WO2022077149A1

WO2022077149A1 - Sensing device based on direct time-of-flight measurement

Info

Publication number: WO2022077149A1
Application number: PCT/CN2020/120329
Authority: WO
Inventors: Long Wang; Patrick Yin Chiang; Frank Fang SHI
Original assignee: PHOTONIC TECHNOLOGIES (SHANGHAI) Co.,Ltd.; Photonic Technologies (Shenzhen) Co.,Ltd.
Priority date: 2020-10-12
Filing date: 2020-10-12
Publication date: 2022-04-21

Abstract

A sensing device includes a transmitter configured to emit a sequence of light pulses toward a target, and a first sequence of time values being indicative of respective times of emitting the light pulses (310); a detector configured to receive optical radiation reflected and to output a second sequence of time values indicative of respective arrival times (320); and a controller. The controller is configured for selecting a part of the second sequence of time values based on comparing a modified first sequence of time values with the second sequence of time values (340), and upon determining that a difference between a total number of the selected part and a total number of the first sequence of time values does not exceed a threshold, using the selected part of the second sequence of time values to calculate a common time of flight (350).

Description

SENSING DEVICE BASED ON DIRECT TIME-OF-FLIGHT MEASUREMENT

TECHNICAL FIELD

The disclosure relates to distance sensing, and more specifically to distance sensing based on direct time-of-flight measurement.

BACKGROUND

Time-of-flight (TOF) imaging techniques are widely used in many implementations, such as light-detection-and-ranging (LiDAR) system, 3D imaging, depth mapping, and SPAD-based sensing devices. The principle of TOF imaging techniques is illuminating an object with pulses of light and detects some pulses reflected from the object. The distance between the detector and the object may be measured indirectly by measuring the phase change between the emission and detection, also called indirect TOF (iTOF) . Alternatively, the distance may be measured directly by measuring a time of flight by calculating the difference between the emission time of the outgoing pulse and the arrival time of the reflected radiation from the corresponding point on the object, also called direct TOF (dTOF) . Because optical power reduces at a square speed of the distance travelled and due to reflection loss, the intensity of the reflected radiation is significantly lower than the intensity of the outgoing light pulses and therefore imposes challenges for detecting such low intensity reflection. While, due to product security concerns and power consumption limitations, the intensity of the outgoing light pulses cannot be increased unlimitedly. As such, the reflected optical radiation is kept with very low intensity, and is affected by the background noise or disturbance, such as the ambient light or sunlight.

Accordingly, there is a need for a technical solution that reduces the impact by the background noise to improve the efficiency and precision of distance measurement based on direct TOF.

SUMMARY OF THE INVENTION

According to the first aspect of the present disclosure, illustrative embodiments provide a sensing device. The sensing device includes: a transmitter configured to emit a sequence of light pulses toward a target, where a first sequence of time values is indicative of respective times of emitting of the sequence of light pulses; a detector configured to receive optical radiation reflected from the target and to output a second sequence of time values indicative of respective arrival times of the received optical radiation; and a controller. The controller is configured for: shifting the first sequence of time values by a time delay to obtain a modified first sequence of time values, where the time delay is determined based on a match between the first sequence of time values and the second sequence of time values; selecting a part of the second sequence of time values based on comparing the modified first sequence of time values with the second sequence of time values; calculating a first difference between a total number of the selected part of the second sequence of time values and a total number of the first sequence of time values; and upon determining that the first difference does not exceed a first threshold, using the selected part of the second sequence of time values to calculate a common time of flight for the sequence of light pulses toward and back from the target.

Therefore, the technical solution as set forth in accordance with the first aspect of the present disclosure, by using the selected part of the second sequence of time values to calculate a common time of flight, achieves the technical effects of reducing the impact by the background noise to improve the efficiency and precision of distance measurement based on direct TOF.

With reference to the first aspect of the present disclosure, illustrative embodiments provide the sequence of light pulses has an equal time interval.

Therefore, situations where emitted light pulses have equal time intervals are covered.

With reference to the first aspect of the present disclosure, illustrative embodiments provide the sequence of light pulses has time intervals that regular change by selecting one of a plurality of predefined time intervals.

Therefore, situations where emitted light pulses have regularly changing time intervals are covered.

With reference to the first aspect of the present disclosure, illustrative embodiments provide selecting the part of the second sequence of time values based on comparing the modified first sequence of time values with the second sequence of time values, includes, for each of the second sequence of time values respectively: identifying a sequential position of one of the second sequence of time values; identifying one or more of the modified first sequence of time values corresponding to the sequential position; comparing the one of the second sequence of time values with the one or more of the modified first sequence of time values to obtain a second difference; and in response to determining that the second difference does not exceed a second threshold, determining that the selected part of the second sequence of time values includes the one of the second sequence of time values.

Therefore, by comparing the one of the second sequence of time values with the one or more of the modified first sequence of time values, valid signals are selected and invalid signals are filtered out.

With reference to the first aspect of the present disclosure, illustrative embodiments provide the one or more of the modified first sequence of time values are located sequentially beginning from a sequential position in the modified first sequence of time values same as the sequential position of the one of the second sequence of time values, where comparing the one of the second sequence of time values with the one or more of the modified first sequence of time values to obtain the second difference, includes: comparing the one of the second sequence of time values with each of the one or more of the modified first sequence of time values respectively so as to select a minimum value as the second difference.

Therefore, by comparing the one of the second sequence of time values with each of the one or more of the modified first sequence of time values respectively, more flexibilities are achieved.

With reference to the first aspect of the present disclosure, illustrative embodiments provide the second threshold is adjustable by the controller.

