JP6225411B2 - 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).

  FIG. 9 shows a TOF measurement method for obtaining a time until light irradiated to an object is reflected by the object and returned. At this time, since the light other than the light irradiated on the object itself is noise, it is necessary to remove the influence of disturbance light such as sunlight. When using a light receiving element that obtains an output substantially proportional to the amount of incident light, the influence of disturbance light can be reduced and the arrival timing of reflected light can be extracted by performing threshold processing after removing the direct current component of the light receiving element. On the other hand, when using a photon count type light receiving element that outputs a voltage pulse in response to incident photons, such as SPAD, the arrival time of the voltage pulse is repeatedly measured to create a histogram, and the maximum value is extracted. Thereby, even if disturbance light exists, correct TOF measurement can be performed.

  In order to extract the maximum value of the histogram of TOF with high accuracy, TOF measurement is required many times, and the measurement time becomes long. When the measurement time becomes long, the distance to the object changes when the object moves, resulting in a measurement error. A method using silicon photomultipliers (SiPM) has been disclosed as a method for correctly obtaining TOF by removing the influence of ambient light in a short measurement time (Patent Document 1, Non-Patent Document 1). SiPM forms a large photodetector as a whole by arranging a plurality of SPADs in an array. An example of a TOF detection circuit using SiPM is shown in FIG. A comparator is provided at the output of the SiPM, and the TOF is measured only when pulses are output simultaneously from a predetermined number or more of SPADs, that is, when a large number of photons arrive at the same time. As a result, the response of disturbance light to photons can be reduced, so that the TOF of irradiation light can be correctly extracted with a small number of measurements. This is a process based on the property that the reflected light of the light irradiated by itself comes in a large number of photons at the same time, whereas the photons of disturbance light arrive at random timing, so the probability of simultaneous arrival is small.

  An optical distance measuring device based on TOF can output brightness information in addition to distance information. The light received by the light receiving element is the sum of the reflected light of the irradiated light on the object, the reflected light of the ambient light (sun or other illumination) on the object, and the light emitted by the object. Light. In the case of a light receiving element that outputs a value substantially proportional to the amount of incident light, the maximum value of the amount of received light can be regarded as irradiation light, and the irradiation light can be extracted separately from other light. By correcting the maximum value of the amount of received light with the distance value, the reflectance of the object is also obtained. Conversely, light other than the irradiation light, that is, disturbance light can be extracted from the light receiving element output during the laser pause period (Patent Document 2).

  On the other hand, in the case of a photon count type light receiving element, brightness information can be extracted from a histogram of TOF. The total value of the histogram values is the total received light amount, and the maximum value is the irradiation light. Moreover, the information of disturbance light can also be obtained from the output of the light receiving element during the laser pause period (Patent Document 3).

  Further, when the photodetector is used outdoors, a wide brightness dynamic range is required. The brightness fluctuation range of the outdoor environment is large, and the illuminance during sunny daytime exceeds 100,000 lux, while the illuminance under night street lights is about several tens of lux. Furthermore, considering the reflectance of the object, a dynamic range of about 6 digits is required. When detecting the light quantity by photon counting, the count number when the light quantity is small is almost proportional to the light quantity. When the amount of light increases and photons are incident at a period shorter than the output voltage pulse width, a plurality of voltage pulses may be combined, thereby decreasing the number of counts. Therefore, as shown in FIG. 11, since the light quantity and the count number do not have a monotonically increasing relationship, the light quantity cannot be measured correctly when the light quantity increases.

  Therefore, the light receiving element is repeatedly reset by the reset means, and it is detected whether there is an incident photon instead of the number of incident photons between the reset pulses, and the detection pulses are counted for a predetermined period. As a result, the number of bits required for counting is reduced, so that the dynamic range can be expanded (Patent Document 4). Further, the detection signal of the photon count type photodetector is A / D converted, and if it is equal to or greater than a preset threshold value, the detection signal is sent to the subsequent photon number calculation circuit as it is. A method of performing processing for sending the reference value to the subsequent stage is disclosed. In the photon number calculation circuit, the light amount is calculated from the area of the acquired detection signal waveform until the light amount measurement is completed (Patent Document 5).

