GB2264411A - Active infrared vehicle detector system - Google Patents

Abstract

The system comprises an IR transmitter and receiver, mounted above a road and pointing substantially downwards. The IR is modulated at a frequency f1 +/- f2 and the received signal is heterodyned with a signal of frequency f1 so as to enable phase comparison at a frequency f2 to take place. The phase difference between the transmitted and received signals gives the range to the road or vehicle underneath, and hence an indication of whether or not a vehicle is present The variation of range with time may be combined with vehicle speed to give vehicle profile and hence vehicle type. Separate transmitter - receiver arrangements may be provided for each lane of the road. <IMAGE>

Description

ACTIVE INFRARED DETECTOR SYSTEM This invention relates to vehicle detection systems.

Some known vehicle detector systems, comprise a buried loop sensor which is used for detecting the presence of vehicles and their passage along a road. A buried loop has the disadvantage that it is positioned beneath the road surface and consequently, it is expensive both to install and to repair or replace if it fails. With repair or replacement, there is an expense associated with the costs of closing off road lanes as well as the direct costs of labour etc. Moreover, the likelihood of a failure occurring is high, because of the great stresses on the road surface which are caused particularly by heavy goods vehicles accelerating or decelerating near locations at which the sensors are placed.

A known alternative to the buried loop type sensor is based on infrared detection. In systems of this kind as currently deployed, infrared beams are radiated from a unit mounted on a lamp standard or similar post onto the road surface, so that each lane is covered as effectively as with the buried loop system. A typical known system relies on radiated infrared light scattered from passing vehicles being collected by a set of photodetectors. The amplitudes of the electronic signals from the photodetectors are proportional to the received optical signal powers. Subsequent processing of these signals enables changes in returned signal caused by the passage of a vehicle to be detected, thus registering the presence or passage of the vehicle.

The main advantages of this known infrared sensor system are that no part of it has to be buried in the road, and also that the illuminated area can be made small, thereby enabling higher spatial resolution to be achieved (if required).

There are however several deficiencies in such a known system, such as for example; (i) It relies upon there being a detectable change in returned scattered infrared signal when a vehicle moves into the beam(s), which does not always occur because in some cases the combination of change in range from the sensor unit and change in surface reflectivity could mean that a net detectable change is not produced in practice, although most vehicles will produce some change as the bonnet, windscreen, roof, rear window and boot pass through the beam.

(ii) However, as a result of (i), the system is normally designed to measure rates of change in signal level, rather than measuring absolute signal levels, and this means that it cannot reliably detect the presence of a stationary vehicle, and is vulnerable to changes in background illumination (see also (iii) below). In addition, the varying reflectivity of a vehicle as it passes can result in multiple counts being registered.

(iii) Changes in illumination that are detected by the system may well be caused by changes in the sun's illumination of the road surface or of the vehicle standing within the beam's detection zone. Although the speed of these changes may be fairly slow in comparison with those due to a passing car, the very large magnitudes of such changes can nevertheless make them readily detectable and a source of detection errors.

It is an object of the present invention to provide an improved infrared system in which the aforesaid disadvantages are obviated at least in part.

According to the present invention a vehicle detector system comprises an infrared (IR) radiator mounted above a vehicle carriageway, roadway or the like, so as to illuminate the carriageway and or a vehicle when positioned therebeneath, signal generator means arranged to drive the IR radiator with a modulated electrical transmission signal thereby to produce corresponding modulated IR radiation, an IR detector positioned to receive reflected modulated IR radiation from surfaces illuminated by the radiator, a receiver responsive to the IR detector for producing an electrical received signal corresponding to the modulated reflected IR energy received by the said detector, and phase detector means operative to produce an output signal indicative of the presence of a vehicle on the carriageway when a predetermined phase relationship obtains between the transmission signal and the received signal.

IR radiation is used, rather than visible radiation, in order to reduce the effect of sunlight on noise in the detector circuit.

It will be appreciated that the phase of the reflected received signal modulation relative to the phase of the transmitted signal modulation will vary in dependence upon the effective path length travelled by the infrared radiation between the radiator and the detector and thus it will be apparent that since the path length will be shortened when a vehicle is present beneath the radiator a corresponding detectable phase change will be produced.

Energy focusing means may preferably be provided to focus IR radiation from the radiator onto the carriageway, or onto a vehicle when present and to focus reflected IR radiation onto the IR detector.

