Distance measuring device

Abstract: A distance measuring device using two kinds of modulation frequencies (frequency f₁, frequency f₂) is disclosed. More particularly, the distance measuring device is an electro-optical distance meter for measuring a distance by combining the results of fine measurement and rough measurement. The fine measurement is effected by detecting a phase difference between a modulation signal generated from a light emitting device (4) and a reflected modulation signal generated from a light receiving device (5), while the rough measurement is effected by measuring a lag time between the modulation signal and the reflected modulation signal. Further, the distance may be measured by detecting a phase difference and a lag time of a multiplex modulation signal, in which the measurements of the phase difference and the lag time may be carried out in parallel with use of a single frequency. Particularly, a measuring time can be shortened as compared with that in an electro-optical distance meter time-divisionally using three frequencies.

Description: BACKGROUND OF THE INVENTION The present invention relates to a distance measuring device according to claim 1. A conventional electro-optical distance meter is constructed of a body of the distance meter and a reflector. A modulation wave generated from the body is reflected by the reflector, and a reflected modulation wave from the reflector is received by the body. A phase difference between the modulation wave from the body and the reflected modulation wave from the reflector corresponds to a distance between the body and the reflector. Therefore, the distance between the body and the reflector can be calculated by detecting the phase difference. In case of measuring a short distance, the reflector such as a reflecting mirror may not be specially provided, but an object located at a measuring point may be utilized as the reflector. In general, the electro-optical distance meter requires a high accuracy of measurement, and it is therefore necessary to use a modulation wave having a relatively short wavelength for the measurement. For instance, in case of obtaining a measurement accuracy of about ±5 mm, it is necessary to use a modulation wave having a first wavelength λ 1 = 20 m for fine measurement. However, the relation between a distance to be measured and a phase of the modulation wave is such that the phase changes in the range of 0 - 2π with a change in the distance of every 10 m (optical path length of 20 m). Accordingly, the distance of 10 m or less can only be measured (i.e., a measuring period is 10 m). To measure a distance more than 10 m, it is necessary to use a modulation wave having a second wavelength λ 2 larger than the first wavelength λ 1 for the fine measurement and thereby carry out rough measurement (approximate measurement). Further, to measure a longer distance, it is necessary to use a modulation wave having a third wavelength λ 3 larger than the second wavelength λ 2 for the rough measurement and thereby carry out very rough measurement. The results of the fine measurement, the rough measurement and the very rough measurement are combined to obtain a final measurement result. That is, the distance measurement is carried out with use of three kinds of frequencies corresponding to the three kinds of wavelengths λ 1 , λ 2 and λ 3 . However, as the conventional electro-optical distance meter uses three or more kinds of frequencies for the measurement, it is necessary to time-divisionally select the wavelengths, causing an increase in total measuring time. To shorten the total measuring time, it is necessary to shorten a partial measuring time corresponding to each measuring frequency, causing a reduction in the measurement accuracy. Furthermore, the partial measuring times corresponding to the respective measuring frequencies are different from one another to result in a difference in turbulent influence by the atmosphere or the like. This difference causes an error in calculation of figure arrangement of each measuring wavelength. Patent abstracts of Japan, Vol. 13, No. 284 (P-892), 29 June 1989, and JP-A-1069983 disclose a measuring device using a multi-modulated signal modulated in strength or amplitude. Also, in Patent Abstracts of Japan, Vol. 7, No. 156 (P-209), 8 July 1989, and JP-A-58066880 a measuring device is disclosed using amplitude modulation. In these measuring devices the amplitude of the reflected light is subjected to influences due to atmospherical conditions or anomalous reflection easily causing measurement errors. It is the object of the present invention to realize a distance measuring device which can shorten the total measuring time without reducing the measurement accuracy. This object is achieved by a measuring device according to claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram illustrating the construction of a first preferred embodiment of the present invention; Fig. 2 is a waveform chart illustrating the operation of the first preferred embodiment; Fig. 3 is a block diagram illustrating the construction of a second preferred embodiment of the present invention; Fig. 4 is a graph illustrating an amplitude spectrum in the second preferred embodiment; Fig. 5 is a block diagram illustrating the construction of a third preferred embodiment of the present invention; Fig. 6 is a graph illustrating an amplitude spectrum in the third preferred embodiment; Fig. 7 is a block diagram illustrating the construction of a fourth preferred embodiment of the present invention; Fig. 8 is a graph illustrating an amplitude spectrum in the fourth preferred embodiment; Fig. 9 is a block diagram illustrating the construction of a fifth preferred embodiment of the present invention; and Fig. 10 is a waveform chart illustrating the operation of the fifth preferred embodiment. DESCRIPTION OF THE INVENTION Some preferred embodiments of the present invention will now be described with reference to the drawings. (First Preferred Embodiment) Referring to Fig. 1 which shows a construction of an electro-optical distance meter according to a first preferred embodiment of the present invention, the electro-optical distance meter is comprised of a reference signal generator 