  By the way, the avalanche phenomenon can be stopped by lowering the bias voltage of the SPAD to the breakdown voltage. Lowering the applied voltage to stop the avalanche phenomenon is called quenching. The simplest quenching circuit is realized by connecting a quenching resistor in series with SPAD. When an avalanche current is generated, the bias voltage of the SPAD drops due to an increase in the voltage between the terminals of the quenching resistor. When the bias voltage drops to the breakdown voltage, the avalanche phenomenon stops. When the avalanche current stops flowing, the voltage between the quenching resistance terminals drops, and a voltage higher than the breakdown voltage is again applied to SPAD. By taking out the voltage rise and fall between the SPAD and the quenching resistor through a buffer, photon incidence can be output as a voltage pulse. 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 lowered, and photons cannot be detected.

When TOF is detected using a photon-counting light receiving element in an environment with large disturbance light such as sunlight, the effective photon detection rate due to dead time decreases. The photon detection rate of photon count type light receiving element lambda, when the dead time is t, the effective photon detection rate lambda _eff is represented by Equation (1).

Equation (1) can be derived as follows. The measurement time T is expressed by Equation (2) as the sum of the total effective time T _eff that can detect photons and the total dead time T _dead that cannot detect photons.

An average count M of photons at the measurement time T is expressed by Equation (3).

Here, since the dead time of M times occurs when the photon count type light receiving element counts M times, the total time of the dead time is expressed by Equation (4).

From Equation (3) and Equation (4), when the dead time total T _dead is eliminated, M / T, that is, Equation (1) of the effective detection rate is derived.

FIG. 12 is a diagram in which the relationship of Equation (1) is plotted with a dead time of 20 ns. When the detection rate λ is small, the detection rate λ is substantially equal to the effective photon detection rate λ _eff . However, as the detection rate λ increases, the effective photon detection rate λ _eff begins to decrease greatly.

In FIG. 12, it can be seen that the characteristics start to change near the detection rate of 5 MHz. The detection period of 5 MHz corresponds to 200 ns, that is, about 10 times the dead time. When the detection cycle becomes short and the dead time approaches, there is a problem that the rate T _eff / T at which photons can be actually detected during the measurement time, that is, the detection time efficiency is lowered.

Next, when the difference between the detection rate and the effective detection rate becomes large, there is a problem in that erroneous detection is caused in the TOF detection by the histogram. FIG. 13A shows an example when gating is not performed on the light receiving element, and FIG. 13B shows an example when gating the light receiving element. The gating of the light receiving element is to prevent the light receiving element from responding for a predetermined time by turning off the bias voltage, for example. If gating is not performed immediately after the emitted light is emitted, a false peak may be formed in the histogram due to unnecessary reflection in which the irradiated light is reflected by the cover of the distance measuring device, the scanning mirror or the like and directly enters the light receiving element. Most of the irradiating light is directed toward the object and only a small amount of light becomes unnecessary reflection, but since it is reflected at a close distance, a large peak is formed in the histogram. This phenomenon is particularly remarkable in the case of a coaxial optical system in which the optical axes of light projection and light reception are the same. Immediately after detecting this stray light (unnecessary reflection), the light receiving element has a dead time in which photons cannot be detected, and then becomes active again. Immediately after becoming active, there is no influence of dead time, so the detection rate is “λ”. Thereafter, when time elapses, the effective detection rate λ _eff converges due to the effect of the dead time. A peak as indicated by a broken line in the figure is formed in the transition time of the detection rate, which may cause a TOF erroneous detection. The false peak increases as the difference between the detection rate λ and the effective detection rate λ _eff increases. As shown in FIG. 13B, when the light receiving element is gated immediately after the irradiation light is emitted, the dead time due to the stray light is eliminated, so that it is possible to detect the near-distance object of the TOF corresponding to the dead time. However, immediately after the light receiving element becomes active after gating, there is a possibility that a false peak may occur due to the transition of the detection rate.

