JP2012048366A - Vehicle sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a vehicle sensor preventing interference when juxtaposed and having a hardware constitution in common.SOLUTION: A vehicle sensor 100 incorporates a distance sensor 10 to be a light wave distance sensor of a phase difference system and decides the presence or absence of a vehicle based on a measurement distance L by the distance sensor 10. In the distance sensor 10, the frequency of a transmission wave can be changed by changing a frequency division ratio N of a frequency divider 14, and only a signal of a reflection wave having the same frequency as that of the transmission wave can be extracted from a reception signal by a two-phase lock-in amplifier 30.

Description

  The present invention relates to a vehicle sensor.

  As a kind of vehicle detector, there is known a detector that irradiates an ultrasonic wave or light toward a road surface and detects the presence or absence of a vehicle based on the reception timing and reception level of a reflected wave (for example, Patent Document 1). , 2).

Japanese Patent Laid-Open No. 6-174832 Japanese Patent Laid-Open No. 10-2222795

  When such a vehicle detector is installed (adjacent) in each of a plurality of lanes, reflected waves from other vehicle detectors are also received, and erroneous detection due to interference has been a problem. In order to prevent this, when a vehicle detector is provided, it is necessary to change the transmission frequency for each vehicle detector. Therefore, manufacturers need to manufacture multiple types of vehicle detectors for each transmission frequency, and vehicle detector installation / users (installers and police agencies) pay attention to detector installation / management. There was a need.

  The present invention has been made in view of the above circumstances, and an object thereof is to realize a new vehicle sensor that prevents interference at the time of installation.

The first form for solving the above problem is
A vehicle sensor (for example, the vehicle sensor 100 in FIG. 1) that is provided in the range where the reflected waves of each other reach and senses the presence or absence of a vehicle in the target lane based on the reflected wave of the own sensor,
A frequency divider (for example, frequency divider 14 in FIG. 4) capable of changing a frequency dividing ratio for dividing a clock signal having a predetermined frequency;
A transmission unit (for example, the light emitting unit 16 in FIG. 4) that generates a transmission wave from the frequency-divided signal by the frequency divider and transmits the transmission wave toward the target lane
A transmission wave frequency setting unit (for example, the frequency setting unit 52 in FIG. 4) that changes the frequency of the transmission wave by changing and setting the frequency division ratio of the frequency divider,
A receiving unit (for example, the light receiving unit 18 in FIG. 4) that receives a reflected wave of the transmission wave;
A phase difference detection unit (for example, a distance calculation unit 22 in FIG. 4) that detects a phase difference between the transmission wave and a reception wave that is a reflected wave received by the reception unit;
A distance calculation unit (for example, the distance calculation unit 22 in FIG. 4) that calculates a measurement distance to the target lane based on the detected phase difference;
A determination unit (vehicle presence determination unit 54) for determining the presence or absence of a vehicle in the target lane based on the measurement distance;
It is a vehicle sensor provided with.

  According to the first aspect, in the vehicle detector that calculates the measurement distance to the target lane based on the phase difference between the transmission wave and the reception wave and determines the presence or absence of the vehicle in the target lane based on the measurement distance. A transmission wave is generated from a frequency-divided signal obtained by frequency-dividing a clock signal having a predetermined reference frequency, and the frequency division ratio of the frequency divider that divides the clock signal can be changed. That is, the frequency of the transmission wave can be changed simply by changing the frequency division ratio of the frequency divider. As a result, it is possible to prevent interference between transmission waves and reflected waves between the vehicle detectors provided side by side, and to make the hardware configuration of a plurality of vehicle detectors provided side by side common.

As a second form, the vehicle detector of the first form,
The phase difference detector is
Having a two-phase lock-in amplifier circuit (for example, the two-phase lock-in amplifier 30 in FIG. 4) that synchronously detects the received wave using the divided signal and extracts a DC component from the synchronously detected signal;
A phase difference between the transmitted wave and the received wave is detected using a positive / negative combination of the extracted two DC components and the DC component.
A vehicle detector may be configured.

  According to the second embodiment, detection of the phase difference between the transmission wave and the reception wave is realized using the two-phase lock-in amplifier. As a result, a reflected wave having the same frequency as the transmitted wave can be easily detected, and a phase difference resistant to noise and accurate can be realized.

