阅读说明：本技术 激光雷达系统 (Laser radar system ) 是由 王超 于 2019-03-07 设计创作，主要内容包括：本发明涉及一种激光雷达系统。一种激光雷达系统,包括：发射装置,用于发射激光信号；分束器,用于将所述激光信号分成本振激光信号和探测激光信号；扫描装置,用于将所述探测激光信号投射至探测区域,还用于接收由探测区域中目标反射的回波激光信号；放大转换模块,用于接收所述扫描装置反射的回波激光信号,并对所述回波激光信号进行放大后输出；耦合器,用于接收所述本振激光信号和放大后的所述回波激光信号,耦合后输出接收激光信号；以及接收装置,用于接收所述接收激光信号,并根据所述接收激光信号计算所述探测区域中目标的位置和/或速度信息。上述激光雷达系统相对于传统的激光雷达系统,信噪比和灵敏度更高。(The present invention relates to a laser radar system. A lidar system comprising: a transmitting device for transmitting a laser signal; the beam splitter is used for splitting the laser signal into a local oscillator laser signal and a detection laser signal; the scanning device is used for projecting the detection laser signal to a detection area and receiving an echo laser signal reflected by a target in the detection area; the amplification conversion module is used for receiving the echo laser signal reflected by the scanning device, amplifying the echo laser signal and outputting the amplified echo laser signal; the coupler is used for receiving the local oscillator laser signal and the amplified echo laser signal, and outputting a received laser signal after coupling; and a receiving device for receiving the received laser signal and calculating the position and/or velocity information of the target in the detection area according to the received laser signal. Compared with the traditional laser radar system, the laser radar system has higher signal-to-noise ratio and sensitivity.)

1. A lidar system, comprising:

a transmitting device for transmitting a laser signal;

the beam splitter is used for splitting the laser signal into a local oscillator laser signal and a detection laser signal;

the scanning device is used for projecting the detection laser signal to a detection area and receiving an echo laser signal reflected by a target in the detection area;

the amplification conversion module is used for receiving the echo laser signal reflected by the scanning device, amplifying the echo laser signal and outputting the amplified echo laser signal;

the coupler is used for receiving the local oscillator laser signal and the amplified echo laser signal, and outputting a received laser signal after coupling; and

and the receiving device is used for receiving the receiving laser signal and calculating the position and/or speed information of the target in the detection area according to the receiving laser signal.

2. The lidar system of claim 1, wherein the amplification conversion module comprises an amplifier and a filter, the amplifier is configured to amplify the received echo laser signal, and the filter is configured to suppress noise generated by spontaneous emission in the amplifier.

3. The lidar system of claim 2, wherein the amplifier is a fiber amplifier.

4. The lidar system of claim 1, wherein an optical coupling mirror is further disposed at one end of the amplification conversion module, and the optical coupling mirror is configured to couple the echo laser signal into the amplification conversion module.

5. The lidar system of claim 2, wherein the amplification conversion module is further configured to receive the detection laser signal and to transmit the detection laser signal to the scanning device.

6. The lidar system of claim 5, wherein the amplification conversion module further comprises a circulator, wherein the circulator allows the probe laser signal to enter from the first port and exit from the second port, and wherein the circulator allows the amplified echo laser signal to enter from the second port and exit from the third port.

7. The lidar system of claim 5, wherein the amplifier is a unidirectional amplifier that amplifies only the echo laser signal.

8. The lidar system of claim 1, wherein the local oscillator laser signal and the amplified echo laser signal enter the coupler through two inlets respectively to interfere with each other to form the received laser signal, and are output through an outlet.

9. The lidar system of claim 1, wherein an optical isolator is disposed between the transmitter and the beam splitter, the optical isolator allowing only one-way passage of the transmitted laser signal.

10. The lidar system of claim 1, wherein the receiving apparatus comprises a signal filter and a receiving array, the signal filter is configured to filter out low-frequency dc signals and high-frequency noise signals introduced by interference in the received laser signal.

Technical Field

The invention relates to the technical field of laser detection, in particular to a laser radar system.

Background

The lidar system is a system for detecting characteristic quantities such as a position and a speed of a target by emitting a laser signal, and sensitivity of the lidar system is one of important performance parameters for determining a ranging range of the lidar system.