  The decrease in detection time efficiency and the formation of false peaks are caused by the detection rate for disturbance light being too large. Therefore, a method of avoiding these influences by adjusting the sensitivity of the light receiving element is considered. For example, a technique for adjusting the bias voltage of SPAD is disclosed (Patent Document 5). Also disclosed is a technique for adjusting the photon detection probability by counting SPAD output pulses and changing the bias voltage of the light receiving element according to the count value (Patent Document 6). Further, a technique is disclosed in which a measurement point is scanned by moving a light projecting lens and a light receiving lens using an electromagnet, and a side lobe of reflected light is measured in the next measurement region. Thus, the gain is adjusted so that the light receiving element is not saturated in the next measurement (Patent Document 7).

JP 2012-60012 A JP 2011-247872 A JP 2010-91378 A Japanese Patent Application Laid-Open No. 07-067043 JP 2012-037267 A US Patent Application Publication 2012/0075615 JP 2007-183246 A

C.Niclass, M.Soga, H.Matsubara, S.Kato, "A 100m-range 10-frames / s 340x96-pixel time-of-flight depth sensor in 0.18μm CMOS", Proceeding of the ESSCIRC, pp.107 -110, September, 2011

  By the way, in the method of repeatedly receiving the reflected light and measuring the photon detection rate while performing the histogram processing for detecting the TOF, the component of the irradiation light is included in addition to the noise component such as sunlight. In particular, when there is an object having a high reflectance at a short distance, the noise component cannot be detected correctly.

  Further, in the method of detecting the side lobe of the reflected light, the disturbance light cannot be accurately measured because the detected light includes the irradiation light. Further, since the focal length of the light receiving lens is adjusted so that the side lobe can be detected, the power of the laser beam is wasted. Further, since the light receiving element for gain adjustment observes the next measurement area, the relative position between the light receiving element for gain adjustment and the light receiving element for measurement must be accurately aligned.

The present invention is an optical distance measuring device for measuring a distance based on a difference between a projection time of irradiation light and a reception time of reflected light, and receives a light from a light source that projects pulsed light and light from an object. Measurement light receiving means including a photon count type measurement light receiving element, scanning means for scanning the light projecting direction and the light receiving direction, and light from a region where the measurement light receiving means measures next time by the scanning means. A reference light receiving unit including a reference light receiving element that receives only light other than the light projected as the irradiation light, and a control for controlling the sensitivity of the measurement light receiving unit according to the amount of light received by the reference light receiving unit And an optical distance measuring device.

Here, the reference light receiving element is composed of a photon count type light receiving element, and the reference light receiving means includes a cumulative means for temporally accumulating pulse signals output from the reference light receiving element and outputting them as cumulative values. Preferably, the control means uses the accumulated value as the amount of light received by the reference light receiving means, and controls the sensitivity of the measurement light receiving means according to the accumulated value. The measurement light receiving means includes a plurality of Geiger mode avalanche photodiodes arranged in an array as the measurement light receiving element, an addition unit for adding outputs of the avalanche photodiodes, and an output of the addition unit is a threshold value. A determination unit that outputs a trigger signal in the case of the above, wherein the control unit changes a bias voltage to the avalanche photodiode, and changes the threshold value of the determination unit It is preferable that the sensitivity of the measurement light receiving means is controlled by at least one of the above.

  ADVANTAGE OF THE INVENTION According to this invention, the dynamic range of the brightness of a photodetector can be expanded appropriately.

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 element in embodiment of this invention. It is a figure which shows the example of the histogram produced | generated with the photodetector in embodiment of this invention. It is a figure which shows the structure of the reference light-receiving means in embodiment of this invention. It is a figure which shows the structure of the optical distance measuring device in embodiment of this invention. It is a figure which shows the example of arrangement | positioning of the measurement light receiving element and reference light receiving element in embodiment of this invention. It is a figure explaining the measurement direction of the optical distance measuring device in embodiment of this invention. It is a figure explaining control of the bias voltage with respect to the measurement light receiving element in embodiment of this invention. It is a figure explaining TOF measurement by an optical distance measuring device. It is a figure which shows the structure of the conventional optical ranging device. It is a figure which shows the relationship between the light reception amount and the count number of a photon in the conventional optical ranging apparatus. It is a figure which shows the relationship between the detection rate in the conventional optical ranging apparatus, and an effective detection rate. It is a figure explaining the subject of the peak detection of TOF in the conventional optical ranging device.

