Patent Application
20220083071
The present disclosure relates to the field of positioning technologies, and in particular, to an apparatus for implementing relative positioning and a corresponding relative positioning method.
The statements in this section are intended only to provide background information relevant to the present disclosure to aid in its understanding. Unless expressly stated, what is described in this section does not constitute prior art.
In many application scenarios, there is a need to determine the position of a device or machine. For example, the position of a vehicle needs to be determined during the navigation of the vehicle. In addition, in some factories, robots or self-driving vehicles are used for distribution or delivery of goods. In a process of distribution or delivery of goods, the positions of these robots or self-driving vehicles need to be determined.
At present, mainstream positioning methods are usually based on wireless signals, for example, GPS positioning, Wi-Fi positioning, and Bluetooth positioning. However, these positioning methods are susceptible to signal interference, making it difficult to obtain accurate positioning results.
Positioning based on visual markers can overcome these disadvantages to some extent. However, limited by imaging accuracy, such methods only find some applications in short-range positioning. For example, visual markers called AR markers are used in some augmented reality applications to determine the position and attitude of a camera in close proximity.
To address the above problems, the present disclosure provides an apparatus that can implement relative positioning with high accuracy and a corresponding relative positioning method.
An aspect of the present disclosure is directed to an apparatus for implementing relative positioning, including: one or more first positioning markers defining a plane; and one or more second positioning markers, at least a subset of the second positioning marker(s) being located outside the plane in which the first positioning marker(s) are located, where the first positioning marker(s) and the second positioning markers emit or reflect light to be acquired by an imaging device.
In some embodiments, the apparatus includes at least three first positioning markers that are located in the plane and are not collinear, and the second positioning marker is located outside the plane in which the first positioning markers are located. In some embodiments, a distance from the second positioning markers to the plane in which the first positioning markers are located is at least 0.2 cm.
In some embodiments, a distance from the second positioning markers to the plane in which the first positioning markers are located is greater than 1/10 of the shortest distance between the first positioning markers.
In some embodiments, the apparatus includes four first positioning markers, and any three of the four first positioning markers are not collinear.
In some embodiments, the four first positioning markers are arranged in the form of a rectangle.
In some embodiments, one or more of the first positioning marker(s) and the second positioning marker(s) are configured as data light sources configured to transmit information.
In some embodiments, the apparatus further includes one or more data light sources or visual markers configured to transmit information.
In some embodiments, the apparatus includes one or more visual markers configured to transmit information, and a subset of the visual marker(s) is used as the first positioning marker or the second positioning marker.
Another aspect of the present disclosure is directed to a relative positioning method implemented by using a positioning apparatus. The positioning apparatus includes one or more first positioning markers and one or more second positioning markers, the first positioning marker(s) defining one plane, and at least a subset of the second positioning marker(s) being located outside the plane in which the first positioning marker(s) are located. The method includes: obtaining an image that is acquired by an imaging device and includes the positioning apparatus; obtaining physical position information of the first positioning marker(s) and the second positioning marker(s); determining imaging position information of the first positioning marker(s) and the second positioning marker(s) based on the image; and determining, according to the physical position information and the imaging position information of the first positioning marker(s) and the second positioning marker(s) in combination with intrinsic parameter information of an imaging component of the imaging device, position information and/or attitude information of the imaging device relative to the positioning apparatus when the image is acquired.
In some embodiments, the physical position information of the first positioning marker(s) and the second positioning marker(s) includes relative physical position information between these positioning markers or absolute physical position information of these positioning markers.
In some embodiments, the physical position information of the first positioning marker(s) and the second positioning marker(s) is obtained at least partially through communication between the imaging device and the positioning apparatus.
In some embodiments, the positioning apparatus further includes one or more data light sources or visual markers used for transmitting information, or one or more of the first positioning marker(s) and the second positioning marker(s) are configured as data light sources configured to transmit information, where the information transmitted by the data light source(s) or the visual marker(s) is recognizable by using the imaging device.
In some embodiments, the information transmitted by the data light source(s) or the visual marker(s) is used for obtaining relative or absolute physical position information of the positioning apparatus, the first positioning marker(s) or the second positioning marker(s).
