CN118226452A - Laser ranging and speed measuring system and method based on echo feedback modulation - Google Patents
Laser ranging and speed measuring system and method based on echo feedback modulation Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/06—Systems determining position data of a target
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
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- G01S17/08—Systems determining position data of a target for measuring distance only
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
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Abstract
The invention discloses a laser ranging and speed measuring system and method based on echo feedback modulation, which belong to the technical field of laser measurement. The outgoing laser of the laser is divided into two paths of signals through an optical beam splitter, one path of the signals is used as detection light to be input to a laser intensity modulator, the other path of the signals is used as a local reference light signal to be coupled with an echo light signal and is subjected to subsequent processing, a modulation signal with the frequency changing along with time is generated to feed back and modulate the laser intensity, so that the detection light signal with the next frequency is emitted, the detection light signal is reciprocated, and finally the measurement of the target distance D and the speed v is realized. The method and the system provided by the invention can be applied to any continuous wave laser receiving and transmitting circuit, and have a certain application value in the aspect of high-precision long-distance measurement.
Description
Technical Field
The invention belongs to the technical field of laser measurement, and particularly relates to a laser ranging and speed measuring system and method based on echo feedback modulation.
Background
The Frequency Modulation Continuous Wave (FMCW) technology is one of laser ranging and speed measuring technologies widely applied in the laser radar at present, and compared with the pulse TOF ranging technology and the amplitude modulation continuous wave laser (AMCW) ranging technology, the Frequency Modulation Continuous Wave (FMCW) technology has the advantages of high ranging precision, wide measuring range and capability of realizing speed measurement of a target object, and is a mainstream measuring technology of the laser radar at present.
However, the implementation of the frequency modulation continuous wave laser ranging and speed measuring technology depends on a high-linearity laser modulation technology and a sweep frequency source with extremely high stability, and the modulation of the high-linearity laser frequency is one of the main difficulties of limiting the development of the frequency modulation continuous wave technology at present. Meanwhile, in the aspect of signal processing, the frequency modulation continuous wave laser ranging and speed measuring technology needs to carry out spectrum analysis on echo signals, and usually, time accumulation of a plurality of modulation periods is needed for completing accurate measurement once, so that the signal processing difficulty is high, and the efficiency is low.
Disclosure of Invention
In order to solve the high standard requirement on laser modulation linearity and sweep source stability in the process of realizing distance measurement and speed measurement of the traditional FMCW, the invention provides a laser distance measurement and speed measurement system and method based on echo feedback modulation. The method and the system provided by the invention can be applied to any continuous wave laser receiving and transmitting circuit, and have a certain application value in the aspect of high-precision long-distance measurement.
The invention is realized by the following technical scheme:
The laser ranging and speed measuring system based on echo feedback modulation comprises a laser 1, an optical beam splitter 2, a laser intensity modulator 3, a laser frequency fixed frequency shift unit 4, an optical circulator 5, an optical transmitting/receiving unit 6, a low-pass filter 7, a single-frequency signal source 8, a signal mixer 9, a signal time-frequency analysis unit 10, a top frequency band-pass filter 11, a balance detector (BPD) 12 and an optical coupler 13;
The outgoing laser of the laser 1 is split into two paths of laser signals by the optical beam splitter 2, one path of laser signals is used as detection light to be input into the laser intensity modulator 3, then the laser signals are sent to the laser frequency fixing and frequency shifting unit 4 for frequency shifting treatment, and then are emitted to a moving target by the optical circulator 5 and the optical transmitting/receiving unit 6, and an echo beam is obtained after being reflected by the moving target; the other path of laser signal is used as a local reference light signal and is coupled with an echo light signal obtained after being reflected by a moving object in an optical coupler 13, a balance detector (BPD) 12 detects the coupled light signal, photoelectric conversion processing is carried out, the obtained electric signal is sent to an upper side frequency band-pass filter 11 for filtering, and the upper side frequency band-pass filter 11 outputs an upper side frequency signal; the upper side frequency signal is divided into two paths, and one path of signal is input to the signal time-frequency analysis unit 10 for calculating the target distance D and the speed v; the other path of signal and local oscillation signal s M (the frequency of the local oscillation signal s M is f M) generated by the single-frequency signal source 8 are mixed in the signal mixer 9, the mixed signal output by the mixer 9 is sent to the low-pass filter 7 for filtering processing to obtain a low-frequency signal component, the low-frequency signal component is sent to the laser intensity modulator 3, and then the laser signal is sent to the laser frequency fixing and frequency shifting unit 4 for frequency shifting processing; the single-frequency signal source 8 sends sine and cosine signals with the frequency of f M to the laser frequency fixed frequency shift unit 4 for modulation, so that the frequency of a laser beam output by the laser frequency fixed frequency shift unit 4 is two upper and lower side frequency signals f 0±fM; the laser signal subjected to frequency shift treatment by the laser frequency fixing and frequency shifting unit 4 is transmitted to a moving target through the optical circulator 5 and the optical transmitting/receiving unit 6, and an echo beam is obtained after being reflected by the moving target; the echo optical signal generated by the echo beam is sent to an optical coupler 13; so reciprocating, realize laser range finding and speed measuring through echo feedback modulation.
