Patent GB-1134074-A (GB)
Title: Light barrier
Inventor: MULLER HARRO
IPC: G01V8/26
Filed: 1966-04-15
Abstract: 1,134,074. Photo-electric machinery guards. E. SICK. 15 April, 1966, No. 16659/66. Heading F2J. [Also in Division G1] In Fig. 1 (not shown), a beam of light (12) from a source (10) is caused by a rapidly rotating polygonal mirror (13) at the focus of a fixed concave mirror (14) to repeatedly sweep across an area of a machine to be protected, a reflector (15) returning the beam via the mirrors (14, 13) and a semi-transparent mirror (16) to a silicon photo-electric device (17), the light to the device (17) being interrupted following each sweep of the beam (12) to produce a monitoring cycle during which the device (17) emits an electrical signal corresponding to zero light-a "dark pulse", and an electronic circuit (Fig. 4, not shown) connected to the device (17) gives a warning output if the light beam is interrupted by an obstruction during a protective sweep or cycle, if the dark pulse is not produced during the monitoring cycle, or if the scanning frequency deviates appreciably from a standard value. The rotating mirror (13) includes alternate reflecting and non-reflecting faces (20, 21, Fig. 2, not shown). Auxiliary mirrors (22) mounted at the boundaries between faces (20, 21) reflect light directly to the mirror (16) so that the protective and monitoring cycles are always separated by bright pulses. Also on the rotating mirror is a ring (79) having reflecting and non-reflecting zones, which reflects light on to diametrically opposed photocells (18, 19) controlling a bi-stable multivibrator 63 to provide electrical signals indicating whether a protective or a monitoring cycle is in progress. The output wave form (23) of the photo-electric device 17 is modified by an amplifier 30, a clamping stage 32, a trigger circuit 34, a differentiating element 36, and a monostable multivibrator 38, to produce an input (41) to a phase sensitive filter 42, comprising pulses (39) corresponding to the monitoring cycle "dark pulses" and, if the beam (12) is obstructed during protective cycles, additional dark pulses (40). The output (45) of the filter 42, which receives a control input from the bi-stable multivibrator 63, comprises only pulses (40) due to obstruction of the beam (12), and is smoothed and fed to two gate circuits 50, 51. The gate 51 also receives a smoothed input from the monostable multivibrator 38, via an amplifier 57, and acts on a switching amplifier 61 to energize a relay 62 to stop the machine when there are no monitoring dark pulses (39) or when obstruction warning pulses (40) are present. An output (68) of the bi-stable multivibrator 63 is fed via an oscillator 70 to a trigger circuit 72 which provides an output only when the scanning frequency, as indicated by the signal (68) is close to a standard value set by the oscillator 70. The trigger output is smoothed and provides the second input to the gate 50, which acts on a switching amplifier 77 to de-energize a relay 78 to stop the machine if obstruction warning pulses are present or if the scanning frequency deviates from the standard value. A feedback from the amplifier 61 to a transistor (81) and a resistor (82) of the trigger circuit 34 automatically controls the sensitivity of the trigger to block the effects of optical dissymmetries of the rotating mirror (13). Transistor (81) is normally conducting, to attenuate the input to the trigger circuit 34, but when pulses indicating an obstacle are received, the signal from amplifier 61 blocks transistor (81) so that the full magnitude input is applied to the trigger, this being equivalent to lowering the trigger threshold.