What is a proximity switch and how it works?
It
is the switch that detects the closeness/ proximity of an object. The switches
are non-contact type sensors that use magnetic, electric, or optical means to
sense the proximity of objects.
The below schematic diagram symbol for a proximity switch with mechanical contacts is the same as for a mechanical limit switch, except the switch symbol is enclosed by a diamond shape, indicating a powered or active device.
The
proximity switch will be in its “normal” condition/status when it is distant
from any actuating object that we have to detect. Due to the non-contact type in
nature, it is often used instead of direct-contact limit switches for the same
purpose of detecting the position of a machine part, with the advantage of never
wearing out over a long time due to repeated physical contact. However, the
greater complexity and cost of a proximity switch over a mechanical limit
switch shows their use in applications where lack of physical contact yields
tangible benefits.
What
are different types of proximity switches?
Most of the proximity, switches are active in design. They have a powered electronic circuit inside them to detect the proximity of an object.
- Inductive proximity switches detect the presence of metallic objects with the help of a high-frequency magnetic field.
- Capacitive proximity switches detect the presence of non-metallic objects with the help of a high-frequency electric field.
- Optical proximity switches detect the interruption of a light beam by an object when it passes through the beam.
- Ultrasonic proximity switches detect the presence of dense matter by the reflection of sound waves by sending the sample waves.
Most
of the proximity, switches have no “dry contact” outputs. Instead of the
contacts output elements are transistors configured as either source current or
sink current. Both “sourcing” and “sinking” will be understood by visualizing
electric current in the direction of electron flow instead of conventional
flow. The below schematic diagrams show the direction of current (conventional
flow notation). The load being driven by each proximity switch is a
light-emitting diode (LED) in both the examples:
What
is sinking and sourcing? or How the switch connected as “NPN” type and “PNP type?
An NPN switch uses a transistor in its output referred to as an electronic switch designed
to sink current through its signal wire. Similarly, a PNP switch is referred to as an electronic switch designed to source current through its signal wire. To understand
this, recognize the emitter terminal of the output transistor is always the one
connected to the power supply rail. The emitter must connect to the negative
rail, necessitating an NPN transistor to do the switching, for a sinking switch.
And the emitter connects to the positive rail for a PNP transistor will suffice
known as a sourcing switch.
Sometimes
sourcing and sinking transistor switches are referred to as high-side switches and
low-side switches, respectively. The emitter terminal is attached
directly to the “high” rail (+) of the DC power supply known as sourcing
transistor (PNP). And the emitter terminal is attached directly to the “low” rail
(-) of the DC power supply known as the sinking transistor (NPN).
There
are two different styles of electronic proximity switch shown below:
Most
of the industries that use proximity switches have built-in LED indicator lamps to
help technicians diagnose circuit problems by directly indicating the switch
status.
The below
photograph shows a proximity switch detecting the passing of teeth on a chain
sprocket, generating a slow square-wave electrical signal as the sprocket rotates.
Those switches may be used as a rotational speed sensor (sprocket speed
proportional to signal frequency) or as a broken chain sensor (when sensing the
rotation of the driven sprocket instead of the drive sprocket):
Proximity switches come in both “normally open” (NO) and “normally closed” (NC) types. Normally-open proximity switches that are sinking (NPN) as well as normally-open proximity switches that are sourcing (PNP), and normally-closed proximity switches in either sinking or sourcing designs as well. The proximity switch detection range is usually a fixed parameter rather than being adjustable.
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