CN113030887B - Installation angle offline calibration method and device based on millimeter wave radar - Google Patents
Installation angle offline calibration method and device based on millimeter wave radar Download PDFInfo
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Abstract
The invention provides a mounting angle off-line calibration method based on millimeter wave radar, which comprises the following steps: acquiring far-near field compensation matrixes under different calibration distances according to far-field direction vectors and near-field direction vectors of the millimeter wave radar; mounting a millimeter wave radar to be detected on a vehicle, and arranging a calibration flat plate at a near-field position right in front of the center of a longitudinal axis of the vehicle; a corresponding far-near field compensation matrix is derived according to the calibration distance; and obtaining a near-field steering vector by subtracting the derived far-near field compensation matrix from the far-field direction vector, and obtaining a calibration angle of the target according to the near-field direction vector and an echo signal of the millimeter wave radar. The invention also provides a corresponding calibration device. According to the calibration method, the antenna direction vector in the near field state is corrected according to the near field model, the far and near field compensation matrix is utilized for performing far and near field conversion to correct the phase relation, so that the angle of the flat target is obtained more accurately, the calibration of the radar installation angle is completed, and the method is applicable to near field conditions.
Description
Technical Field
The invention belongs to the technical field of vehicle millimeter wave radars, and particularly relates to a method and a device for calibrating an installation angle offline based on a millimeter wave radar.
Background
With the vigorous development of the automobile industry, various automobile manufacturers are devoted to the development of intelligent safety systems of automobiles, aiming at reducing traffic accidents, improving road traffic safety, enhancing the traffic capacity of roads and the like, and the systems are required to acquire front environment information, namely the relative distance, speed, angle and the like of a front automobile. A relatively common method is to install a millimeter wave radar at the front end of the vehicle to obtain relevant information of the front vehicle. However, the detection range of the millimeter wave radar is limited, the transverse detection range of the long-distance millimeter wave radar is generally only +/-12 deg, so that the millimeter wave emission plane of the radar is required to be opposite to the front of a vehicle during installation, and the small installation angle or position deviation of the millimeter wave radar can lead to larger error of the detection distance of a vehicle in front, so that the installation position and the gesture of the radar are required to be calibrated.
The existing vehicles for installing millimeter wave radar generally need to install calibration equipment at the tail end of the whole vehicle production line so as to calibrate the installation angle and the position of the millimeter wave radar in a whole vehicle off-line mode.
The existing main flow whole vehicle factory is used for carrying out the offline calibration by fixing the vehicle based on four-wheel positioning and then placing a calibration object at a certain fixed position right in front of the center of the longitudinal axis of the vehicle, wherein the calibration object is generally a flat plate, and the angle of some calibration objects is reversed, so that the whole vehicle factory can purchase the offline calibration equipment to carry out the offline calibration, not only the radar calibration, but also the calibration of a camera, and the like, and the offline calibration equipment is required. Therefore, after the whole vehicle factory is fixed by the off-line calibration equipment, the radar calibration process cannot be executed according to the program parameters on the road, and the algorithm is adjusted to obtain the calibration angle more accurately. In the off-line calibration using a flat plate as a calibration object, the basic requirements are:
1. the method comprises the steps of selecting a flat metal plate with a larger size as a radar target, considering the reflection direction of electromagnetic waves to be the normal direction of the metal plate because of the strong reflection direction characteristic of the electromagnetic waves, fixing the metal plate on a calibration frame, and swinging the metal plate in front of the radar;
2. through strict four-wheel positioning, the longitudinal axis direction of the vehicle is consistent with the normal direction of the metal plate, and then the radar measures the installation angle deviation of the angle corresponding to the radar of the metal plate.
However, if the millimeter wave radar goniometer is required to meet far field conditions, this tends to increase the distance between the calibration plate as a calibration object and the radar (for example, to 4-5 m), which would be very demanding for the environment, and a general whole vehicle factory cannot meet such conditions. Specifically, the general whole vehicle factory requires <2m for calibrating the flat-panel range radar when calibrating the common radar; if the distance between the calibration flat and the radar is increased to 4-5m according to the requirement of calibrating the millimeter wave radar, the calibration station is required to be enlarged, and a metal equal-strength reflector is not required to be arranged in the radius of the distance (4-5 m), so that the radar can detect more peripheral targets, such as equipment workshop walls, vehicles which are queued and pass through a line, and the like, the targets which are at the same distance and the same speed as the calibration flat are more easily formed, the angle measurement phase is influenced, the angle measurement error is increased, and the calibration angle is incorrect.
