A method for measuring distance and velocity based on trapezoidal wave optical frequency-modulated continuous wave.

Abstract

This invention discloses a method for measuring distance and velocity based on trapezoidal wave optical frequency-modulated continuous wave, including a target object, a lidar, and an analog-to-digital converter. The frequency value of the beat signal at the rising edge is denoted as f1, the frequency value at the falling edge as f2, and the frequency value of the beat signal in the invariant region as fd. The beat signal is sampled by a data acquisition card, yielding two main frequency values: f+ and f-, where f+ is the highest peak frequency and f- is the second highest peak frequency, i.e., f+ > f-. This method improves the range and velocity measurement limits without changing the hardware, without sacrificing horizontal resolution or frame rate. Furthermore, the sum of the highest Doppler frequency and the distance frequency that can be sampled is no longer limited by the Nyquist frequency, but can instead reach the sampling frequency of the analog-to-digital converter. The two trapezoidal waves used in this method are equivalent, and the waveforms are easy to implement.

Description

Technical Field

[0001] This invention relates to the field of lidar technology, specifically to a method for measuring distance and velocity based on trapezoidal wave optical frequency-modulated continuous wave. Background Technology

[0002] When measuring the speed and distance of a target using lidar, current technology involves mixing the intrinsic wave and the received wave in a mixer to generate a beat frequency signal. The acquisition card then collects this beat frequency signal at a specific sampling frequency. Therefore, the sampling frequency of the acquisition card limits the highest frequency that can be acquired. Currently, the highest sampling frequency achieved by an acquisition card is 250MHz, corresponding to a maximum acquireable frequency (Nyquist frequency) of 125MHz. When the target has relative speed, the highest frequency of the beat frequency signal that can be acquired is 125MHz. This means the sum of the speed frequency and the distance frequency can only reach a maximum of 125MHz. Beyond 125MHz, aliasing occurs in the sampled waveform. Aliasing refers to the phenomenon where the sampled signal overlaps and becomes distorted when it is restored to a continuous signal. When aliasing occurs, the original signal cannot be recovered from the sampled signal. Aliasing significantly affects the lidar's measurement of the target's distance and speed, causing the obtained data to differ considerably from the true data. Therefore, the Nyquist frequency of the acquisition card limits the acquisition limit for the target's speed and distance. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a distance and velocity measurement method based on trapezoidal wave optical frequency-modulated continuous wave, which solves the problem of the limitation of the Nyquist frequency of the acquisition card on the distance and velocity measurement limit of the target object.

[0004] The present invention is achieved through the following technical solution.

[0005] This invention discloses a method for measuring distance and velocity based on trapezoidal wave optical frequency-modulated continuous wave, comprising a target object, a lidar, and an analog-to-digital converter. The method denotes the frequency value of the beat signal at the rising edge as f1, the frequency value of the beat signal at the falling edge as f2, and the frequency value of the beat signal in the invariant region as fd. The beat signal is sampled by a data acquisition card and has two main frequency values: f+ and f-, where f+ is the highest peak frequency and f- is the second highest peak frequency (i.e., f+ > f-). The beat signal frequency value in the invariant region should be the same as the Doppler frequency value; therefore, both the beat signal frequency value and the Doppler frequency value in the invariant region are denoted as fd, and the distance frequency is denoted as fb. The key feature is that the frequency difference between the intrinsic wave and the received wave in the invariant region is the Doppler frequency shift fd, meaning that the Doppler frequency shift fd can be obtained by sampling the frequency value of the beat signal in the invariant region.

[0006] Given the Doppler frequency shift fd, by converting the frequency difference between the rising and falling regions in the beat frequency signal to obtain the Doppler frequency shift, and comparing it with the known fd, it is possible to determine whether aliasing has occurred, the direction of motion of the target object, and to calculate the correct speed and distance values.

[0007] Whether aliasing occurs in the velocity direction of the target relative to the lidar and the frequency values ​​during sampling can be categorized into the following four cases:

[0008] When the target object moves toward the lidar, aliasing occurs;

[0009] When the target object moves away from the lidar, aliasing occurs;

[0010] No aliasing occurred when the target moved toward the lidar.

