Low power traffic monitoring radar apparatus

Abstract

A vehicle detecting radar apparatus, the radar apparatus comprising: a transmitter 201 which generates a modulated burst of microwave radiation for transmission by an antenna over an area, and a recei

Description

LOW POWER TRAFFIC MONITORING RADAR APPARATUS This invention relates to a low power vehicle detecting radar apparatus, in particular suitable for use with portable applications like temporary traffic lights and remote applications like an illuminated display to provide speed warnings to motorists. The invention may be used in other applications where low power is required. In many traffic applications there is a requirement for a low power traffic monitoring radar. Such applications include speed warning signs and traffic data collection. Figure 1 shows a typical application of a radar apparatus being used to activate a speed warning sign. In this application the radar is usually quite simple and only measures the speed of a vehicle. This type of radar is known as a Doppler radar. This type of radar does not measure the range of a vehicle and often in these applications the amplitude of the radar return signal is used to give an indication of range, which is not reliable. In this application the output of the radar apparatus may cause the sign to be illuminated when an approaching vehicle 4 has a speed, S, that is greater than the speed limit for the road or some other predetermined speed. A typical system can therefore be broken down in to three main elements; a power supply 1, a controllably illuminated sign 2, and a radar apparatus 3. The radar apparatus outputs a signal, typically an relay, opto or serial UART link, which is fed to an input of the sign. The sign includes a signal processor which is configured to illuminate one or more parts of the display dependent on the value of the signal fed from the radar apparatus. These parts may comprise an image, a word or phrase such as SLOW DOWN or a numerical representation of the speed in MPH or KPH, or some combination of these. In many applications there is often no mains power supply and the radar apparatus and sign must rely on a battery power supply that may be topped up from other low power sources such as a solar panel or wind turbine. It is therefore beneficial to minimize the power consumption of the apparatus during use to maximize the battery life. Typically a traffic monitoring radar for temporary traffic lights or speed signs only measure speed. Figure 7 shows a typical diagram of such a radar. To improve the battery life it is known for traffic monitoring radar apparatus that the power consumption may be reduced by alternately enabling and disabling their microwave oscillator 105 following a fixed pattern. The oscillator often has an enable line 111 for enabling and disabling it. When disabled the apparatus turns off the radars oscillator which effectively removes the signal to the transmit antenna 101 and removes the local oscillator signal from the radar’s receiver mixer 110. By enabling the oscillator very infrequently (for example every 0.1 to 4 seconds), the current draw on the battery is reduced but these radar schemes can suffer from poor response times. A vehicle that approaches the radar apparatus at the start of the disabled period will not be detected until the radar is next enabled. This invention describes techniques for further reducing power consumption by the use of a radar that can measure range and speed of a target simultaneously, such a radar could be a frequency modulated continuous wave, FMCW, radar. The range and speed measurements are used to further reduce the number of radar measurements required and hence reduce the amount of on time of the radar oscillator, while maintaining good response times. According to a first aspect the invention provides a vehicle detecting radar apparatus, the radar apparatus comprising: A transmitter which generates burst of frequency modulated microwave radiation for transmission by an antenna over an area, and A receiver which receives a receive signal corresponding to a portion of the transmitted radiation that has been reflected from a vehicular target, the receiver outputting a signal that is the received signal multiplied by the transmitted signal and low pass filtered, and a processing circuit which receives the signals output from the mixer and processes at least one received burst to obtain a measurement of at least two characteristics of the target that includes the range and speed of the target, characterized in that the processing circuit is configured to actively manage the timings of subsequent measurements according to one or more of the measured characteristics of the target or targets detected. The processing circuit may actively manage the timings of measurements by varying the time between bursts of radiation that are transmitted and processed by the processing circuit, according to one or more of the measured characteristics of the target, or both. The radar apparatus may comprise a local oscillator that outputs a microwave frequency signal that is fed to both the transmitter and receiver and the processing circuit may actively manage the timing of subsequent measurements by selectively enabling and disabling the oscillator and actively varying the time which the oscillator is disabled relative to the time it is enabled. The apparatus is arranged such that with the oscillator disabled no measurements are made and no signals are processed by the processing circuit. By making the time between radar measurements dependent on the characteristic of the vehicular target it is possible to vary the power consumption of the traffic monitoring radar apparatus and in some cases within the scope of this invention to reduce the power consumption whilst maintain the level of performance of the apparatus by optimizing the time between measurements. For instance, in many cases the host system is only interested in targets within a certain range. The radar can now use the measured range of a target to decide whether a target is within the desired detection zone. If it is then the time between measurements is decreased, but if not then the time between measurements is kept higher resulting in a power saving. The characteristics may be user definable, and may be set according to the application in which the radar apparatus is to be used. In one preferred