As a third form, the vehicle detector of the first or second form,
The determination unit determines whether or not there is a vehicle in the target lane according to whether or not the measurement distance satisfies a threshold distance condition determined based on a road surface distance that is regarded as a distance from the vehicle sensor to the road surface,
A threshold distance condition changing unit that changes the threshold distance condition by changing the road surface distance so as to follow the measured distance when the determination unit determines that there is no vehicle (for example, vehicle presence determination in FIG. 4). Part 54),
Further comprising
A vehicle detector may be configured.

  According to the third aspect, the presence or absence of a vehicle in the target lane is determined based on whether or not the measurement distance satisfies a threshold condition determined based on the road surface distance. By the way, the actual distance from the vehicle detector to the road surface may vary due to, for example, snow. For this reason, the more accurate determination of the presence or absence of the vehicle is realized by changing the road surface distance so as to follow the measurement distance when it is determined that there is no vehicle.

Installation example of a vehicle detector. The schematic diagram of distance measurement by a light wave distance sensor. Explanatory drawing of the difference in the detection accuracy by a transmission frequency. The block diagram of a vehicle sensor. Correspondence between calculated phase difference and actual phase difference. The data structural example of a phase difference correction table. The correspondence example of a frequency division ratio and the maximum measurable distance. Explanatory drawing of the determination principle of the presence or absence of a vehicle based on measurement distance. The flowchart of a vehicle detection process.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings, but embodiments to which the present invention can be applied are not limited thereto.

[Example of installation]
FIG. 1 is a diagram illustrating an installation example of the vehicle detector 100 according to the present embodiment. As shown in FIG. 1, in a road composed of multiple lanes, a plurality of vehicle detectors 100 for detecting each lane are installed on a pillar installed above the road so as to overlook the lane to be detected. )

  The vehicle sensor 100 has a built-in distance sensor 10 and generates a sensing signal indicating the presence or absence of a vehicle in the sensing target lane based on the distance measured by the distance sensor 10. The sensing signal output from the vehicle sensor 100 is output to the control device 200 that is installed below the pillar and connected to be communicable by wire / wireless, for example.

[Principle of distance sensor]
The distance sensor 10 built in the vehicle detector 100 is a light wave distance sensor using light waves such as infrared rays and laser light, and measures the distance L to the measurement object by a phase method.

FIG. 2 is a schematic diagram of distance measurement by the distance sensor 10. As shown in FIG. 2A, the distance sensor 10 emits a transmission wave, which is a light wave, toward the measurement object 90 and receives a reflected wave reflected by the measurement object 90. Then, as shown in FIG. 2B, the distance L to the measurement object 90 is calculated from the phase difference φ between this transmitted wave and the received reflected wave (hereinafter referred to as “received wave”). The distance L to the measurement object 90 is calculated as L = (C · φ / 2πf) / 2. Here, “C” is the speed of light, and C≈3 × 10 ⁸ m / s.

  Further, in order to calculate the distance L from the phase difference φ between the transmission wave and the reception wave, when the distance L is calculated using only one transmission frequency, the phase difference φ is within one cycle of the transmission frequency. For this reason, the maximum measurable distance Lm (so-called “distance range”) is theoretically determined by the frequency f of the transmission wave and is Lm = λ / 2 = C / (2 · f). For example, when the frequency of the transmission wave is “25 MHz”, the wavelength λ is “12 m” and the maximum measurable distance Lm is “6 m”.

  The calculation accuracy of the measurement distance L by the phase-type distance sensor differs depending on the frequency of the transmission wave. Specifically, the calculation accuracy is better as the frequency of the transmission wave is higher.

  FIG. 3 is a diagram for explaining a difference in calculation accuracy of the measurement distance L due to a difference in frequency of the transmission wave. In the drawing, the upper side shows a transmission wave and a reception wave having a frequency fa, and the lower side shows a transmission wave and a reception wave having a frequency fb (= 3 · fa) higher than the frequency fa.

  When the same distance L is measured, the phase difference φ between the transmission wave and the reception wave differs due to the difference in the wavelength λ when the frequency f is different. Specifically, the higher the frequency, the larger the phase difference φ. In FIG. 3, the phase difference φb at the frequency fb is larger than the phase difference φa at the frequency fa, and φb = 3 · φa.

