CN104730535A - Vehicle-mounted Doppler laser radar distance measuring method - Google Patents

Abstract

The invention discloses a vehicle-mounted Doppler laser radar distance measurement method. The pulse interval module stores the number of transmitted pulses and pulse interval information; the drive module uses the pulse interval information to determine the time to excite the laser emission pulse; the laser emission pulse passes through the beam splitter 100 % is projected to the lens through the optical path, and is collimated and emitted to the target by the lens; the optical signal reflected by the target is collected by the lens, and reaches the photoelectric converter through the 100% reflected optical path of the beam splitter; it is converted into an electrical signal by the photoelectric converter; the signal processing unit module uses The pulse interval information performs pulse position modulation processing on the electrical signal generated by the photoelectric converter, and calculates the distance of the target. The beneficial effect of the present invention is that the vehicle-mounted Doppler lidar transmitter uses pulse position modulation to adjust the transmission pulse interval, and the receiver uses an improved data accumulation method to measure the distance of the road target, while ensuring that the length of the transmitted signal meets the requirements of high-speed scanning, the vehicle-mounted laser can be realized. Radar miniaturization.

Description

A method for distance measurement of vehicle-mounted Doppler lidar

technical field

The invention belongs to the technical field of laser radar, and in particular relates to a vehicle-mounted Doppler laser radar distance measurement method.

Background technique

According to the 2010 "China Road Traffic Safety Blue Book", about 100,000 people die in automobile traffic accidents every year in my country, and nearly 500,000 people are injured in traffic accidents. About two-thirds of these traffic accidents are Caused by collision, loss of control and other factors. In order to reduce unnecessary losses caused by traffic accidents to people's lives and properties, automobile safety has been highly valued by governments around the world and major automobile manufacturers. Automotive safety systems include passive safety systems that can reduce losses after an accident, such as seat belts, airbags, etc., and assisted driving systems before an accident, such as ABS, navigation and positioning, anti-glare panels, and automatic parking systems. The car active safety system that can actively avoid collisions while driving is the most critical technology to greatly reduce road traffic accidents. Using this technology to realize X-by-Wire is the ultimate goal of the world's major car manufacturers in the field of car safety.

The automotive active safety system is composed of on-board distance and speed measuring device, microcomputer, alarm device, execution system and other components, among which the on-board distance and speed measuring device is the core component, which determines the distance, speed and orientation of obstacles in the road environment . At present, the distance speed measurement device using the vehicle-mounted millimeter-wave radar has been used in some high-end models. However, limited by the width of the main lobe of the transmitted signal, the spatial resolution of millimeter-wave radar is poor, and it cannot even distinguish whether obstacles in the road are vehicles or pedestrians. In order to improve the spatial resolution of distance and speed measurement devices and provide more accurate and reliable information for automotive active safety systems, in recent years, major auto parts suppliers such as Ibeo, Bosch, and Denso have begun to develop lidars that use lasers as detection light sources , used to measure the distance of road obstacles. Laser has superior collimation characteristics and can provide very high spatial resolution. According to the distance data measured by lidar, road obstacles can be identified as vehicles, pedestrians or other objects such as road edges.

Two typical methods for LiDAR to measure the target distance are pulsed and continuous wave. The pulse method measures the time difference between the received signal and the transmitted signal, while the continuous wave method measures the phase difference between the received signal and the transmitted signal. Due to the following reasons, most vehicle-mounted laser radars use the pulse method: (1) the price of high-power pulsed semiconductor laser radar is low; (2) the signal processing method is simple; The lower the target, the higher the peak power requirement of the transmitted signal.

The noise performance of a transimpedance amplifier (TIA) that amplifies the received signal is limited by the system bandwidth, which is generally required to be in the hundreds of megahertz. Under this requirement, the TIA chip can detect signals around -40dBm. For weaker signals, TIA chips are difficult to detect. According to the radar equation, in order to measure the distance up to 150m, the diameter of the lens receiving the optical signal needs to be more than 10cm. In order to realize the miniaturization of the vehicle-mounted lidar, it is expected that the diameter of the lens for receiving the optical signal is about 1mm. The lens diameter is reduced to 1/10 of the original, and the signal-to-noise ratio of the received signal will be reduced by 20dB. Under the requirement of hundreds of megabytes of bandwidth, it is currently difficult to manufacture an optical signal capable of detecting -60dBm.

