CN107783107B - Millimeter wave radar altimeter of plant protection rotor unmanned aerial vehicle - Google Patents
Millimeter wave radar altimeter of plant protection rotor unmanned aerial vehicle Download PDFInfo
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Abstract
A millimeter wave radar altimeter of a plant protection rotor unmanned aerial vehicle belongs to the field of radars, and comprises an antenna subsystem, a radio frequency subsystem, a signal conditioning subsystem and a signal processing subsystem in order to solve the problems of calculation of the radar height of the plant protection rotor unmanned aerial vehicle and the calculation accuracy and stability; and the signal processing subsystem is used for acquiring two paths of I/Q intermediate frequency signals output by the signal conditioning subsystem into the AD acquisition channel and processing and outputting radar signals of the radar altimeter system of the plant protection rotor unmanned aerial vehicle. The effect is as follows: not only can far exceed the altitude value after present ultrasonic sensor altitude value settlement, the advantage such as the small size and the low power dissipation of millimeter wave radar can make unmanned aerial vehicle altimeter have better application scene and application.
Description
Technical Field
The invention belongs to the field of radars, and relates to a millimeter wave radar altimeter of a plant protection rotor unmanned aerial vehicle.
Background
Plant protection unmanned aerial vehicle, as the name implies is the unmanned aircraft who is used for agriculture and forestry plant protection operation, and this type unmanned aircraft has flight platform (fixed wing, single rotor, many rotors), GPS to fly to control, spraying mechanism triplex, flies to control through ground remote control or GPS, realizes spraying the operation, can spray medicament, seed, powder etc..
Compared with the traditional plant protection operation, the unmanned aerial vehicle plant protection operation has the characteristics of accurate operation, high efficiency, environmental protection, intellectualization, simple operation and the like, and saves the cost of large machinery and a large amount of manpower for farmers. Plant protection unmanned aerial vehicle has the operation height low, and the drift is few, characteristics such as can hover in the air, and the downdraft that the rotor produced helps increasing the air current to the penetrability of crops when spraying pesticide, if height between control plant protection unmanned aerial vehicle that can be stable and the ground vegetation, at the pesticide spray operation in-process, then can be better make the pesticide efficient spray the vegetation on to improve the prevention and cure effect of pesticide etc. make the pesticide reach the biggest utilization ratio.
Disclosure of Invention
In order to solve the problems of resolving the radar height of the plant protection rotor unmanned aerial vehicle, and resolving accuracy and stability, the invention provides the following technical scheme: a millimeter wave radar altimeter of a plant protection rotor unmanned aerial vehicle is characterized by comprising an antenna subsystem, a radio frequency subsystem, a signal conditioning subsystem and a signal processing subsystem;
the antenna subsystem forms transmitting and receiving beams required by radar detection, radiates a transmitting signal to a designated area and receives a target scattering echo signal in the designated area;
the radio frequency subsystem generates a transmitting signal, and the frequency of the transmitting signal is changed according to the rule of a modulating signal, so that linear frequency modulation continuous waves are output;
the signal conditioning subsystem is used for filtering and amplifying the intermediate frequency analog signal;
the signal processing subsystem enables four paths of I/Q intermediate frequency signals output by the signal conditioning subsystem to be collected into the AD collecting channel, and radar signals of the radar altimeter system of the plant protection rotor unmanned aerial vehicle are processed and output.
Has the advantages that: the invention provides an overall design method and a working principle of a millimeter wave radar altimeter of a vegetation rotor unmanned aerial vehicle; the vegetation unmanned aerial vehicle altimeter system realized by adopting the millimeter wave radar not only can far exceed the altitude value after the settlement of the altitude value of the current ultrasonic sensor, but also has the advantages of small size, low power consumption and the like of the millimeter wave radar, so that the unmanned aerial vehicle altimeter has better application scenes and application fields.
Drawings
Fig. 1 is a schematic structural view of a millimeter wave radar altimeter of a plant protection rotor unmanned aerial vehicle;
FIG. 2 is a graph of the frequency variation of a chirped sawtooth FMCW over a frequency sweep period;
fig. 3 is a flow chart of a signal processing method of a radar altimeter system of a vegetation rotor unmanned aerial vehicle in embodiment 2.
Detailed Description
Example 1: the working frequency of the millimeter wave radar designed by the embodiment is 24GHz or 77GHz, an FMCW continuous wave system is adopted, and the distance resolution is high mainly due to a linear frequency modulation mode. The waveform can adopt a chirp triangular wave FMCW, a sawtooth wave and a constant frequency wave or a combined waveform of the waveforms. Adopt single triangle wave emission waveform, can carry out relative distance's detection to the target, through the relative velocity between the unmanned aerial vehicle that detects out and the vegetation, assume the vegetation is static, then this speed is unmanned aerial vehicle upper and lower flying speed. Accurate settlement of the relative distance of the target can be completed by adopting a single triangular wave. The sawtooth wave can only detect the relative distance of the target, and the constant frequency wave can only detect the speed of the target. Because the Doppler frequency shift that unmanned aerial vehicle vertical flight produced can cause the deviation of certain distance, so can adopt the combination waveform of sawtooth wave and constant frequency wave, realize the compensation to sawtooth wave band distance detection through the constant frequency wave band to the measurement of speed to improve the distance precision between unmanned aerial vehicle and the vegetation. Triangular wave and sawtooth wave and constant frequency wave's combined waveform can all be regarded as the transmission waveform and realize vegetation unmanned aerial vehicle altimeter, can select the transmission waveform according to the application scene of difference to apply to the application field of different vegetation better, in order to reach better distance detection precision.
