JP6653052B2 - Laser ranging device and laser ranging method - Google Patents

Description

  The present invention relates to a laser distance measuring device and a laser distance measuring method.

  2. Description of the Related Art Laser ranging technology that irradiates a measurement target with laser light and receives reflected light to measure a distance has been used in various fields (for example, Patent Documents 1 and 2). In particular, it has been widely used in the field of factory automation (FA), and recently, a laser radar for avoiding collision safety of automobiles has been put into practical use, and further expansion of demand is expected.

  Also, in the construction market, the spread of ICT construction (information-oriented construction) has been promoted, and it has become necessary to measure three-dimensional data before and after construction at construction sites, and the demand for three-dimensional laser scanners for construction applications is growing. ing. 3D laser scanners are not only used in civil engineering and construction, but also in a wide range of applications, including factories and plants, preservation and storage of cultural properties, product inspection, reverse engineering, crime and accident site investigation and analysis, forest research, agriculture, and virtual reality. Etc. are used in various fields.

  In the laser distance measurement technique, particularly for medium to long distance measurement, two distance measurement methods are broadly classified. One is a TOF (Time Of Flight) method, which irradiates a measurement object with a laser pulse and measures a time until the pulse returns, thereby calculating a distance. The other is a phase difference method in which laser light is modulated, and a distance to a target is calculated from a phase difference between modulated signals of transmission light and reception light.

  It is difficult to measure time accurately in the TOF method, and it is said that the phase difference method is superior in terms of accuracy. However, in contrast to the TOF method in which the distance can be obtained with a single pulse, the phase difference method requires a laser irradiation time. For this reason, in the phase difference method, when high-power laser output is required for long-distance measurement, it is difficult to satisfy both accuracy and safety standards. However, at the present time, it is difficult to directly measure the arrival time of light with high accuracy, and therefore, a phase difference method is often used in high-accuracy distance measurement.

  The phase difference method is also roughly classified into two methods. One is a laser wavelength modulation method, and the other is a laser intensity modulation method. The laser wavelength modulation system achieves a very high accuracy of submicron for industrial use or the like, but is expensive and large in size as compared with intensity modulation. For this reason, a laser intensity modulation method is mainly used for distance measurement over a long distance such as a three-dimensional laser scanner.

  As a modulation waveform in the laser intensity modulation method, a pulse is used instead of a sine wave. The reason is that the use of a sine wave causes the laser to emit light constantly.For example, when emitting laser light into space like a three-dimensional laser scanner, there is a possibility that it will be in the human eye and safety This is because there is a problem. As the modulation frequency, a plurality of frequencies are used, such as a low frequency for measuring a long distance, and a high frequency for obtaining high accuracy (for example, Patent Document 2).

JP 2011-511280 A JP 2011-522216 A

  In the laser intensity modulation method, if the modulation frequency is increased, it becomes difficult to process the received light signal at that frequency. As a method of converting a high frequency to a low frequency, a heterodyne method in which a modulation signal is multiplied by a local oscillation signal having a slightly different frequency to obtain a difference signal is well known. However, since the heterodyne method is analog processing, the circuit scale becomes large.

  An object of the present invention is to solve such a problem and to provide a laser distance measuring apparatus capable of directly converting a received light signal into a digital signal and processing the received light signal at a frequency lower than the modulation frequency of the received signal.

  The laser distance measuring apparatus according to the present invention includes: a light emitting unit that generates an intensity-modulated laser light; an optical unit that irradiates an output light of the light emitting unit to an object to be measured; and a reference light that branches the output light of the light emitting unit. A reference light receiving means for receiving the return light from the object to be measured, a reference light signal output from the reference light receiving means, and a return light signal output from the return light receiving means. A / D converter for analog-to-digital conversion, and a signal analyzer for analyzing a phase difference between a reference light signal and a return light signal based on an output of the A / D converter to obtain a distance to a measurement object. A modulation frequency of the laser beam, wherein a plurality of frequencies are partially higher than the Nyquist frequency of the A / D converter, and a modulation frequency lower than the Nyquist frequency and its harmonics; The modulation frequency higher than the Nyquist frequency is set to change with time at a value that does not interfere with the frequency folded at the Nyquist frequency, and the signal analysis circuit is folded at the modulation frequency lower than the Nyquist frequency and the Nyquist frequency. Analyzing each signal of the frequency.

