Proximity sensors are devices that are used to sense the presence of an object without physical contact. Proximity sensor uses different principles to sense objects. Such sensor is used an object should be sensed through nonmetallic contact.
There are different types of sensors, classified based on the principle used to detect. They are chosen depending on the type of material should sense.
Inductive Proximity sensor:
Inductive sensors are used to detect metallic object Inductive proximity sensors operate under the electrical principle of inductance, where a fluctuating current induces an electromotive force (emf) in a target object.
The inductive sensor using the oscillator circuit to generate a high-frequency electromagnetic field. When a metallic object comes to contact with the field, an eddy current will produce at the object surface. The eddy currents on the object absorb some of the radiated energy from the sensor, resulting in a loss of energy and change of strength of the oscillator.
The sensor detection section monitors the change in the strength of the magnetic field and triggers a solid state output level if there is a decrease in the strength of magnetic field.
If the metallic object released from the field, the oscillator turns to the initial stage.
Advantages:
-
Accurate compared to other technologies.
-
Have high switching rate
-
Can work in harsh environment condition
Disadvantages:
-
Detect only metallic target
-
Operating range may be limited
Capacitive sensor:
Capacitive sensors use a similar principle of inductive sensors. Only main difference is that capacitive sensors produce electrostatic field instead of an electromagnetic field.
Sensing surface is formed by two metal electrodes, oscillator produces electrostatic field between the two plates. When a metallic or non-metallic material enters the electrostatic field it changes the capacitance of the oscillator.
As a result, the oscillator circuit begins oscillating and changes the output state of the sensor when it reaches certain amplitude. As the target moves away from the field the oscillator comes to the original state.
Advantages:
-
It can detect both metallic and non-metallic objects.
-
High speed
-
Good stability
-
Good in terms of power usage
-
Low cost
Disadvantages:
-
Affected by temperature and humidity.
-
Not accurate compared to inductive sensors
-
Difficulties in designing