Data integration (Data Integration) is a traditional method for improving the signal-to-noise ratio of radar. The radar using this method transmits pulse trains at equal intervals to road targets, and the signal processing unit at the receiving end accumulates the received signals to improve the signal-to-noise ratio. However, with this signal processing method, the transmitter cannot transmit a pulse until the reflected signal of the previous pulse reaches the receiver. Most of the time is spent waiting to receive the signal, which limits the scanning speed of the lidar. Assuming that the maximum measurement distance is 150m, and a single measurement transmits 100 pulses, the scanning speed will not exceed 10,000 times/second. The scanning speed of the vehicle lidar has exceeded 200,000 times per second.

The present invention aims to propose a method that can realize the miniaturization of the lidar lens while ensuring that the length of the transmitted signal meets the requirements of high-speed scanning.

Contents of the invention

The invention provides a vehicle-mounted Doppler laser radar distance measurement method, which mainly solves the problem that the miniaturization of the current laser radar lens cannot ensure that the length of the transmitted signal meets the requirements of high-speed scanning.

The present invention carries out according to the following steps:

Technical scheme of the present invention is to carry out according to the following steps:

Step 1: The pulse interval module stores the number of transmitted pulses and the pulse interval information;

Step 2: The drive module uses the pulse interval information to determine the time when the excitation laser emits a pulse;

Step 3: The laser emission pulse is projected to the lens through the beam splitter 100% through the optical path, and the lens is collimated and emitted to the target;

Step 4: The optical signal reflected by the target is collected by the lens, and reaches the photoelectric converter through the 100% reflected light path of the beam splitter;

Step 5: Convert to an electrical signal by a photoelectric converter;

Step 6: The signal processing unit module uses the pulse interval information to perform pulse position modulation processing on the electrical signal generated by the photoelectric converter, and calculates the distance of the target.

Further, in the step 1, the pulse interval module performs the interval distribution method of transmitting pulses:

First determine the maximum time length T of the single-point measurement sending pulse sequence according to the requirements of the laser radar scanning speed, and then use the random number generator to generate N random numbers that are not equal to each other, and map these N random numbers to 0- Between T, the smallest random number corresponds to 0, and the largest random number corresponds to T.

Further, the pulse position modulation method in the step 6 is:

The length of the sending pulse sequence is N, and the interval between sending pulses is randomly set as △T ₁ , △T ₂ ,..., △T _N-1 , satisfying

At the receiving end, the signal is sampled for a period of time, and the signal is placed in the first row. The signal processing unit module uses the pulse interval information to shift the row signal by △T ₁ , △T ₂ , ..., △T _{N- 1} , and then placed in the second row, third row, ... Nth row respectively, there is only one pulse in the signal obtained after the addition, and the time position of the pulse corresponds to the flight time of the laser.

The beneficial effect of the present invention is that the vehicle-mounted Doppler lidar transmitter uses pulse position modulation to adjust the transmission pulse interval, and the receiver uses an improved data accumulation method to measure the distance of the road target, while ensuring that the length of the transmitted signal meets the requirements of high-speed scanning, the vehicle-mounted laser can be realized. Radar miniaturization.

Description of drawings

Fig. 1 is the structural principle diagram of lidar of the present invention;

Fig. 2 is the pulse interval distribution mode of pulse position modulation;

Figure 3 shows the working principle of pulse position modulation: (a) transmit pulse sequences at equal intervals; (b) transmit pulse sequences at unequal intervals;

Figure 4 is the comparison of the results of equal interval pulses and pulse position modulation: (a) is the original signal equal interval transmission pulse sequence, (b) is the signal obtained by data accumulation for the restoration of the equal interval pulse situation, (c) is the difference of the original signal Transmitting pulse sequences at equal intervals, (d) is the signal obtained by data accumulation for pulse position modulation recovery;

Figure 5 is the robustness test result of data accumulation algorithm for pulse position modulation.

Detailed ways

The following describes the present invention by enumerating specific embodiments.