Further, the lower cut-off frequency of the upper side band-pass filter 11 is f M, and the bandwidth is Bw; the bandwidth of the low pass filter 7 is B L(BL > Bw).
On the other hand, the invention also provides a laser ranging and speed measuring method based on echo feedback modulation, which comprises the following steps:
Step one: the laser 1 generates a continuous laser signal with constant power, and the emergent laser frequency of the laser 1 is f 0; when the measurement is started for the first time, namely n=0, because no echo signal is received, the laser intensity modulator is not modulated by the echo signal, the laser frequency output by the intensity modulator is still f 0, and the laser frequency is input into the laser frequency fixed frequency shift unit 4 to become a transmitted probe beam output, namely a probe beam transmitted by a first wave; the single-frequency signal source 8 sends sine and cosine signals with the frequency of f M to the laser frequency fixed frequency shift unit 4 for modulation, so that the frequency of a laser beam output by the laser frequency fixed frequency shift unit 4 is two upper and lower side frequency signals f 0±fM, and the signals are transmitted to a moving target through the optical circulator 5 and the optical transmitting/receiving unit 6; an echo signal of the transmitted probe beam after being reflected by the moving object is called as an echo beam of a first wave, and the moving speed of the object causes Doppler frequency shift, so that a frequency spectrum offset f D exists between the echo light signal and the transmitted laser signal;
Step two: the echo optical signal and the local reference optical signal are coupled in an optical coupler 13, a balance detector (BPD) 12 detects the coupled optical signal, the frequency of an electric signal output by the BPD is f M±fD, and an up-conversion signal f M+fD of the electric signal output by the BPD is obtained after the electric signal is filtered by an upper side frequency band-pass filter 11 with lower cut-off frequency f M and bandwidth Bw; the upper side frequency signal extracted by the upper side frequency band-pass filter is divided into two paths, one path of signal is input to the signal time-frequency analysis unit 10 and used for calculating a target distance D and a speed v, the other path of signal is mixed with a local oscillation signal s M generated by the single-product signal source 8 in the signal mixer 9, signals with two frequencies of 2f M+fD and f D are output, finally, a low-frequency signal with the frequency of f D is obtained by filtering by the low-pass filter 7 with the bandwidth of B L, namely, a first wave (n=0) echo feedback modulation signal cos2 pi f D t, and the feedback signal further modulates the laser intensity to emit the detection light speed of a second wave (n=1) with the frequency of f 0±fD;
Step three: setting the frequency of the detection light signal of the (n+1) th wave emitted by modulating the laser intensity by the (N) th wave echo signal as f 0±NfD, wherein n=n-1, N is more than or equal to 1, then generating the frequency spectrum of each photoelectric signal on the basis, and transmitting the frequency spectrum of the light signal to the moving object through the optical circulator 5 and the optical transmitting/receiving unit 6 to have f 0±fM±NfD four frequency components; at this time, the n+1th wave echo optical signal has a spectral offset f D compared with the transmission signal, the echo optical signal and the local reference optical signal are coupled in the optical coupler 13, and the spectrum of the output electric signal passing through the balance detector (BPD) 12 has two frequency components f M±NfD; the output signal of the BPD12 is divided into two paths after being filtered by the upper side band-pass filter 11, one path of the output signal is transmitted to the signal mixer 9 and mixed with the output signal of the single-frequency signal source 8, the output signal of the signal mixer 9 has a low-frequency component and a high-frequency component, then the low-frequency signal component is obtained through filtering of the low-pass filter 7, the signal is the n+1st wave echo feedback modulation signal cos 2pi (n+1) f D t, and the laser intensity is further modulated to obtain the detection light speed and the echo feedback modulation signal of the next wave (n+2th wave); the other path enters a signal time-frequency analysis unit 10, and the distance D and the speed v of the target object are obtained through time-frequency domain analysis of the signal.
