JPH0196580A - Range finder using light wave - Google Patents

Abstract

PURPOSE:To take a measurement even when the S/N is inferior, to maintain accuracy and to measure a longer distance by employing a spectrum spread system for intensity modulation on sent light and the demodulation of a signal from received light. CONSTITUTION:Light is sent out which is intensity-modulated with a signal (spectrum spread signal) spread over a wide frequency range obtained by imposing PSK modulation (phase shift modulation) upon a signal (carrier) of constant frequency with PN codes (pseudo noise code). Then synchronization with a received signal and the demodulation of the carrier signal are performed with the received signal by utilizing the autocorrelation characteristics of the PN codes again. Then respective carriers, the PN codes and the phase difference of a PN code generation clock at the time of modulation and demodulation are measured to calculate a distance. Further, signals other than distance measurement light (or reference light) are inputted, they are not correlated and spread over a wide frequency range and only the distance measurement light (or reference light) is demodulated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a light wave distance meter that photoelectrically measures the straight distance between two points.

(Prior art) Conventionally, light is intensity-modulated at a constant frequency and sent out, the light reflected by the target (hereinafter referred to as distance measurement light) is received, and the distance measurement light and the transmitted light or reference optical path are passed through. A light wave distance meter has been put into practical use that measures distance from the phase difference of intensity modulation with the light received by the light (hereinafter referred to as reference light). When measuring the distance from the distance measuring light and the transmitted light, measurement errors may occur due to drift in the detection circuit, so the phase difference between the ranging light and the transmitted light and the phase difference between the reference light and the transmitted light are Generally, accurate distance measurement is performed from the distance measuring light and the reference light by subtracting the phase difference between the two.

This takes advantage of the fact that the phase difference changes depending on the distance.If the phase difference is Δφ, the distance is the D1 modulation frequency f, and the speed of light is C, the phase difference Δφ can be expressed as Δφ−4πfD/C. The distance can be determined by measuring the phase difference Δφ. In reality, two or more modulated lights at different frequencies are used for measurement, and each digit of the distance value is determined according to the respective resolutions. FIG. 7 shows a block diagram of the conventional system.

(Problem to be solved by the invention) Conventional light wave distance meters perform modulation at a constant frequency, so
A narrowband filter must be used to separate the signal and noise contained in the received light, and if the S/N is poor, for example, at a long distance or when visibility is poor, the signal of the received light may be It is difficult to extract only one. Therefore, in order to improve the S/N ratio, it is necessary to increase the reflection efficiency by, for example, increasing the reflection area of the reflection mirror. However, even with this method, it was often impossible to determine 1llJ or the measured value had an error.

In addition, the interior of the rangefinder (
It is difficult to separate false signals caused by light reflected from the surface of an optical element (such as the surface of an optical element) from signals for distance measurement, and countermeasures such as anti-reflection treatment are required.

In addition, the longest measurement distance depends on the output of the light emitting element and the sensitivity of the light receiving element.
At present, there are practical limits on power usage, dimensions, etc., and the limits have almost been reached.The development of a distance meter that is compact, inexpensive, and capable of measuring longer distances with high precision is now an important issue for surveyors. , is highly desired in fields such as geodesy.

The present invention was made in order to solve the problems seen in the conventional light wave distance meter as described above, and it can perform measurements even when the S/N is poor, maintains accuracy, and can also measure longer distances. The aim is to provide a light wave distance meter that makes it possible to

(Means for solving the problem) In a light wave distance meter that calculates the distance to a target point from the phase difference between the transmitted light and the ranging light, or the phase difference between the ranging light and the reference light, intensity modulation of the transmitted light is used. A spread spectrum method is used to demodulate the signal from the received light, and a wide frequency range obtained by PSK modulating (phase shift keying) a constant frequency signal (carrier wave) using a PN code (pseudo-noise code). Signal spread over a band (spread spectrum signal)
The received light is then synchronized with the received signal and the carrier signal is demodulated using the autocorrelation characteristics of the PN code, and each carrier wave, PN code, and PN code are generated during modulation and demodulation. The distance is calculated by measuring the phase difference between the clocks.

(Function) When signals other than the ranging light (or reference light) are input, these signals are not correlated and are spread over a wide frequency range, and only the ranging light (or reference light) is demodulated.

