GB2390209A - Vehicle detection using emitted radiation - Google Patents

Abstract

An apparatus to detect the presence of a vehicle comprising; a first antenna 2 capable of detecting emitted radiation and positioned in a direction towards the expected location of said vehicle; a reference antenna 1 positioned so as to be pointing away from said expected position; means 9 to compare the output signals from said antennae to give a difference value. Preferably the reference antennae is positioned to point towards the sky. Preferably also the comparison of signals is performed by comparing the output values from the antennas over a time window. The difference value may also be used to provide a threshold value to determine if a vehicle is present. The apparatus and method may also detect stationary vehicles.

Description

IMPROVEMENTS IN OR RELATING TO VEHICLE DETECTION

The present invention relates to an improved method of detecting objects, particularly metal objects such as vehicles and is more particularly 5 concerned with a radiometric system for effecting such detection.

There are many traffic control applications where reliable detection of stationary vehicles is required, the most obvious being the detection of a vehicle, or vehicles, waiting at traffic lights. Currently the most common method uses a wire loop buried in the road, the inductance of this loop is 10 changed by the presence of a large metal object, therefore, the presence or absence of a motor vehicle can be determined by measuring the inductance of the loop. The main problem with inductive loops is that, since they are buried under the road surface, installation and maintenance requires the road to be dug up. This males buried loops a very expensive method of 15 vehicle detection even though the detectors themselves are relatively low cost. Buried loops are also prone to damage.

Another popular method of vehicle detection for traffic control is Doppler radar. A radar vehicle detection system is installed above ground, usually mounted on top of the existing traffic lights. Installation costs are 20 therefore very low. However, there are two main disadvantages of Doppler radar: the Doppler effect requires motion and therefore stationery vehicles cannot be detected; and the radar transmits an RF or microwave signal, the frequency and power of which will be restricted by local regulations.

( - 2 A passive sensor that can be mounted above ground and is capable of detecting stationary vehicles would solve the problems discussed above.

The present invention aims to do this using radiometry.

It is known to use radiometry to detect thermal radiation which 5 depends on the temperature and emissivity of an object. However, reflections from other sources may also be significant. Radiometry is typically used at frequencies close to 35GHz or 95GHz because there are local minima in atmospheric attenuation centred on those frequencies.

Frequencies between these two values tend to have very high attenuation 10 and therefore cannot provide the high resolution required to produce photographic-like quality images of an object. However, components suitable for use at these frequencies are highly specialized, very expensive and difficult to obtain.

The idea of using radiometry to detect vehicles is known. UK Patent 15 GB2,358,269 describes a radiometric system to detect vehicles.

Radiometers are sensors that detect thermal radiation, this radiation depends on the temperature and emissivity of the target, though reflections from other sources may also be significant. However, in order to make the decision as to whether or not a vehicle is present the power received by the 20 radiometer is compared to some threshold. To have a fixed threshold would be impractical since a large amount of amplification of the received signal is required and the gain of these amplifiers can vary considerably due to a number of factors (such as ambient temperature).

A method of getting round this is to store a long-term average (over 25 several minutes) and to compare this with a short term average (over 1/10 second). This works well for vehicles which pass the detection zone

- 3 without stopping, but clearly if the vehicle remains in front of the antenna for a time comparable with, or greater than, the long term averaging period the detector will not incorrectly think a vehicle is not present.

Such a method is therefore unsuitable for the detection of stationary 5 vehicles.

It is an object of the invention to provide an improved method in accurately determining the presence of a vehicle which overcomes the above problems.

According to the invention is provided an apparatus to detect the 10 presence of a vehicle comprising; a first antenna capable of detecting emitted radiation and positioned in a direction towards the expected location of said vehicle; a reference antenna positioned so as to be pointing away from said expected position; means to compare the output signals from said antennae to give a difference value.

15 Preferably the reference antennae is positioned to point towards the sky. Preferably also the comparison of signals is performed by comparing the output values from the antennas over a time window.

The invention will now be described by means of example only.

20 Before a specific example is detailed, some background theory will now be

explained which may be helpful.

Any object emits electromagnetic radiation in the form of random noise. The level of this noise depends on the physical temperature of the body itself, and on the material from which it is made. If the body were a 25 perfect blackbody radiator, that is, all radiation incident on the body was

( absorbed, then the noise power from this body received by an antenna PN = kTB Equation 1 Where k = Boltzmann's constant = 1.38 x 10-23 5 T = physical temperature of body (degrees Kelvin) B = bandwidth of antenna At a given frequency all materials have a characteristic emissivity, reflectivity and transmissivity, given the symbols ú, p and respectively.