[0011] No aliasing occurred when the target moved away from the lidar.

[0012] Distance and velocity measurement methods include the following steps:

[0013] S1, Substitute the values ​​of f1 and f2 into the formulas for the above four cases to calculate the four Doppler frequency shift values;

[0014] S2. Determine under which condition the frequency value fd of the beat frequency signal collected in the invariant region of the trapezoidal wave is consistent with the Doppler frequency shift value calculated by the formula, and then determine the velocity direction of the target object relative to the lidar and whether the frequency value is aliased during the sampling process.

[0015] S3. Based on the judgment, calculate fb using the formula in the same group;

[0016] S4. Substitute the values ​​of fd and fb into the formulas for calculating velocity V and distance L to obtain the velocity and distance.

[0017] Furthermore, when the target moves toward the lidar, f1 < f2, f1 = f-, f2 = f+.

[0018] Furthermore, when the target moves away from the lidar, f1 > f2, f1 = f+, and f2 = f-.

[0019] Furthermore, when the target object moves towards the lidar and aliasing does not occur, the formulas for calculating fd and fb should be:

[0020] 2f d =f2-f1

[0021] 2f b =f2+f1

[0022] Furthermore, when the target object moves away from the lidar and aliasing does not occur, the calculation formulas for fd and fb should be:

[0023] 2f d =f1-f2

[0024] 2f b =f1+f2

[0025] Furthermore, the highest frequency that the acquisition card can acquire is half of the sampling frequency, i.e., the Nyquist frequency. When the value of f+ is greater than the highest frequency that the acquisition card can acquire, aliasing will occur.

[0026] Furthermore, based on the aliasing mechanism, a formula for calculating the aliasing frequency of the analyzed signal can be derived. Let the frequency of the actual signal be fs, the sampling frequency be f, and f < 2*fs. The aliased frequency obtained after sampling analysis is fa, then the following formula applies:

[0027] f a =|f s -n*f|

[0028] in,

[0029]

[0030] Int() is the integer part of the decimal, which is used to round down the integer part of the number.

[0031] When aliasing occurs as the target moves toward the lidar, f+ changes to: f + =f-f2 and f1=f-;

[0032] When the target object moves away from the lidar and aliasing occurs, f+ changes to: f + =f-f1 and f2=f-.

[0033] Furthermore, when the target moves toward the lidar and aliasing occurs, the calculation formulas for fd and fb should be as follows:

[0034] 2f d =f - f2 - f1

[0035] 2f b =f - f2 + f1

[0036] Furthermore, when the target object moves away from the lidar, aliasing occurs, f d The formula for calculating fb should be:

[0037] 2f d =f - f1 - f2

[0038] 2f b =f - f1 + f3

[0039] Furthermore, based on the values ​​of fd and fb, the formulas for the velocity and distance of the target object are as follows:

[0040]

[0041]

[0042] Where λ is the wavelength of the light emitted by the laser, θ is the angle between the velocity direction of the target object and the radial direction (cosθ = 1 when only the radial velocity is considered), τ is the frequency modulation period of the laser, c is the speed of light in the medium, and B is the modulation bandwidth of the laser.

[0043] The beneficial effects of this invention are as follows: Because the trapezoidal wave contains explicit Doppler frequency shift information, this explicit information can be used to determine the velocity direction of the target object and whether frequency aliasing has occurred. This eliminates the influence of the Nyquist frequency on the extreme values ​​of frequency caused by velocity and distance, allowing them to reach the sampling frequency of the analog-to-digital converter, significantly improving the ranging and velocity measurement limits and diversifying measurement scenarios. Furthermore, it avoids the reduction in horizontal resolution and frame rate caused by lowering the laser's modulation frequency to improve the ranging and velocity measurement limits. Since the formulas used to measure targets with different velocity directions differ, this method can distinguish and measure velocities in both the facing and receding directions, especially predicting the danger of high-speed movement towards the lidar. This method improves the ranging and velocity measurement limits without changing the hardware, without sacrificing horizontal resolution and frame rate; the sum of the highest Doppler frequency and the distance frequency that can be sampled is no longer limited by the Nyquist frequency, but can reach the sampling frequency of the analog-to-digital converter; the two trapezoidal waves used in this method are equivalent, and the waveforms are easy to implement. Attached Figure Description