In some embodiments, the determining position information and/or attitude information of the imaging device relative to the positioning apparatus when the image is photographed includes: determining, by using a Perspective-n-Point (PnP) method, the position information and/or the attitude information of the imaging device relative to the positioning apparatus when the image is acquired; and/or determining perspective distortion related to the first positioning marker(s) and the second positioning marker(s); and determining, according to the perspective distortion, the position information and/or the attitude information of the imaging device relative to the positioning apparatus when the image is acquired.
Another aspect of the present disclosure is directed to a non-transitory computer readable storage medium storing a computer program, where the computer program, when executed by a processor, causes the processor to perform the foregoing method.
Still another aspect of the present disclosure is directed to an electronic device, including a processor and a memory, the memory storing a computer program which, when executed by the processor, causes the processor to perform the foregoing method.
Embodiments are further described below with reference to the accompanying drawings, in which:
Detailed descriptions are made with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely examples that should not be deemed limiting.
In order to facilitate the description, an optical communication device is hereinafter described as an example. However, a person of ordinary skill in the art can understand from the description of the present application that the disclosed embodiments can be applied to apparatuses for relative positioning other than the optical communication apparatus.
The optical communication apparatus is also referred to as an optical label. The two terms are used interchangeably herein. The optical label is capable of transmitting information in different light emission manners, and has the advantages of long recognition distances and relaxed requirements on visible light conditions. In addition, information transmitted by the optical label can change with time, so that large information capacity and flexible configuration capabilities can be provided.
The optical label may usually include a controller and at least one light source. The controller may drive the light source in different driving modes to transmit different information to the outside. The controller is further configured to select a corresponding driving mode for each light source according to information to be transmitted. For example, in different driving modes, the controller may use different drive signals to control light emission manners of the light source, so that when the optical label is photographed by a device with an imaging function, the image of the light source may have different appearances (for example, different colors, patterns, and brightness). The driving mode of each light source can be parsed out in real-time by analyzing the image of the light source in the optical label, to parse out information transmitted by the optical label at the moment.
To provide an appropriate service to a user based on an optical label, one piece of identification (ID) information may be assigned to each optical label. The ID information is used for uniquely identifying or recognizing the optical label by a manufacturer, an administrator, a user, or the like of the optical label. Usually, the light source may be driven by the controller in the optical label to transmit the ID information to the outside, and a user may use a device to perform image acquisition on the optical label to obtain the ID information transmitted by the optical label. Using the ID information, the user can access a corresponding service based on the ID information, for example, access a web page associated with the ID information, or obtain other information (for example, position information of the optical label corresponding to the ID information) associated with the ID information. The device may use an image acquisition component (for example, a camera) on the device to perform image acquisition on the optical label to obtain an image of the optical label, and analyze the image of the optical label (or each light source in the optical label) to recognize the information transmitted by the optical label.
The ID information of each optical label or other information such as service information related to the optical label and description information or attributes related to the optical label, for example, the position information, physical size information, physical shape information, attitude or orientation information of the optical label, may be stored on a server. The optical label may also have uniform or default physical size information, physical shape information, and the like. The device may use the recognized ID information of the optical label to search the server for other information related to the optical label. The position information of the optical label may be a physical position of the optical label in the physical world, and may be indicated by geographic coordinate information.
In the embodiment shown in
The data light source 205 may be any light source capable of transmitting information to the outside. For example, the data light source 205 may be a single LED, an array formed by a plurality of LEDs, a display screen or a part thereof, or even an illuminated area of light (for example, an illuminated area of light on a wall) may be used as the data light source 205. The data light source 205 may have any surface shape, for example, a circle, a square, a rectangle or a strip. The data light source 205 may include or may be additionally provided with various common optical components, for example, a light guide plate, a light-subduing plate or a diffuser. In some embodiments, the optical label may be provided with one or more data light sources 205, and the number of the data light sources 205 is not limited to 3.
In an embodiment, a visual marker may be used instead of the data light sources 205 to transmit information to the outside. For example, a printed QR code, applet code, bar code, or the like may be used as a visual marker. The visual marker may be arranged between positioning markers, or may be arranged at another place, for example, above or below the positioning markers.
The first positioning markers 201 and/or the second positioning marker 202 may be a component that does not emit light actively or can emit light actively, for example a lamp, to be used in a scenario without ambient light or with low ambient light. The first positioning markers 201 and/or the second positioning marker 202 may have any appropriate surface shape, for example, a circle, a square, a rectangle, a triangle, a hexagon, an ellipse, etc. The first positioning markers 201 and/or the second positioning marker 202 may be alternatively a three-dimensional positioning marker, for example, a sphere, a cylinder, a cube, etc.
