Proximity sensors are devices that have found varying uses in many different industrial and manufacturing environments. Basically, they are made to sense the presence of objects and perform some action, or to simply flag their the presence or absence. Key to their operation is that they no physical contact is required with the target in order to be sensed. This explains why they’re typically known as non-contact sensors.
There are lots of general sensing techniques that proximity sensors are known to use. Such techniques work to categorize different sensor types on top of other relevant factors like the object that must be detected or the most conducive environmental conditions for detection by a certain sensor type.
Here are the most common types of proximity sensors in use today:
Capacitive
As the term indicates, these sensors work by detecting a change in the capacitance, or the ratio of the change in an electric charge to the matching change in its electric potential as affected by an air gap or the presence of another material between two surfaces. Every time an object to be detected comes into the field of the sensor, the capacitance is changed, and this how the sensor detects the presence of that object.
Inductive
Inductive sensors detect variations in inductance, or inducing voltage in a conductor in response to a different conductor’s changing current. Inductive sensors, however, are only effective with metallic objects because of their inductive properties.
Aside from that, sensing distance also depends on the object sensed. Compared to copper and aluminum, for example, ferromagnetic objects like steel have the most extensive sensing distances.
Photoelectric
These sensors are light-based, meaning, they know when an object is present or absent based on changes in the amount of light. Photoelectric sensors come in two major types – reflective and through-beam. Reflective sensors emit a beam of light that hits the object and is reflected back to the sensor, often in the same physical container as the emitter beam. Through-beam sensors, on the other hand, are made of two separate items – a light source or emitter and a detector or receiver. Every time an object breaks the light beam, the sensor takes note.
Ultrasonic
Lastly, ultrasonic sensors are dependent on sound waves when sensing the presence or absence of objects. A high frequency sound wave that humans cannot detect is emitted by the sensor and returned after it comes in contact with an object, where the distance of that object from the sensor can be computed by considering the amount of time the sound wave took to return. As such, these sensors are used to measure the distance of objects, like in automotive park-assist and in filling applications where fluid levels must be detected as part of the process.
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