GB2203876A - Vehicle detector - Google Patents

Abstract

A vehicle detector suitable for controlling traffic lights is provided for installation in a position above the surface of a road and comprises a first infrared sensor 4 arranged to produce a first output signal when a vehicle is present in a first location furthest from the installation position, a second infrared sensor 5 arranged to produce a second signal when a vehicle is in either of second and third locations nearer to the installation position than the first location, and direction discriminating means 11 for producing an output signal indicative of the detection of a vehicle travelling towards the installation position when the first output signal is provided and then the second output signal is provided within a predetermined time. <IMAGE>

Description

Vehicle Detector This invention relates to vehicle detectors and provides a vehicle detector capable of detecting a vehicle moving in a particular direction. It has particular, though not exclusive, utility in vehicle detectors for traffic controls, such as traffic lights.

Many kinds of detector are known for detecting vehicles for traffic control purposes: flexible pressure tubes or piezo-electric sensors embedded in the surface of the road are known and inductive loop and microwave vehicle detectors are also widely used. It has also been proposed to employ optical gate arrangements using light transmitters and receivers to detect vehicles. All of these devices suffer the disadvantage that they are relatively expensive to install and are difficult to adjust to attain reliable operation.

It has also been proposed in DE-A-3142978 to detect vehicles moving into or out of a measurement field using a pyroelectric detector responsive to the characteristic radiation of a vehicle.

This device is incapable of discriminating the direction of a vehicle and is unsuited to traffic control applications.

Viewed from one aspect the invention provides a vehicle detector comprising radiation detector means arranged to detect radiation from at least two spaced-apart locations and to produce respective signals when a vehicle is present in either of said locations, and including discriminating means for producing an output signal indicative of the detection of the vehicle travelling in a predetermined direction when a predetermined one of said signals first occurs and then the other occurs within a predetermined time.

The radiation detector means- preferably comprises a passive infrared sensor responsive to infrared radiation emitted by the vehicle, such as from a car radiator or headlamps. No other source of radiation is required with such a sensor, which results in a considerable reduction in the difficulties of installation and alignment associated with prior proposals. Preferably two separate infrared sensors are used, each provided with a lens to render it responsive to a respective one of the two spacedapart locations. Such a detector means may be installed in an elevated location, such as at the top of a traffic light standard, so as to be responsive to radiation emanating from two locations spaced apart along a road.

Particularly for the control of traffic lights, the detector means may be arranged to detect radiation from three spaced-apart locations and the apparatus could then be arranged to produce an output signal when a vehicle is detected in the furthest location from the radiation detector means and then in one of the nearer locations within the predetermined time. Clearly three separate signals could be provided from three sensors and an output provided when the first and second or first and third signals occur within the predetermined time. Preferably, however, two sensors are provided with respective lens systems so that one is responsive to radiation from a furthest location and the other is responsive to radiation from either of two nearer locations so that it produces the same signal when a vehicle is in either of the two nearer locations.The apparatus need then merely detect the occurrence of a signal from the first sensor followed by a signal from the second sensor within the predetermined time. Such an arrangement provides a very simple and effective vehicle detector which complies with the standard requirements for traffic light vehicle detectors.

Thus viewed from another aspect the invention provides a vehicle detector for installation in an installation position above the surface of a road, comprising a first infrared sensor arranged to produce a first output signal when a vehicle is present in a first location furthest from the installation position, a second infrared sensor arranged to produce the second signal when a vehicle is in either of second and third locations nearer to the installation position than the first location, and discriminating means for producing an output signal indicative of the detection of a vehicle travelling towards the installation position when the first output signal is provided and then the second output signal is provided within a predetermined time.

The first location may be of the order of 34 to 40m from the installation position; the second and third locations may be of the order of 24m and 14m from the installation position respectively; and the predetermined time of the order of 1.5 seconds.

This allows the reliable detection of vehicles travelling at any speed normally encountered in practice.

Each infrared sensor preferably comprises a dual element pyroelectric sensor with the two elements responsive to radiation from respective spaced-apart positibns within each location. In this arrangement only one element at a time is affected by a vehicle whereas both elements are affected by variations in the ambient radiation. This enables extraneous radiation to be discriminated and improves the reliability of vehicle detection. Each sensor is preferably provided with an optical filter, e.g. arranged to pass a band of wavelengths from 6.5 to 14 microns.

