What are the advantages and disadvantages? Triangulation and TOF lidar analysis!
As the core sensor of many smart devices, lidar has been widely used. Today, we can frequently see lidar in unmanned vehicles, service robots, AGV forklifts, intelligent road administration and transportation, and automated production lines, which is enough to show its indispensable position in the artificial intelligence industry chain.
As far as the mainstream lidar products on the market are concerned, the radars used for environmental detection and map construction can be roughly divided into two categories according to technical routes. One is TOF (Time of Flight) radar and the other is It is a triangular ranging radar. I believe many people are familiar with these two terms, but if you want to say whether these two solutions are better or worse in terms of principle, performance, cost, and application, as well as the reasons behind them, maybe everyone has more or less. Doubted. Today, the editor will give some suggestions and analyze these issues.
1. Principle
The principle of the triangulation method is shown in the figure below. The laser emits laser light. After irradiating the object, the reflected light is received by the linear CCD. Since the laser and the detector are separated by a certain distance, objects at different distances will be imaged on the CCD according to the optical path. Different locations. Calculating according to the trigonometric formula, the distance of the measured object can be derived.
Just look at the principle, do you think it's quite simple?
Figure 1. The principle of triangulation ranging
However, the principle of TOF is simpler. As shown in Figure 2, the laser emits a laser pulse, and the time of emission is recorded by the timer, the return light is received by the receiver, and the return time is recorded by the timer. The two times are subtracted to get the "time of flight" of light, and the speed of light is constant, so the distance can be easily calculated after the speed and time are known.
Figure 2. TOF ranging principle
It is a pity that if everything is as simple as remembering, the world will be wonderful. These two schemes will have their own challenges in the specific implementation, but in comparison, TOF has obviously more difficulties to overcome.
The main difficulties in the realization of TOF radar are:
The first is the timing issue. In the TOF scheme, the distance measurement depends on the time measurement. But the speed of light is too fast, so to obtain a precise distance, the requirements for the timing system become very high. One piece of data is that the lidar needs to measure a distance of 1cm, and the corresponding time span is about 65ps. Students who are a little familiar with electrical characteristics should know what this means behind the circuit system.
The second is the processing of pulse signals. There are two parts:
One is laser: In the delta radar, there is almost no requirement for laser driving. Because the measurement depends on the position of the laser echo, only one continuous light is required to emit. But TOF does not work. Not only does it require a pulsed laser, but the quality is not too bad. At present, the pulse width of the outgoing light of TOF radar is about a few nanoseconds, and the rising edge is required to be as fast as possible. Therefore, the laser driving scheme of each product There are also high and low points.
The other is the receiver. Generally speaking, the echo time identification is actually the time identification of the rising edge. Therefore, when processing the echo signal, it is necessary to ensure that the signal is not distorted as much as possible. In addition, even if the signal is not distorted, since the echo signal cannot be an ideal square wave, the measurement of different objects at the same distance will also cause the front edge to change. For example, the measurement of white paper and black paper at the same position may obtain two echo signals as shown in the figure below, and the time measurement system must measure that the two front edges are at the same time (because the distance is the same distance), which requires Special treatment.
Figure 3. The difference of echo signals with different reflectivity
In addition, the receiving end also faces problems such as signal saturation and noise floor processing, which can be said to be difficult.
Second, the performance PK, knowing the reason, knowing why?
Having said that, in fact, from the point of view of downstream users, you don't care whether it is easy or difficult to implement. Users care about nothing more than two points: performance and price. Let me talk about performance first. If most people who know this industry know that TOF radar is better than triangle radar in performance. But what are the specific aspects and the reasons behind it?
Measuring distance
In principle, TOF radar can measure longer distances. In fact, in some occasions that require distance measurement, such as driverless car applications, almost all TOF radars. There are several reasons why the triangulation radar cannot measure far. First, it is limited in principle. In fact, it is not difficult to observe Figure 1 carefully. The farther the object measured by the triangulation radar, the smaller the position difference on the CCD. After exceeding a certain distance, the CCD can hardly distinguish. The second is that the triangular radar cannot obtain a higher signal-to-noise ratio like the TOF radar. TOF uses pulsed laser sampling and can strictly control the field of view to reduce the impact of ambient light. These are the prerequisites for long-distance measurement.
Of course, the distance does not mean absolute quality, it depends on the specific usage scenarios.
Sampling Rate
When Lidar depicts the environment, it outputs a point cloud image. The number of point cloud measurements that can be completed per second is the sampling rate. In the case of a fixed speed, the sampling rate determines the number of point clouds in each frame of image and the angular resolution of the point clouds. The higher the angular resolution and the greater the number of point clouds, the more detailed the image depicts the surrounding environment.
