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Opposed Mode
Retroreflective
Diffuse
Divergent
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Diffuse Mode Sensing
How It Works
In diffuse-mode sensing, light emitted from the sensor strikes the surface of the object to be detected and is diffused back, i.e. sending some light back to the receiver, which is usually housed with the emitter.

With a diffuse-mode sensor, the object is detected when it "makes" the beam, that is, the object reflects the sensorís transmitted light energy back to the sensor.

Most diffuse-mode sensors use lenses to collimate (convert a divergent beam into a parallel beam) the emitted light rays to gather in more light. These lenses also help to extend the range of diffuse-mode sensors.


Diffuse Sensing

 
  
Collimating Lens
Pros: Convenience
Diffuse-mode sensors have only one item to be mounted: the sensor itself. This is ideal for situations where a sensor can be mounted only on one side.
Pros: Conveyors
A diffuse mode sensor may be used if an opposed or retroreflective-mode sensor isn't practical in a straightforward product presence sensing application. The sensor-to-object distance should be from a few centimeters to a meter.


Simple Close Mounting

 

 

 

 

 


Inaccurate for Glass

 

Cons: Dependant on Object Reflectivity and Shape
The response of a diffuse-mode sensor is dramatically influenced by the surface reflectivity of the object to be sensed. The reflectivity of an object directly affects the distance at which it can be reliably sensed by a diffuse-mode sensor. The shape of the object to be sensed is as significant as reflectivity. Radiused or rounded objects present the sensor with less surface area and therefore have a smaller signal return path.
Cons: Shiny Surfaces
The use of collimating lenses maximizes sensing ranges of diffuse mode sensors. However, if you are using diffuse sensors to detect shiny objects, the collimating lens will affect the sensing angle. The sensor needs to be perfectly parallel to the object being sensed in order to guarantee a return light signal.

Cons: Background Objects
As a general rule, verify that the distance from diffuse mode sensors to the nearest background object is at least four times the distance from the sensor to the object to be detected. However, if the background is more reflective than the object to be detected, more clearance will be required. If the background is not controllable, consider the use of a background suppression sensor (
similar to a diffuse sensor, but with a defined range limit e.g. fixed-field and adjustable-field).

Cons: Small Parts Detection
 Diffuse sensors have less sensing range when used to sense objects with small reflective area than when used to sense objects with a large reflective area. Also the lens of most diffuse mode sensors creates a "blind spot" for small parts that pass close to the lens.
A "Blind Spot" is an area close to a sensor lens, where light energy is returned to the emitter rather than the receiver, rendering the sensor effectively blind. This effect is most pronounced with some retroreflective sensors.

Cons: Low Excess Gain
Most diffuse mode sensors lose their gain very rapidly as dirt and moisture accumulates on their lenses. In some instances, dirt build-up on the sensor lens can channel the light beam from the emitter directly to the receiver, so that the sensor will act as if an object is constantly in front of it. This event is called optical crosstalk.
Cons: Count Inaccuracy
Diffuse mode sensors are usually a poor choice for applications that require accurate counting of radiused parts. Diffuse sensors are particularly unreliable for counting glass or shiny objects, small parts, objects with irregular surfaces, or parts that will pass by the sensor at varying distances.

 

Application - Detecting Capacitor Polarity

Objective:
To determine if a capacitor is properly oriented.

A capacitor can be placed in a circuit board in one of two orientations. Since they are polarity-sensitive, an inspection process is needed to ensure proper insertion. The diffuse rectangular fibre "looks" for the polarity marks on the side of the capacitor. The sensor is programmed to output if the mark is not detected.

Review
In diffuse-mode sensors, the emitter and receiver are housed in the same sensor, like in the retroreflective mode. However, the effective beam is made when the target object bounces the light beam back from the sensor's emitter to the receiver.  Diffuse-mode sensors are very convenient and are often used when opposed or retroreflective-mode sensors aren't practical. Diffuse-mode sensors are significantly affected by the reflectivity of the target object(s). These sensors also tend to lose their excess gain quickly. Diffuse-mode sensors should not be used in applications where small parts need to be detected, in parts counting applications, or where a reflective background is close to the object to be sensed.

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