The present invention relates to a distance measuring device by a phase difference method that emits a distance measuring light toward a target and receives a reflected light from the target.
A conventional distance measuring device will be described with reference to FIG. 1 to FIG. 3.
As shown in the block diagram of FIG. 1, in this distance measuring device, a distance measuring light L transmitted from a light source 20 such as a laser diode is emitted toward a target (such as a prism) 22 placed on a survey point via an unillustrated light transmitting optical system. Since the light source 20 is connected to a modulator 24, the distance measuring light L is transmitted by a signal K′ modulated based on a reference signal K generated by a reference signal oscillator 26.
The distance measuring light L reflected on the target 22 is led to a light receiving element 28 such as a photo diode via an unillustrated light receiving optical system and a diaphragm 27 that adjusts the light amount of the distance measuring light L. Then, by the light receiving element 28, the distance measuring light L is converted to a distance measuring signal M being an electrical signal, and the distance measuring signal M is denoised by a bandpass filter 32 after being amplified by a high-frequency amplifier 30.
Furthmore, this distance measuring signal M is multiplied, by a frequency converter 37 composed of a mixer 34 and a local oscillator 36, with a local oscillation signal Q generated by the local oscillator 36 and is converted to two frequencies of a frequency to be a sum of frequencies of both signals M and Q and a frequency to be a difference of frequencies of both signals M and Q. Here, after screening only the frequency to be a difference of frequencies of both signals M and Q by the low-pass filter 38 to lower the frequency to an intermediate frequency signal IF, this is amplified by an intermediate frequency amplifier 40. The amplified intermediate frequency signal IF is converted to a digital signal by an A/D converter 42, is inputted into a CPU 44 (operation means), and is stored in a memory (storage means) 46.
When carrying out a distance measurement, the reference signal oscillator 26 transmits a synchronizing signal P to the CPU 44 and A/D converter 42 and transmits a reference signal K to the modulator 24. In this manner, the A/D converter 42 carries out sampling in synchronization with the reference signal K. During this distance measurement, the degree of opening of the diaphragm 27 is fixed.
As shown in FIG. 2, sampling timing of the A/D converter 42 is determined so as to always sample one cycle of the intermediate frequency signal IF at fixed phase angle positions, for example, so as to divide one cycle into n (n≧3) equal parts. The intermediate frequency signal IF is consecutively sampled at this sampling timing for more than a few thousands of a large number of cycles. At this time, sampling data of the intermediate frequency signal IF that is beyond an input range of the A/D converter 42 or that is too small relative to the input range is discarded.
In order to store the sampling data in the memory 46, a storage area for n pieces of data equal to one cycle of the intermediate frequency signal IF is prepared in the memory 46, and as shown in FIG. 3, sampling data at the same phase position is added for storage. In this manner, synthetic data S of one cycle of the intermediate frequency signal IF with large amplitude for which the sampling data at the same phase position has been added is created. This synthetic data S is applied to a synthetic sine wave S′ by the method of least square to determine an initial phase β of this synthetic sine wave S′. Since the A/D converter 42 carries out sampling in synchronization with the reference signal K, the initial phase β is equalized to a phase difference between a signal obtained by dividing the reference signal K to the same frequency as that of the intermediate frequency signal IF and the intermediate frequency signal IF, and a distance from the initial phase β to the target 22 is calculated.
Meanwhile, owing to shimmering of air from heat, since the light amount of the distance measuring light L that is made incident into the light receiving element 28 greatly fluctuates even during a measurement, it becomes necessary to correct the initial phase β owing to the fluctuation of the distance measuring light L.
Therefore, in this distance measuring device, at the time of adjustment following completion of assembly, for each machine, after collimating of an identical target, the degree of opening of the diaphragm 27 is changed so as to change the light amount of the distance measuring light L made incident into the light receiving element 28, while an amount of correction of the initial phase necessary at the amplitude (which is a value corresponding to the light amount of the distance measuring light L made incident into the light receiving element 28 or the amplitude of the distance measuring signal M) of the intermediate frequency signal IF inputted into the A/D converter 42 is found, and this is written into a correction table storage area provided in the memory 46. This correction amount is an amount that changes according to amplitude of the intermediate frequency signal IF. Namely, the correction amount is a function of the amplitude.