In some embodiments, there may be three or more than four first positioning markers 201, as long as there are at least three first positioning markers that are not collinear. The three first positioning markers 201 that are not collinear are sufficient to define one plane, and the second positioning marker 202 is located outside the plane defined by the three first positioning markers 201.
In some embodiments, the second positioning marker 202 may be located at another place on the optical label, for example, located below the two bottom first positioning markers 201 and between the left and right first positioning markers 201. In some alternative embodiments, and the optical label may include more than one second positioning marker 202. In an embodiment, the optical label includes two second positioning markers 202 which are located above the two top first positioning markers 201 and below the two bottom first positioning markers 201, respectively.
In some embodiments, the data light sources 205 and the first positioning markers 201 may be located in a same plane or located in different planes. In some embodiments, the data light sources 205 and the second positioning marker 202 may be located in a same plane or located in different planes.
A positioning apparatus that includes a first positioning marker and a second positioning marker is described above by using an optical label as an example. However, a person of ordinary skill in the art may understand that the descriptions can be applied to any apparatus that satisfies the following: the apparatus includes at least three first positioning markers that are located in a same plane but are not collinear, and one or more second positioning markers, where the one or more second positioning markers are located outside the plane defined by the first positioning markers. The foregoing apparatus may be referred to as the “positioning apparatus” hereinafter. In some embodiments, the positioning apparatus may include only a first positioning marker and a second positioning marker used for implementing relative positioning, but does not include a data light source used for transmitting information to the outside. Alternatively, the first positioning marker and/or the second positioning marker in the positioning apparatus may also be configured as a data light source capable of transmitting information to the outside in addition to being used as a positioning marker. In some embodiments, it is advantageous that the positioning apparatus can be used not only for relative positioning and but also for transmitting information to the outside. In this case, the imaging device is enabled to obtain ID information of the positioning apparatus. The ID information can be used for obtaining an absolute position of the positioning apparatus, so that an absolute position of the imaging device can be determined according to the absolute position of the positioning apparatus and a relative position of the imaging device relative to the positioning apparatus. In some embodiments, the positioning apparatus may be used in combination with a visual marker (for example, a QR code, an applet code, a bar code, etc.), and the two may be integrated or arranged together. In some embodiments, a part (for example, some feature points in the visual marker, a corner of the visual marker, etc.) of the visual marker may be used as the first positioning marker or the second positioning marker of the positioning apparatus. For example, one or more feature points in the visual marker may be used as one or more first positioning markers of the positioning apparatus. In an embodiment, the visual marker may be used for providing the ID information of the positioning apparatus or the visual marker, position information of the positioning apparatus or the visual marker in the physical world, or relative or absolute position information of one or more positioning markers in the physical world.
According to different application scenarios, a distance or depth difference between the plane defined by the first positioning markers 201 and the second positioning marker 202 may have different values. Generally, if a required positioning range is relatively large (for example, relative positioning needs to be implemented in a relatively large range around the positioning apparatus), a relatively large depth difference is required. If the imaging device has a relatively low resolution, a relatively large depth difference is required. In contrast, if the required positioning range is relatively small and/or the imaging device has a relatively high resolution, a relatively small depth difference can satisfy the requirements. Theoretically, in a case that there is a depth difference, to observe a change in an imaging position of the second positioning marker relative to the first positioning marker (that is, the change in the imaging position is discernible in pixels), the distance in the image corresponding to the depth difference usually needs to be greater than or equal to two pixels. Assuming that the resolution of the imaging device is R, and the largest distance required for the positioning is D, the smallest depth difference is an actual object length at a distance D corresponding to two pixels when imaging at the resolution R. More specifically, assuming that the resolution of the imaging device is R=Rx * Ry, and the imaging component has a size of L*W, in a coordinate system of the imaging device, a focal length in an x dimension of the imaging device is fx, a focal length in a y dimension is fy, the largest distance required for positioning is D, a size of an object is x*y, and an imaging size of the object is u*v (the foregoing sizes x, y, u, and v are all projected sizes of the object on an x axis and a y axis of a coordinate system of a camera).
