VELODYNE’S HDL-64E: A HIGH DEFINITION LIDAR SENSOR FOR 3-D APPLICATIONS

3 D S e n s o r T e c h n o l o g y O v e r v i e w The use of light pulses to measure distance is well known. The basic concept is to first emit a light pulse, typically using a laser diode. The light travels until it hits a target, when a portion of the light energy is reflected back towards the emitter. A detector mounted near the emitter detects this return signal, and the time difference between the emitted and detected pulse determines the distance of the target. When this pulsing distance measurement system is somehow actuated, a multitude of points (called a “point cloud”) can be collected If no targets are present, then the light would never return. If the light was pointed downward, then the ground would provide some signal return. If a target was positioned within this point cloud, a notch would be seen in the rendered data. The distance and width of the target could be determined from this notch. When this collection of points is rendered, the point cloud begins to resemble a picture. The denser the point cloud the richer the picture becomes. There exist a number of commercial products that can capture distance data in a 2-D (i.e. single plane) point cloud manner. These instruments are often used in industrial applications and are often repurposed for surveying, mapping, autonomous navigation, and other uses. Most of these devices rely on the use of a single laser emitter/detector pair combined with some type of moving mirror to effect scanning across at least one plane. This mirror not only reflects the emitted light from the diode, but also reflects the return light to the detector. Use of a rotating mirror in this application is a means to achieving typically 90 – 180 degrees of azimuth view while simplifying both the system design and manufacturability, as there is only one moving part, the mirror. Figure 1. Single emitter/detector pair rotating mirror lidar design. W h i t e P a p e r Such devices are often used in industrial applications, as shown in Figure 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. Figure 2. Industrial application of a conventional single emitter/detector pair laser. Many applications require more data than just a single 2-D plane. The 2-D point cloud must be expanded to form a 3-D point cloud, where multiple 2-D clouds are used, each pointing at a different elevation angle. One method of achieving this third dimension is to move the emitterdetector pair up and down while the mirror rotates (sometimes referred to as “winking” or “nodding”). This will generate elevation data points, but it will reduce the number of azimuth data points, thus the point clouds are less dense, resulting in a lower resolution system. There also exist “flash lidar” units. These units 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 may someday replace mechanically actuated sensors, as they are solid state and require no moving parts. 3-D point cloud systems exist in several configurations. However, the needs for autonomous vehicle navigation, mobile surveying, and high speed/high density image capture 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 any mobile sensing applications. There are also flash systems that have excellent update rate, but lack field of view and good distance performance. There are single beam systems that can provide useful information, but do not work well with objects that are either too small or fall outside the unit’s field of view. In reality, to adapt to the greatest number of uses for a lidar sensor it is necessary to see everywhere around the collection point a full 360 degrees. In addition, the processed data needs to be presented to the user in real time, so it is necessary to have a minimum of delay between the data gathering and rendering the imaging. For example, in the arena of autonomous navigation, it is generally 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 minimally ten 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 to accommodate dips in the road surface or steep declines.