CN113030887A - Off-line calibration method and device for installation angle based on millimeter wave radar - Google Patents
Off-line calibration method and device for installation angle based on millimeter wave radar Download PDFInfo
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Abstract
The invention provides a millimeter wave radar-based installation angle offline calibration method, which comprises the following steps: acquiring far and near field compensation matrixes at different calibration distances according to the far field direction vector and the near field direction vector of the millimeter wave radar; installing 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; deriving a corresponding near-far field compensation matrix according to the calibration distance; and subtracting the derived far-near field compensation matrix from the far-field direction vector to obtain a near-field steering 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, and the far and near field compensation matrix is used for conducting far and near field conversion to correct the phase relation, so that the angle of the flat target can be obtained more accurately, the calibration of the radar installation angle is completed, and the calibration method is applicable to the near field condition.
Description
Technical Field
The invention belongs to the technical field of vehicle-mounted millimeter wave radars, and particularly relates to a millimeter wave radar-based mounting angle offline calibration method and device.
Background
With the rapid development of the automobile industry, various automobile manufacturers are dedicated to research and development of intelligent automobile safety systems, which aim to reduce traffic accidents, improve road traffic safety, enhance road traffic capacity, and the like, and all of the systems need to acquire front environment information, i.e., relative distance, speed, angle, and the like of a front automobile. At present, a common method is to install a millimeter wave radar at the front end of a vehicle to acquire relevant information of a preceding 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 over against the front of the radar when the radar is installed, and the detection distance of a front vehicle can have a large error due to a small installation angle or position deviation of the millimeter wave radar, so that the installation position and the posture of the radar need to be calibrated.
The existing vehicle provided with the millimeter wave radar generally needs to be provided with calibration equipment at the tail end of a whole vehicle production line so as to carry out whole vehicle offline calibration on the installation angle and position of the millimeter wave radar.
The calibration of inserting a production line is generally all can adopting based on four-wheel location fixed vehicle at present mainstream whole garage, then puts the calibration object at a certain fixed position in the dead ahead of vehicle axis of ordinates center and carry out the calibration of inserting a production line, and wherein, the calibration object generally is dull and stereotyped, and also some calibration objects are that the angle is anti, so whole garage all can purchase such calibration equipment of inserting a production line and carry out the calibration of inserting a production line, not only is radar calibration, often also has the calibration of camera etc., all needs such calibration equipment of inserting a production line. Therefore, after the off-line calibration equipment of the whole factory is fixed, the radar calibration process cannot be executed according to the program parameters on the road, and the algorithm needs to be adjusted to obtain the calibration angle more accurately. In the offline calibration using a flat plate as a calibration object, the basic requirements are:
1. selecting a flat metal plate with a larger size as a radar target, considering that the reflection direction of the electromagnetic wave is the normal direction of the metal plate due to the strong characteristic of the reflection direction of the electromagnetic wave, fixing the metal plate on a calibration frame and placing the metal plate in front of the radar;
2. and (3) through strict four-wheel positioning, the longitudinal axis direction of the vehicle is consistent with the normal direction of the metal plate, and the radar measures the angle of the metal plate corresponding to the installation angle deviation of the radar.
However, if the millimeter wave radar angle measurement needs to satisfy the far field condition, it is necessary to increase the distance between the calibration flat plate as the calibration object and the radar (for example, to 4-5m), which will have very high environmental requirements, and the whole car factory cannot meet such condition. Specifically, the general requirements of a whole car factory in calibrating a common radar are that the calibrated flat plate distance radar is less than 2 m; if the distance between the calibration flat plate and the radar is increased to 4-5m according to the requirement of calibrating the millimeter wave radar, the calibration station is necessarily enlarged, and a metal equal-strength reflector is not required to be arranged in the distance radius (4-5m), so that the radar can detect more peripheral targets, such as walls of equipment workshops, vehicles queuing and the like, and the targets which are at the same distance and the same speed with the calibration flat plate are easier to form, the angle measurement phase is influenced, the angle measurement error is increased, and the calibration angle is incorrect.
Generally, the vehicle-mounted millimeter wave radar measures angles according to direction vectors of far fields. However, when the radar is calibrated in the off-line installation of the whole vehicle, the angle measurement of the millimeter wave radar cannot meet the far field condition, so that the installation angle calculated by the normal angle measurement algorithm is inaccurate, the antenna array layout of the near field can generate great influence on the angle measurement, and the angle errors in different calibration distances are inconsistent.
