A proxy switch is a device that detects an object without physical contact. Proxy switches are used in a variety of devices, from basic home applications to large-scale applications. In proxy switches, we use the Proximity sensor. Proxy switches have a wide range of applications.
How does a proxy switch work?
The main function of proxy switches is to detect objects. Detection of a substance or object through proximity leads to the initiation of a specific action. The action that starts after identification is predefined. Object detection with the help of a proxy sensor without physical contact of the sensor with the object. A number of sensing techniques are used in the proxy sensor for its operation.
Types of proxy switches:
According to the non-contact object detection method, there are five types of proximity switches. they are,
- Induction proxy switch.
- Photoelectric or optical proxy switch.
- Capacitive switch proxy
- Magnetic proxy switch
- Ultrasonic proxy switch.
Induction proxy switch:
The induction proxy switch detects the metal object next to their active side. The sensor operates under the principle of electrical induction, in which an oscillating current of electromotive force (EMF) is induced in the object. They consist of four main components: a winding ferrite core, an oscillator, a Schmidt trigger and an output amplifier.
Principle of operation: The oscillator creates a symmetrical and oscillating magnetic field that radiates from the ferrite core and the coil array at the sensor surface. When an iron target enters this magnetic field, small independent electric currents (eddy currents) are induced on the metal surface. The induction proximity sensor has a frequency range of 10 to 20 Hz in ac or 500 Hz to 5 kHz in direct current dc. Due to the limitations of the magnetic field, induction sensors have a relatively narrow sensory amplitude, ranging from a fraction of a millimeter to an average of 60 millimeters.
For this reason, a load is applied to the sensor, which reduces the amplitude of the electromagnetic field. If the metal object moves towards the proximity sensor, the eddy current increases accordingly. Therefore, the load on the oscillator increases, which reduces the field amplitude.
The Schmidt trigger block monitors the amplitude of the oscillator and turns the sensor trigger circuit on or off at a certain level (preset level). If the metal object or target moves away from the proximity sensor, the oscillator amplitude increases.
The image above shows the oscillator waveform of the proxy sensor in the presence of the target and in the absence of the target.
Induction proxy sensors with different operating voltages are available today. These induction proxies are available in AC, DC, and AC / DC modes (universal modes). The operating range of the proximity sensor circuits is from 10 volts to 320 volts DC and 20 volts to 265 volts AC.
2. Photoelectric or optical proxy switch:
The photoelectric switch is able to detect metallic and non-metallic targets. They are used in many applications such as cell phones and for level measurement. In the phone, the sensor is used to deactivate the touch screen, because the user moves the device closer.
The main components of this sensor are emitter, detector and electronics related. An emitter (light emitting diode, laser diode) emits a beam of light. The detector (photodiode or phototransistor) detects the emitted light. A related electron is needed to amplify the detected signal.
There are three main methods of measuring a photoelectric proximity sensor.
(I) by beam method
(II) Reflection method
(III) Scattered or reflective method
( I ) Beam method:
In this type of method, an emitter sends a beam of light directly to the receiver in the emitter line of sight. When an object refracts this beam of light, it detects it as presence. This type of drive requires two components: an emitter and a separate detector, which makes installation and wiring a bit more complicated. However, the advantage is that it is the most accurate measurement method with the longest measuring range.
Newer models of laser diode emitters can send a well-coordinated 60-meter beam to increase accuracy and detection. At these distances, some laser beam sensors are able to detect an object the size of a fly at close range, which is 0.01 mm. One of the unique capabilities of beam photoelectric sensors is the effective detection of the presence of concentrated pollutants in the air.
( II ) Reflection method:
In this method of detection occurs when the path of light is broken or disturbed. Both light-emitting and light-receiving elements are housed in the same enclosure. Light strikes the reflector from the emitting element and returns to the receiving element. When there is a target, the light goes out.
(Iii) Scattered or reflective method:
Such as sensors, emitters and reflectors that are housed in a housing assembly. In this diffusion method, both the emitting and receiving light elements are fixed in one chamber. The sensor receives light reflected from the target.
Diffuse photoelectric sensors are similar to reflective sensors in some ways. This is because, like reflective sensors, they emit a beam of light in the direction of the object to be detected. However, instead of the reflector used to reflect light to the detector, the object to be sensed acts as a reflector, returning some light to detect and record the object.
Mostly, diffusion sensors are used in public washbasins, where they control automatic valves. The hands under the spray act as a reflector, opening the faucet. Scattered sensors are somewhat color-dependent, with certain versions suitable for detecting dark and light targets in applications that require contrast-based sorting or quality control.
