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Technology of Laser Sensors for Distance Measurement

Laser di­stan­ce sen­sors mea­su­re po­si­tions and di­stan­ces con­tac­tles­sly with laser light. They are pre­ci­se and can be used over long di­stan­ces, as well as in close range. These sen­sors are ideal for pre­ci­se po­si­tion and di­stan­ce mea­su­re­ment or for de­tec­ting ob­jec­ts re­gard­less of color and sur­fa­ce.

How Do Laser Di­stan­ce Sen­sors Work?

Laser sen­sors are pho­toe­lec­tro­nic sen­sors and, thanks to their con­tac­tless mea­su­ring prin­ci­ple and high ac­cu­ra­cy, are well-​suited to ob­ject de­tec­tion, path, po­si­tion and di­stan­ce mea­su­re­ment. wen­glor laser di­stan­ce sen­sors work ac­cor­ding to the tran­sit time mea­su­re­ment prin­ci­ple and use laser trian­gu­la­tion. In both pro­ce­du­res, di­stan­ces are mea­su­red with laser light and out­put as a di­stan­ce value.


          

When to Use a Trian­gu­la­tion Sen­sor and When to Use a Tran­sit Time Sen­sor

Triangulation principle display

Trian­gu­la­tion Sen­sors for Short Di­stan­ces

Pre­ci­se de­ter­mi­na­tion of di­stan­ces in the close range up to 1 m
De­tec­tion of very small ob­jec­ts or dif­fe­ren­ces in di­stan­ce
Li­nea­ri­ty de­via­tion < 1 mm
Very fast mea­su­re­men­ts 
Mea­su­re­ment on dif­fe­rent sha­pes and sur­fa­ces
High pre­ci­sion down to the mi­cron range

Triangulation principle display

Tran­sit Time Sen­sors for Long Di­stan­ces

De­ter­mi­na­tion of long di­stan­ces up to 100 m with re­flec­tors
Wor­king range up to 10 m on ob­jec­ts
Li­nea­ri­ty de­via­tion > 10 mm
Re­si­stant to in­ter­fe­ring in­fluen­ces
Very high am­bient light re­si­stan­ce
Re­pro­du­ci­ble mea­su­re­ment over long di­stan­ces

Pos­si­ble Uses of Laser Sen­sors for Di­stan­ce Mea­su­re­ment

Pre­sen­ce check

Presence check icon

Thic­k­ness mea­su­re­ment

Thickness measurement icon

Dia­me­ter con­trol

Diameter control icon

Edge coun­ting

Edge counting icon

Po­si­tio­ning

Positioning icon

Robot po­si­tio­ning

Robot positioning icon

Stac­king height mo­ni­to­ring

Stacking height monitoring icon

Parts mea­su­re­ment

Parts measurement icon

Dif­fe­ren­tial Mea­su­re­ment

Con­tra­st Re­co­gni­tion

Mo­ni­to­ring of Two-​Layer Ma­te­rials

Sec­tors and In­du­stries which Use Laser Di­stan­ce Sen­sors

Trian­gu­la­tion Sen­sors

Tran­sit Time Sen­sors

In lo­gi­stics cen­ters, shut­tle sy­stems must au­to­ma­ti­cal­ly de­li­ver goods from the ware­hou­se to pro­duc­tion. Time of flight laser di­stan­ce sen­sors with win­tec in­te­gra­ted on the front de­tect in ad­van­ce end po­si­tions or forward-​moving shut­tles in the field of vi­sion up to ten me­ters ahead so that shut­tles can slow down or stop.

The Trian­gu­la­tion Prin­ci­ple

The trian­gu­la­tion prin­ci­ple is a geo­me­tric mea­su­re­ment me­thod that uses the trian­gu­lar re­la­tion­ship. In this me­thod, a light point is pro­jec­ted onto the ob­ject to be mea­su­red. The ob­ject re­flec­ts the light and hits a light-​sensitive CMOS re­cei­ving ele­ment in the sen­sor at a cer­tain angle. The po­si­tion of the light spot on the CMOS line chan­ges de­pen­ding on the di­stan­ce of the ob­ject. In this way, the di­stan­ce to the ob­ject to be mea­su­red can be pre­ci­se­ly de­ter­mi­ned even at small di­stan­ces. 