1, driving circuit 3, light emitting means 4, light receiving means 5, first phase difference detecting means 6A, second phase difference detecting means 6B, microcomputer 8, first frequency divider 91, second freguency divider 92, and AND circuit 10. The reference signal generator I corresponds to the signal generating means of the present invention, and it is provided to generate a modulation signal. The reference signal generator 1 can generate a signal having a frequency f 1 , while it can supply a signal having a frequency f 2 through the first frequency divider 91 to the AND circuit 10. The driving circuit 3 is provided to drive the light emitting means 4, and it can drive the light emitting means 4 according to a signal from the microcomputer 8. The light emitting means 4, which corresponds to the light emitting means of the present invention, is provided to generate a modulation light according to a driving signal received from the driving circuit 3. A laser diode is adopted as the light emitting means 4 in this preferred embodiment. The light receiving means 5, which corresponds to the light receiving means of the present invention, is a photoelectric conversion element for receiving a modulation light reflected by a reflecting mirror located at a measuring point. A reflected multiplex modulation signal obtained by the light receiving means 5 is amplified by an amplifier 51, and is then supplied to the first phase difference detecting means 6A and the second phase difference detecting means 6B. The first and second phase difference detecting means 6A and 6B, which correspond to the phase difference detecting means of the present invention, are provided to detect a phase difference between the modulation signal generated from the light emitting means 4 and the reflected multiplex modulation signal generated from the light receiving means 5, thereby carrying out fine measurement. The first phase difference detecting means 6A is comprised of a first mixer 61A, first low-pass filter 62A, first waveform converting circuit 63A, first phase comparing circuit 64A, and first frequency generator 65A. The first frequency generator 65A, which corresponds to the first local oscillator of the present invention, is provided to form a third period signal having a third frequency (f 3 ) slightly different from the first frequency (f 1 ). The first mixer 61A is provided to form a first synthetic signal by synthesizing the reflected multiplex modulation signal from the light receiving means 5 and the third period signal from the first frequency generator 65A. The first low-pass filter 62A is provided to extract a predetermined frequency component from the first synthetic signal. The first phase comparing circuit 64A is provided to obtain a phase difference between the first synthetic signal and a signal corresponding to the generated signal from the reference signal generator 1. Similarly, the second phase difference detecting means 6B is comprised of a second mixer 61B, second low-pass filter 62B, second phase comparing circuit 64B, and second frequency generator 65B. The second frequency generator 65B, which corresponds to the second local oscillator of the present invention, is provided to form a fourth period signal having a fourth frequency (f4) slightly different from the second frequency (f 2 ). The second mixer 61B is provided to form a second synthetic signal by synthesizing the reflected multiplex modulation signal from the light receiving means 5 and the fourth period signal from the second frequency generator 65B. The second low-pass filter 62B is provided to extract a predetermined frequency component from the second synthetic signal. The second phase comparing circuit 64B is provided to obtain a phase difference between the second synthetic signal and a signal corresponding to the generated signal from the reference signal generator 1. The microcomputer 8 is provided to control the operation of each circuit and carry out computation of a distance. The operation of the first preferred embodiment constructed above will now be described with reference to Figs. 1 and 2. A signal having a frequency f 1 is supplied from the reference signal generator 1 to the AND circuit 10. On the other hand, the frequency f 1 of the signal from the reference signal generator 1 is divided by the first frequency divider 91 to obtain a signal having a frequency f 2 , which is in turn supplied to the AND circuit 10. Further, a measurement start signal (waveform 1) is adapted to be supplied from the microcomputer 8 to the AND circuit 10. Accordingly, when the measurement start signal (waveform 1) is output from the microcomputer 8 to the AND circuit 10, a multiplex amplitude modulation signal (waveform 2) is formed from the signal having the frequency f 1 generated from the reference signal generator and the signal having the frequency f 2 generated from the first frequency divider 91, and is output from the AND circuit 10 (amplitude shift keying system). The multiplex amplitude modulation signal is supplied to the driving circuit 3. The multiplex amplitude modulation signal corresponds to the multiplex modulation signal of the present invention. The driving circuit 3 drives the light emitting means 4 according to the multiplex amplitude modulation signal (waveform 2) input thereinto to generate a multiplex modulation light from the light emitting means 4. The light emitted from the light emitting means 4 is reflected by the reflecting mirror located at the measuring point, and is then received by the light receiving means 5. In the light receiving means 5, photoelectric conversion is carried out to form a reflected multiplex amplitude modulation signal. The reflected multiplex amplitude modulation signal is amplified by the amplifier 51, and is then supplied to the first and second phase difference detecting means 6A and 6B. The first phase difference detecting means 6A operates as follows: The first frequency generator 65A generates a signal having a frequency f 3 (= f 1 - f 5 ) slightly different by a frequency f 5 from the frequency f 1 . The signal having the frequency f 3 is supplied to the first mixer 61A, and it is mixed with the reflected multiplex amplitude modulation signal