Distance measuring device and distance measuring method

Abstract

A distance measuring device includes: a light emitting unit for emitting light having a modulation frequency to an object; a light receiving unit for receiving light emitted from the light emitting unit and reflected by the object and returned; and a controller for measuring the length of time between the time of light emitted by the light emitting unit and the time of light received by the light receiving unit based on the radiation intensity of the light received by the light receiving unit, so as to obtain the distance to the object. The controller performs a first measurement, causing the light emitting unit to emit light of a first irradiation time and a second irradiation time longer than the first irradiation time at a first modulation frequency, thereby measuring the length of time before the light is received by the light receiving unit with a first plurality of times. The controller performs a second measurement, causing the light emitting unit to emit light of a third irradiation time at a second modulation frequency, thereby measuring the length of time before the light is received by the light receiving unit with a second plurality of times less than the first plurality of times. The third irradiation time is longer than the first irradiation time and shorter than the second irradiation time. It is possible to achieve reduction of aliasing and increase of dynamic range.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to a distance measurement device and a distance measurement method. Background Art

[0002] As a method of measuring the distance between a distance measuring device and an object, a method called time of flight (TOF) is known in the art, which irradiates an object with light to calculate the distance to the object based on the time difference between the emitted light and the light reflected by the object. Infrared light is modulated at a modulation frequency of several megahertz (MHz) or more, and the modulated infrared light is emitted to the object in a waveform such as a sine wave, a square wave, or a rectangular wave. A time of flight (TOF) camera using distance measurement using phase detection is known in the art. In other words, in such a TOF camera, the phase of the light returned as reflected by the object is measured by a plurality of light-receiving elements, and the time difference is calculated based on the obtained phase. Finally, the distance to the object is obtained. When the TOF method is adopted and the distance is to be measured with high accuracy, it is effective to increase the modulation frequency of the light. Theoretically, if the modulation frequency is doubled, the variation of the distance measurement can be reduced to half. However, in the distance measurement using phase detection, some indistinguishability or ambiguity due to the periodicity of the phase occurs, which is referred to as aliasing in the description of the present disclosure.

[0003] A time-of-flight (TOF) system that obtains phase data uses multiple modulation frequencies for irradiation light (for example, see PTL 1). In addition, in such a TOF system, phase data obtained based on multiple modulation frequencies are combined to balance noise in the phase data. Due to this configuration, as is known in the art, aliasing can be reduced and distance can be accurately measured.

[0004] Citation List

[0005] Patent Literature

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2012-225807 Summary of the invention

[0007] Technical issues

[0008] According to the technology known in the art (for example, see PTL 1), aliasing can be reduced. However, when both a distant low-reflectivity object and a close-range high-reflectivity object exist in the same scene, the dynamic range of the camera device may inadvertently exceed its limit.

[0009] Solutions to the problem

[0010] The present invention provides a distance measuring device, comprising: a light emitting unit configured to emit light having a modulation frequency to an object; a light receiving unit configured to receive the light emitted from the light emitting unit and reflected by the object and returned; and a controller configured to measure the time length between the time when the light is emitted by the light emitting unit and the time when the light is received by the light receiving unit based on the radiation intensity of the received light by the light receiving unit, so as to obtain the distance to the object, and the controller is configured to perform a first measurement to cause the light emitting unit to emit the light at a first modulation frequency, thereby measuring the time length before the light is received by the light receiving unit in a first plurality of times. The controller is configured to perform a second measurement to cause the light emitting unit to emit the light at a second modulation frequency lower than the first modulation frequency, thereby measuring the time length before the light is received by the light receiving unit in a second plurality of times less than the first plurality of times, wherein the controller is configured to calculate the distance to the object based on the time length obtained in the first measurement and the time length obtained in the second measurement.

[0011] Effects of the Invention

[0012] According to one aspect of the present disclosure, both reduction of aliasing and assurance of dynamic range can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings are intended to describe exemplary embodiments of the present invention and should not be interpreted as limiting its scope. Unless explicitly stated, the accompanying drawings should not be considered to be drawn to scale. In addition, in all the drawings, the same or similar reference numerals represent the same or similar parts.

[0014] Figure 1 FIG. 2 is a diagram showing a hardware configuration of a distance measurement device according to an embodiment of the present disclosure.

[0015] Figure 2 FIG. 1 is a diagram showing the principle of how to calculate and obtain the distance through a distance measuring device according to an embodiment of the present disclosure.

[0016] Figure 3 FIG. 2 is a diagram showing the relationship between the radiation intensity of received light and the accuracy of distance measurement according to an embodiment of the present disclosure.

[0017] Figure 4 Shown is a diagram of high dynamic range measurement related to an embodiment of the present disclosure.

[0018] Figure 5 FIG. 4 is a diagram showing a measurement sequence according to an embodiment of the present disclosure.

[0019] Figure 6FIG. 4 is a diagram showing the relationship between the radiation intensity of received light and the variation in the value of distance measurement in the third measurement process according to the embodiment of the present disclosure.

[0020] Figure 7 FIG. 2 is a diagram illustrating a process of calculating a distance based on a measurement result with high accuracy according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0021] Implementation

[0022] The distance measuring device involved in the embodiment of the present disclosure adopts the time-of-flight (TOF) method to measure the distance to an object. In the time-of-flight method, light is emitted to an object, and the distance is calculated based on the time difference with the light reflected by the object. In the distance measuring device, after emitting infrared light whose radiation intensity has been modulated based on a predetermined radiation pattern from a light source to an object, the light reflected by the object is received by a light receiving element for infrared light. In the distance measuring device, the time difference between the emission time and the light reception time is detected pixel by pixel based on the predetermined radiation pattern of the infrared light to calculate and obtain the distance. In the distance measuring device, the calculated distance values ​​are collected in a bitmap format pixel by pixel and stored as a distance image. A distance measuring device using this method is called a TOF camera.

