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Four types of proximity sensors

new.qq.com | 12/04/2021

Proximity sensors are a type of sensor that uses electromagnetic fields, light and sound to detect the presence or absence of objects.

There are many types of proximity sensors, and each type is suitable for specific application scenarios.

1. Inductive proximity sensor

The working principle of an inductive proximity sensor is to use electromagnetic fields, so it can only detect metal targets. When the metal target enters the electromagnetic field, the inductive characteristics of the metal will change the characteristics of the magnetic field, thereby reminding the proximity sensor of the existence of the metal target. Depending on how sensitive the metal is, the target can be detected at a greater or shorter distance.

The inductive proximity sensor consists of four main parts: a ferrite core with a coil, an oscillator, a Schmitt trigger and an output amplifier.

The oscillator generates a symmetrical oscillating magnetic field, which is emitted from the ferrite core and the coil array at the sensing surface. When the iron target enters the magnetic field, an independent small current called eddy current is generated on the metal surface. This changes the magnetic resistance (natural frequency) of the magnetic circuit, thereby reducing the oscillation amplitude. As more metal enters the induction field, the oscillation amplitude decreases and eventually collapses. (This is the "eddy current suppression oscillator" or ECKO principle.) The Schmitt trigger responds to these amplitude changes and adjusts the sensor output. When the target finally leaves the range of the sensor, the circuit begins to oscillate again, and the Schmitt trigger returns the sensor to its previous output.

Due to the limitation of the magnetic field, the sensing range of the inductive sensor is relatively narrow, ranging from a few millimeters to 60 millimeters on average. However, the inductive sensor's lack of range has been compensated for in terms of environmental adaptability and the diversity of metal induction.

Since there is no wear of moving parts, the inductive proximity sensor has a long service life. However, it should be noted that metal contaminants (such as files in cutting applications) sometimes affect the performance of the sensor. Therefore, inductive sensor housings usually use nickel-plated brass, stainless steel or PBT plastic.

2. Capacitive proximity sensor

Capacitive proximity sensors can detect metallic and non-metallic targets in powder, particle, liquid and solid forms. This, coupled with their ability to sense non-ferrous metal materials, makes it very suitable for observation glass monitoring, tank level detection and hopper powder level identification.

In capacitive sensors, two conductive plates (at different potentials) are housed in the sensor head and are positioned to work like open-circuit capacitors. The air acts as an insulator: at rest, the capacitance between the two plates is very small. Like inductive sensors, these plates are also connected to oscillators, Schmitt triggers and output amplifiers. When the target enters the sensing zone, the capacitance of the two plates increases, which causes the amplitude of the oscillator to change, which in turn changes the Schmitt trigger state and generates an output signal.

It is worth mentioning that please note the difference between inductive sensors and capacitive sensors: inductive sensors oscillate until there is a target, while capacitive sensors oscillate when there is a target.

Since capacitive sensing involves a charging board, it is slower than inductive sensing. The sensing range is 10 to 50 Hz and the sensing range is 3 to 60 mm.

Since capacitive sensors can detect most types of materials, they must be kept away from non-target materials to avoid false triggering. Therefore, if the target contains ferrous materials, an inductive sensor is a more reliable choice.

3. Photoelectric proximity sensor

The photoelectric proximity sensor has a wide range of uses and can detect targets as small as 1 mm in diameter or as large as 60 mm in distance.

All photoelectric sensors are composed of several basic components: each sensor has a transmitter light source (light emitting diode, laser diode), a photodiode or phototransistor receiver for detecting the emitted light, and a receiver for amplification Signal auxiliary electronic equipment.

There are three main types of photoelectric proximity sensors: reflective, through-beam and diffuse. When the light from the sensor is reflected back at the photoreceptor, the reflective proximity sensor detects the object. When the target breaks the light beam between the transmitter and receiver of the sensor, the through-beam sensor detects the target.

The most reliable photoelectric sensor is the through-beam sensor. The transmitter is separated from the receiver by a separate housing to provide a constant beam. When an object passing between the two interrupts the beam, it will be detected.

Although the through-beam type has high reliability, it is the least popular optoelectronic device. Because the transmitter and receiver are installed in two opposite locations (possibly far apart), it is expensive and laborious to install.

A unique function of the through-beam photoelectric sensor is effective sensing in the presence of dense airborne pollutants. If contaminants accumulate directly on the transmitter or receiver, the possibility of false triggering is higher. However, some manufacturers now incorporate the alarm output into the sensor's circuit to monitor the amount of light hitting the receiver. If the detected light drops to a specified level without a target, the sensor will issue a warning via the built-in LED or output line.

The transmitter and receiver of the reflective proximity sensor do not have separate housings, but are located in the same housing and face the same direction. The transmitter generates a laser, infrared or visible light beam and projects it onto a specially designed reflector, which then deflects the beam back to the receiver. When the light path is damaged or other interference, it will be detected.

The advantage of the reflective proximity sensor is that it is convenient to arrange, only need to install the sensor on one side, which can greatly save the cost of components and time.

Like the reflective sensor, the transmitter and receiver of the diffuse sensor are located in the same housing. But the detection target acts as a reflector, so it detects light reflected from a distance.

The transmitter emits a beam of light (the most common is pulsed infrared, visible red or laser), which spreads in all directions to fill a detection area. The target then enters the area and deflects part of the beam back to the receiver. When enough light falls on the receiver, detection occurs, and the output is turned on or off (depending on whether the sensor is on or dark).

A common example of a diffuse sensor is an inductive faucet on a sink in a public restroom. The hand placed under the nozzle acts as a reflector to trigger the opening of the water valve. Note that since the target (hand) is a reflector, the diffuse photoelectric sensor is often limited by the target material and surface characteristics; compared with bright white targets, the sensing range of non-reflective targets (such as matt black paper) Will be greatly reduced.

4. Ultrasonic sensor

Ultrasonic proximity sensors are used in many automated production processes. They use sound waves to detect objects, so color and transparency will not affect them. This makes them ideal for a variety of applications, including remote detection of transparent glass and plastic, distance measurement, continuous liquid and particulate level control, and paper, sheet metal, and wood stacking.

The most common types are the same as those in photoelectric sensing: through-beam, reflection and diffusion.

The ultrasonic diffuse proximity sensor uses a sound wave sensor, which emits a series of sound wave pulses and then listens for the sound they return from the reflective target. Once the reflected signal is received, the sensor sends the output signal to the control device. The sensing range is extended to 2.5 m.

Ultrasonic reflection sensors can detect objects within a specified sensing distance by measuring propagation time. The sensor emits a series of sound pulses that bounce off a fixed opposite reflector (any flat hard surface, a machine, a board). The sound wave must return to the sensor within the time interval adjusted by the user. If not, it is considered that an object is blocking the sensing path, and the sensor sends out an output signal accordingly. Because the sensor listens to changes in propagation time, rather than just returning a signal, it is ideal for detecting sound-absorbing and deflecting materials (such as cotton, foam, cloth, and foam rubber).

Similar to the through-beam photoelectric sensor, the transmitter and receiver of the ultrasonic through-beam sensor are located in a separate housing. When an object destroys the sound beam, the receiver triggers the output. These sensors are ideal for applications that need to detect continuous objects (such as transparent plastic mesh). If the transparent plastic breaks, the output of the sensor will trigger the connected PLC or load.

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