The maximum vertical flight speed of the vegetation rotor unmanned aerial vehicle designed by the embodiment is 35km/h, the range of the millimeter wave radar altimeter of the unmanned aerial vehicle is 1-40 m, and the range precision is 0.2 m.
The working principle of the millimeter wave radar altimeter of the vegetation rotor unmanned aerial vehicle is that the distance and the speed of a target to be measured are determined by using the frequency difference between a transmitting signal and an echo signal of a radar. As shown in fig. 1, the millimeter wave radar altimeter system of the vegetation rotor unmanned aerial vehicle is mainly divided into an antenna subsystem, a radio frequency subsystem, a signal conditioning subsystem, a signal processing subsystem and the like.
The antenna subsystem forms transmitting and receiving beams required by radar detection, radiates a transmitting signal to a designated area and receives a target scattering echo signal in the designated area;
the radio frequency subsystem generates a transmitting signal, and the frequency of the transmitting signal is changed according to the rule of a modulating signal, so that linear frequency modulation continuous waves are output;
the signal conditioning subsystem is used for filtering and amplifying the intermediate frequency analog signal;
the signal processing subsystem makes two ways of signal conditioning subsystem output, and unmanned anticollision because be two passageways, needs the angle measurement, so has four ways IQ, and the altimeter only has a passageway, does not need the angle measurement, and the AD gathers in the passageway is gathered to I/Q intermediate frequency signal to carry out plant protection rotor unmanned aerial vehicle radar altimeter system radar signal processing and output.
The antenna subsystem comprises a transmitting antenna and a receiving antenna, wherein the receiving antenna is composed of a single row of receiving antennas and is formed into a grouped array by using a microstrip rectangular patch; the transmitting antenna and the receiving antenna are connected with the back microwave circuit through the via holes.
The signal processing subsystem comprises an ARM chip, a power supply module, a serial port module and a CAN module, wherein the AMR chip collects two paths of I/Q intermediate frequency signals output by the signal conditioning subsystem into two paths of AD (analog-to-digital) collecting channels carried by the ARM chip, the ARM chip carries out signal processing, and the signals are output through the serial port module and/or the CAN module.
The antenna divides the system to include transmitting antenna and receiving antenna, the radio frequency divides the system to include voltage controlled oscillator and mixer, signal processing divides the system to include signal conditioning circuit and PLL phase-locked loop, signal processing divides the system to include AD converter and ARM chip, and the one end of ARM chip is connected in signal generator, and signal generator connects in voltage controlled oscillator, and voltage-controlled vibrator connects respectively in the first end of transmitter and mixer, and the receiver is connected to the second end of mixer, and signal conditioning circuit is connected to the third end of mixer, and signal conditioning circuit connects the AD converter, and the other end of ARM chip is connected to the AD converter.
In more detail, the main tasks of the antenna subsystem are to form transmitting and receiving beams required by radar detection, radiate a transmitting signal to a designated area and receive a target scattered echo signal in the designated area. The antenna array designed by the embodiment comprises a transmitting antenna and a receiving antenna unit, and the array transmitting and receiving antennas in the form of micro-strip rectangular patches are connected with a back microwave circuit through via holes. The antenna emission beam can be designed according to application scenes, and the angle measurement in the horizontal direction or the angle measurement in the pitching direction can be carried out by selecting a phase comparison method or a amplitude comparison method. The microstrip antenna is selected in the embodiment mainly because the microstrip antenna has the following advantages: small volume, light weight, low profile, low cost, and no damage to the mechanical structure of the carrier except for the lead at the feed point; the performance is diversified, the maximum radiation direction of the designed microstrip element can be adjusted in the range of edge-to-end emission, and various geometric modes are realized; the device can be integrated with active devices and circuits into a unified assembly, is suitable for large-scale production, simplifies the manufacture and debugging of the whole device, and greatly reduces the cost.
The design method of the radio frequency subsystem is mainly designed according to application scenes and functional requirements of the anti-collision millimeter wave radar of the unmanned aerial vehicle, and mainly achieves the task that a voltage controlled oscillator VCO generates a transmitting signal within a certain range under the action of a PLL phase-locked loop, and the frequency of the transmitting signal is changed according to the rule of a modulating signal, so that a linear frequency modulation continuous wave working mode is realized. The radio frequency front end of the radio frequency subsystem mainly comprises a BGT24MTR12 and a phase-locked loop ADF 4158. The British flying radar chip BGT24MTR12 is specially customized for 24G automobile radar by British flying company, and all radio frequency modules including transmitting and receiving channels such as VCO, PA, LNA, MIXER and the like are integrated in the chip; ADF4158 is the only automotive radar-applying PLL introduced by ADI corporation and has versatile functions and is easy and reliable to use. When the frequency divider works, the ADF4158 generates a required transmitting waveform (generally triangular wave, sawtooth wave and combination thereof), then a VCO tuning pin of the radar chip is driven, the VCO generates corresponding radio frequency signals according to the voltage of the tuning pin, wherein one radio frequency signal is amplified by the PA and sent to the transmitting antenna, and the other radio frequency signal is divided by the frequency divider 6 and sent to the ADF4158 for locking. The transmitted signal meets the target reflection, the echo is sent to a low noise amplifier LNA through a receiving antenna, and the LNA amplifies the signal and then down-converts the signal to an intermediate frequency analog signal through a MIXER MIXER to be output. The purpose of locking using ADF4158 is to make the VCO output frequency more stable.