arrangement, if no target vehicle is detected within a predefined threshold range Rp an increased elapsed time may be set for a subsequent measurement compared to the elapsed time that is set if the target is within the threshold range. The threshold range Ry may be chosen to be a value that is less than the maximum detection range of the radar apparatus Ryax. The apparatus may check on each measurement if a target vehicle is within the threshold range and if not may maintain the increased elapsed time between measurements. If a target is detected within the threshold range, the elapsed time may be reduced. The apparatus may apply one of two elapsed times between measurements; one which is relatively short to be used for the measurement immediately after no vehicle is detected within the threshold distance, and another one which is relatively long which is to be for the measurement immediately after a vehicle is detected within the threshold distance . In an alternative to the two elapsed times, the processing circuit may set a variable elapsed time that is a function of the range of the target, which may be infinitely variable between an upper and lower time. The apparatus may include a memory for storing an elapsed time value. The threshold distance Rr may be pre-set or user programmable and the apparatus may include a user interface enabling the threshold range Ry to be selected. The threshold distance may correspond to a threshold distance beyond which the sign is not to be illuminated. When the vehicle range is beyond this, the processing circuit may increase the time between radar measurements. Within this range, the time between radar measurements may be decreased. Therefore, the processing circuit may be configured to actively manage the radar such that radar measurements are taken more frequently once a target is detected that is within a threshold range Rg and less frequently where no target is within the range Rg is detected. The radar apparatus may be configured to output a trigger signal suitable for feeding to an input of an illuminated speed warning sign or other device. It may additionally or alternatively output the measured vehicle speed for a target or any other characteristic that has been determined by the processing circuit. The transmitter and receiver may share a common antenna with signals being routed by a circulator. Alternatively each may be associated with a respective antenna as shown in figure 5. The radar apparatus may use a frequency modulated continuous wave technique to measure the speed and range of a target. Such a radar is shown in figure 5. The radar apparatus may be portable and may include a battery power source. The battery may comprise a rechargeable battery and may be recharged from a portable wind turbine or a portable solar panel or solar array. According to a second aspect the invention provides a radar activated speed warning apparatus comprising: An illuminated display having an input for a trigger signal, and A radar apparatus according to the first aspect that includes an output signal compatible with the input of the illuminated sign. Additionally, the radar measures the speed of targets relative to the radar apparatus. This may be determined as well as range of a target to make the decision. Where the radar apparatus is used to trigger a sign, for example, as the sign commonly only needs to be illuminated when there is a vehicle travelling faster than the road speed limit a reduction in power may be achieved when a vehicle is determined to be travelling through the radar detection zone below that speed. ‘Where the speed is below a threshold speed S or the range is greater than the threshold distance Ry the time between radar measurements may be increased. The threshold distance Rg and threshold speed S may be stored in a memory. The sign when illuminated may display a fixed message, such as SLOW DOWN, or may display a variable message, such as a speed in MPH or KPH. The apparatus may include a battery power source. The apparatus may include a means for securing the radar apparatus to an item of roadside furniture such as a post of gantry. According to a third aspect the invention provides a method of operating a radar apparatus comprising emitting a burst of microwave radiation over an area using a transmitter, receiving from at least one receive antenna a portion of the transmitted radiation that has been reflected from a vehicular target, the receiver outputting a signal that is the multiplication of the received signal with the transmit signal and low pass filtered, processing the received signal corresponding to a burst to measure at least two characteristics of the target that includes the range and speed of the target, and characterized by the steps of actively managing the time of subsequent measurements according to two or more of the measured characteristics of the target. The elapsed time between radar measurements may be calculated by the processing circuit from the target detection information measured by the radar. To begin with the radar apparatus may make measurements with a separation time of X. When a target is detected and its range is less than Ry then from its speed a time Y is calculated for the target to travel a distance of d metres:- Y=d / S d is chosen to be a suitable distance for the target to travel before another update is required. The time Y is now used as the time until the next measurement. In this way the radar only measures the target when it has moved a significant distance of d metres. ‘Where more than one target is detected, a time Y value may be calculated for each valid target and the minimum Y value used for the radar measurement gap time. There will now be described, by way of example only, three embodiments that fall within the scope of the present invention with reference to the accompanying drawings of which: Figure 1 is a plan view of a typical application for a vehicle detecting radar apparatus that triggers a warning sign; Figure 2 is a flowchart that sets out a first algorithm for actively controlling the measurement timing of the radar based on the detected range of any targets measured by the radar; Figure 3 is a flowchart of a generalised version of the algorithm of Figure 2 based on detected characteristics of the target vehicle; Figure 4 is a flowchart of an alternative scheme that actively controls the timings of radar measurements based