Traffic sensor and method for providing a stabilized signal

[0001] The present invention relates generally to road motion sensors and in particular relates to a road motion sensor and a method of providing a stabilized signal.

BACKGROUND OF THE INVENTION [0002] As urban centers are increasing and traffic is becoming more common, the need for accurate and current traffic information is also increasing. Road traffic supervision relies primarily on road motion sensors, such as road motion sensors with induction loops, which are installed in the road surface. Alternatively, vision sensors can also be used to obtain traffic information.

[0003] The installation, replacement and repair of underground sensors with induction loops is expensive due to the required road works. In addition, such road works also cause disruption to traffic. On the other hand, vision sensors are cheaper, but have other disadvantages, such as being unable to work in the dark or in weather conditions that impede visibility, e.g. fog or snow.

[0004] To overcome these drawbacks, radar sensors have been used to obtain traffic information. Radar sensors typically send low-power microwave signals at the location of motion and detect vehicles based on reflected signals. Radar detectors are generally cheaper than road motion detectors with induction loops and, unlike vision sensors, operate in the dark and in very different weather conditions.

[0005] For proper and stable operation, continuous wave frequency modulation (FMCW) radars need to transmit a stable and preferably linear frequency sweep. Any sweeping nonlinearities can reduce the range resolution. In addition, changes in df / dt sweep slope due to temperature drift may reduce the accuracy of the measured ranges, and shifts in the center frequency due to temperature changes may push the transmission signal out of the FCC allocated band.

[0006] One solution to this problem is to digitally synthesize the radar signal in such a way that it is constantly obtained by being some numerical multiple of the piezoelectrically stabilized low frequency reference; however, this approach is rather costly in terms of both financial input in the production process and energy consumption.

[0007] Another approach adopted is based on an analog oscillator to generate a transmit signal. In this case, frequency modulation would be performed by a single capacitive diode. The main disadvantages of this approach are non-linearity and temperature drifts.

One solution is to provide a frequency generating oscillator (FGO) circuit with a memory chip. The numbers defining the function would be entered into this memory₂ derived from the polynomial N (x) = Ax² + Bx + C, where the coefficient A is a non-linear part,

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EP 1 932 015 B1 coefficient B means the original modulation slope and coefficient C refers to the center frequency.

[0008] By testing each transceiver after production, it is possible to determine and store their coefficients A, B and C, which results in a linear sweep and essentially the correct slope and required center frequency. In addition, by combining real-time temperature measurement with statistical analysis of microwave transceiver batches, you can apply a temperature correction to the A, B and C coefficients to stabilize the sweep function and the center frequency, compensating for fluctuations in these coefficients due to temperature. However, despite this alignment, there may be a significant slope drift df / dt. Thus, the disadvantage of this approach, although it is cheaper than the digital solution described above, in terms of both cost and energy consumption, accuracy may be reduced.

[0009] European Patent Application 99200140.4 discloses a Doppler radar traffic measurement system for measuring and monitoring traffic on communication roads such as public roads, railways, waterways, sluices, airports and the like. The system has a radar transmitter / receiver through which two adjacent FMCW radar frequencies are transmitted. The difference frequency phase difference is used to determine the distance that can be converted into a specific lane of road where the traffic participant being measured (vehicle, ship, train) is located.

SUMMARY OF THE INVENTION [0010] According to one aspect of the present invention, there is provided a road motion sensor adapted to be mounted in a fixed location to monitor multiple lanes, the road motion sensor comprising:

a processor unit configured to generate a programmable time-varying frequency modulating signal;

a transceiver unit configured to receive from the processor unit a programmable time-varying frequency modulation signal, and further configured to generate a linear frequency modulated microwave signal based on the programmable time-variable frequency modulating signal;

at least one antenna configured to: (i) receiving a microwave frequency linear frequency modulation signal from the transceiver unit, (II) forming a radiation beam, (iii) emitting a microwave signal linear frequency modulation in the radiation beam; and (iv) receiving the modulated microwave signal reflected from the object;

characterized by a signal stabilizing unit, configured to receive from the transceiver unit a portion of the microwave signal with linear frequency modulation, and further configured to pass the proportional calibration signal back to the processor;

and in that the processor unit is further configured to: (i) receive the calibration signal from the signal stabilizing unit, (ii) measure the frequency of the calibration signal, and (iii)

