Car knowledge small classroom | How to use LiDAR to realize deepness feeling to measure

Car knowledge small classroom | How to use LiDAR to realize deepness feeling to measure

[introduction] the full name of LiDAR is laser of Light Detection And Ranging(is explored reach range finding) , it is a kind of method that uses laser feeling range finding to leave, the time that it can measure laser to come back to use from object reflex and the goal that achieves range finding. According to specific applying, can use different wavelengh, but the most commonly used is infrared ray (IR) .

Most moment, person head is good at concluding the opposite deepness of the object / distance and size, this is a kind of instinct of the mankind, driving especially when car. But it is very difficult however like the system to become accomplish this, especially standard image sensor is to use 2D image to express 3D setting. Two image sensor is used in the stereo layout that is similar to person eye, can collect deepness data, but range finding precision is finite, and can get the influence of ambient light line.


Carry out below the case that use LiDAR wins close reading not to depend on radial condition according to can be in measure, remove the ambiguous issue of image, in accomplishing setting thereby, distinguish reach understanding different object. The smooth pulse that comes back the reflex after launching an object and essence time definitely measure photograph union, can calculate the space that gives an object.

The LiDAR application in car domain is very wide, what be used at SAE level to be L3~L5 especially is semi-automatic drive car, for example, feeling measure the object all round car; Hundreds meters of ahead outside sees on the freeway. LiDAR is commonly used also the application that feels independently at deliver goods robot and other need. The outdoors application scene that this technology applies extensively also at generating the 3D deepness that can handle to pursue quickly with high accuracy — and when use tradition measures a technology, this one process needs to cost several days.

For example, agricultural domain uses LiDAR to measure cropland or land, scale map, assess crop situation, make the farmer can build a model to calculate crop yield thereby, and the pesticide with the rightest choice / fertilizer. To storing the cereal in silo is mixed store the liquid in canister of keep in storage, need to be in only silo / coping of canister of keep in storage installs LiDAR, can measure reserve instantly below the circumstance that does not contact with content content.

Environmental organization often uses LiDAR to assess situation of silvan fell trees, measure the coast to erode a circumstance or monitor glacier subsidise situation. In addition, in these application, through be in nobody drive aircraft (LiDAR of the installation on UAV)/ unmanned aircraft, researcher can undertake survey to unfrequented remote region, and need not head for personally.

Intelligent factory is in guide a car automatically (LiDAR is used on AGV) , carriage raw material undertakes machining, ship finished product hair goods area. When the robot in intelligent factory uses LiDAR, can develop the powerful function of LiDAR oneself adequately, help these robots execute the mission accurately, make they can feel measure all round whether somebody, thereby with considerate work with safe means.

LiDAR can be used at the large construction project such as quick survey railroad or freeway. LiDAR still can regard a kind of safety as auxiliary method, make certain area is avoided suffer needless or accident inbreak. The local sense that this is being put in dangerous material or has large machine job is great. LiDAR can work effectively below all illume condition, in meaning the application in these types, it can be offerred reliable forever on-line protection.

The type of LiDAR

The commonnest LiDAR type is direct flight time (DToF) system, the principle of its backside is very simple: Measure light to pulse reachs a goal and return the time that sensor place uses. Velocity of light is a foregone physical constant, because of this computation emitter / the distance between detector and reflection target is very simple.


Graph 1: DTOF measurable light reachs a goal and return needs time

This technology is used normally by illuminant (the commonnest is laser) those who blast off is individual and special short pulse, blast off to be able to activation at the same time an accurate clock. When smooth pulse hits the object inside limits, it returns reflection the Gao Lingmin that arranges with laser apposition normally to spend smooth sensor. Once be explored,return pulse, clock stops to time, at this moment readable take the time that arrive at an object and returns place to use.

Should know only from send pulse to arrive the time that gets echo course (T) , use velocity of light is invariable (the C) computation distance to target object (D) is very simple.

DToF method is fast and effective, can measure many echo, because this can explore the many objects inside LiDAR eye shot. It can be used at be being mixed remotely short range (application of 0.1 M~300 M) , and stable high accuracy maintains inside whole range.

Method of another kind of LiDAR calls indirect flight time (IToF) , use the successive light wave that comes from laser likewise. This kind of method won’t measure the ToF of the course directly, decide ToF however according to already blasted off and already receiving the phasic difference between weaveform.

IToF technology suits to be measured relative to short distance more (< 10 M) , especially radial condition does not have the has challenge sex so indoor application outdoor, the contrast outdoor should be gotten high normally much. This technology can explore the strongest echo only, because this can explore individual object only.

The 3rd kind of LiDAR is frequency modulation successive wave (FMCW) , suit short range and long-range range finding. This technology is used adjustable humorous laser will generate successive light wave, the light wave that generates will mix in detector place and reflex. This kind mixes can mix in this locality weaveform generation pats frequency between reflection weaveform, calculate from this an object distance and directional speed.

Although FMCW has excellent range finding performance already, still can capture directional rate news, but because want to use,contain a polarization pilot is adjustable humorous laser, and depend on shortwave infra-red wavelengh (requirement laser and detector use special semiconductor) , the total cost of system of this kind of LiDAR raises greatly.


Graph 2: The deepness feeling that is based on LiDAR measures methodological comparing

“Wavelengh great debate “

Around LiDAR one of the most open to question topics are to use which kinds of wavelengh. The visible light of use precede over of IR, should get less because of setting IR much, earning letter a confusion of voices is compared (SNR) is better, make explore thereby it is more easy to return light to become.