Capacitive switch proxy:
Capacitor sensors proxy switches detect changes in capacitance between the sensor body and the sensor. The amount of capacitor varies depending on the size and distance of the sensor body. A typical capacitor proximity sensor is similar to a capacitor with two parallel plates in which the capacitance of two plates is detected. One is the plane of the object being measured (with an imaginary ground) and the other is the sensor surface of the sensor. Detects changes.
In the capacity produced between these two poles. The detection of an object depends on their dielectric constant, but in addition to metals, they also contain resins and water. A typical measuring range for capacitive proximity sensors is from a few millimeters to about 1 inch. (Or 25 mm), and some sensors have a range of up to 2 inches.
Principle of operation: The capacitive proxy switch consists of a high frequency oscillator with a sensor surface composed of two metal electrodes. When an object approaches the surface of the sensor, it enters the electrostatic field of the electrodes and changes the oscillator capacity.
As a result, the oscillator circuit starts to oscillate and changes the output state of the sensor when it reaches a certain range. As the object moves away from the sensor, the oscillator amplitude decreases and the sensor returns to its original state.
4. Magnetic proxy switch
Magnetic sensors are activated by the presence of a permanent magnet. They can detect a magnetic field even through ferromagnetic and non-ferromagnetic materials, it just depends on the strength of the permanent magnet. In addition, these magnetic proximity switches are widely used for liquid level detection, final position monitoring, impact limitation in hydraulic cylinders and so on.
Working principle: Several operating principles are used, including reed switches, GMR induction, variable inclination, magnetic resistance or Hall effect sensor.
Variable reluctance: Proxy sensors consist of a permanent magnet and a collecting coil. It uses a VR sensor as a simple proxy switch that can determine the position of a mechanical link in a piece of industrial equipment.
Crankshaft position sensor (in car engine) is used to provide the angular position of the crankshaft to the engine control unit. The motor control unit can then calculate the motor speed (angular velocity).
A pickup used on an electric guitar or other musical instrument detects the vibrations of metal strings.
Magnetic resistance: Proximity sensors measure the effect of magnetic resistance or the effect of resistance of a ferromagnetic material in the presence of a magnetic field.
Magnetic resistance is the tendency of a substance to change the amount of its electrical resistance in an externally applied magnetic field.
1. Reed switches:
Reed switches are switches that are magnetically actuated. They usually produce two ferromagnetic reeds (contact blades) that work in a glass capsule. It consists of two low-reluctance ferromagnetic reeds enclosed in glass lamps containing inert gas.
A magnetic field from an electric magnet or a permanent magnet causes the reeds to absorb each other to form an electrical circuit. An example of a reed switch is a door detection detection, when used as a proxy switch for a security alarm.
2- The effect of Giant magnetic resistance:
Magnetic sensors use GMR (Giant Magnetic Resistance Effect) technology. The measuring cell is composed of resistors with several very thin, ferromagnetic and non-magnetic layers.
Two of these GMR resistors are used to form a typical Westton bridge bridge circuit, which produces a large signal proportional to the magnetic field when there is a magnetic field. Specifies a threshold value and switches an output signal through the comparator.
The main application of GMR is magnetic field sensors, which are used to read data on hard disks, biosensors, microelectromechanical systems (MEMS) and other devices. GMR multilayer structures are also used in magnetic resistivity random access memory (MRAM) as cells that store one bit of information.
The Hall effect sensor is a device used to measure the magnitude of a magnetic field. Its output voltage is directly proportional to the intensity of the magnetic field through it.
Hall effect sensors are used for proximity measurement, positioning, velocity detection and flow measurement applications. Often, the Hall sensor is combined with a threshold detector to act as a switch.
Ultrasonic proximity switch:
The ultrasonic proxy switch includes an ultrasonic sensor that operates at a frequency of 40 kHz. It uses two specially built ultrasonic transducers: one transmits 40 kHz sound, while the other receives 40 kHz sound and converts it into electrical changes of the same frequency. Ultrasonic proxy sensors work by emitting and receiving high frequency sound waves.
Basics : Ultrasonic proxy sensors emit and receive sound waves. The signal carries a high-frequency, incomprehensible sound wave. They detect the presence of a target object in one of two configurations.
Scattered or reflective sensors pack the transmitter and receiver in one container. When a target enters the measuring range of the device, ultrasonic waves are reflected to the sensor.
Opposite or Thru-Beam sensors package the transmitter and receiver separately. The receiver mounts to the transmitter and blocks the transmitted signal when an object enters the sensor sensor range. Instead of activating the trigger when receiving the frequency, the trigger is activated when the signal is broken.
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