JP6236758B2 - Optical distance measuring device - Google Patents

Description

  The present invention relates to an optical distance measuring device.

  Mobile bodies (vehicles, etc.) equipped with a collision prevention system and the like have been developed with the aim of reducing traffic accidents. In such a system, an environmental sensor equipped with a camera, a millimeter wave radar, or the like is used to observe the external environment.

  Stereo cameras have a relatively wide angle and a high spatial resolution, but on the other hand, the distance accuracy at a distant location is significantly reduced. On the other hand, the millimeter wave radar can detect a distant object of about 200 m, but has a narrow field of view and low angular resolution.

  On the other hand, the optical distance measuring sensor based on the time-of-flight method (TOF: Time Of Flight) has a high spatial resolution (angular resolution) and can measure a wide angle and a long distance. For this reason, the detection accuracy and robustness of the runway and the obstacle can be improved, and the expansion of the function of the safety system can be expected. For example, if a distant obstacle can be detected with high position accuracy, an early warning can be made. Further, if the surrounding environment such as the shape of the parked vehicle can be detected with high accuracy, it is possible to determine the collision slipping with high reliability.

  In such an optical distance measuring device using TOF, an avalanche photodiode (APD) or PIN photodiode is often used as a light receiving element. When photons are incident on the APD, electron-hole pairs are generated, and the electrons and holes are each accelerated by a high electric field, and in succession a collision ionization is generated like an avalanche to generate new electron-hole pairs. . Since sensitivity is enhanced by this internal amplification action, APD is often used particularly when long distance detection is required. The operation mode of the APD includes a linear mode in which the reverse bias voltage is operated below a breakdown voltage (breakdown voltage) and a Geiger mode in which the reverse bias voltage is operated at a breakdown voltage or higher. In the linear mode, the proportion of electron / hole pairs that disappear (exit from the high electrolysis region) is larger than the proportion of electron / hole pairs that are generated, and the avalanche stops naturally. The output current is substantially proportional to the amount of incident light, and is used for measuring the amount of incident light. In the Geiger mode, an avalanche phenomenon can be caused even by the incidence of a single photon, so it is also called a single photon avalanche diode (SPAD).

  In order to obtain high performance in an optical distance measuring device that performs TOF measurement, it is desirable to detect all reflected light of the light irradiated by itself and reduce the amount of disturbance light received as much as possible. For this reason, it is preferable to match the irradiation region of the laser beam to be irradiated with the light receiving field. Therefore, using a perforated mirror or half mirror, the perfect coaxial optical system that perfectly matches the optical axes of the light projecting and receiving light, or the light emitting element and the light receiving element are arranged close to each other so that their optical axes are almost the same. A pseudo coaxial optical system is used. Such an optical system requires alignment for adjusting the positional relationship between the light emitting element and the light receiving element.

  By illuminating the field stop that determines the reception field of view, a part of the light emitted from the reception optical system is bent by the reflecting mirror to form an image on the light receiving element, and a part of the transmission laser light is taken out by the semi-transparent mirror, which is also the light receiving element. A technique for detecting a relative optical axis shift based on whether these two beam spots coincide with each other, and adjusting the direction of the laser device with a direction adjusting device when there is a shift in the optical axis. (Patent Document 1).

  In addition, the light detector position adjuster moves the detector surface linearly at least in one dimension on the surface perpendicular to the optical axis to capture the change in the received light level value. A method is disclosed in which the level value is adjusted to be distributed symmetrically so that the direction of the target matches the set direction. Furthermore, the light receiving detector position adjuster displaces the light receiving detector also in the optical axis direction to capture the change in the light receiving level value so that the light receiving level value becomes a predetermined value, and the correlation with the light receiving spot position or the focal position. A technique for appropriately adjusting the light reception level by adjusting the light intensity is disclosed (Patent Document 2).