[0044] To more clearly illustrate the technical solutions in the embodiments of the invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0045] Figure 1 It is a graph showing the frequency relationship between the intrinsic wave, the received wave, and the beat frequency signal in a triangular frequency-modulated continuous wave when the target object moves away from the lidar and there is a Doppler frequency shift.

[0046] Figure 2 It is a graph showing the frequency relationship between the intrinsic wave, the received wave, and the beat frequency signal in a triangular frequency-modulated continuous wave when the target moves toward the lidar and there is a Doppler frequency shift.

[0047] Figure 3 A diagram showing the frequency relationship between trapezoidal wave frequency-modulated continuous wave and target moving toward the lidar.

[0048] Figure 4 Frequency relationship diagram of trapezoidal wave frequency-modulated continuous wave when the target moves away from the lidar;

[0049] Figure 5 Another type of trapezoidal wave frequency modulation continuous wave frequency relationship diagram. Detailed Implementation

[0050] The following is combined with Figures 1-5 The present invention will be described in detail below.

[0051] In a typical implementation:

[0052] Combined with appendix Figures 1-4 The distance and velocity measurement method based on trapezoidal wave optical frequency-modulated continuous wave, taking the sampling frequency of the highest acquisition card currently available as 250MHz as an example, therefore the highest frequency (Nyquist frequency) that can be acquired without aliasing is 125MHz.

[0053] When the target object moves at a certain speed, the Doppler effect will occur, resulting in a Doppler frequency shift between the received wave and the intrinsic wave. The corresponding frequency diagram is shown below. Figure 1 , Figure 2 As shown, using a triangular wave as a preliminary derivation example, fd in the figure represents the Doppler frequency shift. The Doppler effect refers to the change in wavelength of an object relative to a wave source. In front of a moving wave source, the wave is compressed, the wavelength becomes shorter, and the frequency becomes higher; behind a moving wave source, the opposite effect occurs: the wavelength becomes longer, and the frequency becomes lower; the higher the velocity of the wave source, the greater the effect. The Doppler frequency shift refers to the frequency difference between the received wave and the intrinsic wave after the Doppler effect occurs. By sampling the beat frequency signal using a data acquisition card to obtain the absolute values ​​f+ and f- of the beat frequency signal, the Doppler frequency fd and the range frequency fb can be calculated, thereby determining the velocity and distance of the target object.

[0054] On the one hand, the analysis is based on the situation where the target object moves away from the lidar:

[0055] Figure 1 This diagram illustrates the frequency relationships caused by the Doppler effect when a target moves away from the lidar, resulting in a lower frequency of received light compared to the intrinsic light. The beat frequency signal has two main frequency values, f+ and f-, and their relationships with fd and fb are shown in equations 1, 2, 3, and 4.

[0056] f + =f b +f d Formula (1)

[0057] f - =f b -f d Formula (2)

[0058]

[0059]

[0060] After calculating fd and fb, the velocity V and distance L of the target object can be calculated using formulas 5 and 6:

[0061]

[0062]

[0063] Where λ is the wavelength of the light emitted by the laser, θ is the angle between the velocity direction of the target object and the radial direction (cos θ = 1 when only considering the radial velocity), τ is the frequency modulation period of the laser, c is the speed of light in the medium, and B is the modulation bandwidth of the laser.

[0064] On the other hand, the analysis is based on the situation when the target moves toward the lidar:

[0065] Figure 2 This is a frequency relationship diagram showing the frequency of the received light being higher than the intrinsic light due to the Doppler effect when the target moves toward the lidar. Similarly, the distance and velocity values ​​of the target can also be obtained according to formulas (1)-(6).