amplified by the amplifier 51 to carry out frequency conversion. The components of an output signal (waveform 6) from the first mixer 61A are as follows: f 1 f 2 f 5 f 1 + f 2 f 1 - f 2 f 1 - f 5 f 2 + f 5 f 2 - f 5 2f 1 - f 5 f 1 + f 2 - f 5 f 1 - f 2 - f 5 2f 1 - f 2 - f 5 2f 1 + f 2 - f 5 The output signal from the first mixer 61A is supplied to the first low-pass filter 62A, in which a signal having the predetermined frequency f 5 only is allowed to pass. The signal having passed through the first low-pass filter 62A is shaped in waveform by the first waveform converting circuit 63A (waveform 4). On the other hand, the signal having the frequency f 1 generated from the reference signal generator 1 is divided in frequency by the second frequency divider 92 to obtain a signal having a frequency f 5 ' (same as the frequency f 5 ) (waveform 3). The signal having the frequency f 5 ' is compared in phase with the signal having the frequency f 5 as an output signal from the first waveform converting circuit 63A by the first phase comparing circuit 64A. Similarly, the second phase difference detecting means 6B operates as follows: The second frequency generator 65B generates a signal having a frequency f 4 (= f 2 - f 5 ) slightly different by a frequency f 5 from the frequency f 2 . The signal having the frequency f 4 is supplied to the second mixer 61B, and it is mixed with the reflected multiplex amplitude modulation signal amplified by the amplifier 51 to carry out frequency conversion. The components of an output signal (waveform 7) from the second mixer 61B are as follows: f 1 f 2 f 5 f 1 + f 2 f 2 - f 5 f 1 + f 5 f 1 - f 5 f 1 - f 2 2f 2 - f 5 f 1 + f 2 - f 5 f 1 - f 2 + f 5 f 1 + 2f 2 - f 5 f 1 - 2f 2 + f 5 The output signal from the second mixer 61B is supplied to the second low-pass filter 62B, in which a signal having the predetermined frequency f 5 only is allowed to pass. The signal having passed through the second low-pass filter 62B is shaped in waveform by the second waveform converting circuit 63B (waveform 5). On the other hand, the signal having the frequency f 5 ' output from the second frequency divider 92 is compared in phase with the signal having the frequency f 5 as an output signal from the second waveform converting circuit 63B by the second phase comparing circuit 64B. By utilizing the phase differences obtained by the first and second phase comparing circuits 64A and 64B, the microcomputer 8 can compute the distance between the electro-optical distance meter and the reflecting mirror. Thus, the first and second phase comparing circuits 64A and 64B and the microcomputer 8 correspond to the distance measuring means of the present invention. The above phase comparing operations by the first and second phase comparing circuits 64A and 64B are simultaneously carried out. A typical example of distance measurement will now be described. For instance, assuming that the frequency f 1 is 15 MHz, and the phase difference measured by the first phase difference detecting means 6A is 90 degrees, a reduced distance in the case of using the first phase difference detecting means 6A becomes 2.5 m since the phase difference of 360 degrees corresponds to the distance of 10 m. Similarly, assuming that the frequency f2 is 150 kHz, and the phase difference measured by the second phase difference detecting means 6B is 271 degrees, a reduced distance in the case of using the second phase difference detecting means 6B becomes 753 m since the phase difference of 360 degrees corresponds to the distance of 1000 m. Consequently, the microcomputer 8 synthesizes the two kinds of distances obtained from the first and second phase difference detecting means 6A and 6B to compute an accurate distance to be measured. That is, the distance less than 10 m is computed from the phase difference detected by the first phase difference detecting means 6A (employing the first frequency f 1 ), and the distance of 10 m to 1000 m is computed from the phase difference detected by the second phase difference detecting means 6B (employing the frequency f 2 ). Accordingly, the accurate distance under the above conditions becomes 752.5 m. According to the first preferred embodiment as described above, fine measurement is carried out by the first and second phase difference detecting means 6A and 6B with use of the multiplex amplitude modulation signal of the frequency f 1 and the frequency f 2 . Thus, the measurement can be carried out with a high accuracy by employing a single frequency. Accordingly, it is greatly advantageous that a measuring time can be shortened as compared with that in the conventional electro-optical distance meter using three frequencies. In particular, this preferred embodiment is greatly effective in a high-speed measuring mode (tracking measurement) in a distance meter requiring a wide measuring range in a short time. It is readily appreciated that the values of the frequency f 1 , the frequency f2 and the oscillation frequency of the reference signal generator 1 are not limited to the above values mentioned in the first preferred embodiment, but they may be suitably selected. (Second Preferred Embodiment) A second preferred embodiment of the present invention will now be described with reference to Figs. 3 and 4. Referring to Fig. 3 which shows a construction of an electro-optical distance meter according to the third preferred embodiment of the present invention, the electro-optical distance meter is comprised of a reference signal generator 1, driving circuit 3, light emitting means 4, light receiving means 5, first phase difference detecting means 6A, second phase difference detecting means 6B, microcomputer 8, first frequency divider 91, second frequency divider 92, and amplitude modulator 100. The construction of the second preferred embodiment is basically similar to that of the first preferred embodiment with the exception that amplitude modulator 100 is substituted for the AND circuit 10 employed in the second preferred embodiment and that the measurement start signal is supplied directly to the driving circuit 3. The explanation of the other construction will accordingly be omitted hereinafter. In the second preferred embodiment, amplitude modulation is