CN116794674A - Distance and speed measurement method based on trapezoidal wave optical frequency modulation continuous wave - Google Patents
Distance and speed measurement method based on trapezoidal wave optical frequency modulation continuous wave Download PDFInfo
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Abstract
The invention discloses a distance and speed measuring method based on trapezoidal wave optical frequency modulation continuous wave, which comprises a target object, a laser radar and an analog-digital converter, wherein the beat frequency signal frequency value of a rising edge is marked as f1, the beat frequency signal frequency value of a falling edge is marked as f2, the beat frequency signal frequency value of a constant region is marked as fd, f+ and f-two main frequency values of the beat frequency signal are obtained through sampling by a collecting card, wherein f+ is the highest peak frequency, and f-is the minor peak frequency, namely f+ > f-; the method improves the distance measurement and speed measurement limit without changing hardware and does not sacrifice the horizontal resolution capability and frame rate; the method ensures that the sum of the highest Doppler frequency and the distance frequency which can be sampled is not limited by the Nyquist frequency any more, but can reach the sampling frequency of the analog-digital converter respectively; the two trapezoidal waves available for the method are equivalent, and the waveform is easy to realize.
Description
Technical Field
The invention relates to the technical field of laser radars, in particular to a distance and speed measurement method based on a trapezoidal wave optical frequency modulation continuous wave.
Background
When the speed and distance of a target object are measured by the laser radar, the prior art is that the beat frequency signals are generated after the intrinsic wave and the receiving wave enter the mixer, and the acquisition card acquires the beat frequency signals at a certain sampling frequency, so that the highest frequency which can be acquired is limited by the sampling frequency of the acquisition card. The sampling frequency of the highest acquisition card obtained at present is 250MHz, so the corresponding highest frequency (Nyquist frequency) which can be acquired is 125MHz. When the target object has relative speed, the highest frequency of the acquired beat frequency signal is 125MHz, namely the sum of the speed frequency and the distance frequency can only reach 125MHz, and when the speed frequency exceeds 125MHz, the waveform obtained by sampling will be aliased. Aliasing refers to a phenomenon in which sampled signals overlap each other to be distorted when they are restored to continuous signals. When aliasing occurs, the original signal cannot be restored from the sampling signal, the aliasing has great influence on the distance and the speed of the laser radar on the target object, so that the obtained data and the real data have larger phase difference, and therefore the Nyquist frequency of the acquisition card limits the acquisition limit on the speed and the distance of the target object.
Disclosure of Invention
The invention aims to provide a distance and speed measuring method based on trapezoidal wave optical frequency modulation continuous wave, which solves the problem that the Nyquist frequency of an acquisition card limits the distance measurement and speed measurement limits of a target object.
The invention is realized by the following technical scheme.
The invention relates to a distance and speed measuring method based on trapezoidal wave optical frequency modulation continuous wave, which comprises a target object, a laser radar and an analog-digital converter, wherein the beat frequency signal frequency value of a rising edge is marked as f1, the beat frequency signal frequency value of a falling edge is marked as f2, the beat frequency signal frequency value of a constant region is marked as fd, f+ and f-two main frequency values of the beat frequency signal are obtained through sampling by a collecting card, f+ is the highest peak frequency, f-is the secondary peak frequency, namely, f+ is more than f-, and the beat frequency signal frequency value of a constant region is the same as the Doppler frequency value, so that the beat frequency signal frequency value and the Doppler frequency value of the constant region are marked as fd, and the distance frequency is marked as fb, and the distance measuring method is characterized in that: the frequency difference between the intrinsic wave and the received wave in the constant region is Doppler frequency shift fd, namely the Doppler frequency shift fd can be obtained by sampling the frequency value of the beat signal in the constant region;
on the premise of knowing the Doppler frequency shift fd, comparing the Doppler frequency shift obtained by converting the frequency difference value belonging to the ascending area and the descending area in the beat signal with the known fd, judging whether aliasing occurs or not and the moving direction of the target object, and calculating to obtain correct speed and distance values;
whether the target is aliased relative to the speed direction of the lidar and the frequency value during sampling has the following four conditions:
when the target moves towards the laser radar, aliasing occurs;
when the target object moves back to the laser radar, aliasing occurs;
when the target moves towards the laser radar, no aliasing occurs;
when the target object moves back to the laser radar, aliasing does not occur;
the distance and speed measurement method comprises the following steps:
s1, taking f1 and f2 values into formulas under the four conditions by a root, and calculating four Doppler frequency shift values;
s2, judging which case the beat frequency signal frequency value fd acquired by the trapezoidal wave invariable region is consistent with the Doppler frequency shift value calculated by the formula, and judging whether the speed direction of the target relative to the laser radar and the frequency value in the sampling process are aliased or not;
s3, calculating fb by using the same group of formulas according to the judging condition;
s4, carrying out calculation formulas of the speed V and the distance L according to the values of fd and fb to obtain the speed and the distance.
