| 摘要 |
1,020,631. Electric selective signalling. DAVY & UNITED ENGINEERING CO. Ltd. March 13, 1963 [Dec. 13, 1961], No. 44611/61. Heading G4H. [Also in Division G1] Apparatus for stopping a beam when a preselected length has passed a fixed point, e.g. a saw in a saw-mill, initiates the counting of pulses generated by the displacement when the leading end of the beam has travelled a given distance past the fixed point, the count being compared with a preselected number. Fifty photo-cell units, with one-metre spacing, each deliver a step-function output to a metres coincidence unit on being passed by the leading edge of the beam. A motor driving a roller table carrying the beam is slowed to quarter speed by this unit two metres from the coarse desired position (metres) of the beam. One metre later, the unit gates beam-displacement pulses (one pulse produced per mm. of beam travel) to a counter. The count in the counter is sent to a centimetres coincidence unit to be compared with the fine part (centimetres) of the desired position. At 20 cms, from the desired position, the centimetres coincidence unit gates pulses from the generator to a digital-to-analogue converter which produces a gradually falling output signal which is used to control the motor and reaches zero after the reception of 200 pulses (i.e. when the desired beam position has been reached). If the beam overshoots, further pulses are produced and the converter reverses the motor. The roller now rotates in the opposite direction, producing pulses (one/ mm.) which go to a second input to the converter to suitably change its output signal. In this way the desired position is approached. When the coarse component of the desired position is reached, the metres coincidence unit gates the counter contents (which should be 1000) to a checking unit which indicates to the operator whether or not the count is between 995 and 1005 and if not prevents sawing. When the beam is moved on after sawing and its trailing edge passes the first photo-cell unit the latter resets the system. The outgoing roller table section is started before the ingoing to ensure a gap of at least one metre between the two beam pieces so that the second piece may be cut accurately. The D-A converter may alternatively control the motor via a threshold device whereby the speed is constant until the error signal from the converter falls below a certain level when the motor is cut off. |
