JP2537375B2 - Lightwave rangefinder - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical distance meter for photoelectrically measuring a linear distance between two points.

(Prior Art) Conventionally, intensity-modulated light is transmitted at a constant frequency, the light reflected by a target (hereinafter referred to as distance measuring light) is received, and the distance measuring light and the transmitted light or a reference optical path is passed. An optical wave range finder for measuring a distance from a phase difference of intensity modulation with light received by the above (hereinafter referred to as reference light) has been put into practical use. When the distance is measured from the distance measuring light and the transmitted light, a drift of the detection circuit etc. may appear as a measurement error.Therefore, the phase difference between the distance measuring light and the transmitted light and the phase difference between the reference light and the transmitted light may be detected. In general, the phase difference is obtained, and the two phase differences are subtracted to perform accurate distance measurement from the distance measuring light and the reference light. This utilizes the fact that the phase difference changes according to the distance. The phase difference is Δφ, the distance is D,
Assuming that the modulation frequency is f and the speed of light is C, the phase difference Δφ is Δφ
= 4πfD / C, and the distance D can be obtained by measuring the phase difference Δφ. In practice, modulated lights at two or more different frequencies are used for measurement, and each digit of the distance value is determined according to each resolution. FIG. 7 shows a block diagram of the conventional method.

(Problems to be Solved by the Invention) In the conventional lightwave distance type, since modulation is performed at a constant frequency, a narrow band filter must be used when separating the signal and noise contained in the received light, When the S / N is bad, for example, when the distance is long or the visibility is poor, it is difficult to extract only the received light signal. Therefore, in order to improve the S / N, it is necessary to increase the reflection efficiency by increasing the reflection area of the reflecting mirror. However, even in this case, it was often impossible to measure or there was an error in the measured value.

In addition, it is difficult to separate the fake signal due to the reflected light inside the rangefinder (the surface of the optical element, etc.), which is a major cause of the distance measurement error, from the signal for the side distance. Was needed.

In addition, the longest measurement distance is practically limited by the output of the emitting element, the sensitivity of the light receiving element, the power used, the size, etc. The development of high-performance, compact, and inexpensive rangefinders is highly desired in the fields of surveying and geodetic surveys.

The present invention was made in order to solve the problems found in the conventional light-wave distance meters as described above, the accuracy can be maintained even when the S / N is bad, the accuracy is maintained, and longer distances can also be measured. The purpose is to provide a light-wave rangefinder that enables it.

(Means for Solving Problems) In a light wave rangefinder that calculates the distance to the target from the phase difference between the transmitted light and the distance measuring light or the phase difference between the distance measuring light and the reference light, the intensity modulation It uses a spread spectrum method to demodulate the signal from the received light, and uses a PN code (pseudo noise code) to convert a signal (carrier wave) with a constant frequency
Light intensity-modulated by a signal (spread spectrum signal) spread over a wide frequency band obtained by PSK modulation (phase shift keying) is transmitted, and the autocorrelation characteristic of the PN code is used again from the received light. Synchronizes to the received signal and demodulates the carrier signal, and modifies and demodulates each carrier, PN
The distance is calculated by measuring the phase difference between the code and the PN code generation clock.

(Operation) When signals other than distance measuring light (or reference light) are input, they are not correlated and are spread over a wide frequency range, and only distance measuring light (or reference light) is demodulated.

