Definition of Area Sensors
Area sensors can be defined as multi-beam
photoelectric sensors.
This type of sensor is typically used to monitor
large areas by detecting or measuring objects passing through a specific
barrier.
Also known as "array
sensors”, they can be conceived as a group of photocells
stacked on top of each other.
Area sensors grant several advantages
compared to a set of piled up photocells:
-Simplified wiring:
area sensors require to connect just one or two cables no matter the height of
the area to control.
On the contrary, a stack of photoelectric sensors can
easily result in dozens of cables to connect
-Integrated
signal processing:
area sensors can process
the signals coming from each beam on board eliminating the need for
complex and time-consuming programming on the PLC.
-Application
flexibility:
area sensors typically offer
multiple configuration options to tackle the most common application needs.
For example, they can be programmed to drive an analogue output
proportional to the height of the object that has been detected.
-Ease of
installation:
all optical beams come
factory pre-aligned thus simplifying the mechanical installation in the
field.
-Resolution
and detection accuracy:
thanks to sophisticated
optical design, beam pitch can go down to few millimeters offering very
accurate detection and measurement capabilities.
What are the typical applications and when are they meant to be used?
Area sensors are used in either object detection and
measurement.
Typically, they are installed orthogonally to the travel direction
of the object:
In some applications, area sensors are
required as the objects to detect are irregular or their position is somehow
unpredictable.
Using a traditional single-beam
photoelectric sensor might easily result in unstable or
unreliable object detection.
For
example, food and beverage goods wrapped in soft packages like pasta, chips,
diary or backed products, etc…
Other hard-to-detect examples are transparent
goods or objects with holes like containers, crates, totes or pallets on
conveying systems:
Detection
of irregular shapes with holes
Detection
of glass sheets
Additionally, area sensors are often
used to detect very thin objects like flyers or envelopes in automated sorting
systems.
In this case, single
beam sensors would easily fail in detecting such thin objects
reliably:
Panels’
thickness detection
Area sensors are able not only to detect
but also measure the height or width of
an object.
Typical
examples can be found in end-of-line or logistics applications.
This means that area sensors can be used to measure the height of carton boxes
or pallets.
Another application can be found also in automated painting systems with the aim to measure the dimensions of wooden panels.
Objects
with holes (pallet) detection
Area
sensors are also widely adopted in automated
vending machines to detect goods falling down into the basket
after the user selection and payment.
This control is extremely important to
make sure the machine is properly working and products have been dispensed
correctly.
Vending
machine: detection of falling goods
How does an Area Sensor
work?
Area sensors are usually made by two distinct units:
-the "emitter”: embedding an array of LEDs
-the "receiver”: integrating a group of
photodiodes.
The detection is based on the through-beam optical principle.
The LEDs are emitting light beams that are collected by the photodiodes.
When an object come across the
area sensor, one or more light beams get interrupted thus triggering the
activation of the outputs.
Area sensors can also use the polarized retro-reflection optical principle.
In this case, emitter and receiver are
embedded in one single unit, having a reflector on the opposite
side.
The beam is emitted by the LED, hits the reflector, bounces back and is
collected by the photodiode.
If no light is received back by the photodiode, it
means an object has interrupted the light beam
Through beam sensors or retroreflective sensors: what to choose?
Through beam area sensors are relatively simple devices that require two active units: emitter and
receiver. These units can be powered up and aligned one in front of the other.
They are usually based on non-visible infrared light and they can manage fairly big
working distances (i.e. wide openings) while delivering high detection accuracy
and resolution.
Retroreflective area sensors have just one active unit to be wired up thus reducing the installation
complexity and costs.
The emitted light is usually visible red
to simplify the alignment procedure.
Retroreflective area sensors are suitable for small to mid-size openings.
However, they have lower resolution than through-beam sensors.
This is due to
the light's travel.