Disclosure of Invention
The invention aims to provide a millimeter wave radar-based installation angle offline calibration method and device, which are suitable for calibrating a millimeter wave radar under a near-field condition and reducing errors.
In order to achieve the above object, the present invention provides a millimeter wave radar-based off-line calibration method for an installation angle, including:
s1: taking the distance from the calibration flat plate to the plane of the millimeter wave radar as a calibration distance, and acquiring near-far field compensation matrixes at different calibration distances according to the far field direction vector and the near field direction vector of the millimeter wave radar;
s2: installing 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; deriving a corresponding near-far field compensation matrix according to the calibration distance;
s3: and subtracting the far-near field compensation matrix derived in the step S2 from the far-field direction vector to obtain a near-field steering vector, and obtaining a calibration angle of a target according to the near-field direction vector and an echo signal of the millimeter wave radar, where the calibration angle of the target is an installation angle deviation between a normal direction of the millimeter wave radar and a driving axis direction of the vehicle.
The millimeter wave radar is a time division multiplexing 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 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 a far field compensation matrix under different calibration distances, and storing the far field compensation matrix.
The far field direction vector wFar(θ, m, n) is:
wherein ,
for the Kronecker product, dsin (theta) is the wave path difference, d is the antenna spacing of the transmitting and receiving antennas, and for the far field, d is the spacing d of the receiving antennasrTheta is a direction angle, M is the ordinal number of a transmitting antenna of the millimeter wave radar, N is the ordinal number of a receiving antenna of the millimeter wave radar, M, N is a positive integer, mn is not less than 2, M is the number of transmitting antennas of the millimeter wave radar, N is the number of receiving antennas of the millimeter wave radar, and lambda is the wavelength of the millimeter wave radar;
near field directorQuantity wNear(θ, m, n, r) is:
wherein d' mn-1 is the wave path distance rRmnDistance difference of wave path from distance of wave path rR11, rRmnThe distance of a wave path from an m-th transmitting antenna to an n-th receiving antenna of the millimeter wave radar is defined, 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, theta is a direction angle, and lambda is the wavelength of the millimeter wave radar;
the far and near field compensation matrix is as follows:
gx(θ，m，n，r)＝wFar(θ，m，n)-wNear(θ，m，n，r)，
wherein ,wNear(θ, m, n, r) is the near-field direction vector, wFar(θ, m, n) is a far-field direction vector, gxAnd (theta, m, n, r) is a far-near field compensation matrix.
In the step S1, the calibration angle of the target is calculated by a digital beam forming method according to the near-field direction vector and the echo signal of the millimeter wave radar.
The step S3 includes:
s31: subtracting the far-near field compensation matrix derived in the step S2 from the far-field direction vector stored in the step S1 to obtain a near-field steering vector;
s32: acquiring a conjugate transpose matrix of a near-field direction vector, multiplying the conjugate transpose matrix by an echo signal 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 the angle value theta corresponding to the maximum angle spectrum p (theta) as the calibration angle of the target.
The off-line calibration method for the installation angle based on the millimeter wave radar further comprises the 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 using a computer.
On the other hand, the invention provides a millimeter wave radar-based installation angle offline calibration device, which comprises a millimeter wave radar, a calibration panel and terminal equipment, wherein the millimeter wave radar, the calibration panel and the terminal equipment are installed on a vehicle; the terminal device includes a processor, a storage unit, and one or more computer program modules stored in the storage 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 plate is connected with a controller and used for controlling the position of the calibration flat plate.
The terminal equipment also comprises an input interface and an output interface; the terminal equipment is connected with the millimeter wave radar through the input interface so as to receive an echo signal of the millimeter wave radar; the terminal equipment is connected with the controller of the calibration panel through the output interface so as to control the calibration distance.
According to the off-line calibration method for the installation angle based on the millimeter wave radar, 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 used for conducting 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 off-line calibration method is applicable to the near field condition, the requirements of the far field condition on environment and equipment are met, the operation cost is reduced, and the off-line calibration site of a whole vehicle factory meets the calibration requirements. In addition, the off-line calibration method based on the installation angle of the millimeter wave radar can select a proper far-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 a flowchart of an algorithm of an installation angle offline calibration method based on a millimeter wave radar according to an embodiment of the present invention.