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Correcting the programmable time-varying frequency modulating signal based on the frequency of the calibration signal to increase the linearity of the modulated microwave signal; and in that the processor includes a curve fitting module configured to determine an inverse non-linear function from the frequency of the calibration signal, wherein the processor unit is further configured to generate a programmable time-varying frequency modulating signal based on the inverse non-linear function to compensate for the distortion introduced by the transmitting unit -odbiorczą.

[0011] According to another aspect of the present invention, there is provided a method of operating a road motion sensor adapted to be mounted in a fixed location for monitoring multiple lanes, the method comprising:

(a) generating a programmable time-varying frequency modulating signal;

(b) generating a microwave signal with linear frequency modulation based on a programmable time-varying frequency modulation signal;

(c) emitting a microwave signal with linear frequency modulation in the radiation beam;

(d) providing a proportional calibration signal based on a microwave signal with linear frequency modulation;

(e) measuring the frequency of the calibration signal and (f) correcting the programmable time-varying frequency modulating signal based on the frequency of the calibration signal to increase the linearity of the microwave signal with linear frequency modulation;

wherein:

step (f) further includes determining and periodically updating the inverse non-linear function from the frequency of the calibration signal, and step (a) includes generating a programmable time-varying frequency modulating signal based on the inverse non-linear function to compensate for the distortion introduced in step (b).

BRIEF DESCRIPTION OF THE DRAWINGS [0012] A detailed description of the preferred embodiments is provided below with reference to the following drawings, in which:

Figure 1, in block diagram, shows a road motion sensor according to one aspect of the present invention;

Figure 2, in block diagram, shows the transceiver and signal stabilizing unit of the road motion sensor of Figure 1;

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EP 1 932 015 B1

Figure 3, in block diagram, shows the processor unit of the road motion sensor of Figure 1; and [0013] Figure 4, in a flowchart, illustrates a method of determining a non-linear curve representing a signal fed to a transceiver unit to provide a substantially linear output signal from a transceiver unit, according to another aspect of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0014] With reference to Figure 1, a block diagram of a road motion sensor 20 is shown in accordance with one aspect of the present invention. The road motion sensor 20 includes a processor unit 22, a transceiver unit 24, an antenna 26 and a signal stabilization unit 28. The processor unit 22 generates a programmable time-varying modulation signal 30 that is received by the transceiver unit 24. The transceiver unit 24 receives a programmable time-varying modulation signal and generates a modulated microwave signal 32 that is received by the antenna 26. Antenna 26 then forms a radiation beam and emits a modulated microwave signal 32 in the radiation beam 34 onto an object or objects, such as a vehicle (not shown) or several vehicles. Antenna 26 also receives a reflected modulated microwave signal 36 from the object.

[0015] The signal stabilizing unit 28 adopts the calibration part 38 of the modulated microwave signal 32. The signal stabilizing unit 28 then receives a proportional calibration signal 40 which is sent back to the processor unit 22. The processor unit 22 is further capable of measuring the parameters of the calibration signal 40 and correcting the programmable time-varying modulation signal 30 based on the parameters of the calibration signal 40. The following is a more detailed description of the operation of the transceiver unit 24, signal stabilization unit 28 and processor unit 22.

[0016] With reference to Figure 2, the transceiver unit 24 and the signal stabilizing unit 28 are shown in more detail in a block diagram. As shown, the transceiver unit 24 includes a receiver 24A and a transmitter 24B. The transmitter 24B receives the programmable time-varying modulation signal 30 and transmits the modulated microwave signal 32. The receiver 24A receives the reflected modulated microwave signal 36 from antenna 26.