JP 2000-121724 A JP 2006-47272 A

  By the way, in a configuration that requires a direction adjusting device that adjusts the optical axis deviation, a light source that illuminates a field stop, a light receiving element, a mirror that divides a beam, and the like, there are problems that the device becomes complicated and the manufacturing cost increases.

  In addition, in the configuration that requires the light receiving detector position adjuster, in addition to the complexity of the apparatus, it is necessary to repeat the operation of adjusting the position of the light receiving detector, and there is a problem that the measurement time becomes long.

  In order to obtain high performance in the optical distance measuring device, it is desirable to detect the reflected light of the irradiated light as much as possible, and to reduce the amount of disturbance light received as much as possible. Imaging of the irradiated light on the light receiving surface of the light receiving element Ideally, the spot and the light receiving area are perfectly matched. Therefore, it is possible to design the optical system so that the relative arrangement of the spot of the irradiated light and the light receiving area completely coincides, but if there is a slight deviation of the arrangement due to the measurement conditions, etc. May reduce the amount of received light itself.

  The present invention is an optical distance measuring device that measures a distance based on a difference between a projection time of irradiation light and a reception time of reflected light, and includes a light source that projects irradiation light, and reflected light from an object. A light receiving unit including a plurality of light receiving elements that can receive light and can be turned off independently; and an adding unit that adds outputs of the plurality of light receiving elements, and any one of the light receiving elements according to an output of the adding unit. It is an optical distance measuring device characterized in that it is possible to turn off.

  Here, the light receiving elements are Geiger mode avalanche photodiodes arranged in an array, lowering the bias voltage of the avalanche photodiodes, and turning off the voltage pulse output of the avalanche photodiodes, It is preferable to turn off one of the light receiving elements by at least one of the above.

  Further, any one of an accumulation unit that repeatedly irradiates irradiation light from the light source and accumulates the received light amount over a predetermined time in the light receiving unit, and the light receiving element according to an accumulated value of the accumulation unit Is preferably turned off.

  Further, it is preferable that the accumulation means accumulates the received light amount only when reflected light from a predetermined distance range is detected.

  According to the present invention, it is possible to provide an optical distance measuring device that can reduce only the amount of disturbance light received without reducing the amount of irradiation light received.

It is a figure which shows the structure of the photodetector in embodiment of this invention. It is a figure which shows the example of arrangement | positioning of the light-receiving means in embodiment of this invention. It is a figure which shows the example of the production | generation of the histogram in embodiment of this invention. It is a figure which shows the structure of the optical distance measuring device in embodiment of this invention.

  As shown in FIG. 1, the photodetector 100 in the present embodiment includes a light receiving means 102, a pulse shaping circuit 104, an adding means 106, a comparing means 108, a TDC (Time to Digital Converter) 110, and a histogram generating means 112. Consists of.

  The light receiving means 102 includes a photodiode 10, a quenching resistor 12, a buffer 14, flip-flops 16 and 18, a selector 17, a switching element 20, and an AND element 22.

  One set of the photodiode 10, the quenching resistor 12, the buffer 14, the flip-flops 16 and 18, the selector 17, the switching element 20, and the AND element 22 constitute one light receiving unit 102a.

  The photodiode 10 is a Geiger mode single photon avalanche photodiode (SPAD). That is, the photodiode 10 causes an avalanche phenomenon with respect to the incidence of a single photon by applying a bias voltage higher than the breakdown voltage, and outputs a voltage pulse with respect to the incidence of the photon.

  The quenching resistor 12 is a resistance element for stopping the SPAD avalanche phenomenon. In the present embodiment, the quenching resistor 12 uses a resistance component of a transistor. In the photodiode 10, the avalanche phenomenon can be stopped by lowering the applied voltage to the breakdown voltage. Lowering the applied voltage to stop the avalanche phenomenon is called quenching, and the simplest quenching circuit is realized by connecting a quenching resistor 12 in series with the photodiode 10. When an avalanche current is generated, the bias voltage of the photodiode 10 decreases due to the increase in voltage between the terminals of the quenching resistor 12. When the bias voltage drops to the breakdown voltage, the avalanche phenomenon stops. When the avalanche current stops flowing, the terminal voltage of the quenching resistor 12 drops, and a voltage higher than the breakdown voltage is applied to the photodiode 10 again.