That is, in the present invention, the light receiving signal is directly A / D converted. If the Nyquist frequency _F S / 2 as the modulation frequency _{f H (F S} is the sampling frequency) using the frequency higher than, remains in the not sampled in the _frequency signal F S -f _H, i.e. the Nyquist frequency _F S / 2 (Aliased) (hereinafter referred to as an “alias signal”). Since the phase information of the original signal remains in this alias signal, highly accurate ranging can be performed using this. However, it is necessary to set each frequency used for modulation so that the modulation frequency lower than the Nyquist frequency F _S / 2 and its harmonics do not interfere with the alias signal.

  According to the second aspect of the present invention, an intensity-modulated laser beam is generated, the laser beam is branched, and one of the laser beams is irradiated on the object to be measured, the other is received as reference light, and the laser beam is returned from the object to be measured. In a laser ranging method for receiving light, directly A / D converting respective light reception signals of a reference light and a return light, and analyzing a phase difference between the two light reception signals to obtain a distance to an object to be measured. A plurality of frequencies are used as the modulation frequency of the laser light, and some of the frequencies are higher than the Nyquist frequency of the A / D conversion. The frequency turned back by the frequency is set so that it does not interfere with each other, and it is set to change over time, and the modulation frequency and the Nyquist frequency lower than the Nyquist frequency are set. Characterized by analyzing each of the signals in folded frequency.

FIG. 1 is a block diagram illustrating a configuration of a laser distance measuring apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a phase difference between the reference light signal and the return light signal. FIG. 3 is a diagram illustrating the relationship between a modulation waveform and a spectrum.

  FIG. 1 is a block diagram showing a configuration of a laser distance measuring apparatus according to one embodiment of the present invention. This laser distance measuring device includes a signal processing circuit 1 for performing signal processing for distance measurement, a laser light emitting module 2 as light emitting means for generating an intensity-modulated laser light, and a laser light emitting module 2 for measuring output light. A parallel light lens 3 as an optical means for irradiating the object 4, a light receiving module 6 as a reference light receiving means for splitting the output light of the laser light emitting module 2 and receiving it as reference light, and a condensing lens 5 And a light receiving module 7 as a return light receiving means for receiving the returned light from the measurement object 4. The reference light signal output from the light receiving module 6 and the return light output from the light receiving module 7 are provided in the signal processing circuit 1. An A / D converter (ADC) 13 for directly converting an optical signal into an analog-to-digital signal, and a reference optical signal based on the output of the A / D converter 13 By analyzing the phase difference between the return light signal, and a signal analysis circuit 14 to determine the distance to the measurement object. The signal processing circuit 1 further includes a reference clock generator 11 that generates a reference clock for operating each unit in the signal processing circuit 1, and a modulation that generates a modulation waveform and supplies the modulation waveform to the laser emission module 2 as a modulation signal. And a waveform generator 12.

FIG. 2 is a diagram illustrating a phase difference between the reference light signal and the return light signal. Assuming that the intensity modulation frequency is f, one cycle of the intensity modulation corresponds to λ = c / f. c is the speed of light. Therefore, assuming that the phase of the return light is shifted from the phase of the reference light signal by Δφ, the reciprocating travel distance of the laser light is (Δφ / 2π) λ. That is, the distance d to the measurement object is
d = (Δφ / 2π) λ ÷ 2 = (Δφ / 2π) (c / f) λ ÷ 2 = Δφc / 4πf
Required by For example, when f = 1 MHz, and c = 3 × 10 ⁸ m / s, λ = 300 m. It is not practical to detect the phase difference outside the range of 2π, and the measurement range of the distance to the measurement object is limited to half the wavelength λ. On the other hand, the higher the modulation frequency f, the shorter the wavelength λ, and the higher the measurement accuracy. Therefore, a plurality of frequencies are temporally switched and used as the modulation frequency f, and are selectively used as “large ruler”, “medium ruler”, and “fine ruler”. Thereby, both the measurement range and the measurement accuracy can be improved.