  When detecting the phase difference φ between two signals, the smaller the phase difference φ, the easier the detection error. That is, as the frequency f of the transmission wave is lower, a detection error of the phase difference φ is more likely to occur, and the calculation accuracy of the measurement distance L is reduced. In other words, as the frequency f of the transmission wave is higher, the detection error of the phase difference φ is less likely to occur and the calculation accuracy of the measurement distance L is improved.

  By the way, when the vehicle detector 100 including a distance sensor using an electromagnetic wave such as a light wave as a distance measurement signal is installed (adjacent) in each of a plurality of lanes, interference of a distance measurement signal, that is, other vehicle detection A situation may occur in which the transmitted wave transmitted by the device 100 and its reflected wave are also received. In order to solve this, the vehicle sensor 100 of the present embodiment is configured to be able to change the frequency of the distance measurement signal (transmission wave). Thereby, the frequency of the transmission wave of each of the plurality of vehicle detectors 100 arranged (adjacent) in adjacent lanes can be set to be different, and the influence of interference between the vehicle detectors 100 can be prevented.

[Constitution]
FIG. 4 is a configuration diagram of the vehicle sensor 100. As shown in FIG. 4, the vehicle detector 100 includes a distance sensor 10, a frequency setting unit 52, and a vehicle presence / absence determination unit 54.

  The distance sensor 10 detects a phase difference φ between the transmission wave and the reception wave using a two-phase lock-in amplifier, and includes an oscillator 12, a frequency divider 14, a light emitting unit 16, a light receiving unit 18, A BPF 20 and a two-phase lock-in amplifier 30 are provided.

  For example, the oscillator 12 includes a crystal resonator and generates a reference signal F having a reference frequency f. The reference frequency f is, for example, “25 MHz”.

The frequency divider 14 divides the frequency f of the reference signal F output from the oscillator 12 by 1 / N and outputs it as a transmission signal V _T. The frequency division ratio N of the frequency divider 14 is “N = 2 ⁿ (n is a positive number)”, and is changed and controlled by the frequency setting unit 52.

Emitting unit 16 is composed of, for example, a light-emitting element such as a laser diode, an optical signal whose intensity is modulated at a frequency of the transmitted signal V _T output from the frequency divider 14, and emitted as transmitted waves. That is, a transmission wave having a frequency “f / N” is emitted.

The light receiving unit 18 is configured, for example, a light receiving element such as a photodiode converts the light waves received into an electrical signal, and outputs as a reception signal V _R.

BPF20 is the received signal V _R, is passed through a signal of a predetermined band, for blocking an out-of-band frequency components. The center frequency fo of this BPF20 from the received signal _{V R,} for extracting only the signal of the reflected wave at the same frequency as the transmission wave, the frequency setting unit 52, fo = f / N, is set to.

2 phase lock-in amplifier 30, a transmission signal V _T which is output from the frequency divider 14 as a "reference signal", the received signal V _R as "measurement signal", the synchronous detection in the transmit signal V _T the received signal V _R (Phase detection) and output two alternating signals X and Y orthogonal to each other. In principle, the two-phase lock-in amplifier 30 includes a delay circuit 32, mixers 34 and 36, and LPFs 38 and 40.

The delay circuit 32 delays the phase of the transmission signal V _T (reference signal) by “π / 2 (90 °)”.

The mixer 34 multiplies (synthesizes) the signal output from the BPF 20 and the transmission signal V _T (reference signal) and outputs the result. The mixer 36 multiplies the signal output from the BPF 20 and the signal output from the delay circuit 32.

  The LPF 38 passes a predetermined low-band signal with respect to the output signal of the mixer 34 and blocks out-of-band frequency components. The cut-off frequency fc of the LPF 38 is set to fc = f / 2N by the frequency setting unit 52.

  The LPF 40 passes a predetermined low-band signal with respect to the output signal of the mixer 36 and blocks out-of-band frequency components. The cut-off frequency fc of the LPF 40 is set by the frequency setting unit 52 to fc = f / 2N.

In this distance sensor 10, if the reference signal F generated by the oscillator 12 is F1 = sin (ωt), the output signal of the frequency divider 14, that is, the transmission signal V _T (reference signal) is V _T = sin ( ωt / N). The output signal D of the delay circuit 32 is D = sin (ωt / N−π / 2) = cos (ωt / N).

Further, when the phase difference between the transmitted signal _{V T} and the received signal _{V R} and "phi", the received signal _{V R is, V R = sin (ωt /} N-φ), and becomes. The received signal _{V R} is directly passed through the BPF 20.