Conventional lidar systems often employ solid state lidar. The receiving end of an area array laser radar in the solid-state laser radar generally adopts a pixel-level detector array, and is easily influenced by external environment background light or stray signal light when measuring outdoors, particularly under the conditions of severe weather such as rainy days, foggy days, snowy days and the like, so that the signal-to-noise ratio is deteriorated, and the sensitivity is reduced, thereby limiting the farthest detection distance of the system.

Disclosure of Invention

Therefore, it is necessary to provide a laser radar system to solve the problems of poor signal-to-noise ratio and low sensitivity of the conventional laser radar system.

A lidar system comprising:

a transmitting device for transmitting a laser signal;

the beam splitter is used for splitting the laser signal into a local oscillator laser signal and a detection laser signal;

the scanning device is used for projecting the detection laser signal to a detection area and receiving an echo laser signal reflected by a target in the detection area;

the amplification conversion module is used for receiving the echo laser signal reflected by the scanning device, amplifying the echo laser signal and outputting the amplified echo laser signal;

the coupler is used for receiving the local oscillator laser signal and the amplified echo laser signal, and outputting a received laser signal after coupling; and

and the receiving device is used for receiving the receiving laser signal and calculating the position and/or speed information of the target in the detection area according to the receiving laser signal.

In one embodiment, the amplification conversion module includes an amplifier for amplifying the received echo laser signal and a filter for suppressing noise generated by spontaneous emission in the amplifier.

In one embodiment, the amplifier is a fiber amplifier.

In one embodiment, an optical coupling mirror is further disposed at one end of the amplification conversion module, and the optical coupling mirror is configured to couple the echo laser signal into the amplification conversion module.

In one embodiment, the amplification conversion module is further configured to receive the detection laser signal and transmit the detection laser signal to the scanning device.

In one embodiment, the amplification conversion module further includes a circulator, the circulator enables the probe laser signal to enter from the first port and exit from the second port, and the circulator enables the amplified echo laser signal to enter from the second port and exit from the third port.

In one embodiment, the amplifier is a unidirectional amplifier, and only the echo laser signal is amplified.

In one embodiment, the local oscillator laser signal and the amplified echo laser signal enter the coupler through two inlets respectively to interfere with each other to form the received laser signal, and the received laser signal is output through one outlet.

In one embodiment, an optical isolator is arranged between the transmitting device and the beam splitter, and the optical isolator only allows the transmitting laser signal to pass through in one direction.

In one embodiment, the receiving device comprises a signal filter and a receiving array, wherein the signal filter is used for filtering out low-frequency direct current signals and high-frequency noise signals introduced by interference in the receiving laser signals.

The laser radar system comprises a transmitting device, a beam splitter, a scanning device, an amplification conversion module, a coupler and a receiving device. Laser signals transmitted by the transmitting device are divided into local oscillation laser signals and detection laser signals through a beam splitter; when a target in the detection area is met, an echo laser signal reflected by the target is received by the scanning device and then projected to the amplification conversion module; the amplification conversion module amplifies the echo laser signal and outputs the amplified echo laser signal to the coupler; the coupler couples the received local oscillation laser signal and the echo laser signal to form a received laser signal and outputs the received laser signal; the receiving device receives the received laser signal and calculates position and/or velocity information of the target in the detection area based on the received laser signal. The amplification conversion module amplifies the echo laser signal, so that the influence of ambient background light or stray light can be reduced; even if the echo laser signal is weak in severe weather such as rainy days, foggy days and the like, the echo laser signal can be effectively received and identified by the receiving device after being received and amplified; the system has high signal-to-noise ratio and sensitivity and small limit on the farthest detection distance. And the optical coupling mirror can couple weak echo laser signals into the optical fiber amplifier, although the coupling efficiency of the optical fiber is limited, the mode field matching effect of the optical fiber is equivalent to a filter, and noise introduced by environment or other uncertain factors can be filtered. Before the optical fiber amplifier amplifies the echo laser signal, the optical fiber amplifier can also filter the echo laser signal, so that the noise in the echo laser signal is not amplified, and the signal-to-noise ratio of the system is favorably improved. Moreover, conventional circuit elements such as an amplifier, a detector and the like often generate thermal noise, shot noise and the like under the working state, and the noise is mixed with an actual signal, so that the detection precision of the laser radar system is reduced; the amplification conversion module of the laser radar system comprises a filter and an amplifier, wherein the filter can inhibit noise generated by spontaneous radiation in the amplifier while the amplifier amplifies an echo laser signal, so that the signal-to-noise ratio and the sensitivity of the system are further improved. In addition, the receiving apparatus includes a signal filter and a receiving array; the signal filter can filter low-frequency direct current signals and high-frequency noise signals which are introduced due to interference in the received laser signals, so that the received laser signals received by the receiving array are more accurate, and the overall sensitivity of the system is higher.

