JP2001183458A - Distance sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a distance sensor that uses an inexpensive LED as a light- projecting element and satisfies practical performance both in measured distance and response speed, for preventing collision of automobiles. SOLUTION: High-frequency pulse beams f1, f2 are irradiated by switching frequency from a projector 1, and reflected light from a detected object 2 is received by a light-receiving element 3 and mixed with mixing waves f2, f1 having frequency difference corresponding to an intermediate frequency f3. The beat down signal is compared with an intermediated frequency signal f3 as a phase reference by a phase difference detecting circuit 14, and respective phase delays corresponding to reciprocating distances 2L are measured. A phase delay difference measured between a f1 irradiation time and a f2 irradiation time is determined by a difference detecting circuit 20, and extracted as a detection signal of a distance L. The switching between f1 and f2 is performed at a synchronous time, thereby the duty ratio of the light-projecting element 1 is set below 1/50 radiate a pulse beam with a high crest value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distance sensor using light which can be used for preventing collision of an automobile.

[0002]

2. Description of the Related Art With the shift to the automobile society in recent years, the number of casualties due to traffic accidents has increased remarkably, which has become a serious social problem. As one of the countermeasures, development of a sensor for measuring a distance between vehicles has been advanced.

This sensor is used to detect a vehicle or an obstacle that may cause a collision while the vehicle is running, and to promptly take measures such as applying a brake. Therefore, it is necessary that the detection area has a predetermined width (angle range) and extends to a position more than one hundred and several tens of meters in front of the vehicle, and high-speed response is also required.

[0004] At present, a sensor of interest is 60 GH.
It uses millimeter waves in the Z and 76 GHz bands. The reason for this attention is that when millimeter waves are used, the wavelength is short, so the antenna for setting the above-mentioned detection area can be made large enough to be mounted on a vehicle, and the use of radio waves does not affect the vehicle color. That is, stable detection can be performed.

[0005] This millimeter wave radar is, for example, an FM-CW.
Produced by the method. In the FM-CW method, a transmission wave is FM-modulated with a triangular wave or a waveform close thereto, and a part of the reception wave and the transmission wave that are reflected back from the target and mixed are mixed according to the inter-vehicle distance and the relative speed. The frequency difference between the received wave and the transmitted wave is extracted as a beat signal, and measured to determine the inter-vehicle distance and the relative speed. However, a sensor using a millimeter wave is expensive because it uses special components to operate in the 60 GHz and 76 GHz bands, and has a problem that it is difficult to mount it on a general vehicle.

As a sensor utilizing light waves, an on-vehicle collision prevention device has been proposed which measures a propagation delay time until a radiated laser beam is reflected by a preceding vehicle and returns to determine a distance to the preceding vehicle. (JP-A-6-13159)
8). The reason for using the laser light is to obtain a reflected light with a high intensity that can be observed even during the day by using a laser light that is narrowed down and hardly diffuses. However, when using a laser beam, to cover the detection area,
It is necessary to sweep the detection area with a pulsed spot light. Therefore, it is necessary to provide a mechanism for rotating an optical system such as a pulse motor as a laser spot sweeping / radiating mechanism. Further, since a device for measuring the propagation delay time of laser light is complicated and expensive, it is difficult to provide a device using this laser light at low cost.

[0007] Further, since laser light has a dangerous property when it enters the human eye directly, an eye-safe laser using laser light that is safe to enter the eye has been developed. However, in this case, the sensor is particularly expensive, and practical use as a vehicle-mounted sensor cannot be achieved at all.

[0008] A lightwave distance meter using an LED (Japanese Unexamined Patent Publication No.
160842). This is the LED facing the sensing object
The distance meter measures the distance by measuring the phase delay of the reflected wave by irradiating the light wave emitted from the distance meter.This distance meter is installed in the distance meter because the measured value fluctuates due to changes in the amplification factor of the amplifier due to temperature. The distance of the reference optical path is corrected by the measured value of the phase lag when the same light emitting element and light receiving element are measured.

[0009]

An electro-optical distance meter using an LED can be manufactured at a low price that can be practically used as a sensor for preventing collision, as compared with a distance meter using a millimeter wave radar or a laser beam. .

However, even if the conventional lightwave distance meter described above is manufactured as a collision prevention sensor for an automobile, practical performance cannot be satisfied in both the measured distance and the response speed.

The required measurement distance cannot be obtained because of the LED
This is because the amount of light emission is small. If it is used to prevent collisions of automobiles, it is necessary to aim at a level that can receive the reflected light toward a preceding vehicle that is more than a hundred and several tens of meters away in the daytime and into a predetermined range where there is a possibility of collision. Must be irradiated. However, in a general driving method of the LED,
Cannot illuminate enough light.

The required response speed cannot be obtained because the conventional lightwave distance meter requires a predetermined time for switching between the optical path for measurement and the optical path for reference, and the light receiving levels of the measurement light and the reference light. Mechanical aperture and AGC to keep the pressure constant
This is because it is impossible to follow a change in the amount of reflected light due to a change in the direction of the irradiation position or the color of the detected object.

[0013] Therefore, the above-mentioned conventional lightwave distance meter cannot be applied to collision prevention of automobiles.

