JP2018040656A - Distance measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a distance to be accurately measured even when characteristics of a light receiving element for monitoring vary due to a temperature change or the like.SOLUTION: A distance measuring device 100 comprises: a light projecting unit 1 for projecting a laser beam toward a target TG; a light receiving unit 2 for receiving reflection light that the laser beam has reflected on the target object TG; a monitor light detection unit 3 for detecting, as monitor light, a part of the laser beam projected by the light projecting unit 1; an A/D converter 21 for A/D converting an output signal from the light receiving unit 2; an A/D converter 22 for A/D converting an output signal from the monitor light detection unit 3; and a distance calculator 23 for calculating a distance to the target TG on the basis of outputs of respective A/D converters 21, 22. The monitor light detection unit 3 includes: a photo diode 10 for receiving the monitor light; an amplifier circuit 11 for amplifying an output signal from the photo diode 10 to a predetermined level; and a clamp circuit 12 for fixing a level of an output signal from the amplifier circuit 11 to a clamp level lower than the predetermined level.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a distance measuring device that projects laser light toward an object, receives reflected light reflected by the object, and calculates a distance to the object.

  A distance measuring device using a laser radar is a device that measures the distance to an object based on the time from projecting a laser beam toward the object until receiving the laser beam reflected by the object. It is. In such a distance measuring apparatus, there is a time delay from when a signal instructing laser beam projection is output until the laser beam is actually projected. For this reason, if the distance is calculated by regarding the output time of the projection command signal as the projection time of the laser beam, an error occurs in the calculated distance. Therefore, it has been conventionally performed to correct the error in distance due to the above-described time delay by monitoring the light emitted from the laser and detecting the actual light projection time.

  For example, in the distance measuring device described in Patent Document 1, a part of the laser light emitted from the laser diode is guided by an optical fiber and received by the photodiode, and the time from generation of the laser light to reception is determined. A distance correction value is calculated based on the light guide distance of the optical fiber, and the measurement distance is corrected using this distance correction value.

  Further, in the distance measuring device described in Patent Document 2, a laser beam passage hole is formed in a polygonal pyramid mirror, and a calibration optical fiber for guiding the laser beam that has passed through the passage hole to a light receiving element is provided. The normal distance measurement data by the laser light reflected by the object is corrected based on the distance measurement data by the laser light that has passed through the optical fiber.

JP 2010-204015 A JP 2000-205858 A

  In Patent Documents 1 and 2, a part of the laser light is received as monitor light, and the measurement distance is corrected based on the projection time and the light reception time of the monitor light. However, characteristics such as sensitivity and bandwidth of the light receiving element that receives the monitor light vary depending on the environment such as the ambient temperature and the power supply voltage, or individual differences between elements. For this reason, the waveform of the light reception output (current flowing through the light receiving element) of the monitor light varies, and this causes an error in the light projection time of the laser light and thus the distance calculation result. Therefore, only the correction of the measurement distance by the methods of Patent Documents 1 and 2 cannot solve the above problem, and it is difficult to obtain satisfactory measurement accuracy.

  An object of the present invention is to provide a distance measuring device capable of measuring a distance with high accuracy even when characteristics of a light receiving element for monitoring fluctuate due to a temperature change or the like.

  A distance measuring device according to the present invention includes a light projecting unit that projects laser light toward an object, a light receiving unit that receives reflected light reflected from the object, and a laser projected from the light projecting unit. A monitor light detector for detecting a part of the light as monitor light, a first A / D converter for analog-digital conversion of the output signal of the light receiver, and an analog-digital conversion of the output signal of the monitor light detector A second A / D converter; and a distance calculation unit that calculates a distance to the object based on outputs of the first A / D converter and the second A / D converter. The monitor light detection unit fixes a light receiving element that receives the monitor light, an amplification circuit that amplifies the output signal of the light receiving element to a predetermined level, and a level of the output signal of the amplification circuit at a clamp level lower than the predetermined level. A clamping circuit.

