An article will show you about laser ranging
2023-02-13
According to Maims Consulting, shortly after the world's first ruby laser came out in 1960, the laser ranging technology with precision ranging as the main target was born. Laser ranging * * has been used in the military for a long time, and then, with its strong anti-interference ability and high accuracy, it has played a huge role in many fields, such as aerospace, building surveying and mapping, wind power industry, intelligent transportation, industrial manufacturing and so on.
With the rapid development of industrial automation and machine vision, laser ranging has been proved to be a very important non-contact detection method in many applications such as detection, measurement and control. At the same time, laser ranging, as the premise of high-end technologies such as laser speed measurement, laser tracking, laser three-dimensional imaging and laser radar (LiDAR), is receiving more and more attention. Mimes Consulting will focus on introducing and discussing several current mainstream laser ranging methods.
1. Classification of laser ranging method
According to the basic principle, laser ranging methods can be divided into two categories: time of flight (ToF) method and space geometry method, as shown in Figure 1. Among them, time-of-flight method includes direct ToF method (pulse type) and indirect ToF method (phase type); Spatial geometric methods mainly include triangulation and interferometry.
Pulse laser ranging is a ranging method that laser technology * * * has been used in the field of surveying and mapping for a long time. It obtains the information of target distance by directly measuring the time interval between the emitted light and the received light pulse, as shown in Figure 2. The measured distance can be expressed as:
Where D is the measured distance, c is the speed of light propagation in the air, and ∆ t is the round-trip time of laser beam from emission to reception.
Pulse laser has small emission angle, relatively concentrated energy in space, and high instantaneous power. These characteristics can be used to make various medium-long distance laser rangefinders, laser radars, etc. However, the pulse laser ranging method counts the time between the receiving and receiving pulses through a high frequency clock drive counter, which makes the cycle of the counting clock must be much shorter than the time between the sending pulse and the receiving pulse to ensure sufficient accuracy, so this ranging method is not suitable for short distance measurement.
At present, pulsed laser ranging is widely used in long-distance and low-accuracy surveys, such as topographic and geomorphologic surveys, geological exploration, engineering construction surveys, aircraft altitude surveys, satellite correlation ranging, distance measurement between celestial bodies, etc., as shown in Figure 3.
3. Phase laser ranging - indirect ToF method
Phase laser ranging uses the frequency of the radio band to modulate the amplitude of the laser beam and measure the phase delay generated by the modulation light for one round trip, and then convert the distance represented by the phase delay according to the wavelength of the modulation light. This method indirectly measures time by measuring phase difference, so it is also called indirect ToF method.
As shown in Figure 4, assuming the modulated frequency is f, the modulated waveform λ= C/f, c is the speed of light, and the measured phase shift of modulated light wave signal is ∆ φ， Then the round-trip time of the laser between the measuring point and the target can be calculated ∆ t=∆ φ/ 2 π f, so the measured distance D is:
However, when the target distance D increases, the value of phase delay may be greater than one period of sinusoidal modulated light wave, namely ∆ φ= 2 π (N+∆ N), N and ∆ N are integral and fractional parts of the cycle respectively, so the measured distance D is:
Where, L=c/2f= λ/ 2 is called the length of the measuring ruler, and the length of the phase ranging can be considered as λ/ The distance D is measured with a ruler of 2. The distance can be obtained by determining N and ∆ N. The fractional part ∆ N can be measured, but N is not a fixed value, which causes the problem of multiple solutions. In order to solve this problem, it is necessary to measure the same distance with modulated light wave signals of multiple frequencies, which is also called the ruler frequency in phase ranging. If the measured distance is less than the length of the ruler, N=0, the solution value is * * *. When the accuracy of phase measurement is fixed, the lower the frequency of the measuring ruler, the greater the ranging error, which is not allowed in high-precision ranging. On the contrary, the higher the frequency of the selected ruler, the higher the measurement accuracy, but the N value at this time will be greater than 1, and there is a problem of multiple solutions. To solve this contradiction, in practical applications, usually select a ruler that determines the ranging accuracy of the instrument and several auxiliary rulers that determine the range, which are called fine measuring ruler and rough measuring ruler respectively, and combine the two to obtain high-precision measurement.
The measurement accuracy of phase laser ranging can reach (sub) millimeter level, and the measurement range is from decimeter to kilometer, so it is widely used in short and medium range.