In general, the vehicle millimeter wave radar performs angle measurement according to a far-field direction vector. However, when the whole vehicle is subjected to the off-line installation calibration of the radar, the installation angle calculated by the normal angle measurement algorithm is inaccurate because the millimeter wave radar angle measurement cannot meet far-field conditions, so that the antenna array layout of the near field can have great influence on the angle measurement, and the angle errors on different calibration distances are inconsistent.
Disclosure of Invention
The invention aims to provide a method and a device for calibrating a mounting angle offline based on a millimeter wave radar, which are suitable for calibrating the millimeter wave radar in a near field condition in a small way, and reduce errors.
In order to achieve the above object, the present invention provides a method for calibrating installation angle offline based on millimeter wave radar, comprising:
s1: taking the distance from the calibration flat plate to the plane of the millimeter wave radar as the calibration distance, and acquiring far-near field compensation matrixes under different calibration distances according to the far-field direction vector and the near-field direction vector of the millimeter wave radar;
s2: mounting a millimeter wave radar to be detected on a vehicle, and arranging a calibration flat plate at a near-field position right in front of the center of a longitudinal axis of the vehicle; a corresponding far-near field compensation matrix is derived according to the calibration distance;
s3: and (2) subtracting the far-near field compensation matrix derived in the step (S2) from the far-field direction vector to obtain a near-field guide vector, and obtaining a target calibration angle according to the near-field direction vector and an echo signal of the millimeter wave radar, wherein the target calibration angle is the deviation of the installation angle between the normal direction of the millimeter wave radar and the running axis direction of the vehicle.
The millimeter wave radar is a time division multiplexed MIMO array.
The step S1 includes:
s11: taking the distance from the calibration flat plate to the plane of the millimeter wave radar as a calibration distance, determining a far-field model according to the position parameters of a receiving antenna and a transmitting antenna of the millimeter wave radar so as to obtain a far-field direction vector, and simultaneously determining a near-field model by combining the calibration distance so as to obtain a near-field direction vector;
s12: and subtracting the far field direction vector and the near field direction vector to obtain far and near field compensation matrixes at different calibration distances, and storing the far and near field compensation matrixes.
The far field direction vector w Far (θ, m, n) is:
wherein ,for Kronecker product, dsin (θ) is the difference in wave path, d is the antenna spacing of the transmit and receive antennas, and d is the spacing of the receive antennas d for the far field r θ is a direction angle, m is a ordinal number of a transmitting antenna of the millimeter wave radar, N is a ordinal number of a receiving antenna of the millimeter wave radar, m, N is a positive integer, mn is more than or equal to 2, m is a number of transmitting antennas of the millimeter wave radar, N is a number of receiving antennas of the millimeter wave radar, and λ is a wavelength of the millimeter wave radar;
near field direction vector w Near (θ, m, n, r) is:
wherein d' mn-1 is the path distance r Rmn Distance difference r from distance rR11 Rmn The distance from the mth transmitting antenna to the nth receiving antenna of the millimeter wave radar is the wave distance, m is the ordinal number of the transmitting antenna of the millimeter wave radar, and n is the ordinal number of the receiving antenna of the millimeter wave radar; r is a calibration distance, θ is a direction angle, and λ is the wavelength of the millimeter wave radar;
the far and near field compensation matrix is as follows:
g x (θ，m，n，r)＝w Far (θ，m，n)-w Near (θ，m，n，r)，
wherein ,wNear (θ, m, n, r) is a near field direction vector, w Far (θ, m, n) is a far-field direction vector, g x And (theta, m, n, r) is a far and near field compensation matrix.
In the step S1, the calibration angle of the target is calculated by adopting a digital beam forming technique method according to the near-field direction vector and the echo signal of the millimeter wave radar.
The step S3 includes:
s31: obtaining a near-field steering vector by subtracting the far-field and near-field compensation matrix derived in the step S2 from the far-field steering vector stored in the step S1;
s32: obtaining a conjugate transpose matrix of a near-field direction vector, multiplying the conjugate transpose matrix with echo signals of the millimeter wave radar to obtain a power product, and taking an absolute value of the power product corresponding to each theta value to form an angle spectrum;
s33: and taking an angle value theta corresponding to the maximum of the angle spectrum p (theta) as a target calibration angle.