The signal conditioning subsystem mainly realizes the functions of filtering, amplitude amplification and the like of intermediate-frequency analog signals and comprises a signal amplification part and a signal filtering part. The specific design method can be seen in fig. 2.
The signal processing subsystem hardware part adopts a single ARM processing structure; the main circuit comprises an ARM processing module, a power supply module, a serial port module and a CAN module.
The ARM processing module is mainly used for enabling four paths of I/Q intermediate frequency signal lines output by the signal conditioning circuit to enter four paths of AD acquisition channels of the ARM through the signal conditioning module. And outputting the result through a serial port or a CAN port after certain signal processing. The serial port and the CAN port CAN be selected according to different scenes.
The power supply module provides voltage for the whole signal processing module. And provides 5V and 3.3V voltages to the rf front end module and the signal conditioning module. The power supply input adopts a wide range of input voltage and is compatible with 12V and 24V.
The integral design block diagram of the unmanned aerial vehicle anti-collision radar baseband signal processing module is as shown in figure 3:
the signal processing subsystem software part mainly controls the transmitting waveform of a radio frequency front end phase-locked loop PLL and receives, resolves and outputs a measuring result to an echo signal.
Alarm control divides the system mainly to divide the system to obtain unmanned aerial vehicle the place ahead dangerous barrier distance through dividing signal processing, the further calculation in speed and position, realize that unmanned aerial vehicle main control unit is according to the distance to the real-time update of place ahead target, speed, data information such as angle, carry out processing such as filtering prediction, the controller is according to the data that go out of calculation, combine unmanned aerial vehicle self flight state, including flying speed etc., make warning and control decision in advance, thereby make unmanned aerial vehicle can independently accomplish in the complex environment and keep away the barrier process.
The basic operating principle that this embodiment gives vegetation rotor unmanned aerial vehicle's millimeter wave radar altimeter is:
1. the ARM processor or the DSP processor is used as a main chip and is mainly responsible for tasks such as waveform emission, AD data acquisition, signal processing, alarm control and the like.
The main chip transmits a linear frequency modulation wave (a triangular wave or a combined waveform of a sawtooth wave and a constant frequency wave) by controlling the PLL, namely, outputs a modulation signal with certain amplitude and frequency. The phase-locked loop is mainly used for enabling transmitted waveform data to be more accurate, and therefore the performance of the system is improved.
2. The PLL controls the VCO to generate a transmitting signal within a certain range, and the frequency of the transmitting signal is changed according to the rule of the modulating signal, so that the working mode of the linear frequency modulation continuous wave FMCW is realized.
3. One path of the transmitting signal radiates millimeter waves to a space right below the flight of the plant protection unmanned aerial vehicle through the transmitter, and the other path of the transmitting signal is mixed with an echo signal which is received by the receiver and is reflected by target information. The frequency of the echo signal is changed compared with the previous transmitting signal, and the signal obtained after the echo signal is mixed is the difference frequency signal. The height information can be settled by performing a certain signal processing on the difference frequency signal.
4. Because the difference frequency signal can be input into the main chip for AD sampling only after signal conditioning, the signal conditioning part mainly performs signal amplification and filtering.
5. And further processing the two paths of sampled IQ data in the main chip to complete high settlement. The signal processing part mainly comprises FFT transformation, CFAR threshold detection and height value calculation. The FFT transformation mainly comprises the steps that time domain signals are transformed to a frequency domain, the target frequency of the threshold is obtained through CFAR threshold detection, due to the fact that the points of the threshold are many, spectrum leading edge peak detection, namely the height value of the nearest target, is mainly considered, the target which is the nearest target to the vegetation unmanned aerial vehicle can be displayed in real time, the risk avoiding capability of the unmanned aerial vehicle is improved, and the rapid maneuverability is improved. This example gives the formula of the height settlement for the triangular wave emission waveform as
Where T is the triangular wave period, B is the bandwidth, B is 200MHz, c is the speed of light, c is 3.0 × 108，f0Is the center frequency, f0＝24.125GHz，f_upSweeping the frequency value, f, over a triangular wave for a target_downFrequency values are swept under the triangular wave for the target.
6. The height value and other related information calculated by each triangular wave period are obtained through the last step of signal processing, and due to the influence of certain factors such as interference and the like in the system and the variability of the topographic environment, the height value is unstable and an abnormal value occurs, so that further data processing is needed. The data processing part mainly removes abnormal values, filters and other operations, and the filtering can adopt methods such as an alpha-beta filter or a Kalman filter.
7. The height value after data processing is accessed into the unmanned aerial vehicle main controller through a CAN or other communication modes to be alarmed/controlled, or the output is transmitted back to an upper computer or a mobile phone through a wireless transmission mode to be displayed in real time at a terminal.
Example 2: this embodiment is a method for processing a radar signal of a radar altimeter system of a plant protection rotor unmanned aerial vehicle by using a millimeter wave radar altimeter of the plant protection rotor unmanned aerial vehicle described in embodiment 1, and the method includes the following steps:
s1, AD data acquisition;
AD data acquisition, through AD sampling digital processing, the number of data point N that AD gathered, after removing partial data, the number of the surplus point is N _ s.
