- Spatial Presence
- Inductive Proximity Sensors
Inductive proximity sensors are
widely used in the modern high speed
process control environment for
the detection, positioning and counting
of ferrous and non-ferrous metal
objects. Due to the method of construction
and superior performance of inductive
sensors, they are increasingly used
to replace the traditional limit
switch, thus upgrading speed and
reliability of existing machinery.
Principle of Operation
Inductive proximity sensors respond
to ferrous and non - ferrous metal
objects. They will also detect metal
through a layer of non - metal material.
An inductive sensor consists of
an oscillator circuit (ie. the sensing
part) and an output circuit including
a switching device (eg. transistor
or thyristor), all housed in a resin
encapsulated body. An essential
part of the oscillator circuit is
the inductance coil creating a magnetic
field in front of the sensing face.
When the magnetic field is disturbed,
the output circuit responds by either
closing the output switch (normally
open version type NO) or by opening
the output switch (normally closed
version type NC).
- Capacitive Sensors
Capacitive sensors are often successfully
used in applications which cannot
be solved with other sensing techniques.
Capacitive sensors respond to a
change in the dielectric medium
surrounding the active face and
can thus be tuned to sense almost
any substance. Capacitive sensors
can, also, sense a substance through
a layer of glass, plastic or thin
Some typical applications for
capacitive sensors are:
- Level control of non-conductive
liquids (oil, alcohol, fuel).
- Level control of granular substances
(flour, wheat, sugar).
- Sensing substances through a
protective layer (eg. glass).
The fact that capacitive sensors
respond to most substances, necessitates
some care during the installation,
adjustment and long term operation
of the sensor. The sensitivity of
capacitive sensors is affected by
the moisture content and the density
of the substance to be sensed. Deposits
of excessive dust and dirt on or
around the sensing face of the sensor,
cause erratic response and hence
the sensor may require periodic
cleaning if used in a polluting
Principle of Operation
Capacitive sensors respond to
any substance with a high dielectric
constant (water, oil, fuel, sugar,
paper) without necessarily making
physical contact. They are less
suitable for polystyrene and similar
low density substances. Operation
is based on an internal oscillator
with two capacitive plate-electrodes,
tuned to respond when a substance
approaches the sensing face. When
the target is sensed, the output
switch will either close to activate
a load for a normally open option
or the switch will open to de-activate
the load for a normally closed option.
The LED will illuminate when the
output switch closes.
- Photoelectric or Opto-electronic
Photoelectric sensors offer non-contact
sensing of almost any substance
or object up to a range of 10 meters.
Photoelectric sensors consist of
a light source (usually an LED,
light emitting diode, in either
infrared or visible light spectrum)
and a detector (photodiode). Due
to the high intensity infra-red
energy beam, these sensors have
major advantages over other opto-electronic
systems when employed in dusty enviroments.
With their focused beam and long
range, opto-electronic sensors are
increasingly used in applications
where other sensing techniques are
lacking in sensing distance or accuracy.
Photoelectric sensors are available
in a variety of modes including:
- Infrared Proximity (Diffuse
Proximity type photoelectric
sensors detect the light reflected
by the target itself. Proximity
photoelectric sensors are preferable
for general purpose sensing
applications, particularly where
the detected object is only
accessible from one direction.
- Transmitted Beam (Thru-beam)
Transmitted beam photoelectric
sensors use separate infrared
transmitters and receivers.
Objects passing between the
two parts interrupt the infrared
beam, causing the receiver to
output a signal.
- Retroreflective (Reflex)
sensors operate by sensing the
light beam that is reflected
back from a target reflector.
As with thru beam models, objects
which interrupt the beam activate
an electronic output.
- Polarized Retroreflective
sensors work like normal retroreflective
sensors but use a polarizing
filter in front of the transmitter
and receiver optics. These filters
are designed so that shiny objects
are reliably detected.
- Fiber Optic
Fiber optic sensors use fiber
optic cable to conduct light
from the LED to the sensing
area, and another cable to return
light from the sensing area
to the receiver. By using fiber
optic cables, the electronics
can be protected from hostile
environments such as temperature
extremes and harsh chemicals.
Fiber optics also allow sensing
in extremely confined spaces.
- Background Rejection
STI's background rejection
sensors use a special arrangement
of two sensing zones: the near-field
zone is where objects can be
detected, the far-field zone
is where objects cannot be detected.
There is an extremely sharp
cut-off between these zones.
The cut-off range is adjustable.
These sensors are ideal for
applications where background
objects need to be ignored.
- Ultrasonic sensors
Ultrasonic sensor utilize the
reflection of high frequency (20KHz)
sound waves to detect parts or distances
to the parts. The two basic ultrasonic
sensor types are:
In general, ultrasonic sensors are
the best choice for transparent targets.
They can detect a sheet of transparent
plastic film as easily as a wooden
- Electrostatic - Uses capacitive
effects for longer range sensing
and wider bandwidth with more
- Piezoelectric - These rugged
and inexpensive sensors operate
by a charge displacement during
the strain in crystal lattices.