The following formulas may be obtained:
fx/D=u * (L/Rx)/x,
fy/D=v * (W/Ry)/y,
let u=v=two pixels,
x_min=2 * L * D/(fx * Rx), and
y_min=2 * W * D/(fy * Ry).
The minimum value of x_min and y_min may be used as the minimum value depth_min of the depth difference, that is, depth_min =min(x_min, y_min).
For example, in a case of using an imaging device with 4K resolution (2160*3840), when a recognition distance is 10 m, the image of a line segment of ⅔ cm is approximately two pixels. Therefore, the smallest depth difference needs to be ⅔ cm. If the recognition distance becomes 50 m (for example, for some outdoor positioning scenarios), the smallest depth difference needs to be 10/3 cm. If the recognition distance becomes 1.5 m (for example, for some indoor positioning scenarios), the smallest depth difference needs to be 0.1 cm.
In some embodiments, the distance or depth difference between the plane defined by the first positioning marker and the second positioning marker is at least 0.1 cm, at least 0.2 cm, at least 0.5 cm, at least 0.8 cm, at least 1 cm, at least 1.5 cm, or the like. In some embodiments, the distance or depth difference between the plane defined by the first positioning marker and the second positioning marker may be alternatively determined according to distances between the first positioning markers. In some embodiments, the distance or depth difference between the plane defined by the first positioning markers and the second positioning marker is greater than 1/10, ⅛, ⅕, ¼, ⅓ or the like of the shortest distance between the first positioning markers.
A relative positioning method implemented by using the positioning apparatus according to an embodiment is described below. The relative positioning method is used for determining the position information and/or attitude information of the imaging device relative to the positioning apparatus. The position information and the attitude information may be generally referred to as “pose information”. In some cases, it may not be necessary to obtain both the position information and the attitude information of the imaging device. Instead, only one of the position information and the attitude information may be obtained, for example only the position information of the imaging device.
In an embodiment, to obtain a position and/or an attitude of the imaging device relative to the positioning apparatus, one coordinate system may be established according to the positioning apparatus. The coordinate system may be referred to as a coordinate system of the positioning apparatus. The first positioning marker and the second positioning marker on the positioning apparatus form some spatial points in the coordinate system, and have corresponding coordinates in the coordinate system. After an image of the positioning apparatus is acquired by using the imaging device, for example, image points corresponding to respective spatial points may be found in the image according to a physical structural feature or a geographical structural feature of the positioning apparatus, and imaging positions of the image points in the image are determined. Pose information (R, t) of the imaging device in the coordinate system when the image is acquired can be calculated according to coordinates of the spatial points in the coordinate system, imaging positions of the corresponding image points in the image, and intrinsic parameter information of the camera, where R is a rotation matrix indicating attitude information of the camera in the coordinate system, and t is a displacement vector indicating position information of the camera in the coordinate system. Methods for calculating R and t are known in the prior art. For example, R and t may be calculated by using a 3D-2D Perspective-n-Point (PnP) method. Details of the calculation are not described herein.
During the calculation of the pose information (R, t) of the imaging device, positions of image points corresponding to positioning markers on the positioning apparatus on the image need to be accurately determined. However, this may be challenging if these positioning markers are located in the same plane.
By means of the positioning apparatus in the disclosure, an error in the determined pose information of the imaging device can be greatly mitigated or eliminated.
At step S1101, physical position information of first positioning markers and a second positioning marker on a positioning apparatus is obtained.
