Patent GB-782025-A (GB)
Title: Scale for material carried by a conveyor belt
IPC: G01G23/375
Filed: 1955-01-12
Abstract: 782,025. Photo-electric weighing apparatus. ROCHAR ELECTRONIQUE, [formerly ROCHAR], and ETABLISSEMENTS ELWOR. Jan. 12, 1955 [Jan. 12, 1954; Jan. 6, 1955], No. 999/55. Class 40 (3). A machine for weighing the output of material on a movable conveyer belt comprises a linkage displacing a pointer in accordance with the belt displacement, a device rotating at a speed proportional to that of the conveyer and sensing the pointer position and a photo-electric pulse generator, the arrangement being that the pulses are counted from a datum on the pointer scale until the sensing member locates the pointer position. As shown, deflection of the conveyer belt 4 positions a pointer 16 through a known linkage. The pointer is attached to and rotates with an arm 22 carrying a horseshoe magnet 23. A selsyn receiver 32 is driven by a selsyn transmitter 31 geared to the belt. The receiver drives a disc 37 which is a negative of scale 18, that is the graduations are light transmitting and an opaque portion corresponds to the neutral zone 19 on the graduated disc. Light passes through the apertures in the disc as it rotates to impinge on a photo-cell 54 and generate a series of pulses interrupted only by the portion corresponding to the ungraduated zone. Rotating with disc 37 is an arm 26 carrying coils 47 in which are generated pulses by the passage of the coil past the permanent magnet 23. In operation the passage of the neutral between the photo-electric cell and light source is used to initiate the counting of the subsequent photo-electrically produced light pulses and this counting continues until stopped by a pulse produced as coil 47 passes magnet 23. The speed of the disc is such that several rotations are made during the passage of test length past the weighing point and the meter totals are then divided. Circuit (Fig. 5),-The shaped photo-electric pulses are applied through a restoring diode 96 to the left-hand grid of a pair of cathodecoupled triodes 100 of which the left-hand anode delivers a set of negative pulses 102 and the right a set of positive 104. The positive pulses are applied through long time-constant circuit 105, 106 to the grid of triode 108 which delivers a voltage pulse to its anode only when a slot in the transparent neutral zone passes in front of the cell. The plate potential of this valve is shown at 112, the large tooth representing the passage of the neutral zone. This anode potential is unable to trigger a Schmitt trigger circuit 116 until the centre of the neutral zone passes the photo-cell. The left-hand anode voltage of the trigger is then as shown at 118 and is used to operate to an Eccles-Jordan trigger comprising valves 121. The arrangement is such that the negative pulse from the anode of valve 117 blocks the left-hand valve and, through a divider circuit 125, the left-hand valve of a cathode-coupled pair 132 thus enabling the photo-electric counting pulses 102 to pass to counter 140. When the pulse produced in coil 47 occurs the Eccles-Jordan circuit reverts to a state with the left-hand valve conducting, thus the left-hand valve of pair 132 is blocked and no further pulses are counted. Taring.-A conveyance is made for the tare of the belt by adjustment of the indicating head 17 to a value such that with no load on the belt the pointer indication never falls below zero while a correction proportional to belt length is subtracted by pulses produced by camoperated contacts 176 driven at a rate proportional to belt speed. Modifications.-The neutral zone of scale 19 might be detected by a second magnet placed there. Alternatively the magnetic arrangement may be replaced by a pair of light sources, one moving with the pointer and the other positioned adjacent the neutral zone. The rotating arm then carries a photo-electric cell.