  The buffer 14 is provided to take out the voltage increase and decrease between the photodiode 10 and the quenching resistor 12. Thereby, the incidence of photons on the photodiode 10 can be output as a voltage pulse.

  Note that the time corresponding to the pulse width of this voltage pulse is a dead time in which the avalanche phenomenon cannot be newly induced even if new photons are incident because the SPAD bias voltage is reduced and photons cannot be detected. .

  The light receiving means 102 includes a plurality of light receiving portions 102a. That is, the light receiving means 102 is configured as a silicon photomultiplier (SiPM). In FIG. 1, an example in which m horizontal x vertical n light receiving units 102a are arranged in an array is shown. However, for the sake of simplicity, the configuration of only four light receiving units 102a is illustrated, and other light receiving units 102a are illustrated. Omitted. For example, as shown in the schematic plan view of FIG. 2, the light receiving means 102 may have a configuration in which the photodiodes 10 are arranged in an array. When arranged in an array like this, the total light receiving area is increased, and a larger amount of light can be received.

  The pulse shaping circuit 104 shapes the voltage pulse output from the light receiving means 102 and outputs it. The pulse width of the voltage pulse of the photodiode 10, that is, the dead time is about 10 to 100 nm, which is often longer than the emission pulse width of the laser light. The pulse shaping circuit 104 receives the voltage pulse from each light receiving means 102 and shortens the pulse width of the voltage pulse so as to be substantially equal to the pulse width of the irradiation light from the light source described later.

The adding means 106 adds the outputs from the pulse shaping circuit 104 and outputs the addition result (bit width: N _ADD = [log ₂ (N _SPAD )]). The adding means 106 adds the voltage pulses output from the plurality of light receiving units 102a at the same timing. For example, if two voltage pulses of the plurality of light receiving units 102a included in the light receiving unit 102 are at a high level, the output from the adding unit 106 is “2” (decimal number) of the digital signal.

  As described above, the light receiving unit 102a of the photodetector 100 according to the present embodiment outputs binary information (voltage pulse) with respect to the incidence of photons, and is robust against fluctuations in temperature and the like.

  In addition, since a Geiger mode avalanche photodiode is used as the light receiving portion 102a, it can be mounted on the apparatus at a lower cost and more compactly than other photocounter type light receiving elements such as a photomultiplier tube. In addition, since the avalanche photodiode is a semiconductor element, it is easy to integrate a plurality of light receiving portions 102a. Furthermore, since a technology for realizing an avalanche photodiode by a CMOS process has been developed, it can be mounted on the same chip as the pulse shaping circuit 104, the adding means 106, the comparing means 108, the TDC 110, the histogram generating means 112, and the like. . As a result, the manufacturing process can be simplified and the manufacturing cost can be reduced. Further, since the parasitic capacitance of the avalanche photodiode is reduced, the dead time can be shortened and the dynamic range can be further widened.

  The comparison unit 108 compares the addition value S output from the addition unit 106 with a predetermined threshold Th, and outputs a determination result indicating that a TOF reflection pulse has arrived when the addition value S is equal to or greater than the threshold Th. For example, when the threshold value Th is set to “2”, the output PEAK is set to the high level if the addition value S in the adding means 106 is “2” or more, and otherwise set to the low level.

That is, the time is measured by the TDC 110 in the next stage only when photons are detected in a plurality of (two or more in the present embodiment) photodiodes 10 at the same time. When the output from the comparison means 108 becomes high level at the same time as the gate signal V _GATE is high level, the start signal V _start becomes high level by the flip-flop 30 and the AND element 32, and time measurement at the TDC 110 is started. When the reflected light of the irradiated light is received, a large number of photons arrive at the same time and are received by a plurality of photodiodes 10, whereas when a disturbance light is received, the photons arrive at random times. This process is based on the low probability of arrival at the same time. Since there is a high probability that voltage pulses that are separated in time by more than the pulse width of the laser beam are not due to the same reflected light, the pulse shaping circuit 104 shapes the voltage pulse into the pulse width of the laser beam, and simultaneously outputs a plurality of pulses. Is detected, it can be more reliably detected that the light is reflected light from the same object.