[0066] In summary, regardless of whether the target object moves away from or towards the lidar, the values ​​of fd and fb can be obtained by using the following formulas (1)-(4) based on the relationship between the two main frequency values ​​f+ and f- and fd and fb.

[0067] f + =f b +f d Formula (1)

[0068] f - =f b -f d Formula (2)

[0069]

[0070]

[0071] Theoretically, regardless of whether the target's velocity is facing away from or towards the lidar, the fd calculated by the above formula will always be a positive value.

[0072] However, by determining whether the frequency difference of the rising edge or the falling edge is greater, it is possible to determine whether the speed is positive or negative: if the frequency difference of the rising edge is greater, the speed is negative and the target is moving away from the lidar; if the frequency difference of the falling edge is greater, the speed is positive and the target is moving towards the lidar.

[0073] After calculating fd and fb, the velocity V and distance L of the target object can be calculated using formulas (5) and (6):

[0074]

[0075]

[0076] This allows us to obtain the distance, speed, and direction of motion between the target and the lidar.

[0077] In this embodiment, the principle behind aliasing, its drawbacks, and the calculation method for the distance, velocity, and direction of motion between the target and the lidar when aliasing occurs are explained:

[0078] Taking a triangular wave as an example, the intrinsic wave and the received wave generate a beat frequency signal after entering the mixer. The acquisition card collects the beat frequency signal at a certain sampling frequency. Therefore, the sampling frequency of the acquisition card limits the highest frequency that can be collected. Currently, the highest sampling frequency of the acquisition card is 250MHz. Therefore, the highest frequency (Nyquist frequency) that can be collected without aliasing is 125MHz. When the target object has a relative velocity, the highest frequency f+ that can be collected is 125MHz, that is, the sum of fd and fb can only reach a maximum of 125MHz. Otherwise, the sampled waveform will be aliased.

[0079] Aliasing can lead to misjudgments. For example, suppose the modulation bandwidth of the laser is B = 1 GHz and the modulation frequency is 1 / τ = 57.6 kHz. There is a stationary target 250 m away from the lidar. Substituting into formula (6), the theoretical fb = 192 MHz can be calculated. If a 250 MHz analog-to-digital converter is used to sample the beat frequency signal, the actual fb|250-192| MHz = 58 MHz is obtained. Substituting into formula (6), the corresponding distance value is 75.5 m. When the target has relative velocity, the sampling frequency of the analog-to-digital converter has a more significant impact on the ranging and velocity limits.

[0080] Aliasing refers to the phenomenon where sampled signals overlap and become distorted when they are reconstructed into a continuous signal. When aliasing occurs, the original signal cannot be recovered from the sampled signal. Based on the aliasing mechanism, a formula for calculating the aliasing frequency of the analyzed signal can be derived. Let the frequency of the actual signal be fs, the sampling frequency be f, and f < 2*fs. The aliased frequency obtained after sampling analysis is fa, then the following formula applies:

[0081] f a =|f s -n*f| Formula (7)

[0082] in,

[0083]

[0084] Int() is the integer part of the decimal, which is used to round down the integer part of the number.

[0085] Based on the distance and velocity calculation formulas for triangular waves, we derive the distance and velocity calculation formulas for trapezoidal waves in both directions where aliasing is absent and where aliasing occurs:

[0086] One aspect of this embodiment is:

[0087] First, the calculation methods for aliasing and non-aliasing phenomena are explained when the target object moves towards the lidar. The frequency relationship between the intrinsic wave, the received wave, and the beat frequency signal is shown in the figure below. Figure 3 As shown in the figure, since the analog-to-digital converter cannot distinguish between positive and negative values, the acquired frequency value is always positive. Let the sampling frequency be f, the beat frequency signal acquired from the rising edge of the trapezoidal wave be f1, the beat frequency signal acquired from the invariant region be fd, and the beat frequency signal acquired from the falling edge be f2.

[0088] from Figure 3 In this case, we can see that f1 < f2, and f1 = f-, f2 = f+. Substituting these values ​​into formulas (3) and (4), we get:

[0089] 2f d =f2-f1 Formula (8)

[0090] 2f b =f2+f1 Formula (9)

[0091] The frequency value fd of the beat frequency signal collected from the invariant region of the trapezoidal wave should be consistent with the Doppler frequency shift calculated by formula (8). After obtaining fd and fb, they can be substituted into formulas (5) and (6) to obtain the velocity and distance of the target object.