S2, removing direct current, wherein the step is an optional step;
(1) respectively calculating the average values I _ mean and Q _ mean of respective N _ s data of the I path and the Q path;
(2) wherein I is time domain data of the I path, Q is time domain data of the Q path, I _ mean is the mean value of the I path, and Q _ mean is the mean value of the Q path;
(3) subtracting the average value of each path from each data of the path I and the path Q;
(4) the IQ data DC calculation formula is as follows:
wherein, I 'is data after direct current removal, and Q' is data after direct current removal.
S3, FFT conversion;
preferably, I, Q data after being subjected to direct current removal are combined into an I + jQ data form, the sawtooth wave I + jQ data are subjected to FFT conversion, time domain data are converted into frequency data, of course, AD acquisition data can also be directly subjected to FFT conversion, I, Q data are combined into an I + jQ data form, and the sawtooth wave I + jQ data are subjected to FFT conversion.
S4, CFAR threshold detection;
and carrying out CFAR threshold detection on the complex modulus value of each point after the FFT.
S5, peak value processing;
the peak processing: after CFAR threshold detection, selecting a maximum point as an output peak value, and executing the following method when threshold peak value extraction is carried out:
setting a peak point threshold factor α for limiting the absolute value of the difference between the detected threshold-crossing maximum peak point and the maximum peak point appearing in the previous cycle, so that the absolute value of the difference is not greater than the peak point threshold factor α:
the expression is as follows:
|L_max(k)-L_max(k-1)|≤α；
wherein: l _ max (k) is the maximum peak point coordinate of the threshold passing of the k period, L _ max (k-1) is the maximum peak point coordinate of the last period, and k represents the kth moment; v. ofmaxThe maximum flight speed of the unmanned aerial vehicle is shown, lambda is the wavelength of the millimeter wave radar, fs is the sampling rate, and N is the number of points of FFT;
if the absolute value difference value of the threshold-crossing maximum peak point at the moment k and the threshold-crossing maximum peak point at the moment k-1 is within the set range of the threshold factor alpha of the peak point, the peak point of the kth period is considered to be effective; and if the threshold-crossing maximum peak point exceeds the set peak point threshold factor alpha at the moment k, replacing the peak point output at the moment k with the peak point at the moment k-1.
As a preferable scheme, a peak point sudden change accumulation factor phi is set, and the peak point sudden change accumulation factor phi is defined as that if b periods are continued from the moment k, the value range of b is 5-10, and the threshold crossing maximum peak point is compared with the threshold crossing maximum peak point of the previous period and both exceed the threshold factor a, at the moment k + b, the threshold crossing maximum peak point calculated at the current moment is taken as the threshold crossing maximum peak point at the current moment.
S6, spectrum maximum estimation, wherein the step is an optional step:
obtaining a threshold-crossing maximum peak point for spectrum maximum estimation: setting the coordinates of the threshold-crossing maximum peak point A1 as (a1, k1), wherein a1 represents the value of the threshold-crossing maximum peak point, and k1 represents the amplitude value corresponding to the threshold-crossing peak point; the coordinates of the secondary peak points are A3(A3, k3), the coordinate of the central peak point A is (amax, kmax), and e is amax-a1, the point A1, the coordinate of the point A2 symmetric to the point A is (a2, k1) is (a1+2e, k1), and the zero point A4 of the complex envelope is (a4, k1) is (A3+ e, 0);
wherein: a2, a3 and a4 are the values of the over-threshold maximum peak point of the corresponding point, and k3 and k4 are the amplitude values corresponding to the over-threshold peak point of the corresponding point;
a2, A3 and A4 are approximately a straight line, and the linear relationship is as follows:
order to
Then
Setting error E and deviation E to compare, if | E tint<E, the value of the over-threshold peak point at the moment is the value of the required central peak point, if the deviation E is greater than the set error E,
beta is a correction factor, the value range is 1.5-1.9, the value of e is calculated by changing the correction factor, and the value amax of the central peak point is calculated to be a1+ e.
S7, high settlement;
value amax of the central peak point, whichCorresponding frequency value f _ amax according to the formula
Calculating to obtain the height; where T is the modulation period, B is the bandwidth, and c is the speed of light.
Setting an altitude threshold factor epsilon, which is used for limiting the absolute value of the difference between the current altitude data H (k) and the altitude data H (k-1) appearing in the previous period, so that the absolute value of the difference is not larger than the altitude threshold factor epsilon;
the expression is as follows:
the value of | H (k) | -H (k-1) | is less than or equal to epsilon, and the value range of epsilon is 0.8-1.3;
if the absolute value difference value of the height data at the k moment and the absolute value difference value at the k-1 moment are within the range of the set height threshold factor epsilon, the peak point of the k-th period is considered to be effective; if the height data exceeds the set height threshold factor epsilon at time k, the height data output at time k is replaced with the height data at time k-1.
And setting an altitude abrupt change accumulation factor theta, wherein the altitude abrupt change accumulation factor theta is defined as that if b periods continue from the time k, and the altitude data exceeds a threshold factor theta compared with the altitude data of the previous period, the altitude data obtained by resolving the current time is taken as the altitude data of the current time at the time k + b.
Outputting a height value, namely outputting the height value by adopting a sliding window algorithm for the height data output at a single time; the altitude data at time k is equal to N in the sliding windowcThe average value of the maximum height value and the minimum height value of the individual height value is removed and is output as the final height data, and the calculation formula is
The improvement of the embodiment is that a peak threshold algorithm and a height threshold algorithm are added when the peak point is processed, through the two steps, the stability of data is improved in a matching mode, and the range finding precision can be improved through a designed spectrum maximum estimation algorithm.