Therefore, by providing the second threshold to be adjustable by the controller, more flexibilities are achieved.

With reference to the first aspect of the present disclosure, illustrative embodiments provide the target includes a plurality of pixels each corresponding to a part of the received optical radiation respectively.

Therefore, situations where a target having a plurality of pixels are considered.

With reference to the first aspect of the present disclosure, illustrative embodiments provide the detector includes a plurality of single-photon avalanche diodes (SPADs) and a plurality of time-to-digital converters (TDCs) coupled to the plurality of SPADs; the plurality of SPADs include a plurality sets of SPADs, each set of SPADs includes at least one SPAD and is configured for producing digital pulses in response to a part of the received optical radiation corresponding to a pixel of the target respectively; the plurality of TDCs are configured for generating a plurality of second sequences of time values, each of the plurality of second sequences of time values corresponds to digital pulses produced by a set of SPADs respectively; the controller is configured for calculating a common time of flight for the sequence of light pulses toward and back from a respective pixel of the target based on the first sequence of time values and a corresponding one of the plurality of second sequences of time values.

Therefore, multiple SPADs and multiple TDCs are deployed to calculate respective time of flights corresponding to different pixels of the target, which is beneficial for depth mapping.

With reference to the first aspect of the present disclosure, illustrative embodiments provide the match between the first sequence of time values and the second sequence of time values includes a correlation between a first pattern and a second pattern, the first pattern specifies differences between time values of the first sequence of time values, the second pattern specifies differences between time values of the second sequence of time values.

Therefore, by correlating the first pattern with the second pattern, the match focuses on the differences between time values as opposed to time values themselves and has improved efficiency.

According to the second aspect of the present disclosure, illustrative embodiments provide a method for distance measuring based on direct TOF, including: emitting a sequence of light pulses toward a target, where a first sequence of time values is indicative of respective times of emitting of the sequence of light pulses; receiving optical radiation reflected from the target, where a second sequence of time values is indicative of respective arrival times of the received optical radiation; shifting the first sequence of time values by a time delay to obtain a modified first sequence of time values, where the time delay is determined based on a match between the first sequence of time values and the second sequence of time values, selecting a part of the second sequence of time values based on comparing the modified first sequence of time values with the second sequence of time values, calculating a first difference between a total number of the selected part of the second sequence of time values and a total number of the first sequence of time values, and upon determining that the first difference does not exceed a first threshold, using the selected part of the second sequence of time values to calculate a common time of flight for the sequence of light pulses toward and back from the target.

Therefore, the technical solution as set forth in accordance with the second aspect of the present disclosure, by using the selected part of the second sequence of time values to calculate a common time of flight, achieves the technical effects of reducing the impact by the background noise to improve the efficiency and precision of distance measurement based on direct TOF.

With reference to the second aspect of the present disclosure, illustrative embodiments provide the sequence of light pulses has an equal time interval.

With reference to the second aspect of the present disclosure, illustrative embodiments provide the sequence of light pulses has time intervals that regular change by selecting one of a plurality of predefined time intervals.

With reference to the second aspect of the present disclosure, illustrative embodiments provide selecting the part of the second sequence of time values based on comparing the modified first sequence of time values with the second sequence of time values, includes, for each of the second sequence of time values respectively: identifying a sequential position of one of the second sequence of time values; identifying one or more of the modified first sequence of time values corresponding to the sequential position; comparing the one of the second sequence of time values with the one or more of the modified first sequence of time values to obtain a second difference; and in response to determining that the second difference does not exceed a second threshold, determining that the selected part of the second sequence of time values includes the one of the second sequence of time values.

With reference to the second aspect of the present disclosure, illustrative embodiments provide the one or more of the modified first sequence of time values are located sequentially beginning from a sequential position in the modified first sequence of time values same as the sequential position of the one of the second sequence of time values, where comparing the one of the second sequence of time values with the one or more of the modified first sequence of time values to obtain the second difference, includes: comparing the one of the second sequence of time values with each of the one or more of the modified first sequence of time values respectively so as to select a minimum value as the second difference.

With reference to the second aspect of the present disclosure, illustrative embodiments provide the second threshold is adjustable.

Therefore, by providing the second threshold to be adjustable, more flexibilities are achieved.

With reference to the second aspect of the present disclosure, illustrative embodiments provide the match between the first sequence of time values and the second sequence of time values includes a correlation between a first pattern and a second pattern, the first pattern specifies differences between time values of the first sequence of time values, the second pattern specifies differences between time values of the second sequence of time values.

According to the third aspect of the present disclosure, illustrative embodiments provide a light detection and ranging (LiDAR) system based on direct TOF, including: a laser configured for emitting a sequence of light pulses toward a target, where a first sequence of time values is indicative of respective times of emitting of the sequence of light pulses; and an array of sensing elements, where each sensing elements includes at least one SPAD and is configured to output a second sequence of time values that is indicative of respective arrival times of optical radiation reflected by the target and received by the sensing element; where for each sensing elements respectively: a part of the second sequence of time values is selected based on comparing the second sequence of time values with the first sequence of time values, and the selected part of the second sequence of time values is used to calculate a time of flight for the sequence of light pulses toward and back from the target; where a depth mapping of the target is generated based on a combination of all the calculated time of flights of the array of sensing elements.

Therefore, the technical solution as set forth in accordance with the third aspect of the present disclosure, by using the selected part of the second sequence of time values to calculate a common time of flight, achieves the technical effects of reducing the impact by the background noise to improve the efficiency and precision of distance measurement based on direct TOF.

With reference to the third aspect of the present disclosure, illustrative embodiments provide the array of sensing elements includes a two-dimensional matrix of SPAD pixels.

Therefore, situations where the array of sensing elements are two dimensional are covered.

With reference to the third aspect of the present disclosure, illustrative embodiments provide comparing the second sequence of time values with the first sequence of time values includes correlating a first pattern with a second pattern, the first pattern specifies differences between time values of the first sequence of time values, the second pattern specifies differences between time values of the second sequence of time values.

Therefore, by correlating the first pattern with the second pattern, the comparing focuses on the differences between time values as opposed to time values themselves and has improved efficiency.

With reference to the third aspect of the present disclosure, illustrative embodiments provide comparing the second sequence of time values with the first sequence of time values includes: shifting the first sequence of time values by a time delay to obtain a modified first sequence of time values, where the time delay is determined based on a match between the first sequence of time values and the second sequence of time values, and comparing the modified first sequence of time values with the second sequence of time values.

Therefore, by comparing the second sequence of time values with the modified first sequence of time values, valid signals are selected and invalid signals are filtered out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a sensing device in accordance with one or more embodiments of the present disclosure.

FIG. 2 shows an example sequence of TX pulses emitted by and an example sequence of RX pulses received by the sensing device shown in FIG. 1.

FIG. 3 is a flow diagram illustrating exemplary processes for detecting a time of flight in accordance with one or more embodiments of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Aspects of the present disclosure involves a sensing device. The sensing device includes: a transmitter configured to emit a sequence of light pulses toward a target, where a first sequence of time values is indicative of respective times of emitting of the sequence of light pulses; a detector configured to receive optical radiation reflected from the target and to output a second sequence of time values indicative of respective arrival times of the received optical radiation; and a controller. The controller is configured for: shifting the first sequence of time values by a time delay to obtain a modified first sequence of time values, where the time delay is determined based on a match between the first sequence of time values and the second sequence of time values; selecting a part of the second sequence of time values based on comparing the modified first sequence of time values with the second sequence of time values; calculating a first difference between a total number of the selected part of the second sequence of time values and a total number of the first sequence of time values; and upon determining that the first difference does not exceed a first threshold, using the selected part of the second sequence of time values to calculate a common time of flight for the sequence of light pulses toward and back from the target. Accordingly, the technical solution as provided by the present disclosure achieves the technical effects of reducing the impact by the background noise to improve the efficiency and precision of distance measurement based on direct TOF.

The embodiments of the present disclosure may be implemented in following technical environments, including but not limited to, LiDAR system, 3D imaging, depth mapping, and SPAD-based sensing devices.

With reference to FIG. 1, FIG. 1 is a block diagram illustrating a sensing device in accordance with one or more embodiments of the present disclosure. The sensing device 100 includes a pulsed laser 101, an optical emitter 102, an optical receiver 104, single-photon avalanche diodes (SPADs) 105, time-to-digital converters (TDCs) 106, and a controller 130. The pulsed laser 101 generates a plurality of laser pulses 111 with time intervals specified by a certain temporal pattern. The optical emitter 102 is coupled to the pulsed laser 101 to receive the generated laser pulses 111 and emit a sequence of light pulses, i.e., TX pulses 112, toward a target 103. The target 103 generally has a plurality of points or pixels that reflect the emitted light pulses back to the sensing device 100. It is understood that, the relative distance between the target and the sensing device 100 is still during a detection period during which the emission and reflection happen. As such, TX pulses 112 are emitted from the optical emitter 102 of the sensing device 100 toward the same destination and have the same travelling distance for each respective pulse of TX pulses 112 during the detection period. The optical receiver 104 detects reflected optical radiation 113 from the target 103. Because the TX pulses 112 are emitted to the target 103 and the consequential reflected optical radiation 113 from the target 103 is detected by the optical receiver 104 during the detection period when the target 103 remains still with respect to the sensing device 100, travelling times for each one of the TX pulses 112 toward and back from the target 103 are the same and may be used to calculate the distance between the sensing device 100 and the target 103. When the target 103 has some points or pixels that have different distances from the sensing device 100 with respect to those of other points or pixels, which means, the target 103 has a non-uniform depth distribution, the travelling times corresponding to some pixels of the target may be different from those corresponding to other pixels. The optical emitter 102 may use dividers or collimators or other suitable optical devices to obtain a generally plane wave for the TX pulses 112. Alternatively, the pulsed laser 101 may use one or more vertical-cavity surface-emitting lasers (VCSELs) . As such, the TX pulses 112 toward different pixels of the target 103 share the same temporal patten, and the reflected optical reflection 113 by respective pixels of the target 103 may be compared with the same TX pulses 112 and the same temporal pattern to generate a depth mapping of the target 103.

Still with reference to FIG. 1, the optical receiver 104 is coupled to SPADs 105 each of which is configured to output a signal indicative of a time of incidence of a single photon on the SPADs 105. As the optical receiver 104 receives all of the reflected optical radiation 113 including background noise and transfers the photons of the reflected optical radiation 113 to the SPADs 105, the SPADs 105 outputs a plurality of SPAD signals 114 in response to the received optical radiation 113 including the background noise. The SPADs 105 are coupled to the TDCs 106, and the TDCs 106 generates digital pulses with timing information based on the SPAD signals 114. In some illustrative embodiments, the TDCs 106 are configured to increment counts of respective times of incidences of photons on the SPADs 105 in response to the plurality of SPAD signals 114. In other words, the TDCs 106 provide a statistical result of the timing and respective counts of the plurality of SPAD signals 114. In some illustrative embodiments, the TDCs 106 generates several histograms based on the counts of respective times of incidences of photons on the SPADs 105, and each histogram corresponds to a re-establishment of the reflection of one of the TX pulses 112. Each histogram includes several timing and respective counts, and a peak is selected to represent the histogram and used to calculate the final time-of-flight for the corresponding one of the TX pulses 112. In other words, the TDCs 106 applies a histogram-based filtering to the statistical result of the timing and respective counts of the plurality of SPAD signals 114. The SPADs 105 may have a plurality sets of SPAD sensors, each set of SPAD sensors have one or several SPAD sensors. One set of SPAD sensors correspond to a specific pixel or point of the target 103, so the SPAD signals 114 produced by the set of SPAD sensors correspond to a part of optical radiation reflected by the specific pixel or point of the target 103. The SPADs 105 may also be formed as a two-dimensional array of SPAD sensors.

Still with reference to FIG. 1, the digitalized pulses generated by the TDCs 106, whether subject to a histogram-based filtering or not, are transmitted to the controller 130 as RX pulses 115. Since the RX pulses 115 are generated by the TDCs 106 based on the plurality of SPAD signals 114 that are generated by the SPADs 105 in response to the received optical radiation 113 including the background noise, the RX pulses 115 just like the reflected optical radiation 113 contain some part that may be affected by the background noise. The controller 130 has a variety of modules to identify and filter out the part of the RX pulses 115 affected by the background noise, and to control and operate the sensing device 100. The controller 130 has a synchronization module 131 for providing a global timing or a reference time such that the TX pulses 112 and the RX pulses 115 are recorded with reference to the same global timing or reference time. The controller 130 has a timing count module 132 that cooperates with the synchronization module 131 to record a respective count of the TX pulses 112 and the RX pulses 115. The controller 130 sends a controlling signal 110 to the pulsed laser 101 to drive the pulsed laser 101 to generate the laser pulses 111 in accordance with the specified time intervals between the TX pulses 112 as a result of the cooperation between the synchronization module 131 and the timing count module 132. The controller 130 has a memory 133 for storing the information about the timing and counts of the TX pulses 112 and the RX pulses 115, and instructions for operating the sensing device 100. The controller 130 has a controlling module 134 for providing necessary controlling circuitry and functions to perform the operations such as driving the pulsed laser 101 and recording the timing and counts of the RX pulses 115. The controller 130 optionally may have a histogram module 135 to apply a histogram-based filtering to the statistical result of the timing and respective counts of the plurality of SPAD signals 114 provided by the TDC 106 if the TDC 106 does not have a similar component or function. The controller 130 also has a processor 136 for processing the instructions stored in the memory 133 and provide all kinds of operations. It is noted that, the structures and functions of the components of the controller 130 are for illustrative purposes only, and in some illustrative embodiments the controller 130 may have a different architecture or combination of the components.

With reference to FIG. 2, FIG. 2 shows an example sequence of TX pulses emitted by and an example sequence of RX pulses received by the sensing device shown in FIG. 1. As shown in FIG. 2, the TX pulses 200 correspond to the TX pulses 112 shown in FIG. 1 and have N pulses together, and times of emitting each of the TX pulses 200 are TX1, TX2, TX3, TX4, …TX N-1, and TX N. TXi represents the time of emitting an i-th TX pulse (in the i-th sequential position of the TX pulses 200) , and i is a positive integer not larger than N. The RX pulses 210 correspond to the RX pulses 115 shown in FIG. 1 and have N pulses together, and times of arrival of each of the RX pulses 210 are RX1, RX2, RX3, RX4, …RX N-1, and RX N. N is a positive integer. RXi represents the time of arrival of an i-th RX pulse (in the i-th sequential position of the RX pulses 210) , and i is a positive integer not larger than N. With reference to both FIG. 1 and FIG. 2, the sequence TX1, TX2, TX3, TX4, …TX N-1, and TX N may be called a first sequence of time values (TX sequence) indicative of respective times of emitting the TX pulses 112 toward the target 103. The TX sequence or the first sequence of time values may be generated by the controller 130 that sends the controlling signals 110 to the pulsed laser 101 to drive the pulsed laser 101 to generate the laser pulses 111 following the TX sequence. In some illustrative embodiments, the TX sequence or the first sequence of time values may have time intervals between successive pulses to be equal to a predetermined value. For example, the time intervals between TX1 and TX2, between TX2 and TX3, between TX3 and TX4, and so on, are all equal to 100 ns. In some illustrative embodiments, time intervals between successive pulses of the TX sequence or the first sequence of time values may be switched between a first value and a second value alternatively. For example, the time intervals between TX1 and TX2, between TX3 and TX4, between TX5 and TX6 are 100 ns, while the time intervals between TX2 and TX3, between TX4 and TX5 are 200 ns, such that time intervals are alternatively changed between 100 ns and 200 ns. In some illustrative embodiments, successive light pulses of the sequence of light pulses have time intervals that regular change by selecting one of a plurality of predefined time intervals. For example, the time interval between TX1 and TX2, the time interval between TX2 and TX3, and the time interval between TX3 and TX4 may be 100 ns, 120 ns, and 140 ns respectively. Above-discussed various embodiments about the time intervals between successive pulses of the TX sequence or the first sequence of time values may be described as specific temporal patterns for emitting the sequence of light pulses. For example, the laser pulses 111 may be generated at an equal time interval, and the TX pulses 112 are emitted one by one at 0 ns, 100 ns, 200 ns, 300 ns, and so on. For another example, the TX pulses 112 are emitted sequentially at 0 ns, 100 ns, 300 ns, 400 ns, 600 ns, and so on. As a result, an i-th TX pulse that corresponds to the i-th position in the TX sequence, indicates both the time of emitting the i-th TX pulse and the correlated time interval between the i-th and (i-1) -th pulses as well as the time interval between the i-th and (i+1) -th pulses in the TX sequence. For the convenience of description, the (i-1) -th pulse is understood as the preceding pulse with respect to the i-th pulse, and the (i+1) -th pulse is understood as the subsequent pulse with respect to the i-th pulse.

Still with reference to FIG. 2, the TX sequence shares a common travelling time if the TX pulses corresponding to the TX sequence are emitted toward and back from the same destination, so the reflection of the TX pulses 200 ideally has a common time delay and maintains the same specific temporal pattern of the TX pulses 200. However, due to background noise, the RX pulses 210 do not exactly follow the specific temporal pattern of the TX pulses 200. Specifically, some of the TX pulses after the reflection may be submerged into the background noise and therefore do not appear in the estimated positions of the RX pulses 210, or, the record of the timing may be disturbed. Accordingly, the sequence RX1, RX2, RX3, RX4, …RX N-1, and RX N may be called a second sequence of time values (the RX sequence) with respect to the first sequence of time values or the TX sequence, and the RX sequence must be analyzed to determine whether each of the RX sequence is valid based on the TX sequence. It is noted that while FIG. 2 shows there are a total of N pulses in the TX sequence and a total of N pulses in the RX sequence. However, due to background noise and disturbance caused by jittering and other factors, the total number of RX sequence may be smaller than N.

With reference to FIG. 3, FIG. 3 is a flow diagram illustrating exemplary processes for detecting a time of flight in accordance with one or more embodiments of the present disclosure. A first sequence of time values, the TX sequence 300, indicate respective times of emitting a sequence of light pulses to a target, and a second sequence of time values, the RX sequence 302, indicate respective times of arrival of the reflected optical radiation from the target. It is noted that, the first sequence of time values and the second sequence of time values are used to refer to the TX sequence 300 and the RX sequence 302 respectively for the convenience of description only. The TX sequence 300 is represented as TX1, TX2, TX3, …TX N-1, and TX N, where N is a positive integer and is the total number of the pulses in the TX sequence 300. The RX sequence 302 is represented as RX1, RX2, RX3, …RX M-1, and RX M, where M is a positive integer and is the total number of the pulses in the RX sequence 302. TXi and RXi represent the corresponding pulse in the i-th sequential position of the TX sequence 300 and the RX sequence 302 respectively. For example, TX3 and RX3 both represent the third pulse in the respective sequence. For the convenience of description, the (i-1) -th pulse is understood as the preceding pulse with respect to the i-th pulse, and the (i+1) -th pulse is understood as the subsequent pulse with respect to the i-th pulse. For example, TX4 is a subsequent pulse with respect to TX3, and TX2 is a preceding pulse with respect to TX3. With reference to FIG. 1 and FIG. 3, the TX sequence 300 corresponds to the TX pulses 112 shown in FIG. 1 and the RX sequence 302 corresponds to the RX pulses 115 shown in FIG. 1. The exemplary processes shown in FIG. 3 have the following steps:

Step 310: emitting a sequence of light pulses toward a target, a first sequence of time values, the TX sequence 300, is indicative of respective times of emitting of the sequence of light pulses.

At step 310, the first sequence of time values or the TX sequence 300 (TX1, TX2, TX3, …TX N-1, and TX N) indicates respective times of emitting a sequence of light pulses toward the target. The emission of the light pulses may go through a variety of optical devices such as collimators or lens or other appropriate devices.

Step 320: receiving optical radiation reflected from the target, a second sequence of time values, the RX sequence 302, is indicative of respective arrival times of the received optical radiation.

At step 320, the second sequence of time values or the RX sequence 302 (RX1, RX2, RX3, …RX M-1, and RX M) indicates respective arrival times of the received optical radiation. The reception of the optical radiation may be performed by a variety of suitable optical devices such as lens.

Step 330: shifting the first sequence of time values by a time delay T _shift to obtain a modified first sequence of time values, the TXS sequence 304, the time delay T _shift is determined based on a match between the first sequence of time values TX sequence 300 and the second sequence of time values RX sequence 302.

At step 330, the modified first sequence of time values, the TXS sequence 304, is shifted from the TX sequence 300 by applying a specific time delay T _shift to each of the time values of the TX sequence 300. The TXS sequence 304 may be represented as TXS1, TXS2, TXS3, …TXS N-1, and TXS N, where TXSi corresponds to the shifted TXi of the TX sequence 300. The relationship between the TX sequence 300 and the TXS sequence 304 follows formula (1) :

Within formula (1) , TXi represents the time value in the i-th sequential position of the TX sequence 300, TXSi represents the time value in the i-th sequential position of the TXS sequence 304, T _shift represents the time delay, and N represents the total number of the pulses in the TX sequence 300.

In some illustrative embodiments, the time delay T _shift may be set as the difference between the first one of the second sequence of time values and the first one of the first sequence of time values, i.e., the difference between RX1 and TX1. The time delay T _shift may be determined by selecting from a list of numerical values, or may be determined based on statistical methods such as calculating an overall time delay value between the RX sequence 302 and the TX sequence 300.

Alternatively, the match between the first sequence of time values (TX sequence 300) and the second sequence of time values (RX sequence 302) may include a correlation between a first pattern and a second pattern. The first pattern specifies differences of time values of the TX sequence 300, and the second pattern specifies differences of time values of the RX sequence 302. For example, when the time intervals between successive pulses of the TX sequence 300 are equal to 100 ns, the second pattern of the RX sequence 302 presumably follow the same equal time intervals. However, due to impact by background noise, some of the pulses of the TX sequence 300 after reflection may not be detected and therefore leave some empty positions in the RX sequence 302, making intervals between certain pair of pulses significantly longer than 100 ns. For example, if a light pulse corresponding to TX2 of the TX sequence 300 is reflected but lost, then this light pulse may not be present in the RX sequence 302, while, what is recorded as the second pulse of the RX sequence 302, i.e., RX2 is not exactly corresponding to a reflection of TX2, and is presumably corresponding to a reflection of TX3. As such, while the difference between TX1 and TX2 is 100 ns, the difference between RX1 and RX2 is not 100 ns but rather 200 ns. Then the difference between TX3 and RX2 may be used to identify the appropriate time delay T _shift. As such, by correlating the first pattern of the TX sequence 300 with the second pattern of the RX sequence 302, an appropriate time delay T _shift may be identified.

In some illustrative embodiments, prior to step 330, a preliminary check may be performed by calculating a difference between the total number N of the first sequence of time values TX sequence 300 and the total number M of the second sequence of time values RX sequence 302. When the total number M of the RX sequence 302 is less than the total number N of the TX sequence 300, this indicates that the background noise may cause some disturbance such that the sensing device fails to detect a number of incidents of photons reflected by the target. If the difference is too large, meaning the RX sequence 302 may have a lot of signals lost to background noise, then the whole of the RX sequence 302 may not be suitable for further proceedings.

Step 340: selecting a part of the second sequence of time values based on comparing the modified first sequence of time values TXS sequence 304 with the second sequence of time values RX sequence 302.

At step 340, the TXS sequence 304 is compared with the RX sequence 302 to select a part of the RX sequence 302 to be valid signals. Those of the RX sequence 302 that are not selected are considered loss due to the background noise. In some illustrative embodiments, this may be achieved by comparing each of the RX sequence 302 with a corresponding one of the TXS sequence 304. Take a specific RXi for example, a TXSi having the same sequential position i as the RXi is identified. For example, RX1 is compared with TXS1, RX2 is compared with TXS2, and RX3 is compared with TXS3, and so on. A difference D1 refers to the arbitrary value of the difference between RXi and TXSi. While RXi presumably is larger than TXi, TXSi which is shifted from TXi by a time delay may become larger than RXi. The difference D1 is calculated following formula (2) :

Within formula (2) , RXi represents the time value in the i-th sequential position of the RX sequence 302, TXSi represents the time value in the i-th sequential position of the TXS sequence 304, D1 represents the difference, and M represents the total number of the pulses in the RX sequence 302.

After calculating the difference D1, D1 is then compared with the first threshold TD1, and it is determined that RXi is valid if D1 is smaller than TD1, and it is determined that RXi is not valid if D1 is larger than TD1. The first threshold TD1 is designed as a filter for filtering out the content of the RX sequence 302 that is affected by background noise and other disturbance. In other words, as RXi of the RX sequence 302 is compared with TXSi of the TXS sequence 304, if RXi falls out of a range that is centered on the TXSi that is determined by the first threshold TD1, i.e., D1 is larger than TD1, then the RXi is determined to be an invalid signal, or a lost signal to the background noise. Also, Since the TXS sequence 304 is shifted linearly from the TX sequence 300, the specific temporal pattern of the TX sequence 300 is maintained in the TXS sequence 304. Particularly, since time intervals of successive pulses in the TX sequence 300 are defined in relative ways, the temporal pattern that specifies the way of how time intervals change in the TX sequence 300 is maintained in the TXS sequence 304, which means time intervals of the TXS sequence 304 change in the same way, regardless of how the linear shifting is. As such, by comparing the difference D1 with the first threshold TD1, an layer of filtering out the signals affected by background noise is added. Specifically, the first threshold TD1 may be adjustable depending on the system requirements or specific implementation environments. For example, in an environment where the ambient light is strong and the background noise is intense as a result, the first threshold TD1 may be set to a smaller value than usual so as to impose a stricter standard for filtering out background noise.

In some illustrative embodiments, rather than selecting a single TXSi, a plurality of the modified first sequence of time values TXS sequence 304 are selected sequentially beginning from the sequential position i, while, the difference D1 is the smallest of the differences between the one of the second sequence of time values RXi and each of the selected plurality of the modified first sequence of time values respectively. For example, instead of selecting TXSi, a total of four time values TXSi, TXS i+1, TXS i+2, and TXS i+3 are selected, and D1 is defined as the smallest among the differences between RXi and each of TXSi, TXS i+1, TXS i+2, and TXS i+3, respectively. When the total number M of the RX sequence 302 is less than the total number N of the TX sequence 300, it is likely that the sequential position i of the RXi does not point to the appropriate corresponding TXSi. As such, by extending the comparison range to cover several pulses around the TXSi, the pulse in the TXS sequence 304 closest to the RXi may be located.

Step 350: calculating a difference between a total number of the selected part of the second sequence of time values and a total number of the first sequence of time values TX sequence 300, and upon determining that the difference does not exceed a threshold, using the selected part of the second sequence of time values to calculate a common time of flight for the sequence of light pulses toward and back from the target.

At step 350, after determining whether each of the RX sequence 302 is valid at step 340, a total number of the time values of the RX sequence 302 that are determined valid may be defined, and this total number is compared with the total number N of the TX sequence 300 to calculate a difference D2 and to determine whether the difference D2 exceeds the second threshold TD2. If the difference D2 exceeds the second threshold TD2, this indicates that the background noise may affect the RX sequence 302 to cause too many signals to be determined not valid, therefore warranting that the RX sequence 302 as a whole is not suitable for further proceedings. On the other hand, when the difference D2 does not exceed the second threshold TD2, this indicates that there are sufficient valid signals for calculating a time of flight. Therefore, the impact by the background noise is greatly reduced, and the efficiency and precision of measuring the time of flight and the correlated distance between the sensing device and the target are improved. Specifically, the second threshold TD2 may be adjustable depending on the system requirements or specific implementation environments. For example, in an environment where the ambient light is strong and the background noise is intense as a result, the second threshold TD2 may be set to a smaller value than usual so as to impose a stricter standard for filtering out background noise. After filtering out the impact by the background noise, a common time of flight for the TX sequence 300 toward and back from the target is calculated. A variety of methods may be implemented to calculate the common time of flight. For example, an average value for the differences between each valid RXi and the corresponding TXi may be calculated, or a median value may be selected, or a statistical method may be applied. Overall, since the impact by the background noise is reduced through a scheme containing layers of filtering, the calculated common time of flight has improved efficiency and precision.

With reference to FIG. 1, in accordance with one or more illustrative embodiments of the present disclosure, the SPADs 105 may include a plurality sets of SPADs. Each set of SPADs includes at least one SPAD and is configured for producing digital pulses in response to a part of the received optical radiation corresponding to a pixel of the target 103 respectively. The TDCs 106 are configured for generating a plurality of second sequences of time values, and each of the plurality of second sequences of time values corresponds to digital pulses produced by a set of SPADs respectively. The controller 130 is configured for calculating a common time of flight for the sequence of light pulses toward and back from a respective pixel of the target based on the first sequence of time values and a corresponding one of the plurality of second sequences of time values.

In accordance with one or more illustrative embodiments of the present disclosure, a light detection and ranging (LiDAR) system based on direct TOF is provided. The LiDAR system includes: a laser configured for emitting a sequence of light pulses toward a target, and a first sequence of time values is indicative of respective times of emitting of the sequence of light pulses; and an array of sensing elements. Each sensing elements includes at least one SPAD and is configured to output a second sequence of time values that is indicative of respective arrival times of optical radiation reflected by the target and received by the sensing element. For each sensing elements respectively: a part of the second sequence of time values is selected based on comparing the second sequence of time values with the first sequence of time values, and the selected part of the second sequence of time values is used to calculate a time of flight for the sequence of light pulses toward and back from the target. A depth mapping of the target is generated based on a combination of all the calculated time of flights of the array of sensing elements. In some illustrative embodiments, the array of sensing elements may include a two-dimensional matrix of SPAD pixels. Also, comparing the second sequence of time values with the first sequence of time values may be achieved by correlating a first pattern with a second pattern. The first pattern specifies differences between time values of the first sequence of time values, and the second pattern specifies differences between time values of the second sequence of time values. Alternatively, comparing the second sequence of time values with the first sequence of time values may include: shifting the first sequence of time values by a time delay to obtain a modified first sequence of time values; the time delay is determined based on a match between the first sequence of time values and the second sequence of time values; and comparing the modified first sequence of time values with the second sequence of time values.

Embodiments of the present disclosure include various operations or steps. The steps may be performed by hardware components or may be embodied in machine-executable instructions, which may be used to cause a general-purpose or special-purpose processor programmed with the instructions to perform the steps. Alternatively, the steps may be performed by a combination of hardware, software and/or firmware.

While the present disclosure has been described with reference to various implementations, it will be understood that these implementations are illustrative and that the scope of the disclosures is not limited to them. Many variations, modifications, additions, and improvements are possible. More generally, implementations in accordance with the present disclosure have been described in the context of particular implementations. Functionality may be separated or combined in blocks differently in various embodiments of the disclosure, or described with different terminology. These and other variations, modifications, additions, and improvements may fall within the scope of the disclosure as defined in the claims that follow.

Claims

A sensing device, comprising:

a transmitter configured to emit a sequence of light pulses toward a target, wherein a first sequence of time values is indicative of respective times of emitting of the sequence of light pulses;

a detector configured to receive optical radiation reflected from the target and to output a second sequence of time values indicative of respective arrival times of the received optical radiation; and

a controller, wherein the controller is configured for:

shifting the first sequence of time values by a time delay to obtain a modified first sequence of time values, wherein the time delay is determined based on a match between the first sequence of time values and the second sequence of time values,

selecting a part of the second sequence of time values based on comparing the modified first sequence of time values with the second sequence of time values,

calculating a first difference between a total number of the selected part of the second sequence of time values and a total number of the first sequence of time values, and

upon determining that the first difference does not exceed a first threshold, using the selected part of the second sequence of time values to calculate a common time of flight for the sequence of light pulses toward and back from the target.
The sensing device according to claim 1, wherein the sequence of light pulses has an equal time interval.
The sensing device according to claim 1, wherein the sequence of light pulses has time intervals that regular change by selecting one of a plurality of predefined time intervals.
The sensing device according to claim 1, wherein selecting the part of the second sequence of time values based on comparing the modified first sequence of time values with the second sequence of time values, comprises, for each of the second sequence of time values respectively:

identifying a sequential position of one of the second sequence of time values;

identifying one or more of the modified first sequence of time values corresponding to the sequential position;

comparing the one of the second sequence of time values with the one or more of the modified first sequence of time values to obtain a second difference; and

in response to determining that the second difference does not exceed a second threshold, determining that the selected part of the second sequence of time values includes the one of the second sequence of time values.
The sensing device according to claim 4, wherein the one or more of the modified first sequence of time values are located sequentially beginning from a sequential position in the modified first sequence of time values same as the sequential position of the one of the second sequence of time values,

wherein comparing the one of the second sequence of time values with the one or more of the modified first sequence of time values to obtain the second difference, comprises:

comparing the one of the second sequence of time values with each of the one or more of the modified first sequence of time values respectively so as to select a minimum value as the second difference.
The sensing device according to claim 4, wherein the second threshold is adjustable by the controller.
The sensing device according to claim 1, wherein the target comprises a plurality of pixels each corresponding to a part of the received optical radiation respectively.
The sensing device according to claim 7, wherein

the detector comprises a plurality of single-photon avalanche diodes (SPADs) and a plurality of time-to-digital converters (TDCs) coupled to the plurality of SPADs;

the plurality of SPADs comprise a plurality sets of SPADs, each set of SPADs comprises at least one SPAD and is configured for producing digital pulses in response to a part of the received optical radiation corresponding to a pixel of the target respectively;

the plurality of TDCs are configured for generating a plurality of second sequences of time values, each of the plurality of second sequences of time values corresponds to digital pulses produced by a set of SPADs respectively;

the controller is configured for calculating a common time of flight for the sequence of light pulses toward and back from a respective pixel of the target based on the first sequence of time values and a corresponding one of the plurality of second sequences of time values.
The sensing device according to claim 1, wherein the match between the first sequence of time values and the second sequence of time values comprises a correlation between a first pattern and a second pattern, the first pattern specifies differences between time values of the first sequence of time values, the second pattern specifies differences between time values of the second sequence of time values.
A method for distance measuring based on direct TOF, comprising:

emitting a sequence of light pulses toward a target, wherein a first sequence of time values is indicative of respective times of emitting of the sequence of light pulses;

receiving optical radiation reflected from the target, wherein a second sequence of time values is indicative of respective arrival times of the received optical radiation;

shifting the first sequence of time values by a time delay to obtain a modified first sequence of time values, wherein the time delay is determined based on a match between the first sequence of time values and the second sequence of time values,

selecting a part of the second sequence of time values based on comparing the modified first sequence of time values with the second sequence of time values,

calculating a first difference between a total number of the selected part of the second sequence of time values and a total number of the first sequence of time values, and

upon determining that the first difference does not exceed a first threshold, using the selected part of the second sequence of time values to calculate a common time of flight for the sequence of light pulses toward and back from the target.
The method according to claim 10, wherein the sequence of light pulses has an equal time interval.
The method according to claim 10, wherein the sequence of light pulses has time intervals that regular change by selecting one of a plurality of predefined time intervals.
The sensing device according to claim 10, wherein selecting the part of the second sequence of time values based on comparing the modified first sequence of time values with the second sequence of time values, comprises, for each of the second sequence of time values respectively:

identifying a sequential position of one of the second sequence of time values;

identifying one or more of the modified first sequence of time values corresponding to the sequential position;

comparing the one of the second sequence of time values with the one or more of the modified first sequence of time values to obtain a second difference; and

in response to determining that the second difference does not exceed a second threshold, determining that the selected part of the second sequence of time values includes the one of the second sequence of time values.
The sensing device according to claim 13, wherein the one or more of the modified first sequence of time values are located sequentially beginning from a sequential position in the modified first sequence of time values same as the sequential position of the one of the second sequence of time values,

wherein comparing the one of the second sequence of time values with the one or more of the modified first sequence of time values to obtain the second difference, comprises:

comparing the one of the second sequence of time values with each of the one or more of the modified first sequence of time values respectively so as to select a minimum value as the second difference.
The method according to claim 13, wherein the second threshold is adjustable.
The method according to claim 10, wherein the match between the first sequence of time values and the second sequence of time values comprises a correlation between a first pattern and a second pattern, the first pattern specifies differences between time values of the first sequence of time values, the second pattern specifies differences between time values of the second sequence of time values.
A light detection and ranging (LiDAR) system based on direct TOF, comprising:

a laser configured for emitting a sequence of light pulses toward a target, wherein a first sequence of time values is indicative of respective times of emitting of the sequence of light pulses; and

an array of sensing elements, wherein each sensing elements comprises at least one SPAD and is configured to output a second sequence of time values that is indicative of respective arrival times of optical radiation reflected by the target and received by the sensing element;

wherein for each sensing elements respectively: a part of the second sequence of time values is selected based on comparing the second sequence of time values with the first sequence of time values, and the selected part of the second sequence of time values is used to calculate a time of flight for the sequence of light pulses toward and back from the target;

wherein a depth mapping of the target is generated based on a combination of all the calculated time of flights of the array of sensing elements.
The LiDAR system according to claim 17, wherein the array of sensing elements comprises a two-dimensional matrix of SPAD pixels.
The LiDAR system according to claim 17, wherein comparing the second sequence of time values with the first sequence of time values comprises correlating a first pattern with a second pattern, the first pattern specifies differences between time values of the first sequence of time values, the second pattern specifies differences between time values of the second sequence of time values.
The LiDAR system according to claim 17, wherein comparing the second sequence of time values with the first sequence of time values comprises:

shifting the first sequence of time values by a time delay to obtain a modified first sequence of time values, wherein the time delay is determined based on a match between the first sequence of time values and the second sequence of time values, and

comparing the modified first sequence of time values with the second sequence of time values.