carried out by the amplitude modulator 100 with use of a signal having a frequency f 1 supplied from the reference signal generator 1 as a carrier wave and a signal having a frequency f 2 supplied from the first frequency divider 91 as a modulation wave. That is, the amplitude of the signal having the frequency f 1 is modulated by the signal having the frequency f 2 . Accordingly, as shown in Fig. 6, an amplitude spectrum of a modulation signal is constituted of three carrier waves having the frequencies f 1 , f 1 - f 2 , and f 1 + f 2 . When the measurement start signal is output from the microcomputer 8 to the driving circuit 3, the light emitting means 4 is driven by the driving circuit 3 according to a multiplex amplitude modulation signal to generate a multiplex modulation light. The light emitted from the light emitting means 4 is reflected by the reflecting mirror located at the measuring point, and is then received by the light receiving means 5. In the light receiving means 5, photoelectric conversion is carried out to form a reflected multiplex amplitude modulation signal. The reflected multiplex amplitude modulation signal is amplified by the amplifier 51, and is then supplied to the first and second phase difference detecting means 6A and 6B. The first phase difference detecting means 6A operates as follows: The first frequency generator 65A generates a signal having a frequency f 5 (= f 1 - f 2 - f 4 ) slightly different by a frequency f4 from the frequency (f 1 - f 2 ). The signal having the frequency f 5 is supplied to the first mixer 61A, and it is mixed with the reflected multiplex amplitude modulation signal amplified by the amplifier 51 to carry out frequency conversion. The output signal from the first mixer 61A is supplied to the first low-pass filter 62A, in which a signal having the predetermined frequency f 4 only is allowed to pass. The signal having passed through the first low-pass filter 62A is shaped in waveform by the first waveform converting circuit 63A. On the other hand, the signal having the frequency f 1 generated from the reference signal generator 1 is divided in frequency by the second frequency divider 92 to obtain a signal having a frequency f 4 ' (same as the frequency f 4 ). The signal having the frequency f 4 ' is compared in phase with the signal having the frequency f 4 as an output signal from the first waveform converting circuit 63A by the first phase comparing circuit 64A. Similarly, the second phase difference detecting means 6B operates as follows: The second frequency generator 65B generates a signal having a frequency f 6 (= f 1 - f 4 ) slightly different by a frequency f 4 from the frequency f 1 . The signal having the frequency f 6 is supplied to the second mixer 61B, and it is mixed with the reflected multiplex amplitude modulation signal amplified by the amplifier 51 to carry out frequency conversion. The output signal from the second mixer 61B is supplied to the second low-pass filter 62B, in which a signal having the predetermined frequency f 4 only is allowed to pass. The signal having passed through the second low-pass filter 62B is shaped in waveform by the second waveform converting circuit 63B. On the other hand, the signal having the frequency f 4 ' output from the second frequency divider 92 is compared in phase with the signal having the frequency f 4 as an output signal from the second waveform converting circuit 63B by the second phase comparing circuit 64B. By utilizing the phase differences obtained by the first and second phase comparing circuits 64A and 64B, the microcomputer 8 can compute the distance between the electro-optical distance meter and the reflecting mirror. The above phase comparing operations by the first and second phase comparing circuits 64A and 64B are simultaneously carried out. A typical example of distance measurement will now be described. For instance, it is assumed that the frequency f 1 is 15 MHz; the frequency f2 is 150 kHz; the phase difference measured by the first phase difference detecting means 6A is 196.9 degrees; and the phase difference measured by the second phase difference detecting means 6B is 108 degrees. Under these conditions, a reduced distance in the case of using the first phase difference detecting means 6A becomes 5.525 m since the phase difference of 360 degrees corresponds to the distance of 10.101010 m. Similarly, a reduced distance in the case of using the second phase difference detecting means 6B becomes 3.000 m since the phase difference of 360 degrees corresponds to the distance of 10 m. Furthermore, the distance more than 10 m is computed from the difference between the frequencies f 1 and f 2 by the microcomputer 8, and consequently, the distance between the electro-optical distance meter and the reflecting mirror is displayed as 753.000 m. (Third Preferred Embodiment) A third preferred embodiment of the present invention will now be described with reference to Figs. 5 and 6. Referring to Fig. 5 which shows a construction of an electro-optical distance meter according to the third preferred embodiment of the present invention, the electro-optical distance meter is comprised of a first reference signal generator 1A, second reference signal generator 1B, third reference signal generator 1C, driving circuit 3, light emitting means 4, light receiving means 5, first phase difference detecting means 6A, second phase difference detecting means 6B, microcomputer 8, first frequency divider 91, second frequency divider 92, and frequency modulator 1000. The first, second and third reference signal generators lA, 1B and 1C correspond to the signal generating means of the present invention. The first reference signal generator 1A can generate a signal having a frequency f 1 ; the second reference signal generator 1B can generate a signal having a frequency f 2 ; and the third reference signal generator 1C can generate a signal having a frequency f 3 . The signal having the frequency f 1 from the first reference signal generator 1A is supplied to the frequency modulator 1000, and the signal having the frequency f 2 from the second reference signal generator 1B is also supplied to the frequency modulator 1000. The frequency modulator 1000 receives the