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What is the effect of laser radar in autopilot?

09:26, 26/12/2016 .. 0 comments .. Link

Laser radar is a kind of work in the optical band radar, it has obvious advantages: high resolution laser radar in the optical band, higher than the microwave frequency is 2 ~ 3 orders of magnitude, therefore, compared with microwave radar, laser radar has high range resolution, the angular resolution and velocity resolution; strong anti-interference ability: green laser pointer wave length, can transmit the divergence angle is very small (RAD level) of the laser beam, the multipath effect is small (not directional emission, multipath effect and microwave or millimeter wave), can detect the low altitude / low altitude target; amount of information acquisition: direct access to the target distance, angle, reflection intensity, speed and other information, generate multi-dimensional image; can work all day long: active laser detection, does not depend on the illumination conditions or Radiation characteristics of the target itself. It only needs to launch its own laser beam, through the detection of the echo signal emitted laser beam to obtain the target information. But the biggest drawback of lidar is that it is easy to be influenced by the atmospheric conditions and the smoke of the working environment. It is very difficult to realize the all-weather working environment.

Classification of laser pointer radar, if divided from the system, there are direct detection of laser radar and coherent detection lidar. In fact, at present we mentioned, including automatic driving, robot, mapping used lidar, basically belong to this direct detection type of laser radar. There are special, such as wind speed, such as radar, generally using coherent system. According to application classification, can be divided into laser rangefinder, laser three-dimensional imaging radar, laser Doppler radar, laser atmospheric sounding radar, etc..

At present, we can usually see the ranging method, from the major categories can be divided into: laser time (Time of Fly) method and triangulation. Laser flight time method can be divided into two categories, one is pulse modulation (pulse ranging technology), a class of laser continuous wave intensity modulation, through phase difference to measure the distance information of the phase ranging. We can see in the market rangefinder, or single line, multi line laser radar, basically these three types of ranging methods.

The principle of laser pulse ranging technology is very simple: by measuring the laser pulse between the radar and the target flying time to get the target distance information. Here is a baseline, the speed of light. All measurements must have a baseline, for a beam of laser there are two benchmarks: speed and frequency (the two most accurate benchmark), because TOF benchmark is the laser flight speed. Mentioned above three kinds of distance measurement, I think the biggest technical difficulty is pulse ranging. But it brings the obvious advantages: very fast measurement. Because of the peak value of the high power laser pointer to measure, its strong anti-interference ability of strong light. Shortcomings are difficult to improve the resolution of the range resolution, detection circuit is difficult. For example, if you want to do phase ranging 1.5 mm resolution, we need to clock clock resolution to achieve 10 picoseconds, which is equivalent to 100G bandwidth, which is a very difficult technology.

Laser phase ranging, mainly by measuring the intensity modulated continuous wave laser signal between the radar and the target flying back and forth phase difference to obtain distance information. The biggest advantage is very high ranging resolution, the current market generally can reach the phase meter rangefinder mm resolution. The disadvantage is faster than the slow pulse ranging measurement, after all we have a phase difference measurement, at least dozens or even hundreds of cycles, in fact it is equivalent to the measurement time in the elongated, so its measuring speed is relatively low. Moreover, its measurement accuracy is easily affected by the target shape motion. If measured in the spot, the two targets one after the first, in fact, it measured the specific information is the average distance between the two targets, rather than the previous target information or the latter target information.

But in pulse ranging, it is easy to separate such information. For example, a 100mw laser pointer pulse, if we are able to achieve 10 nanosecond pulse width, then we can put around a target thirty cm apart, through its distinguish echoes. This way it is difficult to distinguish it in phase ranging. Because in the measurement process, its time will be relatively long, the target movement brings in the distance information, introduces it to the measurement value, actually it measured is an average distance information, but is not the real-time information. But laser pulse ranging is actually the current position of real-time information. This is why the laser radar used in vehicle or robot often uses laser pulse ranging technology, not using phase ranging technology.

Triangulation distance measurement is to obtain the distance information by measuring the laser radiation point in the camera imaging position. Triangle distance measurement has the greatest advantage is the technical difficulty is low, the cost is also very low 10mw laser pointer, in close range accuracy is also very high. Such as industrial use can do 100 microns ranging accuracy. But the disadvantage is that its accuracy will gradually become worse with the increase in distance, basically can not be compared with pulse ranging and phase ranging. Triangle ranging is more suitable for robots such as performance requirements are not high scene. Pulse distance measurement in addition to the cost and technical difficulty is relatively large, it is better in all other aspects of performance. Of course, its range accuracy will be slightly lower than the phase ranging accuracy. But this accuracy, according to the current technology, we can basically reach the centimeter order, or even a few millimeters of magnitude ranging accuracy, basically meet the requirements of our many occasions.

Single line laser radar is actually a high frequency pulse laser rangefinder, coupled with a one-dimensional rotation scan. The characteristics of single line laser radar: only a transmitting and a receiving path, relatively simple structure easy to use; high speed, high resolution scanning angle; volume, weight and low power consumption; high reliability; low cost. In the pedestrian detection, obstacle detection (small target detection) and obstacle detection, single line laser than multi line laser radar has the advantage of much more, because the angular resolution of single line laser radar can do better than multi line laser radar, the detection of small objects or pedestrians in very useful. Single line laser radar in the application of assisted driving is pedestrian detection, in fact, this is also a forward anti-collision applications, and car collision basic similar. Because the angular resolution of single line lidar can be higher than that of multi line burning laser pointer radar, pedestrians can be found in the farther distance, and more warning time can be provided for the control system or the driver.

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