This reduces the possibility of errors due to sunlight, fog, rain or other environmental factors. preferably the output of each radiation sensor passes through a bandpass amplifier having a narrow low frequency bandpass characteristic whereby errors due to changing ambient temperature, air movements and radio frequency interference may be reduced to an insignificant level. A 3 dB bandwidth ranging from about 0.2 to 5Hz may be suitable, and this also ensures that only vehicles moving at speeds between 8 and 160 Kph are detected.

Preferably each sensor is provided with a Fresnel lens arranged to direct radiation from the associated location or locations into the sensor. Specifically, the first sensor may be provided with a Fresnel lens to direct radiation from the first location to the first sensor and the second sensor may be provided with a Fresnel lens which directs radiation from the second location to the second sensor and from the third location to the second sensor. For example, a lower portion of the latter Fresnel lens may direct light from the nearest location to the second sensor and an upper portion from the middle one of the three locations.

The discriminating means preferably includes two bistable circuits, the first bistable being arranged to be set when a first signal is produced indicative of a vehicle in the first location and the second bistable being arranged to be set when a second signal is produced indicative of a vehicle in the second location, and connected such that setting of the first bistable enables the discriminating means to produce an output if a said second signal occurs within the predetermined time and the second bistable being arranged to disable the response of the first bistable to the first signal. A separate timer circuit may be provided which is arranged to commence its timing on the occurrence of either signal and to re-set the bistable devices when its timing period expires.If desired a circuit may be provided to reset the bistables a short time, e.g. 0.5 seconds, after the detection of a vehicle. A suitably programmed microprocessor could be employed as the discriminating means.

When the vehicle detector of the invention is embodied as a traffic light controller it may be provided in addition with a device for detecting the presence of a stationary vehicle at the stop line.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings.

Figure 1 is a block diagram of a vehicle detector according to the invention; Figures 2a and 2b show respectively a plan view and an electrical equivalent circuit of a sensor; Figure 3 is a schematic view showing the arrangement of lenses used with the sensors of the detector of Figure 1; Figure 4 is an illustration of the locations on a road surface sensed by the radiation sensors of the detector of Figure 1; Figure 5 is an electrical schematic diagram of a window comparator circuit of the detector of Figure 1; and Figure 6 is an electrical schematic diagram of a direction discriminating circuit of the detector of Figure 1.

Referring to the drawings, Figure 1 shows the main components of a vehicle detector according to the invention for a traffic signal control system.

The detector may be installed in a position above the road surface near a traffic light and may for example be mounted on a traffic light standard.

The detector comprises first and second radiation detector means in the form of infrared sensors 2 and 3. First sensor 2 is provided with an optical system 4 so that radiation from a first location furthest from the detector falls on the sensor 2.

The second sensor 3 is provided with an optical system 5 arranged to direct infrared radiation from second and third locations onto the second sensor 3. The second location is nearer to the detector than the first location and the third location is nearer to the detector than the second location. For example, the first, second and third locations may be 34 to 40 m, 24 m and 14 m from the -detector respectively.

The optical systems 4 and 5 may each comprise a Fresnel lens as deescribed below with reference to Figure 3.

When a vehicle 6 moves into one of the first, second or third locations, the corresponding sensor 2 or 3 produces a positive or negative output signal as described below with reference to Figures 2a and 2b. The output signals pass through respective band pass amplifiers 7 and 8 to respective window comparators 9 and 10. The bandpass amplifiers each comprise two cascaded operational amplifier stages which provide a gain of 64dB over a 3dB bandwidth ranging from 0.2 to 5Hz. The window comparators will be described in detail below with reference to Figure 5. They provide an output signal when a positive or negative input signal is received of an amplitude deemed to be sufficient to indicate the detection of a vehicle.

The outputs of the window comparators 9 and 10 are supplied to a direction discriminating circuit 11 (to be described with reference to Figure 6) and to a central signal processing unit 12. The output of the direction discriminating circuit 11 is also supplied to the central signal processing unit 12.