[0017] The transmitter 24B includes a tunable dielectric resonant voltage oscillator (VT-DRO) 42, which outputs a microwave signal 46 whose frequency is determined by the input of a programmable time-varying modulation signal 30 to the VT-DRO 42. The transmitter 24B further includes a power divider 48 which divides microwave signal 46 into two signals 46a and 46b. Typically, the power divider 48 is a circuit that receives an input signal and provides multiple output signals that are equal in phase and in a fixed amplitude ratio.

[0018] The generation of microwave signal 46 by VT-DRO 42 may introduce unwanted interference at specific frequencies. To remove these unwanted interference from the modulated microwave signal 32 provided to the antenna 26, a bandpass filter 50 is present in the transmitter 24B. The bandpass filter 50 receives the split signal 46a and produces the modulated microwave signal 32 passing through most of the split frequency

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Signal 46a in unchanged form, while suppressing those in the narrow range to very low levels. In one embodiment, the bandpass filter 50 is a 21 gigahertz diffuse bandpass filter selected to remove harmonic distortion introduced into the signal transmitted 46a by VT-DRO 42.

[0019] The receiver 24A receives the reflected modulated microwave signal 36 from the antenna 26. The receiver 24A includes a mixer 52 which effectively simultaneously multiplies the split signal 46b from the power divider 48 and the reflected modulated microwave signal 36 and generates a mixed signal 54. The mixed signal 54 is in the baseband, in which it is defined as the frequency difference between the reflected modulated microwave signal 36 and the signal 46b, which frequency difference represents the time elapsing between transmitting and receiving the modulated microwave signal. The elapsed time, in turn, reflects the distance to the object from which the signal was reflected. The mixed signal 54 is received by the amplifier 56, which produces the amplified signal 58. The amplified signal 58 is then sent to the processor unit 22 as shown in Figure 1.

[0020] The signal stabilizing unit 28, which is also shown in more detail in Figure 2, includes various components for converting down-modulated microwave signal 46 obtained from the VT-DRO 42 to a low baseband signal suitable for digital processing by the processor unit 22. In the specific embodiment illustrated in Figure 2, the voltage coupled oscillator (phase PLL VCO) 60 is locked to clock signal 62 at frequency f1 and outputs the corrected clock signal at frequency f2 = nf1 / m, where n and m are integers usually called frequency divider parameters. In one embodiment, the VCO 60 PLL is an ADF4360 analog device and is configured to produce an output of 2624,72 MHz from an input clock signal of 24.4912 MHz.

[0021] The output of the adjusted clock signal 64 from PLL VCO 60 is provided to a frequency multiplier 66, which receives the adjusted clock signal 64 at frequency f2 and produces a reference signal 68 at frequency Nf2. In other words, frequency multiplier 66 adopts the adjusted clock signal 64 and produces an output signal with the nth harmonic frequency of the input signal 64. In one embodiment, the frequency multiplier 66 is the HMC443LP4 quadrant from Hittite Microwave and produces a 10,4988-gigahertz reference signal from a 2624,72-megahertz input signal.

[0022] The signal stabilizing unit 28 further includes a mixer 70 for mixing the modulated microwave signal 46 and the reference signal 68 from the frequency multiplier 66. The mixer 70 multiplies these two signals to produce two bands of output signals. One of these signals is an up-conversion frequency band with a frequency equal to the sum of the frequencies of the two input signals, while the other output is a down-conversion band and has a frequency equal to the difference between the frequencies of signals 46 and 68. These two output signals 72 from mixer 70 are provided to lowpass filter 74, which passes the conversion signal down the output signals 72, filtering the conversion signal up the output signals 72. In one embodiment, the low pass filter cutoff frequency

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EP 1 932 015 B1 is 50 MHz such that low pass filter 74 passes frequencies lower than 50 MHz, thereby retaining frequencies higher than 50 MHz.