As shown in Figure 1, the structural principle block diagram of the vehicle-mounted laser radar for measuring the target distance is used. The laser used is a high-power semiconductor pulse laser.

Step 1: The pulse interval module stores the number of transmitted pulses and the pulse interval information;

Step 2: The drive module uses the pulse interval information to determine the time when the excitation laser emits a pulse;

Step 3: The laser emission pulse is projected to the lens through the beam splitter 100% through the optical path, and the lens is collimated and emitted to the target;

Step 4: The optical signal reflected by the target is collected by the lens, and reaches the photoelectric converter through the 100% reflected light path of the beam splitter;

Step 5: Convert to an electrical signal by a photoelectric converter;

Step 6: The signal processing unit module uses the pulse interval information to perform pulse position modulation processing on the electrical signal generated by the photoelectric converter, and calculates the distance of the target.

The interval distribution method of the pulse interval module for transmitting pulses:

Fig. 2 shows the distribution mode of pulse position modulation pulse interval.

Assume that in order to measure the distance of a certain point, the transmitter sends N pulses to the target. In traditional radar signal processing, the interval between these N pulses is equal, and this interval must be greater than the flight time of the pulse. The pulse position modulation method adopted in the present invention is to first determine the maximum time length T of the single-point measurement transmission pulse sequence according to the requirements of the scanning speed of the laser radar, and then use a random number generator to generate N random numbers unequal to each other, and use this N random numbers are mapped to 0-T one by one, the smallest random number corresponds to 0, and the largest random number corresponds to T. In this way, the positions of the N pulses within the time interval of 0-T are random, and the intervals of the pulses are no longer equal. This method is called pulse position modulation in the present invention.

Pulse position modulation method:

Figure 3 shows the principle of pulse position modulation to achieve distance measurement: Figure 3(a) shows the traditional situation of sending pulses at equal intervals, the first line receives the signal, and the received signal is shifted by an integer number of pulse intervals from the second line to the last line The multiplied signal is then added to the signals of each row to obtain the processed signal. This signal processing method is called data accumulation. In the case of sending the next pulse without waiting for the arrival of the reflection signal of the previous pulse, the received signal can contain reflection signals of multiple pulses, as shown in the first row of Fig. 3(a). As a result of data accumulation and processing of reflected signals, a signal containing multiple pulses is obtained. The difference between adjacent peak values of these pulses is not large, and the time interval corresponding to the peak value is equal to the time interval between sent pulses. Although the moment of the maximum peak corresponds to the flight time of the laser, in the actual system, due to the existence of noise, the position of the maximum peak does not necessarily correspond to the flight time of the laser, so it is impossible to find the maximum peak pulse from this signal. If the judgment is wrong, it will bring a distance error of k·c△T, where c is the speed of light, △T is the interval of the emission pulse, and k is a positive integer. We call this phenomenon the distance uncertainty problem.

Pulse position modulation can solve the problem of inaccurate distance measurement existing in the traditional transmission of equally spaced pulses. Its working principle is shown in Figure 3(b). In the present invention, the length of the transmitted pulse sequence is N, and the interval between transmitted pulses is randomly set as ΔT ₁ , ΔT ₂ ,..., ΔT _N-1 , satisfy

Among them, the scanning speed and the maximum measurement distance are determined by the lidar specification.

At the receiving end, the signal is sampled for a period of time, denoted as s ₀ (t), and the signal length should include all reflected signals from the target at the maximum distance. As shown in Figure 3(b), this signal is placed in the first row. The signal processing unit module uses the information from the pulse interval module to shift the line signal to the left by △T ₁ , △T ₂ ,..., △T _N-1 , and the shifted signal is denoted as

s ₁ (t)=s ₀ (t+△T ₁ ), s ₂ (t)=s ₁ (t+△T ₂ ), ..., s _N-1 (t)=s _N-2 (t+△T _{N -1} )

As shown in Figure 3(b), they are respectively placed in the second row, the third row, ... the Nth row. Adding the above signals together, i.e.

$\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; the\; s</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>$

There is only one pulse in the signal obtained after the addition, and the time position of the pulse corresponds to the flight time of the laser.