When measuring the distance, as shown in FIG. 3, the synthetic sine wave S′ calculated based on the synthetic data S for which sampling data of the intermediate frequency signal IF has been added for each same phase position for a large number of cycles is used. Therefore, a correction amount Δβ of the initial phase β of this synthetic sine wave S′ is expressed, where amplitude according to an i-th wave of the intermediate frequency signal IF is provided as Ai and a correction amount at this amplitude Ai is provided as Δφi(Ai), as: Δβ=tan−1[Σ{Ai·sin(Δφi(Ai))}/Σ{Ai·cos(Δφi(Ai))}]. Here, since Δφi(Ai) is generally small, it can be approximated as Δβ=tan−1[Σ{Ai·Δφi(Ai)}/ΣAi]=Σ{Ai·Δφi(Ai)}/ΣAi. Consequently, the correction amount Δβ of the initial phase β is a weighted average obtained by weighting the correction amount Δφi(Ai) with the amplitude Ai of the intermediate frequency signal IF. A true initial phase can be obtained by subtracting the foregoing correction amount Δβ from the measured initial phase β.
In this distance measuring device, prior to a distance measurement, the distance measuring device is actuated to carry out preliminary sampling to sample a few cycles of the intermediate frequency signal IF preliminarily. This is carried out to detect sampling positions at the maximum level Amax and the minimum level Amin within one-cycle range of the intermediate frequency signal IF and to thereby simplify the subsequent detection of the amplitude Ai. In addition, in order to obtain frequency distribution of the amplitude Ai, the amplitude range is divided into a plurality of grades, and a storage area to hold frequencies of the respective grades is prepared in the memory 46.
Then, a distance measurement is started, and every time one cycle of the intermediate frequency signal IF has been detected, the synthetic data S (FIG. 3) for which sampling data at the same phase position has been added is stored in the memory 46, and based on ½ of a difference in levels between the sampling positions that have indicated the maximum level Amax and the minimum level Amin determined from the result of the preliminary sampling prior to the distance measurement, a grade of the amplitude Ai of the intermediate frequency signal IF is determined, 1 is added to the frequency of that grade, and frequency distribution of the amplitude Ai is also stored in the memory 46. After completion of the sampling, the initial phase β is determined by use of the synthetic sine wave S′ of the intermediate frequency signal IF, and by use of the frequency distribution of the amplitude Ai, the weighted average Σ{Ai·Δφi(Ai)}/ΣAi obtained by weighting the correction amount Δφi(Ai) with the amplitude Ai of the intermediate frequency signal IF, namely, the correction amount Δβ is calculated. Then, the correction amount Δβ is subtracted from the initial phase β to calculate a distance to the target 22.
As a matter of course, in this distance measuring device as well, the distance measuring light L can be switched to a reference light R that leads to the light receiving element 28 through an internal light path including a diaphragm 27a. By carrying out a distance measurement by use of the reference light R, the distance determined by use of the foregoing distance measuring light L can be corrected. Such a device is disclosed in Japanese Published Unexamined Patent Application No. 2004-264116.
In such a distance measuring device, since the distance has been calculated after consecutively sampling the intermediate frequency signal IF for more than a few thousands of a large number of cycles, a predetermined time has been necessary for a distance measurement. However, in recent years, a further reduction in the time required for a distance measurement has been demanded. Therefore, in order to reduce the time required for a measurement of the distance measuring device, if a reduction in the number of cycles of the intermediate frequency signal IF to be sampled is attempted, in, particularly, the case of a long-distance measurement, deficiency in the receiving light amount of the distance measuring light L frequently occurs owing to shimmering of air from heat, therefore, a problem of degradation in accuracy of the measured value arises.