  When the output PEAK of the comparison unit 108 indicates the arrival of the reflected pulse, the TDC 110 measures the arrival time of the reflected pulse. The TDC 110 outputs the time from the output time of the TOF measurement light (laser light) emitted from the light source, which will be described later, to the time when the reflected pulse is detected by the comparison means 108, and outputs it as a TOF measurement result. The TDC 110 enables measurement with a time resolution of several picoseconds.

  The histogram generation unit 112 generates a histogram by further accumulating the TOF measurement results obtained by the TDC 110 over a predetermined measurement time. As shown in FIG. 3, the histogram is generated by accumulating the TOF measurement results obtained by repeatedly irradiating the irradiation light for measuring the TOF over the measurement time, and the time from the light irradiation time to the arrival ( Bin) indicates the number of times (frequency) at which the comparison means 108 has detected a pulse over the measurement time. Thus, by correctly measuring the arrival time of the pulse signal to create a histogram and extracting the maximum value, correct TOF measurement can be performed even in the presence of disturbance light.

  Note that the histogram generation means 112 may repeatedly irradiate the irradiation light for TOF measurement over the measurement time, and accumulate the addition results of the addition means 106 in each measurement obtained at that time to generate a histogram. Good. By performing such processing, when a large number of photodiodes 10 simultaneously output pulses, more values are added and reflected in the histogram, so that information can be used more effectively to improve accuracy and accuracy. High TOF measurement can be performed.

  When the measurement of the predetermined time is completed, the histogram generation unit 112 outputs the time of the peak value of the generated histogram as the TOF.

  Next, an optical distance measuring device 200 equipped with the photodetector will be described. As shown in FIG. 4, the optical distance measuring device 200 includes a photodetector 100, a light source 202, a hyperboloid mirror 204, and a polygon mirror 206. The optical distance measuring device 200 has a coaxial optical system in which the optical axes of the projected light and the received light are matched.

  The light source 202 irradiates pulse light to the distance measurement target space of the optical distance measuring device 200. The light source 202 can be, for example, a laser diode (LD). The period and pulse width of the pulsed light are not limited to this, but are preferably about several hundreds μs and several ns, respectively. The irradiation time of the pulsed light from the light source 202 is input to the TDC 110 and used for measuring the TOF.

  The light source 202 projects light toward the polygon mirror 206 from a hole 204 a provided at the center of the hyperboloid mirror 204. The light output from the light source 202 may be collimated by a collimating lens or the like.

  The light that has passed through the hole 204a of the hyperboloid mirror 204 is reflected by the polygon mirror 206 and projected onto the distance measurement target space. When an object (for example, a car, a road, a tree, a person, etc.) exists in the distance measurement target space, light is reflected by these objects, and returns to the optical distance measuring device 200 as a reflected wave. The returned light is reflected again by the polygon mirror 206, further reflected by the hyperboloidal mirror 204, and enters the photodetector 100. The hyperboloid mirror 204 works in the same way as a lens, and forms an image of light on a light receiving element provided in the photodetector 100.

  Here, the polygon mirror 206 is a rotating polygon mirror, and has a plurality of mirror surfaces parallel or inclined with respect to the rotation axis. The polygon mirror 206 is rotated about a rotation axis 206a at a predetermined rotation speed, and with the rotation, the direction of the mirror surface is changed with respect to the light from the light source 202 to scan the light into the distance measurement target space. Flood light. The scanning direction is determined by the angle of the mirror surface provided on the polygon mirror 206 with respect to the rotation axis 206a. By making the depression angles of the plurality of mirror surfaces different, it is possible to scan light not only in the horizontal direction but also in the vertical direction. If the difference between the depression angles of each surface is set to be equal to or smaller than the spread angle of the projected light, scanning can be performed in the vertical direction without any gap.