There are many appropriate wavelenghs inside IR spectral limits, include close infra-red (NIR) is spectral (850 Nm, 905 Nm, 940 Nm) and shortwave are infra-red (SWIR) is spectral (1350 Nm, 1550 Nm) . The decision is particular using which kinds of wavelengh is ” wavelengh great debate ” crucial topic for discussion. 3 the most important levels need a consideration, it is the systematic function, total cost that whether has appropriate component and system respectively.

Detector is one of the mainest component in any LiDAR systems. CMOS silicon base detector can explore wavelengh to come in 400 Nm the light inside 1000 Nm limits, accordingly its feel to photosensitive of visible light and NIR, but cannot feel measure SWIR light. To explore SWIR light, must use the III/V such as InGaAs alloy a group of things with common features semiconductor, compare with silicon photograph, inGaAs alloy is very costly.

Package usability is another consideration element, especially with respect to laser emitter character. The brim launchs laser (EEL) is launching laser for perpendicular antrum face gradually (VCSEL) place is replaced, latter is enclosed more easily into array, and inside whole temperature limits wavelengh is stabilized. Although VCSEL is current can effect is inferior, the price is higher also, but extend ceaselessly as what they use range, hopeful of this kind of circumstance gets ameliorative.

Although SWIR EEL has much home supplier, but at present SWIR VCSEL has a supplier only, and NIR VCSEL also has much home supplier. Accordingly, choice NIR enhances the security that supplies chain more likely.

Monitoring area per fire detector is very important, because this can increase practicable reaction time, enhance security thereby. But laser can harm an eye too by force, the largest concessional exposure that provided 1ns laser pulse because of this IEC 60825 (MPE) .

Although NIR must have inferior MPE, but if pulse width shortens, can enhance laser power, and because use acute detector, can reach the range finding range of the longest 300 M. When weather is good, the range finding limits of SWIR will exceed NIR, but SWIR gets more easily damp (like rain or mist) adverse effect, because this is based on the function of the system of NIR to drop speed general under SWIR system, can provide more consistent performance below all sorts of weather conditions thereby.

Be based on above place to narrate, think NIR is the first selection wavelengh of car LiDAR normally. NIR makes we can use silicon radical parts of an apparatus, is not the more expensive stuff such as InGaAs, may more important is, relevant component can be obtained from many supplier place, conduce to build more powerful supply catenary. Although NIR and SWIR can ensure when the job person eye is safe, but while NIR is using inferior power laser, still can satisfy the requirement of car LiDAR.

From the point of commercial angle, the cost of NIR wants low much, and element of a cost is car application respect all the time serious consideration. An investigation of IHS Markit (Amsrud, 2019) shows, the every passageway cost of laser and detector is about 4 to 20 dollars, and to similar SWIR system, every passageway cost is 275 dollars about. Although had further progress, capacity to increase, but the cost of predicting NIR will still compare SWIR low 10~100 times.

LiDAR makes a technology

One of element with any LiDAR the most important systems are the feeling that can capture and quantifies reflection laser measures component. Although can use a variety of technologies to come true this, but silicon photoelectricity multiplier (SiPM) is behaved normally best, because,this basically is it can up to is like 1, 000, the tall gain of 000 amounts class will explore individual photon.

Accordingly, the application of SiPM is wider and wider in recent years, become LiDAR deepness feeling to measure first selection sensor of application already. With avalanche photoelectricity diode (the traditional detector such as APD) (not only gain is small much, still need to undertake integral to passing into signal) photograph comparing, these parts of an apparatus can be Sunday run range finding to provide highest SNR performance below tall contrast condition. Other dominant position includes power source to slant buy lower, uniformity is better, and the sensitivity that changes to temperature is reduced etc, make the ideal of the system that SiPM makes use APD upgrades option. SiPM sensitivity is taller, can use small enclose smooth module, because this makes LiDAR easier and compositive,arrive in car. Because SiPM uses craft of high capacity CMOS to produce, the detector cost lowest of these high-powered parts of an apparatus, drove LiDAR further gain ground.

Ansenmei (the ArrayRDM-0112A20-QFN of Onsemi) is a 1 × that has 0.47 Mm X 1.12 Mm SiPM to resemble element 12 sheet piece array, be based on advanced have technology of RDM SiPM CMOS only, it is implementation to smooth to NIR Gao Lingmin is spent and be developed only, can make below 905 Nm wavelengh photon explores efficiency (18.5% what PDE) achieves an industry to precede. Below this wavelengh, responsibility is more than 100 KA/W.

Because the built-in gain of SiPM is tall, sensitivity can fall to odd photon level, plus tall PDE, can explore go out to return signal faintly. This makes LiDAR system OK the low reflectivity target that explores farther space. This array uses stable and reliable 10 Mm X 5.2 Mm QFN to enclose, can visit 12 independence to resemble element.

This array is car LiDAR system only (include flashlight, machinery or MEMS scanning LiDAR) and design, it is the first array that obtains AEC-Q102 car attestation, already undertook developing according to IATF 16949 standard. As a result of this array cost low, performance is high, can implement the long-range LiDAR plan with efficient economy, raise the security of the car and autonomy level.


LiDAR is a technology that having important sense, because its scanning system can decide deepness well and truly quickly, can undertake sheet nods scanning already, OK also the 3D graph of scale object or large field.

When program LiDAR is designed, the key is to should decide to use light-wave of which kinds of IR to grow. Think integratedly function, whether to have appropriate component and commercial factor, NIR is first selection normally.

In process of most LiDAR implementation, laser illuminant may be relatively simple, but the choice of detector has very big effect to systematic performance. Newest SiPM array of Ansenmei has excellent exploration performance, more important is, to car application, it is first SiPM detector that obtains AEC-Q102 attestation.

Bibliographical reference

Amsrud, p. (On September 25, 2019) . The competition that implements system of low cost LiDAR [conference report] . Car LiDAR 2019, the United States rests closely root city Detroit.

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