Example 3: application publication No. CN 104678397A's patent application discloses an ultrasonic wave altimeter for unmanned aerial vehicle, has adopted the mode of ultrasonic wave to measure the terrain clearance when unmanned aerial vehicle is near the ground, realizes unmanned aerial vehicle's function of independently taking off and landing, but the ultrasonic wave altimeter biggest range finding that describes in this patent is 11m, and this is far away not enough in spraying the pesticide use to plant protection unmanned aerial vehicle. Therefore, in order to improve the ranging range of the unmanned aerial vehicle altimeter between 30 m and 40m and ensure the ranging precision to be about 0.2m, the invention provides the unmanned aerial vehicle altimeter realized based on the millimeter wave radar. Compared with other detection modes, the millimeter wave radar has the advantages of stable detection performance, good environmental adaptation, small size, low price, capability of being used in relatively severe rainy and snowy weather and the like.
As a supplement to the technical solution of embodiment 1, the millimeter wave radar designed in this embodiment has an operating frequency of 24GHz or 77GHz, and adopts an FMCW continuous wave system, mainly because of the chirp mode, its distance resolution is high. The waveform can adopt a chirp triangular wave FMCW, a sawtooth wave and a constant frequency wave or a combined waveform of the waveforms. Adopt single triangle wave emission waveform, can carry out relative distance's detection to the target, through the relative velocity between the unmanned aerial vehicle that detects out and the vegetation, assume the vegetation is static, then this speed is unmanned aerial vehicle upper and lower flying speed. Accurate settlement of the relative distance of the target can be completed by adopting a single triangular wave. The sawtooth wave can only detect the relative distance of the target, and the constant frequency wave can only detect the speed of the target. Because the Doppler frequency shift that unmanned aerial vehicle vertical flight produced can cause the deviation of certain distance, so can adopt the combination waveform of sawtooth wave and constant frequency wave, realize the compensation to sawtooth wave band distance detection through the constant frequency wave band to the measurement of speed to improve the distance precision between unmanned aerial vehicle and the vegetation. Triangular wave and sawtooth wave and constant frequency wave's combined waveform can all be regarded as the transmission waveform and realize vegetation unmanned aerial vehicle altimeter, can select the transmission waveform according to the application scene of difference to apply to the application field of different vegetation better, in order to reach better distance detection precision.
The range finding scope of the vegetation rotor unmanned aerial vehicle radar altimeter system that this embodiment designed is 1 ~ 50m, and the range finding precision is 0.2 m.
The embodiment mainly provides a design of a vegetation rotor unmanned aerial vehicle anti-collision signal processing part based on a millimeter wave radar and a signal processing method.
The radar center frequency f designed by the embodiment is 24.128 GHz. The emission waveform selects a single sawtooth wave, the period is 1ms, the bandwidth is 250MHz, and the sampling rate fs is 320 KHz. The transmit waveform is shown in fig. 1.
The present embodiment only needs to realize the resolution of the target range speed through one path of IQ data. A processing flow chart of the millimeter wave radar signal for preventing collision of the vegetation rotor unmanned aerial vehicle is given as follows, and is shown in fig. 2;
the method comprises the following concrete steps:
1.AD data acquisition, i.e. data processing
Continuous IQ data with height information is subjected to digital processing through AD sampling, the number N of data points acquired by AD is related to the sampling frequency fs and the sweep frequency period T of the system, namely N is fs T. Because partial data in the front of the collected data is abnormal due to system reasons and cannot be used for subsequent data processing, the time domain data of the remaining points are subjected to subsequent processing after partial data needs to be removed. If the number of points to be removed is N _ q, the number of remaining points is N _ s, and N _ s is N-N _ q. The data to be processed subsequently is N _ s pieces of data.
2. Remove direct current
(1) Respectively calculating the average values I _ mean and Q _ mean of the I path and the Q path and the respective N _ s data,
namely, it is
Wherein I is the time domain data of the I path, Q is the time domain data of the Q path, I _ mean is the mean value of the I path, and Q _ mean is the mean value of the Q path.
(2) And subtracting the mean value M obtained by the previous step from each data of the path I and the path Q, thereby finishing the purpose of removing direct current and reducing the influence of a direct current part on target threshold detection.
(3) The IQ data DC calculation formula is as follows:
wherein, I 'is data after direct current removal, and Q' is data after direct current removal.
3. Window function processing
I, Q data after direct current removal are combined into an I + jQ data form, then windowing is carried out, a Hanning window or a Hamming window and the like can be selected, side lobes are reduced, and therefore the detection performance of the target is improved; the hanning window will cause the main lobe to widen and decrease, but the side lobes will decrease significantly.
4. FFT transformation
And performing FFT (fast Fourier transform) on the windowed sawtooth wave I + jQ data, and converting time domain data into frequency data.
5. CFAR threshold detection
And carrying out CFAR threshold detection on the complex modulus value of each point after the FFT. The CFAR threshold detection can select a threshold detection method SO-CFAR with an average selected unit, the protection unit can select 1 to 2 points, and the number of window points can select 15 to 20.
6. Peak processing algorithm design
Considering that each peak point corresponds to a corresponding height value in the module value data of the millimeter wave radar altimeter after FFT. After CFAR threshold detection, the maximum point is selected as the output peak. For the characteristic of using single peak point output, it is easy to cause the peak point to jump in a large range, for example, if the output peak point of the previous period is M ═ 5, and the output peak value of the current period is M ═ 30, then the distance jumps by 15 points, and the distance jumps accordingly. In order to improve the stability of the height data output of the unmanned gyroplane, the following new algorithm design is proposed when threshold peak extraction is performed.