(Example) An example will be described with reference to FIG. The output of the basic oscillator 1 (reference carrier signal 31) is input to the bandpass filter 2 and the frequency divider 3. The output of the bandpass filter 2 becomes a sine wave, and the output of the frequency divider 3 (reference PN code clock 32) in the PSK modulator 5
PSK modulation is performed by the PN code generated by the PN code generator 4. The spread spectrum signal thus obtained becomes the optical signal 41 through the drive circuit 6 and the light emitting element 7, and is transmitted through the objective lens L1. PN from PN code generator 4
The reference epoch pulse 33 is output for each cycle of the code. Distance measuring light reflected back by the target reflector 44, etc.
42 is photoelectrically converted by the light receiving element 11 and is input to the correlators 13a, 13b, 13c via the wide band amplifier 12a and the filter 12b. Reflecting mirrors M1 and M2 are arranged between the light emitting element 7 and the light receiving element 11 and the objective lens L1. When the mirrors M1 and M2 are moved to the optical path by an external operation, The emitted optical signal 41 is reflected by the mirrors M1 and M2,
The light enters the light receiving element 11 via the reference optical path indicated by the reference numeral 43. Each of the correlators 13a, 13b, 13c has a demodulation side PN code generator 19
From P (synchronous) E (leading 1/2 bit to P), L
The PN code of the same code sequence as that generated by the PN code generator 4 (1/2 bit behind P) is input,
The autocorrelation with the PN code of the received spread spectrum signal is taken. The autocorrelation function of the PN code has the characteristics shown in FIG.
When the phase difference of the PN code is within ± 1 bit, the carrier wave is demodulated to the output of the correlator, and the output becomes maximum when the phase difference is 0 (peak position in FIG. 2 (a)).

However, if the phase difference is ± 1 bit or more, the output is minimal and the carrier wave is not demodulated. If a signal other than the ranging light (in the case of the reference light, a signal other than the reference light), for example, an undesired signal such as noise is input, correlation cannot be obtained, and the undesired signal component has a wide frequency range due to the PN code. Since the amount of leakage to the output side of the correlator is very small, even if the transmitted signal and the undesired signal are mixed and input, only the carrier with good S / N is demodulated to the correlator output. . In this way, by using the autocorrelation of the PN code, it is possible to realize an efficient filter that extracts only the desired signal. The demodulation of the carrier wave described above is performed by the correlator 13a, and is output to the phase difference measuring device 20 via the amplifier 14a and the bandpass filter 14b. When used for distance measurement, both the ranging light (or reference light) and demodulation side PN are used for carrier demodulation.
It must be synchronized so that the phase difference between the codes becomes 0, and therefore the correlators 13b and 13c are used for phase synchronization of the PN code and tracking of the received signal. This correlator 13
Even in b and c, the autocorrelation between the PN code of E and L and the PN code of the received signal is taken, and their correlation functions are ±± with respect to that of P as shown in FIGS. 2 (b) and (c). Shifted by 1/2 bit. The output amplitudes of the correlators 13b and 13c are input to the central processing unit 17 via the amplitude detectors 15b and 15c and the A / D converter 16 and compared with the threshold voltage. The central processing unit 17 shifts the phase of the PN code generated by the PN code generator 19 so that both of them are equal to or higher than the threshold voltage. As a result, the PN code of P has a phase within the range R shown in FIG. 2 with respect to the PN code of the received signal, that is, within ± 1/2 bit for perfect synchronization.

Next, in order to achieve perfect synchronization, the central processing unit 17
The difference between the output amplitudes of b and 13c (V _{EL in} FIG. 2 (d)) is obtained, and the numerically controlled oscillator 18 is controlled as soon as the value becomes S (0).

When perfect synchronization is obtained as described above, the PN code generator 19 outputs the PN code P synchronized with the PN code of the received signal.
Then, the demodulated PN code clock 35 and the demodulated epoch pulse 36 are obtained accordingly, and the demodulated carrier signal 34 is obtained from the bandpass filter 14b. Then, the reference signal 31,
The phase difference of these demodulated signals 34, 35, 36 with respect to 32, 33 is measured by the phase difference measuring device 20 and input to the central processing unit 17 to calculate the distance. Demodulated signals 34, 35, 36 are reference signals
The frequencies fc, f _PN , and f _EP are the same as those of 31, 32, and 33, respectively, but the phase changes according to the optical path length. The phase difference between the reference light signal and the reference signal respectively _{φc, φ PN, φ EP,} φ'c the phase difference of the distance measuring light and reference signals, respectively, φ _'PN,
If φ ′ _EP , the difference of each phase difference Δφc = φ′c−φc, Δ
The distance D to the target can be obtained from φ _PN = φ ′ _PN −φ _PN and Δφ _EP = φ ′ _EP −φ _EP . That is, Dc = △ φc C / 4πfc, D _PN = △
φ _PN C / 4πf _PN , D _EP = Δφ _EP C / 4πf _EP , where the frequencies of the reference carrier signal 31, the reference PN code clock 32, and the reference epoch pulse 33 are fc> f _PN > f _EP , Dc ≦ C / Fc, D _PN ≤C /
Since f _PN , D _EP ≤ C / f _EP , if fc, f _PN , and f _EP are selected appropriately, the lower, middle, and upper digits of D can be determined from Dc, D _PN , and D _EP , respectively. Yes, depending on the resolution Dc, D _PN , D
It is also possible to decide with only one or two _EPs , and the distance can be measured.