4. Multi-wavelength interference laser ranging
Interferometric ranging is one of the classical precision ranging methods. According to the interference principle of light, two rows of light with fixed phase difference, and with the same frequency, the same vibration direction or a small angle between the vibration directions overlap each other, which will produce interference phenomenon.
As shown in Figure 6, the schematic diagram of the commonly used Michelson interferometer is shown. The laser emitted by the laser is divided into reflected light S1 and transmitted light S2 through the spectroscope. The two beams are reflected back by the fixed mirror M1 and the movable mirror M2 respectively, and the two converge at the spectroscope to form a coherent beam. Then the combined beam intensity I is:
When distance D=m λ (m is an integer), the combined beam amplitude * *, light intensity * *, forming bright stripes; When D=(2m+1) λ/ At 2 o'clock, the phases of the two beams of light are opposite, the amplitudes of the two beams cancel each other, and the light intensity is * * * small, forming dark stripes. According to this principle, interferometric laser ranging is to convert the light and dark interference fringes from photoelectric detectors into electrical signals, which are counted by photoelectric counters, so as to realize the measurement of distance and displacement.
Due to the wavelength of the laser λ The resolution of interferometric laser ranging can reach nm and the accuracy is very high. However, the traditional laser interferometric ranging technology mentioned above only measures the relative displacement and cannot obtain the distance information of the target. At the same time, in order to ensure the accuracy of continuous measurement, the target must move along a fixed guide rail and the optical path cannot be interrupted. In addition, according to the interference principle, the measurement technology can only obtain the phase value in the range of 0 to 2 π, and considering the laser round-trip distance, it is equivalent to only measuring λ/ If the distance changes within the range of 2, the distance to be measured in a larger range will be uncertain because the 2 π multiple of the phase cannot be determined. this λ/ 2 The range is usually referred to as the unambiguous range of laser * * distance measurement. As follows:
Where D is the measured distance, m and ε Is the integer and decimal order of interference fringe included in the measured distance. The decimal order can be obtained by measurement, while m is an indefinite value.
In order to solve this contradiction, the method of multi-wavelength interference is usually adopted to meet the requirements of high resolution and expansion of non-ambiguity range. The basic principle of multi-wavelength interferometry is to use the decimal multiple method and develop the concept of synthetic wavelength on it.
Multi-wavelength interferometric ranging (MWI) began with the dual-wavelength interference experiment conducted by American scientists Wyant and Polhemus in the early 1970s. This method uses two lasers with different wavelengths λ 1、 λ 2 Perform interference measurement for the unknown distance at the same time, and bring it into the measured distance D of the above formula:
To solve the two equations, there are:
Where is the synthetic equivalent wavelength, ms and ε S are respectively λ S interference fringe integer and decimal order.
If the composite wavelength is regarded as the ranging wavelength, the phase information corresponding to the unknown distance is the difference between the ranging phases of the original two wavelengths, so the unknown distance can be solved. The non-ambiguity range of distance measurement is extended to half of the synthetic wavelength. From the formula, the synthetic wavelength must be greater than λ 1 and λ 2。
In the same way, in order to give consideration to the measurement range and accuracy, the method can be further developed with the idea of multiple rulers. The multi-wavelength laser can be used to measure the distance at the same time to generate multi-level composite wavelengths of different scales. The long synthetic wavelength of * * * is used to achieve the measurement range of * * *, and the distance measurement result obtained is used as the distance reference value of the shorter synthetic wavelength, so as to solve the range measurement result of this level of synthetic wavelength, so as to realize the range measurement with large range and high precision using the small synthetic wavelength of * * * and * * *.
However, this method requires multiple wavelengths of laser, which means that multiple laser sources are required. Considering that each laser source needs its own laser frequency stabilization device, and multiple lasers need high-precision optical beam combination, the structure of the whole laser * * distance measurement system is relatively complex, and the reliability and accuracy of the system will inevitably be affected to a certain extent.
5. FM CW laser ranging
Frequency modulated continuous wave (FMCW) laser ranging is another interferometric method that can realize * * * measurement. It combines the advantages of optical interferometry and radio radar technology. The basic principle of FMCW measurement is to realize interferometry by modulating the frequency of laser beam. Generally, the laser whose frequency of output laser beam changes with time is used as the light source, and the Michelson interferometer is used as the basic interferometric optical path. The frequency difference information is generated according to the different optical path of the reference light and the measurement light. The distance information of the two beams can be obtained after extracting the signal and processing, and the measurement of the * * distance can be realized.