[0023] The signal stabilization unit 28 further includes an amplifier 80 that receives a down conversion component 78 of the output signal 72 received from the low-pass filter 74. The amplifier 80 amplifies this down-converted signal 78 to a level sufficient for direct counting by digital circuits to obtain proportional calibration signal 40, which is sent to processor unit 22 as shown in Figure 1.

[0024] With reference to Figure 3, the processor unit 22 is shown in more detail in a block diagram. The elements of control unit 22 shown in Figure 3 correct the programmable time-varying modulation signal 30 to increase the linearity of the modulated microwave signal 32 generated by the transceiver unit 24 based on the programmable time-varying modulation signal 30. Generally, continuous wave radar with frequency modulation works most effectively when the modulated microwave signal from the transceiver is linearly modulated.

[0025] The programmable time-varying modulation signal 30 is generated by the frequency generating oscillator (FGO) 82 circuit. Since the VT-DRO 42 (as shown in Figure 2) is a non-linear device, in order to produce a line output, the input to the DRO VT-42 must be nonlinear. The non-linear function required to produce a linear output can be approximated₂ by the function N (x) = Ax² + Bx + C, where factor A is the non-linear part, factor B is the original modulation slope and factor C refers to the center frequency. The FGO 82 circuit contains a memory module (MU) that stores a fixed number of function values ​​N (x) = Ax²+ Bx + C.

[0026] The control unit 22 further includes a counter 86, which is used to count from 1 to N and then count backwards from N to 1, where N is the number of function values ​​88 stored in the memory module 84. When the processor unit 22 is operating in the mode normal, the number generated by the counter 86 is sent to the FGO circuit 82, in which it is used to retrieve the function 88 value written at memory module 84 address. After the download, the function value 88 is sent to the DAC 90, which produces a time-varying modulation signal 30 based on the function value 88 received from memory module 84.

[0027] Although the processor unit 22 generally operates in normal mode, it switches to calibration mode at constant intervals. According to one embodiment, the calibration mode interval is set to 3 minutes. In calibration mode, the system runs the curve fitting process shown in the flow chart of Figure 4. The curve fitting process includes₂ determination of three points on the curve represented by the function N (x) = Ax² + Bx + C, and then using these three values ​​to determine the coefficients A, B and C. To determine the point on the N (x) curve, you need to determine the N (x1) value and the x1 value. To specify both N (x1) and x1 microprocessor unit (MPU) 92 changes input signal 94 supplied to DAC 90. More specifically, the function of curve fitting 96 provided by MPU 92 changes input signal 94 supplied to DAC 90 until the signal frequency output 46 from VT-DRO 42 7

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EP 1 932 015 B1 (see Figure 2) reaches the preset frequency. At this stage, the input signal value 94 will be N (x1) and the value x1 is calculated from the required frequency. Later, the MPU 92 will know where the required frequency is on the ideal linear curve. Thus, MPU 92 will know that it must use the value of N (x1) at its equivalent time in its cycle, which is x1.

[0028] With reference to Figure 4, the flowchart illustrates the curve fitting process according to another aspect of this invention. The curve fitting process starts from step 98, in which the curve fitting element 96 from the MPU 92 sends a signal 94 to the DAC 90 trying to generate the frequency f1. Based on signal 94, the DAC 90 generates a programmable time-varying modulation signal 30 that is received by the transceiver unit 24, as shown in Figure 1. As described above in connection with Figure 2, the proportional calibration signal 40 is derived from the output signal 46 with VT-DRO 42. With reference to Figure 3, this proportional calibration signal 40 is supplied back to the analog-to-digital converter (ADC) 100, which generates an equivalent digital signal 102, which is sent to the frequency counter 104, which reads the frequency of the signal 102. This frequency reading equals sum of the frequency of the signal 102 from the ADC 100 and the frequency of the reference signal 68 (Figure 2). The transmission frequency calculated in this way is then compared with f1. If the calculated transmission frequency is not equal to f1 (within the selected tolerance), then the query 108 of the method of Figure 4 returns "NO" and the curve fitting element 96 from MPU 92 returns to step 98. However, when the calculated transmission signal equals f1, then the query 108 returns the value "YES" [yes] and the curve fitting element 96 from the MPU 92 goes to step 110, in which the input value DAC 94 producing f1 is stored as N0.