The radiator may comprise a light emitting diode (LED) and the detector may comprise a photo-diode.

The energy focusing means may comprise lens means operatively associated with the radiator and further lens means operatively associated with the detector.

The signal generator may comprise a master oscillator having a frequency fl, a frequency synthesiser responsive to the master oscillator for producing the modulated electrical transmission signal at a frequency f3 and a reference signal at a frequency f2, such that f3 = fl + or - f2, and the phase detector means may comprise a first mixer responsive to the received signal and to a signal from the signal generator at the frequency fl for producing an intermediate frequency (IF) signal at the frequency f2, and a second mixer responsive to the reference signal and to the IF signal for producing a resultant signal characteristic of the phase difference between the reference signal and the IF signal, from which resultant signal the output signal indicative of the presence or absence of a vehicle is derived.

The first mixer may be arranged to feed the second mixer via a low pass filter amplifier and a signal squarer.

The second mixer may be arranged to feed a further low pass filter and a threshold detector thereby to produce the said output signal.

The signal generator means may be arranged to feed a plurality of IR radiators each arranged to illuminate a predetermined region of carriageway and each operatively associated with an IR detector, each of which detectors being operatively associated with a phase detector means thereby to produce an output signal indicative of the presence or absence of a vehicle in one of the said regions of the carriageway.

One embodiment of the invention will now described by way of example only with reference to the accompanying drawing in which; FIGURE 1 is a generally schematic circuit/block diagram of a vehicle detection system and wherein; FIGURE 2 is a graph illustrating operation of one part of the circuit shown in Figure 1.

Referring now to Figure 1, a vehicle detection system comprises a master oscillator 1 for producing a frequency at fl which is arranged to feed a frequency synthesiser 2. The frequency synthesiser 2 provides a reference signal on a line 3 at a frequency of f2 and an input signal on a line 4 at a frequency of f3 for a light emitting diode (LED) driver unit 5. The frequencies are chosen such that f3 = fl + or - f2. The driver unit 5 is arranged to feed three infrared radiators defined by light emitting diodes 6a, 6b and 6c.

Light from the diode 6a is directed onto a lens 7a shown schematically which serves to focus infrared radiation from the diode 6a onto the surface of a roadway 8. Only one lens assembly (for the diode 6a) is shown in Figure 1, but it will be appreciated that a lens assembly will be provided for each of the diodes 6a, 6b and 6c, so that each diode illuminates a different portion of the roadway 8. Light reflected from the roadway and originating from the diode 6a is focused by means of a further lens 7b and directed onto a diode detector 9a tuned to be receptive to the infrared radiation from the diode 6a modulated at frequency f3. Two further tuned photo-diodes 9b and 9c are provided responsive respectively to radiation from the LED's 6b and 6c and although not shown in Figure 1, each of the photo-diodes 9b and 9c has associated with it a lens similar to the lens 7b.Signals from the photo-diode 9a are fed via a line 10 to a detector circuit shown schematically as being carried on a card 23 The detector circuit comprises an amplifier 11 which is arranged to feed a first mixer/multiplier 12. The mixer 12 is fed on a line 13 with a frequency fl from the frequency synthesiser 2. Output signals from the mixer 12 are fed via a further amplifier 14 to a low pass filter 15 which selects a lower side band signal corresponding to the frequency difference between f3 and fl which corresponds to the frequency f2. A signal from the low pass filter 15 is fed to a signal squarer 16 which is arranged to feed a second mixer 17. The second mixer 17 is fed via the line 3 with the reference signal at the frequency f2.Output signals from the further mixer 17 are fed via a further low pass filter 18 to a threshold detector 19 which is arranged to feed a signal processor 20. The second mixer 17 and the further low pass filter 18 serve in combination to perform the function of a phase detector and accordingly provide a signal for the threshold detector 19 which is proportional to the phase difference between the reference signal on the line 3 at the frequency f2 and the intermediate frequency at the frequency f2 fed to the further mixer 17 from the signal squarer 16.

In practice the master oscillator 1 may be used to produce the frequency fl at 5MHz and then conveniently the reference frequency f2 may be chosen to be 5KHz such that the frequency f3 is 4995KHz.