Further, in the first step, if the target speed direction is opposite to the emission direction of the probe light, the frequency spectrums of the echo light signal and the emission signal are shifted by f D toward the high frequency direction, that is, the frequency of the echo signal is f 0±fM+fD; if the target speed direction is the same as the probe light emission direction, the spectrum is shifted to the low frequency direction by f D, i.e., the frequency of the echo light signal is f 0±fM-fD.
Further, in the third step, the distance D and the velocity v of the target object are obtained by time-frequency domain analysis of the signal, which specifically includes the following contents:
The relationship between the target distance D (T) and the velocity v (T) at any time T in the period D i,Ti is set as follows, where the target distance at the start time of the duration T i in one round of measurement:
Wherein floor (·) represents a downward rounding, Δt is the round trip time of the beam, i.e., Δt=2·d (t)/c, c is the speed of light; Δt is ignored, and then the following is obtained:
Wherein v (t)/c is approximately equal to 0;
the rate of change of the frequency of the output signal of the band-pass filter over time T during the further T i passes is expressed as:
further integration of the above formula can be obtained:
After obtaining the time-frequency data of the band-pass filtering output signal by the time-frequency signal analysis unit, performing linear fitting on the above data to obtain a corresponding linear fitting equation f (t) =a i+Bi t, then
Wherein, f D＝max(fBPD)-max(fBPF),max(fBPD) is the maximum upper frequency signal frequency of the signal output by the BPD, and max (f BPF) is the maximum frequency of the signal output by the maximum upper frequency band-pass filter obtained by the signal time-frequency analysis unit;
The expression substituted into D i can be:
Calculating the target speed according to the proportional relation between Doppler frequency shift and target speed, namely
v＝kD·fD＝kD·[max(fBPD)-max(fBPF)]
Where k D is the scale factor between the Doppler shift and the target speed.
Compared with the prior art, the invention has the following advantages:
According to the laser ranging and speed measuring system and method based on echo feedback modulation, single-frequency modulation is adopted for laser signals, the problems of high technical difficulty and high production cost in the high-frequency linear modulation process are avoided, the echo laser signals are used as feedback signals to realize the modulation of the laser signals, the laser flight time and Doppler frequency shift are converted into frequency domains, the sawtooth waveform periodic variation is realized on the frequency spectrum through time accumulation, and the signal analysis process is simplified. The method has a certain application value in the laser radar ranging and speed measuring field.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. Like elements or portions are generally identified by like reference numerals throughout the several figures. In the drawings, elements or portions thereof are not necessarily drawn to scale.
FIG. 1 is a schematic block diagram of echo feedback modulation laser speed and distance measurement;
in the figure: a laser 1, an optical beam splitter 2, a laser intensity modulator 3, a laser frequency fixed frequency shift unit 4, an optical circulator 5, an optical transmitting/receiving unit 6, a low-pass filter (bandwidth B L) 7, a single-frequency signal source (frequency f M) 8, a signal mixer 9, a signal time-frequency analysis unit 10, a top-side band-pass filter 11, a balance detector (BPD) 12, and an optical coupler 13;
f 0: emitting laser frequency; f M: writing signal frequency by a single-frequency signal source, and fixing the frequency shift frequency of a frequency shift unit by using the optical frequency; and n, echo laser reflection times f D: doppler frequency spectrum offset caused by the movement of a target object;
FIG. 2 is a spectrum of probe light emitted by the (n+1) th wave to the target;
In the figure: f 0: emitting laser frequency; f M: the frequency of the signal output by the single-frequency signal source is fixed to the frequency shift frequency of the frequency shift unit; f D: doppler frequency spectrum offset caused by the movement of a target object;
FIG. 3 is an echo optical signal spectrum of the n+1st wave;
In the figure: f 0: emitting laser frequency; f M: the frequency of the signal output by the single-frequency signal source is fixed to the frequency shift frequency of the frequency shift unit; f D: doppler frequency spectrum offset caused by the movement of a target object;
fig. 4 balances the spectrum of the detector (BPD) output signal;
In the figure: f M: the frequency of the output signal of the single-frequency signal source book, the frequency shift frequency of the frequency shift unit is fixed by the optical frequency; f D: doppler frequency spectrum offset caused by the movement of a target object;
fig. 5 is a spectrum of the output signal of the upper side band pass filter;
In the figure: f M: the frequency of the signal output by the single-frequency signal source is fixed by the optical frequency, and the frequency shift frequency of the frequency shift unit is fixed by the optical frequency; f D: doppler frequency spectrum offset caused by the movement of a target object; n+1: laser echo times represent the (n+1) th wave echo;
FIG. 6 is a frequency spectrum of the signal mixer output signal;
In the figure: f M: the frequency of the signal output by the single-frequency signal source is fixed by the optical frequency, and the frequency shift frequency of the frequency shift unit is fixed by the optical frequency; f D: doppler frequency spectrum offset caused by the movement of a target object; b L: band pass filter bandwidth; n+1: laser echo times represent the (n+1) th wave echo;
The n+1st wave modulated signal spectrum output by the low pass filter of fig. 7;
in the figure: f D: doppler frequency spectrum offset caused by the movement of a target object; n+1: the number of laser echoes indicates the n+1th wave echo.