Further, when the target moves toward the lidar, f1 < f2, f1=f-, f2=f+.
Further, when the target moves away from the laser radar, f1> f2, f1=f+, f2=f-.
Further, when the target object moves towards the laser radar and aliasing does not occur, the calculation formulas of fd and fb should be:
2f d ＝f 2 -f 1
2f b ＝f 2 +f 1
further, when the target object moves away from the lidar and aliasing does not occur, the calculation formulas of fd and fb should be:
2f d ＝f 1 -f 2
2f b ＝f 1 +f 2
further, the highest frequency that the acquisition card can acquire is half of the sampling frequency, namely the nyquist frequency, and when the value of f+ is greater than the highest frequency that the acquisition card can acquire, aliasing occurs.
Further, according to the aliasing mechanism, an aliasing frequency calculation formula of the analysis signal can be obtained, the frequency of the actual signal is fs, the sampling frequency is f, f is less than 2×fs, and the frequency fa after the aliasing obtained by sampling analysis has the following formula:
f a ＝|f s -n*f|
wherein ,
where Int () is a rounding operation, retaining only the integer part of the decimal;
when the target object moves towards the laser radar to generate aliasing, f+ is changed into: f (f) + ＝f-f 2 And f1=f-;
when the target object moves away from the laser radar and is aliased, f+ is changed into: f (f) + ＝f-f 1 And f2=f-.
Further, when aliasing occurs when the target moves toward the lidar, the calculation formulas of fd and fb are strained as follows:
2f d ＝f-f 2 -f 1
2f b ＝f-f 2 +f 1
further, f when aliasing occurs when the target moves away from the lidar d And fb should be calculated as:
2f d ＝f-f 1 -f 2
2f b ＝f-f 1 +f 3
further, the formula for deriving the speed and distance of the target object from the values of fd and fb is as follows:
where λ is the wavelength of the light wave emitted by the laser, θ is the angle between the speed direction of the target object and the radial direction, cos θ=1 when only the radial speed is considered, τ is the frequency modulation period of the laser, c is the speed of the light in the medium, and B is the modulation bandwidth of the laser.