Fig. 2 is a schematic diagram of 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 the near-field model.
Fig. 4 is an analysis diagram of a MIMO array in the near field.
Fig. 5 is a schematic structural diagram of a mounting angle offline calibration device based on a millimeter wave radar according to an embodiment of the present invention.
Fig. 6 is a schematic structural diagram of a terminal device of the installation angle offline calibration apparatus based on the millimeter wave radar according to the present invention.
Fig. 7 is a graph of the results of angle measurements at different distances without compensation according to the prior art.
Fig. 8 is a graph of angular spectra at different distances without compensation according to the prior art.
FIG. 9 is a graph of the results of angle measurements at different distances compensated by the present invention.
FIG. 10 is a graph of angle spectra at different distances compensated for by the present invention.
Detailed Description
The present invention will be further described with reference to the following specific examples. It should be understood that the following examples are illustrative only and are not intended to limit the scope of the present invention.
As shown in fig. 1, the off-line calibration method for installation angle based on millimeter wave radar of the present invention includes:
step S1: taking the distance from the calibration flat plate to the plane of the millimeter wave radar as a calibration distance, and acquiring near-far field compensation matrixes at 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 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 transmit and receive antennas are separate;
step S12: subtracting the far-field direction vector from the near-field direction vector to obtain a far-field compensation matrix under different calibration distances, and storing the far-field compensation matrix and the near-field compensation matrix;
the millimeter wave radar is a time division multiplexing MIMO array. 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 a solidified parameter value generally and cannot be modified along with the change of the calibration distance, so that the far-field direction vector can be stored in the line calibration device at 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 the transmitting antennas is 3, the number of the receiving antennas is 4, and each transmitting antenna corresponds to 4 receiving antennas, if time division MIMO with 3 transmitting antennas is performed, the total number of the transmitting and receiving antennas is 12.
In step S1, (1) as shown in fig. 2, in the far-field model, the far-field direction vector wFar(θ, m, n) (i.e., the rear direction vector) is:
in the formula ,wtx(θ, m) is the directional vector of the transmit antennas of the MIMO array, wrx(theta, n) is a direction vector of the receiving antenna of the MIMO array,
is Kronecker product, 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; for an angle measuring radar, m or n is not less than 2 definitely, the antenna angle measuring principle is calculated according to phase change caused by the wave path difference of echoes of two groups of antennas, and a single antenna cannot generate the wave path difference, so mn is not less than 2 definitely, 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.
It can be known from antenna theory that the distance difference between an object and an antenna can cause the phase of an echo signal to change, and because of a far-field condition, we generally ignore the distance difference between transmitting and receiving antennas, and the phase directional diagram can be regarded as a straight line, i.e. an equiphase plane, and the direction vector of the transmitting antenna and the direction vector of the receiving antenna are respectively expressed as:
wherein ,dt、drThe distance between the transmitting antennas and the distance between the receiving antennas of the millimeter wave radar are respectively, theta is a direction angle (namely an azimuth angle of a normal line of the calibration flat plate relative to the normal line of the millimeter wave radar), M is an ordinal number of the transmitting antennas of the millimeter wave radar, N is an ordinal number of the receiving antennas of the millimeter wave radar, M is the number of the transmitting antennas of the millimeter wave radar, and N is the number of the receiving antennas of the millimeter wave radar.
Thus, under far field conditions, the far field direction vector wFar(θ, m, n) is:
wherein ,
for Kronecker product, dsin (θ) is the wave path difference, where d is the antenna spacing of the transmitting and receiving antenna, and d is d for the far fieldrThe path difference of the receiving antenna is (mn-1)'d, corresponding to the following d'mn-1(ii) a Theta is a direction angle, M is an ordinal number of a transmitting antenna of the millimeter wave radar, N is an ordinal number of a receiving antenna of the millimeter wave radar, M is the number of transmitting antennas of the millimeter wave radar, N is the number of receiving antennas of the millimeter wave radar, and lambda is a wavelength of the millimeter wave radar. In this embodiment, since there are 3 transmitting antennas and 4 receiving antennas, M is 3 and N is 4.
For far-field environments, once the antenna spacing d of the transmitting and receiving antennas is defined, its direction vector is a constant parameter value.
