TECHNICAL FIELD

The described embodiments relate to LIDAR based 3-D point cloud measuring systems.

BACKGROUND INFORMATION

LIDAR systems employ pulses of light to measure distance to an object based on the time of flight (TOF) of each pulse of light. A pulse of light emitted from a light source of a LIDAR system interacts with a distal object. A portion of the light reflects from the object and returns to a detector of the LIDAR system. Based on the time elapsed between emission of the pulse of light and detection of the returned pulse of light, a distance is estimated. In some examples, pulses of light are generated by a laser emitter. The light pulses are focused through a lens or lens assembly. The time it takes for a pulse of laser light to return to a detector mounted near the emitter is measured. A distance is derived from the time measurement with high accuracy.

Some LIDAR systems employ a single laser emitter/detector combination combined with a rotating mirror to effectively scan across a plane. Distance measurements performed by such a system are effectively two dimensional (i.e., planar), and the captured distance points are rendered as a 2-D (i.e. single plane) point cloud. In some examples, rotating mirrors are rotated at very fast speeds (e.g., thousands of revolutions per minute).

In many operational scenarios, a 3-D point cloud is required. A number of schemes have been employed to interrogate the surrounding environment in three dimensions. In some examples, a 2-D instrument is actuated up and down and/or back and forth, often on a gimbal. This is commonly known 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. In a related example, a prism is employed to “divide” the laser pulse into multiple layers, each having a slightly different vertical angle. This simulates the nodding effect described above, but without actuation of the sensor itself.

In all the above examples, the light path of a single laser emitter/detector combination is somehow altered to achieve a broader field of view than a single sensor. The number of pixels such devices can generate per unit time is inherently limited due limitations on the pulse repetition rate of a single laser. Any alteration of the beam path, whether it is by mirror, prism, or actuation of the device that achieves a larger coverage area comes at a cost of decreased point cloud density.

As noted above, 3-D point cloud systems exist in several configurations. However, in many applications it is necessary to see over a broad field of view. For example, in an autonomous vehicle application, the vertical field of view should extend down as close as possible to see the ground in front of the vehicle. In addition, the vertical field of view should extend above the horizon, in the event the car enters a dip in the road. In addition, it is necessary to have a minimum of delay between the actions happening in the real world and the imaging of those actions. In some examples, it is desirable to provide a complete image update at least five times per second. To address these requirements, a 3-D LIDAR system has been developed that includes an array of multiple laser emitters and detectors. This system is described in U.S. Pat. No. 7,969,558 issued on Jun. 28, 2011, the subject matter of which is incorporated herein by reference in its entirety.

In many applications, a sequence of pulses is emitted. The direction of each pulse is sequentially varied in rapid succession. In these examples, a distance measurement associated with each individual pulse can be considered a pixel, and a collection of pixels emitted and captured in rapid succession (i.e., “point cloud”) can be rendered as an image or analyzed for other reasons (e.g., detecting obstacles). In some examples, viewing software is employed to render the resulting point clouds as images that appear three dimensional to a user. Different schemes can be used to depict the distance measurements as 3-D images that appear as if they were captured by a live action camera.

Existing LIDAR systems employ a beam of light to interrogate a particular volume of the surrounding environment at any given time. The detection of return signals includes significant sources of measurement noise that are exacerbated as measurement ranges are extended. In many applications, the signal to noise ratio of measured signals is improved by increasing laser pulse intensity. However, increased laser pulse intensity may result in saturation of the detector, signal conditioning electronics, analog-to-digital converters, or any combination thereof, particularly for short range measurements.

Improvements in power control of LIDAR systems are desired, while maintaining high levels of imaging resolution and range.

SUMMARY

Methods and systems for controlling illumination power of a LIDAR based, three dimensional imaging system based on discrete illumination power tiers are described herein.

In one aspect, the illumination intensity of a pulsed beam of illumination light emitted from a LIDAR system is varied based on the intensity of measured return pulses. In this manner, the intensity of measured return pulses is maintained within a linear range of the ADC for objects detected over a wide range of distances from the LIDAR system and a wide range of environmental conditions in the optical path between the LIDAR system and the detected object. By maintaining the intensity of measured return pulses within the linear range of the ADC, both low signal to noise ratio and saturation are avoided.

In some embodiments, an illumination power control module generates a control signal based on a difference between a desired intensity level and a measured intensity level. The control signal is communicated to an illumination driver IC that causes the illumination driver IC to change the intensity of the illumination generated by an illumination source from one discrete illumination power level to another. In these embodiments, the control signal is a digital signal indicative of a desired illumination power level.

In another aspect, an illumination power control module determines the desired illumination power level based on whether an intensity difference exceeds one of a sequence of predetermined, tiered threshold values.

The values of the intensity difference thresholds are predetermined values (i.e., values are known before the determination of the difference between the desired and measured return pulse intensity). In some embodiments, the value of each intensity difference threshold is stored in a look-up table. In some embodiments, the value of each intensity difference threshold is characterized by a nonlinear function. In some embodiments, the value of each intensity difference threshold depends on the current power level. In some embodiments, the values of each intensity difference threshold vary by a fixed scaling factor. In some embodiments, the value of the each intensity difference threshold depends on whether the difference between the desired and measured return intensity signals is positive or negative.

The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not limiting in any way. Other aspects, inventive features, and advantages of the devices and/or processes described herein will become apparent in the non-limiting detailed description set forth herein.