Setting a peak point threshold factor α for limiting the absolute value of the difference between the detected threshold-crossing maximum peak point and the maximum peak point appearing in the previous cycle, so that the absolute value of the difference is not greater than the peak point threshold factor α:
the expression is as follows:
|L_max(k)-L_max(k-1)|≤α；
wherein: l _ max (k) is the maximum peak point coordinate of the threshold passing of the k period, L _ max (k-1) is the maximum peak point coordinate of the last period, and k represents the kth moment; v. ofmaxThe maximum flight speed of the unmanned aerial vehicle is shown, lambda is the wavelength of the millimeter wave radar, fs is the sampling rate, and N is the number of points of FFT;
if the absolute value difference value of the threshold-crossing maximum peak point at the moment k and the threshold-crossing maximum peak point at the moment k-1 is within the set range of the threshold factor alpha of the peak point, the peak point of the kth period is considered to be effective; and if the threshold-crossing maximum peak point exceeds the set peak point threshold factor alpha at the moment k, replacing the peak point output at the moment k with the peak point at the moment k-1.
And if the absolute value difference value of the threshold-crossing maximum peak point at the moment k and the threshold-crossing maximum peak point at the moment k-1 is within the range of the designed threshold factor, the peak point of the kth period is considered to be effective, and subsequent calculation is carried out, and if the threshold-crossing maximum peak point at the moment k exceeds the designed threshold factor, the peak point output at the moment k is replaced by the peak point at the moment k-1.
As an explanation of the above technical means, in a time unit of an adjacent period, a peak point calculated in a current period and a peak point of a previous period are kept unchanged in the adjacent period if a speed is not changed in the adjacent period, but if a vertical flight speed of the unmanned aerial vehicle is changed in the adjacent period (here, the horizontal flight speed of the unmanned aerial vehicle is directed to an altimeter and if the unmanned aerial vehicle is in collision avoidance), a certain change occurs in the peak point of the current period in the previous period, if the unmanned aerial vehicle is close to the ground, the number of the current period is greater than that of the previous period, and if no unmanned aerial vehicle is close to the groundIf the man-machine is far away from the ground, the number of points in the current period is smaller than that in the previous period, the variation range of the peak point is the designed peak point threshold value factor alpha, and the value range selected by the factor is mainly determined by the maximum flight speed of the unmanned aerial vehicle in the adjacent period, namely a formula
Wherein v ismaxThe maximum flight speed of the unmanned aerial vehicle is shown, lambda is the millimeter wave radar wavelength, fs is the sampling rate, and N is the number of points of FFT.
However, if the height value changes after the flying environment below the plant protection rotor unmanned aerial vehicle changes suddenly, the number of peak points corresponding to the threshold passing may also continuously exceed the designed threshold factor. If the correction is not carried out, after the height is suddenly changed, the threshold-passing maximum peak point detected in each period can exceed the set threshold value factor, and the threshold-passing maximum peak point coordinate can be corrected to be the peak point coordinate at the last moment every time, namely, the height value before the height sudden change can be kept by the same method, and the height value after the sudden change cannot be adapted. In order to improve the adaptability of the radar altimeter of the unmanned aerial vehicle to various environments, a peak point mutation accumulation factor phi is introduced for the adaptability.
The peak point mutation accumulation factor phi is defined as that, if the coordinate value of the threshold-crossing maximum peak point changes suddenly from the time k, continuously for b cycles, that is, the threshold-crossing maximum peak point appearing in the continuously for b cycles is compared with the threshold-crossing maximum peak point in the previous cycle and exceeds the threshold factor a, at the time k + b, the maximum peak point is not replaced by the threshold-crossing maximum peak point in the previous cycle, but the threshold-crossing maximum peak point calculated at the current time is directly used as the threshold-crossing maximum peak point at the current time, so that the task of switching the threshold-crossing maximum peak point in the scene with the height mutation is completed. The value range of b is 5-10.
7. Spectral maximum estimation algorithm
After the threshold-crossing maximum peak point is obtained through the last step, in order to improve the accuracy of height value measurement of the altimeter system of the plant protection rotor unmanned aerial vehicle, a spectrum maximum estimation algorithm for improving the distance measurement accuracy is provided.
Ideally, the frequency spectrum of the echo difference frequency signal has only one spectral line, but actually, in the using process, due to the barrier effect existing in sampling, the spectral line with the maximum amplitude of the discrete frequency spectrum inevitably shifts the position of a spectral peak, so that a certain error exists between the distance value calculated by the peak point and the actual distance. When a spectral peak is shifted, the central spectral line corresponding to the main lobe peak will be shifted to the left or to the right. If the left peak value is larger than the right peak value in the left and right peak values of the threshold-crossing maximum value peak value point, the position of the central spectral line is between the maximum peak value point and the left peak value point, otherwise, the position is between the maximum peak value point and the right peak value point.
