Proximity Sensors | Types, Principle of Operation

Proximity sensors detect the presence of objects or indicate their absence using an electromagnetic field, light or sound. There are many types that are dedicated to specific, often very specific industrial or home applications.

Inductive sensors

https://termipol.pl/category/baza-strona-glowna-czujniki-zblizeniowe

Photo: Inductive Sensor

Such non-contact sensors detect objects made of ferromagnetic materials, such as steel objects. They consist of four main parts: a ferrite core with a coil, an oscillator, a Schmitt trigger, and an output amplifier. The oscillator generates a symmetrical magnetic field that radiates from the core to the detector. When the element we want to detect, made of ferromagnetic material, approaches the sensor, it also enters the so-called detection field, which induces a small current on the metal surface. This leads to a change in reluctance (magnetic resistance) of the magnetic circuit, which in turn reduces the oscillation amplitude. When the object is moved closer to the sensor and its detection field, the oscillations are reduced and even stopped. The Schmitt trigger reacts to these changes and adjusts the output state of the sensor. When the object is removed from the detection field, the circuit begins to oscillate again and the Schmitt trigger switches the output state back to the original state.

Fig. Principle and types of operation of inductive sensors.

Figure Inductive Sensor

When the sensor is in the NO (normal open) configuration - normally open, then the output state is high (ON) when the object enters the sensor's detection zone. Consistently, the NC (normal closed) configuration - normally closed, leads to switching the output to a low state (OFF). The output state is then read by an external pulse counter or PLC controller. Due to the use of a magnetic field and its limitations, sensors of this type have a relatively short range of action ranging from fractions of millimeters (typically from 2 to 8 mm) to a maximum of 60 mm.

The selection of the sensor begins with determining the detection distance, the geometry of the sensor and the finish of the detector itself. A shielded version (flush) is available as well as an unshielded version (non-flush). The shielded version reduces the detection radius to the "straight ahead" area from the sensor, which reduces the detection area compared to the unshielded version.

Inductive sensors do not offer a large detection range but they make up for it with their versatility in detecting metal objects. Due to the lack of moving parts exposed to wear and tear, proper installation guarantees long and trouble-free operation. Termipol's inductive sensors are made in IP67 class of tightness, which allows them to work in contaminated environments or exposure to liquids. The typical housing material for sensors is stainless steel.

Capacitive Sensors

https://termipol.pl/category/baza-strona-glowna-czujniki-zblizeniowe-czujniki-pojemnosciowe

Photo: Capacitive Sensor

Capacitive proximity sensors are able to detect both objects made of ferromagnetic materials (like an inductive sensor) and materials that do not exhibit these properties, such as plastic granules, powders, and liquids. An additional advantage is the ability to detect non-ferrous materials, which makes them suitable for detecting glass, detecting liquid levels in tanks, and measuring the amount of material in machine feeders in plastic processing.

The construction of a capacitive sensor is based on the use of two conducting plates (with different potentials) placed in the measuring head. The air surrounding the sensor acts as an insulator and in the normal state the capacitance of the system is negligible. As with the inductive sensor, the plates are connected to an oscillator, a Schmitt trigger, and an output amplifier. When an object appears in the sensor's detection zone, the capacitance of the system increases. This causes a change in the oscillation amplitude and, consequently, a change in the state of the Schmitt trigger and the output. The difference between the operation of a capacitive sensor and an inductive sensor is that the capacitive sensor oscillates when the object is within its range, while the inductive sensor oscillates when the object is not within its range.

Figure. Principle and types of operation of capacitive sensors.

Figure. Capacitive sensor

Due to the need to charge the plates forming the capacitor, capacitive sensors are characterized by a slower response (from 10Hz to 50 Hz) with a range from 3mm to 60mm. Due to the possibility of detecting very many materials, proper installation is necessary so that objects that are not the "target" are not detected. For this reason, it is recommended to use inductive sensors in the detection of ferritic materials (metal) to avoid interference that may arise due to the presence of other objects. Capacitive sensors have a regulation system in the form of a potentiometer, thanks to which it is possible to adjust the sensor to the application so as to reduce the influence of the background and only the selected object is detected.

