INDUCTIVE PROXIMITY SENSORS
They are commonly known as metal detecting sensors.
Typical image of an inductive sensor
They
use the principle of electromagnetic induction where the presence of metal
changes the electromagnetic field and this detected by an internal circuit. Non-metallic objects do not interfere with the electromagnetic field hence can-not
be detected.
- The sensor consists of 4 main components
- Induction coils
- Oscillator electronics
- Schmitt trigger circuit (comparator circuit)
- Output switching circuit
When
a metallic objects comes within the range of the sensing distance of the sensor
according to Faraday’s first law of induction and E.M.F is induced in the object. The E.M.F leads to current flow of which according to Lenz's law of electromagnetic induction it creates a magnetic field opposing the change causing it.
The opposing magnetic field causes a decrease in the intensity or amplitude of the sensor’s oscillations. As
the object nears the sensors the oscillations reduce to such a point that the internal
circuit termed as Schmitt trigger detects that the intensity/ amplitude has fallen below a
predetermined level. This
predetermined threshold is where the presence of a metal has been confirmed and
the sensor output is switched ‘ON’.
Benefits of inductive proximity sensors
- Ease to use to detect metallic objects.
- Very robust even in harsh environments.
- Their contactless principle means there is no abrasion hence a longer service life.
- Maintenance free.
- High accuracy and repeatability.
Shortcomings of inductive proximity sensors
- Short operating ranges
- Typically output is limited to NO or NC.
TYPICAL APPLICATIONS
- Presence detection
Useful in detection of caps in automated fillers and metallic workpieces in machining centers.
ii. Control / monitor positions like in cams
iii. End position detection