Proximity sensors — often called, proximity switches — detect the presence of nearby objects without making physical contact with them. These objects are called targets. There are several types of proximity sensors, including those used to detect metallic or non-metallic objects. Others are used to detect a specific object.
These sensors also rely on different working principles. However, each one lacks mechanical parts, offers contact-less operation, and is unaffected by its surroundings. They’re also highly durable and suitable for a wide range of applications.
Typically, proximity sensors are classified based on their detection targets, such as a type of material or sense for something in the environment.
According to the method of detection, proximity sensors are classified into five categories:
Optical proximity sensors
Optical proximity sensors are used for light detection and, typically, employ an infrared spectrum of light. The sensor consists of a light source, such as IR LED or laser diode, and a light detector, such as a photodiode or phototransistor. The light source transmits lR radiation, which is reflected by a nearby object. The reflected radiations are detected by the detector, which then generates a voltage output.
Optical sensors offer a short range of detection and can perceive several types of materials. However, its sensing capability can be affected by the nature of an object, the presence of dust, or other environmental factors.
These sensors are commonly used for object detection, product sorting, and contrast detection.
Ultrasonic proximity sensors
As the name suggests, ultrasonic sensors use ultrasonic sound waves to detect objects of various materials. They’re generally used to sense the distance from an object or surface area. The sensor will emit ultrasonic pulses that an object, then, reflects. These reflected waves are identified, and the time it takes to do so is calculated. According to this calculation, the distance from the object is determined.
Ultrasonic sensors can have a range of several meters. As they employ sound waves, these sensors are sensitive to temperature changes and movement. They’re generally used for detecting the distance or motion of nearby objects or surface areas.
Inductive proximity sensors
Inductive sensors use inductance for the detection. These sensors are used to detect metallic objects in the vicinity of their active side by employing an electromagnetic field to induce an eddy current in a nearby object.
The electromagnetic field is produced using an oscillation circuit. Due to the eddy current in the target, a second magnetic field is produced, which is detected by a receiver coil. As the sensor approaches the target, the induction current in the target increases, and the load on the oscillation circuit increases. As a result, the oscillation attenuates or stops.
When the oscillation attenuates, its amplitude is decreased. A circuit detects the change in the oscillation status and operates as an indicator, typically producing a digital or analog output.
It’s important to note that these sensors are only used to detect metallic objects. They have a short range of detection and anything magnetic in nature can affect their operation. These low-cost sensors are often used as metal detectors or to support object detection at tolls or in assembly lines.
Magnetic proximity sensors
These sensors use permanent magnets for detection and are used to locate magnetic objects from a critical distance. When a target enters the sensor’s detection range, it triggers a switching process because of its magnetic characteristics.
Even compact magnetic sensors can locate magnetic objects over a long distance, penetrating through wood, plastic, and other non-magnetizable surfaces.
As an operating principle, they can work based on the:
- Hall effect
- Magneto-resistive effect
- Variable reluctance
- Giant magneto-resistive effect
- Reed switches
Magnetic proximity sensors are particularly sensitive to Reed-techno magnetic field disturbances and the Hall effect.
Capacitive proximity sensors
Capacitive sensors use a change in capacitance to detect an object. They consist of a high-frequency oscillator with a sensing surface that’s formed by a pair of electrodes.
As an object nears the sensor, the capacitance between the electrodes changes because of the electrostatic field. The change in capacitance increases the amplitude of the oscillation. When the amplitude of oscillation peaks a certain threshold, the sensor identifies the presence of the target.
These sensors offer a short range and can detect various types of materials. However, this capability can be affected by the dielectric properties of the target and humidity.
Capacitive proximity sensors are used to identify objects through a surface or to detect different levels of liquid and solid materials.
There are several types of proximity sensors, including optical, ultrasonic, inductive, magnetic, and capacitive types. Each type works on a different operating principle, detecting objects based on a set feature, such as sound waves or capacitance.
Depending on the type, proximity sensors identify metallic, non-metallic, or magnetic objects within a set environment or pathway.
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