Abstract:
A lidar-based 3-D point cloud measuring system and method. An example system includes a base, a housing, a plurality of photon transmitters and photon detectors contained within the housing, a rotary motor that rotates the housing about the base, and a communication component that allows transmission of signals generated by the photon detectors to external components. The rotary component includes a rotary power coupling configured to provide power from an external source to the rotary motor, the photon transmitters, and the photon detectors. In another embodiment, the photon transmitters and detectors of each pair are held in a fixed relationship with each other. In yet another embodiment, a single detector is “shared” among several lasers by focusing several detection regions onto a single detector, or by using a single, large detector.

Description:
PRIORITY CLAIM 
     The application claims the benefit of U.S. Provisional Application Ser. No. 60/807,305 filed Jul. 13, 2006, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The use of a pulse of light to measure distance is well known. As is commonly used in devices such as a police speed detector, the basic concept is that of pulsing a laser emitter, which causes a burst of light to be emitted, usually focused through a lens or lens assembly. Then, the time it takes for that pulse of light to return to a detector mounted near the emitter is measured, and a distance can then be derived from that measurement with high accuracy. 
     When multiple pulses are emitted in rapid succession, and the direction of those emissions is somehow sequentially varied, each distance measurement can be considered a pixel, and a collection of pixels emitted and captured in rapid succession (called a “point cloud”) can be rendered as an image or analyzed for other reasons such as detecting obstacles. Viewers that render these point clouds (today typically PC based) can manipulate the view to give the appearance of a 3-D image. While the data that comes back is lacking color or other characteristics, different schemes can be used to depict the distance measurements that allow the rendering device to show the 3-D image as if it were captured by a live action camera. 
     There exist a number of commercial products that can capture distance points in rapid succession and render a 2-D (i.e. single plane) point cloud. These instruments are often used in surveying, mapping, autonomous navigation, industrial applications, and for other purposes. Most of these devices rely on the use of a single laser emitter/detector combination combined with some type of moving mirror to effect scanning across at least one plane, as shown in  FIG. 1 . 
     Such devices are often used in industrial applications, as shown in  FIG. 2 . Note the scan lines emitting from the unit—the spinning mirror allows the single laser emitter/detector assembly to be aimed along this plane via the use of the rotating mirror. 
     Often, these mirrors are rotated at very fast speeds—in the thousands of RPMs. As stated above, this design inherently renders only a 2-D point cloud. However, a 3-D point cloud is often required. The other dimension is provided for in a number of ways. Most often, the entire instrument is actuated up and down and/or back and forth, often on a gimbal—a process know within the art as winking or nodding the sensor. Thus, a single beam lidar unit can be employed to capture an entire 3-D array of distance points, albeit one point at a time. An example of this approach is shown in  FIG. 3 .  FIG. 3  shows a 2-D scanner employing a single laser emitter/detector pair and a rotating mirror mounted on a gimbal that “nods” the unit up and down, and rotates it back and forth in order to increase field of view. 
     In yet other single laser emitter/detector pair mirror-based prior art devices there exists a prism that “divides” the laser pulse into multiple “layers,” with each layer having a slightly different vertical angle. This simulates the nodding effect described above but with no need for actuation of the sensor itself. 
     In all the above examples, the main premise is a single laser emitter/detector combination, where the light path is somehow altered to achieve a broader field of view than a single sensor can achieve. The device is inherently limited to the number of pixels it can generate due to the limitation of how many pulses per second are possible from a single laser. Any alteration of the laser&#39;s path, whether it is by mirror, prism, or actuation of the device, causes the point cloud to be less dense, but cover a broader area. The goal for sensors of this nature is to maximize the number of pixels to provide a point cloud that covers a broad field of view yet is as dense as possible. 
     It is of course possible to add additional lasers and detectors to a rotating mirror unit. While this can easily be done, the resultant performance does not necessarily scale with the number of lasers used. When multiple laser emitter/detector combinations are employed for a spinning mirror scanner, or when the single laser is divided via the use of a prism, the image also rotates. Therefore, while the beams will fan out vertically in one direction, they will twist so as to align horizontally in the 90-degree rotational direction. While this arrangement can be used for forward-looking-only units, it is less than desirable if a sideways view is also desirable, as is often the case for many applications. 