The invention has the beneficial effects that: because the trapezoidal wave contains definite Doppler frequency shift information, the speed direction of the target object and whether frequency aliasing occurs can be judged by using the definite information, so that frequency extremum caused by speed and distance is not influenced by Nyquist frequency any more, but can reach sampling frequency of an analog-digital converter respectively, the distance measurement and speed measurement limit is greatly improved, and the measurement scene is diversified. And the reduction of the horizontal resolution capability and the frame rate caused by the improvement of the speed and distance measurement limit by reducing the modulation frequency of the laser is avoided. The method can distinguish and measure the speeds in the directions facing and opposite to the directions due to different formulas substituted by targets with different speeds, and particularly can predict the danger of high-speed movement toward the laser radar. The method improves the distance measurement and speed measurement limit without changing hardware and does not sacrifice the horizontal resolution capability and frame rate; the method ensures that the sum of the highest Doppler frequency and the distance frequency which can be sampled is not limited by the Nyquist frequency any more, but can reach the sampling frequency of the analog-digital converter respectively; the two trapezoidal waves available for the method are equivalent, and the waveform is easy to realize.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are used in the description of the embodiments or the prior art will be briefly described, it being obvious that the drawings in the description below are only some embodiments of the invention, and that other drawings can be obtained from these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a graph of the frequency relationship of an eigen wave, a received wave and beat signals in a triangular frequency modulated continuous wave when Doppler shift exists when a target object moves away from a laser radar;
FIG. 2 is a graph of the frequency relationship of the eigen wave, the receive wave and the beat signal in the triangular frequency modulated continuous wave when there is Doppler shift when the target moves toward the lidar;
FIG. 3 is a graph of frequency relationship of trapezoidal wave frequency modulation continuous wave when a target object moves towards a laser radar;
FIG. 4 is a graph of frequency relationship of trapezoidal wave frequency modulation continuous wave when a target object moves away from a laser radar;
FIG. 5 is a graph of another trapezoidal-wave frequency modulation continuous wave frequency relationship.
Detailed Description
The present invention will be described in detail with reference to fig. 1 to 5.
In a general embodiment:
the distance and speed measuring method based on trapezoidal wave optical frequency modulation continuous wave described with reference to fig. 1-4 takes the current highest acquisition card with the sampling frequency of 250MHz as an example, so that the highest frequency (nyquist frequency) that can be acquired without aliasing is 125MHz;
when the target moves at a certain speed, a Doppler effect is generated, doppler frequency shift exists between the received wave and the eigenwave, corresponding frequency diagrams are shown in fig. 1 and 2, triangular waves are taken as preliminary deductions, and fd is the Doppler frequency shift in the diagrams. The Doppler effect is the change in wavelength as an object moves relative to the wave source. In front of the moving wave source, the wave is compressed, the wavelength becomes shorter and the frequency becomes higher; the opposite effect occurs after the moving wave source. The wavelength becomes longer and the frequency becomes lower; the higher the velocity of the wave source, the greater the effect produced. The doppler shift is the difference between the frequencies of the received wave and the eigenwave after the doppler effect occurs. The Doppler frequency fd and the distance frequency fb can be obtained through conversion by sampling the frequency absolute values f+ and f-of the beat frequency signals through the acquisition card, so that the speed and the distance of the target object can be obtained.
On the one hand, the analysis is made on the condition that the target object moves away from the laser radar:
fig. 1 is a frequency relation diagram generated by the doppler effect when a target object moves away from the laser radar and the frequency of the received light is lower than that of the intrinsic light. The beat frequency signal has two main frequency values of f+ and f < - >, and the relation between the f+ and f < - >, and f < f > is shown in formulas 1, 2, 3 and 4:
f + ＝f b +f d formula (1)
f - ＝f b -f d Formula (2)
After fd and fb are calculated, the speed V and the distance L of the target object can be calculated by using formulas 5 and 6:
where λ is the wavelength of the light wave emitted by the laser, θ is the angle between the speed direction of the target and the radial direction (cos θ=1 when only the radial speed is considered), τ is the frequency modulation period of the laser, c is the speed of the light in the medium, and B is the modulation bandwidth of the laser.
On the other hand, the analysis is made with the condition when the target object moves toward the laser radar:
fig. 2 is a frequency relationship diagram generated when the doppler effect occurs and the received light frequency is higher than the eigen light when the target moves toward the laser radar. Similarly, according to the formulas (1) to (6), the distance value and the speed value of the target object can be obtained.
In summary, whether the target moves towards the laser radar or away from the laser radar, the following formulas (1) - (4) can be applied to obtain the values of fd and fb through the relation between the f+ and f-two main frequency values and fd and fb.
f + ＝f b +f d Formula (1)
f - ＝f b -f d Formula (2)
Theoretically, fd calculated from the above equation is positive regardless of whether the target speed is facing away from or toward the lidar.