The invention adjusts the position of the emission pulse on the time axis, so that when the data is accumulated, the pulse corresponding to the position of the laser flight time can be accumulated for the maximum number of times, while the number of pulse accumulation at other times is very small, which solves the problem of traditional equal-interval emission pulses When using data accumulation, the problem of inaccurate distance measurement occurs.

List specific embodiment below and illustrate the present invention:

Embodiment 1: The block diagram shown in FIG. 1 is used to form a vehicle-mounted lidar simulation platform. In order to ensure the requirements of imaging scanning, the length of the transmitted signal does not exceed 5μs, and the last 1μs is used to wait for the reflected signal from the reflector 150m away. According to the introduction of the background technology, when the diameter of the receiving lens is 1mm, the received signal power is -60dBm, and the received signal power needs to be increased by 20dBm, so 100 pulses are placed within 4μs before the transmitted signal.

According to the traditional scheme, 100 pulses are evenly placed in the first 4 μs on the time axis, and the receiving end processes the received reflected signal according to the traditional data accumulation method, and the result shown in Figure 4(b) is obtained. Figure 4(a) is the received reflection signal, the signal is submerged in the noise, and it is impossible to judge the position of the received signal. Through data accumulation, a pulse sequence composed of multiple pulses is obtained, corresponding to the position of the pulse, but the one-to-one correspondence between the transmitted pulse and the received pulse cannot be judged. Therefore, the flight time of the pulse sequence cannot be judged according to the processing results, and then The distance to the target could not be calculated.

In the 5 μs emission time segment, 100 pulses are randomly placed in the first 4 μs according to the time axis, and the last 1 μs is left blank. The intervals between pulses are ΔT ₁ , ΔT ₂ , . . . , ΔT ₉₉ . Time from the moment of transmission, sample the 5μs signal at the receiving end, and place it in the first row. Then shift the received 5μs signal to the left by △T ₁ and place it in the second row. Shift the signal of the second row to the left by ΔT ₂ and place it in the third row. And so on until line 100. Finally, add the 100 lines of signals to get the result in Figure 4(d). Figure 4(c) shows the received reflected signal from the target. The reflected signal is submerged in the noise, and it is impossible to judge the position of the reflected pulse. According to the processing results, there is only one pulse in Fig. 4(d), which corresponds to the location of the reflected signal of the first pulse of the transmitted pulse sequence. Therefore, by calculating the time difference between the position of the first pulse of the transmitted signal and the position of a pulse of the received signal, the flight time of the laser can be calculated, and then the distance of the target can be obtained. The method proposed by the invention overcomes the problem of inaccurate distance measurement caused by traditional equal-spaced transmission.

Algorithm robustness test:

The transmitter adopts pulse position modulation, and the receiver adopts data accumulation. As a result, there is only one pulse in the signal, and the position of the pulse corresponds to the time position of the received signal. However, when the signal-to-noise ratio of the received signal is too low, data accumulation cannot make this pulse appear. In order to judge the effectiveness of the data accumulation algorithm, it is necessary to set a threshold. When the amplitude of the signal is higher than the threshold, the signal is considered to be the pulse being searched; when all the signal amplitudes are lower than the threshold, the data accumulation algorithm is not It can effectively find out the position of the pulse. If the threshold is set to a small value, the probability of missed detection is low, but the probability of false alarm is high; on the contrary, if the threshold is set high, the probability of false alarm is low, but the probability of missed detection is high.

An appropriate threshold is set in the test so that the probability of false alarm is almost zero. The method for setting the threshold is the lowest threshold for a received signal with a high signal-to-noise ratio (such as 0 dB) through 200,000 tests without false alarms, and this threshold is used as a benchmark for judging whether a pulse appears after data accumulation. Test the signal-to-noise ratio between -25dB and 5dB, do 200,000 tests at a specific signal-to-noise ratio, and calculate the probability of missed detection. The results are shown in Figure 5. Fig. 5 is the robustness test result of the data accumulation algorithm for pulse position modulation. The results show that when the signal-to-noise ratio of the received signal is greater than -10dB, the false alarm probability and missed detection probability are close to zero, and the reliable detection of the target can be realized.