[0023] More specifically, in the distance measuring device involved in this embodiment, infrared light is modulated at a modulation frequency of several megahertz (MHz) or more, and the modulated infrared light is emitted to the object in a waveform such as a sine wave, a square wave, or a rectangular wave. Then, in the distance measuring device, the phase of the light returned as reflected by the object is measured by a plurality of light receiving elements, and the time difference is calculated based on the obtained phase. Finally, the distance to the object is obtained. A distance measuring device using this method is called a phase detection TOF camera.

[0024] When performing measurements over a wide range in a TOF camera, the dynamic range of the scene may become a problem. The radiation intensity of the light needs to be increased, or the exposure time needs to be increased, in order to measure the distance to a distant object. However, when measuring the distance to a close object under such conditions, the radiation intensity of the received light may increase excessively, and may inadvertently exceed the capacity of the camera device and reach a saturated level. In addition, the radiation intensity of the received light varies depending on the reflectivity of the object. Therefore, when both a distant low-reflectivity object and a close-up high-reflectivity object are present in the same scene, the dynamic range of the camera device may exceed its limit, and there may be some situations where the distance to the object at a distant low-reflectivity and the distance to the object at a close-up high-reflectivity cannot be measured simultaneously.

[0025] In addition, aliasing may become a problem when measurements are to be performed with high precision in a TOF camera. When it is desired to perform measurements with high precision, it is effective to increase the modulation frequency of the light. Theoretically, if the modulation frequency is doubled, the variation in distance measurement can be reduced to half. On the other hand, in distance measurement using phase detection, the phase has a periodicity of 2π, so when the distance is measured based on the detected phase, the distance obtained may become indistinguishable due to the periodicity of the phase. For example, when light with a modulation frequency of 10 MHz is used for distance measurement, the distance equivalent to one cycle of the phase is approximately 15 meters (m). As a result, some indistinguishability or ambiguity is retained in a cycle of 15 m. In other words, when π is obtained as a result of phase detection, the distance may be "7.5+15×n" [m], where n is a positive integer. In the description of the present disclosure, this indistinguishability or ambiguity due to the periodicity of the phase is referred to as aliasing.

[0026] In order to achieve both measurement with a wide range and measurement with high accuracy in a TOF camera, it is desirable to handle and solve both the problem of dynamic range and the problem of aliasing.

[0027] In order to deal with this situation, the distance measuring device 1 involved in this embodiment adopts a pair of modes including a high-frequency modulation mode and a low-frequency modulation mode. In the high-frequency modulation mode, multiple measurements are performed with a variable exposure time or light intensity. On the other hand, in the low-frequency modulation mode, measurements are performed with an exposure time longer than the maximum exposure time in the high-frequency modulation mode. Alternatively, in the low-frequency modulation mode, measurements are performed with a radiation intensity of light higher than the maximum radiation intensity of the light. Due to such a configuration as described above, it is possible to simultaneously achieve reduction of aliasing and increase of dynamic range.

[0028] More specifically, the distance measuring device adopts a pair of modes including a high-frequency modulation mode and a low-frequency modulation mode, and performs two measurements including a measurement using high-frequency modulated light and a measurement using low-frequency modulated light. In the high-frequency modulation mode, the distance measuring device performs multiple measurements with a variable exposure time or light intensity. Due to this configuration, it is possible to perform measurements with high accuracy while maintaining a wide dynamic range for the signal. In addition, in the low-frequency modulation mode, the distance measuring device performs measurements with an exposure time longer than the maximum exposure time in the high-frequency modulation mode. Optionally, in the low-frequency modulation mode, the distance measuring device 1 performs measurements with a radiation intensity of light higher than the maximum radiation intensity of the light. As a result, the number of measurements performed in the low-frequency modulation mode becomes less than the number of measurements performed in the high-frequency modulation mode. For example, aliasing avoidance in response to a measurement result performed multiple times in the high-frequency modulation mode is performed by performing a measurement in the low-frequency modulation mode. Due to this configuration, aliasing included in such a measurement result can be reduced. As a result, it is possible to simultaneously achieve reduction in aliasing and increase in dynamic range.

[0029] Specifically, the distance measuring device 1 can be configured as follows Figure 1 shown. Figure 1 FIG. 1 is a diagram showing a hardware configuration of the distance measuring device 1 according to the present embodiment. More specifically, as a configuration or structure of the distance measuring device 1, Figure 1 The configuration or structure shown is to combine multiple fisheye TOF cameras with a full viewing angle greater than 180 degrees to realize an omnidirectional TOF camera.

[0030] In the distance measuring device 1, modulated light (illumination light) modulated to a level of several MHz or higher with a square wave, a rectangular wave or a sine wave is emitted to an object whose distance is to be measured, and the modulated light (reflected light) reflected by the object after hitting the object is received to measure the radiation intensity of the light. Figure 1 As shown, the distance measurement device 1 according to the present embodiment includes, for example, a light emitting unit 10 , a light receiving unit 20 , and a control unit 30 .

[0031] The light emitting unit 10 receives a modulation signal from the control unit 30. This modulation signal may be a rectangular wave or a sine wave modulated to a frequency of several MHz or higher. The light emitting unit 10 irradiates modulated light (irradiation light) onto an object whose distance is to be measured based on the modulation signal. Such modulated light is modulated according to the modulation signal. The light emitting unit 10 includes a light source 11 and a lens 12. The light source 11 may be implemented as a two-dimensional array of vertical cavity surface emitting lasers (VCSELs). The lens 12 increases the viewing angle of the modulated light emitted from the light source 11 to a desired angle and irradiates it toward the object. The light source 11 is housed in a housing 1a of the distance measuring device 1, and the lens 12 is exposed to the surface of the housing 1a.