Detailed Description
For a clear and complete description of the technical scheme and the specific working process thereof, the following specific embodiments of the invention are provided with reference to the accompanying drawings in the specification:
Example 1
As shown in fig. 1, the present embodiment provides an echo feedback modulation laser ranging and speed measuring system, which includes a laser 1, an optical beam splitter 2, a laser intensity modulator 3, a laser frequency fixed frequency shift unit 4, an optical circulator 5, an optical transmitting/receiving unit 6, a low-pass filter 7, a single-frequency signal source 8, a signal mixer 9, a signal time-frequency analysis unit 11, a top-side band-pass filter 12, an optical coupler 13 and a balance detector (BPD) 14; the outgoing laser of the laser 1 is split into two paths of signals by an optical splitter 2, one path of the signals is used as detection light to be input to a laser intensity modulator 3, the other path of the signals is used as a local reference light signal to be coupled with an echo light signal obtained after being reflected by a moving object in an optical coupler 13, a balance detector (BPD) 12 detects the coupled light signal and sends the coupled light signal to a top frequency band-pass filter 11 for filtering, and then the top frequency band-pass filter 11 outputs a top frequency signal; the upper side frequency signal is divided into two paths, one path of signal is input to the signal time-frequency analysis unit 10 and is used for calculating the target distance D and the speed v, and the other path of signal and the local oscillation signal s M (the frequency of the local oscillation signal s M is f M) generated by the single-frequency signal source 8; mixing in a signal mixer 9, sending to a low-pass filter 7 for filtering to obtain a low-frequency signal component, sending to a laser intensity modulator 3, and sending the laser signal to a laser frequency fixed frequency shift unit 4 for frequency shift; the single-frequency signal source 8 sends sine and cosine signals with the frequency of f M to the laser frequency fixed frequency shift unit 4 for modulation, so that the frequency of a laser beam output by the laser frequency fixed frequency shift unit 4 is two upper and lower side frequency signals f 0±fM, then the laser signal subjected to frequency shift treatment by the laser frequency fixed frequency shift unit 4 is transmitted to a moving target through the optical circulator 5 and the optical transmitting/receiving unit 6, and an echo beam is obtained after reflection of the moving target; the echo optical signal generated by the echo beam is sent to an optical coupler 13; so reciprocating, realize laser range finding and speed measuring through echo feedback modulation.
Further, the lower cut-off frequency of the upper side band-pass filter 11 is f M, and the bandwidth is Bw; the bandwidth of the low pass filter 7 is B L(BL > Bw).
