GB2266207A

GB2266207A - Vehicle detection system

Info

Publication number: GB2266207A
Application number: GB9307587A
Authority: GB
Inventors: Gary Patrick Stafford; Peter William Justin
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1992-04-13
Filing date: 1993-04-13
Publication date: 1993-10-20
Anticipated expiration: 2013-04-13
Also published as: GB2266207B; GB9208090D0; GB9307587D0

Abstract

A vehicle detection system for the detection of vehicles approaching, as distinct from receding from, traffic lights, employs a homodyne radar operating at 24 GHz. Separate transmitting and receiving patch antennas 1, 3 may be squinted apart slightly to produce a fairly narrow azimuth beam. The received signal is fed to two mixers 7a, 7b in quadrature to give doppler outputs, I, Q which lag or lead each other according to the target approaching or receding. Each channel signal is filtered 11a, 11b to limit the detected speed to about 5 to 70 miles/hour and then fed to further individual mixers 13a, 13b each with a quadrature local oscillator 17 at half the speed range frequency. This process produces a folding of the characteristics and on combination in a summing amplifier eliminates the 'receding' signals when the signals are summed 15. Threshold detection then indicates an approaching vehicle. <IMAGE>

Description

Vehicle Detection Vehicle detection for signalled traffic junctions has traditionally been achieved with inductive loops buried in the road surface. However, above-ground-detectors (AGD) based on microwave Doppler modules began to emerge as an alternative once they had been developed for fitting on top of temporary signals at road works.

Above-ground-detectors are attractive to road authorities because they are potentially cheaper to install with less disruption to traffic, and overcome the poor reliability of loop detectors. Loop detector reliability is a major issue in many urban traffic areas where annual failure rates can exceed 50% due to road wear and tear.

To date, only a small percentage of Pelican crossings have been equipped with AGD. These existing products all employ Gunn diode Doppler modules and rely solely upon antenna pattern for range resolution. Unit alignment is therefore critical and in practice few installations meet the objectives laid down by the Department of Transport.

According to the present invention, a vehicle detection system comprises a homodyne radar including transmitting and receiving antennas, a first pair of mixers in respective channels each mixer being fed by the receiving antennas and the transmitter oscillator but with a quadrature phase displacement between the two mixers to produce doppler output signals of equal frequency but of phase relation dependent upon a target vehicle approaching or receding from the radar, filter means in each channel to restrict the doppler signals to those from vehicles moving in a predetermined speed band relative to the radar, a second pair of mixers in the respective channels supplied with quadrature displaced local oscillator signals of frequency corresponding to an intermediate value within said speed band so that, on combination of signals output from the second pair of mixers a signal is provided corresponding to a vehicle approaching at a speed within said speed band.

The antennas are preferably of micro-stripline form comprising a feedline and a series of stubs extending on each side of the stripline alternately. The stubs may be of graded spacing along the feedline to produce an asymmetric radiation pattern.

The transmitter is preferably pulse modulated with a pulse duration sufficiently short as to exclude coincident transmitted and received pulses in respect of target vehicles beyond a predetermined range limit.

There may be provided modulation means for amplitude modulating the transmitted signal and feeding the resulting double sideband signal back into the quadrature channels as pseudo doppler signals, the modulation means being operable to test the system, preferably periodically.

The combined channel signals are preferably filtered by a digital filter.

The transmitting and receiving antennas preferably have boresights offset in the azimuth plane to reduce the effective beamwidth of the system.

A radar system in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a block diagram of the radar system; Figure 2 is a characteristic of the combined transmitter/ receiver antenna in azimuth with the boresights aligned; Figure 3 is a similar characteristic but with a 140 offset between the transmitter and receiver boresights; Figure 4 is an elevation characteristic for the combined transmitter/receiver.

Referring to Figure 1 of the drawings, the transmit antenna 1 and the receive antenna 3 are of printed microstrip form mounted on a common substrate.