  As shown in FIG. 1, the photodetector 100 in the present embodiment includes a measurement light receiving means 102, a pulse shaping circuit 104, an adding means 106, a comparing means 114, a TDC (Time to Digital Converter) 116, a histogram generating means 118, and A bias unit 120 is included.

  The measurement light receiving means 102 includes a photodiode 10, a quenching resistor 12, and a buffer 14. One set of the photodiode 10, the quenching resistor 12, and the buffer 14 constitute one measurement light receiving unit 102a. The measurement light receiving unit 102a is provided to measure the distance by TOF.

  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 utilizes a resistance component between the source and drain of the FET. 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 measurement light receiving unit 102 may include a plurality of measurement light receiving units 102a. That is, the measurement light receiving means 102 may be configured as a silicon photomultiplier (SiPM). In FIG. 1, for the sake of simplicity, only the two measurement light receiving units 102a are shown, and the other measurement light receiving units 102a are omitted. For example, as shown in the schematic plan view of FIG. 2, the measurement light receiving units 102a may be 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.

  In the pulse shaping circuit 104, the pulse width of the output pulse from the measurement light receiving unit 102a is shaped so as to be equal to the pulse width of the irradiated laser beam. The adding means 106 adds the outputs from the pulse shaping circuit 104.

The comparison unit 114 compares the value S output from the addition unit 106 with a predetermined threshold value N _REF, and outputs a determination result indicating that a TOF reflection pulse has arrived when the value S is greater than or equal to the threshold value N _REF . For example, when the threshold value N _REF is set to “2”, the output PEAK is set to the high level if the value S is “2” or more, and is set to the low level otherwise.

  The TDC 116 measures the arrival time of the reflected pulse when the output PEAK of the comparison unit 114 is at a high level. The TDC 116 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 114 as the TOF measurement result. The TDC 116 allows measurement with a time resolution of several hundred picoseconds.

  The histogram generation means 118 further accumulates the TOF measurement results obtained by the TDC 116 over a predetermined measurement time to generate a histogram. 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 114 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.

  The photodetector 100 includes a reference light receiving means 122 in addition to the measurement light receiving means 102. FIG. 4 shows the reference light receiving means 122 including the reference light receiving part 122a. The reference light receiving unit 122 includes a reference light receiving unit 122a, a first sampling unit 124, an adding unit 126, a second sampling unit 128, an accumulating unit 140, and a latch unit 142.

  The reference light receiving unit 122 includes a photodiode 20, a quenching resistor 22, and a buffer 24. One set of the photodiode 20, the quenching resistor 22 and the buffer 24 constitute one light receiving element 122a, and FIG. 4 shows an example in which the reference light receiving means 122 is constituted by three reference light receiving portions 122a. The number of the reference light receiving parts 122a is not limited to this, and may be one or a plurality of reference light receiving parts 122a may be configured in an array.

The first sampling unit 124 includes a circuit that temporarily holds the output from the reference light receiving unit 122. The first sampling unit 124 can be realized by, for example, the D flip-flop 16. When the reference light receiving unit 122 includes a plurality of reference light receiving units 122a, a D flip-flop 16 is provided for each reference light receiving unit 122a. The first sampling unit 124 samples a binary output signal (pulse voltage: N _SPAD ) output from the reference light receiving unit 122a included in the reference light receiving unit 122 in synchronization with the clock SCLK, and outputs the held value. . The sampling frequency is preferably equal to or higher than the Nyquist frequency of the voltage pulse output from the reference light receiving unit 122a, and is preferably equal to or higher than twice the reciprocal of the voltage pulse width.