The central signal processing unit 12 processes the outputs of the window comparators 9 and 10 and direction discriminating circuit 11 to provide the required traffic signal control signals as defined by the applicable standard specifications. These signals are transmitted to a traffic signal installation controller 13 via a relay or optoisolator output 14.

The traffic signal installation controller 13 may transmit a remote request for a sensor test to the central signal processing unit 12. This activates a sensor test signal generator 15 to cause infrared sources 16 and 17 to supply modulated test radiation to the sensors 2 and 3 representing the radiation emission from a standard saloon car travelling towards the detector at a speed of 48 kph at a range of 40 m.

The traffic signal installation controller 13 then checks for the return of an appropriate vehicle detection signal.

The detector 1 also includes regulated power supplies 18 which may be powered by either a mains supply or a low voltage d.c (e.g. battery) supply 19.

In operation the direction discriminating circuit 11 detects the occurrence of first an acceptable output from window comparator 9 followed within a predetermined time by an output from window comparator 10. If this occurs, an indication of the detection of a vehicle travelling towards the detector is supplied to the central processing unit 12.

Figure 2a is a view of a sensor 2 looking into the window of the sensor, which may be provided with a daylight filter to block all wavelengths except about 6.5 to 14 microns. The sensor comprises two polarized ceramic sensing elements 21 and 22 with opposite polarities facing the window as shown.

The sensing elements 21 and 22 are connected in opposition as shown in Figure 2b and since they are equally responsive to changes in ambient radiation no output is provided as a result of such changes. The equivalent circuit of a sensor as shown in Figure 2b includes a non-linear network 23 connected across the series combination of the two sensing elements and a matching field effect transistor 24 having a gate terminal connected to sensor 21. The field effect transistor 24 provides either a positive or negative output signal depending upon which element 21 or 22 is irradiated. The field effect transistor 24 of sensor 2 is connected to bandpass amplifier 7 via a suitable biasing network and a low noise single transistor 4.8 X gain preamplifying stage (not shown).

Figure 3 shows Fresnel lenses 31 and 32 provided for the sensors 2 and 3 respectively. The lenses are circular low diffraction types and are Positioned one above the other, as of course are the sensors 2 and 3. The upper lens 31 is long range lens arranged to direct radiation from a first location about 34 to 40m from the detector into the sensor 2. The lower lens 32 is b combination of a medium range lens 33 formed in the upper portion thereof and directed at a second location at around 24m from the detector, and a short range lens 34 in the lower portion directed at a third location about 14m from the detector.

By appropriate masking of segments of both lenses the lateral extent of the detection zone of each lens can be controlled so as to cover one, two or three traffic lanes, for example.

Figure 4 shows the first location 41 covered by the first sensor 2 and the second location 42 and third location 43 covered by the second sensor 3. A vehicle travelling from left to right in Figure 4 will produce first a positive output from sensor 2 and then a negative output as radiation falls successively on elements 22 and 21. Similarly, sensor 3 will produce a positive output, a negative output, a positive output and a negative output as indicated by the symbols B+ and B- in Figure 4. The direction discriminating circuit 11 responds to the occurrence of either a positive or negative output from the first sensor 2 followed by either a positive or negative output from the second sensor 3 with a predetermined time to produce a vehicle detection output signal.

The window comparators 9 and 10 are shown in Figure 5. A potential divider consisting of resistors 51, 52 and 53 defines potentials at points 54 and 55 centred around the output bias level of the bandpass amplifiers 7 and 8 and offset therefrom by a sufficient amount to provide a safe margin between a genuine signal response to a moving vehicle and other background noise, such as sensor and amplifier noise or external environmental interference. Point 54 is connected to the non-inverting inputs of comparators 56 and 57 and point 55 is connected to the inverting inputs of comparators 58 and 59. The output of bandpass amplifier 7 is connected to the inverting input of comparator 56 and to the non-inverting input of comparator 58. The output of bandpass amplifier 8 is connected to the inverting input of comparator 57 and to the non-inverting input of comparator 59.Diodes 60 connect the outputs of comparators 56 and 58 to output terminal 61 and diodes 62 connect the outputs of comparators 57 and 59 to output terminal 63. When the output of bandpass amplifier 7 is sufficiently high, the output of comparator 56 goes low and the upper diode 60 conducts thereby making output terminal 61 low. The output of comparator 58 is high and so the lower diode 60 does not conduct. Similarly, if the output of bandpass amplifier 7 is sufficiently low the output of comparator 58 goes low causing output terminal 61 to go low and the output of comparator 56 remains high and this output is blocked from the output terminal 61. Thus the diodes 60 effectively provide a wired-OR function whereby the output terminal 61 goes low when either a high or low input signal is provided from bandpass amplifier 7.Window comparator 10 operates similarly so that output terminal 63 goes low when either a high or low input signal is provided from bandpass amplifier 8.