This tech­no­lo­gy ena­bles di­stan­ce sen­sors to de­tect very small de­tails. The trian­gu­la­tion prin­ci­ple is used by the di­stan­ce sen­sors CP, OCP, YP, P3 se­ries and PNBC

Do Trian­gu­la­tion Sen­sors Have a Blind Spot?

Sen­sors that ope­ra­te ac­cor­ding to the trian­gu­la­tion prin­ci­ple have a so-​called blind spot. This is de­pen­dent on the di­stan­ce from which the re­flec­ted light meets the re­cei­ving ele­ment (CMOS line). If the re­flec­ted light does not hit the CMOS line, no mea­su­re­ment can be taken. The blind spot is below the wor­king range and means that ob­jec­ts lo­ca­ted in this area are not de­tec­ted and no mea­su­red va­lues are out­put

Exam­ple CP24MHT80 laser di­stan­ce sen­sor trian­gu­la­tion: 
Wor­king range: 40…160 mm
Blind apot: 0…40 mm

The CMOS Re­cei­ving Line

The CMOS line is a light-​sensitive re­cei­ving ele­ment with a large num­ber of pi­xels. It is used to eva­lua­te the po­si­tion at which the laser light hits the line. The elec­tri­cal char­ge in the pi­xels of the CMOS sen­sors (Com­ple­men­ta­ry Metal-​Oxide Se­mi­con­duc­tor) is con­ver­ted into a vol­ta­ge. The po­si­tion of the ob­ject can be de­ter­mi­ned based on the light di­stri­bu­tion on the CMOS line. 

The CMOS line ena­bles hi­ghly ac­cu­ra­te di­stan­ce mea­su­re­ment and is ty­pi­cal­ly used in laser di­stan­ce sen­sors based on the trian­gu­la­tion prin­ci­ple.

How to In­stall Trian­gu­la­tion Sen­sors Pro­per­ly

To achie­ve the most sta­ble ob­ject de­tec­tion and mea­su­re­ment pos­si­ble, the fol­lo­wing in­struc­tions must be ob­ser­ved when ad­ju­sting the sen­sor.

Round, Glos­sy, Re­flec­ti­ve Ob­jec­ts

When mea­su­ring shiny or round sur­fa­ces, it should be en­su­red du­ring sen­sor in­stal­la­tion that no di­rect re­flec­tions fall on the re­cei­ving ele­ment.

Tip: Align the sen­sor so that it is po­si­tio­ned in an axis with the round ob­ject. 

Steps, Edges, Re­ces­ses

For all di­stan­ce sen­sors, it should be en­su­red that the re­cei­ving beam has a di­rect line of sight and is not co­ve­red by an ob­sta­cle such as an edge, step, hole or gap.

Tip: Align the sen­sor or­tho­go­nal­ly to the gap cour­se!

Mo­ving Ob­jec­ts

One exam­ple of mo­ving ob­jec­ts to be mea­su­red is con­veyor belts. It is im­por­tant that the ob­ject moves or­tho­go­nal­ly to the sen­sor. This pre­ven­ts di­rect re­flec­tions to the re­cei­ver.

Tip: In­stall the sen­sor or­tho­go­nal­ly!

Color Edges

When mea­su­ring ob­jec­ts with color tran­si­tions, so-​called color edges, it is im­por­tant that the color edge runs or­tho­go­nal­ly to the sen­sor. This pre­ven­ts color er­rors.

Tip: In­stall the sen­sor or­tho­go­nal­ly!

