Proximity Sensor

Proximity Sensor is a sensor to detect the object without touching the object being detected. The detected object can be either metal and non metal.

Proximity Capacitive 875C, 875CP
Tubular by Allen Bradley

This sensor type is widely used because when compared with photoelectric sensor, this sensor is more stable, especially not effected by dirt or dust. Regarding the detection distance, there are two models with fixed detection range and adjustable range detection.
If viewed from the principle of work, or object that is detected is devided into two types:

1. Inductive type, work on the principle of inductive or magnetic field which generated by electric current. while the object that detected is metal. example : all metal products (eg. car).
2. Capacitive type, work on the principle of capacitive. This type can detect both metal and non-metal. example : wood, plastic etc.

E2K-L Capacitive
Proximity by Omron

TLN,TLQ,TLG Inductive
Proximity by Omron

Two pictures above are an example of two type of proximity sensor. Can be seen for type E2K-L functioned as liquid level sensor.
About the maker of this sensor, there are several makers such Allen Bradley, Keyence, Omron, and Fuji. Basically the same type produced. some variations as follows:

1. integrated amplifier and sensor head, This type usually has a fixed  detection range.
2. Separate amplifier and sensor head, the detection range usually can be set using amplifier.
3. Amplifier in cable, It's compact proximity with fix range detection. As the example can be seen in the picture below.

EM Series proximity
sensor by Keyence

Finally, it can be concluded that in choosing the type of proximity will be based on the object (metal-non metal), distance of object as well as possible size according to available space.