[0029] At step 112, the curve fitting element 96 adjusts the value 94 transmitted to the DAC trying to generate a second frequency f₁ + Af, which is currently the required frequency. If the calculated transfer frequency, determined as described above, does not equal the required frequency, then this query 114 returns "NO" and the curve fitting element 96 returns to step 112 to adjust the DAC input value 94. However, when the calculated transfer signal is equal to the required frequency, query 114 returns "YES" and the process proceeds to step 116, where the input DAC value 94, which generated the required transfer frequency, is stored as N1.

[0030] At step 118, the curve fitting element 96 again adapts the DAC input signal 94 in an attempt to generate frequency f₁ + 2ΔΡ which is now the required frequency. If the calculated transfer frequency, determined as described above, does not equal the required frequency, then this query 120 returns "NO" and the curve fitting element 96 returns to step 118 to adjust the DAC input value 94. If, on the other hand, the calculated transfer signal equals the required frequency, then the query 120 returns the value "YES" and the curve fitting element 96 proceeds to step 122, in which the input value DAC 94, producing the required transfer frequency, is stored as N2 .

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[0031] After all N0, N1 and N2 have been determined at steps 110, 116 and 122 respectively, the curve fitting element 96 obtains the value of the factor A, B and C from the values ​​of N0, N1 and N2 at step 124. Then, if the coefficients result in the polynomial N (x), which differs from the one used in the previous normal operating mode, then the new N (x) is used to generate new N (x) values, and the memory module 84 is updated with new N (x) values. The way in which the processor unit 22 switches between normal operation mode and operation calibration mode is described in more detail below.

[0032] As described above, the counter 86 from the control unit 22 is used to count from 1 to N and then count backwards from N to 1, where N is the number of function values ​​stored in memory unit 84. In one embodiment, the module memory takes very little time - i.e. half a millisecond - counting from 1 to N or counting backwards from N to 1. This feature of processor unit 22 is important in switching between normal operation mode and calibration mode in this embodiment as described below.

[0033] According to one embodiment, the processor unit 22 includes a clock 126 which, in addition to providing the clock signal 62 described above with reference to Figure 2, also determines whether a predefined interval for calibration mode has expired, such as, for example, 3 minutes. If this interval for calibration mode has expired, then within half a millisecond, when the counter 86 counts backwards from N to 1, the curve fitting element 96 will temporarily stop the signal 88 sent from memory module 84 to the DAC 90 and instead send signal 94 directly from the element for curve fitting 96 to DAC 90. This signal 94 is adjusted as described above to calibrate the output of the programmable time-varying modulation signal 30 using the DAC to linearize the modulated microwave signal 32 from the transceiver unit 24. It will certainly be obvious to those skilled in the art that calibration may occur more frequently or less frequently, or alternatively it could proceed in parallel to the normal operating mode simply by monitoring the specific proportional calibration value provided by the specific signal 88 from the MU during normal operating mode.

[0034] Preferably, the memory module 84 consists of two separate memories. After the curve fitting element 96 has determined N0, N1 and N2 at stages 110, 116 and 120, respectively, and obtains the value for the polynomial function N (x) at stage 124, one of the components of memory module 84 can still provide values ​​88 to DAC 90 . In fact, this component of memory module 84 can still provide values ​​88 to DAC 90 even when the second component of memory module 84 is updated with new values ​​determined using new N (x) polynomial functions. Then, when all new values ​​are saved, the memory module can be immediately switched from one memory component to another memory component in which new values ​​of this N (x) polynomial are stored. The second memory component can be used to store the value of the next polynomial function N (x) determined during the next calibration mode.

Other variations and modifications of this invention are possible. It is understood that all such modifications and variations are within the scope and scope of the invention as defined in the appended claims.

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EP 1 932 015 B1