[0032] The light receiving unit 20 receives the modulated light (reflected light) reflected by the object after hitting the object, and provides a signal according to the intensity of the received light to the control unit 30. The light receiving unit 20 includes a lens 21 and a light receiving sensor 22. The light receiving sensor 22 can be implemented as a TOF sensor, in which a plurality of light receiving elements are configured as a two-dimensional array. The lens 21 concentrates the modulated light reflected by the object after hitting the object onto the light receiving surface of the light receiving sensor 22. The light receiving sensor 22 generates a signal corresponding to the intensity of the received light for each light receiving element, and provides the generated signal to the control unit 30. The lens 21 is exposed to the surface of the housing 1a, and the light receiving sensor 22 is accommodated inside the housing 1a.

[0033] The control unit 30 controls, for example, the light emission pattern and light emission timing of the light source 11 of the light emitting unit 10, and controls, for example, the light receiving time or timing of the light receiving sensor 22 of the light receiving unit 20 in synchronization with the control of the light emitting unit 10. The control unit 30 measures the phase of the light reflected and returned by the object based on the signal obtained by each light receiving element and provided from the light receiving sensor 22, and calculates the time difference based on the obtained phase. As a result, the distance to the object is obtained. For example, the control unit 30 involved in this embodiment is implemented by a central processing unit (CPU) and is accommodated inside the housing 1a.

[0034] Refer to the following Figure 2 The principle of how to calculate and obtain the distance by the distance measurement device 1 involved in this embodiment is described. Figure 2 The diagram is a diagram showing the principle of how to calculate and obtain the distance through the distance measuring device involved in this embodiment.

[0035] The distance measuring device 1 involved in this embodiment is a time-of-flight (TOF) camera that performs phase detection. The distance measuring device 1 has a pair of charge storage units P1 and P2 for each light-receiving element, and can quickly determine and switch to one of the charge storage unit pairs P1 and P2 in which the charge is to be stored. Such a pair of charge storage units P1 and P2 is configured so that the charge can be accumulated independently of each other. Due to this configuration, a pair of phase signals that are completely opposite to a square wave or rectangular wave can be detected simultaneously. For example, a combination of 0 degrees and 180 degrees or a combination of 90 degrees and 270 degrees can be detected simultaneously. In other words, the distance can be measured based on a process of performing at least two irradiations or light receptions.

[0036] Figure 2 is a diagram of the timing at which each of a pair of charge storage units P1 and P2 accumulates charge in response to irradiated light and reflected light, and accumulates charge as shown in the hatched area. In the present embodiment, irradiated light represents modulated light emitted from the light emitting unit 10 to the object whose distance is to be measured. In the present embodiment, reflected light represents modulated light reflected by the object whose distance is to be measured and received by the light receiving unit 20. However, in reality, in order to increase the amount of accumulated charge, irradiation is not performed with a single rectangular wave. Instead, a repetitive pattern of a rectangular wave with a duty cycle of 50% is performed, and the charge storage units P1 and P2 are repeatedly switched accordingly.

[0037] With reference to the pulse period of the irradiated light, the charge corresponding to the reflected light is accumulated based on the exposure times corresponding to 0°, 90°, 180° and 270° in time, respectively. As a result, four phase signals A0, A90, A180 and A270 are obtained. In other words, the four phase signals A0, A90, A180 and A270 are phase signals divided into four phases of 0°, 90°, 180° and 270° in time. Therefore, the control unit 30 can calculate the phase difference angle φ using the following first equation.

[0038] First equation

[0039] [Mathematical formula 1]

[0040] φ=Arctan[(A90-A270) / (A0-A180)]

[0041] The control unit 30 according to the present embodiment can calculate the delay time Td using the phase difference angle φ based on the following second equation.

[0042] The second equation

[0043] [Mathematical formula 2]

[0044]

[0045] When the pulse width of the irradiation light is assumed to be T0 in the second equation, T=2T0. The control unit 30 according to the present embodiment can calculate the distance value D to the object using the delay time Td obtained in the second equation and the speed C of light based on the third equation described below.

[0046] The third equation

[0047] [Mathematical formula 3]

[0048] D=Td×C / 2

[0049] Next, refer to Figure 3 The relationship between the radiation intensity of the light received by the light receiving unit 20 and the distance measurement accuracy of the control unit 30 will be described. Figure 3 FIG. 2 is a graph showing the relationship between the amount of received light and the distance measurement accuracy involved in this embodiment. Figure 3 In the figure, the vertical axis is the distance measurement accuracy represented by the change in the distance measurement value. The smaller the value, the higher the distance measurement accuracy. Figure 3 , the horizontal axis represents the radiation intensity of light received by the light receiving unit 20 .

[0050] exist Figure 3In the embodiment, it is assumed that the waveform of the light emitted from the light emitting unit 10 is a rectangular wave modulated at 50 MHz. As the radiation intensity of the light received by the light receiving unit 20 increases, the variation of the distance measurement value decreases, and the accuracy of the distance measurement increases. Assume that the accuracy specification of the TOF camera in this embodiment is to be maintained at or less than 50 mm, such as Figure 3 It can be seen that the effective range of the radiation intensity of the received light needs to be within the range from 100LSB to 1000LSB of the digital value. If the effective range of the radiation intensity of the received light is less than 100LSB, the variation of the distance measurement value may inadvertently become equal to or greater than 50mm. If the effective range of the radiation intensity of the received light exceeds 1000LSB, a pair of charge storage units P1 and P2 of each of the multiple light-receiving elements may inadvertently exceed the capacity and reach the saturation level. In other words, the dynamic range is 10 times between 100LSB and 1000LSB.

