1. Working Principle

1.1 Scanning Principle

The working principle of a single-line LiDAR is that the emitter rotates at a constant speed inside the LiDAR. It emits a laser every small rotation angle (referred to as angular resolution), and a complete frame of data is generated after sweeping a certain angle. However, this also means that a single-line LiDAR can only recognize a row of dots at the same height, and can only describe linear information, but not surfaces and heights. The following image shows the principle of scanning:

Xian Gong Intelligent defines that the front of the LiDAR is at a 0° angle. If the LiDAR is 270°, the effective measurement angle is from -135° to 135°. If the LiDAR is 360°, the effective measurement angle is from -180° to 180°. LiDAR generally rotates counterclockwise.

From the description and schematic diagram above, several definitions need to be described:

1.2 Ranging Principle

As shown in the figure above, the ranging principle of a LiDAR is to calculate the distance to an obstacle by calculating the time between the emission pulse and the reception pulse and the propagation speed of the pulse through a timer.
When the laser beam hits most surfaces, it will cause the radar to receive a return signal in the form of diffuse reflection in all directions. The stronger the reflective ability, the easier it is for the radar to receive the return signal. The reflection characteristics of lasers change with the changes of surface material, structure, and color. Compared to surfaces with low reflectivity, surfaces with high reflectivity can be better detected. The following lists the reflectivity of several common objects. Among them, 3M reflective stickers are often used in actual scenes, such as recognition drilling shelves, charger recognition, and even reflective column positioning. For specific principles and usage, please refer to the recognition chapter.

Kinco Intelligence sets the intensity value of the object between 0-255. By setting a threshold, a scan point with an intensity value greater than the threshold is called a high reflection point.
The minimum effective scan distance of the LiDAR can be set by modifying the following parameters:

2. Performance parameters of Lidar

Xingong Intelligent provides a variety of brands and models of single-line Lidar for customers to choose to use, different lidar performance parameters are slightly different, the following is Xingong Intelligent has been adapted to the lidar model and the corresponding performance parameters:

III. Function and Communication

3.1 Positioning Function

When using single-line laser radars, Xian Gong Intelligent classifies them into two categories: obstacle avoidance lasers and navigation lasers. Navigation lasers take into account obstacle avoidance, recognition, and positioning, which require higher product performance, such as angular resolution, maximum measuring distance, frequency, and jitter.

3.2 Ethernet Port

Xian Gong Intelligent selects Ethernet as the interface when choosing navigation lidar, which can realize real-time and high-speed transmission of a large amount of data, and the Ethernet port is not easily loosened.

3.3 Serial Interface

For some Obstacle avoidance laser radars, such as Sick's Tim series, the serial interface is used for communication between the controller and the laser radar. This type of laser radar usually does not require the transmission of raw point cloud data to the controller, as the laser radar can plan a safe area internally, and when there are obstacles within the safe area, it only needs to report the signal of the presence of obstacles.

4. Point Cloud Deduction Function

Xian Gong Intelligent also provides a function to deduct point clouds based on angles. In the configuration list of laser radars that support this function, you can see the following parameters:

5. External Parameters of the Lidar

XianGong Intelligent stipulates that the orthogonal coordinate system of the lidar follows the right-hand rule, with the front being the x-axis and the left being the y-axis.

The position of obstacles scanned by the lidar is obtained with the lidar as the center of the coordinate system. It is necessary to transform the coordinates of these obstacles to the robot base coordinate system. In this case, it is necessary to know the installation position of the lidar in the robot base to obtain the relative position of the lidar coordinate system and the robot base coordinate system, that is, the external parameters of the lidar.
XianGong Intelligent can obtain the external parameters of the lidar through automated calibration. Please refer to the calibration chapter for the principles of this part. For single-line lidars, only three parameters are required for external parameters:

VI. Parameter Configuration Instructions

6.1 Dragging Tail Point Cloud Filtering

By understanding the scanning principle of LiDAR, we know that the LiDAR transmitter emits a pulse, which strikes an object and returns. The receiver receives the echo and calculates the time difference between the two, and measures the distance between objects by multiplying the speed of light. Ideally, the pulse that strikes the object's surface is a perfect light spot, but as the actual pulse has a certain divergence angle, it forms an area on the object. As the distance increases, this area becomes larger. In this case, there is a possibility that when there are two objects in front and behind, and the LiDAR pulse strikes the edge of the front object, some of the laser pulses may strike the back object, causing the "dragging tail" problem of LiDAR.

The principle of the dragging tail elimination algorithm is shown in the following figure:

In the figure, p1 and p2 represent two adjacent points in a frame. Based on the cause of dragging tail formation, we judge the angle a to determine whether a is within the threshold. Points outside the threshold are considered as dragging tail points and removed. If a is greater than the set angle threshold (ShadowMaxAngle) or smaller than ShadowMinAngle, the laser points within the nearby neighborhood range (ShadowNeighbours) are considered invalid points.

7. Error Codes

Xian Gong Intelligent defined some error codes when using the laser radar in order to quickly locate problems in the event of issues during use:
When the laser radar is not connected, a connection failure error will be triggered, as follows:

When the laser radar cannot receive data, a disconnection error will be triggered, as follows:

When the Oulei laser radar has an internal error, an internal error error will be triggered, as follows:

When the sick nano laser radar has an internal error, an internal error error will be triggered, as follows:

When the sick nano laser radar has a parameter configuration error, a configuration parameter error will be triggered, as follows:

When the sick nano laser radar has a port configuration duplication, a port duplication error will be triggered, as follows:

8. FAQ