Drawings

Fig. 1 is a schematic structural diagram of a lidar system in an embodiment.

Fig. 2 is a schematic structural diagram of a laser radar system in another embodiment.

Fig. 3 is a schematic structural diagram of a laser radar system in another embodiment.

Fig. 4 is a schematic diagram of basic principles of FMCW mode-based ranging in an embodiment.

Fig. 5 is a schematic diagram of a basic principle of FMCW mode-based velocity measurement in an embodiment.

FIG. 6 is a diagram illustrating an angle between a moving direction of a target and a direction of a detection laser signal according to an embodiment.

Detailed Description

In order to make the objects, technical solutions and advantages of the present application more apparent, the present application is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.

In one embodiment, as shown in fig. 1, a lidar system includes a transmitting apparatus 100, a beam splitter 200, a scanning apparatus 300, an amplification conversion module 402, a coupler 500, and a receiving apparatus 600.

The transmitting device 100 is used for transmitting a laser signal. Optionally, the laser signal emitted by the emitting device 100 is a Frequency Modulated Continuous Wave (FMCW) laser signal. The transmitting device 100 may include a laser, which is a tuned laser, and a modulator, which is a phase modulator. The tuned laser is used to generate a laser signal with a variable frequency. The phase modulator is used for linearly modulating the laser signal to emit the laser signal with continuous frequency.

The beam splitter 200 is configured to split the laser signal transmitted by the transmitting apparatus 100 into a local oscillator laser signal and a detection laser signal. The laser signal emitted by the emitting device 100 can be transmitted to the beam splitter 200 through an optical fiber, and the beam splitter 200 splits the laser signal in one optical fiber into two optical fibers, wherein one optical fiber is used as a local oscillator laser signal, and the other optical fiber is used as a detection laser signal. The detection laser signal is deflected by an optical element, which may be the scanning device 300, and then projected to a detection area to scan the entire exploration scene.

The scanning device 300 deflects the detection laser signal and projects the signal to the detection area. When the probe laser signal encounters the target 900 in the probe region, the target 900 reflects the echo laser signal. The scanning device 300 is further configured to receive the echo laser signal reflected by the target 900 in the detection area and reflect the echo laser signal to the amplification conversion module 402.

The amplification conversion module 402 is configured to receive the echo laser signal reflected by the scanning device 300, amplify the echo laser signal, and output the amplified echo laser signal. In this embodiment, the lidar system employs an off-axis transceiver structure. Specifically, the detection laser signal is deflected by the scanning device 300 and projected to the detection area. The echo laser signal reflected by the target 900 in the detection area is received by the scanning device 300 and then reflected to the amplification conversion module 402, and the amplification conversion module 402 amplifies the echo laser signal and outputs the amplified echo laser signal to the coupler 500. Only the echo laser signal passes through the amplification conversion module 402, the detection laser signal does not pass through the amplification conversion module 402, the optical axes of the detection laser signal and the echo laser signal are different, and the detection laser signal and the echo laser signal respectively pass through different optical paths to avoid mutual interference.