The output signal MIX1 of the mixer 34 is expressed by the following equation (1).
MIX1 = sin (ωt / N) × sin (ωt / N−φ)
= − (Cos (2ωt−φ) −cosφ) / 2 (1)
The signal MIX1 passes through the LPF 38, and the high frequency component is cut off. The output signal X of the LPF 38 is X = (cosφ) / 2.

Further, the output signal MIX2 of the mixer 36 is expressed by the following equation (2).
MIX2 = cos (ωt / N) × sin (ωt / N−φ)
= (Sin (2ωt−φ) + sinφ) / 2 (2)
This signal MIX2 is blocked from high-frequency components by passing through the LPF 40. The output signal Y of the LPF 40 is Y = (sin φ) / 2.

Thus, these signals X, the Y, the phase difference φ between the transmitted signal _{V T} and the received signal _{V R,} the equation (3).
φ = tan ⁻¹ (Y / X) (3)

However, the actual phase difference [phi] r of the transmission signal _{V T} and the received signal _{V R} is "0 ≦ φr <2π (360 ° ) ". However, the phase difference φ calculated by the above equation (3) is a value of “−π / 2 (−90 °) ≦ φ ≦ π / 2 (90 °)”. For this reason, it is necessary to correct the calculated phase difference φ so as to be the actual phase difference φr. Specifically, the calculated phase difference φ is corrected by a positive / negative combination of the signals X and Y.

FIG. 5 is a diagram showing the relationship between the actual phase difference φr and the positive and negative of the signals X and Y. In Figure 5, the actual phase difference φr of the horizontal axis and the transmission signal V _T and the received signal V _R, the signal X, and Y, respectively, the signal X, a phase difference φ being calculated from the Y, the actual phase difference φr.

  As shown in FIG. 5, when “0 ≦ φr <π / 2 (90 °)”, φ = φr, but when “π / 2 (90 °) ≦ φr <2π (360 °)”, φ ≠ φr. Specifically, in “π / 2 (90 °) ≦ φr <3π / 2 (270 °)”, φ = φr−π (180 °), and in “3π / 2 ≦ φr <2π”, φ = φr−2π. For this reason, it is necessary to correct the calculated phase difference φ so that the actual phase difference φr is obtained. Since the positive and negative combinations of the values of the signals X and Y differ for each quadrant, the necessary correction amount Δφ is determined from the positive and negative combinations of the values of the signals X and Y.

The distance calculation unit 22 includes an arithmetic device (processor) such as a DSP (Digital Signal Processor), and calculates the distance L to the measurement object. That is, the signal X outputted from LPF38,40, based on the Y, according to the above equation (3), calculates the phase difference φ between the transmitted signal _{V T} and the received signal _{V R.} Subsequently, the calculated phase difference φ is corrected according to the phase difference correction table 60 based on the positive / negative combination of the signals X and Y.

  FIG. 6 is a diagram illustrating an example of a data configuration of the phase difference correction table 60. According to FIG. 6, the phase difference correction table 60 stores the correction amount 63 of the phase difference φ in association with each combination of the positive / negative 61 of the value of the signal X and the positive / negative 62 of the value of the signal Y. . The phase difference correction table 60 is stored in the distance calculation unit 22. Then, the measurement distance L is calculated from this phase difference φ. The measurement distance L is given by L = (C · φ · N) / (2 · ω).

  The frequency setting unit 52 is configured to include an arithmetic device (processor) such as a DSP (Digital Signal Processor). For example, the frequency setting unit 52 is operated according to an operation input by an administrator or an external input such as a control signal from the control device 200. Set the frequency of the transmitted wave. Specifically, the frequency division ratio N of the frequency divider 14 is set to a designated value. Further, the center frequency fo of the BPF 20 is set in accordance with the set frequency division ratio N, and the cutoff frequency fc of the LPFs 38 and 40 is set. The center frequency fo is given by fo = f / N, and the cutoff frequency fc is given by fc = f / 2N.

  FIG. 7 is a diagram illustrating a specific setting example of the frequency division ratio N, and shows a correspondence relationship between the frequency division ratio N and the maximum measurable distance Lm. However, it is assumed that the division ratio N that can be set in the present embodiment is “N = 1, 2, 4,...” That is a power of 2, and the reference frequency f is “f = 25 MHz”.