     There also exist “flash lidar” units. These operate by simultaneously illuminating a large area, and capturing the resultant pixel-distance information on a specialized 2-D focal plane array (FPA). Such sensors are complicated and difficult to manufacture, and as a result not widely deployed commercially. However, it is expected that they will someday replace the mechanically scanned sensors, as they are solid state, and require no moving parts.  FIG. 4  shows the framework for the detector array of a flash lidar unit. 
     It is always desirable to collect more points faster. Until flash lidar technology is perfected, there will always be a compromise of sensors that alter the path of the emitter/detector beam in order to achieve a broader field of view. 
     As noted above, 3-D point cloud systems exist in several configurations, the needs for autonomous vehicle navigation place unrealistic demands on current systems. For example, there are numerous systems that take excellent pictures, but take several minutes to collect a single image. Such systems are unsuitable for highway use. There are also flash systems that have excellent update rate but lack field of view lack and good distance performance. There are single beam systems that can provide useful information but do not work well with objects that are too small and fall outside the unit&#39;s field of view. In reality, it is necessary to see everywhere around the vehicle, almost a full 360 degrees, in order to safely navigate today&#39;s highways. In addition, it is necessary to have a minimum of delay between the actions happening in the real world and the imaging/reaction to it. Generally, it is accepted that human response time is in the several tenths of a second. Therefore, it is realistic to provide the navigation computer with a complete fresh update approximately ten times a second. Of course, faster is better, but it may also be possible to navigate successfully with an update rate of 5 times a second. The vertical field of view needs to extend above the horizon, in case the car enters a dip in the road, and should extend down as close as possible to see the ground in front of the vehicle. Of course, it is not possible to see directly in front of the vehicle, since the hood or other parts of the car obstruct the view. 
     While the preferred embodiment uses 64 discrete vertical beams to capture the point cloud data, as few as 16 beams or fewer could be employed, with largely the same result. In addition, it is preferable to disperse the beams such that there is coverage that is more detailed directly horizontally in front of the vehicle, such concentration being useful for highway driving at speed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a lidar-based 3-D point cloud measuring system. An example system includes a base, a housing, a plurality of photon transmitters and photon detectors contained within the housing, a rotary motor that rotates the housing about the base, and a communication component that allows transmission of signals generated by the photon detectors to external components. 
     In one aspect of the invention, the rotary component includes a rotary power coupling configured to provide power from an external source to the rotary motor, the photon transmitters, and the photon detectors, as well as signal in and out of the unit. 
     In another aspect of the invention, the photon transmitters and detectors of each pair are held in a fixed relationship with each other. 
     In another aspect of the invention, a single detector is “shared” among several lasers by focusing several detection regions onto a single detector, or by using a single, large detector. 
     In another aspect of the invention, a single laser beam is divided into several smaller beams, with each smaller beam focused onto its own detector. 
     In still another aspect of the invention, the communication component comprises at least one of a rotary coupling device or a wireless communication device. 
