JP2854590B2 - Distance sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a distance measuring sensor, which is suitable for use in measuring a relatively long distance.

[Prior Art] FIGS. 3 and 4 show a conventional example of a distance measuring sensor using the speed of light. This type of distance measuring sensor has a light emitting element 3
The output light of this is intensity-modulated at high frequency and projected on the reflecting object 17,
The light reflected by the reflecting object 17 is received by the light receiving element 6, it is to measure the distance to the reflecting object 17 from the phase difference of the light receiving signal V _R and the emission signal V _S. As shown in FIG. 4, when the reflective object 17 exists at a position 1 (m) away from the distance measuring sensor S, the light emission signal V _S and the light reception signal V _R can be expressed by the following equations.

V _S = I _S × sin 2πf ₀ t V _R = I _R × sin 2πf ₀ (t−2l / c) where I _S and I _R are the emission signal and light reception signal amplitudes, f ₀ is the modulation frequency, and c is the light speed. is there. As can be seen from the above equation, the phase of the light emission signal V _S and the phase of the light reception signal V _R are shifted by ψ = f ₀ × (2l / c). Therefore, if the phase difference ψ is measured, the distance 1 to the reflecting object 17 can be calculated.
It generates a low frequency signal by mixing the local oscillation signal V _L of a predetermined peripheral f _L to V _S and the light receiving signal V _R, which measures the phase difference.

Now, assuming that the local oscillation signal of V _L = V × sin2πf _L is mixed into the light emission signal V _S and the light reception signal V _R , the mixed output
V _S ′ and _R ′ are as follows.

V _S ′ = (V × I _S / 2) ｛cos 2π (f ₀ −f _L ) t + cos 2π (f <sub>0
+ F L) t} ... V</sub> R '= (V × I R / 2) {cos2π ((f 0 -f L) t-ψ) -co
s2π ((f ₀ + f _L ) t + ψ)… Here, by passing through a filter that selectively passes only the frequency (f ₀ −f _L ), V _S ′ = A × cos 2π (f ₀ −f _L ) t... V _R ′ = B × cos ｛2π (f ₀ −f _L ) t−ψ｝. In addition, even after the conversion into the low frequency, the phase difference の between the two signals is not different from the phase difference に お け る at the high frequency before the conversion. As can be seen from the equation, the phase changes in the frequency difference and the circuit of the modulation frequency f ₀ and the local oscillator frequency f _L is a cause of error in the phase difference [psi.

In general, the phase difference is often represented by the time difference between zero crossings of two input signals, and the time difference τ is given by τ = ψ / (f ₀ −f _L ) = 2lf ₀ / (f ₀ −f _L ) c. . By measuring the time difference τ, the distance 1 is calculated. However, the equation is based on the above signal V _S ′,
This equation holds only when the circuit causes the same phase change when generating V _R ′ .If the signals V _S ′ and V _R ′ are generated and the phase change differs in the circuit system, In
The time difference is τ ′ = τ + Δt, and this Δt is a factor of the error.

Conventional examples for removing an error due to a phase change in such a circuit system are scarce.
In the distance measuring sensor disclosed in Japanese Patent Application Laid-Open Publication No. H11-207, two oscillators for determining the modulation frequency of the light transmission signal and two oscillators for determining the frequency of the local oscillation signal are provided independently. When the frequency of one of the oscillators changes, the phase difference changes greatly.

In JP-A-48-17362, as shown in FIG. 3, an oscillation output from a reference oscillator 1 for determining a modulation frequency f ₀ of a light transmission signal is divided by N by a frequency divider 4. A method of generating a local oscillation frequency f _L by mixing the frequency (f ₀ / N) obtained by frequency division with the original oscillation frequency f ₀ of the reference oscillator 1 by the mixer 18 is disclosed. . In this case, f _L = f ₀ × (N−1) / N. As a result, the equation becomes τ = N × (2la / c), and the phase difference is related to the change in the oscillation frequency f _0. Disappears.

[Problem to be Solved by the Invention] However, since the difference between the oscillation frequency f ₀ of the reference oscillator and the local oscillation frequency f _L is set to be very small as compared with the local oscillation frequency f _L , The selectivity Q of the filter 19 for extracting only the local oscillation frequency f _L from the output needs to be set very high. Therefore, the phase change in the circuit system largely changes due to the subtle change in the frequency, which may increase the phase change in the oscillator.

Japanese Patent Application Laid-Open No. 56-122977 discloses an output of a reference oscillator for determining a modulation frequency of a light transmission signal, and a voltage controlled oscillator (VC) for determining a local oscillation frequency of a light receiving circuit.
O), a configuration using a PLL (phase-locked loop) that mixes the output and feeds back the mixed output as a VCO control signal has been proposed. However, the modulation frequency component of the transmitted light signal is transmitted through a mixer. There is a risk of leakage to the output side of the VCO, that is, to the side of the light receiving circuit system. In general, a distance measuring sensor using light wave reflection receives weak reflected light from an object to be measured, and selectively increases only a high-frequency component for modulating a transmitted signal from the received light signal. Since it is configured to amplify with a gain, it is not preferable that a high frequency for modulating a transmission signal goes around the light receiving circuit system.

