A 2D LiDAR sensor helps a robot measure how far away walls, furniture, and obstacles are by sending out laser light and timing its return. The sensor usually scans in a flat horizontal plane, creating a slice of the surrounding room. This matters because a robot needs reliable distance data to avoid collisions, follow paths, and understand where it is.
LiDAR is widely used in mobile robots, drones, warehouse vehicles, and autonomous navigation systems.
Inside many 2D LiDAR units, a laser beam is aimed outward while a rotating mirror or spinning sensor head sweeps the beam through many angles. For each direction, the sensor measures the time of flight of the light pulse or uses another optical ranging method to calculate distance. Combining angle and distance gives points in polar coordinates, which can be converted into a map of nearby surfaces.
Navigation software uses this scan data for obstacle detection, localization, and mapping.
Key Facts
- Distance by time of flight: d = cΔt/2, where c is the speed of light and Δt is the round-trip time.
- A 2D LiDAR scan measures distance at many angles in one plane, often 180° to 360° around the robot.
- Polar scan point: x = r cos θ and y = r sin θ, where r is range and θ is scan angle.
- Higher angular resolution means smaller angle steps, so the scan has more detail.
- Scan rate, often measured in Hz, tells how many full sweeps the LiDAR completes each second.
- A robot can build an occupancy grid by marking cells as free, occupied, or unknown based on LiDAR returns.
Vocabulary
- LiDAR
- LiDAR is a sensing method that uses laser light to measure distances to objects.
- Time of flight
- Time of flight is the time it takes a light pulse to travel to an object and return to the sensor.
- Scan angle
- Scan angle is the direction in which the LiDAR beam is pointing during a distance measurement.
- Point cloud
- A point cloud is a collection of measured points that represent the positions of surfaces around the sensor.
- Occupancy grid
- An occupancy grid is a map divided into cells that estimate whether each location is empty, blocked, or unknown.
Common Mistakes to Avoid
- Forgetting to divide by 2 in d = cΔt/2 is wrong because the measured time includes the trip to the object and the trip back.
- Assuming a 2D LiDAR sees above and below itself is wrong because it usually scans only one flat plane unless tilted or moved.
- Treating every missing return as empty space is wrong because dark, shiny, glass, or distant surfaces may fail to reflect enough light back.
- Ignoring the robot's motion during a scan is wrong because a moving robot can distort the scan if measurements from different angles are treated as simultaneous.
Practice Questions
- 1 A LiDAR pulse returns after 40 ns. Using c = 3.0 × 10^8 m/s, how far away is the object?
- 2 A 2D LiDAR scans 360° with an angular resolution of 0.5°. How many distance measurements are collected in one full scan?
- 3 A robot sees a flat wall clearly with LiDAR but has trouble detecting a glass door in the same hallway. Explain why this can happen and how the robot could reduce the risk of collision.