The adding unit 126 adds the outputs from the first sampling unit 124 and outputs the addition result (bit width: N _ADD = [log ₂ (N _SPAD )]). The adding means 126 adds the voltage pulses output from the plurality of reference light receiving units 122a at the same timing. For example, if two voltage pulses of the plurality of reference light receiving units 122a included in the reference light receiving unit 122 are at a high level, the output from the adding unit 126 is “2 (decimal number)” = “10 ( Binary number) ”. The second sampling unit 128 outputs a value obtained by sampling and holding the signal output from the adding unit 126 every time the clock SCLK is input. The second sampling means 128 can be configured including the D flip-flop 16.

The accumulating unit 140 accumulates and outputs the addition result output from the adding unit 126 and held in the second sampling unit 128 over a predetermined measurement time. The accumulating means 140 can be constituted by an accumulator. The accumulating means 140 outputs a cumulative value (bit width: N _REG = [log ₂ (N _SPAD · T · f _SCLK )]) obtained by further integrating the added values of the outputs of the plurality of reference light receiving units 122a with time. The value always increases monotonously with an increase in the pulse width of the voltage pulse of the plurality of reference light receiving units 122a.

  That is, when the number of incident photons is small (the amount of incident light is small), an accumulation value proportional to the total pulse width of the voltage pulses of all the photodiodes 20 is output from the accumulation means 140. When the number of incident photons increases (the amount of incident light increases), the output frequency of the voltage pulses of the photodiode 20 increases, and the probability that a plurality of voltage pulses are gradually combined increases. When the voltage pulses start to combine, the number of pulses decreases, but the cumulative value (total time) of the pulse width of the voltage pulse in the accumulating means 140 always increases. Therefore, it is possible to obtain an output with a wide dynamic range that constantly increases monotonously with an increase in the amount of incident light.

  The latch unit 142 outputs a value obtained by sampling and holding the output of the accumulating unit 140 every time the clock SCLK is input. The latch unit 142 can include the D flip-flop 16.

  As described above, the reference light receiving unit 122a outputs binary information (voltage pulse) with respect to the incidence of photons, and is robust against fluctuations in temperature and the like. Therefore, the total time of the pulse width of the voltage pulse is not affected by fluctuations such as the internal amplification gain, and stable light detection can be performed regardless of the temperature.

  Further, since the total time of the pulse widths of the voltage pulses from the plurality of reference light receiving means 122 is output as the amount of light, the dynamic range can be further widened as compared with the case where there is one reference light receiving means 122.

  In addition, since a Geiger mode avalanche photodiode is used as the reference light receiving means 122, it can be mounted on the apparatus at a lower cost and more compactly than other photon count type light receiving elements such as a photomultiplier tube. Further, since the avalanche photodiode is a semiconductor element, it is easy to integrate a plurality of reference light receiving means 122. In particular, it is easy to integrate the reference light receiving means 122 and the measurement light receiving means 102 on one chip. Furthermore, since a technology for realizing an avalanche photodiode by a CMOS process has been developed, it is mounted on the same chip as the first sampling means 124, the adding means 126, the second sampling means 128, the accumulating means 140, the latch means 142, and the like. Is possible. 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 reference light receiving unit 122 includes a plurality of reference light receiving units 122a. However, even if there is only one reference light receiving unit 122a, an effect of widening the dynamic range can be obtained.

  Next, an optical distance measuring device 200 equipped with the photodetector will be described. As shown in FIG. 5, 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 116 and used for the measurement of 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 objects (for example, a car, a road, a tree, a person, etc.) exist in the distance measurement target space, light is reflected by these objects and returns to the optical distance measuring device 200. 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.

  The photodetector 100 includes a measurement light receiving unit 102a for the above-described TOF measurement and a reference light receiving unit 122a for detecting the intensity of disturbance light. As shown in FIG. 6, the measurement light receiving part 102a and the reference light receiving part 122a are arranged in the light receiving element area 208 including the measurement light receiving element area 208a and the reference light receiving element area 208b arranged in an array, respectively. The measurement light-receiving element region 208a is disposed at a position where light emitted from the light source 202 and reflected by an object forms an image. The reference light-receiving element region 208b is disposed adjacent to the measurement light-receiving element region 208a, and is arranged such that the reflected light from the light source 202 does not enter, and disturbance light (environment light) currently incident on the reference light-receiving element region 208b. Is arranged at a position where it enters the measurement light receiving element region 208a in the next TOF measurement time.