The installation angle offline calibration method based on the millimeter wave radar further comprises the following step S4: under the condition that the installation position of the millimeter wave radar is not adjusted, the angle of the target detected by the radar each time is corrected by a computer.
On the other hand, the invention provides an installation angle off-line calibration device based on millimeter wave radar, which comprises the millimeter wave radar installed on a vehicle, a calibration flat plate and terminal equipment; the terminal device includes a processor, a memory unit, and one or more computer program modules stored in the memory unit and configured to be executed by the processor to implement the millimeter wave radar-based installation angle offline calibration method described above.
The calibration flat is connected with a controller and used for controlling the position of the calibration flat.
The terminal equipment also comprises an input interface and an output interface; the terminal equipment is connected with the millimeter wave radar through an input interface so as to receive echo signals of the millimeter wave radar; the terminal equipment is connected with the controller of the calibration flat board through the output interface so as to control the calibration distance.
According to the method for calibrating the installation angle of the millimeter wave radar offline, the antenna direction vector in the near field state is corrected according to the near field model, namely, the far and near field compensation matrix is utilized to carry out far and near field conversion to correct the phase relation, so that the angle of a flat target is obtained more accurately, the calibration of the radar installation angle is completed, the method can be applied to near field conditions, the requirements of far field conditions on the environment and equipment are met, the operation cost is reduced, and the calibration requirements are met in the whole vehicle factory offline calibration field. In addition, the method for calibrating the installation angle offline based on the millimeter wave radar can select a proper far and near field compensation matrix according to the calibration distance between the millimeter wave radar and the calibration flat plate, so that the influence of different distances on angle measurement errors is avoided.
Drawings
Fig. 1 is an algorithm flow chart of a millimeter wave radar-based installation angle offline calibration method according to one embodiment of the invention.
Fig. 2 is a schematic diagram of the calculation of far-field direction vectors and target angles in a far-field model.
Fig. 3 is a schematic diagram of calculation of a near field direction vector and a target angle in a near field model.
Fig. 4 is an analytical schematic of a MIMO array in the near field.
Fig. 5 is a schematic structural view of an installation angle off-line calibration device based on millimeter wave radar according to an embodiment of the present invention.
Fig. 6 is a schematic structural view of a terminal device of the installation angle down-line calibration device based on millimeter wave radar according to the present invention.
Fig. 7 is a graph of angular results for different distances without compensation according to the prior art.
Fig. 8 is a graph of angles at different distances without compensation according to the prior art.
FIG. 9 is a graph of the angular results of different distances after compensation according to the present invention.
FIG. 10 is a graph of the angular spectrum at various distances after compensation in accordance with the present invention.
Detailed Description
The invention will be further illustrated with reference to specific examples. It should be understood that the following examples are illustrative of the present invention and are not intended to limit the scope of the present invention.
As shown in fig. 1, the installation angle offline calibration method based on the millimeter wave radar of the invention comprises the following steps:
step S1: taking the distance from the calibration flat plate to the plane of the millimeter wave radar as the calibration distance, and acquiring far-near field compensation matrixes under different calibration distances according to the far-field direction vector and the near-field direction vector of the millimeter wave radar;
the step S1 includes:
step S11: taking the distance from the calibration flat plate to the plane of the millimeter wave radar as a calibration distance, determining a far field model according to the position parameters of a receiving antenna and a transmitting antenna of the millimeter wave radar to be detected so as to obtain a far field direction vector, and simultaneously determining a near field model by combining the calibration distance (namely the distance between the antenna center of the millimeter wave radar and the calibration flat plate) so as to obtain a near field direction vector; wherein the receiving and transmitting antennas are separated;
step S12: subtracting the far field direction vector from the near field direction vector to obtain a far field and near field compensation matrix under different calibration distances, and storing the far field and near field compensation matrix;
wherein, millimeter wave radar is the MIMO array of time division multiplexing. The position parameters of the receiving antenna and the transmitting antenna of the millimeter wave radar are obtained according to the antenna design index of the millimeter wave radar, and the obtained far field direction vector is generally a solidified parameter value and cannot be modified along with the change of the calibration distance, so that the obtained far field direction vector can be stored in the calibration device for the installation angle of the millimeter wave radar. In the present embodiment, the millimeter wave radar has 3 transmitting antennas and 4 receiving antennas. Since the number of transmitting antennas is 3 and the number of receiving antennas is 4, each transmitting antenna corresponds to 4 receiving antennas, if time division MIMO of 3 transmitting antennas is performed, the total transmitting and receiving antenna pairs thereof are 12.
