Secret of the principle of single-line lidar: Triangular ranging VS ToF ranging

When R&D personnel make 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, which are prone to confusion, and they are not sure which one More suitable for your own usage scenarios.

When R&D personnel make 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, which are prone to confusion, and they are not sure which one More suitable for your own usage scenarios.

This is mainly due to the fact that mechanical lidars currently on the market use different ranging principles, which can be divided into two types: triangular ranging and ToF ranging, which makes the overall design of the radar vary greatly in size, performance, and cost.

Next, we 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. Let’s first 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, and after 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.

It can be seen from the geometric relationship in the figure that for objects located at different distances, the position of the spot formed by the emitted laser on the line array is also different; on the other hand, the internal structure of the ranging module is fixed, the focal length f of the receiving lens, and The two parameters of the offset (ie baseline distance) L between the optical axis of the transmitting optical path and the main optical axis of the receiving lens are known. According to the similarity of the triangles, the distance D of the object can be calculated as follows:

Secret of the principle of single-line lidar: Triangular ranging VS ToF ranging

Secret of the principle of single-line lidar: Triangular ranging VS ToF ranging
Figure 1 Basic principle of triangulation ranging

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 (not the parallel relationship in the figure) , But the basic principle of similar triangles has not changed.

The principle of triangulation ranging determines some technical characteristics of lidar that uses this ranging method.

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 at a far distance, 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 is exceeded, the decrease in resolution will make the measurement results meaningless.

The second is the measurement rate. The mechanical single-line lidar measures targets in different directions while rotating and scanning, so the measurement rate directly determines whether the radar can scan at a faster rate (corresponding to a higher frame rate), and when it completes a scan Can output more measurement points (corresponding to higher angular resolution).

In order to achieve a certain range resolution, the triangular ranging lidar often uses a higher resolution linear image sensor, usually with thousands of pixels. 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, which limits the data rate of the triangulation ranging lidar.

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 object to be measured and scatters in all directions. The receiving optical path of the ranging module receives part of the scattered light energy, converts it into photocurrent through the photoelectric device, and sends it 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 time taken by the laser pulse from emission to return. Use this time value to multiply by the speed of light and divide by two to get the distance between the ranging unit and the measured target. value.

Secret of the principle of single-line lidar: Triangular ranging VS ToF ranging
Figure 2 Basic 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 are very large (peak emission peak) and very weak (pulse echo) And very fast (nanosecond/picosecond level) signal, 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.

From the measurement distance point of view, because ToF ranging emits laser pulses with a very short duration, the instantaneous power of the light pulse can be raised to a 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 measurement principle of triangulation based on geometric similarity, ToF calculates the target distance by measuring the flight time of the light pulse, and the timing accuracy will not change as the distance becomes longer, so that in the entire range, the ToF ranging The range resolution will not change substantially. Finally, ToF ranging processes are all high-speed pulse signals. The benefit from this is that the entire measurement process takes very short time and can easily achieve very high measurement frequencies.

After talking 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 has been applied in the industrial field. Subject to great restrictions.

In addition, because the triangulation distance measurement uses parallel-axis optical paths arranged side by side, the appearance of the radar can be made relatively low and can be used in occasions where the body height is limited; these advantages, combined with the high accuracy of the triangulation distance measurement in close range, This makes the triangular ranging lidar very suitable for use in consumer products. The sweeping robot, which has become popular in recent years, is a good example. As long as it is a model with navigation function, the triangular ranging lidar is almost exclusively used as the main sensor. plan. For service robot products, when the activity scene is not large, or when the blindness and obstacle avoidance need to be filled at close range, the triangular ranging lidar also has application cases.

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 this is also obvious. There are currently many teams working on ToF solutions that can be maintained Based on the industrial-grade stability and high performance of ToF ranging, the cost of the whole machine is reduced to a lower level, which has the potential to replace the application of triangular ranging lidar in the consumer field.

The mainstream ToF single-line lidar products on the market usually have a maximum measurement distance of 10 meters (for 70% reflectivity targets), a data rate of 15 KHz or higher, and scanning frequencies ranging from 15 Hz to 40 Hz, and many All models can be used outdoors, so they are more adaptable to the environment. They are very suitable for mobile platforms that have large activity spaces, high moving speeds, and need to work in strong ambient light (such as outdoors). Currently, ToF ranging lidar is widely used in service robots, AGV/AMR, and low-speed logistics vehicles. In addition, in some static installation occasions, such as industrial safety protection, large screen interaction, security monitoring and other fields, the use of ToF ranging lidar can often be seen.

The Links:   A056DN01-V1 2MI50F-050

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