Page 191 - Sensors and Control Systems in Manufacturing

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the type of application. The certification will ensure the device
has been tested and approved for certain uses.
• Intangibles. These can include the availability of application
support and service, the supplier’s reputation, local availability,
and quality testing statements from the manufacturer.

3.7 Wavelengths of Commonly Used
Light-Emitting Diodes
An LED is a semiconductor that emits a small amount of light when
current flows through it in the forward direction. In most photoelec-
tric sensors, LEDs are used both as emitters for sensing beams and as
visual indicators of alignment or output status for a manufacturing
process. Most sensor manufacturers use visible red, visible green, or
infrared (invisible) LEDs (Fig. 3.4b). This simple device plays a sig-
nificant part in industrial automation. It provides instantaneous
information regarding an object during the manufacturing operation.
LEDs, together with fiber optics, allow a controller to direct a multi-
tude of tasks, simultaneously or sequentially.

3.8 Sensor Alignment Techniques
A sensor should be positioned so the maximum amount of emitted
energy reaches the receiver element in one of three different modes:

• Opposed sensing mode
• Retroreflective sensing mode
• Proximity (diffuse) sensing mode

3.8.1 Opposed Sensing Mode
In the opposed sensing mode, the emitter and receiver are positioned
opposite each other so that the light from the emitter shines directly
at the receiver. An object then breaks the light beam that is estab-
lished between the two. Opposed sensing is always the most reliable
mode.

3.8.2 Retroreflective Sensing Mode
Retroreflective sensing is also called the reflex mode or simply the
retro mode. A retroreflective photoelectric sensor contains both emit-
ter and receiver. A light beam is established between the sensor and a
special retroreflective target. As in opposed sensing, an object is
sensed when it interrupts this beam.
Retro is the most popular mode for conveyer applications where the
objects are large (boxes, cartons, etc.), where the sensing environment