[0051] Refer to the following Figure 4 To illustrate high dynamic range measurement. Figure 4 FIG. 4 is a diagram showing high dynamic range measurement according to the present embodiment.

[0052] In this specific embodiment, it is assumed that the range of the distance to be measured is 1 to 5 m, and the range of the reflectivity is 20 to 80%. As is known in the art, the radiation intensity of the received light is inversely proportional to the square of the distance to the object, and is proportional to the reflectivity of the object. In other words, the radiation intensity of the received light is proportional to "(reflectivity) / (distance) 2 " is proportional.

[0053] On the other hand, in this embodiment, the radiation intensity of the received light is also proportional to the exposure amount. The exposure amount is a controllable parameter that can be changed by changing the exposure time corresponding to the irradiation time or by changing the amplitude of the square wave, rectangular wave or sine wave.

[0054] In short, the radiation intensity of the received light is related to "(exposure) × (reflectivity) / (distance) 2 " is proportional.

[0055] Considering the dynamic range, i.e. the effective range of the radiation intensity of the received light, in order to cover this range, the effective range of the radiation intensity of the received light needs to be 5 2 = 25 times, in order to cover the range of reflectivity, the effective range of the radiation intensity of the received light needs to be 4 times. In other words, as a whole, the dynamic range needs to be 25×4=100 times. In the following description of the present disclosure, such a range is referred to as the dynamic range of the scene.

[0056] As mentioned above, the dynamic range of the TOF camera is ten times. Therefore, the dynamic range of the desired scene, i.e., 100 times, cannot be met. However, if a method of changing the exposure time of the TOF camera and performing multiple shooting operations is adopted, the dynamic range can be increased to exceed the dynamic range of 10 times per shooting operation. In addition, the exposure time refers to the continuous exposure at a predetermined frequency. Figure 2 In other words, in this embodiment, it is assumed that the exposure time is the same as the time length of the period during which the light emitting unit 10 emits light.

[0057] First, the exposure time is set so that the distance is the longest and the reflectivity is low, that is, the distance is 5m and the reflectivity is 20% (corresponding to Figure 4 When point A in the image is able to receive reflected light at 100 LSB, the distance is measured for the first time. For example, assuming that the exposure time is 1 millisecond (msec). In this configuration, by Figure 4 Curve 1 at point A in FIG. 1 represents the condition where the radiation intensity of the received light is 100 LSB. Figure 4 Curve 2 in FIG. 1 represents a condition where the radiation intensity of the received light is 1000 LSB at the same exposure time. In other words, the area bounded by curves 1 and 2 can be measured in the distance measurement performed for the first time.

[0058] Next, the exposure time is reduced by one tenth to 0.1 milliseconds, and the distance measurement is performed a second time. In such a second distance measurement, the radiation intensity of the received light is also reduced by one tenth, and Curve 2 represents the condition where the radiation intensity of the received light is 100LSB. In other words, the condition where the radiation intensity of the received light is 100LSB in the second distance measurement is equivalent to the condition where the radiation intensity of the received light is 1000LSB in the first distance measurement. In addition, in the present embodiment, Curve 3 represents the condition where the radiation intensity of the received light is 1000LSB in the second distance measurement, and Curve 3 passes through Figure 4 , at which point B it is assumed that the distance to the nearest object with the highest reflectivity is measured. In other words, the area bounded by curves 2 and 3 can be measured in the distance measurement performed the second time.

[0059] As described above, when two measurements are performed while changing the exposure time, the area bounded by curve 1 and curve 3 can be measured. As a result, Figure 4 As shown, the measurement of this embodiment can cover the rectangular area to be measured which is bounded by the dotted line.

[0060] In the present embodiment, it is configured so that the condition of the highest radiation intensity (1000LSB) for receiving light in the first measurement and the condition of the lower limit radiation intensity (100LSB) for receiving light in the second measurement match each other. However, it is not limited to this, and it can also be configured so that part of the range of the first measurement overlaps with the range of the second second measurement according to the dynamic range of the light receiving unit 20, the range of the distance measurement to be performed, and the range of the reflectivity, thereby achieving a certain degree of robustness. When the range of the distance or the range of the reflectivity is wider than the dynamic range of the light receiving unit 20, the desired measurement range can be covered by performing three or more measurements.

[0061] Refer to the following Figure 5 to illustrate the measurement timing used to avoid aliasing. Figure 5 FIG. 2 is a diagram showing a measurement sequence according to the present embodiment.

[0062] In order to measure the distance with high accuracy, it is effective to increase the modulation frequency of the irradiated light. Assuming that the resolution of the detected phase is fixed, theoretically, from the following equation 4, if the modulation frequency is doubled, the distance resolution will also be doubled.

[0063] The fourth equation

[0064] [Formula 4]

[0065]

[0066] In the above equation 4, D, C, f, and φ represent distance, light speed, modulation frequency, and detection phase, respectively. In addition, the distance measurement detected by φ has the arbitrariness of a period of 2π. This relationship is shown below.

[0067] 0≤φ<2π

[0068] Assuming k is a desired positive integer, Equation 4 can be modified to the following Equation 5.

[0069] The fifth equation

[0070] [Formula 5]

[0071]

[0072] As shown in the fifth equation above, when a wide range is to be measured and such a wide range exceeds the distance (C / 2f), it becomes difficult to determine a unique value of the distance D. This phenomenon is called aliasing.

