pulse count value is selected, less than the maximum number of pulse counts that could be counted during that portion of time, and if the pulse counts exceed that Case 2561 93~9~ predetermined pulse count value the beam is considered unbro- ken and if not the beam is considered to be broken or obscured. This provides discrimination against, for example, electrical noise and reduces the chance of misinterpretation. The protected area 10 between light source 12 and light receiver 14 is shown in front of a machine tool 17 having a movable ram 18 and a work table 20 with a working surface 21. The portion of the machine tool 17 including the ram supporting structure and ram actuating means is not shown as such machines are well known and the machine itself forms no part of the present invention. Referring to Figure 3, the ram 18 has on the end thereof a working tool 22 with a substantially right angle corner that cooperates with a forming piece 23 mounted on the surface 21 of the working table 20. A piece of plate 24, which is to have a bend made in it, is inserted through the protected area to rest on forming piece 23, as shown. The plate 24 interrupts the light beams received by photodetectors 16Q and 16m as shown. As the machine operates, ram 18 descends and presses the working tool 22 against plate 24 forcing the plate 24 to bend into the forming piece 23. When the operation is completed the plate 24 has a bend substantially as depicted in Figure 4. Plate 24 interrupts the light beams as received by photodetectors 16d, 16e, and 16f. It will be apparent that the end of plate 24 which projects through the light curtain sweeps upwards as the ram 18 presses downwards. If the control circuitry is set to disable the output from photodetectors 16~ and 16m, these will not act to interrupt operation of the machine or affect the machine in any way. However, as the plate begins to sweep upwards, the machine operation would be interrupted as all the other beams are active. Now, in accordance with the invention, the control equipment can be Case 2561 "3~ set to permit, for example, any three light beams to be interrupted regardless of their location and still have the machine operate. The gap allowance of three is, of course, adjustable when setting up the equipment. Thus, as ram 18 descends and the projecting end of plate 24 sweeps upwards, different light beams are interrupted, but as long as no more than three are interrupted the machine can continue to operate. If the operator were, at any time during operation, to place his hand or arm or any other object through the curtain and interrupt light beams so that the total number interrupted was four (in this examplej, the machine would stop or reverse as required~ This provides the necessary operator protection in a simple manner. The safeguard is never muted excessively or turned off. The arrangement requires a minimum of changes in machine setting, nor restriction on operator movement, and minimum delay in production. Each light source/photodetector pair may incorporate a gain control or sensitivity control. For example, the controls may be set so that the beam is considered obstructed 0 if 50~ of the beam cross-sectional area is obscured. This might be considered an average sensitivity. The controls might be set for discrimination at 80% (insensitive) or 20 (highly sensitive). It will, of course, be apparent that the spacing of the light beams from one another, the gap allowance used, and the distance the light curtain is located from the machine's danger area, are selected so that the operator's fingertip cannot reach the pinch point (i.e. the danger area) without the broad part of his hand or arm interfering with sufficient light beams to stop the machine. Referring now to Figure 5, the outputs from photo- detectors 16a - 16p form one input to a respectlve AND gate Case 2561 ~3~ 25a - 25p. Another input signal to each AND gate comes from a control 26. The control 26 not only provides signals to AND gates 25a - 25p on conductors 27 but it provides signals on conductors 28 to energize the light emitting diodes 15 (Figures 1 and 2) in the appropriate sequence. Each time there is a signal from a particular photodetector 16 repre- senting an in~rared beam of light received, that photodetector provides a signal to its respective AND gate 25. That AND gate 25 is enabled if there is a signal from control 26 indicating the respective LED 15 was energized to provide the light received by the particular photodetector. Each of the AND gates 25a - 25p is responsive to a pair of signals at its inp~t to provide an output on a respective conductor 30 to a channel selector 31. Channel selector 31 has a switch which directs all of the input signals on conductors 30 to one of three channels. Only channel 2 is shown in Figure 5. Channels 1 and 3 may either be duplicate back-up channels or may provide some modified form of control. The output from channel selector 31 is applied over ~0 conductors 32 to a bank of "silent/normal" switches 33. There is a switch 33 for each conductor, that is for each photo- detector 16a - 16p. If there are some beams which cannot be used, perhaps because a mounting structure continuously obstructs them, then the appropriate ones of switches 33 are switched to "silent". The outputs of all switches in the "normal" position are connected via conductors 36 as inputs to OR gate 34. The outputs of all the switches in the "silent" position are connected via conductors 37 as inputs to OR gate 35. OR gate 34 has an inverter 40 which provides an inverted output, that is, gate 34 is a NOR gate. Thus, there is a signal on conductor 41 for each beam that is obstructed or obscured. Conductor 41 applies these signals to a counter- Case 2561 decoder 42 which counts the signals representing obscured beams and compares the count with a predetermined number set into the counter-decoder representing the number of beams permitted to be obscured. If the count exceeds the predetermined number a signal is applied on conductor 43 to reverse/freeze selector 44 which provides a control signal to the ram of the machine over conductor 48 to stop it (i.e. free~e it) or over conductor 47 to reverse it. A control signal is applied from control 26 over conductor 45 to counter- 1~ decoder 42 to ensure the count begins with each sequence. The OR gate 35 is connected to receive signals overthose conductors 37 where the respective switches 33 are in the "silent" position. There should not be any signals on these conductors as the switches were placed in the "silent" position because the beams were obstructed. If for some reason light is detected by the respective photodetector 16 it represents a malfunction or an unsafe condition. Therefore, if a signal should occur it is passed b,~ OR gate 35 over conductor 46 to the reverse/freeze selector 44 which provides a control signal to the machine over either conductor 47 to reverse the ram or over conductor 48 to stop the ram. Referring now to Figure 6, an alternate circuit is shown schematically which provides another feature. The switch means 33a has a third position for each of the switches. This third position is connected by conductors 50 to NOR gate 51. The output of NOR gate 51 is connected to reverse/freeze selector 44 by a conductor 52. ~ny switch in the third position, that is where a particular photodiode is connected to NOR gate 51, requires that the beam of light received by 3~ the particular photodiode be unobstructed at all times no matter what happens in the remainder of the circuitry. This may be considered as the opposite effect to having a switch Case 2561 ~3~ 33 in a position connecting a photodiode to OR gate 35. This third position is used when it is known that a particular beam should never be interrupted in the machine operation being performed at the time. It was previously mentioned that the other channels (i.e. channels one or three) could be either back-up channels or channels providing a modified form of control. For example, if the material being operated on was not to project, or if it projected but did not bend upwards during the operation, 1~ the switches 33 could be set to give the necessary protective curtain (for example one or two at the bottom might be switched to "silent" and the counter-decoder set to count all the remaining light beams). This arrangement would thus operate in a general manner similar to the prior art. Interruption of any beam operating in the "normal" condition and exceeding the preset number, would stop the machine.