  According to such a configuration, by setting the gain of the amplifier circuit of the monitor light detection unit to a very large value, the rise of the output signal of the monitor light detection unit becomes steep, so that the characteristics of the monitor light receiving element are Even if it fluctuates according to the voltage or the like, the fluctuation of the waveform of the output signal of the monitor light detection unit is reduced. As a result, the variation in the distance to the object calculated based on the output signal is reduced, and the measurement accuracy is improved.

  In the present invention, the amplifier circuit of the monitor light detection unit may amplify the output signal of the light receiving element to a saturation level exceeding the maximum input voltage value of the second A / D converter.

  In the present invention, a threshold value lower than the clamp level may be set for the output signal of the monitor light detection unit. In this case, the distance calculation unit includes a time when the level of the output signal of the monitor light detection unit exceeds the threshold, a time when the light receiving unit receives the reflected light, and a time corresponding to a response delay of the light receiving element of the monitor light detection unit. Based on the above, the flight time of the laser light projected from the light projecting unit is calculated, and the distance to the object is calculated based on the flight time.

  In the present invention, the clamp level of the clamp circuit may be higher than the threshold and lower than the maximum input voltage value of the second A / D converter.

  In the present invention, the light projecting unit may include a solid-state laser that projects laser light toward an object. Alternatively, the light projecting unit may include a laser diode that projects laser light toward the object.

  ADVANTAGE OF THE INVENTION According to this invention, even if the characteristic of the light receiving element for monitoring fluctuates by temperature changes etc., the distance measuring apparatus which can measure distance with high precision can be provided.

It is the block diagram which showed the distance measuring device which concerns on embodiment of this invention. It is a schematic sectional drawing for demonstrating the detection of monitor light. It is the circuit diagram which showed the specific structure of the monitor light detection part. It is a wave form diagram for demonstrating operation | movement of a monitor light detection part. It is a wave form diagram for demonstrating operation | movement of a monitor light detection part. It is a figure explaining the setting of the threshold value with respect to the output of a monitor light detection part. It is a timing chart which showed operation of a distance measuring device. It is a timing chart which showed operation of a comparative example. It is a wave form chart showing variation in output of a monitor light detection part. It is the block diagram which showed the distance measuring device which concerns on other embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals.

  First, the configuration of the distance measuring device will be described with reference to FIGS.

  In FIG. 1, a distance measuring device 100 is mounted on a vehicle and measures a distance to a target object TG in front of the vehicle. The target object TG includes obstacles on the road in addition to the preceding vehicle 30.

  The distance measuring apparatus 100 includes a light projecting unit 1 that projects laser light toward the object TG, a light receiving unit 2 that receives reflected light reflected by the object TG, and a light projecting unit. The monitor light detector 3 detects a part of the laser light (emitted light) projected from 1 as monitor light, and instructs the light projector 1 to project the laser light, and outputs the light from the light receiver 2. And a controller 4 that calculates a distance to the object TG based on the output of the monitor light detector 3.

  The light projecting unit 1 includes a solid-state laser 5, a laser diode 6, and a light projecting circuit 7. The solid-state laser 5 is composed of a known YAG laser, Nd: YAG laser, or the like, and the laser medium is excited by the incidence of excitation light projected from the laser diode 6 to emit laser light. The light projecting circuit 7 outputs a driving signal for driving the laser diode 6 based on a light projecting command signal (described later) from the control unit 4.

  The light receiving unit 2 includes a photodiode 8 and a light receiving circuit 9. The photodiode 8 receives the reflected light reflected by the object TG from the laser light projected from the light projecting unit 1. A current (photocurrent) corresponding to the intensity of the reflected light flows through the photodiode 8, and this becomes an output signal of the photodiode 8. The light receiving circuit 9 includes an amplifier circuit that performs current-voltage conversion on the output signal of the photodiode 8 to amplify the signal. The output of the light receiving circuit 9 is given to the control unit 4.