Because the spectrum obtained by FFT calculation samples continuous distance spectrum at equal intervals, the maximum point of the spectrum amplitude is necessarily positioned in the main lobe of the curve, and the main lobe has two sampling points. Obtaining a threshold-crossing maximum peak point for spectrum maximum estimation: setting the coordinates of the threshold-crossing maximum peak point A1 as (a1, k1), wherein a1 represents the value of the threshold-crossing maximum peak point, and k1 represents the amplitude value corresponding to the threshold-crossing peak point; the coordinates of the secondary peak points are A3(A3, k3), the coordinate of the central peak point A is (amax, kmax), and e is amax-a1, the point A1, the coordinate of the point A2 symmetric to the point A is (a2, k1) is (a1+2e, k1), and the zero point A4 of the complex envelope is (a4, k1) is (A3+ e, 0);
wherein: a2, a3 and a4 are the values of the over-threshold maximum peak point of the corresponding point, and k3 and k4 are the amplitude values corresponding to the over-threshold peak point of the corresponding point;
a2, A3 and A4 are approximately a straight line, and the linear relationship is as follows:
order to
Then
Setting error E and deviation E to compare, if | E tint<E, the value of the over-threshold peak point at the moment is the value of the required central peak point, if the deviation E is greater than the set error E,
beta is a correction factor, the value range is 1.5-1.9, the value of e is calculated by changing the correction factor, and the value amax of the central peak point is calculated to be a1+ e.
The reason for selecting the correction factor is as follows: due to the initial time
The coordinate of the point a symmetric point a2 is (a2, k1) — (a1+2E, k1), the abscissa of the point a is symmetric to the abscissa of the point a2 about the maximum peak point under the initial condition, that is, the coordinate of the point a2 is a1+2E, if the deviation E is greater than the set error E, it means that the coordinate of the point a2 is selected too large, that is, the maximum peak point is between a1+2E, and the 2-fold deviation E needs to be reduced. The value principle of the correction factor beta can be selected according to the required E value, if the required precision of E is not high, the correction factor beta can be selected to be 1.9 for correction, if the required precision of E is high, multiple iterations are possibly required to meet the requirement, the correction factor beta needs to be selected to be as small as possible, and 1.5 can be selected for correction.
7. High settlement
Calculating the value amax of the central peak point obtained in the last step, and the corresponding frequency value f _ amax according to a formula
Where T is the modulation period, T is 1ms, B is the bandwidth, B is 260MHz, c is the speed of light, and c is 3.0 × 108. Since this embodiment employs sawtooth waves with a very fast period, the difference frequency caused by the maximum speed generated by the vertical ascent and descent of the drone is substantially negligible, so this embodiment does not involve the resolution of the relative speed.
And therefore, the calculation function of the height information of the radar altimeter system of the plant protection rotor unmanned aerial vehicle is completed by single detection.
8. In order to improve the accuracy of altitude information obtained by altimeter calculation and further reduce large-range fluctuation of data, the following data processing algorithm is adopted for further calculation.
The concrete implementation method is as the idea of step 6.
Firstly, an altitude threshold factor epsilon is designed, and the factor is mainly used for limiting the absolute value of the difference between the detected current altitude data H (k) and the altitude data H (k-1) appearing in the previous period not to be larger than the altitude threshold factor epsilon.
I.e., | H (k) | H (k-1) | is less than or equal to epsilon, and the value range of epsilon is generally 0.8-1.3.
And if the absolute value difference value of the height data and the previous time k-1 at the time k is within the range of the designed threshold factor, considering that the peak point of the k-th period is valid, and performing subsequent calculation, and if the height data exceeds the designed threshold factor at the time k, replacing the height data output at the time k with the height data at the time k-1.
Similarly, if the height value changes after sudden change of the flying environment below the plant protection rotor unmanned aerial vehicle, the corresponding height data may also continuously exceed the designed threshold factor. If the height is not corrected, after the height is suddenly changed, the height data detected in each period can exceed the set threshold factor, and each time the height data is corrected into the height data at the previous moment, the height data cannot be well adapted to the suddenly changed height value. To improve the further stability of the altitude output, an altitude jump integration factor θ is introduced for this purpose.
The definition of the height abrupt change accumulation factor theta is that if the height data suddenly changes for b continuous periods from the time k, namely the height data occurring in the b continuous periods is compared with the height data in the previous period and exceeds the threshold factor theta, the height data at the time k + b does not need to be replaced by the height data in the previous period, but the height data calculated at the current time is directly used as the height data at the current time, so that the stability of the height data is improved completely.
For the height data of single output, the sliding window algorithm is adopted to output the height value for the smoothness of the output. I.e. the height data at time k equals N in the sliding windowcThe average value of the maximum height value and the minimum height value of the individual height value is removed and is output as the final height data, and the calculation formula is
The above description is only for the purpose of creating a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can substitute or change the technical solution and the inventive concept of the present invention within the technical scope of the present invention.