(2) As shown in fig. 3, for the near-field condition, the path difference cannot be simply equivalent to the difference caused only by the receiving antenna pitch, and it is necessary to consider the phase difference caused by the distance difference and calculate the actual electromagnetic path length, and the far-field direction vector is not applicable to the scene, and it is necessary to perform compensation correction on the far-field direction vector to obtain the near-field direction vector.
As shown in fig. 4, in the analysis diagram of the near-field MIMO array, for convenience of calculation, a mirror image of the transmitting antenna with respect to the calibration plate is shown as a mirror image point of the transmitting antenna (the mirror image point of the transmitting antenna T1 is schematically shown in fig. 4, and may be a mirror image point of any antenna in practice), and the position of the calibration plate is a long straight line position between the transmitting antenna and the mirror image point. Thus, the near field direction vector wNear(θ, m, n, r) is:
wherein ,d’mn-1Is the distance r of wave pathRmnDistance r from wave pathR11Distance difference of wave path of rRmnThe distance between the mth transmitting antenna Tm and the nth receiving antenna Rn of the millimeter wave radar is represented by the range 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, wherein m or n is not less than 2 for the angle measurement radar, the antenna angle measurement principle is calculated according to the phase change caused by the range difference of echoes of two groups of antennas, and a single antenna cannot generate the range difference, so MN is not less than 2 definitely, MN-1 is 1, … MN-1; r is the calibration distance, namely the distance between the center of the antenna of the millimeter wave radar and the calibration flat plate, and theta is a direction angle.
As shown in FIG. 4, the travel distance difference d'mn-1The calculation method comprises the following steps:
distance of wave path rR11，…，rRmn，…，rRMNIs calculated in relation to the nominal distance r. By range distance is meant the transmission of electromagnetic waves from a transmitting antenna to a target (i.e. calibration)Flat) to the receiving antenna, the distance traveled from the transmitting antenna to the target and then to the receiving antenna, which can be calculated from fig. 4, can be equivalent to a light wave. 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 transmitting and receiving antennas, according to the geometrical relationship of fig. 4, X0 is the coordinate of the antenna center of the millimeter-wave radar, Δ XTDistance of adjacent transmitting antennas, Δ X, for millimeter-wave radarRDistance between adjacent receiving antennas of the millimeter wave radar, Tm is the mth transmitting antenna of the millimeter wave radar, and M is 1, 2, …, M, XTmIs the coordinate of the mth transmitting antenna of the millimeter wave radar, Rn is the nth receiving antenna of the millimeter wave radar, and N is 1, 2, …, N, XRnIs the coordinate of the nth receiving antenna of the millimeter wave radar, and R is the distance from the mth transmitting antenna Tm of the millimeter wave radar to the mirror point thereof (R is the 1 st transmitting antenna T exemplarily shown in the figure1Distance to its mirror point), rRn is the distance from the mirror point to the nth receiving antenna of the millimeter wave radar.
Therefore, the wave path distance r corresponding to different transmitting antennas can be easily obtainedR11，…，rRMN，
R＝[(X0-XTm)*sin(θ)+r]×2 (6)
Wherein R is the distance from the mth transmitting antenna Tm of the millimeter wave radar to the mirror image point thereof, R is the calibration distance, theta is the direction angle, X0 is the coordinate of the antenna center of the millimeter wave radar, and XTmIs the coordinate of the transmitting antenna Tm of the millimeter wave radar.
wherein ,rRmnIs the distance from the mth transmitting antenna Tm to the nth receiving antenna Rn of the millimeter wave radar, R is the distance from the mth transmitting antenna Tm to the mirror image point of the millimeter wave radar, XRnCoordinates, X, of the receiving antenna Rn of the millimeter-wave radarTmIs a transmitting antenna of a millimeter wave radarThe coordinates of the line Tm, θ is the direction angle.
d’mn-1＝rRmn-rR11＝0，d’1，…，d’MN-1 (8)
Wherein MN-1 is 1, … MN-1, rRmnIs the distance of the wave path from the mth transmitting antenna Tm to the nth receiving antenna Rn of the millimeter wave radar.
Thus, the distance difference d 'of the wave path'mn-1By substituting in the above formula (5), the near-field direction vector w can be obtainedNear(θ，m，n，r)。
Then, the near-field direction vector is subtracted from the far-field direction vector, and a far-field and near-field compensation matrix (namely, a correction matrix and a correction antenna direction vector) required by people can be obtained. The far and near field compensation matrix is:
gx(θ，m，n，r)＝wFar(θ，m，n)-wNear(θ，m，n，r)，
wherein ,wNear(θ, m, n, r) is the near-field direction vector, wFar(θ, m, n) is a far-field direction vector, gxAnd (theta, m, n, r) is a far-near field compensation matrix.
