Shulou(Shulou.com)11/24 Report--

Let the editor guess.

There must be friends like editors.

I like to bow my head and play with my cell phone on the road.

And then accidentally

.

The editor ran home crying

Come on, do A Dream.

And then...

Duo La A Meng took out

Great. The editor is going to guide the angels out to play with secret props.

Duo La A Meng said

Hey, wait a minute.

Do you know how the guiding angel protects you?

What you're holding in your hand is actually ~

Lidar lidar is the product of the combination of traditional radar and laser technology. It is an active radar which uses laser beam as information carrier and can carry information with phase, frequency, polarization and amplitude.

Source of lidar: because the laser has the characteristics of high brightness, high directivity, high monochromaticity and high coherence, Baidu lidar can accurately measure distance, measure speed and track, as well as high angular resolution, velocity resolution and range resolution. it can also produce echo signals for smaller-scale targets, which has a unique advantage in detecting fine particles.

It turns out that it is equipped with lidar, so how does it work?

The trigonometric ranging laser emits a laser and reflects back to the detector (CCD) after it encounters an object. There is a fixed distance between the laser and the detector. For objects with different distances and proximity, the reflected light will fall on different positions of the detector. Through the trigonometric function relationship between the position of the reflected spot and the distance of the object, the position information of the object can be obtained.

Source of triangulation schematic diagram: reference 1 according to the schematic diagram, it is known that the length of the baseline s, the angle between the Laser optical axis of the transmitter and β, the distance between the observation object Object and the baseline Q, the focal length of the lens and the CCD chip is o, and the projection point of the reflected light axis on the CCD is x. According to the similarity of triangles:

According to the trigonometric function relationship:

So as to get the distance d of the object.

The time-flying TOF laser emits a laser pulse at T1 time, which is reflected by the object and received by the receiver at T2 time. In general, the speed of light in the air is basically constant. Through the laser beam flight time t1-t2, and the speed of light c, the distance d of the object can be obtained:

Source of TOF schematic diagram: Zhihu

The coherent detection method takes the frequency modulated continuous wave FMCW as an example. The laser emits a continuous signal with a certain bandwidth and a linear change in frequency. The time difference between the two signals is calculated by the frequency difference between the transmitted signal and the received signal. Finally, the corresponding distance value is obtained from the time difference. If the detected object is moving, the reflected signal will have frequency or wavelength change, and the velocity of the detected object can be obtained by Doppler effect.

Coherent detection schematic source: Zhihu, so what is the Doppler effect?

When the vibration wave source moves relative to the observer, the actual frequency received by the observer changes relative to the actual frequency f of the wave source.

Source of Doppler effect: the actual frequency f' received by Baidu objects is:

V is the traveling velocity of the wave in the medium.

V0 is the speed at which the observer moves relative to the medium. If it is close to the emission source, the arithmetic symbol ahead is the + sign, and vice versa, the − sign.

Vs is the speed at which the emission source moves relative to the medium. If it is close to the observer, the forward operation symbol is the − sign, and vice versa, the + sign.

If the wave source emits electromagnetic waves, the received frequency f'is:

Where v is the relative velocity between the wave source and the receiver. When the wave source is close to the observer, v is positive, which is called "purple shift" or "blue shift"; otherwise, v is negative, it is called "redshift".

Are you confused when you look at the formula? At this time, you just need to take out your phone, play a piece of music, and then shake it quickly, and you can feel the changing music sound due to the Doppler effect.

Ahem, back to the point. As you can see from the picture below, the laser emits a beam of continuous signal ft

Source of coherent detection schematic diagram: Zhihu, when the radar (FMCW Rader) and the object (car) are relatively stationary, the signal fr reflected back to the detector delays td in time.

It can be obtained from the geometric relations in the graph (fb is difference frequency)

The distance between the radar and the object is

Source of laser signal when detecting stationary objects: reference 6 when the continuous signal transmitted is a definite waveform, the position information of the object can be obtained by the differential frequency signal fb.

Laser signal when detecting moving object (top); beat signal (bottom) Source: reference 6 when the object moves relative to the radar, the frequency of the signal reflected back to the detector changes due to the Doppler effect, and the Doppler shift is (v).

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