In the above-described lightwave distance meter, the FM
Although it is conceivable to adopt the -CW method, in the frequency range where the light emission can be controlled by the LED, if the light beam frequency is continuously changed, it operates in a frequency range considerably lower than the above-mentioned millimeter wave band. This is not feasible because a wideband amplifier is required and such an amplifier cannot be obtained.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a distance sensor using a light wave having a high-speed response and a detection area extending to a long distance, which can be used for collision prevention.

[0016]

According to a first aspect of the present invention, there is provided a distance sensor for irradiating high-frequency pulsed light toward a predetermined detection range, and detecting reflected light of a detection object within the irradiation range of the light projector. A first high-frequency pulse signal f1, a second high-frequency pulse signal f2, and a first high-frequency pulse signal f2.
A pulse signal generating circuit for generating an intermediate frequency pulse signal f3 having a frequency difference between the second high frequency pulse signal and the second high frequency pulse signal, and switching between the high frequency pulse signals f1 and f2, one of which is supplied to the projector and the other of which is supplied to the mixing circuit. A switching circuit, an intermediate frequency amplifier for amplifying the output of the mixing circuit, a phase comparison circuit for comparing the output of the intermediate frequency amplifier with the intermediate frequency pulse signal f3, and a high frequency pulse signal f1,
and a difference detection circuit for extracting a difference between outputs of the phase comparison circuit obtained by switching of f2 as a detection signal of a distance from the light emitting and receiving device to the detection object.

According to a second aspect of the present invention, in the distance sensor according to the first aspect, each of the first and second high-frequency pulse signals f1 and f2 and the intermediate frequency pulse signal f3 has a common frequency. The frequency of the original oscillation high-frequency pulse signal f0 is determined to be an integer fraction of the frequency of the original oscillation high-frequency pulse signal f0, and the pulse signal generation circuit divides the frequency of the original oscillation high-frequency pulse signal f0 by a counter-type frequency dividing circuit. Pulse signal f
1, f2, and an intermediate frequency pulse signal f3.

According to a third aspect of the present invention, in the distance sensor according to the first or second aspect, the switching by the switching circuit is performed by the first high-frequency pulse signal f1, the second high-frequency pulse signal f2, and the intermediate signal. It is performed when the phase of the frequency pulse signal f3 coincides, and the timing of the phase comparison by the phase comparison circuit is determined at the time when the output of the intermediate frequency amplifier after the frequency switching is stabilized.

According to a fourth aspect of the present invention, in the distance sensor according to any one of the first to third aspects,
A signal indicating the distance to the detected object detected by the difference detection circuit,
Using a relative velocity signal obtained by differentiating this signal, a comparator that determines whether or not there is a danger of collision based on a predetermined reference and outputs a collision prevention signal is provided.

In a distance sensor according to a fifth aspect of the present invention, a light emitting device and a light receiving device are arranged to face each other, and high-frequency pulse light is emitted from the light emitting device of the light emitting device toward the light receiving device of the light receiving device. A distance sensor that measures a distance from a light emitter to a light receiver based on a signal phase delay and determines that there is no object between the light emitter and the light receiver when the distance is within a predetermined range. An optical device that irradiates a pulse light toward a predetermined detection range, a pulse signal generating circuit that generates a first high-frequency pulse signal f1 and a second high-frequency pulse signal f2, and the two high-frequency pulse signals f
A switching circuit for switching and supplying 1, f2 to the projector;
A light receiving device for detecting a pulse light emitted from the light emitter, a mixing circuit for mixing a high frequency pulse signal for mixing with an output of the light receiver, a first high frequency pulse signal f
1, a second high frequency pulse signal f2, an intermediate frequency pulse signal f3 having a frequency difference between the first and second high frequency pulse signals
, A switching circuit for switching and supplying the first and second high-frequency pulse signals f1 and f2 to the mixing circuit, and a high-frequency pulse signal having a frequency different from the frequency of the high-frequency pulse light incident on the light receiver a synchronization pull-in circuit for switching the switching circuit so that f1 and f2 are supplied to the mixing circuit, an intermediate frequency amplifier for amplifying the output of the mixing circuit,
The output of the intermediate frequency amplifier is converted to the intermediate frequency pulse signal f3.
A phase comparison circuit that compares the phases of the signals with each other, a difference detection circuit that extracts a difference between the outputs of the phase comparison circuit obtained by switching the high-frequency pulse signals f1 and f2 by the switching circuit as a distance detection signal between the light emitting and receiving devices; An output circuit that outputs a determination of no object when the detection signal is within a distance range set in accordance with a separation distance between the light projecting device and the light receiving device.

In the distance sensors according to the first to fifth aspects, the following configurations can be added in any combination.

An LED is used as a light emitting element of the projector, and high frequency pulse signals f1 and f2 supplied from the switching circuit to the projector.
Is set to 1/50 or less (claim 6).

A resonant circuit having a variable capacitance diode is connected in series to the light receiving element of the light receiver, and the DC voltage supplied to the variable capacitance diode is changed by interlocking the switching of the high frequency pulse signals f1 and f2. The resonance frequency of the resonance circuit is tuned to the frequency of the high frequency pulse signal to be received (claim 7).

An avalanche photodiode is used as a light receiving element of the light receiver, and the avalanche photodiode is provided with first or second high frequency pulse signals f1 and f for mixing.
By supplying 2 as a voltage, amplification and mixing of the received high-frequency pulsed light are performed simultaneously (claim 8).