     The present invention provides a more compact and rugged unit for gathering 3-D point cloud information. In addition, the present invention provides the ability to discern multiple returns. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIGS. 1-4  illustrate inventions formed according to the prior art; 
         FIG. 5  is a Lidar Terrain mapping and obstacle detection system shown at rest; 
         FIG. 6  is the Lidar Terrain mapping and obstacle detection system shown in operation at 300 RPM; 
         FIG. 7  is a perspective view of the present invention mounted to a vehicle; 
         FIGS. 8A  and B illustrate components of an auto braking and auto steering components; 
         FIGS. 9 and 10  illustrate circuits for performing data acquisition and auto vehicle control; 
         FIGS. 11 and 12  illustrate components of an auto vehicle operating system; 
         FIGS. 13-22  illustrate various views of a scanning device formed in accordance with an embodiment of the present invention; 
         FIGS. 23A  and B illustrate various circuits for driving laser diodes; 
         FIG. 24  illustrates example pulses generated for the laser diode; 
         FIGS. 25-26  illustrate results of operation of the circuit from  FIG. 23A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 5-12  illustrate a Laser Imaging Detection and Ranging (Lidar) terrain mapping and obstacle detection system employed as a sensor for an autonomous vehicle. The Lidar system includes 8 assemblies of 8 lasers each as shown in  FIG. 5  or  2  assemblies of 32 lasers each forming a 64-element Lidar system as shown in  FIGS. 13-26 . The system has a 360-degree horizontal field of view (FOV) and a 26.8-degree vertical FOV. The system is typically mounted on the top center of a vehicle, giving it a clear view in all directions, and rotates at a rate of up to 200 Hz, thereby providing a high point cloud refresh rate, such high rate being necessary for autonomous navigation at higher speeds. At this configuration, the system can collect approximately 1 million time of flight (TOF) distance points per second. The system provides the unique combination of 360 degree FOV, high point cloud density, and high refresh rate. The standard deviation of TOF measurements is equal to or less than 5 cm. The Lidar system has an inertial navigation system (INS) sensor system mounted on it to report exact pitch and roll of the unit that is used by navigational computers to correct for these deviations. The unit generates its own light and uses a proprietary filter to reject sunlight, so it works well under all lighting and most weather conditions. Through the use of digital signal processor (DSP) control, a dynamic power feature allows the system to increase the intensity of the laser emitters if a clear terrain reflection is not obtained by photo detectors (whether due to reflective surface, weather, or other reasons), and to reduce power to the laser emitters for safety reasons if a strong reflection signal is detected by photo detectors. A direct benefit of this feature is that the Lidar system is capable of seeing through fog and heavy rain by increasing laser power dynamically and ignoring early reflections. The unit also has the capability to receive and decipher multiple returns from a single laser emission through digitization and analysis of the waveform generated by the detector as the signal generated from the emitter returns. 
     The Lidar system sends data in the form of range and intensity information via Ethernet output (or similar output) to a master navigational system. Using standard trigonometry, the range data is converted into x and y coordinates and a height value. The height value is corrected for the vehicle&#39;s pitch and roll so the resulting map is with reference to the horizontal plane of the vehicle. The map is then “moved” in concert with the vehicle&#39;s forward or turning motion. Thus, the sensor&#39;s input is cumulative and forms an ultra-high-density profile map of the surrounding environment. 
     This highly detailed terrain map is then used to calculate obstacle avoidance vectors if required and, as importantly, determine the maximum allowable speed given the terrain ahead. The Lidar system identifies of size and distance of objects in view, including the vertical position and contour of a road surface. The anticipated offset of the vehicle from a straight, level path, either vertical or horizontal, at different distances is translated into the G-force that the vehicle will be subject to when following the proposed path at the current speed. That information can be used to determine the maximum speed that the vehicle should be traveling, and acceleration or braking commands are issued accordingly. In all cases the software seeks the best available road surface (and thus the best possible speed) still within the boundaries of a global positioning system (GPS) waypoint being traversed. 
     The system shown in  FIGS. 5-8  includes 64 emitter/detector (i.e. laser diode/photo diode) pairs divided into eight groups of eight. The system shown in  FIGS. 13-24  also includes 64 emitter/detector pairs, but in a configuration of 2 assemblies of 32 pairs. It is also possible to “share” a single detector among several lasers by focusing several detection regions onto a single detector, or by using a single, large detector. By firing a single laser at a time, there would be no ambiguity as to which laser is responsible for a return signal. Conversely, one could also sub-divide a single laser beam into several smaller beams. Each beam would be focused onto its own detector. In any event, such systems are still considered emitter-detector pairs. 
     The laser diode is preferably an OSRAM 905 nm emitter, and the photo diode is preferably an Avalanche variety, but other types can be used. The lenses are preferably UV treated to block sunlight. Each pair is physically aligned in ⅓° increments, ranging from above horizontal (aligned at 500 feet in front of the vehicle) to approximately −24° (aligned to 20 feet in front of the vehicle). Each of the emitter/detector pairs are controlled by one or more DSPs, which determines when they will fire, determines the intensity of the firing based on the previous return, records the time-of-flight, calculates height data based time-of-flight and angular alignment of each pair. Results, including multiple returns if any, are transmitted via Ethernet to the master navigational computer via a rotary coupling. 