Example 2
The embodiment provides an echo feedback modulation laser ranging and speed measuring method, which specifically comprises the following steps:
Step one: the laser 1 generates a continuous laser signal with constant power, and the emergent laser frequency of the laser 1 is f 0; when the measurement is started for the first time, namely n=0, because no echo signal is received, the laser intensity modulator is not modulated by the echo signal, the laser frequency output by the intensity modulator is still f 0, and the laser frequency is input into the laser frequency fixed frequency shift unit 4 to become a transmitted probe beam output, namely a probe beam transmitted by a first wave; the single-frequency signal source 8 sends sine and cosine signals with the frequency of f M to the laser frequency fixed frequency shift unit 4 for modulation, so that the frequency of a detection light beam output by the laser frequency fixed frequency shift unit 4 is two upper and lower side frequency signals f 0±fM, and the detection light beam is transmitted to a moving target through the optical circulator 5 and the optical transmitting/receiving unit 6; an echo signal of the transmitted probe beam after being reflected by the moving object is called as an echo beam of a first wave, and the moving speed of the object causes Doppler frequency shift, so that a frequency spectrum offset f D exists between the echo light signal and the transmitted laser signal;
If the target speed direction is opposite to the emission direction of the detection light, the frequency spectrums of the echo light signal and the emission signal are shifted to the high-frequency direction by f D, namely the frequency of the echo signal is f 0±fM+fD; if the target speed direction is the same as the probe light emission direction, the spectrum is shifted to the low frequency direction by f D, i.e., the frequency of the echo light signal is f 0±fM-fD.
Step two: the echo optical signal and the local reference optical signal are coupled in an optical coupler 13, a balance detector (BPD) 12 detects the coupled optical signal, the frequency of an electric signal output by the BPD is f M±fD, and an up-conversion signal f M+fD of the electric signal output by the BPD is obtained after the electric signal is filtered by an upper side frequency band-pass filter 11 with lower cut-off frequency f M and bandwidth Bw; the upper side frequency signal extracted by the upper side frequency band-pass filter is divided into two paths, one path of signal is input to the signal time-frequency analysis unit 10 and used for calculating the target distance and speed, the other path of signal is mixed with a local oscillation signal s M generated by the single-product signal source 8 in the signal mixer 9, signals with two frequencies of 2f M+fD and f D are output, finally, the signals are filtered by the low-pass filter 7 with the bandwidth of B L to obtain a low-frequency signal with the frequency of f D, namely a first wave (n=0) echo feedback modulation signal cos2 pi f D t, and the feedback signal further modulates the laser intensity to emit the detection light speed of a second wave (n=1) with the frequency of f 0±fD;
Step three: the frequency component of the probe light velocity of the second wave (n=1) emitted after the echo of the first wave (n=0) modulates the laser intensity is f 0±fD. Further, by analogy with the above measurement procedure, it is assumed that the frequency component of the detection light signal of the n+1th wave emitted after the laser intensity is modulated by the N (where n=n-1, n+.1) th wave echo signal is f 0±NfD, and then the frequency spectrum of each photoelectric signal will be generated on the basis, and the frequency spectrum of the light signal emitted to the moving object through the optical circulator 5 and the light emitting/receiving unit 6 will have f 0±fM±NfD four frequency components, the frequency spectrum of which is shown in fig. 2; at this time, the n+1st wave echo optical signal has an overall spectral offset f D compared with the emission signal, the spectrum of which is shown in fig. 3, wherein (a) in fig. 3 is an echo spectrum in which the target movement speed is opposite to the emission direction of the light beam, and (b) in fig. 3 is an echo spectrum in which the target movement speed is in the same direction as the emission direction of the light beam; the echo optical signal and the local reference optical signal are coupled in an optical coupler 13, and the spectrum of the output electrical signal via a balanced detector (BPD) 12 has two frequency components f M±NfD, the spectrum of which is shown in fig. 4. The output signal of the BPD12 is then filtered 11 by the upper band-pass filter having the filtering characteristic shown by the dashed box in fig. 4, and the frequency spectrum of the electric signal output by the upper band-pass filter 11 is shown in fig. 5, and it can be seen that the frequency of the output signal of the upper band-pass filter 11 becomes larger as n becomes larger. The output signal of the BPD12 is divided into two paths after being filtered by the upper side band-pass filter 11, and one path is transmitted to the signal mixer 9 to be mixed with the output signal of the single frequency signal source 8. The output signal spectrum of the signal mixer 9 is shown in fig. 6, the signal having two frequency components, a low frequency component and a high frequency component. Finally, the low-frequency signal component is obtained by filtering with a low-pass filter 7 with a filtering characteristic shown by a dashed box in fig. 6, and the output signal spectrum of the low-pass filter 7 is shown in fig. 7. The low-frequency signal component obtained through filtering of the low-pass filter 7 is an (n+1) th wave echo feedback modulation signal cos2 pi (n+1) f D t, and the laser intensity is further modulated to obtain the detection light speed and the echo feedback modulation signal of the next wave (n+2) th wave; the other path is input into a signal time-frequency analysis unit, and the distance D and the speed v of the target object are obtained through time-frequency domain analysis of the signal.