Take sawtooth modulation as an example. It is a sine signal whose frequency changes linearly with time in a sawtooth shape. The instantaneous frequency of the measured light and the reference light changes with time, as shown in Figure 7.
Set the frequency of the reference light as ft, the frequency of the measurement light as fr, the modulation bandwidth as ∆ F, the modulation period as T, and the distance as D. The measurement light will have a time delay relative to the reference light due to different transmission paths as τ， Where ft changes periodically between f0 and fm according to sawtooth wave, then the expression of ft and fr is as follows:
Then the generated beat signal is fIF:
So the measured distance:
The frequency modulated continuous wave laser ranging takes laser as the carrier, and all environmental interference only affects the light intensity of the measured signal, but not the frequency information. Therefore, it can obtain high ranging accuracy and strong ability to resist environmental light interference, and the accuracy can reach the micron level. It is currently a research hotspot in large size and high-precision measurement applications. However, this measurement method requires high stability and linearity of the laser beam frequency, which makes the realization of the system more complex, and the measurement range is limited by the period T.
6. Triangular laser ranging
Triangular laser ranging means that the light source, the measured object surface and the light receiving system form a triangular optical path together. The light emitted by the laser source is focused by the collimating lens and then incident on the measured object surface. The light receiving system receives the scattered light from the incident point and images it on the sensitive surface of the photoelectric detector. It is a measurement method to measure the moving distance of the measured object surface through the displacement of the light point on the imaging surface.
According to the angle relationship between the incident laser beam and the normal line of the measured object surface, there are generally two ranging methods: oblique and direct, as shown in Figure 8. In general, the direct laser triangulation method is simpler in geometric algorithm than the oblique laser triangulation method, and the error is relatively small, and the volume can be designed to be more compact and compact. In industry, the direct laser ranging method is often used.
Compared with phase laser ranging and frequency modulated continuous wave laser ranging, triangulation laser ranging has many advantages, such as simple structure, fast testing speed, flexible and convenient use, low cost, etc. However, the accuracy of triangulation laser ranging will gradually deteriorate with the increase of distance, and since in the laser triangulation system, the photoelectric detector receives the scattered light from the target surface to be measured, this ranging method is generally suitable for indoor close work, It is not suitable for working in outdoor or indoor strong light background. Therefore, the application range of triangulation laser ranging is mainly small displacement measurement, which is widely used in the measurement of object surface contour, width, thickness and other quantities, such as body model surface design, laser cutting, sweeping robot, etc. in the automobile industry.
With the rapid development of industrial automation and machine vision, laser ranging has been proved to be a very important non-contact detection method in many applications such as detection, measurement and control. At the same time, laser ranging, as the premise of high-end technologies such as laser speed measurement, laser tracking, laser three-dimensional imaging and laser radar (LiDAR), is receiving more and more attention. Mimes Consulting will focus on introducing and discussing several current mainstream laser ranging methods.
1. Classification of laser ranging method
According to the basic principle, laser ranging methods can be divided into two categories: time of flight (ToF) method and space geometry method, as shown in Figure 1. Among them, time-of-flight method includes direct ToF method (pulse type) and indirect ToF method (phase type); Spatial geometric methods mainly include triangulation and interferometry.
2. Pulse laser ranging - direct ToF method
Pulse laser ranging is a ranging method that laser technology * * * has been used in the field of surveying and mapping for a long time. It obtains the information of target distance by directly measuring the time interval between the emitted light and the received light pulse, as shown in Figure 2. The measured distance can be expressed as:
Where D is the measured distance, c is the speed of light propagation in the air, and ∆ t is the round-trip time of laser beam from emission to reception.
Pulse laser has small emission angle, relatively concentrated energy in space, and high instantaneous power. These characteristics can be used to make various medium-long distance laser rangefinders, laser radars, etc. However, the pulse laser ranging method counts the time between the receiving and receiving pulses through a high frequency clock drive counter, which makes the cycle of the counting clock must be much shorter than the time between the sending pulse and the receiving pulse to ensure sufficient accuracy, so this ranging method is not suitable for short distance measurement.
At present, pulsed laser ranging is widely used in long-distance and low-accuracy surveys, such as topographic and geomorphologic surveys, geological exploration, engineering construction surveys, aircraft altitude surveys, satellite correlation ranging, distance measurement between celestial bodies, etc., as shown in Figure 3.
3. Phase laser ranging - indirect ToF method
Phase laser ranging uses the frequency of the radio band to modulate the amplitude of the laser beam and measure the phase delay generated by the modulation light for one round trip, and then convert the distance represented by the phase delay according to the wavelength of the modulation light. This method indirectly measures time by measuring phase difference, so it is also called indirect ToF method.
