Lidar range measurement principles include triangulation, phase method, pulsed time of flight (TOF) and frequency modulated continuous wave (FMCW)
Triangulation: Using the triangular geometry principle, the distance of the target is calculated by measuring the displacement of the scattered light on the imaging surface of the receiver, which is simple, low cost and high accuracy at short distances.
Phase method: The distance measurement is achieved by measuring the phase of the echo signal and comparing it with the phase of the transmitted signal or by arithmetic, but the long-range measurement and high accuracy cannot be satisfied at the same time.
Frequency modulated continuous wave (FMCW) is also considered as one of the ultimate solid-state lidar solutions because of its long detection distance, its ability to measure the velocity information of objects, its ultra-high resolution, and its ability to achieve interference-free and true solid-state structure, but the lidar with FM continuous wave (FMCW) measurement principle is limited by various aspects such as technical system, device maturity and material cost However, it is difficult to be practical in the short term within five or six years.
TOF measurement method of technology is more mature, more widely used according to the scanning method and internal structure and can be divided into:
Mechanical LIDAR
The transmitter and receiver modules are rotated 360 degrees by electric motors. Multiple laser beams are arranged in the vertical direction, and the transmitter module emits laser lines at a certain frequency to achieve dynamic scanning by continuously rotating the transmitter head. The maximum beam is currently up to 128 lines.
Hybrid Solid State LIDAR
Classified according to the scanning mode:
Multi-sided rotating prism type
-The transceiver module remains motionless, and the motor reflects the beam to a certain area of space while driving the rotating mirror to achieve scanning detection.
MEMS micro-vibrator type
-MEMS micro-vibrating mirrors are integrated on the silicon chip, and the mirrors are suspended between a pair of torsion bars at a certain harmonic frequency, and the rotating micro-vibrating mirrors reflect the light from the laser to achieve scanning. However, the large size mirror MEMS microvibrator suspension beam material fatigue problems cannot pass the impact, vibration, high and low temperature and other automotive certification, the service life of instability.
Wedge mirror type
-Non-repetitive scanning method, with the increase of scanning time, the field of view coverage can be close to 100%; but its speed is very fast, the device is easy to fatigue damage, short service life, low reliability, and non-repetitive scanning method is not conducive to the fusion of automatic driving algorithm matching.
Mechanical oscillator + rotating mirror type
-In order to reduce the size of LIDAR and achieve high precision scanning of multiple beams, mechanical mirrors are used to achieve vertical angle scanning coverage.
Pure Solid-State Lidar
Lidar internal transceiver and scanning module are not moving, divided into:
3D Flash LIDAR
-Laser is emitted directly to a large detection area, and the distance information corresponding to each pixel is subsequently calculated by the receiver array. No delay in response to external environment, no moving parts, higher stability, more mature solution at the transmitter side, lower cost; easy to pass vehicle specification level inspection
OPA Optical Phased Array LIDAR
-Different emission units emit light waves with specific phase and intensity, forming a high-intensity laser beam with certain directionality through interference in the far field of space. However, the “side flap” will be formed outside the main beam, which affects the detection distance and angular resolution, and the microarray chip is difficult to manufacture.