Advantage of the present invention also has:

1. Use pulse position modulation to measure the distance of the road environment target with high precision, and compress the length of the pulse sequence sent by a single measurement to the order of microseconds, which meets the requirements of the scanning speed of the vehicle-mounted lidar. At the same time, the diameter of the receiving optical lens can be reduced, and the common optical lens for transmitting and receiving can be designed to realize coaxial optical lidar. The present invention mainly aims at the practical problem of the large volume of the vehicle-mounted laser radar, and studies the miniaturization of the vehicle-mounted laser radar by reducing the diameter of the laser radar receiving lens. The reduction of the diameter of the receiving lens will lead to the reduction of the signal-to-noise ratio of the received signal. Through the pulse position modulation and data accumulation method of the present invention, the signal-to-noise ratio of the received signal can be improved, and reliable detection of the received signal can be realized in the case of a small lens.

2. The present invention uses pulse position modulation at the transmitting end of the vehicle-mounted laser radar to continuously transmit pulses at unequal intervals, so that the length of the transmitted signal for a single measurement is at the microsecond level to meet the needs of high-speed scanning and imaging.

3. The pulse position modulation method proposed by the present invention has a random pulse interval, and the driving of the light source can be realized through an external modulation method, which has the advantages of simple operation and low price.

4. The present invention uses data accumulation in the receiving end of the vehicle-mounted laser radar to improve the signal-to-noise ratio of the received signal, reduce the requirement for transmitting optical power, and improve the stability of the receiving end.

5. The present invention improves the signal-to-noise ratio of the received signal, reduces the diameter of the receiving lens, and realizes the miniaturization of the vehicle-mounted laser radar.

6. The invention reduces the diameter of the receiving lens so that it is equivalent to the diameter of the transmitting lens, and in the structural design, the transmitting and receiving share one lens to realize the coaxial vehicle-mounted laser radar.

7. The present invention uses the pulse position modulation method, the pulse interval is relatively large, and the interference between reflected signals from different paths is weak, which can be used for distance measurement in a multi-target environment.

The above description is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Any simple modifications made to the above embodiments according to the technical essence of the present invention, equivalent changes and modifications, all belong to this invention. within the scope of the technical solution of the invention.

Claims (3)

1. a vehicle-mounted Doppler lidar distance measuring method, is characterized in that: carry out according to the following steps: Step 1: The pulse interval module stores the number of transmitted pulses and the pulse interval information; Step 2: The drive module uses the pulse interval information to determine the time when the excitation laser emits a pulse; Step 3: The laser emission pulse is projected to the lens through the beam splitter 100% through the optical path, and the lens is collimated and emitted to the target; Step 4: The optical signal reflected by the target is collected by the lens, and reaches the photoelectric converter through the 100% reflected light path of the beam splitter; Step 5: Convert to an electrical signal by a photoelectric converter; Step 6: The signal processing unit module uses the pulse interval information to perform pulse position modulation processing on the electrical signal generated by the photoelectric converter, and calculates the distance of the target. 2. according to a kind of vehicle-mounted Doppler lidar distance measuring method according to claim 1, it is characterized in that: in the described step 1, the pulse interval module carries out the interval distribution method of transmitting pulse: First determine the maximum time length T of the single-point measurement sending pulse sequence according to the requirements of the laser radar scanning speed, and then use the random number generator to generate N random numbers that are not equal to each other, and map these N random numbers to 0- Between T, the smallest random number corresponds to 0, and the largest random number corresponds to T. 3. according to a kind of vehicle-mounted Doppler lidar distance measuring method according to claim 1, it is characterized in that: in the described step 6, the pulse position modulation method is: The length of the sending pulse sequence is N, and the interval between sending pulses is randomly set as △T ₁ , △T ₂ ,..., △T _N-1 , satisfying At the receiving end, the signal is sampled for a period of time, and the signal is placed in the first row. The signal processing unit module uses the pulse interval information to shift the row signal by △T ₁ , △T ₂ , ..., △T _{N- 1} , and then placed in the second row, third row, ... Nth row respectively, there is only one pulse in the signal obtained after the addition, and the time position of the pulse corresponds to the flight time of the laser.