     It is advantageous to fire only several lasers, or preferably just one, at a time. This is because of naturally occurring crosstalk, or system blinding that occurs when the laser beam encounters a retroreflector. Such retroreflectors are commonly used along the roadways. A single beam at a time system is thus resistant to retroreflector blinding, while a flash system could suffer severe image degradation as a result. 
     Another advantage of firing only a small number of lasers at a time is the ability to share, or multiplex, the detection circuitry among several detectors. Since the detection circuitry consists of high speed A-D&#39;s, such as those made by National Semiconductor, considerable cost savings can be had by minimizing the use of these expensive components. 
     In the preferred embodiment, the detectors are power cycled, such that only the desired detector is powered up at any one time. Then the signals can simply be diode-ored together to obtain the desired multiplexing. An additional benefit of power-cycling the detectors is that total system power consumption is reduced, and the detectors therefore run cooler and are therefore more sensitive. 
     A simple DC motor controller driving a high reliability brushed motor controls the rotation of the emitter/detectors. A rotary encoder feeds rotational position to the DSPs that use the position data to determine firing sequence. Software and physical failsafes ensure that no firing takes place until the system is rotating at a minimum RPM. 
     In one embodiment, the navigational system uses Dual GPS receivers. The first is a Navcom 2050G using the Starfire subscription service and the second is a Novatel ProPak-LB receiver using the Omnistar subscription service. These subscription services typically deliver 2-inch accuracy under full sky-in-view conditions when operating in dual-differential mode. Any high precision GPS system could be used. The GPS receivers are used to correct the errors in the INS. The INS includes gyros, such as fiber optic gyros (FOG). In addition, there is a 6-axis inertial system mounted on the Lidar head that is used to correct the Lidar signal as well as provide pitch and roll information for correcting the FOG gyro signal. 
     Vehicle control is accomplished through the actuation of 2 20 HP brushless motors for brake and steering respectively (see  FIGS. 8A , B), controlled by Texas Instruments C2400 DSP chips. The acceleration is accomplished electronically by tying into the dual voltage acceleration system of the vehicle. The present invention can be retrofitted on virtually any vehicle—land, air, sea, or space vehicles. 
       FIG. 9  illustrates an example of the circuit components within a rotating head unit. The rotating head unit includes multiple detector and emitter circuits each having its own processor. The data produced by each circuit is output to external components via a rotary coupling. Other data transmission techniques may be used, such as wireless transmission via any of a number of different wireless protocols. 
       FIG. 10  shows the key navigational aspects of a system  50  and how they interconnect. The terrain map as generated from the Lidar terrain-mapping system (not shown) is fed via a single channel high-speed connection to a serial-to-parallel converter  52 , which then populates a FIFO memory array  54  (later embodiments transfer information via an ETHERNET connection). A DSP  60  then receives the Lidar information from the FIFO array  54  along with information from two GPSs, two INSs, FOG Gyro, Odometer, and remote kill switch. This data is evaluated to decide the vehicle&#39;s path followed by serial commands issued to control the vehicle (note the steering, brake, throttle, and siren outputs). The steering and brake motors are controlled by DSPs  62 ,  64 , and the acceleration is controlled through an electronic interface via a custom interface board. The DSP  60  also controls a video display  68  that allows viewing of the Lidar image for observation and debugging purposes. 
     A small footprint is achieved through the use of embedded DSP technology. All PC boards for decision making, sensing, motor control and navigational data are proprietary, designed exclusively for this purpose, and fit in a single motherboard/daughterboard case. All major navigational components fit in a box mounted on the roof of the truck cab (shown in  FIGS. 7 and 11 ). The only other modifications to the vehicle are a steering motor integrated with the steering mechanism in the engine compartment, a brake motor that sits on the floor of the cab, and the acceleration interface (in this case electronic). 
     The result of the preferred embodiment design is a truck that is street legal and that affords full passenger comforts even while in autonomous mode. The entirety of system testing is performed while riding inside the vehicle and actuating three switches for gas, steering and brake over the driver&#39;s head, shown in  FIG. 12 . 