It will be apparent that the phase difference between the reference frequency f2 on the line 3 and the signal fed from the signal squarer 16 to the second mixer 17 which is also at a frequency of f2 will be determined in dependence upon the distance of the roadway 8 from the LED's 6a, 6b and 6c and the photo-diodes 9a, 9b and 9c. Thus it will be appreciated that if a car is positioned on the roadway such that the path length as travelled from the relevant LED 6a, for example, to the corresponding photo-diode 9a, for example, will be shortened and that therefore a corresponding phase change will be detected between the reference signal on the line 3 and the output signal from the signal squarer 16.

The frequency of modulation of the radiated IR illumination is chosen such that the phase change produced by the presence or absence of a vehicle does not exceed 1800 whereby ambiguities are avoided. By utilising an intermediate frequency, some of the signal processing required can be effected at a lower frequency which permits the use of less expensive components than might otherwise be required for high frequency processing. It will be appreciated that although the circuit carried by the card 23 which is fed from the tuned photo-diode 9a only, has hereinbefore been described, the tuned photo-diodes 9b and 9c will be feeding similar detector circuits carried by cards 21 and 22 respectively and moreover it will be appreciated also that the photo-diodes 9a, 9b and 9c will each be responsive to infrared radiation reflected from different portions of the roadway 8.

By using a signal threshold detector 19, traffic detection reliability is significantly improved since noise levels associated with reflection from the roadway can be arranged to be below a detection threshold and therefore ignored. Thus, referring to Figure 2, in operation of the circuit shown in Figure 1, an output signal from the low pass filter 18 will be produced which is proportional to the height of vehicles above the roadway.

The threshold level is set at Vt corresponding to a height above ground level of Ht. Thus, it will be appreciated that signals below this level will be ignored. Because of the effects of noise, signals from the ground and signals from vehicles will tend to fluctuate in amplitude. It will therefore be appreciated that the setting of the threshold level is chosen to give good spurious ground signal rejection whilst ensuring reliable vehicle detection. Thus, if for example, the threshold voltage is set to correspond to say 2 feet, vehicle detection reliability will increase progressively for target heights above 2 feet, until at target heights of say 4 feet and above, excellent detection reliability is achieved.

The system as hereinbefore described uses the attributes of infrared illumination mentioned above, i.e. no need to be buried, highly flexible and well-defined, small illumination area. It overcomes the deficiencies of known amplitude dependent detection systems by measuring the range from the unit to the road or the vehicle in its field of view. Therefore, as long as there is sufficient returned signal to achieve this measurement, the precise amount of light scattered back is of no consequence. The device can be designed for manufacture at a price which is compatible with the needs of traffic engineering and similar markets.

The measurement of range may be achieved by using the infrared beam as a carrier of modulation with a fundamental wavelength of around 100 to 30 metres (3 to 10 MHz), and as can be seen from the drawings, each infrared channel consists of a light emitting diode (LED) driven from a common signal (f3). The infrared emission from the LED may be collected by an optical arrangement consisting of one or more lenses, which focuses the radiation into a well defined area in the vicinity of the road surface. The focussing range of the beam is dependent upon the design trade-off between signal return from the road surface and from passing vehicles. The signal returns are also dependent upon the way in which the transmitter and receiver fields of view overlap, and again trade-offs are available.

Detailed operation will now be considered. The radiation returned from the target area and received by the photo detectors 9a, 9b and 9c, which may be silicon or other semiconductor are converted to an RF narrow band electronic signal. This narrow bandwidth RF signal is applied to a tuned circuit (not shown) connected to shunt each photo detector, which has the effect of allowing the direct and slowly varying currents from the detector to flow to earth, whilst presenting a high impedance to the RF. The selected RF signal is passed to the amplifier 11 and then to the RF mixer 15. The mixer also has the signal (fl) applied to it offset from f3 by the frequency f2, which is at a much lower frequency (typically 5KHz).

The phase of the RF signal, which is dependent upon the range of the target surface is preserved throughout this mixing process, and so the range (and variations in it) can be determined by any suitable phase detector It may be necessary to set up the relative phases of signals fl, f2 and f3 by means of the frequency synthesiser 2 such that the second mixer 17 operates over its unambiguous range for the full range of suitable vehicle heights.

Amplification and bandpass filtering then takes place at this 'intermediate frequency', followed by limiting (i.e. driving the signal to the voltage rails and thereby squaring up the waveform). In this way, information on the amplitude of the signal is lost, leaving only the phase information.