(Example) This will be explained with reference to FIG. The output of the reference oscillator 1 (reference carrier signal 31) is input to a bandpass filter 2 and a frequency divider 3. The output of the bandpass filter 2 becomes a sine wave, which is output from the frequency divider 3 by the PSK modulator 5 (reference PN code clock 32).
The signal is modulated into PS by the PN code generated by the PN code generator 4. The spread spectrum signal obtained in this way passes through the drive circuit 6 and the light emitting element 7, and is then sent to the optical signal 4.
1 and is sent out through the objective lens LL. The PN code generator 4 outputs a reference epoch pulse 33 for each period of the PN code. The distance measuring light 42 reflected and returned by the target reflecting mirror 44 is photoelectrically converted by the light receiving element 11, and is input to the correlators 13a Sb, c through the broadband amplifier 12a and filter 12b. Light emitting element 7
Interlocking reflecting mirrors M1 and M2 are arranged between the light receiving element 11 and the objective lens LL, and when the mirror ML M2 is moved onto the optical path by external operation, the optical signal output from the light emitting element 7 is Reference numeral 41 indicates the old mirror, which is reflected by M2 and passes through the reference optical path indicated by reference numeral 43 to the light receiving element 11.
to go into. In the correlators 13a, b, and c, P on the demodulation side is
The N code generator 19 generates P (synchronous), E (172 bits ahead of P), L (1/2 bit behind P) of the same code sequence as that generated by the PN code generator 4. A PN code is input, and an autocorrelation with the PN code of the received spread spectrum signal is taken. P
The autocorrelation function of the N code has the characteristics shown in Figure 2 (a), and if the phase difference between the two PN codes to be correlated is within ±1 bit, the carrier wave will be demodulated in the output of the correlator. , the output reaches its maximum when the phase difference is 0 (the peak position in FIG. 2(a)).

However, if the phase difference is ±1 bit or more, the output becomes minimum and the carrier wave is not demodulated. If a signal other than the ranging light (in the case of a reference light, a signal other than the reference light), for example, an undesired signal such as noise, is manually generated, the correlation will not be established, and the undesired signal component will have a wide frequency range due to the PN code. Since the amount leaking to the correlator output side becomes very small, even if a mixture of the sending signal and the undesired signal is input, only the carrier wave with a good S/N will be demodulated to the correlator output. . By utilizing the autocorrelation of the PN code in this way, it is possible to realize an efficient filter that extracts only the desired signal. The carrier wave demodulation described above is performed by a correlator 13a, an amplifier 14a, and a bandpass filter 14b.
The signal is then output to the phase difference measuring device 20. In order to demodulate the carrier wave used for distance measurement, it is necessary to synchronize the distance measurement light (or reference light) and the PN code on both the demodulation side so that the phase difference becomes 0. Therefore, the correlator 13b, c.
is used for phase synchronization of the PN code and tracking of received signals. This correlator tab Sc also takes the autocorrelation between the PN codes of E and L and the PN code of the received signal, and their correlation function is compared to that of P as shown in Figure 2 (b) and (C). Each is shifted by ±1/2 bits. The output amplitude of the correlator 13b s e is determined by the amplitude detector 15
b Sc SA/D converter 16 to central processor 1
7 and is compared with a threshold voltage. The central processor 17 shifts the phase of the PN code generated by the PN code generator 19 so that both codes are equal to or higher than the threshold voltage. As a result, the PN code of P is in phase with respect to the PN code of the received signal in the range R1 shown in FIG. 2, ie, ±1/21/2 bits relative to perfect synchronization.

Next, in order to achieve complete synchronization, the central processor 17
Difference in output amplitude between b and 13c (Fig. 2(d) Vp-t
), and the numerically controlled oscillator 18 is controlled so that the value becomes 5 (0).