Because the near-field direction vector is related to the calibration distance r, the obtained far-near field compensation matrix changes along with the calibration distance r, and different calibration distances r can be substituted into the far-near field compensation matrix gxAnd (theta, m, n, r) to obtain the far and near field compensation matrixes at different calibration distances. Therefore, the radar can be converted from a far-field model to a near-field condition according to the near-field model, direction vectors at different calibration distances are output, and the angle of the target is estimated according to the near-field direction vectors and the array signals.
Step S2: installing 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 center of a longitudinal axis of the vehicle, and enabling the normal line of the calibration flat plate to be parallel to the driving axis of the vehicle as far as possible so as to determine a calibration distance (namely the distance between the center of an antenna of the millimeter wave radar and the calibration flat plate); deriving a corresponding near-far field compensation matrix according to the calibration distance for correcting the phase relation;
the near-far field compensation matrix is obtained by subtracting the near-field direction vector from the far-field direction vector calculated in advance in step S1, and the near-field direction vector is related to the calibration distance, and finally the near-far field compensation matrix (i.e. the correction matrix) at different calibration distances is obtained, so in step S2, the near-far field compensation matrix corresponding to the environment of the actual test site can be selected and obtained according to the actually determined calibration distance, that is, the range distances of each group of transceiving antennas at different calibration distances are calculated, so as to obtain the true phase relationship, and obtain the actual derivation result of the near-far field compensation matrix.
In this embodiment, the calibration flat plate is made of metal, and the calibration distance is determined by obtaining the position coordinates of the calibration flat plate through detection and measurement of the laser calibration device. In other embodiments, since the actually required position parameter is only the calibration distance, i.e. the distance from the calibration flat plate to the plane of the millimeter wave radar, the distance from the calibration flat plate to the plane of the millimeter wave radar can be directly measured by the meter ruler after being corrected by the laser calibration device. In addition, in other embodiments, it is also possible to directly measure the distance by using the millimeter wave radar to be measured, and the distance between the millimeter wave radar and the millimeter wave radar is nearly equal to each other in actual measurement. Because the far and near fields of the radar only affect the angle information of the target, and the radial distance, the speed and other information of the target are not affected, the distance and the speed measurement are just the advantages of the millimeter-wave radar.
Step S3: and (4) subtracting the far-near field compensation matrix derived in the step (S2) from the far-field direction vector stored in the step (S1) to obtain a near-field guide vector, and obtaining a calibration angle of a target according to the near-field direction vector and an echo signal of the millimeter wave radar, wherein the calibration angle of the target is the installation angle deviation between the normal direction of the millimeter wave radar and the driving axis direction of the vehicle.
As described above, the far-field direction vector is a fixed parameter value, and once the calibration distance between the calibration panel and the radar is determined in step S2, the far-field and 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-field and near-field conversion is performed on the direction vector of the radar through the far-field and near-field compensation matrix at the time of calibration, that is, the far-field direction vector is subtracted from the near-field compensation matrix to obtain the near-field direction vector.
And calculating the calibration angle of the target according to the near-field direction vector and the echo signal of the millimeter wave radar by adopting a DBF (digital beam forming technology) method. Among them, DBF (digital beam forming technology) 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 include a virtual signal S11(t)…Smn(t)…SMN(t)。
Referring to fig. 1 again, the specific process of step S3 is as follows:
step S31: subtracting the far-near field compensation matrix derived in the step S2 from the far-field direction vector stored in the step S1 to obtain a near-field steering vector;
step S32: acquiring a conjugate transpose matrix of a near-field direction vector, multiplying the conjugate transpose matrix by an echo signal 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 addition and subtraction, and x is product operation, where the conjugate transpose matrix of the near-field direction vector is multiplied by the echo signal of the millimeter wave radar, and each element of the near-field direction vector is multiplied by the echo signal of a different receiving channel and accumulated. In which fig. 2 shows the calculation principle in the far-field model of the prior art and fig. 3 shows the calculation principle in the near-field model according to the invention.