[0073] For example, when the modulation frequency is 50 MHz, the arbitrariness of the distance D is a period of 3 meters (m). For example, when the modulation frequency is 12.5 MHz, the arbitrariness of the distance D is a period of 12 m.

[0074] like Figure 4 As shown, in the high dynamic range measurement performed multiple times, the measurement results are included in the area bounded by curves 1 and 3. In this configuration, the distance to the object is less than 12 m. Therefore, when a modulation frequency of, for example, 12.5 MHz is used, the occurrence of aliasing can be prevented.

[0075] However, when it is desired to measure distance with high accuracy, a modulation frequency of about 12.5 MHz is not sufficient. Figure 3 As described above, in order to satisfy the condition that the variation of the distance value is equal to or less than the threshold value (ie, 40 mm), it is necessary to perform measurement at a modulation frequency of 50 MHz and a radiation intensity of received light of 100 to 1000 LSB. In such a configuration, aliasing may occur at a period of 3 m.

[0076] In order to deal with such technical problems, for example, the measurement of the modulation frequency at 12.5 MHz can be additionally performed. When the measurement involved in this embodiment is based on the high dynamic range measurement performed multiple times, as will be described in detail later, the measurement is usually performed using a variable modulation frequency under the condition that the exposure time is the same. In this embodiment, the exposure time refers to the length of time during which the light emitting unit 10 irradiates the modulated light at a predetermined frequency and completes one measurement.

[0077] First time: Measurement with a modulation frequency of 50 MHz and an exposure time of 1 msec.

[0078] Second time: measured at a modulation frequency of 50 MHz and an exposure time of 0.1 msec.

[0079] The third time: measured with a modulation frequency of 12.5MHz and an exposure time of 1msec.

[0080] Fourth time: measured at a modulation frequency of 12.5 MHz and an exposure time of 0.1 msec.

[0081] However, in this embodiment, the measurement at low frequency is performed only once to avoid aliasing. Alternatively, the measurement at low frequency can be performed multiple times, but the number of measurements performed at low frequency needs to be less than the number of measurements performed at high frequency. Specifically, the measurement is performed as follows.

[0082] First time: Measurement with a modulation frequency of 50 MHz and an exposure time of 1 msec.

[0083] Second time: measured at a modulation frequency of 50 MHz and an exposure time of 0.1 msec.

[0084] The third time: measured with a modulation frequency of 12.5 MHz and an exposure time of 0.8 msec.

[0085] In the third measurement performed at a low frequency, the value of the expected exposure time is greater than one of the exposure time of the first measurement performed at a high frequency and the exposure time of the second measurement performed at a high frequency, and the value of the expected exposure time is less than the other of the exposure time of the first measurement performed at a high frequency and the exposure time of the second measurement performed at a high frequency. In addition, in the measurement performed for the third time, data in the region where the radiation intensity of the received light is 80 to 1000LSB is used as valid data. The data obtained in the third measurement is used to avoid aliasing, and it is sufficient as long as it can be determined which cycle of the 3m cycle in the first and second measurements to which the data obtained in the third measurement belongs. Therefore, the expected level of measurement accuracy is not very high. For example, as long as the maximum value of the variation of the distance value is less than 3m. Therefore, data of the radiation intensity of the received light smaller than the lower limit radiation intensity of the received light measured with high precision is used.

[0086] Figure 5 Curve 1 in represents the condition that the lower limit radiation intensity of the received light is 80 LSB in the measurement performed for the third time, and the condition that the lower limit radiation intensity of the received light is 80 LSB in the measurement performed for the third time is equivalent to the condition that the radiation intensity of the received light is 100 LSB in the measurement performed for the first time. Figure 5 Curve 4 in FIG. 4 represents the condition where the maximum radiation intensity of the received light is 1000 LSB. In other words, the measurement result performed for the third time is included in the area defined by Curve 1 and Curve 4.

[0087] As described above, the area bounded by curves 1 and 3 represents the range in the measurements performed for the first and second times. As a result, the measurement performed for the third time fails to cover the area bounded by curves 3 and 4, and the radiation intensity of the received light inadvertently exceeds 1000LSB and reaches the saturation level. However, such an area corresponds to a range of less than 3m. In other words, it is known that when the data of the radiation intensity of the received light in the measurement performed for the third time has reached the saturation level, the distance to the object is shorter than 3m. In the measurements performed for the first and second times, the period of aliasing is 3m. For this reason, if it is known that the distance to the object is less than 3m, aliasing can be avoided and the distance to the object can be determined.

[0088] Refer to the following Figure 6 The following is a graph illustrating the relationship between the radiation intensity of the received light and the change in the distance measurement value during the third measurement. Figure 6 FIG. 4 is a diagram showing the relationship between the radiation intensity of light received and the variation in the value of distance measurement in the third measurement process according to the present embodiment.

[0089] When the third measurement is performed, the modulation frequency is 12.5 MHz. Therefore, the variation of the distance measurement value can be increased by a factor of four compared to the first and second measurements performed at a modulation frequency of 50 MHz. Figure 6 The curve diagram is shown in FIG. Figure 6 From the curve graph, we can see that when the lower limit radiation intensity is 80LSB, the distance variation is about 220mm. This variation in distance provides sufficient accuracy to identify one of the multiple distance candidates in the 3m period.

[0090] In the region where the radiation intensity of the received light is greater than 80 LSB, the variation of the distance measurement value is sufficiently small. Therefore, it can be seen that the measurement accuracy in the range from 80 to 1000 LSB is sufficient to avoid aliasing.