  The monitor light detection unit 3 includes a photodiode 10, an amplifier circuit 11, and a clamp circuit 12. The photodiode 10 is a light receiving element for monitoring that receives the monitor light described above. A current (photocurrent) corresponding to the intensity of the monitor light flows through the photodiode 10 and becomes an output signal of the photodiode 10. The amplifier circuit 11 performs current-voltage conversion on the output signal of the photodiode 10 and amplifies the signal to a predetermined level. The clamp circuit 12 fixes the level of the output signal of the amplifier circuit 11 to a predetermined clamp level lower than the predetermined level. The operations of the amplifier circuit 11 and the clamp circuit 12 will be described in detail later. The output of the clamp circuit 12 is given to the control unit 4.

  FIG. 2 is a diagram for explaining an example of the monitor light detection method, and shows a schematic cross-sectional view of the distance measuring apparatus 100. In FIG. 2, a case 101 is made of a transparent resin such as polycarbonate resin, and the solid laser 5, laser diode 6, photodiodes 8 and 10, and circuit boards 102 and 103 are accommodated in the case 101. . Other parts are not shown. Laser light (emitted light) emitted from the solid-state laser 5 passes through the case 101 and is projected onto the object, but a part of the laser light is reflected by the inner surface of the case 101 as indicated by a broken line. Monitor light, which is received by the photodiode 10. In order to increase the light intensity of the monitor light, a mirror surface may be formed on a part of the inner surface of the case 101 by metal vapor deposition or the like.

  Returning to FIG. 1, the control unit 4 includes an MCU (Micro Controller Unit), and includes a projection command generation unit 20, A / D (analog-digital) converters 21 and 22, and a distance calculation unit 23. Yes. The projection command generator 20 outputs a projection command signal to the projection unit 1 regularly or irregularly. The A / D converter 21 samples the output signal of the light receiving unit 2 and A / D converts the output signal. The A / D converter 22 samples the output signal of the monitor light detection unit 3 and A / D converts the output signal. The distance calculation unit 23 converts the output signal of the light receiving unit 2 A / D converted by the A / D converter 21 and the output signal of the monitor light detection unit 3 A / D converted by the A / D converter 22. Based on this, the distance to the object TG is calculated. The A / D converter 21 corresponds to a “first A / D converter” in the present invention, and the A / D converter 22 corresponds to a “second A / D converter” in the present invention.

  FIG. 3 shows a specific circuit configuration of the monitor light detection unit 3. The cathode of the photodiode 10 is connected to the DC power supply V 1, and the anode is connected to the input terminal of the amplifier circuit 11. The amplifier circuit 11 includes an operational amplifier 11a and a resistor 11b. The input terminal of the operational amplifier 11 a is connected to the anode of the photodiode 10, and the output terminal is connected to the input terminal of the clamp circuit 12. The resistor 11b is provided across the input end and the output end of the operational amplifier 11a. The clamp circuit 12 includes a diode 12a and a diode 12b. The cathode of the diode 12 a is connected to the DC power supply V <b> 2 and the anode is connected to the output terminal of the amplifier circuit 11. The cathode of the diode 12b is connected to the output terminal of the amplifier circuit 11, and the anode is grounded. The voltage of the DC power supply V1 is 20 volts, for example. The voltage of DC power supply V2 is 5 volts, for example.

  FIG. 4A shows the waveform of the current (photocurrent) flowing through the photodiode 10 before being amplified by the amplifier circuit 11. FIG. 4B shows a voltage waveform after the current is converted into a voltage by the amplifier circuit 11 and amplified. A broken line is a voltage waveform amplified to the saturation level after passing through the amplifier circuit 11. The saturation level means a level exceeding the maximum input voltage value of the A / D converter 22 as will be described later. The solid line is a voltage waveform after passing through the clamp circuit 12, that is, a voltage waveform clamped to a certain level. For example, in FIG. 3, when the voltage of the DC power supply V2 is 5 volts and the forward voltage drop of the diode 12a is 0.6 volts, the diode 12a is forwarded if the voltage is amplified to the saturation level by the amplifier circuit 11. As a bias, the output voltage of the amplifier circuit 11 is clamped to 5.6 volts.

