Solid-State vs Mechanical LiDAR: Scanning Methods for Robots

How a LiDAR measures distance

A LiDAR sends pulses or a modulated beam of laser light and measures the light that returns from surfaces. Most robot LiDAR uses time of flight: the sensor times how long a pulse takes to return and converts it to distance. Some designs use frequency-modulated continuous wave (FMCW) measurement, which can also measure the radial speed of each point. The scanning method decides where the beams point, how many points are produced and how the field of view is covered.

Mechanical spinning LiDAR

A mechanical LiDAR mounts several laser emitters and detectors on a head that rotates, typically several to twenty times per second. Each emitter traces a ring around the sensor, so a 32- or 64-channel unit produces 32 or 64 rings stacked vertically. The result is a full 360° horizontal field of view from one sensor, which is why spinning units are widely used on mobile robots, autonomous vehicles and mapping systems.

Single-channel 2D spinning LiDAR, which measures one plane, remains common for indoor navigation and obstacle avoidance on service robots and AMRs because it is compact and inexpensive.

Semi-solid-state LiDAR

Semi-solid-state designs keep a small moving element but no large rotating head:

MEMS mirror scanning. A tiny mirror on a silicon chip oscillates to steer the beam across the field of view. The mechanism is compact, but the field of view is limited by the mirror's deflection.

Rotating prisms or polygon mirrors. Optical elements rotate inside a fixed housing to steer the beam. Some designs use pairs of prisms to draw a non-repetitive pattern, so coverage of the field of view increases the longer the sensor integrates. These units often provide hemispherical or wide forward coverage.

Solid-state LiDAR

Flash LiDAR illuminates the whole scene at once and captures returns on a detector array, like a camera. It has no moving parts but its range is limited by how much laser power can be spread over the field of view.

Optical phased arrays steer the beam electronically by controlling the phase of light across many emitters. They promise very compact sensors, but few are in volume use.

Electronic scanning with addressable arrays fires individual emitters and reads matching detector pixels in sequence, scanning the scene without moving parts.

Wavelength and eye safety

Most robot LiDAR operates near 905 nm, where silicon detectors are inexpensive. Some long-range units use about 1550 nm, which permits higher pulse energy within eye-safety limits but needs more expensive detectors. Sensors sold for use around people are normally specified as Class 1 laser products under IEC 60825-1. Ask for the laser classification on the datasheet.

What matters on a robot

Field of view and blind zone. Check the vertical field of view and the minimum detection distance. A long-range unit may not see obstacles close to the robot's base, so mobile robots often combine a main LiDAR with short-range sensors.

Point density where you need it. Channel count alone does not describe density. Compare the angular resolution in the region of interest, and whether the scan pattern repeats or fills in over time.

Reflectivity and sunlight. Range is quoted at a stated target reflectivity. Dark targets, glass and bright sunlight reduce effective range; compare figures at the same reflectivity.

Vibration and environment. Legged robots and outdoor vehicles subject sensors to shock and vibration. Check the operating temperature, IP rating and shock and vibration specifications.

Data and synchronisation. Check the Ethernet interface, point rate, time synchronisation support such as PTP, and driver support for your software stack.

Sources

• Livox Mid-360 specifications
• Hesai product information
• RoboSense product information