Proximity switch internal structure

- Oct 11, 2019-

There are many types of proximity switches, among which the high frequency oscillation type is the most commonly used, which accounts for more than 80% of the total proximity switch output. The high-frequency oscillation type proximity switch is mainly composed of a sensor (induction head), an oscillator, a switching circuit, an output circuit, and a regulated power supply. The structure of the proximity switch is shown in Figure 2-31. Proximity switches are mostly composed of a high frequency oscillator and a shaping amplifier. The working principle is that after the oscillator oscillates, an alternating magnetic field is generated on the sensing surface of the switch. When the metal object approaches the sensing surface, the metal body generates an eddy current, which absorbs the energy of the oscillator, so that the oscillation is weakened and the vibration is stopped. Two different states of oscillation and stop vibration are converted into binary switching signals by the shaping amplifier to achieve the purpose of detecting the position.


Proximity switches can be divided into high frequency oscillation type, inductive bridge type, Hall effect type, photoelectric type, permanent magnet and magnetosensitive element type, capacitance type and ultrasonic type, among which high frequency oscillation type is the most common, which is produced in China. Most of the proximity switches are of this type, and they are mainly composed of an inductive head, an oscillator, a switching element, an output, and a voltage regulator. The shape of the commonly used travel switch and proximity switch.


High-frequency oscillating proximity switch detection, when a metal detector (usually a ferromagnetic member) mounted on a moving part of a production machine approaches the induction head, the inside of the detector in the magnetic field of the high-frequency oscillator coil is caused by electromagnetic induction Eddy current and hysteresis loss are generated, so that the oscillation circuit is weakened due to an increase in internal resistance and an increase in loss until the oscillation is stopped. At this time, the transistor switching element is turned on and outputs a signal through an output device (electromagnetic relay), thereby functioning as a control.


2. The proximity switch model and the electrical symbol proximity switch are very rich in product types and various models. At present, the domestic proximity switches have 3SG, LJ, CJ, SJ, AB, LX10 and other series.


(a) normally open contact


(b) Graphic symbols and text symbols of the normally closed contact proximity switch. The text symbols are the same as the travel switch and can be regarded as one type of travel switch.


3. Selection of proximity switch This switch can contactless, no pressure, no spark and quickly issue electrical commands, accurately reflect the position and stroke of moving parts, and its positioning accuracy is high, response speed is fast, service life is long, and installation and adjustment are convenient. Advantages such as strong application ability. However, the price is high, so it is often used in applications where the operating frequency is high, reliability and accuracy are high. When selecting, the model, specification and output form should be selected according to the response distance, output requirements, and corresponding speed.


Closer to home, the proximity switch, as its name implies, is a device that can detect nearby objects and give a switching signal. Its internal structure is shown below:


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Although it looks small and even small, it is internally composed of a sensor coil, an oscillator, a digital-to-analog converter, an amplifier, and an output. The principle of detection is as shown below:


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As shown in the left figure above, close to the area in front of the sensing surface of the switch, the LC oscillator always generates a high-frequency oscillating electromagnetic field (left sine wave as shown in the figure above), when a ferromagnetic metal enters the electromagnetic field shown in the figure. In the region, according to the principle of electromagnetic induction, the eddy current is induced inside the ferromagnetic metal, and according to Lenz's law, this eddy current generates another electromagnetic field that adversely affects the direction of the electromagnetic field of the sensor itself, because the two directions are opposite, then After the vector is superimposed, the amplitude of the electromagnetic field will be attenuated (as shown in the waveform on the right side of the figure above). At this time, the internal processor will detect the attenuation of this amplitude, and then pass the signal through digital-to-analog conversion and amplifier output. At this time, we use You can see that the status of the sensor switch light has changed. So from the principle of understanding, we can know why the inductive proximity switch can only detect metal. (Of course, some people will also ask, since only ferromagnetic metals can produce eddy currents, why can non-ferromagnetic metals such as aluminum and copper be detected? Here, by the way, it is popular for everyone. In order to solve this problem, many manufacturers The method is to detect the oscillation frequency while detecting the amplitude of the oscillating electromagnetic field. When non-ferromagnetic metals such as aluminum and copper enter the sensing area, the oscillation frequency will change. By detecting these comprehensive indexes, all the metals can be detected.


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From left to right, pig iron, stainless steel, lead, brass, aluminum, copper


The second point to note is that when the proximity switch is installed on site, there will be a parameter called flush and non-flush (also called shielded, unshielded). As shown below. The so-called flush means that the proximity switch sensing surface is flush with the surrounding metal casing. In this installation mode, the proximity switch can be buried in the metal carrier, as long as the sensing surface is not lower than the carrier surface. The non-flat type must have a sensing surface that is higher than the surrounding metal carrier during installation (non-metallic carriers do not require this). (In this case, why do you have a non-flushing method: because the non-flushing sensing surface has a large electromagnetic field, the sensing distance can be increased under the same size of the housing, and the large sensing distance is required for many applications. So there is a non-flush design).