  As shown in FIG. 2, the photodiodes 10 are mounted in an array on the light receiving means 102. In the example of FIG. 2, the light receiving means 102 has a configuration in which a plurality of silicon photomultipliers (SiPM) 10 a composed of a plurality of photodiodes 10 are further arranged vertically. FIG. 2 shows the light receiving state of each photodiode 10 when the reflected light of the irradiated light is incident on the light receiving means 102. In FIG. 2, the region A where the reflected light of the irradiated light is incident is indicated by hatching, and the photodiode 10 where the reflected light of the irradiated light is not incident is indicated by white.

  In the optical distance measuring device 200, the light receiving area of the light receiving means 102 is provided larger than the spot of the reflected light, only the photodiode 10 on which the area A where the reflected light is incident is turned on, and the reflected light is transmitted. The photodiode 10 in the non-incident area is turned off. Specifically, flip-flops 16 and 18, a selector 17, a switching element 20 and an AND element 22 are provided for each light receiving unit 102 a of the light receiving unit 102 shown in FIG. 1, and these are used to turn on / off the light receiving unit 102 a. .

  “1 (high level)” for the light receiving portion 102a to be turned on, and “0 (low level)” for the light receiving portion 102a to be turned off with respect to the shift register constituted by the flip-flops 16 of the plurality of light receiving portions 102a. The data Din is sequentially input serially and transferred in synchronization with the clock signal SCLK. After ON / OFF data is transferred to the flip-flop 16 for each of the light receiving units 102a, data is set from the flip-flop 16 to the flip-flop 18 for each light receiving unit 102a via the selector 17 by the load signal LOAD. When “1 (high level)” is set in the flip-flop 18, the switching element 20 is turned on, a bias voltage is applied to the photodiode 10, and a voltage pulse output of the photodiode 10 is output through the AND element 22. It becomes effective. On the other hand, when “0 (low level)” is set in the flip-flop 18, the switching element 20 is turned off, no bias voltage is applied to the photodiode 10, and it becomes inactive with respect to the incidence of photons. Further, “0 (low level)” is also input to the AND element 22, and the output from the photodiode 10 becomes invalid.

  For example, data is set in the flip-flop 16 and the flip-flop 18 of each light receiving unit 102a according to the added value of the voltage pulse from the adding means 106. The data setting is repeated so that the addition value of the voltage pulse in the adding means 106 becomes the largest and the number of the light receiving parts 102a turned on is minimized, and the “flip-flop 18 of each light receiving part 102a” “1 (high level)” or “0 (low level)” is set. Thereby, it is possible to turn on only the light receiving unit 102a in which the region A where the reflected light is incident is applied, and to turn off the light receiving unit 102a in the region where the reflected light is not incident.

  Further, data may be set in the flip-flop 16 and the flip-flop 18 of each light receiving unit 102a according to the result of accumulating the output from the adding means 106. For example, the bin value forming the peak of the histogram generated over a predetermined period in the histogram generation means 112 is the accumulated value. Data setting is repeated so that the accumulated value becomes the largest and the number of light receiving units 102a turned on is minimized, and “1 (high level)” is set for the flip-flop 18 of each light receiving unit 102a. Alternatively, “0 (low level)” is set. Thereby, it is possible to turn on only the light receiving unit 102a in which the region A where the reflected light is incident is applied, and to turn off the light receiving unit 102a in the region where the reflected light is not incident.