As shown in Figure 4, assuming the modulated frequency is f, the modulated waveform λ= C/f, c is the speed of light, and the measured phase shift of modulated light wave signal is ∆ φ， Then the round-trip time of the laser between the measuring point and the target can be calculated ∆ t=∆ φ/ 2 π f, so the measured distance D is:
However, when the target distance D increases, the value of phase delay may be greater than one period of sinusoidal modulated light wave, namely ∆ φ= 2 π (N+∆ N), N and ∆ N are integral and fractional parts of the cycle respectively, so the measured distance D is:
Where, L=c/2f= λ/ 2 is called the length of the measuring ruler, and the length of the phase ranging can be considered as λ/ The distance D is measured with a ruler of 2. The distance can be obtained by determining N and ∆ N. The fractional part ∆ N can be measured, but N is not a fixed value, which causes the problem of multiple solutions. In order to solve this problem, it is necessary to measure the same distance with modulated light wave signals of multiple frequencies, which is also called the ruler frequency in phase ranging. If the measured distance is less than the length of the ruler, N=0, the solution value is * * *. When the accuracy of phase measurement is fixed, the lower the frequency of the measuring ruler, the greater the ranging error, which is not allowed in high-precision ranging. On the contrary, the higher the frequency of the selected ruler, the higher the measurement accuracy, but the N value at this time will be greater than 1, and there is a problem of multiple solutions. To solve this contradiction, in practical applications, usually select a ruler that determines the ranging accuracy of the instrument and several auxiliary rulers that determine the range, which are called fine measuring ruler and rough measuring ruler respectively, and combine the two to obtain high-precision measurement.
The measurement accuracy of phase laser ranging can reach (sub) millimeter level, and the measurement range is from decimeter to kilometer, so it is widely used in short and medium range.
4. Multi-wavelength interference laser ranging
Interferometric ranging is one of the classical precision ranging methods. According to the interference principle of light, two rows of light with fixed phase difference, and with the same frequency, the same vibration direction or a small angle between the vibration directions overlap each other, which will produce interference phenomenon.
As shown in Figure 6, the schematic diagram of the commonly used Michelson interferometer is shown. The laser emitted by the laser is divided into reflected light S1 and transmitted light S2 through the spectroscope. The two beams are reflected back by the fixed mirror M1 and the movable mirror M2 respectively, and the two converge at the spectroscope to form a coherent beam. Then the combined beam intensity I is:
When distance D=m λ (m is an integer), the combined beam amplitude * *, light intensity * *, forming bright stripes; When D=(2m+1) λ/ At 2 o'clock, the phases of the two beams of light are opposite, the amplitudes of the two beams cancel each other, and the light intensity is * * * small, forming dark stripes. According to this principle, interferometric laser ranging is to convert the light and dark interference fringes from photoelectric detectors into electrical signals, which are counted by photoelectric counters, so as to realize the measurement of distance and displacement.
Due to the wavelength of the laser λ The resolution of interferometric laser ranging can reach nm and the accuracy is very high. However, the traditional laser interferometric ranging technology mentioned above only measures the relative displacement and cannot obtain the distance information of the target. At the same time, in order to ensure the accuracy of continuous measurement, the target must move along a fixed guide rail and the optical path cannot be interrupted. In addition, according to the interference principle, the measurement technology can only obtain the phase value in the range of 0 to 2 π, and considering the laser round-trip distance, it is equivalent to only measuring λ/ If the distance changes within the range of 2, the distance to be measured in a larger range will be uncertain because the 2 π multiple of the phase cannot be determined. this λ/ 2 The range is usually referred to as the unambiguous range of laser * * distance measurement. As follows:
Where D is the measured distance, m and ε Is the integer and decimal order of interference fringe included in the measured distance. The decimal order can be obtained by measurement, while m is an indefinite value.
In order to solve this contradiction, the method of multi-wavelength interference is usually adopted to meet the requirements of high resolution and expansion of non-ambiguity range. The basic principle of multi-wavelength interferometry is to use the decimal multiple method and develop the concept of synthetic wavelength on it.
Multi-wavelength interferometric ranging (MWI) began with the dual-wavelength interference experiment conducted by American scientists Wyant and Polhemus in the early 1970s. This method uses two lasers with different wavelengths λ 1、 λ 2 Perform interference measurement for the unknown distance at the same time, and bring it into the measured distance D of the above formula:
To solve the two equations, there are:
Where is the synthetic equivalent wavelength, ms and ε S are respectively λ S interference fringe integer and decimal order.