The intermediate frequency at f2 is then mixed with the reference f2 derived from the master oscillator 1, and the output is low pass filtered in the filter 18. The threshold circuit shown after the filter is used to set the range at which the presence of a vehicle is detected. Typically, this will correspond to a range mid-way between the road and a typical vehicle height. If appropriate, the subsequent processing can control the threshold circuit, either to adapt to changing conditions or to enable software control of threshold height to be set up on installation.

An attribute of this system is that, in the absence of vehicles, it is continuously measuring the range to the road surface. This allows the subsequent (probably low power digital) circuitry to adapt to any drifts in phase in the circuits, perhaps due to temperature changes, and thereby ensures that the threshold level can remain fixed, once it is set up.

To ensure that in a wide range of conditions the signal from the road surface is detectable, it would be beneficial to apply reflective or retroreflective paint to the road surface in the vicinity of the illuminated areas. A useful improvement would be achievable with the white paint currently used for road markings. Alternatively, a surface application that is reflective mainly in the near-infrared region would avoid the need for markings that are visible to the road user.

As with known infrared sensors using only amplitude detection, it is possible to set up multiple beams and detection channels (as shown) each of which is used to monitor a defined part of the road.

By this means, a vehicle lane can be fully covered. A common source of modulation frequencies is provided in this example, although separate receivers and electronics must be used. The outputs are then combined logically in the subsequent processing stages, to provide the required vehicle detection presence/passage information.

A further use of the Infrared Detector as described herein may be as part of a vehicle classification system. For this purpose a system according to the invention would be used in combination with a vehicle speed measuring device to enable the variation of a vehicle's height along its length to be determined, thus giving its characteristic profile for identification purposes as may be required for a traffic survey for example.

Claims (9)

CLAIMS:

1. A vehicle detector system comprises an infrared (IR) radiator mounted above a vehicle carriageway, roadway or the like, so as to illuminate the carriageway and or a vehicle when positioned therebeneath, signal generator means arranged to drive the IR radiator with a modulated electrical transmission signal thereby to produce corresponding modulated IR radiation, an IR detector positioned to receive reflected modulated IR radiation from surfaces illuminated by the radiator, a receiver responsive to the IR detector for producing an electrical received signal corresponding to the modulated reflected IR energy received by the said detector, and phase detector means operative to produce an output signal indicative of the presence of a vehicle on the carriageway when a predetermined phase relationship obtains between the transmission signal and the received signal.

2. A vehicle detection system as claimed in Claim 1, wherein energy focusing means are provided to focus IR radiation from the radiator onto the carriageway, or onto a vehicle when present and to focus reflected IR radiation onto the IR detector.

3. A system as claimed in Claim 1 or Claim 2, wherein the radiator comprises a light emitting diode (LED) and the detector comprises a photo-diode.

4. A system as claimed in Claim 2 and Claim 3, wherein the energy focusing means comprises lens means operatively associated with the radiator and further lens means operatively associated with the detector.

5. A system as claimed in any preceding claim, wherein the signal generator comprises a master oscillator having a frequency fl, a frequency synthesiser responsive to the master oscillator for producing the modulated electrical transmission signal at a frequency f3 and a reference signal at a frequency f2, such that f3 = fl + or - f2, and the phase detector means comprises a first mixer responsive to the received signal and to a signal from the signal generator at the frequency fl for producing an intermediate frequency (IF) signal at the frequency f2, and a second mixer responsive to the reference signal and to the IF signal for producing a resultant signal characteristic of the phase difference between the reference signal and the IF signal, from which resultant signal the output signal indicative of the presence or absence of a vehicle is derived.

6. A system as claimed in Claim 5, wherein the first mixer is arranged to feed the second mixer via a low pass filter and a signal squarer.

7. A system as claimed in Claim 6, wherein the second mixer is arranged to feed a further low pass filter and a signal limiter thereby to produce the said output signal.

8. A system as claimed in Claim 7, wherein the signal generator means is arranged to feed a plurality of IR radiators each arranged to illuminate a predetermined region of carriageway and each operatively associated with an IR detector, each of which detectors being operatively associated with a phase detector means thereby to produce an output signal indicative of the presence or absence of a vehicle in each of the said regions of the carriageway.

9. A system as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.