  In the present embodiment, the gain of the operational amplifier 11a of the amplifier circuit 11 is set to a very large value (for example, 3000 to 10,000 times). Therefore, the input signal of the amplifier circuit 11 is reliably amplified to the saturation level, and the signal that has passed through the amplifier circuit 11 becomes a signal having a steep rise as shown in FIG. Further, the signal that has passed through the clamp circuit 12 also maintains its steep rise as it is, although the level is lowered by the clamp.

  As shown in FIG. 5A, when the input signal of the amplifier circuit 11, that is, the current waveform of the photodiode 10, fluctuates like P and Q according to the ambient temperature, the power supply voltage, etc., the signal level reaches the threshold value. The exceeding timing variation δ1 increases. However, by amplifying these signals to the saturation level by the amplifier circuit 11 and making the rise steep, the fluctuation of the waveform is reduced as shown in FIG. 5B, and the timing at which the signal level exceeds the threshold value. The variation δ2 is reduced. Note that this threshold value is necessary when calculating the flight time of the laser light projected from the solid-state laser 5, as will be described later.

  FIG. 6 shows the relationship between the threshold value Vth set for the output signal of the monitor light detector 3 (voltage clamped after saturation amplification) and various voltage levels. The threshold value Vth is a threshold value necessary for calculating the flight time of the laser light as described above, and is stored in an internal memory (not shown) provided in the control unit 4.

  The saturation level Ls is a voltage level that is saturated and amplified by the amplifier circuit 11, and specifically, a level that exceeds the maximum input voltage value of the A / D converter 22. The MCU port rated level Lm is the level of the input allowable voltage at the input port corresponding to the A / D converter 22 among the input ports of the control unit 4, that is, the maximum input voltage value of the A / D converter 22 described above. . The clamp level Lc is the level of the clamp voltage when the saturated and amplified voltage is clamped by the clamp circuit 12. The noise level Lz is a voltage level of a noise signal that appears due to the gain of the amplifier circuit 11 being large.

  As can be seen from FIG. 6, the threshold value Vth is set to a level lower than the clamp level Lc and higher than the noise level Lz (Lz <Vth <Lc). Thereby, erroneous detection of monitor light due to noise can be avoided. The clamp level Lc is set to a level higher than the threshold Vth and lower than the MCU port rated level Lm (the maximum input voltage value of the A / D converter 22) (Vth <Lc <Lm). Thereby, it can be avoided that an overvoltage is applied to the input port of the MCU and the element is destroyed.

  FIG. 7 is a timing chart showing the operation of the distance measuring apparatus 100. (A) is the output (excitation light) of the laser diode 6, (b) is the output (emitted light) of the solid-state laser 5, (c) is the output of the monitor light detector 3, and (d) is the output of the light receiver 2. Represents. Hereinafter, a method for measuring the distance to the object TG will be described with reference to FIG.

  In FIG. 7, as shown in FIG. 7A, the laser diode 6 projects excitation light onto the solid-state laser 5 at time x <b> 1. Note that, unlike a solid state laser, a laser diode has almost no time delay until light is emitted, and therefore the time x1 can be regarded as the time when a light projection command signal is output from the light projection command generator 20. At a time x2 when a predetermined time has elapsed from this point, the solid-state laser 5 emits a laser beam as shown in (b). A part of the laser light is received as monitor light by the photodiode 10 of the monitor light detector 3 (FIG. 2). As a result, as shown in (c), a detection signal is output from the monitor light detection unit 3 with a slight delay from the time x2. The level of this detection signal exceeds the threshold value Vth at time x3. The time x3 at this time is a time when the time t1 has elapsed from the time x2, and is a time when the time t2 has elapsed from the time x1. The laser light projected from the solid-state laser 5 is reflected by the object TG and is received by the photodiode 8 of the light receiving unit 2 as reflected light. As a result, as shown in (d), a light reception signal is output from the light receiving unit 2 at time x4 when time t3 has elapsed from time x1.