In the step S1, (1) as shown in FIG. 2, in the far field modelMiddle, far field direction vector w Far (θ, m, n) (i.e., the rear direction vector) is:
in the formula ,wtx (θ, m) is the direction vector of the transmit antenna of the MIMO array, w rx (θ, n) is the direction vector of the receive antennas of the MIMO array,the product is Kronecker, m is the ordinal number of a transmitting antenna of the millimeter wave radar, and n is the ordinal number of a receiving antenna of the millimeter wave radar; for the angle measuring radar, m or n is not less than 2, the antenna angle measuring principle is calculated according to phase change caused by wave path difference of two groups of antenna echoes, and a single antenna cannot generate wave path difference, so mn is not less than 2, that is, m and n are positive integers, and mn is not less than 2; tx is the transmit antenna and rx is the receive antenna.
According to the antenna theory, the distance difference between the object and the antenna can cause the change of the phase of the echo signal, because the distance difference between the receiving and transmitting antennas is ignored, the phase direction diagram can be regarded as a straight line, namely an equiphase plane, and the direction vectors of the transmitting antenna and the receiving antenna are respectively expressed as follows:
wherein ,dt 、d r Respectively, the spacing of the transmitting antennas and the spacing of the receiving antennas of the millimeter wave radar, θ is the direction angle (i.e., the azimuth angle of the normal of the calibration flat plate relative to the normal of the millimeter wave radar), m is the ordinal number of the transmitting antennas of the millimeter wave radar, n is the ordinal number of the receiving antennas of the millimeter wave radar,m is the number of transmitting antennas of the millimeter wave radar, and N is the number of receiving antennas of the millimeter wave radar.
Thus, under far field conditions, the far field direction vector w Far (θ, m, n) is:
wherein ,for Kronecker product, dsin (θ) is the difference in wave path, where d is the antenna spacing of the transceiver antenna, d=d for far field r The wave path difference of the receiving antenna is (mn-1) ×d, corresponding to the following d' mn-1 The method comprises the steps of carrying out a first treatment on the surface of the θ is a direction angle, M is a ordinal number of transmitting antennas of the millimeter wave radar, N is a ordinal number of receiving antennas of the millimeter wave radar, M is a number of transmitting antennas of the millimeter wave radar, N is a number of receiving antennas of the millimeter wave radar, and λ is a wavelength of the millimeter wave radar. In the present embodiment, since the number of transmitting antennas is 3 and the number of receiving antennas is 4, m=3, n=4.
For far-field environments, once the antenna spacing d of the transceiver antenna is defined, its direction vector is a constant parameter value.
(2) As shown in fig. 3, in the near-field condition, the wave path difference cannot be simply equivalent to the difference caused by the receiving antenna spacing, and the phase difference caused by the distance difference and the actual electromagnetic wave path are considered, so that the far-field direction vector is not suitable for the scene, and compensation correction is required for the far-field direction vector to obtain the near-field direction vector.
As shown in fig. 4, in the analysis schematic diagram of the near-field MIMO array, for the sake of calculation, the mirror image of the transmitting antenna with respect to the calibration flat is shown as the mirror image point of the transmitting antenna (the mirror image point of the transmitting antenna T1 is schematically shown in fig. 4, which may be the mirror image point of any antenna in practice), and the position of the calibration flat is a long straight line position right between the transmitting antenna and the mirror image point. Thus, near field direction vector w Near (θ，m, n, r) is:
wherein ,d’mn-1 For the wave path distance r Rmn Distance from wave path r R11 Is the difference of the wave distance, r Rmn For the distance from the m-th transmitting antenna Tm to the n-th receiving antenna Rn of the millimeter wave radar, m is the ordinal number of the transmitting antenna of the millimeter wave radar, n is the ordinal number of the receiving antenna of the millimeter wave radar, wherein for the angle measuring radar, m or n is not less than 2, the antenna angle measuring principle is calculated according to the phase change caused by the wave path difference of the echoes of the two groups of antennas, and the single antenna cannot generate the wave path difference, so MN is not less than 2, MN-1 is not less than 1, and … MN-1; r is the calibration distance, namely the distance between the antenna center of the millimeter wave radar and the calibration flat plate, and θ is the direction angle.