  The vehicle detector 100 is usually installed at a position where the height from the road surface is about “5 to 6 m”. That is, the measurable distance Lm may be set to “5 to 6 m” or more, that is, the frequency division ratio N may be set to “N = 1” or more. Moreover, the higher the frequency of the transmission wave, the higher the measurement accuracy. Therefore, when a plurality of vehicle detectors 100 are provided, the frequency division ratio N of each vehicle detector 100 frequency divider 14 is assigned in the order of “N = 1, 2, 4,...”. Specifically, for example, when two vehicle detectors 100a and 100b are provided side by side, the division ratio N of one vehicle detector 100a is set to “N = 1”, and the division of the other vehicle detector 100b is set. Let the circumferential ratio N be “N = 2”. When three vehicle detectors 100a to 100c are also provided, the division ratio N of the first vehicle detector 100a is set to “N = 1”, and the frequency division of the second vehicle detector 100b is performed. The ratio N is “N = 2”, and the frequency division ratio N of the third vehicle detector 100c is “N = 4”.

  The vehicle presence / absence determination unit 54 includes an arithmetic device (processor) such as a DSP (Digital Signal Processor), and determines the presence / absence of a vehicle in the target lane based on the measurement distance L calculated by the distance calculation unit 22. , A sensing signal as a determination result is output.

  FIG. 8 is a diagram for explaining the principle of determining the presence or absence of a vehicle based on the measurement distance L. As shown in FIG. 8, a “threshold level” is defined based on a “road surface level” that is a distance from the vehicle detector 100 to the road surface. Specifically, the distance from the road surface level to a position that is higher by a predetermined distance (for example, about 30 cm) determined assuming the height of the vehicle is determined as the threshold level. If the measured distance L satisfies the threshold distance condition “does not reach the threshold level”, it is determined that the vehicle is present, and the threshold distance condition is not satisfied (that is, the measured distance L reaches the threshold level). In the case of "No vehicle".

  Note that the actual height of the road surface is not always constant and varies depending on, for example, snow accumulation. For this reason, based on the measurement distance L, the road surface level is made to follow the fluctuation | variation of the actual road surface height.

  Specifically, when the current road surface level substantially matches the actual road surface, if a vehicle exists, the measurement distance L does not reach the threshold level, and if there is no vehicle, the measurement distance L Should almost match the road level. For this reason, when the determination result is “no vehicle”, the road surface level is gradually changed so as to approach the measurement distance L. That is, when the measured distance L exceeds the road surface level, the road surface level is gradually increased, and when the measured distance L is between the threshold level and the road surface level, the road surface level is gradually decreased. At this time, the amount of change in the road surface level is larger for the increase amount than for the decrease amount. The threshold level also varies according to the variation in the road surface level.

  Two or all of the distance calculation unit 22, the frequency setting unit 52, and the vehicle presence / absence determination unit 54 may be configured by the same control unit.

[Process flow]
FIG. 9 is a flowchart for explaining the flow of the vehicle detection process executed by the vehicle presence / absence determination unit 54. According to FIG. 9, the vehicle presence / absence determination unit 54 first determines whether or not the measurement distance L is normally calculated by the distance calculation unit 22. Here, the case where the measurement distance L is not normally calculated is, for example, a case where a transmission wave is transmitted but a reflected wave is not received.

  If the calculation of the measurement distance L is not performed normally (step A1: NO), if the sensing result up to this point is “abnormal” continuously for a predetermined time (for example, 5 minutes) (step A3: YES) Then, the sensing result is set to “abnormal” (step A5), and if not (step A3: NO), the sensing result is set to “present” (step A7).

  On the other hand, if the measurement distance L is normally calculated (step A1: YES), the calculated measurement distance L is compared with the road surface level. If the measured distance L exceeds the road surface level (step A9: YES), the road surface level is increased by a predetermined increase amount (step A11), and the threshold level is changed in accordance with the road surface level after the increase (step A11). A13).

  If the measured distance L matches the road surface level (step A15: YES), the sensing result is “none” (step A17).

If the measured distance L does not reach the road surface level (step A15: NO), then
The measurement distance, L, and threshold level are compared. If the measurement distance L does not reach the threshold level (step A19: YES), the sensing result is “present” (step A21).