Example 3
The working principle of the echo feedback modulation laser ranging and speed measuring system is specifically described by combining the system described in the accompanying drawings and the embodiment 1 with the method described in the embodiment 2:
the embodiment specifically illustrates the working principle of the echo feedback modulation laser ranging and speed measuring system and the calculation method of the target distance and speed by combining the working procedures of the system described in the accompanying drawings and the embodiment 1 and the working procedure described in the embodiment 2, and specifically comprises the following steps:
The laser signal is reciprocally emitted and fed back as in the above process, so that time-frequency data of the frequency of the feedback signal varying with time can be obtained, and as known from the filtering characteristic of the up-conversion band-pass filter 11, when the frequency of the up-conversion signal output by the BPD12 is greater than f M +bw, the signal will be suppressed by the band-pass filter, at this time, the output signal of the band-pass filter is 0, the echo feedback modulation signal (output of the low-pass filter) is 0, and the system is equivalent to a state that returns to the start measurement time, so that the measurement is automatically restarted.
From the start measurement time to the restart of the start measurement state, a round measurement is completed, and the duration length T i (i=0, 1,2, …, M) is used to characterize the measurement process of a round, so that the signal time-frequency analysis unit 10 in fig. 1 collects the data of the output signal of the upper side frequency band-pass filter 11 in a longer period T through the measurement of a plurality of rounds, and the target distance D and the speed v are obtained through the time-frequency analysis and calculation of the data. From the above qualitative analysis, the frequency f of the output signal (and the feedback modulation signal) of the band-pass filter obtained by the multiple-pass measurement is a curve of a sawtooth-like shape according to the change relation of time t.
Let the target distance D i at the start time (corresponding to the start measurement time) of one of the round measurement processes T i be, in general, the relationship between the target distance D (T) and the velocity v (T) at any time T in the period T i is:
In the formula, floor (·) represents a downward rounding, Δt is the round-trip flight time of the light beam, i.e., Δt=2·d (t)/c, and c is the light speed. Because Δt is small, the above formula can be further expressed as
The speed of macroscopic objects is much smaller than the speed of light, i.e. v (T)/c.apprxeq.0, and the sum term is shown as 0, in other words, the distance of the target is a constant value D i during one round of measurement, and the target can be regarded as constant motion, i.e. f D, because the measurement time T i of one round is also very small. In practice, a special object, for example a rotating or vibrating object at a fixed position, is a constant distance object with a movement speed.
From the above, the rate of change of the frequency of the output signal of the upper side band pass filter 11 with time T during the T i rounds of measurement can be expressed as:
further integration of the above formula can be obtained:
After obtaining the time-frequency data of the output signal of the upper edge band-pass filter (11) by the time-frequency signal analysis unit, performing linear fitting on the above data to obtain a corresponding linear fitting equation f (t) =A i+Bi t, then
As mentioned above, when the frequency of the up-converted signal output by the BPD is greater than f M +bw, the signal will be suppressed by the upper band pass filter 11, so the maximum frequency of the signal output by the upper band pass filter 11 cannot exceed f M +bw, however, in any complete round of measurement, the frequency of the signal output by the BPD12 will appear as an upper frequency component greater than f M +bw, i.e. the maximum upper frequency signal frequency of the signal output by the BPD12 will appear, denoted as max (f BPD), which can also be obtained by the signal time-frequency analysis unit, while the signal time-frequency analysis unit can also obtain the maximum frequency of the signal output by the maximum band pass filter, denoted as max (f BPF), which is known from the feedback process of the signal of the above system:
fD＝max(fBPD)-max(fBPF)
substituting the above formula into the expression of D i yields:
The target speed can be calculated according to the proportional relation between Doppler frequency shift and target speed, namely
v＝kD·fD＝kD·[max(fBPD)-max(fBPF)]
Where k D is a scale factor between the doppler shift and the target speed, and the value can be previously defined by setting a specific target speed value to measure the doppler shift.
The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the specific details of the above embodiments, and various simple modifications can be made to the technical solution of the present invention within the scope of the technical concept of the present invention, and all the simple modifications belong to the protection scope of the present invention.