However, if it is determined whether the frequency difference of the rising edge is large or the frequency difference of the falling edge is large, it is possible to determine whether the velocity is a positive value or a negative value: if the frequency difference of the rising edge is large, the speed is a negative value, and the target moves back to the laser radar; if the frequency difference of the falling edge is large, the speed is positive, and the target moves towards the laser radar.
After fd and fb are calculated, the speed V and the distance L of the target object can be calculated by using formulas (5) and (6):
thus, the distance between the target object and the laser radar and the speed movement direction are obtained.
In the embodiment, the principle of aliasing, the drawbacks caused by the principle of aliasing, the distance between the target object and the laser radar when the aliasing occurs, and the calculation mode of the velocity motion direction:
taking triangular waves as an example, the intrinsic waves and the received waves generate beat frequency signals after entering the mixer, and the acquisition card acquires the beat frequency signals at a certain sampling frequency, so that the highest frequency which can be acquired is limited by the sampling frequency of the acquisition card. The sampling frequency of the highest acquisition card obtained at present is 250MHz, so the highest frequency (Nyquist frequency) that can be acquired without aliasing is 125MHz. When the target has relative speed, the highest frequency of f+ which can be acquired is 125MHz, namely the sum of fd and fb can only reach 125MHz at the highest, otherwise, the sampled waveforms will be aliased.
The occurrence of aliasing can lead to erroneous decisions, such as: let the modulation bandwidth b=1 GHz of the laser, the modulation frequency 1/τ=57.6khz of the laser, there is a stationary target object at 250m from the laser radar, and substituting the stationary target object into the formula (6) can calculate the theoretical fb=192 MHz. If the beat frequency signal is sampled by a 250MHz analog-digital converter, the actually obtained fb|250-192|MHz=58 MHz is substituted into the formula (6) to obtain the corresponding distance value of 75.5m. When the target object has relative speed, the sampling frequency of the analog-digital converter has more obvious limitation on the distance measurement limit and the speed measurement limit.
Aliasing refers to a phenomenon in which sampled signals overlap each other to be distorted when they are restored to continuous signals. When aliasing occurs, the original signal cannot be restored from the sampled signal. According to the aliasing mechanism, an aliasing frequency calculation formula of the analysis signal can be obtained, the frequency of the actual signal is fs, the sampling frequency is f, f is less than 2 fs, and the frequency after aliasing obtained by sampling analysis is fa, and then the following formula is provided:
f a ＝|f s -n f|formula (7)
wherein ,
where Int () is a rounding operation, only the integer part of the decimal is reserved.
Based on a distance and speed calculation formula taking triangular waves as an example, deducing a distance and speed calculation formula of each direction in which the trapezoidal waves are not aliased and the aliased phenomenon occurs:
on the one hand, the embodiment comprises the following steps:
firstly, the calculation mode of the aliasing phenomenon and the non-aliasing phenomenon of the target object when the target object moves towards the laser radar is described, and a frequency relation diagram among the eigenwave, the receiving wave and the beat frequency signal is shown in fig. 3. In the figure, the frequency value acquired is always positive because the analog-digital converter cannot distinguish positive from negative. Setting the sampling frequency as f, setting the beat frequency signal frequency value acquired from the rising edge of the trapezoidal wave as f1, setting the beat frequency signal frequency value acquired from the non-variable region as fd, and setting the beat frequency signal frequency value acquired from the falling edge as f2.
From fig. 3, it can be seen that f1 < f2, and f1=f-, f2=f+ are substituted into formulas (3), (4):
2f d ＝f 2 -f 1 formula (8)
2f b ＝f 2 +f 1 Formula (9)
The beat frequency signal frequency value fd acquired from the trapezoidal wave invariable region is consistent with the Doppler frequency shift calculated by the formula (8), and after fd and fb are obtained, the beat frequency signal frequency value fd can be substituted into the formulas (5) and (6) to obtain the speed and distance of the target object.