on the speeds of targets detected by the radar; Figure 5 is a block diagram showing the key components of a radar apparatus within the scope of the first aspect of the invention; and Figure 6 shows the combination of the radar apparatus of Figure 5 with a power supply and a warning sign. Figure 7 is a block diagram showing the key components of a typical radar apparatus that is used for traffic monitoring. The radar shown only measures the speed of targets and not range. Figure 5 shows a vehicle detecting radar apparatus 100 according to the present invention. A transmit antenna 201 is driven by a voltage controlled oscillator, VCO, 205 whose modulating input is connected to the output of the waveform generator 203. The generator 203 generates an asymmetric triangular waveform in order that the VCO produces a frequency ramped waveform as used in many frequency modulated continuous wave, FMCW, radars. The VCO 205 also has a enable line that is used to enable or disable the oscillator. The VCO 205 also feeds a signal to the mixer 210 that is replica of the signal fed to the transmit antenna 201. This signal is used to downconvert the received radar return signals from the receive antenna 202 to I and Q baseband signals using a mixer 210. These baseband signals are typically filtered and then amplified by the amplifiers 206 and then converted to the digital domain using analogue to digital converters 207,ADC. The digitized I and Q signals are input to the signal processor 208 for further digital signal processing to obtain target range and speed information. As will be described the processing circuit is configured to actively manage the timings of subsequent measurements according to one or more of the measured characteristics of the target. In the example of Figure 5 this is accomplished by the processing circuit outputting a signal 211 that enables or disables the oscillator to control the timing of the microwave bursts that are transmitted. When disabled the power consumption of the radar apparatus is reduced, but at a cost that no radar measurements can be made during the disabled time. The processing circuit also generates a trigger signal 209 encoding the speed of the detected vehicle which is fed to an output. This may alternatively simply encode whether the speed is above or below a trigger threshold speed S, or if the vehicle is within a threshold range Rg, or combination of speed S and threshold range Rg. The values of S and Rr may be stored in an area of electronic memory within the processing circuit, and may be set using a user operable interface (not shown). The whole circuit as described may be contained in a single housing which may be secured to a pole at a roadside or on an overhead gantry or other structure. In the first embodiment shown in Figure 6 the radar apparatus 100 is paired with an illuminating sign 200 and a portable power supply 300 such as a battery. The battery is shown separate from the radar apparatus and sign but could be integrated into either or both the radar and sign. The trigger signal output of the radar apparatus 100 is used to trigger the vehicle speed warning sign 200 to warn approaching vehicles that they are speeding or to inform them of their speed. As shown the trigger signal is sent over an communication link. The requirements of the installation of the sign 200 are that it is only illuminated if a vehicle is approaching at a speed above a set threshold speed S. The sign also will not need to be illuminated unless a vehicle is within a range Ry of the sign. In use of the exemplary radar apparatus 100 of Figure 5, the processing circuit of the radar will make a measurement and determine the ranges and speeds of potential targets. In the application only targets with a range less than Ry are of interest. Therefore if any of the targets have a range less than Ry the radar waits a short time Y between each radar measurement for at least N measurements. If after N measurements have been made and no valid target has been detected in any of the radar measurements the interval between radar measurements is increased to X. The power saving the radar makes is made by this waiting the longer time X, when no targets of interest are present, instead of time Y before the next radar measurement. The algorithm flow diagram for this is shown in Figure 2. In a second embodiment of this invention the radar instead of using just the range to decide on whether to wait time X or time Y before the next radar measurement, it uses speed and range of a target to make the decision. For a speed sign application this makes perfect sense as the sign only needs to be illuminated when there is a vehicle travelling faster than the road speed limit. Figure 3 shows the flow diagram for this algorithm where the range test has been replaced with ‘does a target meet the match criteria’. The match criteria could be any combination of tests on the range and speed measurements made by the radar to suit a specific application. For radar target tracking to be successful the radar measurement rate needs to be high enough that target detections can be associated with track information. In a third embodiment of this invention the time between measurements is calculated from the radars track information. To begin with the radar makes measurements with a separation time of X. When a target is detected and its range is less than Ry then from its speed a time Y is calculated for the target to travel a distance of d metres: - Y=d / S d is chosen to be a suitable distance for the target to travel before another update is required. The time Y is now used as the time until the next measurement. In this way the radar only measures the target when it has moved a significant distance of d metres. Figure 4 shows the algorithm for controlling the radar measurements in this case. In all three exemplary embodiments it can be seen that the radar apparatus actively manages the function of the oscillator dependent on one or more characteristics of the target vehicle. This invention reduces the power consumption of a radar apparatus by actively managing the number of radar measurements made so that the time between measurements is greater when the apparatus has determined that it is acceptable to do so. This in turn reduces the amount of processing the radar performs and reduces the amount of on time for the microwave oscillator and associated circuitry.