When complete synchronization is obtained as described above, the PN code generator 19 generates a PNN synchronized with the PN code of the received signal.
A code is obtained, and a demodulated PH code clock 35 and a demodulated epoch pulse 3B are obtained accordingly, and a demodulated carrier signal 34 is obtained from the bandpass filter 14b. Then, the phase difference between these demodulated signals 34, 35, and 36 with respect to the reference signal 31, 32, and 33 is measured by the phase difference measuring device 20, and is input to the central processor 17 to calculate the distance. The demodulated signal 34.35.3B is the reference signal 81.32.
33 and the same frequency re, rpNS1'EP, respectively.
However, the phase changes depending on the optical path length. The phase difference between the reference optical signal and the standard signal is φC1φPNsφE, respectively.
The phase difference between the P% ranging optical signal and the reference signal is φ°C1, respectively.
φPNsφ゛I! , then the difference between each phase difference Δφe=φ
°C−φeSΔφPN″φ PN−φPNSΔφEP−
The distance to the target is determined from φBP-φgp.

That is, Dc=ΔφcC/4πfc, DPN−ΔφPNC/
4 πrpHSDap-AφgpC/ 4 yr
fgps Here, each frequency of the reference carrier signal 31, reference PN code clock 32, and reference epoch pulse 33 is f
'c>rpN>fgp, DC≦C/Fc S
DPN≦C/ f'pNs Dgp≦C/ fgp
If you select fcStpNs f'gp appropriately, you can determine the lower, middle, and upper digits of D from D e SD PNSD EP, and depending on the resolution, one or two digits of DCSDPN% DEP can be determined. It is also possible to determine the distance by itself, and the distance can be measured.

In this way, the distance to the target point can be measured based on the phase difference between the distance measuring light and the transmitted light. Further, by moving the mirrors M1 and M2 onto the optical path, the phase difference between the reference light and the transmitted light that has passed through the reference optical path 43 is determined, and the target point is determined from the difference between the phase difference between the ranging light and the transmitted light. It is possible to measure distances up to
In this case, since errors within the signal propagation circuit are eliminated, accurate values can be obtained. In addition to the synchronization tracking system using the delay lock loop described above, there is also a synchronization tracking system using a delay 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 carrier wave is PSK based on the PN code.
Send out light whose intensity is modulated by the modulated spread spectrum signal, and apply PN to the signal obtained by receiving the light.
Distance measurement can be performed by synchronizing the PN code for demodulation and demodulating the carrier signal with good S/H by using the autocorrelation characteristic of the code, and checking the phase of each signal 31 to 3B after synchronization. In this configuration, synchronization-related control is performed digitally by the central processor 17, but in this embodiment, it is performed analogously.

In FIG. 3, the output of the amplitude detector 15b Sc is compared with a threshold voltage by a threshold detection circuit 51b Sc, and each output is input to the synchronization determination circuit 52, and the range R (FIG. 2) A signal is sent out to indicate whether or not someone has entered the room. If it is not within the predetermined range R, the synchronization determination circuit 52 outputs an output from the voltage controlled oscillator 53, PN
The phase of each PN code of P, E, and L is shifted by controlling the coco generator 19.

Once within the range R, the output of the amplitude detector 15b Sc is subtracted (VE-L) by the subtraction circuit 53a and then input to the voltage controlled oscillator 53, and the subtracted value is shown in FIG.
) The phase of the PN code is controlled so that it becomes the eight points shown in ). In this way, perfect synchronization is obtained and the distance can be determined as described above.

FIG. 4 shows a method for acquiring a synchronization determination signal according to another embodiment of the present invention. In this method, the amplitude of the carrier wave signal demodulated by the correlator 13a is detected and compared with a threshold voltage to determine whether it falls within the range R and perform control. Control can be done either analog or digitally.

This method can increase the threshold voltage compared to the above two examples. In the previous two examples, the threshold voltage is ■ because the voltages v8 and V are detected to determine whether the voltage is within the range R shown in FIG. However, in this example, since the output of the amplitude detector 15a, that is, the voltage of V, is detected, the threshold voltage may be v1/2. This has the advantage that the determination near both ends of the range R can be performed correctly.

FIG. 5 is a block diagram showing the correlator portion of another embodiment of the present invention. The correlator in this example is called a heterodyne type and consists of a two-stage correlator, with the first correlator 13°a113
°b, a signal of constant frequency fc+fa at 11°C and P,
A product of E and L with the PN code is created and input to the second correlator 13a s 13b 513c, respectively. By doing this, the correlators 13a, 13b s 13
The center frequency is converted from rc to f at the input and output of c. In addition, for distance measurement, a frequency converter 61 generates a reference signal B2 from the reference oscillator 1 and demodulates f.
This is done by determining the phase difference between the signal 63 and the signal 63.