Wherein the power product y (t) is:
y(t)＝wH(θ，m，n)S(t)，
the angular spectrum p (θ) is:
p(θ)＝|y(t)|
wherein S (t) is the echo signal of the millimeter wave radar composed of S11(t)…Smn(t)…SMN(t) is a linear chain; w is aH(θ, m, n) is the conjugate transpose of the near-field direction vector.
Step S33: and taking the angle value theta corresponding to the maximum angle spectrum p (theta) as the calibration angle of the target.
Further, step S4 may be further included: 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 through the computer.
The calibration angle of the target obtained by calculation is the installation angle deviation of the normal line 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 (read only 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 to obtain the corrected target angle, so that the real position of the radar detection target relative to the vehicle is obtained. Steps S2-S3 may also be repeated subsequently to determine whether the calibrated angle of the target is correct.
As shown in fig. 5, the off-line calibration apparatus for an installation angle based on a millimeter wave radar, based on the above-mentioned off-line calibration method for an installation angle based on a millimeter wave radar, specifically includes: the system comprises a millimeter wave radar 1 installed on a vehicle, a calibration flat plate 2 which is arranged at a near-field position right in front of the center of the longitudinal axis of the vehicle and the normal line of which is parallel to the running axis of the vehicle as much as possible, 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 is in communication with the millimeter wave radar 1 and the calibration panel 2, respectively.
Fig. 6 is a schematic structural diagram of a terminal device of the installation angle offline calibration apparatus based on the 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 to the millimeter wave radar 1 through the input interface to receive an echo signal of the millimeter wave radar, and the terminal device 3 is connected to the controller of the calibration panel 2 through the output interface to control the calibration distance (i.e., the distance between the center of the antenna of the millimeter wave radar 1 and the calibration panel 2). The computer program module is stored in the storage unit and configured to be executed by the processor to implement the millimeter wave radar-based mounting angle offline calibration method described above.
And (3) analyzing an experimental result:
to illustrate the effectiveness of the method, the result analysis of the angle measurement of the metal plate target at different distances is carried out
Under the same state, according to the prior art, when the compensation is not carried out, the angle measurement fluctuation of different distances ranges from 0.5 degrees to 0.7 degrees, as shown in fig. 7, 0.9m to 2.2m respectively represent the calibration distance (i.e. the distance between the calibration flat plate and the millimeter radar), the angle measurement result in the graph shows that the jitter exists between frames, the abscissa represents the number of frames, and the ordinate represents 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 the antenna phase fluctuates between each frame of the radar detection echo signal for a fixed calibration object, the angle fluctuates.
According to the prior art, the angular spectrum of a target of millimeter radar waves at different distances when uncompensated is shown in fig. 8, where the abscissa is the angle (°) and the ordinate is the normalized power gain (dB). Wherein, each target has an angle spectrum, the range of the angle spectrum is the angle range of the near field direction vector, generally-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 distance between the calibration flat plate and the radar is different, the angle measurement result is also different, the angle spectrum at different distances is drawn on a graph in the graph 8, the angle value corresponding to the maximum 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.
Under the same state, after compensation of the invention, the angle measurement fluctuation of different distances is between 0.1 and 0.2 degrees, the angle measurement result is basically near 0.5 degrees and is similar to the angle value after 2m of a far field, as shown in fig. 9, the abscissa is the frame number, and the ordinate is the measurement angle. This indicates that the farther the distance is, the more the far-field condition is satisfied, the closer the two angle measurement results are consistent. The compensation of the invention is to modify the parameters of the direction vector of the radar, and does not change the echo data detected by the radar.
The angle spectra of the target of millimeter radar waves obtained after compensation according to the present invention at different distances are shown in fig. 10, in which the abscissa is the angle (°) and the ordinate is the normalized power gain (dB). It can be seen that the consistency of the compensated angle spectrum is better than that of the uncompensated angle spectrum.
In conclusion, through result analysis, after far-near field conversion compensation, compared with an uncompensated far-field angle measurement result, under the same state, the angle measurement errors at different distances can be reduced from 0.5-0.7 degrees to 0.1-0.2 degrees, so that the error of near-field angle measurement is reduced, and the precision of near-distance offline angle calibration is improved.
The above embodiments are merely preferred embodiments of the present invention, which are not intended to limit the scope of the present invention, and various changes may be made in the above embodiments of the present invention. All simple and equivalent changes and modifications made according to the claims and the content of the specification of the present application fall within the scope of the claims of the present patent application. The invention has not been described in detail in order to avoid obscuring the invention.