In this way, it is possible to measure the distance from the phase difference between the distance measuring light and the back-emitted light to the target point. Further, the mirrors M1 and M2 are moved to the optical path to obtain the phase difference between the reference light and the outgoing light that has passed through the reference optical path 43, and the target point is determined from the difference between the phase difference between the distance measuring light and the outgoing light. The distance can be measured up to, and in this case, an accurate value can be obtained because there is no error in the signal propagation circuit. In addition to the sync tracking system using the delay lock loop described above, there is a sync tracking system using the tau dither lock loop. However, in this case, the S / N is slightly reduced.

FIG. 3 is a block diagram of a portion for controlling synchronization tracking showing another embodiment of the present invention.

In the first embodiment, the light intensity-modulated by the spread spectrum signal obtained by PSK-modulating the carrier wave by the PN code is transmitted, and the signal obtained by receiving the light has the characteristic of the autocorrelation of the PN code. PN for demodulation by using
The code is synchronized, the carrier signal with good S / N is demodulated, and the distance measurement can be performed by examining the phase of each signal 31-36 after synchronization. The device 17 is designed to perform digitally, but in this embodiment, it is designed to be performed analogically.

In FIG. 3, the outputs of the amplitude detectors 15b and 15c are compared with the threshold voltage by the threshold detection circuits 51b and 51c, and the respective outputs are input to the synchronization determination circuit 52 and the range R (second (Fig.) A signal is sent out as to whether or not it entered. If it is not within the predetermined range R, the synchronization determination circuit 52 outputs it to control the voltage controlled oscillator 53 and the PN code generator to shift the phase of each P, E and L PN code.

Then, within the range R, the outputs of the amplitude detectors 15b and 15c are subtracted (V _EL ) by the subtraction circuit 53a and then input to the voltage controlled oscillator 53, and the subtracted value is S shown in FIG. 2 (d). The phase of the PN code is controlled so that it becomes a point. In this way perfect synchronization is obtained and the distance is determined as described above.

FIG. 4 shows a method of acquiring a synchronization discrimination signal according to another embodiment of the present invention. This is a method in which the amplitude of the carrier signal demodulated by the correlator 13a is detected and compared with the threshold voltage to determine whether it is within the range R to perform control. Control can be performed either in analog or digital.

This method can increase the threshold voltage as compared with the above two examples. In the previous two examples, the threshold voltage must be close to V _o because the voltages of V _E and V _L are detected to determine whether or not it is within the range R of FIG. 2, but in this example Then, the output of the amplitude detector 15a, that is, V
_Since the voltage of _P is detected, the threshold voltage V
It may be the _1/2. This has the advantage that it can be performed correctly near both ends of the range R.

FIG. 5 is a block diagram showing a correlator portion of another embodiment of the present invention. The correlator in this example is called a heterodyne type and is composed of two stages of correlators, and the first correlator 13'a, 1 '
3'b and 13'c signals of constant frequency f _c + f _d and P, E, L
And the PN code of
Input to a, 13b and 13c. By doing this, the center frequencies at the inputs and outputs of the correlators 13a, 13b, 13c are changed from f _c to f _d.
Is converted to. Also, for distance measurement, a reference signal 62 of f _d is made by the frequency converter 61 from the reference oscillator 1 and demodulated.
This is done by finding the phase difference between the f _{d and} the signal 63.

Normally, f _c > f _d , so the frequency of the signal is low, and the circuit design after the correlator becomes simple.

FIG. 6 is a block diagram showing a portion for obtaining a demodulated carrier signal 34 according to another embodiment of the present invention. In this example, the output of the frequency f _c is obtained by the oscillator 18 or 53, and the output is input to the frequency divider 3 to obtain the demodulated PN code clock 35. When perfectly synchronized with the received signal, the output of the oscillator 18 or 53 becomes the demodulated carrier signal 34, which can be used as a signal for distance measurement.