Claims

Claims Ls A vehicle detecting radar apparatus, the radar apparatus comprising: a transmitter which generates a modulated burst of microwave radiation for transmission by an antenna over an area, and a receiver which receives a receive signal corresponding to a portion of the transmitted radiation that has been reflected from a vehicular target, the receiver outputting baseband I and Q signals, and analogue to digital converters that convert the baseband I and Q signals to the digital domain, and a processing circuit which receives the digitized I and Q signals from the analogue to digital converters and processes at a burst length of data to obtain target range and speed data, characterized in that the processing circuit is configured to actively manage the timings of subsequent measurements according to one or more of the radar measured characteristics of the target.

2. A vehicle detecting radar apparatus according to claim 1 in which the processing circuit actively manages the timings of measurements by varying the time at which bursts of radiation are transmitted. 3 A vehicle detecting radar apparatus according to claim 1 or claim 2 in which the processing circuit actively manages the timings of measurements by varying the time at which signals output from the receive antenna are processed by the processing circuit.

4. A vehicle detecting radar apparatus according to any preceding claim in which the radar apparatus comprises a local oscillator that outputs a microwave frequency signal that is fed to both the transmitter and receiver and the processing circuit actively manages the timing of subsequent measurements by selectively enabling and disabling the oscillator and actively varying the time which the oscillator is disabled relative to the time it is enabled.

5. A vehicle detecting radar apparatus according to any preceding claim in which the characteristics used to control the radar timings are user definable according to the application in which the radar apparatus is to be used.

6. A vehicle detecting radar apparatus according to any preceding claim in which the processing circuit is configured so such in the event that no target vehicle is detected within a predefined threshold range Ry an increased elapsed time is set for a subsequent measurement compared to the elapsed time that is set if the target is within the threshold range.

7. A vehicle detecting radar apparatus according to any preceding claim in which the processing circuit is configured such that in the event that a target vehicle is detected within a predefined threshold range Rg the elapsed time to a subsequent measurement is reduced.

8. A vehicle detecting radar apparatus according to any preceding claim in which at least one radar measured characteristic comprises the speed S of the target relative to the radar apparatus.

9. A vehicle detecting radar apparatus according to claim 8 in which the processing circuit is configured such that where the speed is below a threshold speed and the range is greater than a threshold distance Rr the time between radar measurements is actively increased.

10. A vehicle detecting radar apparatus according to claim 8 or claim 9 in which the processing circuit is configured so such that the elapsed time between measurements is calculated from track information of the target.

11. A vehicle detecting radar apparatus according to any preceding claim in which the processing circuit is configured to output a signal suitable for feeding to an input of an illuminated speed warning sign or other device.

12. A radar activated speed warning apparatus comprising: an illuminated display having an input for a signal, and a radar apparatus according to any preceding claim that includes an output signal compatible with the input of the illuminated sign.

13. A method of operating a radar apparatus comprising emitting a plurality of bursts of microwave radiation over an area using a transmitter, receiving from at least one receive antenna a portion of the transmitted radiation that has been reflected from a vehicular target, the receiver outputting a signal each time a reflected portion of a burst is received, processing at least one received signal corresponding to a burst to measure at least one characteristic of the target that includes the range of the target, and characterized by the steps of actively managing the timing of subsequent measurements according to one or more of the measured characteristics of the target or targets.

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