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 inductive properties of the metal change the magnetic field characteristics, alerting the proximity sensor to the presence of a metal target. Depending on how much the metal is perceived, the target can be detected at a greater or shorter distance.
An inductive proximity sensor consists of four main parts: a coil ferrite core, an oscillator, a Schmitt trigger, and an output amplifier.
This oscillator generates a symmetrically oscillating magnetic field emitted by an array of coils located at the ferrite core and the sensing surface. When an iron target enters a magnetic field, a small independent electric current is generated on the surface of the metal, called an eddy current. This changes the magnetoresistance (natural frequency) of the magnetic circuit, reducing the amplitude of the oscillation. As more metal enters the induction field, the amplitude of the oscillation decreases and eventually collapses. (This is the "vortex 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 Schmidt 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, averaging between a few millimeters and 60 millimeters. However, the environmental adaptability of inductive sensors and the versatility of metal sensing make up for their shortcomings in range. Inductive proximity sensors have a long service life due to the absence of wear and tear on moving parts. However, it should be noted that metal contaminants, such as files in cutting applications, can sometimes affect the performance of the sensor. For this reason, the housing of inductive sensors is usually 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 sensor head and positioned to operate like open-circuit capacitors. Air acts as an insulator: at rest, the capacitance between the two plates is small. Like inductive sensors, these boards are connected to oscillators, Schmitt triggers, and output amplifiers. When the target enters the sensing area, the capacitance of the two plates increases, causing the amplitude of the oscillator to change, changing the Schmitt trigger state and generating an output signal.
It is worth mentioning that it is important to note the difference between inductive and capacitive sensors: inductive sensors oscillate to a target, and capacitive sensors oscillate to a target. Since capacitive induction involves a charging pad, it is slower than inductive induction, with an inductive range of 10 ~ 50Hz and an inductive range of 3 ~ 60mm. 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, inductive sensors are a more reliable choice.
3. Photoelectric proximity sensor:
Photoelectric proximity sensors are widely used to detect targets as small as 1 mm in diameter or as large as 60 mm in distance. All photosensors consist of several basic components: each sensor has an emitter, a light source (light-emitting diode, laser diode), a photodiode or phototransistor receiver to detect the emitted light, and auxiliary electronics to amplify the received signal.
There are three main types of photoelectric proximity sensors: reflective, reflective, and diffuse.
When the light emitted by the sensor is reflected back by the photoelectric receiver, the reflective proximity sensor will detect the target. When the target disconnects the beam between the sensor transmitter and receiver, the opposing sensor will detect the target.
A reliable photoelectric sensor is the opposite type of sensor. The transmitter and receiver are separated by a separate enclosure to provide a constant beam. The beam is detected when it is interrupted by an object that passes through the two. Trans optoelectronic devices, despite their high reliability, are undesirable optoelectronic devices. Because it is expensive and laborious to install the transmitter and receiver in two opposite positions, which can be very far away.
A unique feature of radiant photoelectric sensors is the effective perception of the presence of strong air pollutants. If contaminants accumulate directly on the transmitter or receiver, there is a higher chance of false triggering. However, some manufacturers now integrate the alarm output into the sensor's circuitry to monitor the amount of light emitted to the receiver. When the detected light drops to the specified brightness in the absence of an object, the sensor warns via the built-in LED or output line.
The transmitter and receiver of a reflective proximity sensor do not have separate housings, but are located within the same housing and facing the same direction. The emitter produces a beam of laser, infrared, or visible light and projects it onto a specially designed reflector, which then deflects the beam back to the receiver. The optical path is detected when it is damaged or otherwise interfered with.
The advantage of reflective proximity sensors is that they are easy to arrange. It only needs to be mounted on one side, which can greatly save parts and time costs.
Like reflective sensors, the transmitter and receiver of a reflex sensor are located in the same housing. However, the detection target acts as a reflector, so it detects light reflected from a distance. The transmitter emits a beam of light (typically pulsed infrared, visible, or laser) that diffuses in all directions to fill the detection area. The target then enters the area and deflects part of the beam back to the receiver. When there is enough light on the receiver, detection occurs and the output is turned on or off (depending on whether the sensor is on or off).
A common example of a diffuse sensor is a sensor faucet on a public toilet sink. The hand placed under the nozzle acts as a reflector, triggering the opening of the water valve. Note that since the target (hand) is a reflector, diffuse photoelectric sensors are often affected by the properties of the target material and surface; The sensing range of non-reflective targets, such as matte black paper, will be greatly reduced compared to 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 particle level control, and paper, sheet metal, and wood build-up.
The common types are the same as photoelectric induction: inverted, reflective, and diffusive.
Ultrasonic diffuse proximity sensors use an acoustic sensor that emits a series of sound pulses and then listens for their return from the reflected 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 meters.
Ultrasonic reflex sensors can detect objects within a specified sensing distance by measuring the time of propagation. The sensor emits a series of sound pulses that reflect back from a fixed opposite reflector (any flat hard surface, machine, plate). The sound waves must be returned to the sensor at user-adjusted intervals. If not, it is assumed that an object is blocking the sensing path, and the sensor emits a corresponding output signal. Since the sensor detects 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 an opposing photocell
