JP7534786B2 - Foreign object detection system - Google Patents

Description

This invention relates to a foreign object detection system using radar. More specifically, this invention relates to a foreign object detection system using radar that suppresses interference between radars.

International Publication WO 2020-262535 describes a foreign object detection system. This foreign object detection system differs in the length of optical fiber from one signal source to multiple radars, thereby making the arrival time of the signal different and thereby preventing interference between the radars. This foreign object detection system has a problem in that a time lag occurs when switching the optical fiber length. In particular, with this foreign object detection system, there are cases in which the target goes outside the range of the main lobe while switching the optical fiber.

International Publication No. 2020-262535 Brochure

The invention described in this specification aims to provide a foreign object detection system that does not suffer from the problem of time lag that occurs when switching optical fiber lengths in conventional foreign object detection systems.

The above problem is basically based on the knowledge gained from the embodiment that by using a dual-frequency radar in which one of two signals is used as the local signal, signals resulting from false images can be removed by performing AND processing on the received signal.

The first invention relates to a foreign object detection system. The foreign object detection system 1 includes a first radar 9.

The first radar 9 includes a first output unit 3, a first receiving unit 5, and a first foreign object analysis unit 7.

The first output unit 3 is an element for outputting a first radio signal. The first radio signal includes a first signal and a second signal that has the same frequency change pattern as the first signal but has a different output timing. It is preferable that the first signal and the second signal are frequency modulated continuous waves (FMCW).

The first receiving unit 5 is an element for receiving a first reflected wave based on a first wireless signal. The first reflected wave includes a first reflected signal based on the first signal and a second reflected signal based on the second signal.

The first foreign object analysis unit 7 is an element for analyzing the presence or absence of a foreign object using the first reflected wave received by the first receiving unit. The first foreign object analysis unit 7 removes a false image caused by interference using the first reflected signal and the second reflected signal. It is preferable that the first foreign object analysis unit 7 analyzes the presence or absence of a foreign object using either the first signal or the second signal as a local signal.
The first foreign object analysis unit 7 regards a signal that is analyzed to be present at a position common to the first reflected signal and the second reflected signal as a signal originating from a foreign object,
It is preferable that signals that are analyzed to be present at different positions in the first reflected signal and the second reflected signal are considered to be signals originating from an artifact.

The foreign object detection system 1 may include multiple radars.
For example, this foreign object detection system 1 further includes a second radar 11 .
And the second radar 11:
a second output unit 13 that outputs a second wireless signal;
a second receiving unit 15 that receives a second reflected wave based on a second wireless signal;
and a second foreign object analyzing section 17 that analyzes the presence or absence of a foreign object using the second reflected wave received by the second receiving section.
In this case, the second radio signal is
a third signal having the same frequency change pattern as the first signal;
The fourth signal has the same frequency change pattern as the third signal, but has a different output timing.
When the difference in output timing between the first signal and the second signal is Δt _d1 and the difference in output timing between the third signal and the fourth signal is Δt _d2 , Δt _d1 and Δt _d2 are different.

This provides a foreign object detection system that does not suffer from the problem of time lag that occurs when switching optical fiber lengths, as occurs in conventional foreign object detection systems.