  That is, as shown in FIG. 7, the measurement light receiving means 102 and the reference light receiving means 122 are observed in different directions at a certain time during light scanning by the polygon mirror 206, and the measurement direction A of the reference light receiving means 122 is always the next time. The measurement light receiving means 102 is disposed at a relative position where the measurement direction B is observed. The angle difference in the observation direction is determined by the distance between the measurement light receiving means 102 and the reference light receiving means 122 and the focal length of the hyperboloid mirror 204. Further, the angle difference between the measurement direction B at a certain time of the measurement light receiving means 102 and the next measurement direction A, that is, the horizontal resolution is determined by the rotational speed of the polygon mirror 206, the measurement time interval, the incident angle of the irradiation light, and the like. . Therefore, the measurement light receiving means 102 and the reference light receiving means 122 may be arranged in consideration of these conditions. For example, when the horizontal resolution and the focal length of the hyperboloid mirror 204 are constant, the reference light receiving unit 122 always observes the next measurement direction A by adjusting the distance between the measurement light receiving unit 102 and the reference light receiving unit 122. Can be.

  When a plurality of measurement light receiving units 102a are provided, it is preferable to provide a plurality of reference light receiving units 122 for each measurement light receiving unit 102a or for a set of several measurement light receiving units 102a. In the example of the photodetector 100 shown in FIG. 1, the reference light receiving unit 122 is provided for each set of a plurality of measurement light receiving units 102 a arranged in the horizontal direction of the measurement light receiving unit 102. Of course, one reference light receiving means 122 may be provided for each measurement light receiving means 102.

In order to suppress a decrease in photon detection time efficiency and formation of a false peak in a histogram when disturbance light such as sunlight is strong, the sensitivity of the measurement light receiving unit 102a may be reduced when disturbance light is strong. The measurement light receiving unit 102a is a Geiger mode operated by applying a voltage larger than the breakdown voltage. The sensitivity increases when the difference (excess voltage) between the bias voltage and the breakdown voltage is increased, and the sensitivity decreases when the voltage is decreased. Sensitivity is the probability of photon detection, and is the probability of outputting a voltage pulse in response to photon incidence. When the excessive voltage becomes lower than the threshold value V _TH , the operation in the Geiger mode cannot be performed. Conversely, when the excessive voltage is excessively increased, the frequency of generation of false pulses (noise) increases.

Therefore, the bias unit 120 of the photodetector 100 adjusts the bias voltage of the measurement light receiving unit 102a according to the output signal _LBG indicating the intensity of the disturbance light output from the reference light receiving unit 122.

As shown in FIG. 1, the bias unit 120 includes a bias voltage setting unit 18 and a digital / analog conversion unit (DAC) 19. The bias voltage setting unit 18 sets an appropriate bias voltage value according to the output signal _LBG output from the reference light receiving unit 122. For example, the bias voltage setting unit 18 refers to a look-up table in which an output signal L _BG previously obtained through experiments or the like is associated with an appropriate bias voltage and registered, and an appropriate bias voltage value according to the output signal L _BG. Set. Alternatively, the appropriate bias voltage may be calculated by expressing the output signal L _BG and an appropriate bias voltage as a function and substituting the output signal L _BG into the function. The DAC 19 converts the bias voltage set by the bias voltage setting unit 18 into an analog value and outputs the analog value. The bias voltage output from the DAC 19 controls the negative potential of the photodiode 20 included in the measurement light receiving unit 102a. As shown in FIG. 8, the bias voltage is set so that the negative potential of the photodiode 20 increases as the output signal _LBG increases.