signal having the frequency f 3 from the third reference signal generator 1C, and alternately outputs the signal having the frequency f 1 and the signal having the frequency f 2 according to the period of the signal having the frequency f 3 . Accordingly, multiplex modulation is effected by frequency modulation. As shown in Fig. 6, an amplitude spectrum of a modulation signal is constituted of a side band of carrier waves having reference frequencies f 1 and f 2 and adjacent frequencies spaced therefrom by the frequency f 3 . When the measurement start signal is output from the microcomputer 8 to the driving circuit 3, the light emitting means 4 is driven by the driving circuit 3 according to a multiplex frequency modulation signal to generate a multiplex modulation light. The light emitted from the light emitting means 4 is reflected by the reflecting mirror located at the measuring point, and is then received by the light receiving means 5. In the light receiving means 5, photoelectric conversion is carried out to form a reflected multiplex frequency modulation signal. The reflected multiplex frequency modulation signal is amplified by the amplifier 51, and is then supplied to the first and second phase difference detecting means 6A and 6B. The first and second phase difference detecting means 6A and 6B, which correspond to the phase difference measuring means of the present invention, are provided to detect a phase difference between the modulation light generated from the light emitting means 4 and the reflected multiplex frequency modulation signal generated from the light receiving means 5, thereby carrying out distance measurement. The first phase difference detecting means 6A is comprised of a first mixer 61A, first low-pass filter 62A, first waveform converting circuit 63A, and first phase comparing circuit 64A. While the first phase difference detecting means 6A in the fourth preferred embodiment does not include any frequency generator as 65A employed in the third preferred embodiment, the signal having the frequency f 2 from the second reference signal generator 1B is supplied to the first mixer 61A. Accordingly, the second reference signal generator 1B in the third preferred embodiment corresponds to the first local oscillator of the present invention. Similarly, the second phase difference detecting means 6B is comprised of a second mixer 61B, second Low-pass filter 62B, second waveform converting circuit 63B, and second phase comparing circuit 64B. While the second phase difference detecting means 6B in the fourth preferred embodiment does not include any frequency generator employed in the third preferred embodiment, the signal having the frequency f 1 from the first reference signal generator 1A is supplied to the second mixer 61B. Accordingly, the first reference signal generator 1A in the fourth preferred embodiment corresponds to the second local oscillator of the present invention. The operation of the first and second phase difference detecting means 6A and 6B will now be described. As mentioned above, the multiplex frequency modulation signal amplified by the amplifier 51 is supplied to both the first and second phase difference detecting means 6A and 6B. The first phase difference detecting means 6A operates as follows: The signal having the frequency f 2 from the second reference signal generator 1B is supplied to the first mixer 61A, and it is mixed with the reflected multiplex frequency modulation signal amplified by the amplifier 51 to carry out frequency conversion. The components of an output signal from the first mixer 61A are as follows: f 1 f 2 f 1 - f 2 f 1 + f 2 2f 2 The output signal from the first mixer 61A is supplied to the first low-pass filter 62A, in which a signal having the predetermined frequency (f 1 - f 2 ) only is allowed to pass. The signal having passed through the first low-pass filter 62A is shaped in waveform by the first waveform converting circuit 63A to obtain a signal having the frequency (f 1 - f 2 ). On the other hand, the signal having the frequency f 1 generated from the first reference signal generator 1A is divided in frequency by the first frequency divider 91 to obtain a signal having a frequency f 5 ' (= f 1 - f 2 ). The signal having the frequency f 5 ' is compared in phase with the signal having the frequency (f 1 - f 2 ) as an output signal from the first waveform converting circuit 63A by the first phase comparing circuit 64A. Similarly, the second phase difference detecting means 6B operates as follows: The signal having the frequency f 1 from the first reference signal generator 1A is supplied to the second mixer 61B, and it is mixed with the reflected multiplex frequency modulation signal amplified by the amplifier 51 to carry out frequency conversion. The components of an output signal from the second mixer 61B are as follows: f 1 f 2 f 1 - f 2 f 1 + f 2 2f 1 The output signal from the second mixer 61B is supplied to the second low-pass filter 62B, in which a signal having the predetermined frequency (f 1 - f 2 ) only is allowed to pass. The signal having passed through the second low-pass filter 62B is shaped in waveform by the second waveform converting circuit 63B to obtain a signal having the frequency (f 1 - f 2 ). On the other hand, the signal having the frequency f 1 generated from the first reference signal generator 1A is divided in frequency by the second frequency divider 92 to obtain a signal having a frequency f 5 ' (= f 1 - f 2 ). Then, the signal having the frequency f 5 ' output from the second frequency divider 92 is compared in phase with the signal having the frequency (f 1 - f 2 ) as an output signal from the second waveform converting circuit 63B by the second phase comparing circuit 64B. By utilizing the phase differences obtained by the first and second phase comparing circuits 64A and 64B, the microcomputer 8 can compute the distance between the electro-optical distance meter and the reflecting mirror. The above phase comparing operations by the first and second phase comparing circuits 64A and 64B are simultaneously carried out. A typical example of distance measurement will now be described. For instance, it is assumed that the frequency f 1 is 15 MHz; the frequency f2 