FIG. 1 is a schematic diagram for explaining a foreign object detection system. FIG. 2 is a conceptual diagram showing various signals in the foreign object detection system. FIG. 3 is a conceptual diagram for explaining the process by which the foreign object detection system removes peaks resulting from artifacts. FIG. 4 is a conceptual diagram showing an outline of a two-frequency FMCW radar. FIG. 5 is a diagram showing an outline of a transmission signal and a reflected signal when the transmission signal 1 is a local signal. FIG. 6 is a diagram showing an outline of a transmission signal and a reflected signal when the transmission signal 2 is a local signal. 7 is a PPI (Plane Position Indication) image in place of a drawing showing a PPI image in which _Δtd is 0.5 microseconds and transmitted signal 1 is the local signal. The vertical and horizontal axes indicate distance (m). 8 is a spectrum in place of a drawing showing a spectrum when _Δtd is 0.5 microseconds and the transmission signal 1 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m). 9 is a PPI image in place of a drawing showing a simulation result when _Δtd is 0.5 microseconds and the transmission signal 2 is a local signal. The vertical and horizontal axes indicate distance (m). 10 is a spectrum in place of a drawing showing a spectrum when _Δtd is 0.5 microseconds and the transmission signal 2 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m). 11 is a PPI image in place of a drawing showing a simulation result when _Δtd is 0.25 microseconds and the transmission signal 1 is a local signal. The vertical and horizontal axes indicate distance (m). 12 is a spectrum in place of a drawing showing a spectrum when _Δtd is 0.25 microseconds and the transmission signal 1 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m). 13 is a PPI image in place of a drawing showing a simulation result when _Δtd is 0.25 microseconds and the transmission signal 2 is a local signal. The vertical and horizontal axes indicate distance (m). 14 is a spectrum in place of a drawing showing a spectrum when _Δtd is 0.25 microseconds and the transmission signal 2 is a local signal. The vertical axis shows intensity (dBm) and the horizontal axis shows distance (m). FIG. 15 is a PPI image showing the results of AND processing of a two-frequency FMCW radar. 16 is a diagram showing an outline of the transmission signal and the reflected signal of a two-frequency FMCW linear cell radar when the transmission signal 1 is a local signal. The vertical axis indicates frequency, and the horizontal axis indicates time. Figure 17 is a diagram showing an overview of the transmission signal and the reflected signal of a two-frequency FMCW linear cell radar when the transmission signal 2 is a local signal. The vertical axis shows frequency, and the horizontal axis shows time. FIG. 18 is a conceptual diagram showing a linear cell radar experimental system. FIG. 19 is a side view of FIG. Figure 20 is a PPI image showing the simulation results when the transmission signal 1 is a local signal. The vertical and horizontal axes indicate distance (m). 21 is a spectrum, instead of a diagram, showing the spectrum when the transmission signal 1 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m). 22 is a PPI image showing the simulation results when the transmission signal 2 is a local signal. The vertical and horizontal axes indicate distance (m). 23 is a spectrum, instead of a diagram, showing the spectrum when the transmission signal 2 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m). FIG. 24 is a PPI image in place of a drawing showing the result of ANDing two PPI images (FIGS. 20 and 22). 25 is a PPI image in place of a drawing showing a simulation result when _Δtd of RAU1 is 0.5 microseconds, _Δtd of RAU2 is 0.25 microseconds, and transmission signal 1 is a local signal. The vertical and horizontal axes indicate distance (m). 26 is a spectrum instead of a diagram showing a spectrum when _Δtd of RAU1 is 0.5 microseconds, _Δtd of RAU2 is 0.25 microseconds, and transmission signal 1 is a local signal. The vertical axis indicates intensity (dBm) and the horizontal axis indicates distance (m). 27 is a PPI image in place of a drawing showing a simulation result when _Δtd of RAU1 is 0.5 microseconds, _Δtd of RAU2 is 0.25 microseconds, and transmission signal 2 is a local signal. The vertical and horizontal axes show distance (m). 28 is a spectrum instead of a diagram showing a spectrum when _Δtd of RAU1 is 0.5 microseconds, _Δtd of RAU2 is 0.25 microseconds, and transmission signal 2 is a local signal. The vertical axis indicates intensity (dBm) and the horizontal axis indicates distance (m). FIG. 29 is a PPI image in place of a drawing showing the result of ANDing two PPI images (FIGS. 25 and 27).

Below, the embodiments for carrying out the present invention will be explained using the drawings. The present invention is not limited to the embodiments described below, but also includes appropriate modifications of the embodiments below within the scope that would be obvious to a person skilled in the art.

Figure 1 is a schematic diagram for explaining a foreign object detection system. As shown in Figure 1, the foreign object detection system 1 includes a first radar 9.

The foreign object detection system 1 relates to a system for detecting foreign objects present on, for example, an airplane runway 21. An example of a foreign object is an object that is not present on the runway under normal conditions, such as an obstacle that impedes the progress of an airplane, and that impedes or may impede the safe operation of an airplane. In the example of an airplane, the foreign object is present on the runway. In other words, a foreign object is an object that is usually present in an area that a vehicle may pass through during normal operation. The following explanation will focus on a foreign object detection system for an airplane runway. Of course, this foreign object detection system 1 can be used for various purposes, such as a foreign object detection system for roads and a foreign object detection system for navigation. This system can also be installed on a road and used to notify self-driving vehicles of the presence of a foreign object.

The first radar 9 includes a first output unit 3, a first receiving unit 5, and a first foreign object analysis unit 7. Note that the output unit and the receiving unit may be physically the same.

As the radar 9, a known radar can be appropriately adopted. The radar can receive a synchronization signal (e.g., an optical signal), convert the received synchronization signal into a radio signal by a signal converter, or generate a signal by an oscillator synchronized with the synchronization signal, and emit the radio signal from a radio output unit. The radar can also receive a radio signal by its radio receiving unit, convert the received radio signal into a signal (e.g., an optical signal) by a signal converting unit, and output it from a received signal output unit. The radar can be a continuous wave (FMCW) radar or a pulse radar. In an FMCW radar, the transmitted wave and the received wave are input to a mixer in an analyzer, and the frequency difference between them is measured, so that the distance from the radar to a foreign object can be calculated. Fiber radio technology that superimposes a radio signal onto an optical fiber may be adopted. In this case, the synchronization signal is light modulated by the radar signal waveform itself. The synchronization signal may be an intermediate frequency band waveform that is in the middle of the radar signal waveform generation process. The synchronization signals transmitted to multiple radars are synchronized, and the timing can be adjusted accordingly. Fiber radio units and fiber radio systems are publicly known, as described in JP2010/001438A. The signal source may generate and output a synchronization signal using such a publicly known system.

As shown in FIG. 1, the radars are preferably installed at a predetermined interval on both ends of the runway. Some radars are installed on vehicles (airplanes, automobiles, motorcycles, bicycles, helicopters, drones). It is preferable that the radar 9 is a radar installed on the ground. In this case, the radar group may be located near the runway, avoiding the part where the airplane runs. For example, the radar group (first radar group) located on one end of the runway is also represented as RAU _1j (j is 1, 2, ...), and the radar group (second radar group) located on the other end of the runway is also represented as RAU _2i (i is 1, 2, ...). An example of the distance of the radars located in each group is 1 m or more and 5 km or less, and may be 10 m or more and 1 km or less, 100 m or more and 1 km or less, 200 m or more and 1 km or less, or 10 m or more and 100 m or less.

The first output unit 3 is an element for outputting a first wireless signal. The first wireless signal includes a first signal and a second signal that has the same frequency change pattern (pattern in which the frequency changes: waveform) as the first signal but has a different output timing. These two wireless signals may be physically output from one output device (e.g., an antenna), or may be physically output from multiple output devices. However, it is preferable that the two wireless signals are output from one output device.

The first signal and the second signal are preferably frequency modulated continuous waves (FMCW). The first signal and the second signal are preferably transmitted as one transmission wave. The signal may have a frequency that changes in a triangular wave shape with respect to time, or may have a frequency that changes in a sawtooth wave shape with respect to time. The difference in output timing between the first signal and the second signal is Δt _d1 , and the period or sweep width of each signal is ΔT. Then, Δt _d1 is preferably 1/100000 or more and 1/2 or less of ΔT, may be 1/10000 or more and 1/100 or less of ΔT, may be 1/5000 or more and 1/500 or less of ΔT, may be 1/3000 or more and 1/500 or less of ΔT, or may be 1/3000 or more and 1/1000 or less of ΔT. An example of Δt _d1 is 0.01 microseconds or more and 10 microseconds or less, and may be 0.1 microseconds or more and 1 microsecond or less.

The output unit 3 may be controlled by a signal source and a computer that controls the output of the signal source. The signal source and the output unit 3 may be connected by optical fiber. One signal source may be connected to multiple radars by an optical fiber network, and light from one signal source may control the output of multiple radars.

A computer has an input unit, an output unit, a control unit, a calculation unit, and a memory unit, and each element is connected by a bus or the like so as to be able to send and receive information. For example, the memory unit may store a control program, or various information. When specific information is input from the input unit, the control unit reads out the control program stored in the memory unit. The control unit then reads out the information stored in the memory unit as appropriate and transmits it to the calculation unit. The control unit also transmits the input information to the calculation unit as appropriate. The calculation unit performs calculation processing using the various information it receives, and stores the results in the memory unit. The control unit reads out the calculation results stored in the memory unit and outputs them from the output unit. In this manner, various processes are performed.

The first receiving unit 5 is an element for receiving a first reflected wave based on a first wireless signal. The first reflected wave includes a first reflected signal based on the first signal and a second reflected signal based on the second signal.

Figure 2 is a conceptual diagram showing various signals in a foreign object detection system. Note that in Figure 2, only one period of the first signal and the second signal is drawn. However, in reality, each signal may be a continuous wave. In the example of Figure 2, two FMCW signals that differ only in transmission timing are output as one transmission wave. In response to this transmission wave, the reflected wave from a target (e.g., a foreign object) also becomes two FMCW signals. One of the reflected waves becomes the desired signal, and a false image is generated by the other signal. Depending on which of the two FMCW signals is selected as the local signal, the analysis is performed so that the false image is generated at a different position. In general, in a radar system, the difference frequency between the received signal and the local signal from the local oscillator is calculated. The invention described in this specification, for example, uses a signal derived from one of the signals included in the transmission wave as a local signal, and calculates the difference frequency with the signal derived from the remaining signal included in the transmission wave.

The first foreign object analysis unit 7 is an element for analyzing the presence or absence of a foreign object using the first reflected wave received by the first receiving unit. The first foreign object analysis unit 7 may be implemented by a computer connected to the receiving unit 5. The first foreign object analysis unit 7 uses the first reflected signal and the second reflected signal to remove false images caused by interference. The interference may be caused by the inclusion of two signals in the output, or may be caused by something other than a foreign object.

It is preferable that the first foreign object analysis unit 7 analyzes the presence or absence of a foreign object using either the first signal or the second signal as a local signal. It is preferable that the first foreign object analysis unit 7 analyzes a signal that is analyzed to be present at a position common to the first reflected signal and the second reflected signal as a signal originating from a foreign object, and analyzes a signal that is analyzed to be present at a different position in the first reflected signal and the second reflected signal as a signal originating from a false image.

Figure 3 is a conceptual diagram for explaining the process of removing peaks due to false images in a foreign object detection system. Figure 3(a) is a conceptual diagram showing a PPI image when the first signal is a local signal. Figure 3(b) is a conceptual diagram showing a PPI image when the second signal is a local signal. Figure 3(c) is a conceptual diagram showing a PPI image obtained when performing AND processing (processing to extract only peaks common to both PPI images) on Figure 3(a) and Figure 3(b). A PPI image (planar position display image) is, for example, an image for displaying the presence of an object at a planar position from a certain radar. As shown in Figure 3, by obtaining a PPI image that displays only peaks present in both a PPI image obtained when one of the two signals is a local signal and a PPI image obtained when the other is a local signal, it is possible to extract only the signal (desired signal) originating from a foreign object. As shown in Figures 3(a) and 3(b), the desired signal in the PPI image (peak due to a foreign object) is located at the same position in the two PPI images. On the other hand, as shown in Figures 3(a) and 3(b), the signal (peak) due to an artifact in the PPI image is located at a different position in the two PPI images. Therefore, as shown in Figure 3(c), the peak due to the artifact can be removed by performing the above image processing (AND processing).

As shown in FIG. 1, the foreign object detection system 1 may include multiple radars. For example, the foreign object detection system 1 includes a second radar 11. As explained above, the foreign object detection system 1 is preferably one in which multiple radars are installed at a predetermined interval on both ends of the runway. The second radar 11 has a second output unit 13 that outputs a second radio signal, a second receiving unit 15 that receives a second reflected wave based on the second radio signal, and a second foreign object analysis unit 17 that analyzes the presence or absence of a foreign object using the second reflected wave received by the second receiving unit. These are similar to those of the first radar explained above.

In this case, the second radio signal includes a third signal having the same frequency change pattern as the first signal, and a fourth signal having the same frequency change pattern as the third signal but different output timing. The third signal and the first signal may be output at the same timing or at different timings. When the difference in output timing between the first signal and the second signal is Δt _d1 and the difference in output timing between the third signal and the fourth signal is Δt _d2 , it is preferable that Δt _d1 and Δt _d2 are different. In this case, too, an AND process may be performed on the obtained PPI image. In this way, false images caused by adjacent radars can also be removed.