[0092] If f+ > f / 2, aliasing will occur when the beat frequency signal exceeds the acquisition mode. At this time, according to formula (7), we can know that:

[0093] f + =f-f2 Formula (10)

[0094] At this point, the formulas for calculating fd and fb should be:

[0095] 2f d=f - f2 - f1 Formula (11)

[0096] 2f b =f - f2 + f1 Formula (12)

[0097] After obtaining fd and fb, they can be substituted into formulas (5) and (6) to obtain the velocity and distance of the target object.

[0098] Another aspect of this embodiment:

[0099] Next, we discuss the situation when the target object moves away from the lidar, explaining the calculation methods for aliasing and non-aliasing phenomena. The frequency relationship diagram between the intrinsic wave, the received wave, and the beat frequency signal is shown in the figure below. Figure 4 As shown.

[0100] from Figure 4 From this, we know that f1 > f2, f1 = f+, f2 = f-. Substituting these into formulas (3) and (4), we get:

[0101] 2f d =f1-f2 Formula (13)

[0102] 2f b =f1+f2 Formula (14)

[0103] The frequency value fd of the beat frequency signal collected from the invariant region of the trapezoidal wave should be consistent with the Doppler frequency shift calculated by formula (13). After obtaining fd and fb, they can be substituted into formulas (5) and (6) to obtain the velocity and distance of the target object.

[0104] If f+ > f / 2, aliasing will occur when the beat frequency signal exceeds the acquisition signal. According to formula (7), the following can be obtained:

[0105] f + =f-f1 Formula (15)

[0106] At this point, the formulas for calculating fd and fb should be:

[0107] 2f d =f - f1 - f2 Formula (16)

[0108] 2f b =f - f1 + f2 Formula (17)

[0109] After obtaining fd and fb, they can be substituted into formulas (5) and (6) to obtain the velocity and distance of the target object.

[0110] The above four situations, including the four sets of formulas in pairs (8)(9), (11)(12), (13)(14), (16)(17), are four main methods for calculating speed and distance values, covering all situations where the target object moves towards and away from the lidar, and aliasing occurs or does not occur during acquisition.

[0111] The Doppler frequency shift fd caused by any speed value within the speed measurement limit must be equal to the fd calculated by one of the formulas (8)(11)(13)(16), so that the corresponding fb can be obtained, and it can be determined whether the velocity direction of the target object and the beat frequency signal have been mixed.

[0112] like Figure 5 Another one shown is... Figure 3 and Figure 4 The effects are equivalent across different waveforms, and the above formula set can still be used.

[0113] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand and implement the present invention. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A distance and velocity measurement method based on trapezoidal wave optical frequency modulation continuous wave, comprising a target object, a laser radar, an analog-to-digital converter, a rising edge beat signal frequency value denoted as f1, a falling edge beat signal frequency value denoted as f2, an unchanged region beat signal frequency value denoted as fd, and two main frequency values f+ and f- of the beat signal obtained by a sampling card, wherein f+ is the highest peak frequency and f- is the second highest peak frequency, i.e. f+ > f-, the beat signal frequency value of the unchanged region should be the same as the Doppler frequency value, and therefore the beat signal frequency value of the unchanged region and the Doppler frequency value are both denoted as fd, and a distance frequency is denoted as fb, characterized in that: The frequency difference of eigenwave and received wave in the invariant region is Doppler shift fd, that is, the Doppler shift fd can be obtained by sampling the frequency value of beat signal in the invariant region; Under the premise of knowing the Doppler shift fd, the Doppler shift obtained by converting the frequency difference of beat signal belonging to the rising region and the falling region is compared with the known fd, whether aliasing occurs, the motion direction of the target object, and the correct speed and distance values are calculated; The speed direction of the target object relative to the laser radar and whether aliasing occurs in the frequency value during sampling has the following four cases: When the target object moves towards the laser radar, aliasing occurs; When the target object moves away from the laser radar, aliasing occurs; When the target object moves towards the laser radar, aliasing does not occur; When the target object moves away from the laser radar, aliasing does not occur; The distance and speed measurement method comprises the following steps: S1, the f1 and f2 values are brought into the formula in the above four cases, and four Doppler shift values are calculated; S2, judging whether the frequency value fd of the beat signal collected in the trapezoidal wave invariant region is consistent with the Doppler shift value calculated by the formula, the speed direction of the target object relative to the laser radar and whether aliasing occurs in the frequency value during sampling can be judged; S3, according to the judgment, the fb is calculated by using the same group of formulas; S4, the values of fd and fb are brought into the calculation formula of speed V and distance L to obtain the speed and distance.