The coupler 500 receives the local oscillator laser signal split by the beam splitter 200 and the echo laser signal amplified by the amplification conversion module 402, and couples the local oscillator laser signal and the echo laser signal into a bundle of optical fibers to form a received laser signal and output the received laser signal, that is, the local oscillator laser signal and the echo laser signal interfere in the coupler 500, and the received laser signal is a laser signal obtained after the local oscillator laser signal and the echo laser signal interfere. Coupler 500 is a passive device in optical communications. It is a device that distributes and combines light from multiple input and output optical fibers. It can couple light from several optical fibers to several other optical fibers at the same wavelength, or can separate light from one optical fiber to several optical fibers. The coupler 500 couples the local oscillator laser signal in one optical fiber and the echo laser signal in the other optical fiber to one optical fiber, and the coupled laser signals are received laser signals. Optionally, coupler 500 is a 3dB coupler. One of the important parameters of a coupler is the coupling ratio, which represents the amount of power coupled from the input channel to a given output channel. A 3dB coupler refers to a coupler with a coupling ratio of 50: 50.

The receiving device 600 receives the received laser signal output by the coupler 500. Since the received laser signal is obtained by interfering the local oscillator laser signal and the echo laser signal, the received laser signal includes difference frequency information of the local oscillator laser signal and the echo laser signal, and information such as the position and the speed of the target 900 in the detection area can be calculated according to the difference frequency information.

The laser radar system includes a transmitting device 100, a beam splitter 200, a scanning device 300, an amplification conversion module 402, a coupler 500, and a receiving device 600. The laser signal transmitted by the transmitting device 100 is divided into a local oscillator laser signal and a detection laser signal by the beam splitter 200; wherein, the local oscillator laser signal is received by the coupler 500; the detection laser signal is projected to the detection area through the scanning device 300, when encountering the target 900 in the detection area, the echo laser signal reflected by the target 900 is received by the scanning device 300 and projected to the amplification conversion module 402, and the amplification conversion module 402 amplifies the echo laser signal and outputs the amplified echo laser signal to the coupler 500; the coupler 500 couples the received local oscillation laser signal and the echo laser signal to form a received laser signal and outputs the received laser signal; the receiving device 600 receives the received laser signal and calculates position and/or velocity information of the target 900 in the detection area from the received laser signal. The influence of the ambient background light or stray light can be reduced by amplifying the echo laser signal through the amplification conversion module 402; even if the echo laser signal is weak in bad weather such as rainy days and foggy days, the echo laser signal can be ensured to be received by the coupler 500; moreover, echo laser signals in the laser radar system can be transmitted through the optical fiber, although the coupling efficiency of the optical fiber is limited, the mode field matching effect of the optical fiber is equivalent to a filter, and noise introduced by environment or other uncertain factors can be filtered. The amplification conversion module 402 can also filter the echo laser signal while amplifying the echo laser signal, so that the noise in the echo laser signal is not amplified, which is beneficial to improving the signal-to-noise ratio of the system. The system has high signal-to-noise ratio and sensitivity and small limit on the farthest detection distance.

In this embodiment, the amplification conversion module 402 includes an amplifier and a filter. The amplifier is used for amplifying the received echo laser signal. The filter is used for suppressing noise generated by spontaneous radiation in the amplifier. Conventional circuit components, such as amplifiers, tend to generate thermal noise, shot noise, etc. during operation. The noises are mixed with the actual echo laser signals, so that the detection precision of the laser radar system is reduced; the amplification conversion module 402 of the laser radar system includes a filter and an amplifier, and the filter can suppress noise generated by spontaneous radiation in the amplifier while the amplifier amplifies the echo laser signal, so that the signal-to-noise ratio and the sensitivity of the system are further improved, and the final measurement result is more accurate. Optionally, the filter in the amplification conversion module 402 is a band pass filter. In other embodiments, the filter may also be an optical fiber disposed inside or outside the amplifier, and the mode field matching effect of the optical fiber may also be used to filter noise generated by the amplifier itself.

In this embodiment, the amplifier is a fiber amplifier. An Optical Fiber Amplifier (OFA) is a novel all-Optical amplifier that can be used in an Optical communication line to amplify signals. The conventional optical fiber transmission system adopts an optical-electrical-optical regenerative repeater, and the OFA can directly perform all-optical amplification on echo laser signals without complex processes such as photoelectric conversion, electro-optical conversion, signal regeneration and the like. And the input end and the output end of the OFA are both optical fiber connectors, so that the echo laser signal can be filtered by using the mode field matching effect of the optical fiber while the echo laser signal is amplified, the noise in the echo laser signal is not amplified, and the signal-to-noise ratio of the laser radar system is higher. Optionally, the amplifier is an erbium doped fiber amplifier.