As described above, by providing the reference light receiving unit 122 and the bias unit 120, it is possible to decrease the bias voltage of the photodiode 20 and reduce the sensitivity of the photodiode 20 when disturbance light is strong. With this configuration, it is possible to adjust the bias voltage in a range from the voltage obtained by adding the threshold voltage V _TH to the breakdown voltage V _BD to the power supply voltage V _{SPAD with} respect to the photodiode 20. That is, it is possible to appropriately adjust the sensitivity of the measurement light receiving unit 102a for TOF observation according to the intensity of disturbance light from the next measurement direction observed by the reference light receiving unit 122. Therefore, a decrease in detection time efficiency and occurrence of a false peak in the histogram can be suppressed, and the TOF detection efficiency and accuracy can be increased.

Furthermore, it is good also as a structure which changes the threshold value _NREF of the comparison means 114 according to the intensity | strength of disturbance light. By increasing the threshold value N _REF as the disturbance light increases, the false peak of the histogram can be further suppressed. In particular, it is effective to adjust the threshold value N _REF when the number of measurement light receiving units 102a constituting the measurement light receiving unit 102 is large or when the dead time of the TDC 116 is long.

  Further, it is preferable that the measurement light receiving unit 102a and the reference light receiving unit 122a are light receiving elements having the same structure. Thereby, since the difference in characteristics between the measurement light receiving unit 102a and the reference light receiving unit 122a is reduced, the influence of the dark count (dark current) of the measurement light receiving unit 102a can be compensated by the output value of the reference light receiving unit 122a. Furthermore, it is preferable to mount the measurement light receiving unit 102a and the reference light receiving unit 122a on the same semiconductor substrate (chip). As a result, the difference in characteristics between the measurement light-receiving unit 102a and the reference light-receiving unit 122a becomes smaller, so that the bias control of the measurement light-receiving element and the adjustment of the threshold value of the comparison unit can be performed more accurately. Furthermore, the relative positional relationship between the measurement light receiving means 102 and the reference light receiving means 122 can be accurately determined, and the arrangement with other components can be performed more accurately and easily.

  10 photodiode, 12 quenching resistor, 14 buffer, 16 flip-flop, 18 bias voltage setting unit, 19 DAC, 20 photodiode, 22 quenching resistor, 24 buffer, 100 photodetector, 102 measurement light receiving means, 102a measurement light receiving 104 pulse shaping circuit, 106 addition means, 114 comparison means, 116 TDC, 118 histogram generation means, 120 bias means, 122 reference light receiving means, 122a reference light receiving section, 124 first sampling means, 126 addition means, 128 second Sampling means, 140 accumulating means, 142 latch means, 200 optical distance measuring device, 202 light source, 204 hyperboloid mirror, 204a hole, 206 polygon mirror, 208 light receiving element region, 208a measurement light receiving element Region, 208b Reference light receiving element region.

Claims (3)

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 that emits pulsed light;
A measurement light receiving means having a variable sensitivity and a photon count type measurement light receiving element for receiving light from an object;
Scanning means for scanning the light emitting direction and the light receiving direction;
Reference light receiving means comprising a reference light receiving element that receives only light other than the light projected as the irradiation light, which is light from a region that the measurement light receiving means next measures by the scanning means;
Control means for controlling the sensitivity of the measurement light receiving means in accordance with the amount of light received by the reference light receiving means;
An optical distance measuring device comprising: The optical distance measuring device according to claim 1,
The reference light receiving element is composed of a photon count type light receiving element,
The reference light receiving means includes accumulating means for accumulating a pulse signal output from the reference light receiving element in time and outputting it as an accumulated value,
The optical distance measuring device according to claim 1, wherein the control means uses the accumulated value as the amount of light received by the reference light receiving means, and controls the sensitivity of the measurement light receiving means according to the accumulated value. The optical distance measuring device according to claim 1,
The measurement light receiving means includes
As the measurement light receiving element, a plurality of Geiger mode avalanche photodiodes arranged in an array,
An adder for adding the outputs of the avalanche photodiode;
A determination unit that outputs a trigger signal when the output of the addition unit is equal to or greater than a threshold; and
Comprising
The control means includes
Controlling the sensitivity of the measurement light receiving means by at least one of changing a bias voltage to the avalanche photodiode and changing the threshold value of the determination unit;
An optical distance measuring device.

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