Claims (7)
1.A millimeter wave radar altimeter of a plant protection rotor unmanned aerial vehicle is characterized by comprising an antenna subsystem, a radio frequency subsystem, a signal conditioning subsystem and a signal processing subsystem;
the antenna subsystem forms transmitting and receiving beams required by radar detection, radiates a transmitting signal to a designated area and receives a target scattering echo signal in the designated area;
the radio frequency subsystem generates a transmitting signal, and the frequency of the transmitting signal is changed according to the rule of a modulating signal, so that linear frequency modulation continuous waves are output;
the signal conditioning subsystem is used for filtering and amplifying the intermediate frequency analog signal;
the signal processing subsystem is used for acquiring two paths of I/Q intermediate frequency signals output by the signal conditioning subsystem into the AD acquisition channel, and processing and outputting radar signals of the radar altimeter system of the plant protection rotor unmanned aerial vehicle;
a method for processing radar signals of a radar altimeter system of a plant protection rotor unmanned aerial vehicle comprises the following steps:
s1, AD data acquisition;
s2, FFT conversion;
s3, CFAR threshold detection;
between the step S1 and the step S3, there is a step of dc removal, which includes:
(1) respectively calculating the average values I _ mean and Q _ mean of respective N _ s data of the I path and the Q path;
(2) wherein I is time domain data of the I path, Q is time domain data of the Q path, I _ mean is the mean value of the I path, and Q _ mean is the mean value of the Q path;
(3) subtracting the average value of each path from each data of the path I and the path Q;
the IQ data DC calculation formula is as follows:
wherein, I 'is data after direct current removal, and Q' is data after direct current removal;
s4, peak value processing;
the peak processing: after CFAR threshold detection, selecting a maximum point as an output peak value, and executing the following method when threshold peak value extraction is carried out:
setting a peak point threshold factor α for limiting the absolute value of the difference between the detected threshold-crossing maximum peak point and the maximum peak point appearing in the previous cycle, so that the absolute value of the difference is not greater than the peak point threshold factor α:
the expression is as follows:
|L_max(k)-L_max(k-1)|≤α；
wherein: l _ max (k) is the maximum peak point coordinate of the threshold passing of the k period, L _ max (k-1) is the maximum peak point coordinate of the last period, and k represents the kth moment; v. ofmaxThe maximum flight speed of the unmanned aerial vehicle is shown, lambda is the wavelength of the millimeter wave radar, fs is the sampling rate, and N is the number of points of FFT;
if the absolute value difference value of the threshold-crossing maximum peak point at the moment k and the threshold-crossing maximum peak point at the moment k-1 is within the set range of the threshold factor alpha of the peak point, the peak point of the kth period is considered to be effective; if the threshold-crossing maximum peak point exceeds the set peak point threshold factor alpha at the moment k, replacing the peak point output at the moment k with the peak point at the moment k-1;
and S5, high settlement.
2. The millimeter wave radar altimeter for a plant protection unmanned rotorcraft according to claim 1, wherein a peak point jump accumulation factor Φ is set, the peak point jump accumulation factor Φ is defined as that if b is continuously set for b cycles from the time k, b ranges from 5 to 10, and the threshold crossing maximum peak point exceeds the threshold crossing maximum peak point of the previous cycle by a threshold factor a, then the threshold crossing maximum peak point calculated at the current time is set as the threshold crossing maximum peak point at the current time at the time k + b.
3. The millimeter wave radar altimeter for plant protection unmanned rotorcraft of claim 1, wherein between steps S4 and S5, there is a step of spectral maximum estimation: obtaining a threshold-crossing maximum peak point for spectrum maximum estimation: setting the coordinates of the threshold-crossing maximum peak point A1 as (a1, k1), wherein a1 represents the value of the threshold-crossing maximum peak point, and k1 represents the amplitude value corresponding to the threshold-crossing peak point; the coordinates of the secondary peak points are A3(A3, k3), the coordinate of the central peak point A is (amax, kmax), and e is amax-a1, the point A1, the coordinate of the point A2 symmetric to the point A is (a2, k1) is (a1+2e, k1), and the zero point A4 of the complex envelope is (a4, k1) is (A3+ e, 0);
wherein: a2, a3 and a4 are the values of the threshold-crossing maximum peak point of the corresponding point, and k3 is the amplitude value corresponding to the threshold-crossing peak point of the corresponding point;
a2, A3 and A4 are straight lines, and the linear relationship is as follows:
order to
Then
Setting error E and deviation E to compare, if | E tint<E, the value of the over-threshold peak point at the moment is the value of the required central peak point, if the deviation E is greater than the set error E,
beta is a correction factor, the value range is 1.5-1.9, the value of e is calculated by changing the correction factor, and the value amax of the central peak point is calculated to be a1+ e.
4. The plant protection rotor unmanned aerial vehicle's millimeter wave radar altimeter of claim 1, wherein the height settlement: the value amax of the central peak point, and its corresponding frequency value f _ amax, according to the formula
Calculating to obtain the height; where T is the modulation period, B is the bandwidth, and c is the speed of light.
5. The millimeter wave radar altimeter for a plant protection unmanned rotorcraft according to claim 1, wherein in altitude resolution, an altitude threshold factor e is set for limiting an absolute value of a difference between current altitude data H (k) and altitude data H (k-1) occurring in a previous cycle so that the absolute value of the difference is not greater than the altitude threshold factor e;
the expression is as follows:
the value of | H (k) | -H (k-1) | is less than or equal to epsilon, and the value range of epsilon is 0.8-1.3;
if the absolute value difference value of the height data at the k moment and the absolute value difference value at the k-1 moment are within the range of the set height threshold factor epsilon, the peak point of the k-th period is considered to be effective; if the height data exceeds the set height threshold factor epsilon at time k, the height data output at time k is replaced with the height data at time k-1.
6. The millimeter wave radar altimeter of a plant protection unmanned rotary wing aircraft according to claim 5, wherein an abrupt altitude change accumulation factor θ is set, the abrupt altitude change accumulation factor θ being defined such that if the altitude data exceeds a threshold factor θ for b consecutive periods from time k, the altitude data calculated at time k + b is taken as the altitude data at the current time.
7. The millimeter wave radar altimeter for a plant protection rotary-wing drone of claim 1, further comprising an altitude output, wherein for altitude data of a single output, a sliding window algorithm is used for outputting altitude values;
the altitude data at time k is equal to N in the sliding windowcThe average value of the maximum height value and the minimum height value of the individual height value is removed and is output as the final height data, and the calculation formula is
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