Optical Proximity Switches - Diffuse
These photoelectric proximity switches combine the transmitter and the receiver
all in one housing, the light emitted by the transmitter is reflected by the target
object and the receiver evaluates this reflected light. The advantage of this
method is that no reflector is required.
As the proximity switch evaluates the reflected light and its energy, the scanning
range of conventional sensors (also called energetic optical sensors) depends to
a great extent on the colour of the target object and on its surface.
Due to the poor reflection properties of dark materials, the sensing distances of
diffuse sensors are reduced on those surfaces. The type of reflection depends
on the structure of the surface so that very rough, inhomogeneous surfaces reflect
light in a diffuse way, i.e. in all directions. Only a small amount of the
light sent out is reflected to the receiver and in this case, the scanning range
is very short.
Proximity switches, which are based on energetic evaluation, are ideally suited
to the detection of larger objects or of objects whose colour and surface structure
are constant. However, attention must be paid to the fact that the quantity
of light reflected by the background should not be greater than the quantity of
light reflected by the object itself. This effect occurs, for example, with
a black object in front of a white background. In this case, detection with
a diffuse proximity switch is impossible, so we recommend the use of a proximity
switch with background suppression in these applications.
If the background is beyond the detection range, as is the case when a diffuse proximity
switch points across a conveyor belt, reliable detection of objects is still possible,
the sensor is 'taught' what to look for by an adjusting screw on the sensor itself.
During a detection application without a background, the sensor can be set to its
maximum scanning range, however for applications with a background, an accurate
setting must be made.
The sensor 'senses' the light energy bouncing back in to the receiver, sometimes
called an 'energetic' type sensor.
Proximity Switches - Retro Reflective
The retro-reflective light barrier also combines the transmitter and the receiver
in one housing, the light emitted by the transmitter is reflected by a reflector
(target)and the receiver evaluates the reflected light. The small size of
the reflector is advantageous for accurate detection and in addition, it is easy
to install, being a passive element not requiring connection to an electrical supply.
As with through-beam sensors, retro-reflective light barriers are often selected
according to their scanning range. As the light has to travel the distance
between the sensor and the reflector twice, it is often called a two-way light barrier.
The reflector does however, cause additional dispersion, which has an unfavourable
effect on the scanning range. In simplified terms, the transmitter sends out
a cone of light and the diameter of the light cone increases with scanning range.
This is the reason why for large scanning ranges the reflector is larger than for
small scanning ranges. A data sheet should indicate the scanning range for
each reflector type.
Modern manufacturing techniques allow the 'light cone' to be tightly controlled,
laser sensors produce a near parallel light beam which is extremely small and parallel
across the whole detection area. This advantage is particularly useful for
the reliable detection of very small objects inside the whole working range.
Regardless of their physical principle, all SensoPart retro-reflective sensors possess
a polarizing filter,these are optical filters which are comparable to a line grid.
Incoming light, which oscillates in all directions, is filtered and only light which
oscillates parallel to the grid plane is let through. By the use of polarizing filters
in combination with triple reflectors, retro-reflective light barriers can detect
even mirror-like objects reliably.
Proximity Switches - Background Suppressed
Different object colours and surfaces can heavily affect the detection behaviour
of a diffuse (energetic) proximity switch. With purely diffuse evaluation,
it is not possible to detect a black object in front of a white background as the
white background reflects more light than the black object itself.
In order to deal with such applications reliably, the background suppression procedure
was developed. In this procedure, the light reflected by the background and
by the object is evaluated, the light being captured by two different receiver elements.
With this procedure, it is possible to reliably detect a dark object on a light
colour conveyor belt.
There are two different methods for the physical implementation of background suppression,
a distinction being made between a fixed and an adjustable background suppression.
With a fixed background suppression, the transmitter and receiver elements are rigidly
mounted. One receiver element captures the light from the object, and the
other receiver captures light from the background. It is not the returning
energy which is evaluated but rather the geometric position of the object to be
detected. The overlap of the transmitter and receiver angles fixes the field of
detection. Objects outside the field of detection cannot be detected.
With adjustable background suppression, the parameters for the detection of objects
can be mechanically adjusted with an adjusting screw or electronically taught.
This enables much greater flexibility of use.
Laser units are particularly suitable for the detection of very small objects, with
larger objects, a red-light or infra-red sensor should be used. Background
suppression must be used to enable the detection of a dark object on a light conveyor
Proximity Switches -Through Beam
Simplicity itself - The transmitter and receiver are in separate housings and the
beam is broken to detect an object.
The transmitter and receiver of through-beam sensors are separate devices which
means that the emitted light has to travel the distance between the transmitter
and the receiver only once. Therefore, they are called through-beam sensors
and can reach very large scanning ranges.
The sensing range of a through beam sensor is very application dependant, particularly
if they are used in dusty or steamy conditions where it is important to pay attention
that the sensors are not run close to their sensing range limits. Attenuation
often results in these cases, which reduces the sensing range.
The sensing range listed in the data sheet should not be exceeded in order to guarantee
functionality in poor operating conditions. If corner mirrors are used, the
total distance to be monitored should be within the indicated sensing range.
A special feature available to one-way sensors has been incorporated in the Sensopart
FLS/FLE 18 laser sensor which has been equipped with special focusing optics.
This makes it possible to alter the diameter of the light spot. The scanning distance
of this sensor is a maximum of 50 meters and the light spot can be reduced to a
diameter of 0.03 mm. As a result it is possible to reliably detect even the
smallest objects at large distances.
Fibre Optic Amplifiers
A Fibre Optic Amplifier can be considered as a simple light source and detector,
a particularly useful feature of such fibre optic sensors is that they can provide
distributed sensing over distances of up to several meters.
A major benefit of fibre optic sensors is their ability to reach places which are
otherwise inaccessible such as ATEX installations, confined machine tooling and
high temperature areas, typically -30'C up to +160'C.
Optical fibres are very versatile and can be used as sensors for through beams,
reflective or analogue type applications.
An optical fibre is a glass or plastic fibre that carries light along its length.
Fibres are used instead of metal wires because signals travel along them
with less loss and they are also immune to electromagnetic interference. Specially
designed fibres are used for a variety of other applications, including sensors
and fibre lasers.
Light is kept in the core of the optical fibre by total internal reflection, this
causes the fibre to act as a waveguide. Joining lengths of optical fibre is
more complex than joining electrical wire or cable as the ends of the fibres must
be carefully cleaved, and then spliced together, either mechanically or by fusing
them together with an electric arc. Special connectors are used to make removable
Fibres have many uses in remote sensing. In some applications, fibre may be
used because of its small size, or the fact that no electrical power is needed at
the remote location.
Fibre Optic Leads
Fibre optics are basically divided into two categories - glass and plastic transmission
media. Fibre optics with glass are referred to as glass fibres and use of
a very high-quality mineral glass achieves much better performance compared with
plastic fibres. Attenuation is less with glass fibres than with plastic fibres
which makes it possible to cover considerably longer transmission paths. It
is also possible to use glass fibres in very high temperatures with the appropriate
With such a large product range and the ability to customise requirements we are
not going to cover the range here but please feel free to contact us with your requirements.