As far as the products on the market are concerned, the sampling rate of the triangulation radar is generally below 20k, while the TOF radar can achieve higher (for example, the star-second TOF radar PAVO can reach a sampling rate of up to 100k). The reason is that TOF only needs one light pulse to complete a measurement, and real-time time analysis can also respond quickly. But three
To
The calculation process required for angular radar is longer.
Figure 4. Imaging effects of different sampling rates for objects at the same location
(A): Low sampling rate point cloud pattern; (B): High sampling rate point cloud pattern (PAVO)
Precision
Lidar is essentially a distance measuring device, so distance measurement accuracy is undoubtedly the core indicator. At this point, the accuracy of trigonometry is very high at close distances, but as the distance gets farther and farther, the accuracy of its measurement will become worse and worse. This is because the measurement of trigonometry is related to the angle, and with the distance Increase, the angle difference will become smaller and smaller. Therefore, the triangular radar usually uses percentage marking when marking accuracy (common such as 1%), then the maximum error at a distance of 20m is 20cm. The TOF radar relies on flight time, and the time measurement accuracy does not change significantly with the increase in length. Therefore, most TOF radars can maintain an accuracy of several centimeters within a measurement range of tens of meters.
Speed (frame rate)
In mechanical radar, the image frame rate is determined by the speed of the motor. As far as the two-dimensional lidar currently on the market is concerned, the maximum speed of the triangular radar is usually below 20Hz, while the TOF radar can achieve about 30Hz-50Hz. Generally, the triangular radar usually adopts the structure of upper and lower split bodies, that is, the upper part is responsible for laser emission, reception and collection, and the lower part is responsible for motor driving and power supply. The excessively heavy motion components limit higher speed. The TOF radar usually adopts an integrated semi-solid structure, and the motor only needs to drive the mirror, so the power consumption of the motor is small, and the speed that can be supported is also higher.
Of course, the difference in speed mentioned here is just an objective analysis of existing products. In fact, there is no essential connection between the speed and the radar adopting TOF or the triangulation method. The mainstream multi-line TOF radar also adopts the upper and lower split structure. After all, the optical design of the coaxial structure is subject to many restrictions. The speed of multi-line TOF radar is generally below 20 Hz.
However, high speed (or high frame rate) is meaningful for point cloud imaging. The high frame rate is more conducive to capturing high-speed moving objects, such as vehicles driving on highways. In addition, when building a map by itself, the moving radar map will be distorted (for example, if a stationary radar scans a circle as a circle, then when the radar moves in a straight line, the scanned image becomes an ellipse ). Obviously, high speed can better reduce the influence of this kind of distortion.
3. Cost
If you only look at the performance comparison, it seems that the performance of the TOF radar completely overwhelms the triangular radar. However, product competition is not just a competition of performance parameters, users also care about price, stability and service.
At least in terms of cost, the current cost of triangular radar is lower than that of TOF radar, and the cost of short-range triangular radar is already at the level of 100 yuan. At present, the price of imported TOF radar is more than 10,000 yuan. It can be said that the high price is an important factor restricting the further expansion of TOF lidar applications.
However, with the rise of domestic TOF radar manufacturers in recent years, the cost of TOF radar has been greatly reduced. Compared with imported brands, the price of domestic TOF radar products has become quite competitive. In the future, with the improvement of production technology and the further increase of shipments, it is believed that the cost of TOF radar will be further compressed, and it is not impossible to drop to a level similar to that of triangular radar.
Four, application scenarios
The scene of the triangular radar is mainly used in indoor short-distance applications, and the most typical scene is the sweeping robot. In scenes with a large detection range (such as shopping malls, airports or stations), as well as outdoor scenes, TOF is more widely used. In addition, it is worth mentioning that the delta radar, which is exposed and rotating, makes its products very fragile in terms of dust and water resistance. In some special applications, such as the workshop where the AVG car works, there is often a lot of dust. In the environment, the motor of the triangle radar is very easy to damage. In contrast, the semi-solid design adopted by the TOF radar can provide better protection and a longer working life.
Figure 5. Star-second TOF lidar
At present, the domestic TOF radar is developing rapidly. The 2D safety laser scanner launched by cyndar can reach a measurement distance of 20m, a point cloud rate of 100kHz, a maximum angular resolution of 0.036°, and an IP65 protection level. Its application has been involved Unmanned driving, robotics, AGV, security, road administration and many other fields are excellent representatives of China's TOF radar.
