GB1499371A - Doppler effect kinemometers - Google Patents

Abstract

1499371 CW Doppler radar SOC DE FABRICATION D'INSTRUMENTS DE MESURE SFIM 12 March 1975 [17 July 1974] 10395/75 Heading H4D In a CW vehicle speed measuring Doppler radar, the durations of one or more cycles of the Doppler frequency signal are measured and a signal internally generated having the same frequency as said Doppler frequency, utilizing said measurements. This allows the use of the Doppler frequency for speed and distance travelled measurements after the actual Doppler signal has stopped due to the vehicle leaving the radar beam. By comparing the durations of successive cycles of the actual Doppler signal, any instability (e.g. due to the presence of two vehicles in the beam) in the signal can be detected and the regeneration of the Doppler frequency inhibited. A 10 GHz CW Doppler radar head 10, Fig. 1 (possibly bidirectional to give the sense of the vehicle movement) produces a possibly unstable, noise spoiled Doppler signal. A threshold, "squarer" circuit 14, produces a square wave FD synchronous with the cross-overs of the Doppler signal and removing the noise. The first 50 of the Doppler square wave pulses Fd are produced when the vehicle first enters the beam and are not reliable. A divider/counter 16 determines the arrival of the 50th pulse and causes a trigger circuit 18 to produce a start of analysis pulse MP1. The Doppler pulses FD are fed via a clock and sync. circuit 50 to an analysis circuit 46 where three sequences of analysis are carried out under the control of a circuit 48 fed with pulses FD. The following pulses FD are arranged into groups of ten by "by ten" frequency divider counter 20 and trigger circuit 22. The groups often pulses are marked by pulse MP2. In analysis circuit 46, the three sequences S 1 , S2, S 3 are: S I Using a clock frequency fr from circuit 50 the duration in terms of a number of clock pulses of the 51st pulse is determined. A new clock frequency is then produced from fr/<SP>K</SP>/ n , e.g. fr.<SP>100</SP>/ K , for the remaining pulses. S2 Durations (i.e. number of clock pulses) of the next 10 pulses are determined using the new frequency. If durations are a 1 , a 2 ... a 10 and these vary too much, an invalidating signal is produced. The durations are also progressively summed to give This gives the average value of the pulses' durations. S3 Sequence S2 is repeated for the following groups of 10 pulses to give averages A2, A3 &c. Too much change in the averages again produces an invalidating signal. If K/ n = X, then a 1 = fr/ xfd and, A1= 10fr/ x .<SP>1</SP>/ FD . A programmable divider in the circuit 46 therefore produces, from the clock frequency, a signal <SP>10fr</SP>/ XA1 , or FD i.e. the regenerated Doppler signal. The regenerated Doppler signal is fed to speed measuring circuit 26 where marking pulse MP2 produces a gate of predetermined duration T from circuit 28. The speed is then determined in counter 30 as the number of cycles of the regenerated Doppler signal occurring in the gate T. A threshold device 34 determines when the speed is excessive and part enables an AND gate 44. A counter 40 receiving the regenerated Doppler signal and started by an "end-of FD" signal, calculates a delay sufficient for the speeding vehicle to travel 15 metres past the radar, and completes the enablement of the gate 44 whereby a camera photographs the vehicle. The speed is displayed at 32.