GB1371173A - Coordinate measuring apparatus - Google Patents

Abstract

1371173 Acoustic position-finder M S INSTRUMENTS Ltd 19 Jan 1973 [19 Jan 1972] 2477/72 Heading H4D [Also in Division F3] Apparatus for determining the co-ordinates of a pressure-wave-emitting object (e.g. a bullet) in a plane includes two elongated members arranged in the plane and with a given angle between them, a pair of transducers on each member, the transducers of each pair being spaced along the member and responsive to a vibration travelling along the member and initiated at a point on the member by a pressurewave from the object, and a circuit for timing the interval between the arrival of the vibration at the two transducers of each pair. In the target embodiment shown in Fig. 1, transducers 3, 4, 5, 6 are on orthogonal rods 1, 2 and detect rod vibrations induced by impingement of a bullet's circular shock-front on the rods. If the impingement point is displaced from the rod centre the vibration arrival time difference for transducer pair 3, 4 or 5, 6 is measured digitally by a respective timing circuit 8 or 9 which also gives an indication of which of the two associated transducers first received the vibration. The measurement is compensated for vibration velocity changes with temperature (sensed at 10, 11), and may be recorded on an XY plotter. Each transducer is coupled to the (remote) timing circuit through a similar processing circuit, detailed in Fig. 2. Thus the output of transducer 3 is fed via a high-pass (>10kHz) amplifier 13 and threshold detector 17 to trigger a monostable 22 whose output is converted at 31 to positive and negative versions for driving a transmission line. The line receiver (not shown) receives these versions in a differential amplifier whose output pulse is steered by a logic circuit to either the start or stop input, as appropriate, of the timing circuit 8. The use of dual-polarity versions enables in-phase noise pick-up on the line to be rejected. To prevent re-triggering of monostable 22 while the rod vibration is decaying, a lock-out monostable 26 is provided. A calibration facility comprises a push-button 47 operating a pulse generator 44 for energizing transducers 3 and 6 as transmitters, resulting vibrations being received by transducers 4 and 5 and their propagation times along the rods being registered by the circuits 8, 9. In modifications rods 1 and 2 may cross each other, and a third rod othogonal to the plane may be added, together with associated transducers and timing circuitry, for obtaining a three-dimensional measure of the bullet's path. If several targets are in use, the output of a transducer associated with a particular firearm can be used to enable the apparatus of a particular target only when that firearm is fired.