The physical position information of the first positioning markers and the second positioning marker may be obtained in various manners. For example, in some application scenarios (for example, in an automated factory), the positioning apparatus has a fixed specification or model. In this way, the imaging device, the server, or the like may know the physical position information of the first positioning markers and the second positioning marker on the positioning apparatus in advance. In some application scenarios, the positioning apparatus has a communication function, and the imaging device may communicate with the positioning apparatus (for example, through a wireless signal or light), to obtain the physical position information of the first positioning markers and the second positioning marker on the positioning apparatus. The imaging device may directly obtain the physical position information of the first positioning markers and the second positioning marker on the positioning apparatus, or may obtain other information (for example, ID information, specification information, or model information) of the positioning apparatus, and make a search or analysis by using the other information to determine the physical position information of the first positioning markers and the second positioning marker. For example, for the positioning apparatus in the form of the optical label shown in
The physical position information of the first positioning markers and the second positioning marker may be relative physical position information, or may be absolute physical position information. In some embodiments, the physical position information of the first positioning markers and the second positioning marker may be a relative position relationship (for example, a relative distance and a relative direction) between positioning markers. In some embodiments, the physical position information of the first positioning markers and the second positioning marker may be coordinate information of the positioning markers in the coordinate system established according to the positioning apparatus. For example, one positioning marker may be used as the origin of the coordinate system, and the positions of the positioning markers can be represented by the coordinate information in the coordinate system. In some embodiments, the physical position information of the first positioning markers and the second positioning marker may be absolute physical position information of the positioning markers in the real world. A person of ordinary skill in the art can understand that the absolute physical position information of the positioning markers is not essential for determining the relative pose information between the imaging device and the positioning apparatus. However, by using the absolute physical position information of the positioning markers, the absolute pose information of the imaging device in the real world can be further determined based on the relative pose information between the imaging device and the positioning apparatus.
At step S1102, an image that is acquired by an imaging device and includes the positioning apparatus is obtained.
The imaging device mentioned herein may be a device (for example, a mobile phone, a tablet computer, smart glasses, a smart helmet, a smart watch, etc.) carried or controlled by a user. However, it can be understood that, the imaging device may alternatively be a machine capable of autonomous movement, for example, an unmanned aerial vehicle, a self-driving car, a robot, etc. An imaging component, for example, a camera, is mounted on the imaging device.
At step S1103, the image is analyzed to obtain imaging position information of the first positioning markers and the second positioning marker on the image.
After the image that is acquired by the imaging device and includes the positioning apparatus is obtained, the imaging positions of the first positioning markers and the second positioning marker of the positioning apparatus on the image can be determined by analyzing the image. The imaging positions may be represented by for example corresponding pixel coordinates.
At step S1104, position information and/or attitude information of the imaging device relative to the positioning apparatus when the image is acquired is determined according to the physical position information and the imaging position information of the first positioning markers and the second positioning marker in combination with intrinsic parameter information of an imaging component of the imaging device.
The imaging component of the imaging device may have corresponding intrinsic parameter information. Intrinsic parameters of the imaging component are parameters related to characteristics of the imaging component, for example, a focal length or a number of pixels of the imaging component. The imaging device may obtain the intrinsic parameter information of its imaging component at the time of acquiring an image. The other device or apparatus (for example, the server) may alternatively receive the intrinsic parameter information from the imaging device. For example, when uploading the image to the server, the imaging device may additionally upload the intrinsic parameter information of its imaging component. In some embodiments, the imaging device may alternatively upload model information of its imaging component to the server, and the server may obtain the intrinsic parameter information of the imaging component according to the model information.
After the physical position information of the first positioning markers and the second positioning marker, the imaging position information of the first positioning markers and the second positioning marker, and the intrinsic parameter information of the imaging component of the imaging device are obtained, the position information and/or the attitude information of the imaging device relative to the positioning apparatus can be determined by using various methods (for example, a 3D-2D PnP method, also referred to as a solvePnP method) known in the field. Representative methods include a P3P method, an iterative method, an EPnP method, a DLT method, and the like.
In some embodiments, after the physical position information of the first positioning markers and the second positioning marker, the imaging position information of the first positioning markers and the second positioning marker, and the intrinsic parameter information of the imaging component of the imaging device are obtained, the position information and/or the attitude information of the imaging device relative to the positioning apparatus when the image is acquired can be determined by analyzing perspective distortion of these positioning markers, a pattern formed by these positioning markers, or the like. For example, the imaging device may determine distance information and direction information of the imaging device relative to the positioning apparatus in various manners. In an embodiment, the imaging device may determine a relative distance between the imaging device and the positioning apparatus by analyzing an actual size of the pattern formed by the positioning markers and an imaging size of the pattern (the larger the imaging size, the smaller the distance; the smaller the imaging size, the larger the distance). The imaging device may further determine the direction information of the imaging device relative to the positioning apparatus by comparing an actual shape of the pattern formed by the positioning markers and the imaging shape of the pattern. For example, for the image shown in
With the optical label according to an embodiment of the present application, when the optical label is photographed at different positions, imaging differences of the positioning markers on the optical label can be observed more clearly, so that an error in the determined pose information of the imaging device can be greatly mitigated or eliminated.