As shown in Figure 1, when a foreign object detection system includes multiple radars, it is preferable that the output timing difference between the two signals is different at least for adjacent radars. Also, in the example shown in Figure 1, it is preferable that the output timing difference between the two signals is different for two opposing radars. In this way, interference from other radars can be avoided and false images resulting from signals from other radars can be suppressed. However, when a foreign object detection system includes multiple radars, it is not necessary to make the output timing difference between the two signals different for all radars.

Overview of dual-frequency FMCW radar Below, we propose a dual-frequency FMCW radar and examine it through simulation. First, we introduce the overview of the dual-frequency FMCW radar. Next, we introduce the results of a simulation using an actual dual-frequency FMCW radar. After that, we examine a system in which a dual-frequency FMCW radar is incorporated into a linear cell radar, and perform a simulation of the dual-frequency FMCW linear cell radar.

The radar image that undergoes AND processing must satisfy the following three conditions:
Condition 1: The target's appearance position (relationship between the transmitted wave and the desired wave) must not change in the two radar images.Condition 2: The appearance position of the false image must change in the two radar images.Condition 3: Two radar images must be acquired simultaneously to perform AND processing.We will consider whether the two-frequency FMCW radar meets these conditions.Figure 4 shows an overview of the signals used in the two-frequency FMCW radar.

In Figure 4, the vertical axis indicates frequency and the horizontal axis indicates time. In Figure 4, frequency means frequency, transmitted 1 means transmitted signal 1, transmitted 2 means transmitted signal 2, Reflected 1 means reflected signal 1, Reflected 2 means reflected signal 2, Desired Signal means desired signal, and Undesired Signal means undesired signal including false images. These terms are the same in the following. Note that RAD means radar (or radar head). Interference 1 means interference signal 1, and Interference 2 means interference signal 2.

Figure 4 is a conceptual diagram showing the outline of a dual-frequency FMCW radar. As shown in Figure 4, a dual-frequency FMCW radar uses a signal that combines two types of FMCW signals for the transmission wave. All parameters of these FMCW signals are the same except for the timing of the start of the sweep. Due to this transmission wave, the reflected wave from the target also becomes two types of FMCW signals. Therefore, in a dual-frequency FMCW radar, false images due to narrowband interference always occur.
First, let us consider condition 1. Depending on which of the two FMCW signals of the transmitted waves is designated as the local signal, one of the FMCW signals of the reflected waves will be the desired signal, and the other signal will generate a false image due to narrowband interference. Figure 5 shows a schematic diagram when transmitted signal 1 is considered as the local signal, and Figure 6 shows a schematic diagram when transmitted signal 2 is considered as the local signal.

That is, Figure 5 is a diagram showing an overview of the transmission signal and the reflected signal when transmission signal 1 is the local signal. Figure 6 is a diagram showing an overview of the transmission signal and the reflected signal when transmission signal 2 is the local signal.

As shown in Fig. 5, if transmission signal 1 is considered to be a local signal, reflected signal 1 is the desired signal. On the other hand, as shown in Fig. 6, if transmission signal 2 is considered to be a local signal, reflected signal 2 is the desired signal. The time from transmitting transmission signal 1 to receiving reflected signal 1 is assumed to be Δt. Then, the time from transmitting transmission signal 2 to receiving reflected signal 2 is also Δt. Therefore, transmission signal 1 and transmission signal 2 are all the same signals except for the transmission start timing. Therefore, regardless of which is considered to be the local signal, the difference frequency between the transmission signal and the desired signal will be the same frequency, and the distance at which the image of the target appears will be the same. Moreover, the difference frequency (beat frequency) f _b between the transmission signal and the desired signal at this time is expressed by equation (1).

f _b =2f _w ×Δt/ΔT (1)
where _fw and ΔT are the frequency sweep width and sweep time, respectively.

Next, let us consider condition 2. First, if we consider the transmission signal 1 as a local signal, a false image will be generated by the reflected signal 2. If the delay time between the transmission signal 1 and the transmission signal 2 is _Δtd , the time _Δt1 from when the transmission signal 1 is transmitted to when the reflected signal 2 is received is expressed by equation (2).