Dif­fe­ren­ces Bet­ween Sphe­ri­cal and Asphe­ri­cal Len­ses

Sphe­ri­cal Lens

  • The lens has a sphe­ri­cal sur­fa­ce

  • In­co­ming light on the edge area is more stron­gly re­frac­ted than in the cen­tral area

  • Bund­ling of the light beams leads to a loss of pre­ci­sion 

Asphe­ric Lens

  • The lens has an une­ven cur­va­tu­re

  • The light beam is even­ly bro­ken over the en­ti­re sur­fa­ce

  • Lens shape re­du­ces ima­ging er­rors

  • Focus point is map­ped pre­ci­se­ly on the line

  • Very high mea­su­ring ac­cu­ra­cy

Time of Flight Prin­ci­ple (ToF)

The ToF (Time-​of-Flight) laser sen­sors for di­stan­ce mea­su­re­ment com­bi­ne re­pro­du­ci­ble mea­su­re­ment re­sul­ts, re­lia­bi­li­ty and a large mea­su­ring range. This makes them sui­ta­ble for a va­rie­ty of ap­pli­ca­tions at di­stan­ces of up to one hun­dred me­ters with re­flec­tors or ten me­ters to ob­jec­ts. 


The Time-​of-Flight mea­su­ring prin­ci­ple, also known as tran­sit time mea­su­re­ment, cal­cu­la­tes the di­stan­ce L to the ob­ject via light pul­ses. The diode in the sen­sor emits laser pul­ses that are re­flec­ted by the ob­ject. The time in­ter­val from the emis­sion of the light pulse to the ob­ject and back again is mea­su­red. The time T and the light speed C then pro­vi­de the cor­re­spon­ding di­stan­ce to the ob­ject

The fol­lo­wing phy­si­cal for­mu­la is used to de­ter­mi­ne the di­stan­ce:
 
L = ½ × C × T 

The Time-​of-Flight mea­su­ring prin­ci­ple is used by the di­stan­ce sen­sors P1PY, P2PY, P1KY and OY

The Most Im­por­tant Facts About the Speed of Light at a Glan­ce

The speed of light is a fun­da­men­tal con­stant of phy­sics. In va­cuum, it is 299,792,458 m/s. No­thing moves as fast as light.

Do ToF Sen­sors Have a Blind Spot?

ToF sen­sors have no blind spot. Ob­jec­ts can be de­tec­ted below the set­ting range and the sen­sor swit­ches, but can­not pro­vi­de any mea­su­re­ment re­sul­ts.

At What Co­ve­ra­ge of the Light Spot Does the Sen­sor Switch?

The sur­fa­ce fi­nish of the ob­ject plays a de­ci­si­ve role in de­ter­mi­ning which co­ve­ra­ge of the light spot the sen­sor swit­ches. Bright sur­fa­ces lead to swit­ching of the ToF sen­sor even with low co­ve­ra­ge of the light spot, as the num­ber of pho­tons re­qui­red for de­tec­tion of the light pulse is rea­ched fa­ster. Dark sur­fa­ces, on the other hand, re­qui­re grea­ter co­ve­ra­ge to achie­ve the same ef­fect.

If am­bient light, such as sun­light or il­lu­mi­na­tion, in­crea­ses, the ob­ject ap­pears to be­co­me dar­ker for the sen­sor. In such cases, a lar­ger area of the light spot must hit the ob­ject to en­su­re re­lia­ble de­tec­tion.

Due to the op­tics of the sen­sor, there is also a small pro­por­tion of scat­te­red light that oc­curs ou­tsi­de the ac­tual light spot. With hi­ghly re­flec­ti­ve, glos­sy sur­fa­ces, this can lead to the ob­ject being de­tec­ted be­fo­re the light spot ac­tual­ly rea­ches it. It is the­re­fo­re im­por­tant to avoid di­stur­bing, shiny struc­tu­res near the light beam.

