GB1408521A - Apparatus for determining the length of a moving object - Google Patents

Abstract

1408521 Measuring the length of moving objects DSO RESPROM 25 April 1973 [27 April 1972] 19699/73 Heading G1A [Also in Division G4] The invention relates to the photo-electric measurement of the length of a moving object (e.g. vehicle) and subsequently determining whether the length is greater or less than one or more reference values. The length is measured by the ratio of the time taken for the front of the vehicle to traverse the distance between two photo-cells and the time for the whole vehicle to pass one photocell. Two separate light sources may be used Fig.1 (not shown) or a single source 8 and a beamsplitter 9 with two photo-cells 10, Fig.3. The circuit arrangement is symmetrical to accept vehicles 21 travelling in either direction, and comprises a photo-cell output detector 11 arranged to set a flip-flop 13 (e.g. in the top channel) as vehicle 21 obscures the first beam. The flip-flop 13 is reset by the signal from the lower channel (as the vehicle enters the second beam) resulting in a pulse of width t x at the output of gate 14. This triggers monostable 16 to enable gate 15 to generate a pulse of width tk from the lower channel signal. These pulses are fed to a comparator circuit 17 where the ratio t k :t x (representing the length of the vehicle in terms of s the effective distance between the light beams), is compared with a reference value. A plurality of reference values may be provided, so that the vehicle can be classified into one of a number of length ranges. As shown the comparator outputs are fed to a device 18 to operate counters 19. Alternatively the pulse-width-signals may be converted to a digital form before comparison.