Δt ₁ =Δt+Δt _d (2)

Therefore, the difference frequency f _b1 between the transmission signal 1 and the reflected signal 2 is expressed by equation (3).

f _b1 =2f _w × (Δt+Δt _d )/ΔT (3)

From equations (1) and (3), f _b1 is greater than f _b . Therefore, the false image appears at a position farther away than the target. On the other hand, if the transmission signal 2 is considered as a local signal, a false image is generated by the reflected signal 1. The time Δt ₂ from the transmission of the transmission signal 2 to the reception of the reflected signal 1 is expressed by equation (4).

Δt ₂ = Δt−Δt _d (4)

Therefore, the difference frequency f _b2 between the transmission signal 2 and the reflected signal 1 is expressed by equation (5).
f _b2 =2f _w × (Δt−Δt _d )/ΔT (5)

From equations (.1) and (.5), f _b2 is smaller than f _b . Therefore, the false image appears closer to the target. From equations (3) and (5), we can see that the false image appears in a different position depending on the local signal.

Finally, we consider condition 3. With a dual-frequency FMCW radar, two FMCW signals are combined and received as a single reflected wave. In other words, one received wave contains information on two radar images, so if the receiver can process two local signals considered under conditions 1 and 2 simultaneously, this is equivalent to simultaneously acquiring two radar images with different fiber lengths. From the above, a dual-frequency FMCW radar meets the three conditions that can resolve the issues involved in switching optical fiber lengths, and it is possible to observe signals at the receiver while simultaneously suppressing interference through AND processing.

Simulation of dual-frequency FMCW radar A dual-frequency FMCW radar was used to verify whether it was possible to control only the position where the false image was generated. Two signal sources were placed at the same position. Although the same signal was transmitted from the two signal sources, a dual-frequency FMCW signal could be simulated by delaying only one of the signals. First, the time difference between the two FMCW signals of the transmission signal was set to 0.5 us (microseconds), the same as when 100 m of optical fiber was added, and the distance from the transmission source to the target was set to 200 m. The other parameters were set as shown in Table 1 below. Figures 7 and 8 show the results of the simulation when transmission signal 1 was set to a local signal as in Figure 5. Figures 9 and 10 show the results of the simulation when transmission signal 2 was set to a local signal as in Figure 6. In addition, in order to investigate whether the position of the false image changes depending on the delay time of the optical fiber length in a dual-frequency FMCW radar, a simulation was also performed using a signal delayed by 0.25 us, the same as when 50 m of optical fiber was added. The results are shown in Figures 11 to 14.

7 is a PPI (Plane Position Indication) image in place of a drawing showing a PPI image in which _Δtd is 0.5 microseconds and transmitted signal 1 is the local signal. The vertical and horizontal axes indicate distance (m).
8 is a spectrum in place of a drawing showing a spectrum when _Δtd is 0.5 microseconds and the transmission signal 1 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m).
9 is a PPI image in place of a drawing showing a simulation result when _Δtd is 0.5 microseconds and the transmission signal 2 is a local signal. The vertical and horizontal axes indicate distance (m).
10 is a spectrum in place of a drawing showing a spectrum when _Δtd is 0.5 microseconds and the transmission signal 2 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m).
11 is a PPI image in place of a drawing showing a simulation result when _Δtd is 0.25 microseconds and the transmission signal 1 is a local signal. The vertical and horizontal axes indicate distance (m).
12 is a spectrum in place of a drawing showing a spectrum when _Δtd is 0.25 microseconds and the transmission signal 1 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m).
13 is a PPI image in place of a drawing showing a simulation result when _Δtd is 0.25 microseconds and the transmission signal 2 is a local signal. The vertical and horizontal axes indicate distance (m).
14 is a spectrum in place of a drawing showing a spectrum when _Δtd is 0.25 microseconds and the transmission signal 2 is a local signal. The vertical axis shows intensity (dBm) and the horizontal axis shows distance (m).

From these results, it was confirmed that, as expected from theory, when transmission signal 1 is the local signal, the false image occurs at a position farther away than the target image, and when transmission signal 2 is the local signal, the false image occurs at a position closer to the target image. In addition, regardless of which signal is used as the local signal, the absolute value of the difference between the beat frequency of the desired signal and the frequency at which the false image appears is the same, so it was confirmed that the false image appears at a position close to and far from the transmission source by the same distance, with the target image at 200 m as the boundary. Note that the size of the false image on the PPI (plan position indicator) image is different. This is due to the spread of the antenna beam.
Next, comparing Figure 7 and Figure 11, it can be seen that the position where the false image appears changes depending on the difference in delay time. Therefore, even in a two-frequency FMCW radar, it is possible to control the movement of the false image occurrence position by adding optical fiber length in the same way.
FIG. 15 shows the result of performing an AND process on FIG. 7 and FIG.