[0091] As described above, it can be seen that when the radiation intensity of the received light accidentally exceeds 1000LSB and reaches the saturation level, the measurement result is as follows Figure 5 The region shown is included in the region bounded by curves 3 and 4. Since the range is less than 3 m, one of the multiple distance candidates of a period of 3 m that is uniquely shorter than 3 m can be determined.

[0092] In the above-mentioned embodiment, the exposure time or irradiation time of the first measurement performed at a high frequency is different from the exposure time or irradiation time of the second measurement performed at a high frequency. Alternatively, it can be configured so that the irradiation time or exposure time of one measurement is constant, and multiple measurements are performed. Due to this configuration, a similar effect can be achieved. This alternative embodiment of the present disclosure is described in detail below.

[0093] In this embodiment, a vertical cavity surface emitting laser (VCSEL) two-dimensional array of semiconductor lasers is used as the light emitting unit 10. The continuous irradiation time of the light emitting unit 10 using such semiconductor lasers that can irradiate modulated light with stable light emission intensity may be limited due to various factors.

[0094] Specifically, when the light emitting unit 10 continuously irradiates light for a long time, the light radiation intensity may change due to the heat generated by the light emitting unit 10 itself, and the measurement accuracy may deteriorate. In addition, the measurement time may be limited due to, for example, the scale of the calculation circuit of the control unit 30. In order to deal with this situation, the configuration in this embodiment is to set the irradiation time of one measurement to a certain value and perform multiple measurements.

[0095] For example, in one measurement, if the length of time during which the light emitting section 10 can stably and continuously irradiate light is 0.5 milliseconds, the control section 30 performs measurement control as shown below.

[0096] First time: Measurement was performed with a modulation frequency of 50 MHz and an irradiation time of 0.5 msec.

[0097] Second time: Measurement was performed at a modulation frequency of 50 MHz and an irradiation time of 0.5 msec.

[0098] The third time: measurement was performed with a modulation frequency of 50 MHz and an irradiation time of 0.1 msec.

[0099] Fourth time: Measurement was performed at a modulation frequency of 12.5 MHz and an irradiation time of 0.4 msec.

[0100] Fifth time: Measurement was performed with a modulation frequency of 12.5 MHz and an irradiation time of 0.4 msec.

[0101] In this specific embodiment, the first to third measurements correspond to the first measurement with a high modulation frequency, and the fourth and fifth measurements correspond to the second measurement with a low modulation frequency. Between each set of measurements from the first measurement to the fifth measurement, a stop time may be set to stabilize the temperature of the light emitting unit 10.

[0102] The control unit 30 involved in this embodiment adds the first measurement result of the radiation intensity of the received light and the second measurement result of the radiation intensity of the received light. Due to this structure, it is possible to obtain a measurement result that is the same as the measurement result obtained when the measurement is performed at a modulation frequency of 50 MHz and an irradiation time of 1.0 msec. Similarly to the above, the control unit 30 involved in this embodiment adds the fourth measurement result of the radiation intensity of the received light and the fifth measurement result of the radiation intensity of the received light. Due to this structure, it is possible to obtain a measurement result that is the same as the measurement result obtained when the measurement is performed at an irradiation time of 0.8 msec. As described above, multiple measurement results can be averaged to reduce the overall variation of the measurement value.

[0103] As described above, the irradiation time of one measurement is set to be equal to or shorter than the continuous irradiation time during which the light emitting unit 10 can irradiate modulated light with a stable light radiation intensity. As a result, it is possible to perform measurement with a stable exposure amount. In addition, it is possible to obtain a measurement accuracy equivalent to that when the irradiation time is extended by performing an addition operation on a plurality of measurement results, and aliasing can be reduced.

[0104] Furthermore, the irradiation time in the measurement with a modulation frequency as high as 50 MHz and the irradiation time in the measurement with a modulation frequency as low as 12.5 MHz can be made equal to each other. For example, the measurement can be performed as follows.

[0105] The first to tenth times: Measurements were performed at a modulation frequency of 50 MHz and an irradiation time of 0.1 msec.

[0106] Eleventh time: Measurement was performed with a modulation frequency of 50 MHz and an irradiation time of 0.1 msec.

[0107] The twelfth to nineteenth times: measurements were performed at a modulation frequency of 12.5 MHz and an irradiation time of 0.1 msec.

[0108] By making the irradiation time of all the measurements performed for the first to nineteenth times 0.1 msec, the conditions for driving the light emitting unit 10 can be made the same. In this configuration, the amount of heat generated can be kept at a certain amount, and the light can be irradiated with a stable light radiation intensity. In this specific embodiment, the measurements performed for the first to tenth times and the measurements performed for the eleventh time (i.e., eleven measurements) correspond to the first measurement, and the measurements performed for the twelfth to nineteenth times (i.e., eight measurements) correspond to the second measurement.

[0109] The control unit 30 involved in this embodiment obtains and adds the radiation intensity of the received light from the light receiving unit 20 each time the first to tenth measurements are performed. As a result, the radiation intensity of the received light after the addition can be obtained. In the same way as described above, the control unit 30 involved in this embodiment obtains the radiation intensity of the received light from the light receiving unit 20 each time the twelfth to nineteenth measurements are performed, and obtains the radiation intensity of the received light after the addition. Based on the radiation intensity of the received light after the addition, the control unit 30 measures the time difference between the irradiation time and the light receiving time, and calculates the value of the distance based on the measured time length. Although the irradiation time and the exposure time in one measurement are as short as 0.1 milliseconds (msec), the radiation intensity of the received light can be added so that the change in the measurement value in the first to tenth measurements is equivalent to the change in the measurement performed with an irradiation time of 1 millisecond. The change in the measurement value in the twelfth to nineteenth measurements is equivalent to the change in the measurement performed with an irradiation time of 0.8 milliseconds.