In the present embodiment, as shown in fig. 1, an optical coupling mirror 406 is further disposed at one end of the amplification conversion module 402. The optical coupling mirror 406 is disposed on a side of the amplification conversion module 402 that receives the echo laser signal. The purpose of the optical coupling mirror 406 is to couple light into the optical fiber. The optical fiber connectors used at the input and output of the OFA cannot be directly coupled with the free space. Since there is a free space between the scanning device 300 and the amplification conversion module 402, an optical coupling mirror 406 is required to couple the echo laser signal reflected from the scanning device 300 into the optical fiber connector, so as to enter the OFA. The echo laser signal may propagate through the optical fiber between the amplification conversion module 402 and the coupler 500, and thus the optical coupler 406 is not required. Optionally, coupling mirror 406 is a tapered coupling mirror. The cone angle of the cone coupling mirror is connected with the optical fiber joint of the OFA. The tapered coupling mirror can couple weak echo laser signals into the OFA, and the sensitivity of the laser radar system is high.

In another embodiment, the amplification and conversion module 402 is further configured to receive the detection laser signal in addition to the reflected laser signal, and transmit the detection laser signal to the scanning apparatus 300, that is, as shown in fig. 2, the lidar system employs a coaxial transceiver structure. Specifically, the detection laser signal comes out of the beam splitter 200 and sequentially passes through the amplification conversion module 402, the optical coupling mirror 406 and the scanning device 300, the scanning device 300 scans the whole detected region, and when encountering the target 900, the echo laser signal reflected by the target 900 sequentially passes through the scanning device 300, the optical coupling mirror 406 and the amplification conversion module 402, and finally is transmitted to the coupler 500. The detection laser signal and the echo laser signal both pass through the amplification conversion module 402, the optical coupling mirror 406 and the scanning device 300, and the optical paths are coaxial. The coaxial transceiving structure can avoid the ranging error brought by the structure, and the design of the coaxial transceiving structure is simpler.

In this embodiment, the amplifier is a unidirectional amplifier. In the above coaxial transmission/reception structure, the amplifier does not amplify the passing probe laser signal, but amplifies only the echo laser signal.

In this embodiment, the amplification conversion module 402 further includes a circulator. The circulator is a multi-port device, and enables a detection laser signal to be input from a first port and output from a second port (not shown); meanwhile, the circulator also enables the amplified echo laser signal to be input from the second port and output from the third port; the port for inputting the detection laser signal to the circulator and the port for outputting the amplified echo laser signal from the circulator are not the same port. In other embodiments, a second optical coupler 404 may be disposed between the amplification conversion module 402 and the coupler 500, as shown in fig. 3. The second optical coupler 404 functions as the circulator, and ensures that the port for inputting the detection laser signal and the port for outputting the amplified echo laser signal are not the same port.

In this embodiment, the local oscillator laser signal and the amplified echo laser signal respectively enter the coupler 500 through two inlets. The local oscillator laser signal and the amplified echo laser signal interfere in the coupler 500 to form a received laser signal, and the received laser signal is output through an outlet.

In the present embodiment, an optical isolator 700 is disposed between the transmitter 100 and the beam splitter 200. Optical isolator 700 is a passive optical device that allows only one-way light to pass through. The optical isolator 700 allows the laser signal emitted from the emitting device 100 to be transmitted only in the direction of the beam splitter 200.

In the present embodiment, the receiving apparatus 600 includes a signal filter and a receiving array. The receiving array is used for receiving the receiving laser signal output by the coupler 500. Optionally, Single Photon Avalanche Diodes (SPAD) are used in the receive array. The single photon avalanche diode has extremely high sensitivity, and is beneficial to further improving the detection rate of the laser radar system, thereby improving the sensitivity and the detection threshold of the system and realizing the detection of the target 900 at a longer distance. A signal filter is disposed between the coupler 500 and the receive array. The signal filter is used for filtering low-frequency direct current signals and high-frequency noise signals which are introduced due to interference in the received laser signals output by the coupler 500, so that the received laser signals received by the receiving array are more accurate, and the overall sensitivity of the system is higher.