As a phase difference detection circuit, a saw-tooth wave generated in synchronization with the intermediate frequency pulse signal f3 as a phase reference is sampled and held at the rising or falling timing of the intermediate frequency pulse signal output from the mixing circuit. A phase detector that performs phase detection is used (claim 9).

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A description will be given of a first embodiment in which a distance sensor according to the present invention is configured as a collision prevention sensor.

In FIG. 1, 1 is a floodlight, 2 is a detection object,
3 is a light receiver. The light projector 1 is configured by a light emitting element such as an LED having a lens disposed in front thereof, and irradiates high frequency pulsed light to a range in which a detection object 2 such as a tail of a vehicle traveling ahead can be reliably irradiated. The light receiver 3 detects reflected light of the detection object 2 within the irradiation range of the light projector 1 and performs photoelectric conversion, and includes a lens and a light receiving element such as a phototransistor.

Reference numeral 4 denotes a pulse signal generation circuit which converts a high-frequency pulse signal f0 oscillated by the original oscillator 5 into a 1 / N1 frequency divider 6,
The frequency is divided by the 1 / N2 divider 7 and the 1 / N3 divider 8,
High-frequency pulse signal f1 and second high-frequency pulse signal f
An intermediate frequency pulse signal f3 having a frequency difference between the second and first and second high frequency pulse signals is generated. These first,
Each frequency of the second high-frequency pulse signals f1 and f2 and the intermediate frequency pulse signal f3 is determined to be an integer fraction of the frequency of the common original oscillation high-frequency pulse signal f0. Also,
The 1 / N1 frequency divider 6, the 1 / N2 frequency divider 7, and the 1 / N3 frequency divider 8 output a single pulse when counting the number of pulses corresponding to the frequency division ratio. A frequency divider is used. When such frequency division is performed, the pulse signals f1, f2, and f3 obtained do not have jitter (phase fluctuation) generated when a PLL circuit is used, and the distance is measured by phase measurement according to the present invention. High accuracy can be maintained. In addition, the phase matching timing as shown in FIG. 2 can be known with high accuracy.

Reference numeral 9 denotes a two-input / two-output switching circuit which converts the first high-frequency pulse signal f1 and the second high-frequency pulse signal f2 into two.
It receives at one input terminal and outputs it as f4 and f5 from two output terminals. The outputs of f4 and f5 are performed with a predetermined intermittent period as shown in FIG. 3, and the output direction is switched in the middle of each output period. Further, the output start time after the intermittent period and the output direction switching time are the times when the phases of f1, f2 and f3 match as shown in FIG.

The reason why the switching is performed at the time when the phases match with the intermittent period in between is to take a sufficiently long pause time of energization (if the phases match, the output period is shortened for the reason described later). it can). Thus, by setting the duty ratio to, for example, 2% or less, the magnitude of the light emission pulse current is increased by the maximum allowable current (continuous rating)
For example, it can be several hundred times, and with a low-cost LED,
It is possible to irradiate a vehicle traveling ahead by more than a hundred tens of meters during the day with light having an intensity sufficient to obtain a sufficient amount of reflected light while securing a necessary irradiation area.

Reference numeral 10 denotes a gate circuit, which generates gate signals G1, G2 and G3 for determining the operation timing from a signal obtained by dividing the intermediate frequency pulse signal f3 output from the pulse signal generation circuit 4 by the 1 / N4 frequency divider 11. I do. Gate signal G1
Is used as a switching signal of a switching circuit 9 that switches between the first high-frequency pulse signal f1 and the second high-frequency pulse signal f2 and outputs the signals as f4 and f5. Gate signals G2, G3
Determines the timing of the phase measurement.

Reference numeral 12 denotes a high-frequency amplifier for amplifying the output of the light receiver 3, to which the gate signal G1 is input in order to switch the resonance frequency of the internal resonance circuit in accordance with the light emission frequency.

Reference numeral 13 denotes a mixing circuit for performing heterodyne detection, which mixes the received light output f6 amplified by the high-frequency amplifier 12 and the high-frequency pulse signal f5 to generate a beat-down signal. The received light output f6 is a high-frequency pulse light (signal) f4 incident on the light receiver 3 with a phase delay corresponding to the distance 2L from the projector 1, and the high-frequency pulse signal f5 is input from the switching circuit 9 without phase delay. Things. The high-frequency pulse signals f4 and f5 are obtained by alternately replacing the first and second high-frequency pulse signals f1 and f2 by the switching circuit 9, and the frequency of the beat-down signal (intermediate frequency signal) becomes constant. .

An intermediate frequency amplifier 14 amplifies the beatdown signal output from the mixing circuit 13 and outputs the amplified signal as f7.

Reference numeral 15 denotes a phase comparison circuit, which is an intermediate frequency amplifier 1
4 is compared with the intermediate frequency pulse signal f3 used as a phase reference, and a voltage signal V1 corresponding to a phase delay is output. The timing of this phase comparison is
This is a period during which the gate signals G2 and G3 output from the gate circuit 10 are generated. The OR of the gate signals G2 and G3 is obtained by the OR gate 16, and the intermediate frequency pulse signal f3 is passed through the AND gate 17 opened by the OR signal. Is supplied. As shown in FIG. 3, the generation timings of the gate signals G2 and G3 are made to coincide with the timing when the output f7 of the intermediate frequency amplifier 14 is stabilized (after the start of irradiation and after the switching of the frequencies of f4 and f5). It is.