[0110] Refer to the following Figure 7 To illustrate the process of calculating distance based on high-precision measurement results. Figure 7 FIG. 1 is a diagram showing a procedure for calculating a distance based on a measurement result with high accuracy according to the present embodiment.

[0111] In this embodiment, this step should be performed after the first measurement, the second measurement and the third measurement are completed. However, this step can be started before all measurements are completed.

[0112] In step S1, the control unit 30 of the distance measuring device 1 calculates a plurality of distance candidates and periodicity based on the measurement data obtained in the first and second measurements. In addition, the control unit 30 may adopt a distance calculation formula of a phase detection TOF camera known in the art. The distinction between the measurement data obtained in the first measurement and the measurement data obtained in the second measurement may be performed as follows. If the data of the radiation intensity of the received light of the first measurement performed with a relatively long exposure time has reached a saturation level, the measurement data obtained in the second measurement may be used.

[0113] In step S2, the control unit 30 determines whether the third measurement result has reached the saturation level. When it is determined that the third measurement result has reached the saturation level ("Yes" in step S2), in step S3, the distance measurement device 1 can adopt the distance candidate in the first cycle as the appropriate distance and terminate the processing. This is because, when it is determined that the measurement result performed for the third time has reached the saturation level, it can be determined that the measurement result is included in the range less than 3m.

[0114] When it is determined that the third measurement result has not reached the saturation level ("No" in step S2), in step S4, the control unit 30 calculates the distance to the object based on the measurement data obtained for the third time. As is known, the distance measuring device 1 involved in this embodiment has a period of 12m, and the object whose reflectivity is less than 100% and whose distance has been successfully measured must exist within the range of 12m, so it is possible to obtain a distance without indistinguishability or ambiguity caused by periodicity. However, due to the low modulation frequency, the measurement accuracy is not very high.

[0115] In step S5, the control unit 30 adopts the distance closest to the distance obtained in step S4 among the plurality of distance candidates obtained in step S1 as the appropriate distance. With the above configuration, the distance measurement device 1 according to the present embodiment can avoid aliasing of the distances obtained in step S1.

[0116] As described above, the distance measuring device 1 involved in the present embodiment adopts a pair of modes including a high-frequency modulation mode and a low-frequency modulation mode, and performs multiple measurements with a variable exposure time or light intensity in the high-frequency modulation mode. In addition, in the low-frequency modulation mode, measurement is performed with an exposure time longer than the maximum exposure time in the high-frequency modulation mode. Alternatively, in the low-frequency modulation mode, measurement is performed with a light radiation intensity higher than the maximum light radiation intensity. Due to such a configuration as described above, it is possible to simultaneously achieve a reduction in aliasing and an increase in dynamic range.

[0117] Furthermore, the distance measuring device 1 according to the present embodiment changes the exposure time while performing a plurality of measurements at the first modulation frequency. Due to this configuration, it is possible to perform measurements with an increased dynamic range.

[0118] Furthermore, when performing a plurality of measurements at the first modulation frequency, the distance measuring device 1 according to the present embodiment can perform at least two distance measurements while fixing the exposure time to a maximum value that the distance measuring device 1 can adopt. Due to this configuration, although the dynamic range cannot be effectively increased, the maximum distance can be extended.

[0119] When the distance measurement device 1 according to the present embodiment performs distance measurement at the second modulation frequency, the exposure time is equal to or shorter than the maximum exposure time but longer than the minimum exposure time in a plurality of distance measurements performed at the first modulation frequency. Due to this configuration, data for avoiding aliasing can be obtained from a plurality of distance measurement results, and an intermediate exposure time can be obtained.

[0120] In addition, the distance measurement device 1 according to the present embodiment performs aliasing avoidance on the distance measurement data obtained using the first modulation frequency based on information on whether the radiation intensity of the received light in the distance measurement data obtained using the second modulation frequency has reached a saturation level. Due to this configuration, part of the aliasing avoidance can be performed based on information on whether the radiation intensity of the received light in the distance measurement data has reached a saturation level.

[0121] In the distance measuring device 1 involved in the present embodiment, a lower limit radiation intensity is set for the distance measurement performed multiple times. The measurement data is determined to be valid when the radiation intensity of the received light is equal to or greater than the lower limit radiation intensity, and the lower limit radiation intensity in the second modulation frequency is less than the lower limit radiation intensity in the first modulation frequency. Due to this configuration, it is not necessary to obtain data with high accuracy for avoiding aliasing. Therefore, the lower limit radiation intensity can be reduced and the measurement can be performed with an increased dynamic range.

[0122] In view of the above teachings, a variety of additional modifications and variations are possible. It should therefore be understood that, within the scope of the appended claims, the disclosure of the present invention may be implemented in a manner different from that specifically described herein. For example, within the scope of the present disclosure and the appended claims, elements and / or features of different illustrative embodiments may be combined with each other and / or replaced with each other. For example, some elements described in the above embodiments may be removed. In addition, the elements involved in each embodiment or variation may be appropriately combined.

[0123] Any of the above operations may be performed in various other ways, for example, in an order different from that described above. Each of the functions of the embodiments may be implemented by one or more processing circuits or circuits. The processing circuits include a programmed processor that includes a circuit system. The processing circuits also include devices such as application specific integrated circuits (ASICs), digital signal processors (DSPs), field programmable gate arrays (FPGAs), and conventional circuit elements arranged to perform the functions.

[0124] This patent application is based upon and claims the benefit of priority from Japanese patent applications No. 2020-048495 and 2021-006693 filed in the Japan Patent Office on March 18, 2020 and January 19, 2021, respectively, the disclosures of which are incorporated herein in their entirety by reference.