The problem here is that while the higher the modulation frequency f, the higher the measurement accuracy, the higher the modulation frequency f, the higher the processing speed of the processing system or the higher the frequency. It is necessary. In the present invention, when A / the Nyquist frequency D converter F _{S /} 2 (F _S is the sampling frequency) and inputs the signal of frequency higher than, folded at the Nyquist frequency (which is aliased) signal frequency is generated Take advantage of what you do. Generally, such signals should be removed as alias noise. In the present invention, this signal is used not as noise but as a signal for analyzing a phase difference. For this reason, this signal is specifically referred to herein as an "alias signal".

That is, in the embodiment illustrated in FIG. 1, the modulation waveform generator 12 determines that the plurality of frequencies are partially higher than the Nyquist frequency F _S / 2 of the A / D converter 13 and that the Nyquist frequency F _S / a low modulation frequency and its harmonics than 2, at the Nyquist frequency F _{S /} higher modulation frequencies than 2 do not interfere with the Nyquist frequency F _{S /} 2 and frequencies folded in each other value, is set to change temporally The modulated waveform is supplied to the laser light emitting module 2 as a modulation signal. Further, the signal analysis circuit 14 analyzes each signal of the modulation frequency lower than the Nyquist frequency F _S / 2 and the frequency folded back at the Nyquist frequency.

FIG. 3 is a diagram illustrating the relationship between a modulation waveform and a spectrum. Modulation waveform generator 12, for each period of the frequency f _1, sequentially generates a plurality of pulse trains having different frequencies. In the example of FIG. 3 (a), for each cycle of frequency _{f 1,} sequentially generates pulses at a frequency _f 2, _{f h.} Frequency _{f h} is assumed higher than the Nyquist frequency _F S / 2 of the A / D converter 13. At this time, the output of the A / D converter 13, as shown in FIG. 3 (b), the frequency _f 1, in addition to _{f 2,} the frequency _{f h} is the Nyquist frequency _F S / 2 in folded frequency _{F S} alias signal of the -f _h occurs. Since this alias signal is also utilized as a signal for analyzing the phase difference, the value of the frequency f _1, f _2, f _h is the frequency f _1, f ₂ and its harmonics does not interfere in the alias signal . In addition, the output of the A / D converter 13 includes harmonics of the modulation signal.

here,
_{F S -f h = n k ×} f k
The number becomes a _{n k,} where k = 1,2, ..., to define. When n _k is an _integer, for an integer multiple of F S -f _h is _{f k,} overlapping harmonics _{f k} is the _F S -f _h, interference _occurs, error in the phase of F S -f _h is Occurs. On the other hand, if _{n k} is a _non-integer, F S -f _h is not an integer multiple of _{f k,} will not be harmonics of _{f k} interferes with _F S -f _h. If F _S -f _h is than condition is met that rather than an integral multiple of _{f k, f k (k =} 2, ...) and _{f h} may be an integer multiple of _{f 1.}

As a specific example,
F _S = 1 GHz,
f ₁ = 1 MHz,
f ₂ = 20 MHz,
f _h = 600 MHz
Consider the case (frequency example 1). in this case,
_{F S -f h = 400f 1 =} 30f 2
_Next, harmonics of f 1, _{f 2} is superimposed over the _F S -f _h, so that the error occurs in the phase of F S -f _h.

In contrast,
F _S = 1 GHz,
f ₁ = 1.2 MHz,
f ₂ = 24 MHz,
f _h = 600 MHz
(Frequency example 2)
F _S -f _h = 333.333... × f ₁ = 16.666... × f _2
Next, f 1, _{f 2} harmonics 2.4GHz _(F S -f _h and _f 1, the lowest frequency among the frequencies overlapping the folded result _F S -f _h at the Nyquist frequency of the common multiple of _{f 2} If it is less than) it will _not interfere against F S -f _h.

_However, f 1, higher harmonics of _{f 2,} specifically, 2.4GHz, when such 3.6 GHz, their alias _frequencies, but becomes coincident with the F S -f _h, A / D avoided in front of the transducer 13 to 600MHz it is passed, by inserting the LPF (low pass filter) to cut off the more harmonics 2.4GHz, interference against higher harmonics of _F S -f _h It is possible to Alternatively, the LPF can be omitted if a light receiving module whose band is appropriately restricted is used.

On the other hand, in the frequency Example 1, the harmonics lower than f _h since may interfere in F _S -f _h, techniques for blocking higher harmonics than f _h only LPF can not be applied. Accordingly, the present invention employing a combination such as frequency Example 2, f _1, harmonic f ₂ can be eliminated from interfering against F _S -f _h, can cause errors are eliminated in the phase The Rukoto.

In the embodiment described above, the distance can be measured by the laser intensity modulation method at a modulation frequency higher than the Nyquist frequency F _S / 2 using the existing A / D converter 13. By appropriately setting the modulation frequency used at that time, interference between digitally converted signals can be prevented, and highly accurate measurement can be performed.

1 signal processing circuit 2 laser light emitting module (light emitting means)
3 Parallel light lens (optical means)
5 Condensing lens 6 Light receiving module (reference light receiving means)
7. Light receiving module (return light receiving means)
11 Reference clock generator 12 Modulation waveform generator 13 A / D converter 14 Signal analysis circuit

Claims (2)

Light-emitting means for generating intensity-modulated laser light,
Optical means for irradiating the output light of the light emitting means to the object to be measured,
Reference light receiving means for branching the output light of the light emitting means and receiving it as reference light,
Return light receiving means for receiving return light from the measurement object,
An A / D converter for directly analog-to-digital converting the reference light signal output from the reference light receiving means and the return light signal output from the return light receiving means, respectively;
A signal analysis circuit that analyzes a phase difference between the reference light signal and the return light signal based on an output of the A / D converter to obtain a distance to the measurement target;
With
As the modulation frequency of the laser beam, a plurality of frequencies are partially higher than the Nyquist frequency of the A / D converter, and are lower than the Nyquist frequency and higher harmonics thereof, and higher than the Nyquist frequency. The modulation frequency is set so that the frequency folded back at the Nyquist frequency does not interfere with each other, and changes with time.
The signal analysis circuit analyzes each signal of the modulation frequency lower than the Nyquist frequency and the frequency folded at the Nyquist frequency.
A laser ranging device characterized by the above-mentioned. Generate intensity modulated laser light,
The laser beam is branched, one is irradiated on the object to be measured, and the other is received as reference light,
Receiving return light from the measurement object,
In the laser ranging method, the light receiving signals of the reference light and the return light are directly A / D converted, and the phase difference between the two light receiving signals is analyzed to determine the distance to the object to be measured.
As the modulation frequency of the laser light, a plurality of frequencies, some of which are higher than the Nyquist frequency of the A / D conversion, and whose modulation frequency is lower than the Nyquist frequency and its harmonics, and which are higher than the Nyquist frequency The frequency is set so that the frequency turned back at the Nyquist frequency does not interfere with each other and changes with time,
Analyzing each signal of the modulation frequency lower than the Nyquist frequency and the frequency folded back at the Nyquist frequency,
A laser ranging method characterized by the above-mentioned.

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