Reference numerals 18 and 19 denote first and second memories which store the voltage signal V1 output from the phase comparator 15 at the falling timing of the gate signals G2 and G3. 20
Is a difference detection circuit that calculates the difference between the voltage signals V2 and V3 (each output of the phase comparison circuit 15 obtained by switching the high-frequency pulse signals f1 and f2) stored in the memories 18 and 19, and outputs the difference between the light emitter 1 and the light receiver 3. Is output as a distance detection signal V4 representing the distance L from the object to the detection object 2.

Numeral 21 denotes a differentiating circuit for differentiating the output of the difference detecting circuit 20 and outputting it as a relative speed signal V5 between the vehicle and the detected object 2. A comparator 22 receives the input of the distance detection signal V4 and the relative speed signal V5, and determines whether or not there is a danger of collision. When there is a danger, a collision warning signal V6 is output. When the collision warning signal V6 is generated, the indicator lamp 23 and the buzzer 24 operate.

The principle by which the distance L from the light emitter 1 and the light receiver 3 to the object 2 can be measured by the distance sensor A shown in FIG. 1 will be described.

As shown in FIG. 4, the phase relationship between the transmitted wave VP having the frequency f1 (= ω1 / 2π) emitted from the light projector 1 and the received wave VR which is reflected on the detection object 2 and returns to the light receiver 3 is as follows. It looks like this:

If the transmission wave VP emitted from the projector 1 is VP = A · cos [ω1 · t1 + φ1] (1) (where φ1 is the phase at t1), this is reflected by the detection object 2 and received. The received wave VR incident on the device 3 is expressed as follows: VR = B · cos [ω1 · t1 + φ1 + 2πN + φR] (2)

However, 2πN + φR means that at the time point t1, the previously irradiated transmission wave is transmitted from the projector 1 to the receiver 3
The phase change occurring in the round trip path 2L up to
πN + φR = (2L) / C · 2πf1. Where C
Is the speed of light αR = ω1 · t1 + φ1 + 2πN + φR, Equation (2) can be expressed as VR = B · cosαR (2) ′.

The mixing wave (high-frequency pulse signal f5) input to the mixing circuit 13 is expressed as VM = C · cos (ω2 · t1 + φ2) (where φ2 is the phase of the mixing wave at t1), and β2 = ω2 · If t1 + φ2, VM = C · cosβ2. (3) The composite wave Vc generated by the mixing is as follows: Vc = VR × VM = B · cosαR · C · cosβ2 =
(1/2) BC [cos (αR + β2) + cos (α
R-β2)] (4). Intermediate frequency amplifier 14
Extracts the component having the frequency difference of each wave from the composite wave VC generated by the mixing.
4 is f7 = D · cos (αR−β2). By substituting αR and β2, f7 = D · cos [(ω1 · t1 + φ1 + 2πN + φ)
R) − (ω2 · t1 + φ2)] = D · cos [(ω1−
ω2) · t1 + (φ1−φ2) + (2πN + φR)]
... (5).

According to the equation (5), the phase delay (2πN) occurring in the received wave VR from the transmission to the return of the reflected wave is shown.
+ ΦR) is found as it is in the output f7 of the intermediate frequency amplifier 14 that reduces the frequency (angular velocity) to ω1−ω2.

As shown in FIG. 2, the phase comparison circuit 15
Compared with f3 synchronized with f1 and f2, the above (5)
Measure the phase of the equation. That is, (ω1−ω2) · t1
Is defined as 0, φD = (φ1−φ2) + (2πN + φ
R)... (6) is measured.

This measured value φD is stored in the first memory 18 at the falling timing of the gate signal G2.

Next, the phase of f1 and f2 is changed by φ as described above.
1, while keeping the same by setting φ1 = φ2 = 0 as shown in FIG. 2, for example, and changing the frequency (angular velocity) of the transmission wave f4 from f1 (ω1) to f2
(Ω2), the output VE of the intermediate frequency amplifier 14
Is VE = E · cos [(ω2−ω1) · t1 + (φ2−φ
1) + (2πM + φS)] (7)

The phase of the equation measured by the phase comparison circuit 15 is φE = (φ2−φ1) + (2πM + φS) (8) This φE is stored in the second memory 19 at the falling timing of the gate signal G3.

When the difference between φD and φE stored in the memory is calculated by the difference detection circuit 20, Δφ = φD−φE = (φ1−φ2) + (2πN + φR)
− [(Φ2−φ1) + (2πM + φS)] = 2π (N−
M) + (φR−φS) (9)

As shown in FIG. 5, the difference between the phase difference (2πN + φR) while the light of ω1 travels between the reciprocating distances of 2L and the difference of the light of ω2 between the reciprocating distances of 2L as shown in FIG. Means that the difference of the phase change (2πM + φS) is obtained while the difference Δφ is equal to ω3 (=
ω1-ω2) represents a phase change when the virtual light travels between 2L reciprocating distances.

That is, Δφ = (2L) / C · (2πω
L = Δφ · C / 4π · ω3 (10) where C is the speed of light, and the distance L can be obtained directly from Δφ.

Note that the phase detection circuit 15 has a function of 2πN in FIG.
Since φD and φE are measured with 0 and 2πM being 0, φR> φs does not always hold. Therefore, when φR <φS as shown in FIG. 5, Δφ = φR + 2π−φ
S is calculated.