Turning now to Figure 6, the direction discriminating circuit 11 will be described in detail. Output signals from terminals 6-1 and 63 of the window comparators 9 and 10 are supplied to input terminals 65 and 66 of the direction discriminating circuit 11 respectively.

As mentioned above, these signals take the form of a negative-going pulse of a duration substantially equal to the time a vehicle remains in the field of view of a sensor 2 or 3. The main components of the circuit are flip-flops 67 and 68, to which the respective input signals are supplied via logic elements, an output flip-flop 69 and a timer reset circuit 70. The flip-flops each comprise a pair of cross-coupled NAND gates. NAND gates 71 and 72 connected to the respective input terminals 65 and 66 are arranged as inverters and provide positive-going pulses whenever a sensor output signal is provided. The outputs of gates 71 and 72 are coupled respectively to NAND gate logic elements 73 and 74 and both are connected to inputs of NOR gate 75.The inverted output pin 10 of flip-flop 68 is connected to the other input pin 1 of gate 73 and the inverted output pin 10 of flip-flop 67 is connected to the other input pin 1 of gate 74.

Assuming the flip-flops are all initially reset, a logical 0 appears at each output pin 11 thereof and a logical 1 at each inverted output pin 10'. Thus in the absence of a sensor signal pulse all of the inputs of gates 73 and 74 are low and the outputs thereof high. If a negative pulse appears at terminal 66 the output of gate 74 provides a negative pulse to set flip-flop 68.

A low output from pin 10 of flip-flop 68 is thus supplied to input pin 1 of gate 73 to disable gate 73. In this condition any subsequent pulse arriving at input terminal 65 has no effect on flip-flop 67. The positive pulse at the output of gate 72 is also supplied to timer circuit 70 which provides a reset signal on line 76 to reset all the flipflops if no further pulses are received in a predetermined time, e.g. 1.5 seconds.

If, on the other hand, a pulse first arrives at input terminal 65, the timer 70 is again started and flip-flop 67 is set. The upper output of flipflop 67 enables NAND gate 77 and the lower output disables gate 74 so that flip-flop 68 is rendered unresponsive to pulses received at terminal 66.

Should a pulse be received at terminal 66 within the predetermined time, it is inverted by gate 72 and passes through gate 77 to set output flipflop 69. A continuing output is then provided from output terminal 78 to the central signal processing unit 12, until the flip-flops are all reset.

Timer circuit 70 includes an integrated circuit timer 80. The output of NOR gate 75 is connected to the trigger input thereof and via a differentiating capacitor C1 to the reset input thereof. Capacitor C2 is a frequency compensation component and resistor R3 and capacitor C3 define the timing period of the circuit (which is substantially 1.1 R3 C3).

The output of gate 75 is a negative-going pulse for the duration of the input pulse at terminal 65 or 66. Capacitor C1 provides a very brief negativegoing pulse at the fallinq leading edge of the output' pulse of gate 75. The output at pin 3 of timer 80 is high whilst it is timing and becomes low at the end of the timing period. If another input pulse is received during the timing period of the timer, the output briefly goes low and high again as the timer is reset and retriggered. Capacitor C4 and NAND gates 81 and 82 prevent this spurious negative-going pulse from providing a reset signal to line 76: capacitor C4 provides a brief negativegoing pulse which is inverted by gate 81. However, at this time a negative input pulse is present at the upper input terminal of NAND gate 82 and so this gate is disabled.When the timer 80 reaches the end of its timing period there is unlikely to be a low signal at the output of gate 75 and so gate 82 is enabled. The falling edge of the output of timer 80 is differentiated by capacitor C4 to provide a brief negative-going pulse. This is inverted by gate 81 and again by gate 82 to provide a negative pulse on reset line 76 to reset all three flip-flops 67, 68 and 69.