Claims (10)
1. A millimeter wave radar-based installation angle offline calibration method is characterized by comprising the following steps:
step S1: taking the distance from the calibration flat plate to the plane of the millimeter wave radar as a calibration distance, and acquiring near-far field compensation matrixes at different calibration distances according to the far field direction vector and the near field direction vector of the millimeter wave radar;
step S2: installing 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; deriving a corresponding near-far field compensation matrix according to the calibration distance;
step S3: and subtracting the far-near field compensation matrix derived in the step S2 from the far-field direction vector to obtain a near-field steering vector, and obtaining a calibration angle of a target according to the near-field direction vector and an echo signal of the millimeter wave radar, where the calibration angle of the target is an installation angle deviation between a normal direction of the millimeter wave radar and a driving axis direction of the vehicle.
2. The millimeter wave radar-based mounting angle offline calibration method according to claim 1, wherein the millimeter wave radar is a time division multiplexing MIMO array.
3. The millimeter wave radar-based mounting angle offline calibration method according to claim 1, wherein the step S1 includes:
step S11: determining a far-field model according to 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: and subtracting the far field direction vector and the near field direction vector to obtain a far field compensation matrix under different calibration distances, and storing the far field compensation matrix.
4. The millimeter wave radar-based mounting angle offline calibration method according to claim 3, wherein the far-field direction vector wFar(θ, m, n) is:
wherein ,
for the Kronecker product, dsin (theta) is the wave path difference, d is the antenna spacing of the transmitting and receiving antennas, and for the far field, d is the spacing d of the receiving antennasrTheta is a direction angle, M is the ordinal number of a transmitting antenna of the millimeter wave radar, N is the ordinal number of a receiving antenna of the millimeter wave radar, M, N is a positive integer, mn is not less than 2, M is the number of transmitting antennas of the millimeter wave radar, N is the number of receiving antennas of the millimeter wave radar, and lambda is the wavelength of the millimeter wave radar;
near field direction vector wNear(θ, m, n, r) is:
wherein ,d’mn-1Is the distance r of wave pathRmnDistance r from wave pathR11Distance difference of wave path of rRmnThe distance of a wave path from an m-th transmitting antenna to an n-th receiving antenna of the millimeter wave radar is defined, 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, theta is a direction angle, and lambda is the wavelength of the millimeter wave radar;
the far and near field compensation matrix is as follows:
gx(θ，m，n，r)＝wFar(θ，m，n)-wNear(θ，m，n，r)，
wherein ,wNear(θ, m, n, r) is the near-field direction vector, wFar(θ, m, n) is a far-field direction vector, gxAnd (theta, m, n, r) is a far-near field compensation matrix.
5. The millimeter wave radar-based mounting angle offline calibration method according to claim 1, wherein in the step S3, the calibration angle of the target is calculated according to the near-field direction vector and the echo signal of the millimeter wave radar by using a digital beam forming technique.
6. The millimeter wave radar-based mounting angle offline calibration method according to claim 5, wherein the step S3 includes:
step S31: subtracting the far-near field compensation matrix derived in the step S2 from the far-field direction vector stored in the step S1 to obtain a near-field steering vector;
step S32: acquiring a conjugate transpose matrix of a near-field direction vector, multiplying the conjugate transpose matrix by an echo signal 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 the angle value theta corresponding to the maximum angle spectrum p (theta) as the calibration angle of the target.
7. 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 using a computer.
8. An installation angle offline calibration device based on a millimeter wave radar is characterized by comprising the millimeter wave radar, a calibration panel and terminal equipment, wherein the millimeter wave radar, the calibration panel and the terminal equipment are installed on a vehicle; the terminal device comprises a processor, a storage unit, and one or more computer program modules stored in the storage 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 to 7.
9. The millimeter wave radar-based mounting angle offline calibration device according to claim 7, wherein the calibration flat plate is connected with a controller for controlling the position of the calibration flat plate.
10. The millimeter wave radar-based mounting angle offline calibration device according to claim 7, wherein the terminal device further comprises an input interface and an output interface; the terminal equipment is connected with the millimeter wave radar through the input interface so as to receive an echo signal of the millimeter wave radar; the terminal equipment is connected with the controller of the calibration panel through the output interface so as to control the calibration distance.
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