     The present invention performs at a frame rate that permits high-speed navigation, provides recognition of both positive and negative obstacles, provides exceptional point cloud density, provides full 360 degree HFOV, provides broad VFOV, and provides high accuracy rates. 
       FIG. 13  illustrates a perspective view of a 64 emitter/detector pair lidar component  150 . The component  150  includes a housing  152  that is opened on one side for receiving a first Lidar system  154  located above a second Lidar system  156 . The second Lidar system  156  is positioned to have line of sight with a greater angle relative to horizontal than the first Lidar system  154 . The housing  152  is mounted over a base housing section  158 . 
     As shown in  FIGS. 14 and 15 , the section  158  includes a magnetic rotor  159  and stator  160 . A rotary coupling  161 , such as a three-conductor Mercotac model  305 , passes through the center of the section  158  and the rotor  159 . The three conductors facilitated by the rotary coupling are power, signal, and ground. A bearing  162  mounts on the rotary coupling  161 . A rotary encoder  163  has one part mounted on the rotary coupling  161  and another part mounted on the base section  158  of the housing  152 . The rotary encoder  163 , such as a U.S. Digital Model number E6s-1000-750-T-PKG1 provides information regarding to rotary position of the housing  152 . The magnetic rotor  159  and stator  160  cause rotary motion of the base section  158  and thus the housing  152  about the rotary coupling  161 . 
       FIG. 16  illustrates a perspective view of the first Lidar system  14 . The Lidar system  154  includes a face section that has two cavities  174  for mounting emitter lenses and one larger cavity  170  for mounting the single detector lens. As shown in  FIG. 22 , behind each of the lenses in the cavity  174 , are 16 laser emitters organized relatively horizontally, thereby combining for 32 total emitters. Behind the lens of the cavity  170  are 32 detectors that are positioned within a tube  176  of the unit  154 . As shown in  FIG. 17  the second lidar unit  156  is somewhat comparable to the first Lidar system  154 , but includes a shorter tube  178  and has a downward looking pitch angle.  FIG. 18  illustrates a rearview of the second Lidar system  156 . 
     Behind each of the lenses of the cavity  174 , are emitter packages  180 . Each emitter package  180  includes 16 distinct emitters  188  (per side). Each emitter  188  of an emitter unit  180  is positioned within unit  196  and of laser bracket  195  (see  FIGS. 19 and 20 ). Each emitter is aligned to the corresponding receiver on the laser detector board  166  than attached with adhesive to each emitter and to bracket laser unit  196  and bracket block strap  195 . 
       FIGS. 23A  and B illustrate circuits used for controlling the firing of a laser diode. With regard to  FIGS. 23A and 24 , the DSP sends a charge/on signal to a FET  200 , thereby charging an inductor  204 , which in turn charges a capacitor  206 , which in turn causes a laser  210  to fire. The DSP turns off the FET  200  after a predetermined period of time as previously determined by return intensity measurements from the last pulse. The charging pulse is on for ˜5 microseconds and the firing pulse is on for ˜20 nanoseconds. variable power laser diode firing circuit. It can be seen that the energy stored in the inductor is ½*L*I^2. When the FET is turned off, this energy is transferred into the discharge capacitor via a diode. The energy in the capacitor is ½° C.*V^2. It is apparent then that the voltage that is in the capacitor is proportional to the on duration of the FET. Therefore, the DSP can use a simple algorithm to predict the proper amount of voltage in the capacitor. For example, if the return pulse is ½ as large as desirable, from a noise and measurement accuracy point of view, then the DSP simply charges the inductor for twice as long for the next pulse. Of course, such a system cannot see into the future, so it is not always possible to get the perfect return intensity every time. Nevertheless, the technique works well enough most of the time for the system to benefit from the technique. 
       FIG. 23B  includes two FETs. When FET 1  is on during a charging pulse ( FIG. 24 ), an inductor  240  charges a capacitor  242 . When the FET 2  is on during the firing pulse (see  FIG. 24 ), FET 2  causes the capacitor  242  to discharge thereby firing a laser diode  244 . 
       FIG. 25  illustrates current and luminance output of the circuits of  FIGS. 23A  and B.  FIG. 26  shows digitized sensed values at the photo diode of the receiving side. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.