Tran­sit Time Sen­sors with Re­flec­tor

The use of re­flec­tors can si­gni­fi­can­tly ex­tend the area over which the time-​of-flight sen­sors can be used. The ToF sen­sors focus ex­clu­si­ve­ly on the light re­flec­ted by the re­flec­tor and ef­fec­ti­ve­ly sup­press all other si­gnals. This en­su­res that mea­su­re­men­ts are only taken on re­flec­tors, while re­flec­ti­ve ob­jec­ts and other shiny sur­fa­ces are not de­tec­ted as re­flec­tors and igno­red ac­cor­din­gly.

This func­tio­nal prin­ci­ple is par­ti­cu­lar­ly ad­van­ta­geous if in­cor­rect mea­su­re­men­ts due to back­ground ob­jec­ts are to be avoi­ded. A ty­pi­cal ap­pli­ca­tion exam­ple is the con­trol of ove­rhead con­veyor sy­stems, where the di­stan­ce to the ve­hi­cle in front must al­ways be re­lia­bly re­cor­ded. Espe­cial­ly when cor­ne­ring, mea­su­re­men­ts are pre­ven­ted from being taken er­ro­neou­sly on ob­jec­ts in the back­ground, as these could lead to in­cor­rect con­trol com­mands. 

This tech­no­lo­gy is also ideal for ap­pli­ca­tions re­qui­ring large wor­king ran­ges.

Com­pa­ri­son of Tran­sit Time and Trian­gu­la­tion Wor­king Ran­ges

The sen­sor at the top of the image is a tran­sit time sen­sor, while the sen­sor below ope­ra­tes ac­cor­ding to the trian­gu­la­tion prin­ci­ple.

Key
Red area: Blind spot (ob­jec­ts are not re­lia­bly de­tec­ted)
Green area: Wor­king range (ob­jec­ts are re­lia­bly de­tec­ted)
Yel­low range: Set­ting range/mea­su­ring range (set swit­ching poin­ts/mea­su­red va­lues are di­splayed) 

Out­put of Di­stan­ce Va­lues

Di­gi­tal Swit­ching Out­put

Di­stan­ces can be taught in via di­gi­tal swit­ching out­pu­ts. As soon as the taught-​in di­stan­ce is rea­ched, the sen­sor out­pu­ts a swit­ching si­gnal at the out­put. This ena­bles ob­jec­ts to be de­tec­ted and po­si­tions to be de­tec­ted.

Ana­log Out­put

The di­stan­ce value is out­put as a li­near­ly pro­por­tio­nal cur­rent (4…20 mA) or vol­ta­ge value (0…10 V) via an ana­log out­put. The cha­rac­te­ri­stic curve can be set wi­thin the en­ti­re mea­su­ring range by tea­ching in. 

IO-​Link

IO-​Link tech­no­lo­gy is used around the world for stan­dar­di­zed com­mu­ni­ca­tion with sen­sors and ac­tua­tors. This is point-​to-point com­mu­ni­ca­tion.

In­du­strial Ether­net

In­du­strial Ether­net is a ge­ne­ric term for all Ether­net stan­dards for real-​time data tran­smis­sion bet­ween the con­trol and sen­sor. In­du­strial Ether­net pro­to­cols in­clu­de, for exam­ple, Ether­CAT, Ether­net/IP or PRO­FI­NET.
 

What Is Ac­cu­ra­cy?

High ac­cu­ra­cy means that the an­ti­ci­pa­ted mea­su­re­ment re­sul­ts are achie­ved. This term is only used for qua­li­ta­ti­ve sta­te­men­ts. This means that it is not a tech­ni­cal pa­ra­me­ter. Ac­cu­ra­cy is made up of pre­ci­sion and cor­rect­ness. The ac­cu­ra­cy al­ways de­pends on the mea­su­ring prin­ci­ple used.
 

 Pre­ci­sion

Pre­ci­sion, also known as re­pe­ti­tion ac­cu­ra­cy, can be de­ter­mi­ned by suc­ces­si­ve mea­su­re­men­ts under con­si­stent con­di­tions. A very pre­ci­se value the­re­fo­re de­li­vers al­mo­st con­stant mea­su­re­men­ts. The pre­ci­sion of a sen­sor is quan­ti­fied by re­pro­du­ci­bi­li­ty.

