Convergent Mode Sensing

How It Works
Convergent-mode sensors use reflective energy very efficiently, unlike divergent-mode sensors. Convergent-mode sensors use a lens system to focus emitter and the receiver elements to an exact point in front of the sensor. Like diffuse-mode and divergent-mode sensors, convergent-mode sensors detect an object when that object completes or "makes" the light beam. This design produces a small, intense, and well-defined sensing area, at a fixed distance from the sensor lens. It is a very efficient use of reflective energy.

Pros: High Excess Gain
Convergent-mode sensors make the most efficient use of reflective sensing energy. This is a good mode to use to detect objects with low or high reflectivity, when you can’t use an opposed or retroreflective mode sensor. However, the small beam of the convergent-mode sensor can be easily deflected by highly specular or shiny surfaces.

Pros: Counting Radiused Objects
Convergent-mode sensors allow the sensing light energy to be focused on the nearest point (i.e. the tangent point) of a radiused object. For this reason, it is a good choice for counting bottles, jars, or cans, where there is no space between adjacent products.

Pros: Accurate Positioning
Convergent-mode sensors have well-defined effective beams, especially at the focal point. Therefore it is a good sensing choice, after opposed mode, for accurately detecting the position of edges. However, convergent-mode sensing becomes a first choice for accurate position sensing of clear materials.

Pros: Fill Level Applications
Convergent-mode sensors may be used in some applications for detecting the fill level of materials in an open container, where an ultrasonic sensor would not work.

Pros: Colour Sensing
Convergent-mode sensors are available with a wide variety of LED colors such as blue, green, and red. These sensors work well in colour registration applications.

Cons: Depth of Field
Convergent-mode sensors require that the surface to be detected pass at (or close to) the focal point from the sensor lens. Avoid the use of convergent-mode sensors for detection of objects that pass at an unpredictable distance from the sensor.

Cons: Effect of Surface Reflectivity
As with any photoelectric proximity mode, consider the reflectivity of the surface to be detected. The distance within which a convergent-mode sensor will detect an object is relative to that object’s optical reflectivity.

If a shiny background object returns unwanted light, tilt or rotate the sensor to move the sensing beam so that it is not perpendicular to the shiny surface.

Application - Ink Jet Printing Registration
 

Objective:
To sense the leading edge of bottles and provide the trigger signal to an ink jet printer.

Bottles are channeled through guide rails to control the distance to both the sensor and the ink jet printer. The convergent-mode sensor consistently triggers the printer at the same point on the circumference of each bottle to provide accurate printing registration.

Review

The convergent mode makes very efficient use of reflective energy. It uses a lens to focus its beam to an exact point, which produces a small, intense and well-defined sensing area at a fixed distance in front on the sensor. Convergent-mode sensors are excellent in applications involving accurate positioning, counting radiused objects, and colour sensing.

Reconsider the use of convergent sensors when the object to be detected passes by at unpredictable distances, or if the object is very shiny.