FIG. 15 is a PPI image showing the results of AND processing of a two-frequency FMCW radar.
In Figure 15, it can be seen that only the target image remains and only the false image has been removed. Therefore, it was confirmed that interference suppression processing can be performed even in a two-frequency FMCW radar.
From the above, it can be said that the two-frequency FMCW radar is a system that can solve the problems of conventional systems.

Overview of dual-frequency FMCW linear cell radar We consider the case where the dual-frequency FMCW signal described above is applied to a linear cell radar system including multiple RAUs (radars). In this system, all RAUs transmit dual-frequency FMCW signals, so the signals observed as interference waves are also dual-frequency FMCW signals. Figures 16 and 17 show the cases where transmission signal 1 and transmission signal 2 are local signals, respectively.

16 is a diagram showing an outline of the transmission signal and the reflected signal of a two-frequency FMCW linear cell radar when the transmission signal 1 is a local signal. The vertical axis indicates frequency, and the horizontal axis indicates time.
Figure 17 is a diagram showing an overview of the transmission signal and the reflected signal of a two-frequency FMCW linear cell radar when the transmission signal 2 is a local signal. The vertical axis shows frequency, and the horizontal axis shows time.

If the delay time Δt _d between the transmission signal 1 and the transmission signal 2 is the same in all RAUs in the two-frequency FMCW linear cell radar, the delay time between the interference signal 1 and the interference signal 2 observed as interference waves will also be Δt _d . As a result, the time from when the transmission signal 1 is transmitted until the interference signal 1 is observed will be the same as the time from when the transmission signal 2 is transmitted until the interference signal 2 is observed, and the difference frequency between them will also have the same value of f _i . In other words, the position where the false images caused by those signals are generated is the same, and the false images cannot be removed even by performing AND processing on the radar image. Therefore, in order to suppress interference by AND processing in the two-frequency FMCW linear cell radar, it is preferable to change the value of Δt _d at least between adjacent RAUs in order to make the difference frequency between the transmission wave and the interference wave different depending on the local signal.

Simulation of dual-frequency FMCW linear cell radar A simulation was performed on a dual-frequency FMCW linear cell radar using a dual-frequency FMCW signal in the experimental system shown in Figures 18 and 19. First, the simulation results when the delay times Δt _d of each RAU were all 0.25 us are shown in Figures 20 to 24, respectively.

FIG. 18 is a conceptual diagram showing a linear cell radar experimental system.
FIG. 19 is a side view of FIG.
Figure 20 is a PPI image showing the simulation results when the transmission signal 1 is a local signal. The vertical and horizontal axes indicate distance (m).
21 is a spectrum, instead of a diagram, showing the spectrum when the transmission signal 1 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m).
22 is a PPI image showing the simulation results when the transmission signal 2 is a local signal. The vertical and horizontal axes indicate distance (m).
23 is a spectrum, instead of a diagram, showing the spectrum when the transmission signal 2 is a local signal. The vertical axis shows the intensity (dBm) and the horizontal axis shows the distance (m).
FIG. 24 is a PPI image in place of a drawing showing the result of ANDing two PPI images (FIGS. 20 and 22).

From Fig. 24, it was confirmed that in a two-frequency FMCW linear cell radar using RAUs with the same delay time _Δtd , even if AND processing is performed on two radar images acquired by changing the local signal, false images that cannot be completely removed occur. In order to suppress this false image by AND processing, the delay time _Δtd of RAU1 is set to 0.5 us and the delay time _Δtd of RAU2 is set to 0.25 us (microseconds), and the simulation results are shown in Figs. 25 to 29, respectively.