In the foregoing embodiments, the description is mainly made with dot-shaped positioning markers. However, a person of ordinary skill in the art may understand that in some embodiments, the first positioning markers and/or the second positioning marker may have another shape. For example, in an embodiment, a strip-shaped positioning marker may be used to replace two first positioning markers 201 shown in
Position calculation results obtained by the positioning apparatus according to an embodiment through repeated experiments are described below.
I. Experiments in an X Direction (a Transverse Direction)
Relative positioning results toward the left and toward the right are tested at a location one meter in front of the positioning apparatus. A photographing position of the imaging device is moved toward the left and right by 0.5 meters, 1 meter or 1.5 meters, respectively. The positioning apparatus is taken as the origin of a spatial coordinate system. The experiment results are shown in Table 1. When the imaging device performs photographing on the left side, the X coordinate of the imaging device is a negative value. When the imaging device performs photographing on the right side, the X coordinate of the imaging device is a positive value. All the data is in millimeter (mm). X, Y, and Z represent calculated coordinates of the imaging device, and X0, Y0, and Z0 represent actual coordinates of the imaging device when the positioning apparatus is photographed.
II. Verification Experiments in a Y Direction (a Vertical Direction)
Relative positioning results toward the top and bottom are tested at two meters in front of the positioning apparatus. A photographing position of the imaging device is moved toward the top and bottom by 0.5 meters, 1 meter or 1.5 meters, respectively. The experiment results are shown in Table 2. When the imaging device performs photographing on an upper side, the Y coordinate of the imaging device is a negative value. When the imaging device performs photographing on a lower side, the Y coordinate of the imaging device is a positive value. All data is in millimeter (mm). X, Y, and Z represent calculated coordinates of the imaging device, and X0, Y0, and Z0 represent actual coordinates of the imaging device when the positioning apparatus is photographed.
As can be seen from the foregoing experimental results, by means of the positioning apparatus in an embodiment, when an imaging apparatus performs photographing at a position several meters away from the positioning apparatus, errors between X, Y, and Z coordinate values of the imaging apparatus that are calculated through relative positioning and actual coordinate values are usually only tens of millimeters, thereby providing relatively high accuracy of relative positioning.
References herein to “individual embodiments,” “some embodiments,” “an embodiment,” or “embodiments” refer to that particular features, structures, or properties described in combination with the embodiments are included in at least one embodiment. Therefore, the phrases “in various embodiments,” “in some embodiments,” “in one embodiment,” or “in an embodiment” and the like throughout this specification do not necessarily refer to the same embodiment. In addition, particular features, structures, or properties may be combined in any suitable manner in one or more embodiments. Therefore, a particular feature, structure, or property shown or described in combination with an embodiment may be combined in whole or in part with features, structures, or properties of one or more other embodiments without limitation, so long as the combination is not illogical or does not work. Expressions such as “according to A”, “based on A”, “through A” or “using A” appearing herein are intended to be non-exclusive, that is, “according to A” may cover “solely according to A” or “according to A and B”, unless specifically stated or the context clearly indicates that the meaning is “solely according to A”. In the present application, some schematic operational steps are described in a certain order for clarity, but it will be understood by those having ordinary skill in the art that each of these operational steps is not essential and some of them can be omitted or replaced by other steps. These operational steps also do not have to be performed sequentially in the manner shown; instead, some of these operational steps may be performed in a different order, or in parallel, depending on practical requirements, as long as the new execution is not illogical or does not work.
Several aspects of at least one embodiment are described, and it will be understood that those having ordinary skill in the art may easily make various changes, modifications, and improvements. Such changes, modifications, and improvements are intended to be within the spirit and scope of the present disclosure. Although the present the disclosure is made through several embodiments, the present invention is not limited to the embodiments described herein, but defined by the claims below as well as their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201910485778.9 | Jun 2019 | CN | national |
| 201920841515.2 | Jun 2019 | CN | national |
The present application is a by-pass continuation application of PCT International Application No. PCT/CN2020/093689 filed Jun. 1, 2020, which claims priority to Chinese Patent Application No. 201910485778.9 filed Jun. 5, 2019, and Chinese Patent Application No. 201920841515.2 filed Jun. 5, 2019, all of which are hereby incorporated by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/CN2020/093689 | Jun 2020 | US |
| Child | 17536752 | US |