The distance to the object TG is calculated as follows by the distance calculation unit 23 using the times t1, t2, and t3. In FIG. 7, the time T from the laser beam projection time x2 to the reflected light reception time x4 is the time for the laser beam to reciprocate between the distance measuring device 100 and the object TG, that is, the flight time of the laser beam. (TOF: Time of Flight). Therefore, when the distance to the object TG is D and the speed of light is c (c = 3 × 10 ⁸ m / s),
D = (T / 2) · c
The relationship is established. And as you can see from Figure 7,
T = t3-t2 + t1
Therefore, the distance D is calculated by the following equation.
D = [(t3−t2 + t1) / 2] · c (1)

Here, the light projection time x1 of the excitation light, the time x3 when the detection signal of the monitor light exceeds the threshold value Vth, and the light reception time x4 of the reflected light are times that the control unit 4 can directly grasp. Therefore, the time t2 and the time t3 can be obtained from the following equations.
t2 = x3-x1
t3 = x4-x1

  On the other hand, since the control unit 4 cannot directly grasp the laser beam projection time x2, the time t1 cannot be obtained by the calculation of t1 = x3-x2. However, this time t1 is a time corresponding to the response delay of the photodiode 10, and is a value unique to the element, and may be regarded as a substantially constant value. Therefore, the value of the time t1 is stored as a parameter (constant) in the internal memory of the control unit 4, and the distance D to the object TG is calculated by referring to this parameter when calculating the above equation (1). can do. In consideration of the temperature dependence of the time t1, a table in which the temperature and the time value are associated may be provided in the internal memory.

  By the way, the solid-state laser is provided with a Q switch for switching the Q value (performance index) of the resonator in order to emit high-energy pulsed light. The Q switch includes an active Q switch that switches the Q value electrically or mechanically and a passive Q switch that automatically switches the Q value using a saturable absorber. Among these, in particular, in a solid-state laser using a passive Q switch, the time from excitation light incidence to laser light emission, that is, the laser light projection time (time x2 in FIG. 7) is the ambient temperature, power supply voltage, etc. It varies widely according to. Accordingly, along with this, the time when the output signal of the monitor light detection unit 3 exceeds the threshold value Vth (time x3 in FIG. 7) varies greatly, but as described above, the time t1 from the time x2 to the time x3 is almost equal. Since it is constant, even if the solid-state laser 5 is of the passive Q switch type, the influence of the variation in the projection time is small.

  In the present embodiment, as described with reference to FIG. 5, the rising of the output signal of the monitor light detection unit 3 is made steep by amplifying the current of the photodiode 10 to the saturation level by the amplifier circuit 11. For this reason, even if the characteristics of the photodiode 10 fluctuate according to temperature, voltage, etc., the variation in timing (time x3 in FIG. 7) when the output signal exceeds the threshold Vth (δ2 in FIG. 5B) is kept small. be able to. As a result, variations in the times t1 and t2 in FIG. 7 are reduced, so that variations in the value of the distance D calculated by the above equation (1) are also reduced, and measurement accuracy is improved.

  FIG. 8 shows a comparative example when saturation amplification is not performed in the amplifier circuit 11. In this case, the output signal of the monitor light detector 3 is a signal having a gradual rise as shown in (c). Therefore, when the characteristics of the photodiode 10 fluctuate according to temperature, voltage, etc., the waveform of the output signal changes as shown in FIG. 9, for example, and the timing when the output signal exceeds the threshold value Vth ′ is τ1 to τ3 Greatly varies. For this reason, the variations in the times t1 'and t2' in FIG. 8 are increased, so that the variation in the value of the distance D calculated by the above equation (1) is also increased, and the measurement accuracy is lowered.