When L is obtained as described above, a phase shift due to a temperature drift or the like is removed, and high measurement accuracy can be secured. The phase shift due to the temperature drift occurs in the light emitting element, the light receiving element, and the first-stage amplifier, and the temperature drift of the light emitting element, which generates a large amount of heat, is particularly large. In the present invention, the frequency of the transmission wave f4 is switched between f1 and f2, and φD and φE in the above equations (6) and (8) are measured. The error due to the temperature drift superimposed on φD and φE can be considered to be substantially the same since the measurements of φD and φE are performed at substantially the same time, and Δφ in equation (9) is used.
= ΦD−φE, these are eliminated by being offset.

When the frequency of the transmission wave is switched from f1 to f2, the magnitude of the noise may slightly fluctuate.
This variation cannot be removed by the above method. This variation ± φ
N is generated, for example, by white noise generated in the light receiving element and the first-stage amplifier that amplifies the output when receiving light. In order to reduce the error due to the noise fluctuation, the following arithmetic method can be adopted instead of the above equation (10).

According to equation (9), (2πN + φ in FIG. 5)
R) and (2πM + φS).
Assuming that the difference between (2πN + φR) and (2πM + φS) is within 2π, from this difference Δφ, φR, φS,
N and M can be obtained as independent values. This is because the frequencies f1, f2, and f1-f2 are known. Therefore, the calculation is performed using an arithmetic expression of L = (2πN + φR) · C / 4π · ω1 or L = (2πM + φS) · C / 4π · ω2. In these equations, the error rate due to the variation ± φN is ± φN / (2πN + φ
R) or ± φN / (2πM + φS), and the error given to L is greatly reduced as compared with the case of Expression (9).
Further, measurement accuracy can be improved.

The maximum detection angle that can be measured by the phase detection circuit 15 is 360 °, and assuming that a reciprocating distance of 2 L is measured at one wavelength of ω3, the maximum detectable distance LMAX is f1 =
When 4.5 MHz and f2 = 4.05 MHz, C (speed of light) =
Since 3 × 108 (m), LMAX = [(3 × 108) / 2] · ｛1 / [(4.5
−4.05) × 106]｝ = 333 (m).

The distance sensor A shown in FIG. 1 will be further described. The high-frequency amplifier 12 of FIG. 1 can be configured as shown in FIG. 6, for example. In FIG. 6, reference numeral 25 denotes an LC parallel resonance circuit connected in series with the light receiving element of the light receiving device 3, and a DC power supply (+ V) is connected to both ends of the series circuit with the light receiving element and appears at both ends of the resonance circuit 25. The voltage is input to the high frequency amplifier circuit 26 as a light receiving signal. Resonant circuit 2
5, a variable capacitance diode 28 is connected in parallel via a coupling capacitor 27 for cutting a direct current component, so that the resonance frequency of the resonance circuit 25 is changed. The voltage that changes the capacitance of the variable capacitance diode 28 is supplied from two voltage regulators 29 and 30 and a changeover switch 31 that receives these adjustment voltages and selectively supplies one of the voltages to the variable capacitance diode 28. The changeover of the changeover switch 31 is performed by the gate signal G1. This switching is to make the resonance frequency of the resonance circuit 25 coincide with the frequency of the high-frequency pulse signal to be received. The DC resistance of the series circuit of the light receiving elements is reduced to prevent saturation due to disturbance light and to reduce the signal frequency impedance. -The dance is enlarged to improve the sensitivity. In the present invention, the high-frequency pulse signal is received, and the rising and falling timings of the high-frequency pulse signal need only be detected. Since the waveform distortion due to saturation is not a problem, the amplification by the high-frequency amplifier circuit 26 is sufficiently performed.

The output f7 of the intermediate frequency amplifying circuit 14 changes the amplitude toward a stable state while changing the phase according to the Q of the resonance circuit, and the phase measurement waits for the stabilization of this amplitude. There is a need. One light emission time is determined corresponding to the time required for the stabilization, and the measurement timing during the light emission time is determined by the gate signals G2 and G3. However, if one waits until f7 is sufficiently stabilized, one light projection time becomes longer. From the viewpoint of shortening the measurement time and reducing the duty ratio of the light emitting element, it is preferable to shorten the light emitting time. This is because when the duty ratio is reduced, the peak value of the irradiation pulse can be increased, and measurement at a longer distance becomes possible. Therefore, before complete stabilization, for example, when the amplitude of f7 rises to 80% is regarded as a tentative stable point, measurement is performed, and how much the light projection time can be reduced is considered.

First, a time at which an accuracy of 1/1000 is obtained in amplitude is calculated. This is the accuracy used in the conventional lightwave distance meter, and it is sufficient to calculate the time to obtain 0.999 in the expression of 1-et. The result is ta = 6.91. Next, the time for the amplitude to rise to 80% (0.8) is calculated using the same equation. This results in tb = 1.61. These ratios t
b / ta is 0.23, which is 23% as compared with the conventional method.
It can be shortened to the following. Next, we consider whether such a shortening is actually possible.

F at the start of light emission at the light emission frequency f1
The rising edge of the seven signals shows a constant rising characteristic because there is no signal before that. However, the rising characteristic of the f7 signal when the light emission frequency is switched to f2 greatly varies depending on the phase relationship between f1 and f2. For example, assuming that the Q of the intermediate frequency amplifier circuit 14 is 100, the time required for the first rise of the f1 signal is about 85 μs, but when this is switched to f2, if the phases of f1 and f2 are shifted by 180 °. Since the frequency of the resonator coincides with the frequency of the beat signal, as shown by the dotted line in FIG. 3, the frequency temporarily drops to near 0 level, and the subsequent rise takes about 140 μs. In the state where the rising characteristics after switching greatly vary, the light projection time cannot be reduced.