The present invention has been made in view of such a point,
The purpose is to generate the modulation frequency and local oscillation frequency of the light transmission signal from the output of the same reference oscillator without using a filter with high selectivity in a distance measuring sensor using the speed of light. An object of the present invention is to stabilize phase detection characteristics and increase measurement accuracy.

[Means for Solving the Problems] In the distance measuring sensor according to the present invention, in order to solve the above problems, as shown in FIG. And a light-receiving element 6 for receiving light transmitted from the light-emitting element 3 and reflected by the reflective object 17, and reflected from a phase difference between the received light and the transmitted light. In a distance measuring sensor for measuring a distance to an object 17, a reference oscillator 1 for oscillating a reference frequency for modulating light transmitted from the light emitting element 3, and a voltage controlled oscillator for changing the oscillation frequency according to a voltage signal 5 and
A first frequency divider 4 for dividing the reference frequency oscillated by the reference oscillator 1 by N, a second frequency divider 10 for dividing the oscillation frequency from the voltage controlled oscillator 5 by (N-1), 1st and 2nd
A first phase detector 11 for detecting a phase difference between the divided outputs of the frequency dividers 4 and 10, and a voltage signal for controlling the oscillation of the voltage controlled oscillator 5 from the output of the first phase detector 11. A loop filter 12 to be generated, a mixer 8 that mixes the received light output from the light receiving element 6 with an oscillation output of the voltage controlled oscillator 5, a beat output obtained by the mixer 8, and a division by the first frequency divider 4. And a second phase detector 9 for detecting a phase difference from the peripheral output.

As shown in FIG. 2, the modulation frequency of the light emitting element 3 is not directly used by the oscillation output of the reference oscillator 1, but is once divided by N, and then the voltage controlled oscillator 17 and the frequency divider 1 are divided.
8. The modulation frequency may be obtained by using the loop filter 19 and the phase detector 20.

[Operation] In the configuration shown in FIG. 1, the output signal of the reference oscillator 1 is set to N
The divided signal and the output signal of the voltage controlled oscillator 5 are (N−
1) Since the oscillation of the voltage controlled oscillator 5 can be controlled so that the frequency and phase of the frequency-divided signal become the same, a local oscillation signal having a slight frequency difference from the output signal of the reference oscillator 1 is output from the voltage controlled oscillator 5. Can be obtained, and the local oscillation signal and the output signal of the reference oscillator 1 have a stable phase relationship. Therefore, it is not necessary to use a filter with high selectivity, and the phase detection characteristics can be stabilized. Further, since the output signal of the reference oscillator 1 is not directly mixed with the output signal of the voltage control oscillator 5, the modulation frequency component of the light transmission signal goes to the light receiving circuit system as disclosed in Japanese Patent Application Laid-Open No. 56-122977. Such a problem does not occur.

Also, in the configuration shown in FIG. 2, the modulation frequency is generated based on the signal once divided by N without using the oscillation output of the reference oscillator 1 directly.
Even if the variation in the oscillation frequency of the reference oscillator 1 is somewhat large, the variation in the frequency is reduced when the reference frequency is divided by 1 / N, so that the configuration of the reference oscillator 1 is simplified.

Embodiment 1 FIG. 1 is a block diagram of one embodiment of the present invention. Reference oscillator 1 oscillates a reference frequency f ₀ for determining the modulation frequency. The oscillation output of the reference oscillator 1 is supplied to the light emitting element 3 via the drive circuit 2 including an amplifier, and modulates the luminance of the light emitting element 3. The oscillation output of the reference oscillator 1 is also input to the frequency divider 4 and is divided by (1 / N). The voltage control oscillator 5 is an oscillator (so-called VCO) whose oscillation frequency changes according to a control voltage. The oscillation output of the voltage controlled oscillator 5 is supplied to the mixer 8 as a local oscillation signal.
On the other hand, the output of the light receiving element 6 is input to the mixer 8 through a light receiving circuit 7 including an amplifier and a filter. A beat signal of the received light signal and the local oscillation signal is obtained from the mixer 8, and a beat signal having a frequency much lower than the received light signal is extracted by the filter 13. The signal extracted by the filter 13 is input to the phase detector 9 together with the output of the frequency divider 4, converted into a signal corresponding to the phase difference, and supplied to the distance calculator 14. The oscillation output of the voltage controlled oscillator 5 is also input to the frequency divider 10 and is divided into a frequency of 1 / (N-1). However, N is an integer of 3 or more, and is usually set to a sufficiently large value. The phase detector 11 detects a phase difference between the frequency divider 4 and the frequency divider 10, and outputs a signal corresponding to the phase difference to the loop filter 12. The output of loop filter 12 is
It works so as to control the oscillation of the voltage controlled oscillator 5 so that the frequency and the phase of the frequency divider 4 and the frequency of the frequency divider 10 match. As a result, the output frequency f _L of the voltage controlled oscillator 5 is always f _L = f _{0 with} respect to the frequency f _{0 of the} reference oscillator 1 in a stable state.
× (N-1) / N. Therefore,
The output frequency of the mixer 8 is given by f ₀
Although + f _L = (2 + 1 / N) f ₀ and f ₀ −f _L = (1 / N) f ₀ are included as beat signals, there is a large difference between the two frequencies. Thus, only the low frequency component (f ₀ −f _L ) can be extracted.

