Patent GB-931527-A (GB)
Title: Apparatus for sensing of hot moving material
IPC: B21B38/00
Filed: 1961-02-08
Abstract: 931,527. Photo-electric control devices. M. C. PFISTER. Feb. 8, 1961 [Feb. 20, 1960], No. 4657/61. Class 40 (3). In apparatus for signifying the presence or absence of a material in a zone through which the material is moved longitudinally and which is wider than the width of the material, a radiation focusing device repeatedly scans the zone in a direction transverse to the direction of movement of the material and focuses radiation emanating from the material on to a radiation sensitive cell once during each scanning movement, and an electrical receiver connected between the cell and a switching device responds to the presence in the output of the cell of electrical impulses, each significant of the incidence of focused radiation thereon, by switching the switching device from a first state to a second state so signifying the presence of material in the zone and from which the switching device is arranged to be returned to the first state by the absence of the electrical pulses in the output from the cell. A prior art statement refers to known apparatus in which a lead sulphide cell is followed by an electrical differentiating network. The signal from the apparatus may be used to operate a deflector or shears, or to initiate an operation in an electronically controlled rolling mill. The invention is stated to obviate false operation due to transient departures of the material from the field of view and due to the presence of regions in the material of low radiation level (" black " regions). In the embodiment, a sensing head comprises a revolving reflector 2 having concave side walls each forming a mirror to focus radiation emitted by horizontallytravelling hot wire 5 on to a photo-conductive cell 4; air or water cooling fluid is led through pipe 6. The cell impulses are applied to a cathode-follower 8, in the sensing head, which feeds a pre-amplifier 11 and thence a valve 15 ; a capacitor 16 charges through a resistor 17 and each time valve 15 conducts the capacitor is discharged. Thus each positive pulse at the input of valve 15 produces the same sawtooth wave shape irrespective of pulse amplitude and duration within wide limits. The sawtooth waveform is coupled to diode-connected triode 22 via a capacitor 21, and each time the diode cuts off a constant charging current flows to capacitor 21 through resistor 33 to generate a positive bias voltage which is applied to the control grid of a pentode 23; the control grid is also connected via resistor 33 to a voltage derived from a rectifier bridge 31. It is arranged that only small increases in positive bias, during flow of intermittent charging current through capacitor 21, are sufficient to increase anode current sufficiently in valve 23 to cause operation of a relay 24; a resistor 38 is thereby introduced into the screen grid circuit to so reduce anode current than any further fall thereof, caused by removal of bias generated across resistor 33, ensures de-energization of the relay. A signal lamp 27 is thereby deenergized and a control circuit to terminal 102 is made. The polygonal mirror may be replaced by a vibrating mirror or a lens drum.