However, as shown in FIG. 2, when switching is performed when the phases of f1 and f2 match, f7 maintains a stable state as shown by the solid line in FIG. 3, and this characteristic becomes constant. This makes it possible to reduce the light projection time, for example, when measuring up to 80%. When the light projection time is shortened in this way, the phase measurement value is slightly reduced, but the measurement timing is determined and the rate of reduction is constant, so that correction may be performed later.

When the time is shortened, the accuracy of measuring the relative distance may slightly decrease, but the repetition accuracy that the same measurement value is obtained when the same distance is measured is kept high. This is a preferable condition when the present invention is applied to a collision prevention device. It is important for the anti-collision device to have high accuracy in measuring the relative speed with respect to the preceding vehicle, and high repeatability when measuring the relative distance means that the relative speed difference obtained by differentiating this relative distance is high. This is because the accuracy is high.

As the phase comparison circuit 15 in FIG. 1, for example, a circuit as shown in FIG. 7 can be used. FIG. 8 shows operation waveforms of the circuit 15. This circuit 15 generates a sawtooth wave having the same phase as the intermediate frequency pulse signal f3 as a phase reference by the sawtooth wave generator 32, extracts the zero cross point of the output f7 of the intermediate frequency amplifier 14 by the comparator 33, and converts the zero cross point into a rectangular wave. The sampling and holding circuit 35 samples the sawtooth wave at the falling edge of the rectangular wave detected by the differentiating circuit 34 and integrates the sampled value by the integrating circuit 36 to obtain an A / D converter 37. Digitize with V
Output as 1. The phase of the circuit 15 is measured by adjusting the gate signals G2 and G3 matched with the period during which the beat signal is stabilized.
Is performed at the occurrence of.

The voltage V1 is φD,
It represents φE, and is taken in and stored in the first and second memories 18 and 19 shown in FIG. 1 at the falling timing of the gate signals G2 and G3. In this storage method, for example, old data is discarded, new data is stored, and data for a certain period is retained.

The difference detection circuit 20 of FIG. 1 outputs the difference between the moving averages of the data stored in the first and second memories 18 and 19 as a signal representing the distance L to the detection object 2. Since this value is a difference between the measured values at substantially the same time, the light emitting element,
The influence of the characteristic fluctuation due to the temperature in the light receiving element and each amplifier is canceled out, and the influence is not received. This cancellation can also eliminate the phase shift due to the Doppler effect due to the speed difference from the detected object. Since the signal representing the distance measured in this way is hardly affected by disturbance due to temperature or the like by taking the difference, the repeatability is high, and the measurement can be performed at high speed. , It is possible to accurately obtain a relative speed signal V5 between the vehicle and the detection object 2.

The distance detection signal V4 and the relative speed signal V5 obtained as described above are input to the comparator 22 to determine whether there is a danger of collision. This comparison / judgment is performed from the relationship between the traveling speed and the braking distance, taking into account the idling distance. When there is a danger, the collision warning signal V6 is generated, and the indicator light 23 and the buzzer 24 operate. The collision warning signal V6 can be used as a control signal for a brake operation or the like.

It is practical that the first and second memories 18 and 19, the difference detecting circuit 20, the differentiating circuit 21 and the comparator 22 are constituted by software processing by a computer.

In the embodiment of FIG. 1, the light receiving element of the light receiver 3, the high-frequency amplifier 12, and the mixing circuit 13 used in the embodiment of FIG. Can be achieved. Distance sensor B according to the embodiment in this case
Is shown in FIG.

In FIG. 9, the portions denoted by the same reference numerals as those in FIG. 1 represent the same components. The avalanche photodiode 38 is used as a light receiving element, and its output is output to the intermediate frequency amplifier 14. The avalanche photodiode 38 utilizes an avalanche photo phenomenon in which, when a high electric field is formed in the depletion layer, electrons generated by incident light are accelerated and collide with atoms to generate secondary electrons. It also has the function of a high-frequency amplifier. If a high-frequency pulse signal to be mixed is used as a voltage for generating a high electric field, a function as a mixer can be provided at the same time. Therefore, a high voltage generating circuit 39 is provided in the sensor, and a voltage adder 40 which is opened by the high frequency pulse signal f5 output from the switching circuit 9 and applies the output voltage of the high voltage generating circuit 39 to the avalanche photodiode 38 is provided. Provide.

Although the embodiment shown in FIG. 1 and the embodiment shown in FIG. 9 constitute the present invention as a collision preventing device, the present invention can be applied to a facing photoelectric switch.

The opposing photoelectric switch has a light emitter and a light receiver opposed to each other, and detects the presence or absence of an object between the light emitter and the light receiver based on whether or not light emitted from the light emitter enters the light receiver. In this case, if a large number of photoelectric switches are arranged in the same premises, a malfunction may occur if the light from the projector of another photoelectric switch accidentally enters its own light receiver.

Therefore, utilizing the distance detection function of the present invention,
Only when light is received from the photoelectric switch within the set distance range, a signal indicating that there is no object between the light emitting and receiving devices is output.