As shown in FIG. 4, the wave distance difference d' mn-1 The calculation method of (1) is as follows:
distance of wave path r R11 ，…，r Rmn ，…，r RMN Is related to the calibration distance r. Wave Cheng Juli is defined as the distance that an electromagnetic wave from a transmitting antenna emits to a target (i.e., a calibration plate) and then reflects to a receiving antenna, and can be equivalent to an optical wave, and the distance that an electromagnetic wave travels from the transmitting antenna to the target and then to the receiving antenna can be calculated from fig. 4. Therefore, the difference of the calibration distances will result in the difference of the wave path distances, thereby obtaining different near field direction vectors.
Knowing the relative position coordinates of the transmit-receive antenna, X0 is the coordinates of the antenna center of the millimeter wave radar, ΔX, according to the geometric relationship of FIG. 4 T Is the distance between adjacent transmitting antennas of millimeter wave radar, deltaX R For the distance between adjacent receiving antennas of the millimeter wave radar, tm is the M-th transmitting antenna of the millimeter wave radar, m=1, 2, …, M, X Tm Is the coordinate of the m-th transmitting antenna of the millimeter wave radar, rn is the N-th receiving antenna of the millimeter wave radar, n=1, 2, …, N, X Rn The coordinates of the nth receiving antenna of the millimeter wave radar, R is the millimeter wave radarThe distance of the mth transmitting antenna Tm to its mirror point (R is shown as 1 st transmitting antenna T in the figure 1 Distance to its mirror point), rRn is the distance from the mirror point to the nth receive antenna of the millimeter wave radar.
Therefore, the wave distance r corresponding to different transmitting antennas can be easily obtained R11 ，…，r RMN ，
R＝[(X0-X Tm )*sin(θ)+r]×2 (6)
Wherein R is the distance from the m-th transmitting antenna Tm of the millimeter wave radar to the image point, R is the calibration distance, θ is the direction angle, X0 is the coordinate of the antenna center of the millimeter wave radar, X Tm Is the coordinates of the transmitting antenna Tm of the millimeter wave radar.
wherein ,rRmn The distance from the m-th transmitting antenna Tm to the n-th receiving antenna Rn of the millimeter wave radar is the distance from the m-th transmitting antenna Tm to the image point of the millimeter wave radar, and R is X Rn Is the coordinate, X of the receiving antenna Rn of the millimeter wave radar Tm The coordinates of the transmitting antenna Tm of the millimeter wave radar, θ is the direction angle.
d’ mn-1 ＝r Rmn -r R11 ＝0，d’ 1 ，…，d’ MN-1 (8)
Wherein MN-1=1, … MN-1, r Rmn Is the distance between the mth transmitting antenna Tm and the nth receiving antenna Rn of the millimeter wave radar.
Thereby, the wave distance difference d 'is reduced' mn-1 Is carried into the above formula (5) to obtain the near-field direction vector w Near (θ，m，n，r)。
Then subtracting the near-field direction vector from the far-field direction vector to obtain the far-near field compensation matrix (i.e. correction matrix and correction antenna direction vector) needed by us. The far and near field compensation matrix is:
g x (θ，m，n，r)＝w Far (θ，m，n)-w Near (θ，m，n，r)，
wherein ,wNear (θ, m, n, r) is a near field direction vector, w Far (θ, m, n) is a far-field direction vector, g x And (theta, m, n, r) is a far and near field compensation matrix.
Since the near field direction vector is related to the calibration distance r, the resulting far and near field compensation matrix varies with the calibration distance r, and the far and near field compensation matrix g can be obtained by substituting different calibration distances r into the far and near field compensation matrix g x And (theta, m, n, r) to obtain far and near field compensation matrixes under different calibration distances. Therefore, the radar can be converted into near-field conditions from the far-field model according to the near-field model, and the direction vectors on different calibration distances are output, so that the angle of the target is estimated according to the near-field direction vectors and the array signals.
Step S2: mounting a millimeter wave radar to be detected on a vehicle, arranging a calibration flat plate at a near-field position right in front of the longitudinal axis center of the vehicle, and enabling the normal line of the calibration flat plate to be parallel to the running axis of the vehicle as far as possible so as to determine a calibration distance (namely the distance between the antenna center of the millimeter wave radar and the calibration flat plate); a corresponding far-near field compensation matrix is derived according to the calibration distance and is used for correcting the phase relation;
the far-near field compensation matrix is obtained by subtracting the near-field direction vector from the far-field direction vector calculated in advance in the step S1, and the near-field direction vector is related to the calibration distance, and finally the far-near field compensation matrix (i.e. the correction matrix) under different calibration distances is obtained, so in the step S2, the far-near field compensation matrix corresponding to the environment of the actual test field can be selected and obtained according to the actual determined calibration distance, namely the wave path distance of each group of receiving and transmitting antennas on different calibration distances is calculated, and the actual phase relation is obtained, and the actual deriving result of the far-near field compensation matrix is obtained.