If desired a further reset circuit 85 may be provided to reset the flip-flops a predetermined time, e.g. 0.5 seconds, after a vehicle is detected.

Normally a capacitor C6 is discharged but when an output is provided to terminal 78 it begins to charge. Eventually the output of gate 86 will fall and capacitor C5 will provide a negative-going pulse to reset line 76.

If a number of vehicles pass the detector in rapid succession, i.e. within 1.5 seconds, the timer 70 is continually reset and does not time out. Thus a continuous high output remains at output terminal 78. After a quiet period without detecting the presence of a vehicle in any of the three locations for 1.5 seconds, the direction discriminating circuit 11 is reset to enable it to detect another vehicle. This is recognised by an output from the first sensor 2 followed within 1.5 seconds by an output from sensor 3.

Thus it may be seen that the invention provides a possible direct replacement for conventional vehicle detectors, such as induction loop and microwave arrangements and offers discrete and multiple lane coverage, vehicle direction discrimination and high false detection immunity. Other attractive features include simple installation, low maintenance and low power consumption.

Claims (13)

1. A vehicle detector comprising radiation detector means arranged to detect radiation from at least two spaced-apart locations and to produce respective signals when a vehicle is present in either of said locations, and including discriminating means for producing an output signal indicative of the detection of the vehicle travelling in a predetermined direction when a predetermined one of said signals first occurs and then the other occurs within a predetermined time.

2. A vehicle detector as claimed in claim 1 wherein said radiation detector means comprises at least one passive infrared sensor responsive to infrared radiation emitted by said vehicle.

3. A vehicle detector as claimed in claim 1 or 2 wherein said radiation detector means comprises two separate infrared sensors, each provided with a lens system to render it responsive to a respective one of said spaced apart locations.

4. A vehicle detector as claimed in any preceding claim wherein said detector means are arranged to detect radiation from three spaced-apart locations and said vehicle detector means is arranged to produce said output signal when a vehicle is detected in the furthest location from said radiation detector means and then in one of the nearer locations within said predetermined time.

5. A vehicle detector as claimed in claim 4, including two sensors each provided with respective lens systems so that one is responsive to radiation from the furthest location and the other is responsive to radiation from either of the two nearer locations so that it produces the same signal when a vehicle is in either of said two nearer locations.

6. A vehicle detector for installation in an installation position above the surface of a road, comprising a first infrared sensor arranged to produce a first output signal when a vehicle is present in a first location furthest from said installation position, a second infrared sensor arranged to produce a second signal when a vehicle is in either of second and third locations nearer to the installation position than said first location, and discriminating means for producing an output signal indicative of the detection of a vehicle travelling towards the installation position when the first output signal is provided and then the second output signal is provided within a predetermined time.

7. A vehicle detector as claimed in claim 6 wherein each of said infrared sensors comprises a dual element pyroelectric sensor'with the two elements responsive to radiation from respective spaced-apart positions within each location.

8. A vehicle detector as claimed in claim 6 or 7 wherein each sensor is provided with an optical filter.

9. A vehicle detector as claimed in claim 6, 7 or 8 wherein the output of each radiation sensor passes through a bandpass amplifier having a narrow low frequency bandpass characteristic.

10. A vehicle detector as claimed in any of claims 6 to 9 wherein each sensor is provided with a Fresnel lens arranged to direct radiation from said associated location or locations into said sensor.

11. A vehicle detector as claimed in any of claims 6 to 10 wherein said discriminating means includes two bistable circuits, the first bistable being arranged to be set when a first signal is produced indicative of a vehicle in the first location and the second bistable being arranged to be set when a second signal is produced indicative of a vehicle in the second or third locations, and connected such that setting of the first bistable enables the discriminating means to produce an output if a said second signal occurs within the predetermined time and the second bistable being arranged to disable the response of the first bistable to the first signal.

12. A vehicle detector as claimed in claim 11 wherein a separate timer circuit is provided which is arranged to commence its timing on the occurrence of either of said signals and to re-set said bistables when its timing period expires.

13. A vehicle detector substantially as hereinbefore described with reference to the accompanying drawings.