is 15 MHz - 150 kHz; the phase difference measured by the first phase difference detecting means 6A is 108 degrees; and the phase difference measured by the second phase difference detecting means 6B is (360 - 196.9) degrees. Under these conditions, a reduced distance in the case of using the first phase difference detecting means 6A becomes 3.000 m since the phase difference of 360 degrees corresponds to the distance of 10 m. Similarly, a reduced distance in the case of using the second phase difference detecting means 6B becomes 5.525 m since the phase difference of 360 degrees corresponds to the distance of 10.101010 m. Furthermore, the distance more than 10 m is computed from the difference between the frequencies f 1 and f 2 by the microcomputer 8, and consequently, the distance between the electro-optical distance meter and the reflecting mirror is displayed as 753.000 m. (Forth Preferred Embodiment) A forth preferred embodiment of the present invention will now be described with reference to Figs. 7 and 8. Referring to Fig. 7 which shows a construction of an electro-optical distance meter according to the forth preferred embodiment of the present invention, the electro-optical distance meter is comprised of a reference signal generator 1, driving circuit 3, light emitting means 4, light receiving means 5, first phase difference detecting means 6A, second phase difference detecting means 6B, microcomputer 8, first frequency divider 91, second frequency divider 92, and phase modulator 10000. The construction of the forth preferred embodiment is basically similar to that of the second preferred embodiment with the exception that phase modulator 10000 is substituted for the amplitude modulator 100 employed in the second preferred embodiment. The explanation of the other construction will accordingly be omitted hereinafter. In the forth preferred embodiment, phase modulation is carried out by the phase modulator 10000 with use of a signal having a frequency f 1 supplied from the reference signal generator 1 as a carrier wave and a signal having a frequency f2 supplied from the first frequency divider 91 as a modulation wave. That is, the phase of the signal having the frequency f 1 is modulated by the signal having the frequency f 2 . Accordingly, as shown in Fig. 7, an amplitude spectrum of a modulation signal-is constituted of a carrier wave f 1 and a side wave of frequency interval f2 having the frequencies f 1 - f 2 , and f 1 + f 2 . When the measurement start signal is output from the microcomputer 8 to the driving circuit 3, the light emitting means 4 is driven by the driving circuit 3 according to a multiplex phase modulation signal to generate a multiplex modulation light. The light emitted from the light emitting means 4 is reflected by the reflecting mirror located at the measuring point, and is then received by the light receiving means 5. In the light receiving means 5, photoelectric conversion is carried out to form a reflected multiplex phase modulation signal. The reflected multiplex phase modulation signal is amplified by the amplifier 51, and is then supplied to the first and second phase difference detecting means 6A and 6B. The first phase difference detecting means 6A operates as follows: The first frequency generator 65A generates a signal having a frequency f 5 (= f 1 - f 2 - f 4 ) slightly different by a frequency f4 from the frequency (f 1 - f 2 ). The signal having the frequency f 5 is supplied to the first mixer 61A, and it is mixed with the reflected multiplex phase modulation signal amplified by the amplifier 51 to carry out frequency conversion. The other operation is the same as that in the third preferred embodiment with the exception that the phase modulation is substituted for the amplitude modulation employed in the third preferred embodiment. A typical example of distance measurement will now be described. For instance, it is assumed that the frequency f 1 is 15 MHz; the frequency f 2 is 150 kHz; the phase difference measured by the first phase difference detecting. means 6A is 196.9 degrees; and the phase difference measured by the second phase difference detecting means 6B is 108 degrees. Under these conditions, a reduced distance in the case of using the first phase difference detecting means 6A becomes 5.525 m since the phase difference of 360 degrees corresponds to the distance of 10.101010 m. Similarly, a reduced distance in the case of using the second phase difference detecting means 6B becomes 3.000 m since the phase difference of 360 degrees corresponds to the distance of 10 m. Furthermore, the distance more than 10 m is computed from the difference between the frequencies f 1 and f 2 by the microcomputer 8, and consequently, the distance between the electro-optical distance meter and the reflecting mirror is displayed as 753.000 m. (Fifth Preferred Embodiment) A fifth preferred embodiment of the present invention will now be described with reference to Figs. 9 and 10. Referring to Fig. 9 which shows a construction of an electro-optical distance meter according to the fifth preferred embodiment of the present invention, the electro-optical distance meter is comprised of a reference signal generator 1, synchronizing circuit 2, driving circuit 3, light emitting means 4, light receiving means 5, first phase difference detecting means 6A, second phase difference detecting means 6B, lag time measuring means 7, microcomputer 8, first frequency divider 91, second frequency divider 92, and AND circuit 10. The synchronizing circuit 2 is provided to input an AND output of the signal having the frequency f 1 from the reference signal generator 1 and the signal having the frequency f2 from the first frequency divider 91 and generate a signal synchronized with the signal having the frequency f 2 from the first frequency divider 91. The driving circuit 3 is provided to drive the light emitting means 4, and it can drive the light emitting means 4 according to the synchronous signal from the synchronizing circuit 2. The light receiving means 5 is a photoelectric conversion element for receiving a modulation light reflected by a reflecting mirror located at a measuring point. A reflected modulation signal obtained by the light receiving