2. The method of claim 1, wherein the method is based on trapezoidal wave optical frequency modulated continuous wave (FMCW) range and velocity measurement. When the target object moves towards the laser radar, f1 < f2, f1 = f-, and f2 = f+.

3. The method of claim 1, wherein the method is based on trapezoidal wave optical frequency modulated continuous wave (FMCW) range and velocity measurement. When the target object moves away from the laser radar, f1 > f2, f1 = f+, and f2 = f-.

4. The method of claim 1, wherein the method is based on trapezoidal wave optical frequency modulated continuous wave (FMCW) range and velocity measurement. When the target object moves towards the laser radar and aliasing does not occur, the calculation formula of fd and fb should be: 2f d = f2 - f1 2f b = f2 + f1.

5. The method of claim 1, wherein the method is based on trapezoidal wave optical frequency modulated continuous wave (FMCW) range and velocity measurement. When the target object moves away from the laser radar and aliasing does not occur, the calculation formula of fd and fb should be: 2f d = f1 - f2 2f b = f1 + f2.

6. The method of claim 1, wherein the method is based on trapezoidal wave optical frequency modulated continuous wave (FMCW) range and velocity measurement. The highest frequency that the acquisition card can collect is half of the sampling frequency, that is, the Nyquist frequency, when the value of f+ is greater than the highest frequency that the acquisition card can collect, aliasing phenomenon occurs.

7. The method of claim 6, wherein the method is based on trapezoidal wave optical frequency modulated continuous wave (FMCW) range and velocity measurement. According to the aliasing mechanism, the aliasing frequency calculation formula of the analysis signal can be obtained, assuming that the frequency of the actual signal is fs, the sampling frequency is f, and f < 2*fs, the aliasing frequency obtained by sampling analysis is fa, and the following formula is obtained: f a =|f s -n*f| Wherein, Wherein Int() is the integer operation, only the integer part of the decimal is reserved; When the target object moves towards the laser radar, aliasing occurs, f+ changes to: + = f - f2 and f1 = f - When the target object is moving away from the laser radar, f+ changes to: + = f - f1 and f2 = f - 8. The method of claim 7, wherein the method is based on trapezoidal wave optical frequency modulated continuous wave (FMCW) range and velocity measurement. When the target object moves towards the laser radar and aliasing occurs, the calculation formula of fd and fb should be changed to: 2f d = f - f2 - f1 2f b = f - f2+ f1.

9. The method of claim 7, wherein the method is a distance and velocity measurement method based on trapezoidal wave optical frequency modulated continuous wave. When the target is moving away from the lidar, aliasing occurs when f d The formula for calculating fband fshould be: 2f d = f - f1 - f2 2f b = f - f1 + f2.

10. The distance and speed measurement method based on trapezoidal wave optical frequency modulation continuous wave according to any one of claims 4, 5, 8, and 9, characterized in that: The formula for obtaining the speed and distance of the target object according to the values of fd and fb is as follows: Wherein, λ is the wavelength of the light wave emitted by the laser, θ is the included angle between the speed direction of the target object and the radial direction, cosθ = 1 when only the radial speed is considered, τ is the frequency modulation period of the laser, c is the speed of light in the medium, and B is the modulation bandwidth of the laser.

Images