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Contrast Sensor Technology

Con­tra­st sen­sors de­tect con­tra­st and color dif­fe­ren­ces on a wide range of ma­te­rials and sur­fa­ces. They can ac­cu­ra­te­ly de­tect the po­si­tion of con­tra­st mar­kings even at high ma­chi­ne speeds. With white light LED or red laser light, con­tra­st sen­sors re­lia­bly de­tect print marks and ob­jec­ts based on dif­fe­ren­ces in con­tra­st.
Mode of Operation
Light
Color Spaces
Reflective Properties
Technology
Industries
Teach-In
Outputs
Installation

How Do Con­tra­st Sen­sors Work?

Con­tra­st sen­sors are pho­toe­lec­tro­nic sen­sors that work ac­cor­ding to the prin­ci­ple of a re­flex sen­sor: Emit­ted light is re­flec­ted by the ob­ject, reab­sor­bed and eva­lua­ted. Con­tra­st sen­sors are used to de­tect mar­kings based on dif­fe­ren­ces in con­tra­st. They are cha­rac­te­ri­zed by high swit­ching speeds and low jit­ter. Dif­fe­rent con­tra­st de­tec­tion tech­no­lo­gies are avai­la­ble to achie­ve the best pos­si­ble de­tec­tion:
 
  • Laser red light + mo­no­chro­me pho­to­dio­de
  • White light + mo­no­chro­me pho­to­dio­de
  • White light + RGB pho­to­dio­de


What Is the Dif­fe­ren­ce Bet­ween Light and Laser Light?

Light con­sists of elec­tro­ma­gne­tic ra­dia­tion vi­si­ble to the human eye that pro­pa­ga­tes in dif­fe­rent wa­ve­length ran­ges when emit­ted by a light sour­ce. Laser light is crea­ted by sti­mu­la­ted emis­sion that am­pli­fies light, i.e. high con­cen­tra­tions of rec­ti­fied light waves are bund­led into a beam.

What Is a Color Space and Which Is Par­ti­cu­lar­ly Re­le­vant for Sen­sors?

A color space is a de­fi­ned color coor­di­na­te sy­stem used to sy­ste­ma­ti­ze and stan­dar­di­ze co­lors. This de­ter­mi­nes the amount of co­lors and the me­thod of color re­pre­sen­ta­tion. In a color space, co­lors are de­scri­bed by coor­di­na­tes where the axes re­pre­sent dif­fe­rent color pro­per­ties, such as color in­ten­si­ty, bright­ness, tone or sa­tu­ra­tion. The RGB and CMYK color spa­ces are par­ti­cu­lar­ly re­le­vant for in­du­stry. The RGB color space is used for sen­sors, be­cau­se they col­lect color in­for­ma­tion in the form of red, green and blue color chan­nels.

 

RGB Color Space

RGB uses light to crea­te co­lors by com­bi­ning red, green and blue.

CMYK Color Space

The CMYK color space is main­ly used in prin­ting and pro­du­ces co­lors with cyan, ma­gen­ta, yel­low and black.

What Is White Light Based on the RGB Color Space?

The RBG color model works as an ad­di­ti­ve color mix. White light is crea­ted by mi­xing red, green and blue light. Black re­sul­ts in the ab­sen­ce of light of any wa­ve­length. 
 

What Are the Re­flec­ti­ve Pro­per­ties of Co­lo­red Sur­fa­ces?

Con­tra­st Sen­sor Tech­no­lo­gies

How Do Con­tra­st Sen­sors with White Light Work?

Con­tra­st sen­sors with white light LED emit white light in the form of a rec­tan­gu­lar light spot that is re­flec­ted back from the sur­fa­ce to the re­cei­ver of the sen­sor, which then eva­lua­tes this light.

Re­cei­ver with RGB Pho­to­dio­de (P1PWxxx)

The re­cei­ver con­sists of three areas cor­re­spon­ding to the RGB color space. De­pen­ding on the color of the con­tra­st mar­king, the light is re­flec­ted back to the re­cei­ver ac­cor­ding to the re­flec­tion pro­per­ties and eva­lua­ted based on the three chan­nels.

Re­cei­ver with Mo­no­chro­me Pho­to­dio­de (WM03xxx)

The pho­to­dio­de of the re­cei­ver con­sists of a sin­gle con­ti­nuous sur­fa­ce. De­pen­ding on the color of the con­tra­st mar­king, the light is re­flec­ted back to the re­cei­ver ac­cor­ding to the re­flec­tion pro­per­ties. The re­cei­ver eva­lua­tes the re­cei­ved bright­ness and sorts it into a gray­sca­le.

How Do Con­tra­st Sen­sors with Laser Light Work?

Con­tra­st sen­sors with laser light emit a spot of red laser light, the re­flec­tion of which is eva­lua­ted by the re­cei­ver. The re­cei­ver uses a mo­no­chro­me pho­to­dio­de. Con­tra­st sen­sors with laser light are avai­la­ble in a di­gi­tal and an ana­log ver­sion:

  • Di­gi­tal ver­sion (YM24PAH, YP11VAH): The swit­ching th­re­shold is set to a cer­tain bright­ness level. This bright­ness value is then com­pa­red with the amount of light re­flec­ted by the ob­ject.
  • Ana­log ver­sion (YP11MGV): The ana­log out­put out­pu­ts a vol­ta­ge pro­por­tio­nal to the bright­ness. The out­put vol­ta­ge in­crea­ses along with the de­gree of bright­ness.