Cor­rect­ness

Cor­rect­ness is a qua­li­ta­ti­ve value. It is de­fi­ned by li­nea­ri­ty de­via­tion, tem­pe­ra­tu­re drift, switch-​on drift and swit­ching di­stan­ce de­via­tion.

The fi­gu­re shows how cor­rect­ness, pre­ci­sion and ac­cu­ra­cy are re­la­ted. The red dots re­pre­sent suc­ces­si­ve mea­su­re­men­ts from a sen­sor, while the tar­get in­di­ca­tes the cor­rect value. If the mea­su­red va­lues are far apart and far away from the tar­get, this in­di­ca­tes a low pre­ci­sion and cor­rect­ness. Ideal­ly, mea­su­re­men­ts should be cor­rect and ac­cu­ra­te, mea­ning they are close to­ge­ther and wi­thin the tar­get range.

Re­pro­du­ci­bi­li­ty and Li­nea­ri­ty in Com­pa­ri­son: When is Which Value Used?

ab­so­lu­te mea­su­re­ment

Li­nea­ri­ty and re­pro­du­ci­bi­li­ty va­lues are im­por­tant for ab­so­lu­te mea­su­re­men­ts, such as de­ter­mi­ning the ac­tual di­stan­ce of an ob­ject or a dia­me­ter. A good re­pro­du­ci­bi­li­ty value pro­vi­des re­pea­ta­ble va­lues. High li­nea­ri­ty en­su­res cor­rect mea­su­red va­lues. Ove­rall, both li­nea­ri­ty and re­pro­du­ci­bi­li­ty are im­por­tant fac­tors when it comes to ob­tai­ning cor­rect and ac­cu­ra­te mea­su­red va­lues in ab­so­lu­te mea­su­re­men­ts.

po­si­tio­ning tasks

The sen­sor de­li­vers re­pro­du­ci­ble mea­su­red va­lues for re­pea­ted mea­su­re­men­ts. It al­ways hits the same point or po­si­tion, i.e. it is re­pea­ta­ble. This is cru­cial to en­su­re ac­cu­ra­te and re­lia­ble po­si­tio­ning of an ob­ject. The main goal is to al­ways place the ob­ject in the same place. Re­pe­ti­tion ac­cu­ra­cy is of great im­por­tan­ce, while li­nea­ri­ty is less im­por­tant for po­si­tio­ning tasks. High pre­ci­sion is cru­cial here, and the cor­rect­ness can be ne­glec­ted. 

 

Star­ting point
A di­stan­ce mea­su­re­ment is car­ried out and the ma­xi­mum pos­si­ble de­via­tion is de­ter­mi­ned. It is al­ways mea­su­red on the same ob­ject to rule out color er­rors. The am­bient tem­pe­ra­tu­re may vary by 10 °C.

Va­lues from the data sheet: 

  • Re­pro­du­ci­bi­li­ty: 3 mm
  • Li­nea­ri­ty de­via­tion: 10 mm
  • Tem­pe­ra­tu­re drift:  0.4 mm/K

Cal­cu­la­tion
Pre­ci­sion (re­pro­du­ci­bi­li­ty) + cor­rect­ness (li­nea­ri­ty de­via­tion, tem­pe­ra­tu­re drift) = ac­cu­ra­cy 
mm + 10 mm + (0.4 mm * 10 °C) = 17 mm

What De­ter­mi­nes the Ac­cu­ra­cy of the Mea­su­re­ment Re­sul­ts?

Time-​of-flight laser di­stan­ce sen­sors achie­ve high mea­su­ring ran­ges of up to 10 m on ob­jec­ts and 100 m on re­flec­tors. Trian­gu­la­tion laser di­stan­ce sen­sors, on the other hand, is very ac­cu­ra­te. Ho­we­ver, the mea­su­ring range is re­stric­ted to max. 1,000 mm. Va­rious set­tings can be made to op­ti­mi­ze the ac­cu­ra­cy of the sen­sors for di­stan­ce mea­su­re­ment de­pen­ding on the ap­pli­ca­tion. This means that the ac­cu­ra­cy can be fur­ther in­crea­sed by fil­ter func­tions.