[0125] Reference numerals list

[0126] 1 Distance measurement device

[0127] 10. Light-emitting part

[0128] 20 Light receiving part

[0129] 30 Controller

Claims

1. A distance measuring device, comprising: a light emitting unit configured to emit light having a modulated frequency toward an object; a light receiving unit configured to receive the light emitted from the light emitting unit and reflected by the object and returned; and a controller configured to control the light emitting unit and the light receiving unit, The controller is configured to perform a first measurement to cause the light emitting unit to emit the light at a first modulation frequency, thereby measuring the length of time before the light is received by the light receiving unit at a first plurality of times, wherein the controller is configured to perform a second measurement to cause the light emitting unit to emit the light at a second modulation frequency lower than the first modulation frequency, thereby measuring the length of time before the light is received by the light receiving unit at a second plurality of times less than the first plurality of times; Wherein, the controller is configured to perform measurement so that the illumination time of the second measurement is greater than or equal to the illumination time of the first measurement.

2. The distance measuring device according to claim 1, wherein: The controller is configured to calculate the distance to the object based on the length of time obtained in the first measurement and the length of time obtained in the second measurement.

3. The distance measuring device according to claim 1, wherein: The first measurement includes a measurement in which the light emitting section emits the light for a first irradiation time at the first modulation frequency, and a measurement in which the light emitting section emits the light for a second irradiation time longer than the first irradiation time at the first modulation frequency. The second measurement includes a measurement in which the light emitting section emits the light for a third irradiation time at the second modulation frequency, and The third irradiation time is longer than the first irradiation time and shorter than the second irradiation time.

4. The distance measuring device according to claim 1, wherein: The first measurement includes a measurement in which the light emitting section emits the light for a first irradiation time at the first modulation frequency, wherein the measurement is performed a plurality of times.

5. The distance measuring device according to claim 1, wherein: The second measurement includes a measurement in which the light emitting section emits the light for a third irradiation time at the second modulation frequency, and Therein, the measurement is performed multiple times.

6. The distance measuring device according to claim 1, wherein: The first measurement includes causing the light emitting section to emit the light for a first irradiation time at the first modulation frequency, wherein the measurement is performed multiple times, The first measurement includes causing the light emitting section to emit the light at the first modulation frequency for a second irradiation time shorter than the first irradiation time, The second measurement includes a measurement in which the light emitting section emits the light for a third irradiation time at the second modulation frequency, and The third irradiation time is equal to the first irradiation time or the second irradiation time.

7. The distance measuring device according to claim 4, wherein: The controller is configured to obtain, in the first measurement, a radiation intensity of the received light from the light receiving unit for the first irradiation time each time the measurement of causing the light emitting unit to emit the light for the first irradiation time at the first modulation frequency is performed, The controller is configured to add the radiation intensity of the received light each time the measurement of causing the light emitting unit to emit the light for the first irradiation time at the first modulation frequency is performed in the first measurement, The controller is configured to measure, in the first measurement, a time between the time when the light is emitted by the light emitting section and the time when the light is received by the light receiving section based on the added radiation intensity of the received light.

8. The distance measuring device according to claim 6, wherein: The controller is configured to obtain, in the first measurement, a radiation intensity of the received light from the light receiving unit for the first irradiation time each time the measurement of causing the light emitting unit to emit the light for the first irradiation time at the first modulation frequency is performed, The controller is configured to add the radiation intensity of the received light each time the measurement of causing the light emitting unit to emit the light for the first irradiation time at the first modulation frequency is performed in the first measurement, The controller is configured to measure, in the first measurement, a time between the time when the light is emitted by the light emitting section and the time when the light is received by the light receiving section based on the added radiation intensity of the received light.

9. The distance measuring device according to any one of claims 1 to 8, wherein: The controller is configured to process data obtained in the first measurement to avoid aliasing based on information of whether the radiation intensity of the received light of the light receiving portion has reached a saturation level in the second measurement.

10. The distance measuring device according to any one of claims 1 to 8, wherein: The controller is configured to determine that the measurement data is valid when the radiation intensity of the received light of the light receiving portion is equal to or greater than a lower limit radiation intensity in the first measurement and the second measurement, and The lower limit radiation intensity in the second measurement is smaller than the lower limit radiation intensity in the first measurement.

11. A distance measurement method, comprising: emitting light at a first modulation frequency to measure a length of time for receiving said light reflected by an object a first plurality of times as a first measurement; emitting the light at a second modulation frequency lower than the first modulation frequency, thereby measuring the length of time for receiving the light reflected by the object at a second plurality of times less than the first plurality of times as a second measurement, calculating the distance to the object based on the length of time obtained in the first measurement and the length of time obtained in the second measurement; The irradiation time of the second measurement is greater than or equal to the irradiation time of the first measurement.

12. The method according to claim 11, wherein: emitting the light at the first modulation frequency comprises emitting the light at the first modulation frequency for a first illumination time, and emitting the light at the first modulation frequency for a second illumination time longer than the first illumination time, emitting the light at the second modulation frequency comprises emitting the light at the second modulation frequency for a third illumination time, and The third irradiation time is longer than the first irradiation time and shorter than the second irradiation time.

13. The method according to claim 11, wherein: emitting the light at a first modulation frequency comprises emitting the light for a first illumination time a plurality of times, and emitting the light for a second illumination time shorter than the first illumination time at the first modulation frequency, emitting the light at the second modulation frequency comprises emitting the light for a third illumination time, and The third irradiation time is equal to the first irradiation time or the second irradiation time.

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