When R&D personnel makes a technical selection of the lidar carried by the robot, they will often find that it is the same mechanical single-line radar. Different models of different manufacturers have huge differences in appearance, performance parameters, and price ranges.
This is mainly due to the fact that mechanical lidars currently on the market use different ranging principles. There are two types of Single-line Lidar ranging: triangular ranging and ToF ranging.
Next, I will introduce the technical principles of the two types of ranging methods in detail to help R&D personnel deepen their understanding and choose a lidar solution that is more suitable for their own scenarios.
Triangular ranging lidar
First, let’s look at the triangular ranging lidar. The basic principle of this ranging method is shown in Figure 1. The radar ranging module emits infrared laser light. After it is incident on the measured object, part of the scattered light is converged on the linear image sensor (CCD/CMOS) through the receiving lens for imaging.
From the geometric relationship in the figure, it can be seen that for objects located at different distances. The position of the spot formed by the outgoing laser on the line array is also different. According to the similarity of triangles, the distance D of the object can be calculated as follows:
The above principle is described as the most simplified situation. In practical applications, in order to improve the distance resolution and make full use of the pixel resources of the linear image sensor, the optical axis of the transmitting light path and the main optical axis of the receiving lens are usually arranged at a certain oblique angle (rather than the parallel relationship in the figure). But the basic principle of similar triangles has not changed.
The principle of triangular ranging determines some technical characteristics of lidar that use this ranging method.
Measurement distance of triangulation lidar
Let’s first look at the measurement distance. For the triangulation lidar, this parameter is closely related to the distance resolution. The so-called range resolution is the ability to distinguish targets at different distances; in other words, when the distance of the target changes, how much change is required to change the range value of the radar output, which is equivalent to using a ruler to measure the length. What is the minimum scale of the ruler? A major feature of the triangulation distance measurement is that the scale of this “ruler” is uneven.
It is easy to know from Figure 1 that for a relatively short distance range, the change of the target distance will cause a significant change in the position of the imaging point; when the target is located far away, even if the distance changes greatly, it will only be reflected in the imaging point. A little bit of movement, that is to say, the range resolution of the triangulation range will decrease rapidly as the distance gets farther. This limits the maximum practical measurement distance of the triangulation distance measurement. After this distance, the decrease in resolution will make the measurement results meaningless.
Measurement rate
The second is the measurement rate. The mechanical single-line lidar measures target in different directions while rotating and scanning. Therefore, the measurement rate directly determines whether the radar can scan at a faster rate, and when completing a scan can output more measurement points.
In order to achieve a certain range resolution, the triangular ranging lidar often uses a higher resolution linear image sensor. And the gray value of these pixels needs to be read out each time the range is measured. And it is handed over to DSP for processing. The entire readout and processing process takes a certain amount of time. It limits the data rate of the triangular ranging lidar.
ToF ranging
After talking about triangulation ranging, let’s look at ToF ranging. ToF is the abbreviation of Time of Flight, that is, the time-of-flight ranging method. Its basic principle is shown in Figure 2:
When starting distance measurement, the pulse drive circuit drives the laser to emit a light pulse with a very short duration but very high instantaneous power, and the timing unit starts timing at the same time.
After the light pulse exits through the emission light path, it reaches the surface of the measured object and scatters in all directions. The receiving optical path of the ranging module receives part of the scattered light energy, which is converted into photocurrent by the optoelectronic device and sent to the echo signal processing circuit;
The echo signal processing circuit converts the photocurrent into a voltage signal, and after one or several stages of amplification and conditioning, an electric pulse corresponding to the echo signal is obtained, which is used to trigger the timing unit to stop timing.
At this time, the time interval recorded by the timing unit represents the total time taken by the laser pulse from emission to return. Use this time value to multiply by the speed of light and divide by 2 to get the distance between the ranging unit and the measured target.
Principle of ToF ranging
The principle of ToF ranging is easy to understand. But there are many technical difficulties that need to be resolved in the implementation of specific projects. This is mainly due to the extreme working conditions of ToF ranging, which puts forward high requirements on the drive capability, bandwidth, and noise suppression level of the entire circuit. But its advantage is that as long as these problems are solved, the entire ranging system can achieve very high performance.
Because ToF ranging emits laser pulses with a very short duration, the instantaneous power of the light pulse can be raised to a very high level under the premise of meeting the safety requirements of the human eye, so that it can be detected A longer-distance goal.
Different from the triangulation, ToF calculates the target distance by measuring the flight time of the light pulse. And the timing accuracy will not change due to the longer distance. So in the entire range, the ToF ranging resolution will not change substantially. Finally, ToF ranging processes are all high-speed pulse signals. The advantage is that the entire measurement process takes a short time and can achieve very high measurement frequencies.
Which type of single-line lidar is better?
We talked about the working principles of triangulation ranging and ToF ranging. The next natural question is, which type of single-line lidar has more advantages? The answer is to look at the specific usage scenarios according to the characteristics of each type of radar.
The advantage of the triangular ranging lidar is mainly reflected in the cost. Because of its mature design, the cost can be reduced to a very low level in mass production. But because it is not stable in actual use, it is applied in the industrial field. Subject to great restrictions.
The system design of ToF ranging lidar is more complicated than triangular ranging radar. So the cost will be higher, but the performance improvement brought by it is also obvious. There are currently many teams working on ToF solutions. They can be maintained based on the industrial-grade stability and high performance of ToF ranging. The cost of the whole machine is at a lower level. It has the potential to replace the application of triangular ranging lidar in the consumer field.
At present, ToF ranging lidar is widely used in service robots, AGV/AMR, and low-speed logistics vehicles. We can see ranging lidar in some occasions, such as industrial security protection, large screen interaction, and other fields.