In addition, the specific features described in the above embodiments may be combined in any suitable manner, and in order to avoid unnecessary repetition, various possible combinations are not described further.
Moreover, any combination of the various embodiments of the invention can be made without departing from the spirit of the invention, which should also be considered as disclosed herein.
Claims (5)
1. The laser ranging and speed measuring system based on echo feedback modulation is characterized by comprising a laser (1), an optical beam splitter (2), a laser intensity modulator (3), a laser frequency fixed frequency shift unit (4), an optical circulator (5), an optical transmitting/receiving unit (6), a low-pass filter (7), a single-frequency signal source (8), a signal mixer (9), a signal time-frequency analysis unit (10), a top frequency band-pass filter (11), a balance detector (BPD) (12) and an optical coupler (13);
The method comprises the steps that the outgoing laser of a laser (1) is divided into two paths of laser signals through an optical beam splitter (2), one path of laser signals is used as detection light to be input into a laser intensity modulator (3), then the laser signals are sent to a laser frequency fixed frequency shifting unit (4) for frequency shifting treatment, and then the laser signals are emitted to a moving target through an optical circulator (5) and an optical emitting/receiving unit (6) to obtain echo beams after being reflected by the moving target; the other path of laser signal is used as a local reference light signal and is coupled with an echo light signal obtained after being reflected by a moving object in an optical coupler (13), the coupled light signal is detected by a balance detector (12), photoelectric conversion processing is carried out, the obtained electric signal is sent to an upper side frequency band-pass filter for filtering (11), and the upper side frequency band-pass filter (11) outputs an upper side frequency signal; the upper side frequency signal is divided into two paths, and one path of signal is input to a signal time-frequency analysis unit (10) for calculating a target distance D and a speed v; the other path of signal and local oscillation signal s M generated by a single frequency signal source (8) are mixed in a signal mixer (9), the mixed signal output by the mixer (9) is sent to a low-pass filter (7) for filtering processing to obtain a low-frequency signal component, the low-frequency signal component is sent to a laser intensity modulator (3), and then the laser signal is sent to a laser frequency fixed frequency shifting unit (4) for frequency shifting processing; the single-frequency signal source (8) sends sine and cosine signals with the frequency of f M to the laser frequency fixed frequency shift unit (4) for modulation, so that the frequency of a laser beam output by the laser frequency fixed frequency shift unit (4) is two upper and lower side frequency signals f 0±fM; the laser signal subjected to frequency shift treatment by the laser frequency fixed frequency shift unit (4) is transmitted to a moving target through the optical circulator (5) and the optical transmitting/receiving unit (6), and an echo beam is obtained after being reflected by the moving target; an echo optical signal generated by the echo light beam is sent to an optical coupler (13); so reciprocating, realize laser range finding and speed measuring through echo feedback modulation.
2. The laser ranging and velocity measuring system based on echo feedback modulation according to claim 1, wherein the lower cut-off frequency of the upper side band-pass filter (11) is f M, and the bandwidth is Bw; the bandwidth of the low-pass filter (7) is B L, wherein B L > Bw.
