Understanding the Functionality of Proximity Sensors

Proximity sensors are widely used in a variety of applications from industrial automation to mobile phones. They are designed to detect the presence of an object without physical contact making them a valuable tool for many industries. In this article, we will explore the working principle of proximity sensors.

How Does a Proximity Sensor Work?

The working principle of an inductive or capacitive proximity sensor is based on a high-frequency oscillator that creates a field in the close surroundings of the sensing surface. The presence of a metallic object (inductive) or any material (capacitive) in the functional area causes a change in the oscillation amplitude.

The rise or fall of such oscillation is identified by a threshold circuit that changes the output state of the sensor. The operating distance of the sensor depends on the actuator’s shape and size and is strictly linked to the nature of the material. A screw placed on the back of the capacitive sensor allows regulation of the operating distance. This sensitivity regulation is useful in applications such as the detection of full containers and the non-detection of empty containers.

In the magnetic type, the sensors are actuated by the presence of a permanent magnet. Their operating principle is based on the use of reed contacts which consist of two low-reluctance ferromagnetic reeds enclosed in glass bulbs containing inert gas. The reciprocal attraction of both reeds in the presence of a magnetic field, due to magnetic induction, establishes an electrical contact.

Materials to be detected

Choosing the right sensor is not always so easy at first glance. The material to be detected is a key selection parameter.

Object material to be detected	Proximity sensor technology
Metals Steel, aluminum, brass, copper, etc	Inductive
Liquids Water, acid, etc.	Capacitive
Bulk materials Powders, granules, etc.	Capacitive
Other solids Wood, paper, etc.	Capacitive
Magnets Permanent magnets, etc.	Magnetic

Shielded and non-shielded models

The proximity sensors differ between shielded and non-shielded. The shielded models have the sensitive part completely shielded from the sensor body. The field generated by the sensor is only present on the active face and therefore only detects a front-positioned target. These models can be mounted completely embedded in the metal body of the machine. The non-shielded models have the sensitive part jutting out from the sensor body. The detection range is also present laterally to the active face. Thus, a more significant extension makes the detection range greater than the shielded models. These models must be mounted jutting out from the metal body of the machine.

Features of proximity sensors

Proximity sensors detect an object without touching it and they, therefore, do not cause abrasion or damage to the object.

No contacts are used for output, so the sensor has a longer service life.

Unlike optical detection methods, they are suitable for use in locations where water or oil is used.

Proximity sensors provide high-speed response, compared with switches that require physical contact.

They can be used in a wide temperature range and are not affected by colors.

Unlike switches, which rely on physical contact, proximity sensors are affected by ambient temperatures, surrounding objects and other sensors.

They must not be mutually influenced. For this reason, a minimum distance between them must be provided.

If not otherwise specified, proximity sensors have minimum IP65 protection against dust and water jets.

Proximity sensors may incorporate one or more color indicators.

Each application has a specific space available for the sensor and each application has a requirement for how close the sensor can be mounted to the sensed object.