In this embodiment, the calibration flat is made of metal, and the calibration distance is determined by detecting and measuring the position coordinates of the calibration flat by the laser calibration device. In other embodiments, since the actually required position parameter is only the calibration distance, that is, the distance from the calibration flat plate to the millimeter wave radar plane, the calibration flat plate to the millimeter wave radar plane can be measured directly by the meter ruler after calibration by the laser calibration device. In addition, in other embodiments, the distance measurement can be performed by directly using the millimeter wave radar to be measured, and the distance between the two is approximately equal when the distance is actually measured. Because the far and near fields of the radar only affect the angle information of the target, the radial distance, speed and other information of the target are not affected, and the distance and speed measurement is also the advantage of the millimeter wave radar.
Step S3: obtaining a near-field guiding vector by subtracting the far-field compensation matrix derived in the step S2 from the far-field direction vector stored in the step S1, and obtaining a target calibration angle according to the near-field direction vector and an echo signal of the millimeter wave radar, wherein the target calibration angle is the deviation of the installation angle between the normal direction of the millimeter wave radar and the running axis direction of the vehicle.
As described above, the far-field direction vector is a cured parameter value, and once the calibration distance between the calibration plate and the radar is determined in step S2, the far-near field compensation matrix corresponding to the calibration distance is directly derived and written into the radar memory, so that in step S3, the far-near field conversion is performed on the direction vector of the radar by using the far-field direction vector minus the far-near field compensation matrix.
The target calibration angle is calculated by adopting a DBF (digital beam forming technology) method according to the near-field direction vector and the echo signal of the millimeter wave radar. Among them, DBF (digital beam forming technique) is a method of calculating an angle.
As shown in fig. 2 and 3, the echo signals of the millimeter wave radar are directly detected by the radar and respectively correspond to different receiving channel data, which comprises a virtual signal S 11 (t)…S mn (t)…S MN (t)。
Referring to fig. 1 again, the specific process of step S3 is as follows:
step S31: obtaining a near-field steering vector by subtracting the far-field and near-field compensation matrix derived in the step S2 from the far-field steering vector stored in the step S1;
step S32: obtaining a conjugate transpose matrix of a near-field direction vector, multiplying the conjugate transpose matrix with echo signals of the millimeter wave radar to obtain a power product, and taking an absolute value of the power product corresponding to each theta value to form an angle spectrum;
as shown in fig. 2 and 3, + is an addition and subtraction operation, and x is a multiplication operation, where the conjugate transpose matrix of the near-field direction vector is multiplied by the echo signal of the millimeter wave radar, and the elements corresponding to the near-field direction vector are multiplied by the echo signals of different receiving channels and added. Wherein fig. 2 shows the calculation principle under the far field model of the prior art and fig. 3 shows the calculation principle under the near field model according to the present invention.
Wherein the power product y (t) is:
y(t)＝w H (θ，m，n)S(t)，
the angle spectrum p (θ) is:
p(θ)＝|y(t)|
wherein S (t) is an echo signal of the millimeter wave radar, which is formed by S 11 (t)…S mn (t)…S MN (t) is composed of; w (w) H (θ, m, n) is the conjugate transpose of the near field direction vector.
Step S33: and taking an angle value theta corresponding to the maximum of the angle spectrum p (theta) as a target calibration angle.
In addition, step S4 may be further included: under the condition that the installation position of the millimeter wave radar is not adjusted, correcting the angle of the target detected by the radar each time through a computer.
The calculated calibration angle of the target is the installation angle deviation of the normal of the radar and the running axis of the vehicle, so that after the installation angle deviation is obtained, the installation angle deviation is written into a ROM memory of the radar under the condition that the installation position of the millimeter wave radar is not adjusted, and then the target angle detected by the radar each time is corrected on the radar software level according to the installation angle deviation value, so that the corrected target angle is obtained, and the real position of the radar detection target relative to the vehicle is obtained. Steps S2-S3 may then be repeated to determine if the target' S nominal angle is correct.