In this embodiment, the receiving apparatus 600 further includes a processor. The processor is configured to calculate information such as a distance and a speed of the target 900 with respect to the lidar system based on the received laser signal. Thus, the user can directly know the distance, speed, etc. of the target 900 in the detection area through the laser radar system. The specific calculation process is as follows.

It will be appreciated that the transmitting device 100 described above is capable of transmitting a frequency modulated continuous laser signal, the lidar system being an FMCW-based lidar system. The laser radar system based on FMCW obtains distance information of the target 900 by comparing a difference between an echo laser signal frequency at an arbitrary time and a local oscillator laser signal frequency at the time, and the distance is proportional to a frequency difference between the two frequencies. The radial velocity and distance of the target 900 can be obtained by processing the measured difference between the two frequencies. The requirement of the laser radar system based on FMCW on the energy of the detectable echo laser signal is lower than that of the laser radar system based on the traditional Time of flight (TOF) method, the performances of low transmitting power, anti-interference capability and high detection precision which cannot be achieved by the laser radar system in the TOF mode on the premise of low cost and safety can be effectively compensated, and the distance and speed information of the target 900 in the detection area can be obtained simultaneously. FMCW calculates the absolute distance of the target 900 by using the relationship between the difference frequency and the time delay of the continuous spectrum, and fig. 4 is a correlation curve of the frequency and the time of the local oscillator laser signal, the echo laser signal, and the difference frequency signal of the local oscillator laser signal and the echo laser signal when the target 900 and the laser radar system are relatively stationary. In the figure, the dotted line represents an echo laser signal, and the solid line represents a local oscillator laser signal. After the delay of time τ, a difference frequency Δ f is generated, and the absolute distance R between the target 900 and the laser radar system is related to the delay τ by the following equation:

where c is the speed of light. In fig. 4, it can be seen from the similar geometrical relationships:

wherein, Δ F is the peak value of the local oscillation laser signal frequency, and T is the period of the local oscillation laser signal. And since the laser is linear frequency conversion, the frequency modulation speed V of the laser can be expressed as:

from the above equations (1), (2), (3), the absolute distance R at which a radar measurement can be obtained can be expressed as:

where V is the frequency modulation speed (in Hz/s) of the laser.

When resolving the speed information of the target 900 based on the FMCW lidar speed measurement principle, the intermediate frequency of the signal is obtained by combining a certain signal processing algorithm with the measurement signal, and then the doppler shift generated by the movement of the target 900 can be obtained according to the doppler effect, so that the movement speed of the target 900 can be calculated according to the doppler shift formula, specifically, as shown in fig. 5, the doppler shift △ f can be obtained according to the frequency shift characteristic of the doppler effect_DoppRelationship to velocity v of target 900:

as shown in FIG. 6, the angle between the moving direction of the target 900 and the direction of the detection laser signal projected onto the target 900 is α, c is the speed of light, f₀The frequency of the laser signal is transmitted for the transmitting device 100.

Also, from the geometrical relationship in fig. 5, it can be derived:

Δf_{Diff_down}+Δf_{Diff_up}＝2Δf_Dopp(6)

the moving speed of the target 900 can be obtained by calculating from the above equations (5) and (6), that is, the relative speed of the target 900 and the lidar system can be obtained by analyzing the received laser signal and performing corresponding calculation. In this embodiment, a balanced photodetector may also be included in the lidar system. A balanced photodetector is disposed between the coupler 500 and the receiving array. The balanced photoelectric detector is actually provided with two built-in channels, two photodiodes with completely similar characteristics are used as photoelectric conversion, one channel is added with a delay line, or the front end of the balanced photoelectric detector is provided with a Mach-Zehnder interferometer to adjust the phase reverse bias of the other channel. The back end uses a differential amplifier to amplify the differential mode signal and suppress the common mode signal. After the two paths are added, the noises are completely balanced, and the output amplitude is greatly amplified. Balanced photodetectors are often used for weak signal detection and Differential Phase Shift Keying (DPSK) and Differential detection applications, among others.

The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.