Technology Comparison of Contrast Sensors with Laser Light and LED White Light

LED white light Laser Light (red)
Near field up to 50 mm Di­stan­ces up to 150 mm
Print and color mark de­tec­tion De­tec­tion of con­tra­st dif­fe­ren­ces
Very high speed up to 50 kHz High speed up to 20 kHz
Rec­tan­gu­lar light spot Small, round light spot


Contrast Sensor Applications

Checking the Material End with Visual Marks

Film material is processed on rolls in production technology. During production, the material is unwound from the roll. Marking stripes are applied to mark the end of the roll. These are detected by contrast sensors to ensure that a roll change is initiated in good time.

Color Check of Objects

To ensure high product quality, the visual characteristics must also be monitored and controlled at all times in the production process. Contrast sensors can detect differences in color, enabling quality control based on visual appearance characteristics. For example, the color of bottle caps is checked to ensure that the correct caps are always used in a container.

Print Mark Recognition

Print marks are colored markings or gray blocks in various percentages printed on packaging materials. They can be recognized photoelectronically and then trigger control commands on a connected processing machine. Print marks are used to control various processes:

  • Alignment and positioning of objects
  • Cutting, welding and bonding processes

Detection of Contrast Differences

Contrast differences are different brightness levels of objects. Brightness levels are compared with reference values to solve various applications:

  • Differentiation of objects based on contrast values
  • ​​​​​Presence check of inserts in an assembly process
  • Presence check of very flat objects against a background

Checking the Material End with Visual Marks

Film material is processed on rolls in production technology. During production, the material is unwound from the roll. Marking stripes are applied to mark the end of the roll. These are detected by contrast sensors to ensure that a roll change is initiated in good time.

Color Check of Objects

To ensure high product quality, the visual characteristics must also be monitored and controlled at all times in the production process. Contrast sensors can detect differences in color, enabling quality control based on visual appearance characteristics. For example, the color of bottle caps is checked to ensure that the correct caps are always used in a container.

Print Mark Recognition

Print marks are colored markings or gray blocks in various percentages printed on packaging materials. They can be recognized photoelectronically and then trigger control commands on a connected processing machine. Print marks are used to control various processes:

  • Alignment and positioning of objects
  • Cutting, welding and bonding processes

Detection of Contrast Differences

Contrast differences are different brightness levels of objects. Brightness levels are compared with reference values to solve various applications:

  • Differentiation of objects based on contrast values
  • ​​​​​Presence check of inserts in an assembly process
  • Presence check of very flat objects against a background

In­du­stries for Pre­fer­red Use of White Light LED Con­tra­st Sen­sors

Pac­ka­ging In­du­stry

Con­tra­st sen­sors with LED white light are used for print mark re­co­gni­tion in the pac­ka­ging in­du­stry.

Food In­du­stry

Con­tra­st sen­sors with LED white light are used in the food in­du­stry to de­tect print marks. 

Prin­ting In­du­stry

Con­tra­st sen­sors with white light LED de­tect a wide range of print marks in the prin­ting in­du­stry.

In­du­stries for Pre­fer­red Use of Con­tra­st Sen­sors with Laser Light

Au­to­mo­ti­ve In­du­stry

Con­tra­st sen­sors with laser light are used for con­tra­st de­tec­tion in the au­to­mo­ti­ve in­du­stry.

Pla­stics In­du­stry

Con­tra­st sen­sors with laser light are used for con­tra­st de­tec­tion in the pla­stics in­du­stry.

How Does Tea­ching in the Con­tra­st Sen­sors to the Ap­pli­ca­tion Work?

Con­tra­st sen­sors with laser light are ad­ju­sted using a po­ten­tio­me­ter. This means that the th­re­shold value is set ma­nual­ly.

Con­tra­st sen­sors with white light LED are taught in at the touch of a but­ton by the sen­sor au­to­ma­ti­cal­ly cal­cu­la­ting the swit­ching th­re­shold via the teach-​in key. Three dif­fe­rent teach-​in modes are avai­la­ble for this:
  • Two-​point teach-​in: With two-​point teach-​in, the ob­ject, i.e. the mark, is taught in du­ring one teach-​in pro­cess and the back­ground in ano­ther teach-​in pro­cess. Using both mea­su­red va­lues ac­qui­red and saved in this way, the sen­sor cal­cu­la­tes the swit­ching point so that it lies bet­ween the two teach-​in poin­ts.
  • Dy­na­mic teach-​in: With dy­na­mic teach-​in, the sen­sor is swit­ched to re­cor­ding mode ope­ra­tion, al­lo­wing for au­to­ma­tic teach-​in. Mea­su­red va­lues are re­cor­ded in this mode. After the re­cor­ding mode has been exi­ted, a swit­ching point is cal­cu­la­ted bet­ween the mi­ni­mum and ma­xi­mum mea­su­red va­lues.

Out­pu­ts and In­ter­fa­ces

Di­gi­tal Swit­ching Out­put

Con­tra­st va­lues can be taught in via a di­gi­tal swit­ching out­put via teach-​in or po­ten­tio­me­ter. If the taught-​in con­tra­st value is rea­ched, the sen­sor out­pu­ts a swit­ching si­gnal at the out­put, whe­re­by con­tra­st or print marks are de­tec­ted, for exam­ple.

Ana­log Out­put

The con­tra­st value is out­put as a li­near­ly pro­por­tio­nal cur­rent (4…20 mA) or vol­ta­ge value (0…10 V) via an ana­log out­put. This al­lo­ws the bright­ness cur­ves to be mo­ni­to­red pre­ci­se­ly.



IO-​Link

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




 

Moun­ting In­struc­tions for Con­tra­st Sen­sors

The sen­sor is ali­gned pa­ral­lel to the print mark.

In the case of shiny ob­jec­ts, the sen­sor is moun­ted at the spe­ci­fied angle.

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