As shown in fig. 5, the installation angle offline calibration device based on the millimeter wave radar based on the installation angle offline calibration method based on the millimeter wave radar specifically includes: the vehicle-mounted millimeter wave radar 1, a calibration flat 2 which is disposed at a near-field position just in front of the center of the longitudinal axis of the vehicle and whose normal line is as parallel as possible to the traveling axis of the vehicle, and a terminal device 3. The calibration plate 2 is connected with a controller for controlling the position of the calibration plate 2. The terminal device 3 may be a computer, a mobile terminal, etc., and the terminal device 3 communicates with the controllers of the millimeter wave radar 1 and the calibration flat 2, respectively.
Fig. 6 is a schematic structural view of a terminal device of the installation angle down-line calibration device based on millimeter wave radar according to the present invention. In this embodiment, the terminal device 3 includes: an input interface 31, an output interface 32, a processor 33, a storage unit 34, and one or more computer program modules 35. The terminal device 3 is connected with the millimeter wave radar 1 through an input interface to receive an echo signal of the millimeter wave radar, and the terminal device 3 is connected with a controller of the calibration flat plate 2 through an output interface to control the calibration distance (i.e. the distance between the antenna center of the millimeter wave radar 1 and the calibration flat plate 2). The computer program modules are stored in the memory unit and configured to be executed by the processor to implement the millimeter wave radar-based installation angle off-line calibration method described above.
Analysis of experimental results:
to illustrate the effectiveness of the method, analysis of the results of angle measurement of metal plate targets at different distances was performed
In the same state, according to the prior art, when uncompensated, the angle measurement fluctuation of different distances is between 0.5 and 0.7 degrees, as shown in fig. 7, 0.9m-2.2m are respectively calibrated distances (namely, the distances between a calibrated flat plate and a millimeter radar), the angle measurement result in the figure shows that jitter exists between frames, the horizontal coordinate is the frame number, and the vertical coordinate is the measurement angle. The millimeter radar wave is provided with a network packet acquisition card, and then the data of each frame is obtained by acquiring network packet data through a computer. Since for a fixed calibration object the antenna phase between each frame of the radar probe echo signal is fluctuating, resulting in angular fluctuations.
According to the prior art, the angular spectrum of a target of a millimeter radar wave at different distances is shown in fig. 8, where the abscissa is the angle (°), and the ordinate is the normalized power gain (dB), when uncompensated. Each target has an angle spectrum, the range of the angle spectrum is the range of the near-field direction vector, which is typically-60 degrees to 60 degrees, and the peak position of the angle spectrum is the angle of the target. Because of the influence of the near field, the distances between the calibration flat and the radar are different, and the angle measurement results are also different, in fig. 8, the angle spectrum under different distances is drawn on a graph, the angle value corresponding to the maximum value point of the angle spectrum in the graph is the angle value of the target, and the position of the peak point of the angle spectrum can be seen to be different along with the change of the calibration distance.
In the same state, after the compensation of the invention, the angle measurement fluctuation of different distances is between 0.1 and 0.2 degrees, the angle measurement results are basically near 0.5 degrees and approximate to the angle value after 2m of the far field, as shown in fig. 9, the abscissa is the number of frames, and the ordinate is the measurement angle. This means that the farther the distance, the more the far field condition is met, the more closely the two goniometers are consistent. The compensation of the invention is to correct the parameters of the direction vector of the radar itself, and not to change the echo data detected by the radar.
The angle spectrum of the millimeter radar wave target obtained after the compensation of the invention under different distances is shown in fig. 10, wherein the abscissa is the angle (°), and the ordinate is the normalized power gain (dB). It can be seen that the compensated angular spectrum has better consistency than the uncompensated angular spectrum.
In conclusion, after the result analysis is carried out, the far-field and near-field conversion compensation is carried out, the uncompensated far-field angle measurement result is compared, and under the same state, the angle measurement errors of different distances can be reduced from 0.5-0.7 degrees to 0.1-0.2 degrees, so that the error of the near-field angle measurement is reduced, and the accuracy of the near-field off-line angle calibration is improved.
The foregoing description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention, and various modifications can be made to the above-described embodiment of the present invention. All simple, equivalent changes and modifications made in accordance with the claims and the specification of the present application fall within the scope of the patent claims. The present invention is not described in detail in the conventional art.