25 is a PPI image in place of a drawing showing a simulation result when _Δtd of RAU1 is 0.5 microseconds, _Δtd of RAU2 is 0.25 microseconds, and transmission signal 1 is a local signal. The vertical and horizontal axes indicate distance (m).
26 is a spectrum instead of a diagram showing a spectrum when _Δtd of RAU1 is 0.5 microseconds, _Δtd of RAU2 is 0.25 microseconds, and transmission signal 1 is a local signal. The vertical axis indicates intensity (dBm) and the horizontal axis indicates distance (m).
27 is a PPI image in place of a drawing showing a simulation result when _Δtd of RAU1 is 0.5 microseconds, _Δtd of RAU2 is 0.25 microseconds, and transmission signal 2 is a local signal. The vertical and horizontal axes show distance (m).
28 is a spectrum instead of a diagram showing a spectrum when _Δtd of RAU1 is 0.5 microseconds, _Δtd of RAU2 is 0.25 microseconds, and transmission signal 2 is a local signal. The vertical axis indicates intensity (dBm) and the horizontal axis indicates distance (m).
FIG. 29 is a PPI image in place of a drawing showing the result of ANDing two PPI images (FIGS. 25 and 27).

Figure 29 shows that in a two-frequency FMCW linear cell radar, the delay time Δt _d can be changed for each RAU to make the difference frequency between the transmitted wave and the interference wave different values depending on the local signal, and interference suppression can be performed by AND processing on the two obtained radar images.

The dual-frequency FMCW radar solved the problem of the time lag in radar image acquisition caused by switching the optical fiber length, which was a challenge in avoiding interference in conventional systems, and demonstrated that it is possible to simultaneously acquire radar images from two different fiber lengths. Simulations demonstrated that the principle of the dual-frequency FMCW radar is correct, and we succeeded in suppressing interference by AND processing using radar images obtained by the dual-frequency FMCW radar. Furthermore, we investigated the points to note when incorporating a dual-frequency FMCW signal into a linear cell radar, and demonstrated that interference suppression by AND processing is possible by changing the value of _Δtd between adjacent RAUs.

The invention described in this specification relates to a foreign object detection system and can be used, for example, in the information and communications industry, aviation industry, automotive industry, and construction industry.

REFERENCE SIGNS LIST 1 Foreign object detection system 3 First output section 5 First receiving section 7 First foreign object analysis section 9 First radar 11 Second radar 13 Second output section 15 Second receiving section 17 Second foreign object analysis section

Claims (5)

a first output unit (3) that outputs a first wireless signal;
A first receiving unit (5) that receives a first reflected wave based on a first wireless signal;
a first foreign object analyzing unit (7) that analyzes the presence or absence of a foreign object using the first reflected wave received by the first receiving unit;
A foreign object detection system (1) including a first radar (9) having
the first wireless signal includes a first signal and a second signal having the same frequency change pattern as the first signal but different output timing from the first signal;
the first reflected wave includes a first reflected signal based on the first signal and a second reflected signal based on the second signal;
The first foreign object analysis unit (7) performs AND processing on a PPI image obtained when the first reflected signal is used as a local signal, and a PPI image obtained when the second reflected signal is used as a local signal, thereby removing a false image caused by interference.
Foreign object detection system. 2. A foreign object detection system according to claim 1,
A foreign object detection system, wherein the first signal and the second signal are frequency modulated continuous waves (FMCW). 2. A foreign object detection system according to claim 1,
The first foreign matter analysis unit (7)
Either the first signal or the second signal is used as a local signal to analyze the presence or absence of a foreign object.
Foreign object detection system. 2. A foreign object detection system according to claim 1,
The first foreign matter analysis unit (7)
A signal that is analyzed to be present at a position common to the first reflected signal and the second reflected signal is determined to be a signal originating from a foreign object;
A signal that is analyzed to be present at a different position in the first reflected signal and the second reflected signal is defined as a signal originating from the false image.
Foreign object detection system. 2. A foreign object detection system according to claim 1,
Further comprising a second radar (11),
The second radar (11)
a second output unit (13) that outputs a second radio signal;
a second receiving unit (15) that receives a second reflected wave based on a second wireless signal;
a second foreign object analyzing unit (17) that analyzes the presence or absence of a foreign object using the second reflected wave received by the second receiving unit;
Having
The second wireless signal is
a third signal having the same frequency change pattern as the first signal;
a fourth signal having the same frequency change pattern as the third signal and a different output timing;
When the difference in output timing between the first signal and the second signal is Δtd1 and the difference in output timing between the third signal and the fourth signal is Δtd2, Δtd1 and Δtd2 are different.
Foreign object detection system.

Images