Laser Clas­ses and Their Modes of Ac­tion

Use of Red and Blue La­sers

wen­glor’s laser di­stan­ce sen­sors work with red or blue laser light. Whe­ther red or blue light is used de­pends on the ap­pli­ca­tion. Red laser light has a wa­ve­length of 650 nm. Blue la­sers work with a wa­ve­length of 405 nm and the­re­fo­re have a shor­ter wa­ve­length. This means that the blue laser beam pe­ne­tra­tes less dee­ply into the ob­ject to be mea­su­red and de­li­vers pre­ci­se and sta­ble re­sul­ts. Glo­wing sur­fa­ces in par­ti­cu­lar are not af­fec­ted by the blue laser. Laser di­stan­ce sen­sors with blue diode are very well sui­ted for or­ga­nic sur­fa­ces, po­li­shed me­tals, shiny pla­stic sur­fa­ces or dark pain­ts.

What Is the Dif­fe­ren­ce Bet­ween Nor­mal Light and Laser Light?

Nor­mal Light

Di­sper­sion di­rec­tionLight waves are di­sper­sed in all di­rec­tions
Wa­ve­leng­thsCon­si­st of many dif­fe­rent wa­ve­leng­ths
Phase equi­va­len­ceWaves oscil­la­te out of phase
Di­ver­gent light beam with large spot dia­me­ter

Laser Light

Light waves are stron­gly di­rec­ted
Con­sists of one wa­ve­length (mo­no­chro­ma­ti­ci­ty)
Waves oscil­la­te syn­chro­nou­sly
-> Strong bund­ling ena­bles small light spot dia­me­ters at great di­stan­ces.

Why Is There Red and Blue Laser Light?

The light spec­trum con­sists of dif­fe­rent wa­ve­leng­ths. Each has a dif­fe­rent color. A color can be as­si­gned to each wave in the color spec­trum. Red light dif­fers from blue light in its wa­ve­length and ener­gy den­si­ty.
 
Wa­ve­length blue: 380 – 500 nm
Wa­ve­length red: 640 – 675 nm

What Is Light?

Light is the part of elec­tro­ma­gne­tic ra­dia­tion vi­si­ble to the human eye. The ra­dia­tion pro­pa­ga­tes in dif­fe­rent wa­ve­length ran­ges when emit­ted by a light sour­ce, for exam­ple a light bulb. The wa­ve­length range lies bet­ween UV ra­dia­tion (shor­ter wa­ve­leng­ths) and in­fra­red ra­dia­tion (lon­ger wa­ve­leng­ths).

What Is Color?

The color of ob­jec­ts is a su­b­jec­ti­ve im­pres­sion crea­ted by ob­jec­ts ab­sor­bing dif­fe­rent wa­ve­leng­ths and re­flec­ting others. These wa­ve­leng­ths re­pre­sent dif­fe­rent co­lors. The color re­flec­ted by the ob­ject can be per­cei­ved by the human eye. 

What Is a Laser?

The term laser stands for Light Am­pli­fi­ca­tion by Sti­mu­la­ted Emis­sion of Ra­dia­tion. A laser beam can be ge­ne­ra­ted over a wide range of the op­ti­cal spec­trum. In sim­ple terms, this means that di­rec­ted light waves are bund­led into a beam in high con­cen­tra­tion.

Dif­fe­ren­ces Bet­ween Laser Di­stan­ce Sen­sors and Ul­tra­so­nic Sen­sors

  • Di­stan­ce sen­sors and ul­tra­so­nic sen­sors dif­fer in the size of the de­tec­tion range

  • Ul­tra­so­nic sen­sors work with a wide sonic cone 

  • Laser di­stan­ce sen­sors work with a fine laser beam
     

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