  Further, the data of the flip-flop 16 and the flip-flop 18 may be set according to the output of each light receiving unit 102a. First, “1 (high level)” is set in the flip-flop 18 through the flip-flop 16 of only one light receiving unit 102a, and the output of the adding means 106 is examined. When the output becomes 1 within a predetermined period, it is set to “1 (high level)”, and when the output is 0, it is set to “0 (low level)”. By examining this for all the light receiving parts 102a, only the light receiving part 102a on which the area A where the reflected light is incident is turned on, and the light receiving part 102a in the area where the reflected light is not incident is turned off. Can do. Instead of checking the output of the adding means 106, the threshold value of the comparing means 108 may be set to 1, and the output may be checked. Furthermore, the threshold value of the comparison means 108 is set to 1, and the accumulated value for each of the light receiving units 102a can be obtained from the peak value of the histogram generated over a predetermined period.

  As described above, in the optical distance measuring device 200 according to the present embodiment, the reflected light of the light irradiated by itself is completely detected, and each light receiving unit 102a is selected to be turned on or off so as to reduce the amount of disturbance light received as much as possible. Can be turned off. As a result, the imaging spot of the reflected light of the light irradiated on the light receiving surface of the light receiving means 102 and the light receiving area can be adaptively matched. In addition, even when there is a slight shift in arrangement due to the measurement conditions, etc., the light receiving area and the reflected spot of the reflected light of the irradiated light without reducing the amount of reflected light of the irradiated light itself Can be matched.

  Since the clock frequency of the flip-flop 16 constituting the shift register can be about several hundreds MHz, even if the number of the light receiving units 102a is about several thousand, the light receiving unit 102a is turned on in about several tens of ms. / Can be set to off state.

  In the present embodiment, a flip-flop 18 is provided separately from the flip-flop 16 constituting the shift register. While the measurement is performed with the light receiving unit 102a turned on / off according to the data set in the flip-flop 18, the next data can be transferred by the flip-flop 16 constituting the shift register. This makes it possible to set the next data at the same time while maintaining the measurement state, thereby improving the measurement efficiency. However, the flip-flop 18 may not be provided, and the switching element 20 and the AND element 22 may be controlled by the output of the flip-flop 16 constituting the shift register.

10 photodiode, 12 quenching resistor, 14 buffer, 16 flip flop, 17 selector, 18 flip flop, 20 switching element, 22 AND element, 30 flip flop, 32 AND element, 100 photodetector, 102 light receiving means, 102a light receiving Part, 104 pulse shaping circuit, 106 addition means, 108 comparison means, 112 histogram generation means, 200 optical distance measuring device, 202 light source, 204 hyperboloid mirror, 204a hole, 206 polygon mirror, 206a rotation axis.

Claims (4)

An optical distance measuring device that measures a distance based on a difference between a projection time of irradiation light and a reception time of reflected light,
A light source for projecting irradiation light;
A light receiving means including a plurality of light receiving elements that receive reflected light that is reflected coaxially with the irradiation light from an object and that can be turned off independently;
Adding means for adding outputs of the plurality of light receiving elements;
With
Co the output of said adding means is maximized, optical distance measuring the number of the light receiving element that is turned on is characterized and to Turkey off one of the light receiving element so as to minimize apparatus. An optical distance measuring device that measures a distance based on a difference between a projection time of irradiation light and a reception time of reflected light,
A light source for projecting irradiation light;
A light receiving means including a plurality of light receiving elements that receive reflected light that is reflected coaxially with the irradiation light from an object and that can be turned off independently;
Accumulating means for repeatedly irradiating irradiation light from the light source and accumulating the amount of light received by the light receiving means over a predetermined time;
With
Co the output of the accumulator means is maximized, photometric number of light receiving elements which are turned on is characterized and to Turkey off one of the light receiving element so as to minimize Distance device. The optical distance measuring device according to claim 2,
The accumulating means accumulates the amount of received light only when reflected light from a predetermined distance range is detected. The optical distance measuring device according to any one of claims 1 to 3,
The light receiving element is a Geiger mode avalanche photodiode arranged in an array,
Wherein lowering the bias voltage of the avalanche photodiode, and optical characterized and to Turkey off one of said turning off the voltage pulse output of the avalanche photodiode, via at least one of said light receiving element Distance measuring device.

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