Principle and Application Analysis of Lidar Sensors

Laser radar can help robots quickly obtain information about their surroundings, and has the advantages of wide detection range, high precision, strong anti-interference ability, etc. It is an important part of a series of ground autonomous mobile robots, such as autonomous vehicle, sweeping robots, storage robots, etc. However, currently industrial grade LiDARs are often expensive. For unmanned vehicles like those made by Google and Baidu, the cost of their LiDARs even exceeds the value of the vehicle itself, making ordinary people hesitant. Even a single line LiDAR with a detection range of only 25m has been sold for thousands of yuan on a certain platform.



Effect demonstration

Let's first take a look at the parameters of the LiDAR after the project is completed. This may be the most concerning issue for everyone, after all, setting aside accuracy to negotiate price is just playing tricks

Scanning speed: 15 times per second
Resolution: Approximately 1% of the detection distance, but not better than ± 2 cm
Minimum detection distance: 5 cm
Maximum detection distance: approximately 25 meters (white reflection plane)
Angular resolution: 0.5 degrees
Scanning frequency: approximately 11 kHz
Scanning angle range: approximately 230 degrees, there is still room for improvement by changing the corresponding structure
Power: 5 V/0.1 A (i.e. 0.5 watts), the voltage may exceed 0.8A at the beginning of startup
Size: 50x50x120 mm
The following figure shows the CAD model and finished product drawing of the LiDAR.
The detection effect in the author's apartment is shown in the following figure
Principle and Application Analysis of Lidar Sensors
Principle and Application Analysis of Lidar Sensors
The project author stated that the total cost of producing the large components of the LiDAR is only about $114, and the optical lenses used for the laser transmitter and receiver are both standard lenses for surveillance cameras. Have you seen this and can't help but want to try it out on your own? Below, we will explain the production process one by one.
The principle and manufacturing process of LiDAR ranging
The LiDAR adopts the TOF (Time of Flight) ranging principle. As the subsequent lens selection, installation, debugging, and calibration all involve knowledge of TOF principles, we will briefly introduce it here.


Distance measurement principle

As shown in the figure below, the laser emits a series of light pulses, which are captured by the photosensitive element after being reflected by an object. The electrical signal generated by the photosensitive element is amplified, and the rising or falling edge is captured by an operational circuit to calculate the flight time of the light pulse in the air. Combined with the speed of light, the distance between the measured object and the LiDAR can be obtained.
Principle and Application Analysis of Lidar Sensors
Principle and Application Analysis of Lidar Sensors
Schematic diagram of TOP ranging principle (Image source: Wikipedia)
We can see from the principle of TOP ranging that the following factors determine the measurement performance of LiDAR:
1. The color of objects in the environment and their reflective surfaces
Due to the fact that TOP ranging works by calculating the time difference between the emission and reception of light pulse signals, if the emitted light pulses are absorbed by objects in the environment or reflected outside the measurement range of photosensitive components, the LiDAR cannot collect correct measurement data. Taking an extreme example here, stealth fighter jets achieve stealth by using coatings that can absorb radar and a smaller radar reflection area.
Of course, as most indoor walls are predominantly white or light colored, there is no need to worry. It should be noted that when there are mirrors or objects with darker colors in the environment, the measurement accuracy of LiDAR will be greatly affected.

2. The relative position between the laser and the photosensitive element and its lens focal length
To effectively capture the pulse signal emitted by the laser, the photosensitive element needs to be placed at a suitable distance from the laser. And it is necessary to install lenses with appropriate focal lengths for the laser and photosensitive components according to measurement requirements. The following image shows the installation of the LiDAR lens in this project.
3. Laser signal generator pulse width, amplified current, and operational unit resolution
Another crucial factor affecting sensor performance is the pulse width of the laser signal transmitter, which determines the maximum distance that the LiDAR can measure. Assuming the speed of light is c=300000000 m/s, the farthest measurement distance is 7.5 m when the width of the light pulse is 50 ns.
Due to the need for amplification processing of the electrical signals generated by photosensitive components, key indicators such as gain, bandwidth, noise, power suppression ratio, common mode suppression ratio, linearity, and output impedance of the corresponding amplification components also need to be considered during the manufacturing process. Finally, the resolution of the computing unit determines whether the sensor can accurately calculate the time difference between laser pulses traveling back and forth.


Production process

Firstly, the production of laser pulse circuits. The author chose the relatively inexpensive but powerful pulse laser diode SPL PL903, which operates at a wavelength of 905 nm and can generate laser pulses with a power of up to 75 watts. The corresponding driving circuit diagram is shown in the bottom left. In the detection circuit section, the author selected avalanche photodiodes (APDs) as photodetectors and designed DC-DC converters to generate a sufficiently high bias voltage. Laser pulse detection requires an operational amplifier with a large signal bandwidth and low input capacitance, so the author chose MAX3658 as the operational amplifier. This chip is specifically designed for photodiodes, with an amplification factor of 18000 and a bandwidth of 580 MHz. In addition, the chip also includes a built-in filter that can reduce low-frequency noise. The detection circuit diagram is shown in the figure:
Principle and Application Analysis of Lidar Sensors
Principle and Application Analysis of Lidar Sensors
Principle and Application Analysis of Lidar Sensors
Principle and Application Analysis of Lidar Sensors
Pulse laser driving circuit diagram; Pulse detection unit circuit diagram.

In the time node measurement section, the author selected the ready-made chip TDC-GP21 for ultrasonic liquid flow meters. The time resolution of this microcircuit is around 90 ps, and the MCU can control it through the SPI bus. The author chose STM32F303CBT6 as the microcontroller to achieve the transmission and reception of laser pulses, reading and calibration of measurement data, control of mirror motor speed, and communication with the upper computer. The completed PCB is shown in the following figure.

Front schematic diagram+back schematic diagram.

In terms of optical components, a standard M12 lens is selected as the laser lens with a divergence angle of approximately 0.45 degrees. And a lens with a focal length of 25 mm is selected for the photosensitive element, which has a large aperture and can allow as much reflected light to fall on the photodiode as possible to achieve a higher signal-to-noise ratio. If using LiDAR under strong light conditions (such as outdoors), the author suggests installing an interference filter specifically designed for the 905 nm wavelength between the photodiode and lens. As this project was tested under indoor lighting conditions, it was not installed.
There are two ways for rotary LiDAR to detect the surrounding environment, which are to rotate the entire rangefinder or to scan by rotating a mirror tilted 45 degrees relative to the LiDAR optical axis. The corresponding working mode is shown in the following figure.
Principle and Application Analysis of Lidar Sensors
Principle and Application Analysis of Lidar Sensors
Principle and Application Analysis of Lidar Sensors
Principle and Application Analysis of Lidar Sensors
These two methods have their own advantages and disadvantages. As the second method only requires rotating the reflector, while the first method needs to consider the power supply and communication issues of the rangefinder during rotation, the scanning method that only rotates the reflector is adopted here. After installing the reflector and driving motor, the entire hardware of the LiDAR has been completed. Now you can use the upper computer software provided by the project author for debugging and measurement.

Principle and Application Analysis of Lidar Sensors
Principle and Application Analysis of Lidar Sensors
Schematic diagram of upper computer software.
The use of LiDAR in SLAM can be said to be a relatively mature technology nowadays, which has advantages such as high accuracy, fast speed, and strong anti-interference ability. However, due to its high cost, its popularity in the field of civilian unmanned driving is currently limited. Even the Tesla Model S, priced at over 1 million RMB, still uses cameras as the main detection solution. Perhaps such low-cost open-source projects for LiDAR can provide us with convenience in understanding this technology.