  On the other hand, if the measured distance L reaches the threshold level (step A19: NO), the road surface level is decreased by a predetermined decrease amount (step A23), and the threshold level is changed in accordance with the road surface level after the decrease (step A23). A25). Then, the detection result is set to “none” (step A27).

  Thereafter, it is determined whether or not to end the vehicle sensing. If not (step A29: NO), the process returns to step A1, and if finished (step A29: YES), the vehicle sensing process is terminated.

[Action / Effect]
As described above, the vehicle sensor 100 according to the present embodiment incorporates the distance sensor 10 that is a phase difference type light wave distance sensor, and determines the presence or absence of the vehicle based on the distance L measured by the distance sensor 10. In the distance sensor 10, the frequency of the transmission wave can be changed by changing the frequency division ratio N of the frequency divider 14, and the reflected wave having the same frequency as the frequency of the transmission wave is received from the received signal by the two-phase lock-in amplifier 30. Only the signal can be extracted.

  Thereby, the hardware of the several vehicle detector 100 installed (adjacent) for the adjacent lane can be made common. Then, by setting the frequency division ratio N of the frequency divider 14 in each vehicle detector 100 to a different frequency division ratio N and changing the frequency of the transmission wave, the reflected wave between the vehicle sensors 100 provided in the vehicle detector 100 is different. Interference can be prevented.

[Modification]
It should be noted that embodiments to which the present invention can be applied are not limited to the above-described embodiments, and can be appropriately changed without departing from the spirit of the present invention.

(A) Threshold distance condition For example, in the above-described embodiment, the threshold distance condition serving as the vehicle presence / absence determination criterion is “the measurement distance L does not satisfy the threshold level”, but other conditions may be used. Specifically, for example, the threshold distance condition is “the difference between the measured distance and the road surface level is equal to or greater than a predetermined distance”, and the difference between the measured distance L and the road surface level is determined based on the height of the vehicle. If it is equal to or greater than (for example, about 30 cm), it is determined that there is a vehicle, and if it is equal to or less than a predetermined distance, it is determined that there is no vehicle.

(B) Distance sensor 10
In the above-described embodiment, the distance sensor 10 incorporated in the vehicle detector 100 is a light wave distance sensor. However, the present invention can be similarly applied to a distance sensor using electromagnetic waves or ultrasonic waves.

DESCRIPTION OF SYMBOLS 100 Vehicle detector 10 Distance sensor 12 Oscillator, 14 Frequency divider, 16 Light emission part, 18 Light reception part, 20 BPF
30 Two-phase lock-in amplifier 32 Delay circuit, 34, 36 Mixer, 38, 40 LPF, 22 Distance calculation unit 52 Frequency setting unit, 54 Vehicle presence / absence determination unit

Claims (3)

It is a vehicle detector that is installed in the reach of each other's reflected waves and detects the presence or absence of a vehicle in the target lane based on the reflected waves of its own detector,
A frequency divider capable of changing a frequency dividing ratio for dividing a clock signal of a predetermined frequency;
A transmission unit that generates a transmission wave from the frequency-divided signal by the frequency divider and transmits the signal toward the target lane;
A transmission wave frequency setting unit that changes the frequency of the transmission wave by changing and setting the frequency division ratio of the frequency divider,
A receiver for receiving a reflected wave of the transmission wave;
A phase difference detection unit that detects a phase difference between the transmission wave and a reception wave that is a reflected wave received by the reception unit;
A distance calculation unit that calculates a measurement distance to the target lane based on the detected phase difference; and
A determination unit for determining the presence or absence of a vehicle in the target lane based on the measurement distance;
Vehicle sensor equipped with. The phase difference detector is
A two-phase lock-in amplifier circuit that synchronously detects the received wave using the divided signal and extracts a DC component from the synchronously detected signal;
A phase difference between the transmitted wave and the received wave is detected using a positive / negative combination of the extracted two DC components and the DC component.
The vehicle detector according to claim 1. The determination unit determines whether or not there is a vehicle in the target lane according to whether or not the measurement distance satisfies a threshold distance condition determined based on a road surface distance that is regarded as a distance from the vehicle sensor to the road surface,
A threshold distance condition changing unit that changes the threshold distance condition by changing the road surface distance so as to follow the measured distance when the determination unit determines that there is no vehicle;
Further comprising
The vehicle detector according to claim 1 or 2.

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