means 5 is amplified by a first amplifier 51, and is then supplied to the first and second phase difference detecting means 6A and 6B and the lag time measuring means 7. As the constructions of the reference signal generator 1 and the first and second phase difference detecting means 6A and 6B are similar to those employed in the first preferred embodiment, the explanation thereof will therefore be omitted hereinafter. The operation of the fifth preferred embodiment constructed above will now be described with reference to Figs. 9 and 10. A signal having the frequency f 1 is supplied from the reference signal generator 1 to the AND circuit 10. Further, the signal having the frequency f 1 from the reference signal generator 1 is divided in frequency by the first frequency divider 91 to obtain a signal having the frequency f 2 , which signal is in turn supplied to the AND circuit 10. Accordingly, the AND circuit 10 forms an AND signal (waveform 1) of the signal having the frequency f 1 from the reference signal generator 1 and the signal having the frequency f 2 from the first frequency divider 91. Then, such a multiplex modulation signal of the frequency f 1 and the frequency f 2 (i.e., the AND signal) is supplied to the synchronizing circuit 2, and is synchronized with the signal having the frequency f2 from the first frequency divider 91. When a measurement start signal (waveform 2) is output from the microcomputer 8 to the synchronizing circuit 2, the multiplex modulation signal of the frequency f 1 and the frequency f 2 is supplied from the synchronizing circuit 2 to the driving circuit 3, and simultaneously a start signal (waveform 4) is supplied from the synchronizing circuit 2 to the counter 72 of the lag time measuring means 7. The light emitting means 4 is driven by the driving circuit 3 according to the multiplex modulation signal input into the driving circuit 3 to generate a multiplex modulation light. The multiplex modulation light generated from the light emitting means 4 is reflected by the reflecting mirror located at the measuring point, and is received by the light receiving means 5. The light receiving means 5 carries out photoelectric conversion to form a reflected multiplex modulation signal. The reflected multiplex modulation signal is amplified by the first amplifier 51, and is then supplied to the first and second phase difference detecting means 6A and 6B. At the same time, the reflected multiplex modulation signal amplified by the first amplifier 51 is further amplified by a second amplifier 52, and is then supplied to the lag time measuring means 7. The lag time measuring means 7 is provided to measure a lag time between the modulation signal generated from the light emitting means 4 and the reflected modulation signal generated from the light receiving means 5, thereby carrying out rough measurement. The lag time measuring means 7 is comprised of a comparator 71, counter 72, and reference voltage generator 73. The microcomputer 8 is provided to control the operation of each circuit and carry out computation of a distance or the like. The lag time measuring means 7 operates as follows: The reflected modulation signal amplified by the second amplifier 52 is input into the comparator 71. The comparator 71 compares the reflected modulation signal input thereinto with a reference voltage input from the reference voltage generating circuit 73, and outputs a stop signal when the reflected modulation signal exceeds the reference voltage. The stop signal from the comparator 71 and the start signal from the synchronizing circuit 2 are input into the counter 72. The counter 72 counts the number of clocks generated from the reference signal generator 1 and input during a lag time between the start signal and the stop signal. The microcomputer 8 can compute a distance between the reflecting mirror and the electro-optical distance meter according to the number of clocks counted above and the oscillation frequency of the reference signal generator 1. The time lag measuring means 7 and the microcomputer 8 correspond to the second distance measuring means of the present invention. The first and second phase difference detecting means 6A and 6B operate in the same manner as those in the first preferred embodiment, and the explanation thereof will therefore be omitted hereinafter. As described above, the fifth preferred embodiment adopts the construction of the first preferred embodiment for the first and second phase difference detecting means 6A and 6B, and also adopts the construction of the above embodiment for the time lag measuring means 7. Consequently, the microcomputer 8 synthesizes the three kinds of distances obtained from the first and second phase difference detecting means 6A and 6B and the time lag measuring means 7 to compute an accurate distance to be measured. That is, the distance less than 10 m is computed from the phase difference detected by the first phase difference detecting means 6A; the distance of 10 m to 1000 m is computed from the phase difference detected by the second phase difference detecting means 6B; and the distance more than 1000 m is computed from the lag time measured by the lag time measuring means 7. Accordingly, the accurate distance under the above conditions becomes 3752.5 m. According to the fifth preferred embodiment as described above, fine measurement is carried out by the first and second phase difference detecting means 6A and 6B with use of the multiplex modulation signal of the frequency f 1 and the frequency f 2 , and rough measurement is carried out by the lag time measuring means 7 with use of the same multiplex modulation signal of the frequency f 1 and the frequency f 2 . Thus, the measurement can be carried out with a high accuracy by employing a single frequency. Accordingly, it is greatly advantageous that a measuring time can be shortened as compared with that in the conventional electro-optical distance meter using three frequencies. It is readily appreciated that the values of the frequency f 1 , the frequency f 2 and the oscillation frequency of the reference signal generator 1 are not limited to the above values mentioned in the sixth preferred embodiment, but they may be suitably selected.