(Effect of the Invention) As described above, according to the present invention, the intensity of the transmitted light is modulated by the spread spectrum signal obtained by PSK modulating the carrier signal with the PN code, and the autocorrelation characteristic of the PN code is used. Since the received signal is synchronized and the carrier signal is demodulated, it is possible to demodulate a signal having a very good S / N even when the signal is buried in noise. As a result, it is possible to measure distances even under long-distance or poor visibility conditions, and at the same time, it is possible to extend the longest measurement distance. can do.

In addition, although the selection of the reference light and the distance measuring light is currently performed by mechanically switching the optical path, due to the characteristics of the spectrum expansion method (the property that only a synchronized signal can pass through the correlator), The reference light, the distance measuring light, or the internal reflected light is discriminated from the phase difference of the synchronizing signal, that is, the phase difference between the reference light, the internal reflected light (if necessary) and the reference signal (because it is geometrically determined. It will be a certain value)
It is possible to determine whether the synchronized signal is the reference light, the internal reflection light, or the distance measuring light by comparing the phase difference of the synchronized signal with the phase difference obtained in advance. If the synchronization is shifted so that the desired signal can be obtained, the reference light and the distance measuring light can be selected. That is, the desired signal can be obtained even when the reference light, the internal reflected light, and the distance measuring light are constantly received without using the optical path switching mechanism, and the control is performed so as to synchronize with the desired signal. This eliminates the need for mechanical parts such as an optical path switching mechanism and eliminates the need for special measures such as antireflection treatment, which is very advantageous in terms of cost.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a PN code autocorrelation function diagram of a delay locked loop, and FIG. 3 is a block diagram of an embodiment in the case of analog synchronous tracking.
4 is a block diagram showing another method for determining synchronization, FIG. 5 is a block diagram showing a heterodyne correlator, and FIG. 6 is a block diagram showing a portion for obtaining a demodulated carrier signal according to another embodiment of the present invention. FIG. 7 is a block diagram of a conventional method. 1 ... Reference oscillator, 2 ... Bandpass filter 3 ... Divider, 4, 19 ... PN code generator 5 ... PSK modulator, 6 ... Light emitting element drive circuit 7 ... Light emitting element, 11 ... Photodetector 12 …… Broadband amplifier and filter 13a, b, c …… Correlator 14 …… Amplifier and bandpass filter 15a, b, c …… Amplitude detector 16 …… A / D converter, 17 …… Center Processor 18 …… Numerically controlled oscillator, 20 …… Phase difference measuring instrument 21 …… Display, 31 …… Reference carrier signal 32 …… Reference PN code clock 33 …… Reference epoch pulse 34 …… Demodulation carrier signal 35 …… Demodulated PN code clock 36 …… Demodulated epoch pulse 41 …… Outgoing light, 42 …… Distance measuring light 43 …… Reference optical path, 44 …… Target reflector 51b, c …… Threshold detection circuit 52 …… Synchronous discrimination circuit, 53 ...... a voltage-controlled oscillator 61, 64 ...... frequency converter 62 ...... f _d reference signal, 63 ...... f _d demodulated signal 64 ... Frequency sorter

Claims (2)

(57) [Claims] 1. A carrier signal of the outgoing light, comprising: an optical sending means for intensity-modulating light by a carrier signal and sending the light toward a target point; and an optical receiving and demodulating means for receiving and demodulating reflected light at the target point. In the lightwave rangefinder for calculating the distance to the target point from the phase difference between the carrier signal of the reflected light and the reflected light, the light transmitting means uses a PN code (pseudo noise code) for the carrier signal of a constant frequency.
The intensity-modulated light is transmitted by a signal (spread spectrum signal) spread over a wide frequency band obtained by PSK modulation (phase shift keying) by means of the PN code based on the received light. An optical distance meter using a spread spectrum method for demodulating a carrier signal by the autocorrelation characteristic of 2. A reference optical path is provided between the light transmitting means and the light receiving and demodulating means, and a distance from a phase difference between a carrier signal of reflected light and a carrier signal of light passing through the reference optical path to a target point. The lightwave rangefinder according to claim 1, characterized in that