An embodiment in this case is shown in FIG. 10 and FIG. FIG. 10 is a configuration diagram of the light projecting device C. The light projecting device 1 irradiates a pulse light toward a predetermined detection range, and a pulse signal generating device that generates a first high-frequency pulse signal f1 and a second high-frequency pulse signal f2. A circuit having a circuit 4 and a switching circuit 9 for switching and supplying the two high-frequency pulse signals f1 and f2 to the projector 1 is used. These components are shown in FIG.
Are constituted by the same parts represented by the same reference numerals as those of the embodiment described in FIG.

FIG. 11 is a block diagram of the light receiving device D, which is constructed by taking out circuit portions (common portions having the same reference numerals) related to the processing of the light receiving signal from the circuit of FIG. ing. However, since the light receiving device D cannot know the timing of the irradiation and switching of the high frequency pulse signal emitted from the opposing light projecting device C, the high frequency of the high frequency pulse light different from the frequency of the high frequency pulse light incident on the light receiver 3 A synchronization pull-in circuit 41 that switches the switching circuit 9 so that the pulse signals f1 and f2 are supplied to the mixing circuit 13 is provided. The synchronization pull-in circuit 41 includes the intermediate frequency amplifier 14
Level detector 42 detects that the output of
, The incidence of high-frequency pulsed light from the light projecting device C is detected, the timing is measured by the timer circuit 43, and a timing signal for frequency switching is supplied to the gate circuit 10. Since the light receiving device D determines whether there is an object between the light emitting device C and the light receiving device D, the output of the difference detection circuit 20 is compared with a predetermined threshold value by the comparator 22. Then, the presence or absence of the object is determined. This determination result is output to the indicator light 23 and the control output circuit 44 for external output.

[0074]

According to the first aspect of the present invention, the modulation frequencies f1 and f2 of the light that determine the measurement accuracy are set high, and irradiation and reflected light are received for each frequency to reach the detection object. The phase lag generated according to the distance is detected, and the phase change with respect to the distance is increased by taking the difference of the phase lag to perform the measurement. Therefore, high-precision measurement is possible in spite of long-distance measurement. In addition, in this measurement method, noise due to temperature drift or the like is removed at the same time by taking the difference, and processing such as temperature correction is unnecessary, so that high-speed measurement becomes possible.

According to the invention of claim 2 of the present invention, the first and second high-frequency pulse signals f1 and f2 and the intermediate frequency pulse signal f3 are generated by a counter-type frequency divider, and the jitter, which is a phase fluctuation, is generated. Does not occur. by this,
The measurement accuracy of the device of the present invention in which the distance detection is performed by phase measurement can be improved.

According to the third aspect of the present invention, the time required for the output of the intermediate frequency amplifier to stabilize is reduced, and the duty ratio of the light emitting element is further reduced, whereby the light emission amount of the light emitting element is reduced. It can increase the measurement distance.

The invention according to claim 4 of the present invention provides a configuration for generating an output necessary for preventing collision of a vehicle.

According to a fifth aspect of the present invention, in a factory or the like where a plurality of opposing photoelectric switches are arranged, a malfunction due to the interference is added as a factor of detecting the distance between the light emitting and receiving devices. Can be prevented.

According to the invention of claim 6 of the present invention, irradiation of a large amount of pulse light capable of measuring a long distance even during the day can be achieved by an inexpensive LED.

In the invention according to claim 7 of the present invention, the light receiving sensitivity can be increased by connecting in series a resonance circuit having a variable capacitance diode as a component to the light receiving element of the light receiving device.

The invention according to claim 8 of the present invention enables downsizing by using an avalanche photodiode for the light receiving element of the light receiving device.

According to the ninth aspect of the present invention, by performing the phase measurement in an analog manner, the phase comparison circuit can be simplified and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a distance sensor of the present invention configured as a collision prevention sensor.

FIG. 2 is a timing chart of frequency switching in the apparatus of FIG. 1;

FIG. 3 is a signal waveform diagram of a main part of the apparatus of FIG. 1;

FIG. 4 is a diagram showing a transmission wave emitted from a projector and a reflected wave thereof.

FIG. 5 shows transmission waves ω1, ω2 in the distance sensor of FIG.
And the Δ which is actually measured as a signal representing the distance L
The figure which shows the relationship with (phi).

FIG. 6 is a diagram illustrating a circuit example of a high-frequency amplifier.

FIG. 7 is a circuit diagram showing a configuration example of a phase comparison circuit.

FIG. 8 is a signal waveform diagram of the circuit of FIG. 7;

FIG. 9 is a circuit diagram showing an embodiment of the present invention using an avalanche photodiode.

FIG. 10 is a circuit diagram of a light emitting device when the present invention is configured as a facing photoelectric switch.