3. The method of an echo feedback modulation based laser ranging and velocimetry system of claim 1, comprising the steps of:
Step one: the laser (1) generates a continuous laser signal with constant power, and the emergent laser frequency of the laser (1) is f 0; when the measurement is started for the first time, namely n=0, because no echo signal is received, the laser intensity modulator is not modulated by the echo signal, the laser frequency output by the intensity modulator is still f 0, and the laser frequency is input into the laser frequency fixed frequency shift unit (4) to become a transmitted probe beam output, namely a probe beam transmitted by a first wave; the single-frequency signal source (8) sends sine and cosine signals with the frequency of f M to the laser frequency fixed frequency shift unit (4) for modulation, so that the frequency of a laser beam output by the laser frequency fixed frequency shift unit (4) is two upper and lower side frequency signals f 0±fM, and the signals are transmitted to a moving target through the optical circulator (5) and the optical transmitting/receiving unit (6); an echo signal of the transmitted probe beam after being reflected by the moving object is called as an echo beam of a first wave, and the moving speed of the object causes Doppler frequency shift, so that a frequency spectrum offset f D exists between the echo light signal and the transmitted laser signal;
Step two: the echo optical signal and the local reference optical signal are coupled in an optical coupler (13), a balance detector (BPD) (12) detects the coupled optical signal, the frequency of an electric signal output by the BPD is f M±fD, and an up-conversion signal f M+fD of the electric signal output by the BPD is obtained after the electric signal is filtered by an upper side frequency band-pass filter (11) with lower cut-off frequency f M and bandwidth Bw; the upper side frequency signal extracted by the upper side frequency band-pass filter is divided into two paths, one path of signal is input to a signal time-frequency analysis unit (10) and used for calculating a target distance D and a speed v, the other path of signal is mixed with a local oscillation signal s M generated by a single product signal source (8) in a signal mixer (9), signals with two frequencies of 2f M+fD and f D are output, finally a low-pass filter (7) with the bandwidth of B L filters to obtain a low-frequency signal with the frequency of f D, namely a first wave (n=0) echo feedback modulation signal cos2 pi D t, and the feedback signal further modulates the laser intensity to emit the detection light speed of a second wave (n=1) with the frequency of f 0±fD;
Step three: setting the frequency of a detection light signal of an (n+1) th wave emitted by modulating the laser intensity by an (N) th wave echo signal as f 0±NfD, wherein n=N-1, N is more than or equal to 1, then generating a frequency spectrum of each photoelectric signal on the basis, and transmitting the frequency spectrum of the light signal to a moving target through an optical circulator (5) and an optical transmitting/receiving unit (6) to have f 0±fM±NfD four frequency components; at this time, the n+1th wave echo optical signal has a spectral offset f D compared with the transmission signal, the echo optical signal and the local reference optical signal are coupled in the optical coupler (13), and the frequency spectrum of the output electric signal passing through the balance detector (BPD) (12) has two frequency components f M±NfD; the output signal of the BPD (12) is divided into two paths after being filtered by the upper side band-pass filter (11), one path of the output signal is transmitted to the signal mixer (9) and mixed with the output signal of the single-frequency signal source (8), the output signal of the signal mixer (9) is provided with a low-frequency component and a high-frequency component, then the low-frequency signal component is obtained through filtering of the low-pass filter (7), and the signal is an (n+1) th wave echo feedback modulation signal cos2 pi (n+1) f D t, and the laser intensity is further modulated to obtain the detection light speed and the echo feedback modulation signal of the next wave (n+2 wave); the other path enters a signal time-frequency analysis unit (10), and the distance D and the speed v of the target object are obtained through time-frequency domain analysis of the signal.
4. The method for measuring distance and speed by laser based on echo feedback modulation as claimed in claim 3, wherein in the first step, if the target speed direction is opposite to the emission direction of the probe light, the frequency spectrums of the echo light signal and the emission signal are shifted to the high frequency direction by f D, that is, the frequency of the echo signal is f 0±fM+fD; if the target speed direction is the same as the probe light emission direction, the spectrum is shifted to the low frequency direction by f D, i.e., the frequency of the echo light signal is f 0±fM-fD.
5. The method for measuring distance and speed by laser based on echo feedback modulation as claimed in claim 3, wherein in step three, the distance D and speed v of the target object are obtained by time-frequency domain analysis of the signal, which comprises the following contents:
The relationship between the target distance D (T) and the velocity v (T) at any time T in the period D i,Ti is set as follows, where the target distance at the start time of the duration T i in one round of measurement:
Wherein floor (·) represents a downward rounding, Δt is the round trip time of the beam, i.e., Δt=2·d (t)/c, c is the speed of light; Δt is ignored, and then the following is obtained:
Wherein v (t)/c is approximately equal to 0;
the rate of change of the frequency of the output signal of the band-pass filter over time T during the further T i passes is expressed as:
further integration of the above formula can be obtained:
After obtaining the time-frequency data of the band-pass filtering output signal by the time-frequency signal analysis unit, performing linear fitting on the above data to obtain a corresponding linear fitting equation f (t) =a i+Bi t, then
Wherein, f D＝max(fBPD)-max(fBPF),max(fBPD) is the maximum upper frequency signal frequency of the signal output by the BPD, and max (f BPF) is the maximum frequency of the signal output by the maximum upper frequency band-pass filter obtained by the signal time-frequency analysis unit;
The expression substituted into D i can be:
Calculating the target speed according to the proportional relation between Doppler frequency shift and target speed, namely
v＝kD·fD＝kD·[max(fBPD)-max(fBPF)]
Where k D is the scale factor between the Doppler shift and the target speed.
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