The emissivity of a blackbody would be equal to one. However, most 10 bodies do not absorb perfectly; these are called grey bodies. For a grey body, E + P + r = I Equation 2 If, as in most cases, the body is homogeneous and is several skin depths thick, the effects of transmission can be ignored, and Equation 2 is 15 reduced to: p = 1 Equation 3 If a grey body reflects the sky, its noise brightness temperature is related to the ernissivity and reflectively of its material by the following expression: 20 To = sT pTskyo Equation 4 Where To = object brightness temperature = emissivity of object p - reflectivity of object Tskyey brightness temperature of the sky and is dependent on 25 angle of incidence.

The noise temperature of an antenna can be calculated using the integral below.

TA - 41; J^To(u)G(u)dQ Equation 5 5 4 Where TA antenna noise temperature G = antenna gain as a function of direction TO = object noise temperature as a function of direction u = direction vector 10 Q = solid angle If the Object fills the antenna beamwidth (ie., To is constant) this integral can be reduced to: TA = TO Equation 6 Combining equations 1, 4 and 6, the noise power received by an 15 antenna, the beam width of which is filled by one object of constant TO which reflects the sky, is therefore; PN = kB(6T + pits) Equation 7 ie., the power received by the antenna is a function of the emissivity of the objects in front of the antenna.

20 The emissivity of materials from which roads are constructed differs greatly from that of metals; so the detection of cars should be possible, see table below.

- 6 Material Emissivity Heavy Vegetation 0.93 Dry Grass 0.91 Dry Snow 0. 88 Asphalt 0.83 Concrete 0.76 Metal O Table 1 - Material Emissivities The invention will now be described by way of example only and 5 with reference to Figure 1.

Figure I shows a preferred embodiment of the invention and includes a reference antenna 1, and a detection antenna 2. The reference antenna is positioned so as to be pointing up to the sky and the detection antenna to an area of a road when the presence of vehicles are to be 10 detected. The antennae are connected by a SPDT switch 3 (that switches between the two) to a pair of amplifiers 4 and diode detector 5 to a second SPDT 6 switch which switches either to a reference integrator 7 or detection integrator 8. These integrators integrate the amplifier signal readings from the respective antennas.

15 The values of the output of the integrator are compared by feeding them with a differential amplifier 9. This gives a measure of the difference in signal strengths. The output is then fed to a threshold detector 10 for

- 7 comparison so that a signal from the threshold detector is only given if the difference is above a set threshold.

A suitable stable reference signal could be achieved by pointing the reference antenna at the sky. Other reference sources may be possible, 5 other than the sky.

The device used to switch between the two antennas may be a mechanical, MEMS or PIN diode switch. The switch used to alternate between the two integrators would most likely be a solid state analogue switch IC, but other methods such as bipolar transistors, FETs or relays 10 could be used.

Claims (8)

f - 8 CLAIMS

1. An apparatus to detect the presence of a vehicle comprising; (a) a first antenna capable of detecting emitted radiation and positioned in a direction towards the expected location of said 5 vehicle; (b) a reference antenna positioned so as to be pointing away from said expected position; (c) means to compare the output signals from said antennae to give a difference value.

2. An apparatus as claimed in claim l wherein said reference antennae is positioned to point towards the sky.

3. An apparatus as claimed in any preceding claim wherein said 15 difference value is obtained by comparing the antennae signals over a finite time period.

4. A method to detect the presence of a vehicle comprising: (a) providing a first antenna capable of detecting emitted 20 radiation and positioned in a direction towards the expected location of said vehicle; (b) providing a reference antenna positioned so as to be pointing away from said expected position; (d) comparing the output signals from said antennae to provide a 25 difference value.

( - 9 -

5. A method as claimed in claim 4, wherein said reference antennae is positioned to point towards the sky.

6. A method as claimed in claims 4 or S wherein said vehicle is 5 stationary.

7. A method as claimed in any of claims 4 to 6 wherein said difference value is obtained by comparing the antennae signal magnitude over a time window.

8. A method as claimed in claims 4 to 7 wherein said difference value is compounded to a pre-set threshold to determine the presence of the vehicles.