Claims: A distance measuring device for measuring a distance by detecting a reflected light from reflecting means located at a measuring point; said distance measuring means comprising signal generating means (1000, 10000) for generating a multiplex modulation signal; light emitting means (3, 4) for generating a multiplex modulation light having a modulated factor according to said modulation signal generated from said signal generating means; light receiving means (5) for receiving said multiplex modulation light generated from said light emitting means (3, 4) and reflected by said reflecting means to form a reflected multiplex modulation signal; first phase signal forming means (61A 62A, 63A) for forming a first phase signal containing a phase component of a first modulation signal from said reflected multiplex modulation signal and for forming the first phase signal having lower frequency than the frequency of the first modulation signal; second phase signal forming means (61B, 62B, 63B) for forming a second phase signal containing a phase component of a second modulation signal, which is different in frequency from said first modulation signal, from said reflected multiplex modulation signal and for forming the second phase signal having lower frequency than the frequency of the second modulation signal; and distance measuring means (8) for measuring said distance to said measuring point with use of a phase difference between said first phase signal and a reference signal corresponding to said multiplex modulation signal and with use of a phase difference between said second phase signal and said reference signal. The distance measuring device according to claim 1, wherein said signal generating means (10000) generates a phase modulation signal having a plurality of frequency components; said first phase signal forming means comprises a first local oscillator (65A) for forming a third frequency signal having a third frequency slightly different from a first frequency in said plural frequency components of said multiplex phase modulation signal, and first mixing means (61A) for mixing said third frequency signal with said reflected multiplex modulation signal to form said first phase signal and for forming the first phase signal having lower frequency than the frequency of the first modulation signal: and said second phase signal forming means comprises a second local oscillator (65B) for forming a fourth frequency signal having a fourth frequency slightly different from a second frequency in said plural frequency components of said multiplex phase modulation signal, said second frequency being different from said first frequency, and second mixing means (61B) for mixing said fourth frequency signal with said reflected multiplex modulation signal to form said second phase signal and for forming the second phase signal having lower frequency than the frequency of the second modulation signal. The distance measuring device according to claim 1, wherein said signal generating means (1000) generates a multiplex frequency modulation signal having a plurality of frequency components; said first phase forming means (61A, 62A, 63A) comprises a first local oscillator (1B) for forming a third frequency signal, and first mixing means (61A) for mixing said third frequency signal with said reflected multiplex modulation signal to form said first phase signal and for forming the first phase signal having lower frequency than the frequency of the first modulation signal; said third frequency signal has a frequency such, that a frequency of said first phase signal formed by said first mixing means becomes equal to a frequency of said reference signal: said second phase signal forming means comprises a second local oscillator (1C) for forming a fourth frequency signal, and second mixing means (61B) for mixing said fourth frequency signal with said reflected multiplex modulation signal to form said second phase signal and for forming the second phase signal having lower frequency than the frequency of the second modulated signal; and said fourth frequency signal has a frequency such, that a frequency of said second phase signal formed by said second mixing means becomes equal to a frequency of said reference signal. The distance measuring means according to claim 2 or 3, wherein said first mixing means (61A) includes a first low-pass filter (62A) for extracting said first phase signal having the same frequency as that of said reference signal, and said second mixing means (61B) includes a second low-pass filter (62B) for extracting said second phase signal having the same frequency as that of said reference signal.

Inventors: KIMURA KAZUAKI, KUMAGAI KAORU, KIMURA, KAZUAKI, KUMAGAI, KAORU
Applicants: TOPCON CORP, KABUSHIKI KAISHA TOPCON
Application year: 1991
Main classification: G01S17
IPC codes: G01S17