If f+ > f/2, aliasing will occur when the beat signal exceeds the acquisition model, and it can be known from formula (7):
f + ＝f-f 2 formula (10)
At this time, the calculation formulas of fd and fb should be:
2f d ＝f-f 2 -f 1 formula (11)
2f b ＝f-f 2 +f 1 Formula (12)
After fd and fb are obtained, equations (5) and (6) can be substituted to obtain the speed and distance of the target.
Another aspect of this embodiment is:
and secondly, the situation when the target object moves away from the laser radar is discussed, the calculation mode of the aliasing phenomenon and the calculation mode of the aliasing phenomenon is described, and the frequency relation diagram among the eigenwave, the receiving wave and the beat frequency signal is shown in fig. 4.
As can be seen from fig. 4, f1> f2, f1=f+, f2=f-, and can be obtained by substituting the following formulas (3) and (4):
2f d ＝f 1 -f 2 formula (13)
2f b ＝f 1 +f 2 Formula (14)
The beat frequency signal frequency value fd acquired from the trapezoidal wave invariable region is consistent with the Doppler frequency shift calculated by the formula (13), and after fd and fb are obtained, the beat frequency signal frequency value fd can be substituted into the formulas (5) and (6) to obtain the speed and distance of the target object.
If f+ > f/2, aliasing will occur when the beat signal exceeds the acquisition model, and this is obtained according to equation (7):
f + ＝f-f 1 formula (15)
At this time, the calculation formulas of fd and fb should be:
2f d ＝f-f 1 -f 2 formula (16)
2f b ＝f-f 1 +f 2 Formula (17)
After fd and fb are obtained, equations (5) and (6) can be substituted to obtain the speed and distance of the target.
The four conditions, including four groups of formulas (8), (9), (11), (12), (13), (14), (16) and (17), are four main methods for calculating speed and distance values, and cover all conditions of aliasing and non-aliasing during acquisition when the target moves towards the back to the laser radar.
The Doppler frequency shift fd caused by any speed value in the speed measurement limit is necessarily equal to the fd calculated by one of the formulas (8), (11), (13) and (16), so that the corresponding fb can be obtained, and whether the speed direction of the target object and the beat signal are mixed or not is judged.
Another waveform, different from fig. 3 and 4, as shown in fig. 5 is equivalent in effect application, and the above-described set of equations can still be used.
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 the present invention and implement it without limiting the scope of the present invention. All equivalent changes or modifications made in accordance with the spirit of the present invention should be construed to be included in the scope of the present invention.
Claims (10)
1. The utility model provides a distance and speed measurement method based on trapezoidal wave optical frequency modulation continuous wave, including the target object, the laser radar, analog-digital converter, it is denoted to mark the beat frequency signal frequency value of rising edge as f1, the beat frequency signal frequency value of falling edge is denoted as f2, the beat frequency signal frequency value of invariable district is denoted as fd, sample through the collection card and obtain that beat frequency signal exists f+ and f-two kinds of main frequency values, wherein f+ is the highest peak frequency, f-is the minor peak frequency, namely f+ is f-, the beat frequency signal frequency value and the Doppler frequency numerical value of invariable district should be the same, consequently, beat frequency signal frequency value and Doppler frequency numerical value of invariable district are all denoted as fd, distance frequency is denoted as fb, its characterized in that: the frequency difference between the intrinsic wave and the received wave in the constant region is Doppler frequency shift fd, namely the Doppler frequency shift fd can be obtained by sampling the frequency value of the beat signal in the constant region;
on the premise of knowing the Doppler frequency shift fd, comparing the Doppler frequency shift obtained by converting the frequency difference value belonging to the ascending area and the descending area in the beat signal with the known fd, judging whether aliasing occurs or not and the moving direction of the target object, and calculating to obtain correct speed and distance values;
whether the target is aliased relative to the speed direction of the lidar and the frequency value during sampling has the following four conditions:
when the target moves towards the laser radar, aliasing occurs;
when the target object moves back to the laser radar, aliasing occurs;
when the target moves towards the laser radar, no aliasing occurs;
when the target object moves back to the laser radar, aliasing does not occur;
the distance and speed measurement method comprises the following steps:
s1, bringing f1 and f2 values into the formulas under the four conditions, and calculating four Doppler frequency shift values;
s2, judging which case the beat frequency signal frequency value fd acquired by the trapezoidal wave invariable region is consistent with the Doppler frequency shift value calculated by the formula, and judging whether the speed direction of the target relative to the laser radar and the frequency value in the sampling process are aliased or not;
s3, calculating fb by using the same group of formulas according to the judging condition;
s4, carrying out calculation formulas of the speed V and the distance L according to the values of fd and fb to obtain the speed and the distance.