Since ra>f+ is usually satisfied, the signal frequency is low and the circuit design after the correlator is simplified.

FIG. 6 is a block diagram showing a portion for obtaining a demodulated carrier signal 34 in another embodiment of the present invention. In the second example, the oscillator ■8
Or get an output of frequency ``6'' at 58, and send that output to frequency divider 3.
A demodulated PN code clock 35 is obtained. When completely 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.

(Effects 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 utilized. Since synchronization with the received signal and demodulation of the carrier signal are performed, even when the signal is buried in noise, it is possible to demodulate a signal with very good S/H. This makes it possible to measure distances even at long distances or in poor visibility conditions, maintaining accuracy, and at the same time extending the maximum measurement distance, making it useful for general surveying and long-distance geodetic surveying, especially for earthquake prediction. can do.・Also, currently the selection of the reference light and ranging light is done by mechanically switching the optical path, but due to the characteristics of the spread spectrum method (the property that only synchronized signals cannot pass through the correlator) , to determine whether it is a reference light, ranging light, or internally reflected light based on the phase difference of the synchronization signal, that is,
Measure and find the phase difference between the reference light, internally reflected light (if necessary), and the reference signal (determined geometrically, so it will be a certain value) in advance, and then calculate the phase difference between the synchronized signal and the phase difference between the reference signal and the reference signal. By comparing the phase difference, it is possible to determine whether the synchronized signal is the reference light, internally reflected light, or ranging light, so if you shift the synchronization to obtain the desired signal, the reference light, This allows you to select the distance measuring light. In other words, the desired signal can be obtained even when the reference light, internally reflected light, and ranging light are constantly being received without using an optical path switching mechanism. This eliminates the need for mechanical parts such as an optical path switching mechanism, and also eliminates the need for special antireflection treatment, which is extremely advantageous in terms of cost.

[Brief explanation of the drawing]

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, FIG. 3 is a block diagram of an embodiment for analog synchronization tracking, and FIG. The figure is a block diagram showing another method for determining synchronization.
FIG. 6 is a block diagram showing a heterodyne correlator, FIG. 6 is a block diagram showing a part for obtaining a demodulated carrier signal according to another embodiment of the present invention, and FIG. 7 is a block diagram of a conventional system. 1... Reference oscillator 2... Bandpass filter 3.
...Frequency divider 4.19...PN coco generator 5
...PSK modulator 6...Light emitting element drive circuit 7...Light emitting element 1 ■... Light receiving element 12.
... Wideband amplifier and filter 13a s b Sc - ... Correlator 14 ... Amplifier and bandpass filter 15a s b Sc ... Amplitude detector 1B ... A/
D converter 17...Central processor 18...Numerically controlled oscillator 20...Phase difference measuring device 21...Display device 31...Reference carrier wave signal 32...Reference PN code clock 33...Reference Epoch pulse 34... Demodulated carrier wave signal 35... tLliPN code blocker 3B... Demodulated epoch pulse 41... Sending light 42... Ranging light 43...
...Reference optical path 44...Target reflector 51b
s c...Threshold detection circuit 52...Synchronization determination circuit 53...Voltage controlled oscillator 61, B4
...Frequency converter 62...f, reference signal 63...r6 demodulated signal 64...Frequency selector procedure dispute report (method) 1. Indication of the case Patent Application No. 254308, filed in 1988 2, Name of the invention Optical distance meter 3, Person making the amendment Relationship to the case Patent applicant January 26, 1988 5, Drawing subject to the amendment

Claims (1)

[Claims] 1. Means for intensity modulating light and sending it toward a target point, means for receiving and demodulating the reflected light at the target point, and detecting the target point from the phase difference between the transmitted light and the reflected light. 1. A light wave distance meter characterized in that a spread spectrum method is used for intensity modulation of transmitted light and demodulation of a signal from received light. 2. A reference optical path is provided between the light sending means and the light receiving and demodulating means, and the distance to the target point is calculated from the phase difference between the reflected light and the light passing through the reference optical path. A light wave distance meter according to claim 1, characterized in that: