Patent Publication Number: US-2022221559-A1

Title: Scanning lidar system and method with compensation for transmit laser pulse effects

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure is related to LiDAR systems and, in particular, to a scanning coaxial LiDAR system and method of using same with compensation for the effects caused by the transmit laser pulse, which can be used in an automotive or other motor vehicle application. 
     2. Discussion of Related Art 
     Vehicles often include detection systems which can be used for collision avoidance, self-driving, cruise control, and the like. LiDAR, which is an acronym for light detection and ranging, can be used to detect and range objects as part of such a system. 
     In a pulsed LiDAR system, a light pulse is sent out and the returning reflection has to be detected to determine the time-of-flight and the range to the object. The output laser power has to be as low as possible due to power consumption, laser lifetime, laser cost, and eye safety constraints. This results in low signal levels that, in combination with strong ambient light, gives very low signal to noise levels. There is therefore a need to optimize the method of detecting the signal peak to increase the range performance of the LiDAR system, especially when detecting dark, low reflectivity objects. 
     A coaxial LiDAR system uses the same optics for both transmit and receive light. There are many benefits with a coaxial configuration, including good matching between transmit and receive beams and high stability as the laser transmitters and detectors can be placed close to each other. One of the main problems with a coaxial system is that the transmit laser pulse saturates the detector for a short period of time and/or distorts the detector signal for some time period, usually due to reflections and scattering in the LiDAR. This effect, sometimes referred to as “self pulse” or “big bang”, makes it difficult to detect peaks and objects close to the LiDAR system. 
     As such, there is a need for a coaxial detection system, and method of operating same, that can accurately detect objects at low signal levels. 
     SUMMARY 
     In at least one aspect, the subject technology relates to a LiDAR system. The system includes an optical source for generating an optical signal. The system includes a first movable optical element having an at least partially optically reflective surface for redirecting the optical signal. The first movable optical element is movable through a plurality of positions corresponding to a respective plurality of directions of the optical signal reflected from the movable optical element, the directions including a plurality of first directions resulting in an output optical signal being directed though an output of the LiDAR system to an external region, and at least one second direction resulting in the optical signal being reflected from the movable optical element to a reference optical element of the system. The reference optical element is adapted to return a reference signal indicative of a self-pulse of the LiDAR system. The system includes a detector configured to detect optical energy and generate an electrical signal indicative of the optical energy detected by the detector. The system includes a processor for receiving and storing the electrical signal indicative of the optical energy and the reference signal. The processor is configured to determine a position of at least one object by adjusting the electrical signal indicative of the optical energy detected by the detector using the reference signal. 
     In at least some embodiments, the processor adjusts the electrical signal by subtracting the reference signal. In some cases, the reference optical element is a light trap. 
     In some embodiments, first movable optical element is rotatable about an axis and is polygonal in a cross-section taken orthogonal to the axis. The first movable optical element can be rotatable about the axis to scan the output optical signal over the external region in a first dimension. 
     In some embodiments, the system can include a second movable optical element disposed optically between the optical source and the first movable optical element. The first movable optical element can then be rotatable about an axis and polygonal in cross section orthogonal to the axis, while the second movable optical element is a wedge mirror. Further, the first movable optical element can be rotatable to scan the output optical signal over the external region in a first dimension. The second movable optical element can then be movable to scan the output optical signal over the external region in a second dimension orthogonal to the first dimension. 
     In some embodiments, the processor is configured to determine whether a pulse shape of the reference signal is stable and adjust the electrical signal by subtracting the reference signal from the electrical signal only when the pulse shape of the reference signal is stable. 
     In at least one aspect, the subject technology relates to a method for detecting objects in an external region using a LiDAR system. The method includes transmitting a plurality of optical signals with a plurality of LiDAR transmitters. A plurality of LiDAR detectors are positioned coaxially with the LiDAR transmitters. A first movable optical element is provided, the first movable optical element having an at least partially optically reflective surface. The first movable optical element is positioned to redirect the optical signals from the LiDAR transmitters such that the first movable optical element is configured to redirect light between a plurality of first directions and at least one second direction. The first directions result in an output optical signal being output into the external region and the at least one second direction results in the optical signals being transmitted to a reference optical element. The method includes detecting, by the LiDAR detectors, returning optical signals, and generating an electrical signal indicative of optical energy detected. The reference optical element generates a reference signal indicative of a self-pulse of the LiDAR system in response to the optical signal being transmitted to the reference optical element. A position is determined of at least one object in the external region by adjusting the electrical signal indicative of optical energy detected using the reference signal. 
     In some embodiments, the method includes adjusting, by the processor, the electrical signal by subtracting the reference signal. The reference optical element can be a light trap. 
     In some embodiments, the first movable optical element is rotatable about an axis and is polygonal in a cross-section taken orthogonal to the axis. The first movable optical element can then be rotated about the axis to scan the output optical signal over the external region in a first dimension. 
     In some embodiments, a second movable optical element is provided, the second movable optical element having an at least partially optically reflective surface. The second movable optical element can be positioned optically between the LiDAR transmitters and the first movable optical element, the second optical element being configured to redirect light between the LiDAR transmitters and the first movable optical element. The first movable optical element can be rotated about an axis, the first movable optical element being polygonal in a cross section orthogonal to the axis. In some cases, the second movable optical element is a wedge mirror. Further, the first movable optical element can then be rotated to scan the output optical signal over the external region in a first dimension. Additionally, the second movable optical element can be moved to scan the output optical signal over the external region in a second dimension orthogonal to the first dimension. 
     In some embodiments, the processor is configured to determine whether a pulse shape of the reference signal is stable and adjust the electrical signal by subtracting the reference signal from the electrical signal only when the pulse shape of the reference signal is stable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the present disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings. 
         FIG. 1  is a block diagram of a LiDAR system in accordance with the subject technology. 
         FIG. 2  is an overhead view of the LiDAR system of  FIG. 1 . 
         FIG. 3  is an overhead perspective view of the LiDAR system of  FIG. 1 . 
         FIG. 4  is a block diagram of the LiDAR system of  FIG. 1  showing an alternative position of the system components. 
         FIG. 5  is an overhead view of the LiDAR system of  FIG. 4 . 
         FIG. 6  is an overhead perspective view of the LiDAR system of  FIG. 4 . 
         FIG. 7  is a cross-sectional view of  FIG. 5 , taken along the x-y plane through the center of the system. 
         FIG. 8  is a block diagram of a method of operating a LiDAR system in accordance with the subject technology. 
         FIG. 9  represents graphs depicting subtracting the self-pulse of a LiDAR system in accordance with the method shown in  FIG. 8 . 
         FIG. 10  is a block diagram of a method of operating a LiDAR system in accordance with the subject technology. 
         FIG. 11  represents graphs depicting subtracting the self-pulse of a LiDAR system in accordance with the method shown in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     The subject technology overcomes many of the prior art problems associated with vehicle detection systems. In brief summary, in at least some embodiments, the subject technology provides a LiDAR system which makes adjustments based on a reference signal, as discussed in more detail herein. The advantages, and other features of the systems and methods disclosed herein, will become more readily apparent to those having ordinary skill in the art from the following detailed description of certain preferred embodiments taken in conjunction with the drawings which set forth representative embodiments of the present invention. Like reference numerals are used herein to denote like parts. Further, words denoting orientation such as “upper”, “lower”, “distal”, and “proximate” are merely used to help describe the location of components with respect to one another. For example, an “upper” surface of a part is merely meant to describe a surface that is separate from the “lower” surface of that same part. No words denoting orientation are used to describe an absolute orientation (i.e. where an “upper” part must always be at a higher elevation). 
     Referring now to  FIGS. 1-7 , a scanning LiDAR system  100  for a vehicle in accordance with the subject technology is shown. The system  100  can be mounted on or within a vehicle (not distinctly shown) and can be used generally to gather information and data on the surrounding external environment. 
     The LiDAR system  100  includes the necessary components for target detection using LiDAR as are found in typical LiDAR systems known in the art. To that end, the LiDAR system  100  includes an optical source  102  for transmitting optical signals into the surrounding environment, and detectors  104  for receiving returning optical energy that has reflected off objects within the environment. The optical source  102  can be an array of LiDAR transmitters, each transmitter being a laser diode configured to generate a pulsed laser or light beam  110 . The transmitted light beams  110  pass through a lens  106  within a lens housing  108 , the lens  106  being one or more panels of curved glass which acts to collimate the transmitted (and returning) light beams  110 . The transmitted light beams  110  are redirected by two movable optical elements  112 ,  114  with reflective surfaces. 
     In the embodiment shown, the movable optical elements  112 ,  114  include a wedge mirror  112  and a reflecting member  114 , although it should be understood that different movable optical elements (or different positioning of the optical elements) can be used in other embodiments. An actuator (not distinctly shown) causes the wedge mirror  112  to rotate or oscillate around an axis to redirect transmitted light beams  110  between the lens  106  and the reflecting member  114 . Movement of the wedge mirror  112  changes the optical path of the transmitted light beams  110  in the azimuth and/or elevation direction, causing the LiDAR system  100  to scan in the elevation and azimuth directions when the transmitted light beams  110  are output into the environment. Likewise, an actuator (not distinctly shown) rotates the reflective member  114  around an axis “a” extending in the vertical (“z”) direction. The reflective member has a polygonal shape, and in the embodiment shown, has four different sides  124 . While the exemplary reflective member  104  is a four sided polygon, which has been found to be effective, different numbers of sides, or even a differently shaped reflective member  104 , can be used. As the reflective member  114  rotates, different sides are exposed to the transmitted light beams  110  and the angle of each side changes with respect to the transmitted light beams  110 . This results in a sweep in the azimuth direction when the transmitted light beams  110  are output into the surrounding environment. 
       FIGS. 1-3  show a positioning of the movable optical elements  112 ,  114  that causes the transmitted light beams  110  to be output from the system  100 . After being output, some of the transmitted light beams  110  reflect off objects within the surrounding environment and are returned to the detection system. The returning light beams  120  follow the reverse path of the transmitted light beams  110 , being redirected from the reflective member  114  to the wedge mirror  112 , and then through the lens  106 . After passing through the lens  106 , the retuning light beams  120  are received by the detectors  104 . A beams splitter  122  is disposed between the lens  108  and both the optical source  102  and detectors  104 , allowing the optical source  102  and detectors  104  to be arranged in a coaxial configuration. The beam splitter  122  can be one of the many types of known beam splitters, such as wavelength dependent or polarized. In other cases, the beam splitter  122  can simply reflect a portion of all light that hits it while allowing a portion of the light to pass through. As shown, the beam splitter  122  is configured to redirect some light (e.g. light transmitted from the optical source  102 ) while allowing other light to pass through (e.g. returning light for receipt by detectors  104 ). The detectors  104  generate an electrical signal indicative of the optical energy detected from the returning light beams  120 . The system  100  includes, or is connected to, a processor (not distinctly shown) which processes the electrical signal to generate data related to the position of detected objects within the environment. 
       FIGS. 4-7  show a positioning of the movable optical elements  112 ,  114  where transmitted light beams  110  are redirected from the reflective member  114  into a portion of the lens housing  108  containing a reference optical element  126 . The reference optical element  126  can include a reference surface or a light trap and generates a reference signal based on the received transmitted light beam  110 . After a reference signal is generated, the processor uses the reference signal to adjust the electrical signal from the detectors  104  to make a more accurate determination of the position of external objects, as will be discussed in more detail below. Since the environment is not being scanned by the transmitted light beams  110  in the position of  FIGS. 4-7 , this position occurs only occasionally within a given scan cycle. 
     Referring now to  FIG. 8 , a block diagram  800  of a method of adjusting a detected electrical signal in accordance with the subject technology is shown. The diagram  800  shows a detection system operating in a low gain environment. During operation, the detection system scans the environment, gathering and storing raw measurements of the detected signals  801 . Included within those measurements are self-pulses which are detected at block  802 . This can be done by utilizing a reference surface (e.g. a light trap) as described above. The transmitted LiDAR beams are periodically directed towards the reference surface, at which point a reference signal is generated, indicative of the self-pulse of the system. Self-pulses are then stored at block  804 . Measurements representative of targets between 0.1 m and 2 m away may be subject to inaccuracies due to self-pulse detection in the low gain system. Therefore, at block  806 , the stored self-pulse is subtracted from the raw measured electrical signal for targets between 0.1 m and 2 m. On the other hand, raw measurements for targets between 2-30 m are stored at block  808 , and no adjustment is made based on the stored self-pulse. 
     The data is combined at block  810 , and the mixed data is then provided to a finite impulse response (FIR) filter  812  for processing within a processor. The processor first identifies target peaks for potential targets within a 0.1 m to 30 m range at block  814 . After potential target peaks are identified, the candidate target peaks are checked at block  816 , to see if the peak is above a defined threshold. During this process, features are extracted including target pulse width and target amplitude difference, and a classifier is applied to separate the real target from pulse noise. After the candidate target peaks are checked, the processor carries out 3-point quadratic interpolation at block  818 . The initial range index from block  816  is an integer and can have large quantization error. The 3-point interpolation uses the amplitude of three samples to obtain a fractional range part. The processor then outputs the final determination of the position of any targets within the scanned area at block  820 , based on the addition of the integer range and fractional range parts. 
     Referring now to  FIG. 9 , graphs  900   a - 900   c  (generally  900 ) show a FIR algorithm utilizing a method of self-pulse subtraction in accordance with the subject technology, with the y axis representing signal amplitude and the x axis representing a sample number for a single laser firing. In particular, in graph  900   a , the y axis is ADC raw amplitude. In graph  900   b , the y axis is amplitude—a mean amplitude. In graph  900   c , the y axis is the amplitude represented by the difference of the raw data amplitude and the self pulse amplitude. The graphs  900  represent exemplary calculations carried out by the FIR filter  812  as part of the process shown in  FIG. 8 . 
     A FIR matched filter is an optimal detector for Gaussian noise. However, FIR is normally ineffective for close targets in low gain. This can be because the target pulse and self- pulse are merged into one peak which results in one peak for FIR output. Further, low reflective targets with low pulse amplitudes can be located in the negative section of self-pulse which results in missed detection of low reflective target. 
     Referring now to graph  900   a , a graph of the raw signal measurement is shown. Graph line  902  represents an exemplary received signal which includes the detection of a target at 202 cm. The signal experiences a large spike  904  self-pulse. The signal  902  also includes a peak  906  caused by the detection of a target, which is obscured by the self-pulse  904  in graph  900   a . 
     Graph  900   b  shows the signal line of the self-pulse alone, represented by graph line  908 . If the self-pulse  908  is removed from the raw signal measurement  902 , the FIR algorithm can be much more accurately applied to detect very close and close targets. Therefore, graph  900   c  shows a graph line  910  with the self-pulse  908  subtracted from the raw signal measurement  902 . Once the self-pulse  908  is subtracted, the true target peak  912  is much more easily identifiable. This method of subtracting the self-pulse is effective to detect low reflective targets in low gain, even when those targets are very close (e.g. between 0.1 m and 2 m), particularly when the self-pulse is stable. 
     If the self-pulse varies significantly (amplitude, pulse width, negative overshooting, positive overshooting), it is sometimes more difficult to remove the self-pulse  908  from the raw signal measurement  902 . This is due to potential false peaks, such as false peak  914 . Therefore, in such conditions, attempting self-pulse subtraction may generate false peaks in low gain environments. The false target can be recognized by the classifier, as mentioned in step  816 . In particular, the first peak  914  represents the false peak caused by self-pulse which can be present if self-pulse is not subtracted out corrected. Even with the attempted subtraction of self-pulse, there may be some residual from the peak  914 . In some cases, the first peak  914  can be assumed to be a residual peak caused due to subtraction of the self-pulse. The first peak  914  can then be considered to be at range 0 m, and the second peak  912  will have a distance=(position of the second peak  912 —position of the residual peak  914 ) * resolution. In any case, the system can make a determination as to whether the self-pulse is stable and only carry out the method of self-pulse subtraction if the self-pulse is stable. 
     Referring now to  FIG. 10 , a block diagram  1000  of a method of adjusting a detected electrical signal in accordance with the subject technology is shown. The method shown in diagram  1000  is similar to the method of diagram  800 , except as otherwise shown and described. In particular, the method of block diagram  1000  is carried out in a high gain environment, rather than low gain. 
     As the LiDAR system scans, the detectors detect a raw signal  801 . Periodically, self-pulses are detected from the generated reference signal, at block  802 , which are then stored at block  804 . In the high gain environment, LiDAR systems are ordinarily blind for the first 1.5 m due to the effects of self-pulse. However, utilizing the methods described herein, the system is able to reliably reduce the blind range from 1.5 m to 0.5 m. Therefore, at block  1006 , the stored self-pulse is subtracted from the raw measured electrical signal for targets between 0.5 m and 5 m. A larger threshold of 0.5 m to 5 m is used (for the high gain system) because significant overshooting exists in high gain, and a larger threshold results in the removal of false positives from overshooting at close distances. 
     Raw measurements for targets between 5-30 meters are stored at block  1008 , and no adjustment is made based on the stored self-pulse. The data is combined at block  810  and provided to a FIR filter  812  for processing within a processor. The processor then identifies target peaks for potential targets within a 0.5 m to 30 m range at block  1014 , checks the candidate target peaks at block  816 , and then carries out 3-point quadratic interpolation at block  818 . The processor then generates and outputs the final determination of the position of any targets within the scanned area at block  820 . 
     Referring now to  FIG. 11 , graphs  1100   a - 1100   c  (generally  1100 ) showing a FIR algorithm utilizing a method of self-pulse subtraction in accordance with the subject technology are shown. The graphs  1100  represent exemplary calculations carried out by the FIR filter  812  as part of the high gain process shown in  FIG. 10 . 
     Referring now to graph  1100   a , a graph of the raw signal measurement from a detector is shown, represented by graph line  1102 , which includes a target detection at 310 cm. The signal experiences a large spike due self-pulse  1104 . The signal also includes a peak  1106  caused by the detection of a target, which is obscured by the self-pulse  1104  in graph  1100   a . 
     Graph  1100   b  shows the signal of the self-pulse alone, represented by graph line  1108 . The self-pulse  1108  is then subtracted from the raw signal measurement  1102  to generate graph line  1110  of graph  1110   c . Once self-pulse  1108  is subtracted, the true target peak  1112  is much more easily identifiable. However, in the example of graph  1110   c , variations in self-pulse amplitude and width also result in false peaks  1114 . Therefore, in some cases, the method of self-pulse subtraction can be selectively carried out only when a determination is made that no variations in self-pulse are present. In this way, the subject technology is able to eliminate blind areas at close range and improve detection accuracy of the LiDAR system. 
     It is noted that the present disclosure describes one or more LiDAR systems which can be installed in a vehicle, such as an automobile. It will be understood that the embodiments of LiDAR systems of the subject disclosure are applicable to any kind of vehicle, e.g., bus, train, etc. Also, the scanning LiDAR systems of the present disclosure need not be associated with any kind of vehicle. 
     Whereas many alterations and modifications of the disclosure will become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Further, the subject matter has been described with reference to particular embodiments, but variations to those particular embodiments may occur in systems and methods practicing the subject technology described herein, as should be understood by one skilled in the art. It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present disclosure. 
     While the present inventive concept has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the inventive concept as defined by the following claims.