If the composite wavelength is regarded as the ranging wavelength, the phase information corresponding to the unknown distance is the difference between the ranging phases of the original two wavelengths, so the unknown distance can be solved. The non-ambiguity range of distance measurement is extended to half of the synthetic wavelength. From the formula, the synthetic wavelength must be greater than λ 1 and λ 2。
In the same way, in order to give consideration to the measurement range and accuracy, the method can be further developed with the idea of multiple rulers. The multi-wavelength laser can be used to measure the distance at the same time to generate multi-level composite wavelengths of different scales. The long synthetic wavelength of * * * is used to achieve the measurement range of * * *, and the distance measurement result obtained is used as the distance reference value of the shorter synthetic wavelength, so as to solve the range measurement result of this level of synthetic wavelength, so as to realize the range measurement with large range and high precision using the small synthetic wavelength of * * * and * * *.
However, this method requires multiple wavelengths of laser, which means that multiple laser sources are required. Considering that each laser source needs its own laser frequency stabilization device, and multiple lasers need high-precision optical beam combination, the structure of the whole laser * * distance measurement system is relatively complex, and the reliability and accuracy of the system will inevitably be affected to a certain extent.
5. FM CW laser ranging
Frequency modulated continuous wave (FMCW) laser ranging is another interferometric method that can realize * * * measurement. It combines the advantages of optical interferometry and radio radar technology. The basic principle of FMCW measurement is to realize interferometry by modulating the frequency of laser beam. Generally, the laser whose frequency of output laser beam changes with time is used as the light source, and the Michelson interferometer is used as the basic interferometric optical path. The frequency difference information is generated according to the different optical path of the reference light and the measurement light. The distance information of the two beams can be obtained after extracting the signal and processing, and the measurement of the * * distance can be realized.
Take sawtooth modulation as an example. It is a sine signal whose frequency changes linearly with time in a sawtooth shape. The instantaneous frequency of the measured light and the reference light changes with time, as shown in Figure 7.
Set the frequency of the reference light as ft, the frequency of the measurement light as fr, the modulation bandwidth as ∆ F, the modulation period as T, and the distance as D. The measurement light will have a time delay relative to the reference light due to different transmission paths as τ， Where ft changes periodically between f0 and fm according to sawtooth wave, then the expression of ft and fr is as follows:
Then the generated beat signal is fIF:
So the measured distance:
The frequency modulated continuous wave laser ranging takes laser as the carrier, and all environmental interference only affects the light intensity of the measured signal, but not the frequency information. Therefore, it can obtain high ranging accuracy and strong ability to resist environmental light interference, and the accuracy can reach the micron level. It is currently a research hotspot in large size and high-precision measurement applications. However, this measurement method requires high stability and linearity of the laser beam frequency, which makes the realization of the system more complex, and the measurement range is limited by the period T.
6. Triangular laser ranging
Triangular laser ranging means that the light source, the measured object surface and the light receiving system form a triangular optical path together. The light emitted by the laser source is focused by the collimating lens and then incident on the measured object surface. The light receiving system receives the scattered light from the incident point and images it on the sensitive surface of the photoelectric detector. It is a measurement method to measure the moving distance of the measured object surface through the displacement of the light point on the imaging surface.
According to the angle relationship between the incident laser beam and the normal line of the measured object surface, there are generally two ranging methods: oblique and direct, as shown in Figure 8. In general, the direct laser triangulation method is simpler in geometric algorithm than the oblique laser triangulation method, and the error is relatively small, and the volume can be designed to be more compact and compact. In industry, the direct laser ranging method is often used.
Compared with phase laser ranging and frequency modulated continuous wave laser ranging, triangulation laser ranging has many advantages, such as simple structure, fast testing speed, flexible and convenient use, low cost, etc. However, the accuracy of triangulation laser ranging will gradually deteriorate with the increase of distance, and since in the laser triangulation system, the photoelectric detector receives the scattered light from the target surface to be measured, this ranging method is generally suitable for indoor close work, It is not suitable for working in outdoor or indoor strong light background. Therefore, the application range of triangulation laser ranging is mainly small displacement measurement, which is widely used in the measurement of object surface contour, width, thickness and other quantities, such as body model surface design, laser cutting, sweeping robot, etc. in the automobile industry.
