Types:
Because there are no moving parts within a proximity,
except if it uses a relay to switch the output, it has unlimited lifespan
and requires no maintenance whatsoever.
It is for this reason I recommend to use the solid
state type to switch the relay externally because then the switch itself is
ultra-reliable.
Inductive sensor: Applying a frequency to
a coil creates a magnetic field around it.
Mount such a coil in a housing, then when a metallic object enters, the
magnetic field
is modified. This field modification is amplified to switch an output stage
e.g. relay, transistor(for DC) or triac(for AC).
The type of material (steel, aluminium, copper, brass
etc.) has a higher/lower effect on the field.
Capacitive sensor: These detectors also
have an oscillator but they monitor the dielectric of air, and whenever it
is different it knows there is something different in the field.
To phrase it a bit more technically correct is to say:
When an ac signal is applied to 2 plates with a dielectric medium between
the plates, it forms a capacitance circuit. The value of capacitance is
determined by size/types of plates, type of dielectric medium, distance
between the plates and so forth. For capacitive sensors this medium is air
and any change within this field is amplified.
It is for this reason capacitive sensors are said to be
able to sense any material and even works through glass and other mediums,
if set correctly.
Namur sensor: This refers to the 2 wire
technology where the sensor uses the "leakage" (off/on state) current through the
load to power the electronics of the sensor. The device resistance changes
with activation of the sensing field. Essentially it is a sensor with no
output amplifier and they can be designed around any sensing technology:-
Inductive, capacitive, magnetic, photo electric etc..
Namur sensors works with specific
controllers(amplifiers) only and cannot work with normal loads such as
relays. To give an idea of how specific the criteria is: For N/C, when not
activated, the circuit mA is >2-3mA (Normal closed condition), the normal
open condition <1mA. Usually normal closed but one can get normal open
versions.
Namur sensors comes into its own in intrinsic safe
environments (explosive environments) where the amount of energy available
in the different EX zones are explicitly defined by law. In this case the
namur sensor would then be used in conjunction with INTRINSIC SAFETY
BARRIER/ I.S BARRIER/ GALVANIC ISOLATION BARRIER.
Photo electric sensors: Light beams
(optical)...that is what makes these tick. This includes Infrared, laser
light beams.
Different solutions dependant on range required
PS: The noted ranges is from what I am aware of.