An optical path switch 15 using a chopper or the like is disposed on the front surface of the light emitting element 3 to reflect light from the light emitting element 3 to a reflecting object.
A first optical path for transmitting light to the light receiving element 17 and causing the light receiving element 6 to receive the reflected light from the reflecting object 17;
A second optical path to be immediately received by the light receiving element 6 is alternately selected. The optical path detector 16 detects which optical path is selected by the optical path switch 15, and when the first optical path is selected, outputs the output of the phase detector 9 to the distance calculator 14.
When the second optical path is selected, the output of the phase detector 9 is stored in the second storage section A of the distance calculation section 14.
Is stored in the storage unit B. The distance calculation unit 14 outputs a distance signal by taking the difference between the two storage units A and B.

Embodiment 2 FIG. 2 is a block diagram of another embodiment of the present invention. In this embodiment, the oscillation output of the reference oscillator 1 is
, And the voltage-controlled oscillator 17 and the frequency divider 18, the loop filter 19, and the phase detector 20 convert the frequency to N times again, thereby obtaining the modulation frequency of the light emitting element 3. In this case, even if the variation in the oscillation frequency of the reference oscillator 1 is somewhat large, Since the variation is reduced by a factor of two, the configuration of the reference oscillator 1 is simplified.

Note that, in the present invention, the optical path switch 15 is not an essential component, and if a phase difference serving as a reference is stored by performing calibration measurement on a reflective object existing at the reference distance, the optical path switch 15 may be used at a distance other than the reference distance. The distance measurement can be performed for an existing reflecting object.

[Effects of the Invention] According to the present invention, a single reference oscillator can generate a frequency for modulating a light transmission signal and a local oscillation frequency, and it is not necessary to use a filter with high selectivity. There is an effect that characteristics are stable, accurate phase comparison can be performed, and ranging accuracy can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention, FIG. 2 is a block diagram of another embodiment of the present invention, FIG. 3 is a block diagram of a conventional example, and FIG. . 1 is a reference oscillator, 2 is a drive circuit, 3 is a light emitting element, 4 is a frequency divider, 5 is a voltage controlled oscillator, 6 is a light receiving element, 7 is a light receiving circuit, 8 is a mixer, 9 is a phase detector, and 10 is A frequency divider, 11 is a phase detector, and 12 is a loop filter.

Claims (2)

(57) [Claims] A light-emitting element for transmitting light modulated at a predetermined frequency to a reflective object; and a light-receiving element for receiving light transmitted from the light-emitting element and reflected by the reflective object. In a distance measuring sensor that measures the distance to the reflecting object from the phase difference between the light and the transmitted light, a reference oscillator that oscillates a reference frequency for modulating the light transmitted from the light emitting element and a voltage signal A voltage-controlled oscillator whose oscillation frequency is changed by the first frequency divider, a first frequency divider for dividing the reference frequency oscillated by the reference oscillator by N, and a second frequency divider for dividing the oscillation frequency from the voltage-controlled oscillator by (N-1) Divider and first and second frequency dividers
A first phase detector that detects a phase difference of a divided output of the frequency divider, a loop filter that generates a voltage signal for controlling oscillation of the voltage controlled oscillator from an output of the first phase detector, A mixer for mixing the light receiving output from the light receiving element with the oscillation output of the voltage controlled oscillator, and a second for detecting a phase difference between a beat output obtained by the mixer and a frequency divided output of the first frequency divider. A distance measuring sensor comprising a phase detector. 2. A light-emitting element for transmitting light modulated at a predetermined frequency to a reflective object, and a light-receiving element for receiving light transmitted from the light-emitting element and reflected by the reflective object. In a distance measuring sensor for measuring a distance to a reflecting object from a phase difference between light and transmitted light, a reference oscillator oscillating a reference frequency, and first and second voltages whose oscillation frequencies change according to a voltage signal. A control oscillator; a first frequency divider for dividing the reference frequency oscillated by the reference oscillator by N;
A second frequency divider that divides the oscillation frequency from the voltage-controlled oscillator by (N−1), a third frequency divider that divides the oscillation frequency from the second voltage-controlled oscillator by N, And a first phase detector for detecting a phase difference between frequency-divided outputs of the second frequency divider;
A first loop filter for generating a voltage signal for controlling the oscillation of the first voltage-controlled oscillator from an output of the first phase detector; and a light-receiving output from the light-receiving element, the oscillation output of the first voltage-controlled oscillator. A second phase detector for detecting a phase difference between the beat output obtained by the mixer and the frequency-divided output of the third frequency-divider, and a first and third frequency-divider Phase detector for detecting a phase difference between frequency-divided outputs of the detector, and a second loop for generating a voltage signal for controlling oscillation of a second voltage-controlled oscillator from an output of the third phase detector A distance measuring sensor comprising a filter.