Fig. Capacitive Sensor Operating Principle

The full range of capacitive sensors from Termipol can be seen on the website:

https://termipol.pl/category/baza-strona-glowna-czujniki-zblizeniowe-czujniki-pojemnosciowe

Photoelectric Sensors

Photoelectric sensors are one of the most widely used in industry. They are able to detect without contact, from a distance of 1mm to more than 60m. Classification of these sensors is based on the method by which the light is emitted and intercepted by the detector. All photoelectric sensors consist of three basic components: an emitter acting as a light source (LED, laser), a photodiode or phototransistor as a light detector, and signal amplifying circuits.

Photoelectric sensors also share the same operating principle. When an object is in the path of the beam of light emitted by the sensor, then depending on the configuration, the output state changes to high or low.

Through-beam Type

They are the most reliable group of photoelectric sensors. The emitter and detector are housed in separate enclosures. The emitter continuously emits a beam of light and detection occurs when an object crosses the beam and blocks the detector. Despite its high reliability, this type of sensor is rarely chosen due to difficult installation. However, when we need to detect objects located at a great distance (25 m or more), "Through-beam" sensors may be the right solution. The typical operating frequency of this type of sensor is 500 Hz and the distinguishing feature of this type of sensor is the ability to work in an environment with contaminated atmosphere, e.g. dust. The reason is the principle of operation based on the reaction directly to the beam crossing between the opposite detector and emitter without the need to take into account issues related to the reflection of radiation, physical properties of the object, etc.

In addition to industrial applications, "Throug-beam" sensors can also be useful in the home. In order to detect obstacles on the way of automatic garage doors and protect objects such as cars, bicycles, toys from damage, sensors of the "Through –beam" type are used.

Type with Reflector Element

Reflective element sensors also have a large detection range (up to 10 m). They work on the same principle as "Through-beam" sensors, an object is detected when the light beam between the sensor and the reflector is interrupted. The emitter and detector are housed in one enclosure. The emitter emits laser light, infrared radiation or a visible light beam which is reflected from the reflective element and reaches the detector. This type of sensor is characterized by easier installation compared to "Through-beam" sensors and lower costs due to the placement of the emitter and detector in one enclosure. It may be problematic to detect objects with a shiny housing or glass that can reflect radiation and interfere with the measurement. Some manufacturers have solved this problem by using polarizing filters that narrow the detection only to light from the reflector.

Type "Diffuse"

https://termipol.pl/category/baza-strona-glowna-czujniki-optyczne-czujniki-dyfuzyjne

Photo: Diffuse Sensor

Diffuse sensors have an emitter and a detector built into one housing, however in this type of construction it is the object detected that reflects the radiation and thus plays the role of a reflector. The emitter sends out a beam of light (pulsed infrared radiation, visible red light or laser) which spreads out and fills the detection zone. When an object enters this zone and reflects part of the radiation sent out by the sensor, then detection occurs and the output state changes.

This type of sensors are used in public toilets where they control the water flow when hands are placed under the tap. Due to the large dependence on the physical properties of the detected objects, diffuse sensors should be selected for the appropriate applications. Only objects that are able to reflect enough light and at the right angle can be detected by this type of sensor. In practice, most objects meet these requirements and only a few are undetectable.

Thanks to their properties, these sensors can also be used to distinguish between light and dark objects. Easy installation is another advantage of this type of sensors and advanced systems for eliminating interference introduced by objects in the background allow for trouble-free operation.

Ultrasonic Sensors

https://termipol.pl/category/baza-strona-glowna-czujniki-poziomu-cieczy-ultradzwiekowe-czujniki-poziomu

Ultrasonic proximity sensors are widely used in industrial applications. They use acoustic waves, making them insensitive to the transparency or color of objects. The use of this type of sensor allows for difficult applications to be solved by previous types of sensors, such as the detection of transparent glass or plastic plates at a long distance, continuous liquid level measurement or bulk material measurement. Subtypes of ultrasonic sensors, similar to photoelectric sensors, can be divided into: “Through-beam” with a reflective element and “Diffuse” type.

Ultrasonic “Diffuse” sensors consist of a sound transducer that emits a pulsed acoustic wave and then registers the response in the form of a reflected wave. The range of operation for this type of sensor is up to 2.5 m in standard solutions. The sensitivity of the sensor can be adjusted using a potentiometer mounted on the housing. In addition, some types offer an analog output in which the current (4-20 mA) or voltage (0 to 10 Vdc) changes depending on the distance of the object from the sensor.

Figure. Principle and types of operation of photoelectric sensors.