Each antenna (not shown in detail) is formed as a feed line with a series of stubs on alternate sides the stubs having a fixed spacing. This arrangement gives each antenna an approximately symmetrical characteristic in azimuth and elevation. The antennas are spaced apart on the substrate approximately 10-12 centimetres and the boresights are normal to the substrate. The unit is arranged with the feed lines of the antennas in a vertical plane and in operation the unit is arranged with the antenna boresights directed downwardly to a point approximately 30-40 metres from the traffic light pole on which the radar unit would normally be mounted.

The net effect of this basic arrangement is shown (in azimuth) by the characteristic of Figure 2.

In a modified design of the antenna system, the transmit and receive antennas are 'squinted' apart at a small angle so that the two radiation patterns only partly overlap and consequently produce an effectively narrower composite characteristic. It is desirable to limit the azimuth beam width to avoid responding to movement on pavements adjacent to the carriageway of interest. Figure 3 shows the effect of a 'squint', or offset, of 14" between transmit and receive beams.

In a further modification of the basic antenna design, the elevation radiation pattern is tailored to reduce side lobes on the 'sky side' and transfer the power to the 'ground side'. This is illustrated in Figure 4 where the sky lobes are on the left and the ground lobes on the right.

The basis of the transmitter is a dielectric resonant oscillator 5 which operates at about 24 GHz.

The radar is a homodyne system, this DRO oscillator acting as both transmitter and reference source for the receiver. The receive antenna 3 is coupled to a pair of mixers 7a and 7b by a 3 dB coupler 11 so providing received signals of equal amplitude to the two mixers 7a and 7b. The other inputs to the mixers 7a and 7b are derived from the oscillator 24 as mentioned above, mixer 7a by way of a quadrature phase shifter.

The result of this quadrature phase displacement is that the two mixers provide doppler signals of equal amplitude but one or other leading in phase quadrature according to whether the original target vehicle is approaching or receding from the radar. This phase relation provides a means for discriminating between approaching and receding targets, and in particular rejecting receding targets.

The doppler signal in each channel, arising from any target having a velocity component in line with the radar, is applied to band pass filters lia, lib, which limit the frequency range to about 300 Hz to 5 kHz, corresponding to a target speed band of about 2 metres/second to 30 metres/second. This excludes the average pedestrian and vehicles travelling illegally fast. The filtered signals are then applied to low noise FET amplifiers 9a and 9b.

A phase correction between the channels is now effected by a bleed resistance 12 feeding a small proportion of one channel signal into the other channel. The quadrature phase relation is thus re-established.

As indicated above, there is as yet no discrimination between approaching and receding targets. This is effected by a further mixer stage comprising mixers 13a and 13b in the respective quadrature channels. The local oscillator signals to these mixers are derived from a clock generator 17 and are in quadrature (18). The frequency chosen for the local oscillator is about 2.8 k Hz, which corresponds to a target velocity approximately in the middle of the target speed band. One effect of this mxing process is to fold the output signal characteristic about 2.8 k Hz so that zero frequency corresponds to about 15 metres/second target velocity. Subsequent filtering of the signal is thereby simplified. At the same time the 900 shift (18) produces receding target signals in phase opposition in the two channels and approaching target signals in phase alignment.

Subsequent combination of the two signals in an amplifier 15 eliminates the unwanted receding target part of the characteristic, leaving the wanted, approaching target portion. This signal is filtered in a digital filter 19 - an 8th order elliptic filter is preferred. The filtered output is then full wave rectified (20), to produce a degree of smoothing and a single polarity signal and then fed to the detector circuitry 23. This operates on a simple amplitude threshold to determine the presence of a target vehicle approaching within the above speed band and range criterion.

Control logic 25 responds to the detector 23 and operates a relay 27 and an LED on the unit. The relay controls the traffic control lights and the LED gives an external indication of vehicle detection.

In addition to controlling the traffic system the control logic 25 operates a self-test generator 29 periodically. The transmitter oscillator 5 is controlled by a pulse driver circuit 31 which determines the transmitted pulse width (400 n Secs) and pulse repetition frequency 200 k Hz so as to limit the detected target range to the specified limit, i.e. 60 metres.