11 is a circuit diagram of the light receiving device of the opposing photoelectric switch shown in FIG. 10. 1 Floodlight 2 Detected object 3 Light receiver 4 Pulse signal generation circuit 9 Switching circuit 10 Gate circuit 12 High frequency amplifier 13 Mixing circuit 14 Intermediate frequency amplifier 15 Phase comparison circuit 18, 19 First and second memories 20 Difference detection circuit 28 Variable capacitance diode 32 Sawtooth wave generator 33 Comparator 35 Sample hold circuit 37 A / D converter 38 Avalanche photodiode

Continued on front page F term (reference) 2F065 AA06 BB05 CC11 DD01 FF12 FF13 GG07 GG12 HH02 HH12 JJ01 JJ15 JJ18 KK01 PP22 QQ13 QQ23 QQ25 QQ47 SS09 SS15 UU05 5J084 AA05 AB01 AC02 AD02 CA19 CA43 CA69 EA22 EA29

Claims (9)

[Claims] 1. A projector for irradiating high-frequency pulsed light toward a predetermined detection range, a light-receiving device for detecting reflected light of a detection object within an irradiation range of the light-emitting device, and a high-frequency pulse for mixing the output of the light-receiving device. A mixing circuit for mixing with the signal; a pulse signal generating circuit for generating an intermediate frequency pulse signal f3 having a frequency difference between the first high frequency pulse signal f1, the second high frequency pulse signal f2, and the first and second high frequency pulse signals A switching circuit for switching the high-frequency pulse signals f1 and f2, one of which is supplied to the projector and the other of which is supplied to the mixing circuit; an intermediate frequency amplifier for amplifying the output of the mixing circuit; A phase comparator for comparing the phase with the frequency pulse signal f3, and a difference between respective outputs of the phase comparator obtained by switching the high frequency pulse signals f1 and f2 by the switching circuit, Distance sensor, characterized by comprising a differential detection circuit for taking out a detection signal of the distance al detection object. 2. The first and second high-frequency pulse signals f1, f
2, and each frequency of the intermediate frequency pulse signal f3 is determined to be an integer fraction of the frequency of the common original oscillation high frequency pulse signal f0.
Is divided by a counter-type frequency dividing circuit to generate first and second high-frequency pulse signals f1, f2, and an intermediate frequency pulse signal f3. Distance sensor. 3. The switching by the switching circuit is performed when the phases of the first high-frequency pulse signal f1, the second high-frequency pulse signal f2, and the intermediate frequency pulse signal f3 match,
3. The distance sensor according to claim 1, wherein the timing of the phase comparison by the phase comparison circuit is determined at a time point when the output of the intermediate frequency amplifier is stabilized after the frequency is switched. 4. A signal indicating a distance to a detected object detected by a difference detection circuit and a relative speed signal obtained by differentiating the signal are used to determine whether or not there is a danger of collision on a predetermined basis. The distance sensor according to claim 1, further comprising a comparator that outputs a collision prevention signal. 5. A light-emitting device and a light-receiving device are arranged opposite to each other, and high-frequency pulse light is emitted from the light-emitting device of the light-emitting device toward the light-receiving device of the light-receiving device. A distance sensor that measures a distance and determines that there is no object between the light emitting device and the light receiving device when the distance is within a predetermined range. A projector for irradiating pulse light, a pulse signal generating circuit for generating a first high-frequency pulse signal f1 and a second high-frequency pulse signal f2,
A switching circuit for switching the two high-frequency pulse signals f1 and f2 to the light emitter; a light-receiving device detecting a pulse light emitted from the light emitter; and a high-frequency pulse for mixing in an output of the light receiver. A mixing circuit for mixing the signals, and a first high-frequency pulse signal f
1, a second high frequency pulse signal f2, an intermediate frequency pulse signal f3 having a frequency difference between the first and second high frequency pulse signals
, A switching circuit for switching and supplying the first and second high-frequency pulse signals f1 and f2 to the mixing circuit, and a high-frequency pulse signal having a frequency different from the frequency of the high-frequency pulse light incident on the light receiver a synchronization pull-in circuit for switching the switching circuit so that f1 and f2 are supplied to the mixing circuit, an intermediate frequency amplifier for amplifying the output of the mixing circuit,
The output of the intermediate frequency amplifier is converted to the intermediate frequency pulse signal f3.
A phase comparison circuit for comparing the phase with the output signal, a difference detection circuit for extracting a difference between outputs of the phase comparison circuit obtained by switching of the high-frequency pulse signals f1 and f2 by a switching circuit as a distance detection signal between the light emitting and receiving devices, A distance sensor, comprising: an output circuit that outputs a determination of no object when a detection signal is within a distance range set in accordance with a distance between the light emitting device and the light receiving device. 6. The high frequency pulse signals f1 and f supplied from the switching circuit to the light projector, wherein the light emitting element of the light projector is an LED.
The distance sensor according to any one of claims 1 to 5, wherein a duty ratio of 2 is set to 1/50 or less. 7. A resonance circuit having a variable capacitance diode as a constituent element is connected in series to the light receiving element of the light receiver, and changes a DC voltage supplied to the variable capacitance diode in conjunction with switching of the high frequency pulse signals f1 and f2. The distance sensor according to any one of claims 1 to 6, wherein the resonance frequency of the resonance circuit is tuned to the frequency of a high-frequency pulse signal that receives light. 8. An avalanche photodiode is used as a light receiving element of the light receiver, and the avalanche photodiode is provided with a first or second high-frequency pulse signal f1 for mixing.
The distance sensor according to any one of claims 1 to 7, wherein by supplying f2 as a voltage, amplification and mixing of the received high-frequency pulsed light are performed simultaneously. 9. A saw-tooth wave generated by a phase difference detection circuit in synchronization with an intermediate frequency pulse signal f3 as a phase reference,
9. A phase detection circuit which samples and holds the phase at the rising or falling timing of the intermediate frequency pulse signal output from the mixing circuit to detect the phase.
A distance sensor according to any one of the above.