GB1096651A - Control apparatus and method - Google Patents

Abstract

1096651 Electric selective signalling DAVY & UNITED ENGINEERING CO Ltd 18 Jan 1965 [17 Jan 1964] 2098/64 Headings G4D D7X D6Y D6C1 D6C2 D6L D1D11 and D1D9G Apparatus for monitoring the position of an object comprises at least two detectors arranged to detect the passage of the object, a pulse generator for emitting a pulse for each unit movement of the object, a counter for counting the pulses, and means for checking the number of pulses emitted by the generator between passage of the object past successive detectors and for correcting the count of the counter if said number does not correspond to the distance between successive detectors. Thirty-five photo-cells ("detectors") spaced at one-metre intervals along the path of a beam in a saw mill provide a coarse indication of the beam position which is compared with the coarse part of the desired position in a metres coincidence unit. When the coarse difference is 9 metres, a roller is lowered into contact with the beam and actuates the pulse generator to produce a pulse for each 2 mm. travel of the beam, on a forward or reverse line depending on the direction of travel. When the coarse difference is 7 metres, the forward pulses are fed to a counter the count of which is compared to the fine part (cm.) of the desired position in a centimetres coincidence unit. When the latter indicates that the position is exactly 7 metres from the desired position, the forward pulses are gated to the counter referred to in the first sentence above. This counter controls the motors driving the roller tables supporting the beam, via a digitalto-analogue converter, so as to bring the beam to rest at the desired position. The pulses on the reverse line from the pulse generator are always fed to a subtract input of the last-mentioned counter without hindrance. Possible slip of the roller is corrected for using two checking units alternately. Each checking unit comprises a counter which receives the forward pulses starting at the time of arrival (via the metre coincidence unit) of a signal from one of the photocells. When the count reaches 500 a first flip-flop is set. When the signal from the next photo-cell arrives a second flip-flop is set. When both have been set, input to the counter is terminated and the counter and flip-flops are reset. In the absence of roller slip, the two flip-flops will be set simultaneously, but otherwise for as long as one flip-flop is set and the other is not, additional pulses (obtained by delaying the forward pulses) are fed to the add or subtract input of the counter controlling the D/A converter, depending on which of the flip-flops is set. The first of the photo-cells resets the system on detecting the trailing edge of the beam.