  FIG. 10 shows another embodiment of the present invention. In the embodiment of FIG. 1, the solid-state laser 5 is used as a light source that projects laser light toward the object TG. However, in the embodiment of FIG. 10, as a light source that projects laser light toward the object TG. A laser diode 15 which is a semiconductor laser is used. The light projecting circuit 16 outputs a drive signal for driving the laser diode 15 based on the light project command signal from the control unit 4. Since other configurations are the same as those in FIG. 1, description of overlapping portions is omitted. 10 can measure the distance to the object TG with high accuracy according to the same principle as in FIG.

  In addition to the embodiments described above, the present invention can employ the following various embodiments.

  In the above embodiment, the output signal (current) of the photodiode 10 is amplified to the saturation level in the amplifier circuit 11, but it is not always necessary to amplify to the saturation level. For example, it may be amplified to a level near the saturation level and lower than the saturation level.

  In the above embodiment, the photodiodes 8 and 10 are used as the light receiving elements, but a phototransistor or the like may be used instead of the photodiode.

  In the above-described embodiment, the example in which the A / D converters 21 and 22 are provided inside the MCU configuring the control unit 4 has been described, but the A / D converters 21 and 22 are provided outside the MCU. It may be done.

  In the above-described embodiment, the distance measuring device 100 mounted on the vehicle is taken as an example, but the present invention can also be applied to a distance measuring device used in fields other than the vehicle.

DESCRIPTION OF SYMBOLS 1 Light projection part 2 Light reception part 3 Monitor light detection part 5 Solid state laser 10 Photodiode (light receiving element)
DESCRIPTION OF SYMBOLS 11 Amplifier circuit 12 Clamp circuit 15 Laser diode 21 A / D converter (1st A / D converter)
22 A / D converter (second A / D converter)
23 Distance calculation unit 100 Distance measuring device TG Object Ls Saturation level Lm MCU port rated level (Maximum input voltage value of A / D converter 22)
Lc Clamp level Vth threshold T Flight time

Claims (6)

A light projecting unit that projects laser light toward an object;
A light receiving unit that receives the reflected light reflected by the object by the laser beam;
A monitor light detector that detects a part of the laser light projected from the light projector as monitor light;
A first A / D converter for analog-digital conversion of an output signal of the light receiving unit;
A second A / D converter for analog-digital conversion of an output signal of the monitor light detection unit;
A distance calculation unit that calculates a distance to the object based on outputs of the first A / D converter and the second A / D converter;
The monitor light detector is
A light receiving element for receiving the monitor light;
An amplification circuit for amplifying the output signal of the light receiving element to a predetermined level;
A distance measuring device comprising: a clamp circuit that fixes a level of an output signal of the amplifier circuit to a clamp level lower than the predetermined level. The distance measuring device according to claim 1,
The distance measuring device, wherein the amplifier circuit amplifies the output signal of the light receiving element to a saturation level exceeding a maximum input voltage value of the second A / D converter. In the distance measuring device according to claim 1 or 2,
For the output signal of the monitor light detection unit, a threshold value that is lower than the clamp level is set,
The distance calculation unit
Based on the time when the level of the output signal of the monitor light detection unit exceeds the threshold, the time when the light receiving unit receives the reflected light, and the time corresponding to the response delay of the light receiving element of the monitor light detection unit A distance measuring device that calculates a flight time of the laser light projected from the light projecting unit and calculates a distance to the object based on the flight time. The distance measuring device according to claim 3,
The distance measuring device according to claim 1, wherein the clamp level is higher than the threshold and lower than a maximum input voltage value of the second A / D converter. The distance measuring device according to any one of claims 1 to 4,
The distance measuring device, wherein the light projecting unit includes a solid-state laser that projects laser light toward the object. The distance measuring device according to any one of claims 1 to 4,
The distance measuring device, wherein the light projecting unit includes a laser diode that projects laser light toward the object.

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