The pulse driver 31 is controlled by the self-test generator to amplitude modulate the transmitted pulse at about 1/4 kHz and produce 1.4 kHz double sidebands in the transmitted signals. These sidebands are arranged to be of such (low) power that the spillover from transmit antenna to receive antenna corresponds to an echo signal received from the smallest target of interest at the maximum range of interest. These sidebands appear as pseudo doppler signals corresponding to a target moving at about 9 metres/second.

The control logic 25 monitors the detection of this test 'vehicle' and flags up a fault if not detected. The periodic test is inhibited by the control logic in the presence of a real target detection.

The antenna, printed on the same stripline circuit board as the mixers, amplifiers and other circuitry, is designed for maximum efficiency by controlling the sideband energy. The sidelobes that would be directed at the ground are degraded (i.e. increased) and those directed at the sky have been lowered in amplitude to produce an asymmetric radiation pattern. This is achieved by creating a progressive phase shift across the array (in the elevation plane) resulting in a near cosec2 radiation pattern. This complements the amplitude of the echo signal which otherwise falls with the 5th power of the range. The echo signal thus remains approximately constant with range over the area of interest.

In the azimuth plane the transmit and receive characteristics are offset by 14" to narrow the detection region.

Figure 2 shows the azimuth echo amplitude for zero offset, i.e.

aligned, transmit and receive antennas, and Figure 3 shows the same characteristic but with the offset.

Figure 4 shows the elevation characteristic and illustrates the gradation of amplitude by progressive phasing the right hand side being downward and the left hand side upward.

The particular processing method employed, using two mixer stages, provides superior rejection of receding vehicles in heavy traffic conditions compared to existing vehicle detection systems.

Claims

1. A vehicle detection system comprising a homodyne radar including transmitting and receiving antennas, a first pair of mixers in respective channels each mixer being fed by the receiving antennas and the transmitter oscillator but with a quadrature phase displacement between the two mixers to produce doppler output signals of equal frequency but of phase relation dependent upon a target vehicle approaching or receding from the radar, filter means in each channel to restrict the doppler signals to those from vehicles moving in a predetermined speed band relative to the radar, a second pair of mixers in the respective channels supplied with quadrature displaced local oscillator signals of frequency corresponding to an intermediate valve within said speed band so that, on combination of signals output from the second pair of mixers a signal is provided corresponding to a vehicle approaching at a speed within said speed band.

2. A vehicle detection system according to Claim 1 comprising separate transmitting and receiving antennas having boresights offset in the aximuth plane to reduce the effective beamwidth of the system.

3. A vehicle detection system according to Claim 2, wherein said antennas are of microstripline form and each comprise a feedline and a series of stubs extending on each side of the stripline alternately.

4. A vehicle detection system according to Claim 3, wherein said stubs are of graded spacing along the feedline to produce an asymmetric radiation pattern.

5. A vehicle detection system according to any preceding claim, wherein the transmitter is pulse modulated with a pulse duration sufficiently short as to exclude coincident transmitted and received pulses in respect of target vehicles beyond a predetermined range limit.

6. A vehicle detection system according to any preceding claim, including modulation means for amplitude modulating the transmitted signal and feeding the resulting double sideband signal back into the quadrature channels as pseudo doppler signals, said modulation means being operable to test the system.

7. A vehicle detection system according to Claim 6, wherein said modulation means is operable periodically.

8. A vehicle detection system according to Claim 6 or Claim 7, wherein said modulation means is adapted to produce sidebands of plus and minus 1.4 kilohertz.

9 A vehicle detection system according to any preceding claim wherein the combined channel signals are filtered by a digital filter.

10. A vehicle detection system according to any preceding claim, including detector means for assessing the combined channel signals and determining whether a target vehicle is approaching within said speed band.

11. A vehicle detection system substantially as hereinbefore described with reference to the accompanying drawings.