Claims (6)
1. The method for calibrating the installation angle offline based on the millimeter wave radar is characterized by comprising the following steps of:
step S1: taking the distance from the calibration flat plate to the plane of the millimeter wave radar as the calibration distance, and acquiring far-near field compensation matrixes under different calibration distances according to the far-field direction vector and the near-field direction vector of the millimeter wave radar;
step S2: mounting a millimeter wave radar to be detected on a vehicle, and arranging a calibration flat plate at a near-field position right in front of the center of a longitudinal axis of the vehicle; a corresponding far-near field compensation matrix is derived according to the calibration distance;
step S3: obtaining a near-field guiding vector by subtracting the far-field compensating matrix derived in the step S2 from the far-field direction vector, and obtaining a target calibration angle according to the near-field direction vector and an echo signal of the millimeter wave radar, wherein the target calibration angle is the deviation of the installation angle between the normal direction of the millimeter wave radar and the running axis direction of the vehicle;
the step S1 includes:
step S11: determining a far-field model according to the position parameters of a receiving antenna and a transmitting antenna of the millimeter wave radar so as to obtain a far-field direction vector, and simultaneously determining a near-field model by combining a calibration distance so as to obtain a near-field direction vector;
step S12: subtracting the far field direction vector from the near field direction vector to obtain a far field and near field compensation matrix under different calibration distances, and storing the far field and near field compensation matrix;
the far field direction vector w Far (θ, m, n) is:
wherein ,for Kronecker product, dsin (θ) is the difference in wave path, d is the antenna spacing of the transmit and receive antennas, and d is the spacing of the receive antennas d for the far field r θ is a direction angle, m is a ordinal number of a transmitting antenna of the millimeter wave radar, N is a ordinal number of a receiving antenna of the millimeter wave radar, m, N is a positive integer, mn is more than or equal to 2, m is a number of transmitting antennas of the millimeter wave radar, N is a number of receiving antennas of the millimeter wave radar, and λ is a wavelength of the millimeter wave radar;
near field direction vector w Near (θ, m, n, r) is:
wherein ,d’mn-1 For the wave path distance r Rmn Distance from wave path r R11 Is the difference of the wave distance, r Rmn The distance from the mth transmitting antenna to the nth receiving antenna of the millimeter wave radar is the wave distance, m is the ordinal number of the transmitting antenna of the millimeter wave radar, and n is the ordinal number of the receiving antenna of the millimeter wave radar; r is a calibration distance, θ is a direction angle, and λ is the wavelength of the millimeter wave radar;
the far and near field compensation matrix is as follows:
g x (θ,m,n,r)＝w Far (θ,m,n)-w Near (θ,m,n,r)，
wherein ,wNear (θ, m, n, r) is a near field direction vector, w Far (θ, m, n) is a far-field direction vector, g x (θ, m, n, r) is a near-far field compensation matrix;
in the step S3, the calibration angle of the target is calculated by adopting a digital beam forming technique method according to the near-field direction vector and the echo signal of the millimeter wave radar;
the step S3 includes:
step S31: obtaining a near-field steering vector by subtracting the far-field and near-field compensation matrix derived in the step S2 from the far-field steering vector stored in the step S1;
step S32: obtaining a conjugate transpose matrix of a near-field direction vector, multiplying the conjugate transpose matrix with echo signals of the millimeter wave radar to obtain a power product, and taking an absolute value of the power product corresponding to each theta value to form an angle spectrum;
step S33: and taking an angle value theta corresponding to the maximum of the angle spectrum p (theta) as a target calibration angle.
2. The millimeter wave radar-based mounting angle offline calibration method according to claim 1, wherein the millimeter wave radar is a time division multiplexed MIMO array.
3. The millimeter wave radar-based mounting angle offline calibration method according to claim 1, further comprising step S4: under the condition that the installation position of the millimeter wave radar is not adjusted, the angle of the target detected by the radar each time is corrected by a computer.
4. The device is characterized by comprising a millimeter wave radar, a calibration flat plate and terminal equipment, wherein the millimeter wave radar, the calibration flat plate and the terminal equipment are arranged on a vehicle; the terminal device comprises a processor, a memory unit, and one or more computer program modules stored in the memory unit and configured to be executed by the processor to implement the millimeter wave radar-based installation angle offline calibration method according to one of claims 1-3.
5. The millimeter wave radar-based mounting angle offline calibration device of claim 4, wherein the calibration plate is coupled to a controller for controlling the position of the calibration plate.
6. The millimeter wave radar-based mounting angle offline calibration device according to claim 4, wherein the terminal device further comprises an input interface and an output interface; the terminal equipment is connected with the millimeter wave radar through an input interface so as to receive echo signals of the millimeter wave radar; the terminal equipment is connected with the controller of the calibration flat board through the output interface so as to control the calibration distance.
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