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Home > News > Principle of Ranging for Single-line Lidar

Principle of Ranging for Single-line Lidar

2019-01-09

At present, the measurement principle of lidar is mainly divided into three types: pulse method, coherent light method and trigonometry method.

Among the main measurement principles of lidar applications, the pulse method and the coherent light method have high requirements on the hardware of the Laser Radar but the measurement accuracy is much higher than the laser triangulation method. So it is mostly used in the military field. In contrast, the laser triangulation method meets most industrial and civil requirements due to its accuracy and low cost. Most of the domestic laser radar manufacturers use the triangulation method and the phase ranging method.

One is the coaxial pulsed TOF technology, Time of Flight. It is a way of measuring the time of light flight. In this way, as the name implies, a laser is emitted, and then a diode is used to detect the echo of the laser. We use a high-precision timer to measure the time difference between the emission of the light and the feedback caused by the return of the target. However, we all know that the speed of light is invariant, and then the time difference is multiplied by the speed of light to obtain the distance of the target object. This is the way of distance measurement adopted by the current mainstream industrial-grade laser radars. The thresholds of technology (optics, algorithms, etc.) are relatively high. At present, only SDKELI laser radars in China use this technology. In fact, this measurement method has a big problem: its cost is very high. Because the measurement of the speed of light requires a very high precision timer. This timer does not mean that as long as it can distinguish between milliseconds and microseconds. It need to achieve the measurement accuracy of the picosecond level. In addition, very high quality are required for laser emitter and sensing devices.

The other is the measurement method of the phased TOF.
The phase type is a continuous emission laser. However, the received echo signal will have a difference in phase due to the characteristics of the speed of light propagation. When checking the phase, you can turn around to handle this distance. The advantage of this approach is that the cost is relatively cheaper, but the main problem is that the speed of measurement cannot be improved. The laser rangefinder currently sold online is the phase method of laser ranging.
Laser triangulation method. Basic principle: a laser beam is used to illuminate the object to be measured at a certain angle of incidence. The laser reflects and scatters on the surface of the object. At another angle, the lens is used to converge the reflected laser. The spot is imaged in a CCD (Charge-coupled Device). Component) on the position sensor. When the measured object moves along the laser direction, the spot on the position sensor will move, and the displacement corresponds to the moving distance of the measured object. Therefore, the distance between the measured object and the baseline can be calculated by the algorithm by the algorithm. value. Since the incident light and the reflected light form a triangle, the geometric triangle theorem is applied to the calculation of the spot displacement. Therefore, the measurement method is called the laser triangulation method. According to the angle relationship between the incident beam and the surface normal of the measured object, it can be generally divided into Both oblique and direct type of distance measurement.
The biggest advantage of this method is that its cost will be greatly reduced compared to TOF (Time of Flight). The triangulation method is essentially a camera plus a processing chip. Of course, this approach also has some drawbacks. Just like taking a photo, it has a resolution limit. If the resolution is not high and the object is far away, it may not be visible. In the same way, the triangulation method is not very clear for long-distance objects, so there are high challenges to the algorithm here. If the algorithm is not good enough, even if you measure objects that are four or five meters away, problems can occur.

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