Banner Sensors - An introduction to the four types of proximity sensors

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

There are many types of proximity sensors, each of which is suitable for a specific application.

1. Inductive proximity sensor

Inductive proximity sensors work on the principle of using electromagnetic fields, so they can only detect metal targets. When a metal target enters an electromagnetic field, the sensing properties of the metal change the characteristics of the magnetic field, alerting the proximity sensor to the presence of a metal target. Depending on how well the metal is sensitive, the target can be detected at a greater or shorter distance.

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

The oscillator generates a symmetrical oscillating magnetic field that emanates from an array of coils at the ferrite core and sensing surface. When an iron target enters this magnetic field, a separate small electric current called eddy currents is generated on the metal surface. This changes the magnetic resistance (natural frequency) of the magnetic circuit, which in turn reduces the amplitude of the oscillation. As more metal enters the induction field, the amplitude of the oscillation decreases and eventually collapses. (This is the "eddy current suppression oscillator" or ECKO principle.) The Schmidt 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 limitations of the magnetic field, the sensing range of the inductive sensor is relatively narrow, from a few millimeters to 60 millimeters on average. However, the lack of range of inductive sensors makes up for it in environmental adaptability and the diversity of metal sensing.

With no wear and tear on moving parts, inductive proximity sensors have a long service life. However, it is important to note that metal contaminants, such as files in cutting applications, can sometimes affect the performance of the sensor, so inductive sensor housings are typically made of nickel-plated brass, stainless steel, or PBT plastic.

2. Capacitive proximity sensor

Capacitive proximity sensors can detect metallic and non-metallic targets in powder, granular, liquid, and solid form. This, combined with their ability to sense non-ferrous materials, makes them ideal 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 sensing head and positioned to work like open-circuit capacitors. In it, air acts as an insulator: at rest, the capacitance between the two plates is small. Like inductive sensors, these plates are connected to oscillators, Schmitt triggers, and output amplifiers. When the target enters the sensing region, the capacitance of the two plates increases, which causes the oscillator amplitude to change, which in turn changes the Schmitt trigger state and generates an output signal.

It is worth mentioning that it is important to note the difference between inductive and capacitive sensors: inductive sensors oscillate until a target is present, while capacitive sensors oscillate when a target is present.

Since capacitive sensing involves a charging pad, it is a bit slower than inductive sensing, which has a sensing range of 10 to 50 Hz and a sensing range of 3 to 60 mm.

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

3. Photoelectric proximity sensor

Photoelectric proximity sensors are versatile and can detect targets as small as 1 mm in diameter or up to 60 mm in distance.

All photosensors consist of several basic components: each sensor has an emitter light source (light-emitting diode, laser diode), a photodiode or phototransistor receiver to detect the emitted light, and auxiliary electronics to amplify the receiver signal.

There are three main types of photoelectric proximity sensors: reflective, through-beam, and diffuse. When the light emitted from the sensor is reflected back at the photoelectric receiver, the reflective proximity sensor detects the object. A through-beam sensor detects a target when it disconnects the beam between the sensor's transmitter and receiver.

The most reliable photoelectric induction is a thrift-beam sensor. The transmitter is separated from the receiver by a separate housing, which provides a constant beam. Detection occurs when an object passing between the two interrupts the beam.

Despite the high reliability of the through-beam type, it is the least popular optoelectronic device. Because the installation of the transmitter and receiver in two opposite positions, which can be very far apart, is expensive and laborious.

A unique feature of thru-beam photoelectric sensors is effective sensing in the presence of dense airborne contaminants. If contaminants accumulate directly on the transmitter or receiver, there is a higher chance of false triggering. However, some manufacturers now incorporate the alarm output into the sensor's circuitry to monitor the amount of light shining onto the receiver. If the detected light in the absence of a target drops to a specified level, the sensor warns via the built-in LED or output line.

The transmitter and receiver of the reflective proximity sensor do not have separate housings, but are both located in the same housing and facing the same direction. The emitter generates a laser, infrared, or visible beam and projects it onto a specially designed reflector, which then deflects the beam back to the receiver. Detection occurs when the optical path is disrupted or otherwise disturbed.

The advantage of reflective proximity sensors is that they are easy to arrange, and only one side of the sensor needs to be mounted, which can greatly reduce component and time costs.

Like reflective sensors, diffuse sensors have a transmitter and receiver in the same housing. But the detection target acts as a reflector, so the light reflected from a distance is detected.

The emitter emits a beam of light (most commonly pulsed infrared, visible red, or laser) that diffuses in all directions, filling a detection area. Then the target enters the area and deflects part of the beam back to the receiver. Probing occurs when enough light falls on the receiver, and the output is turned on or off (depending on whether the sensor is on or off).

A common example of diffuse sensors is a sensor-operated faucet on a public restroom sink. The hand placed under the sprinkler acts as a reflector, triggering the opening of the water valve. It should be noted that since the target (hand) is a reflector, diffuse photoelectric sensors are often subject to the target material and surface properties; Non-reflective targets, such as matte black paper, will have a much lower sensing range than bright white targets.

4. Ultrasonic sensor

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

The most common types are the same as those found in photoelectric induction: through, reflection and diffusion.

Ultrasonic diffuse proximity sensors employ acoustic sensors that emit a series of pulses of sound waves and then listen for their return from a reflected target. As soon as 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 reflex sensors can detect objects within a specified sensing distance by measuring the time of travel. The sensor emits a series of sound pulses that bounce off a fixed opposing reflector (any flat hard surface, a machine, a board). The sound waves must be returned to the sensor at user-adjusted intervals. If not, it is assumed that something is blocking the sensing path and the sensor sends out an output signal accordingly. Because the sensor listens for 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 through-beam photoelectric sensors, the transmitter and receiver of ultrasonic through-beam sensors are located in separate housings. When an object destroys the sound beam, the receiver triggers the output. These sensors are ideal for applications that require the detection of continuous objects, such as transparent plastic meshes. If the transparent plastic breaks, the output of the sensor triggers the connected PLC or load.

Banner Sensors - An introduction to the four types of proximity sensors

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