2. The method for measuring the distance and the speed of the optical frequency modulation continuous wave based on the trapezoidal wave according to claim 1, wherein the method comprises the following steps of: when the target moves towards the lidar, f1 < f2, f1=f-, f2=f+.
3. The method for measuring the distance and the speed of the optical frequency modulation continuous wave based on the trapezoidal wave according to claim 1, wherein the method comprises the following steps of: when the target moves away from the laser radar, f1> f2, f1=f+, f2=f-.
4. The method for measuring the distance and the speed of the optical frequency modulation continuous wave based on the trapezoidal wave according to claim 1, wherein the method comprises the following steps of: when the target moves towards the laser radar and aliasing does not occur, the calculation formulas of fd and fb should be:
2f d ＝f 2 -f 1
2f b ＝f 2 +f 1 。
5. the method for measuring the distance and the speed of the optical frequency modulation continuous wave based on the trapezoidal wave according to claim 1, wherein the method comprises the following steps of: when the target object moves away from the laser radar and aliasing does not occur, the calculation formulas of fd and fb should be:
2f d ＝f 1 -f 2
2f b ＝f 1 +f 2 。
6. the method for measuring the distance and the speed of the optical frequency modulation continuous wave based on the trapezoidal wave according to claim 1, wherein the method comprises the following steps of: the highest frequency that the acquisition card can acquire is half of the sampling frequency, namely the Nyquist frequency, and when the value of f+ is greater than the highest frequency that the acquisition card can acquire, the aliasing phenomenon can occur.
7. The method for measuring the distance and the speed of the optical frequency modulation continuous wave based on the trapezoidal wave according to claim 6, wherein the method comprises the following steps of: according to the aliasing mechanism, an aliasing frequency calculation formula of the analysis signal can be obtained, the frequency of the actual signal is fs, the sampling frequency is f, f is less than 2 fs, and the frequency after aliasing obtained by sampling analysis is fa, and then the following formula is provided:
f a ＝|f s -n*f|
wherein ,
where Int () is a rounding operation, retaining only the integer part of the decimal;
when the target object moves towards the laser radar to generate aliasing, f+ is changed into: f (f) + ＝f-f 2 And f1=f-;
when the target object moves away from the laser radar and is aliased, f+ is changed into: f (f) + ＝f-f 1 And f2=f-.
8. The method for measuring the distance and the speed of the optical frequency modulation continuous wave based on the trapezoidal wave according to claim 7, wherein the method comprises the following steps of: when the target object moves towards the laser radar and aliasing occurs, the calculation formulas of fd and fb are as follows:
2f d ＝f-f 2 -f 1
2f b ＝f-f 2 +f 1 。
9. the method for measuring the distance and the speed of the optical frequency modulation continuous wave based on the trapezoidal wave according to claim 7, wherein the method comprises the following steps of: f when aliasing occurs when the target moves away from the laser radar d And fb should be calculated as:
2f d ＝f-f 1 -f 2
2f b ＝T-f 1 +f 2 。
10. a method for measuring distance and speed based on trapezoidal wave optical frequency modulated continuous wave according to any one of claims 4, 5, 8 and 9, wherein:
the formula for deriving the speed and distance of the target from the values of fd and fb is as follows:
where λ is the wavelength of the light wave emitted by the laser, θ is the angle between the speed direction of the target object and the radial direction, cos θ=1 when only the radial speed is considered, τ is the frequency modulation period of the laser, c is the speed of the light in the medium, and B is the modulation bandwidth of the laser.
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