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Proximity switch I. Performance characteristics Among various types of switches, there is a component that is capable of detecting objects approaching it – the displacement sensor. By utilizing the sensitive property of displacement sensors to detect approaching objects, it is possible to control whether a switch is on or off; this is what a proximity switch does. Only when an object moves toward the proximity switch and reaches a certain distance does the displacement sensor detect it, and then the switch operates. This distance is usually called the “detection distance”. Different proximity switches have different detection distances. Sometimes, the objects to be tested move toward the proximity switch one after another at regular intervals, and then move away one after another, repeating this process continuously. Different proximity switches have varying response capabilities to the objects they detect. This response characteristic is called the “response frequency”. II. Types: Since displacement sensors can be manufactured using different principles and methods, and different displacement sensors employ various approaches to \"sense\" objects, the common types of proximity switches are as follows: 1. Eddy current proximity switch – this type of switch is sometimes also referred to as an inductive proximity switch. It works by using a conductive object; when this object comes close to a proximity switch that generates an electromagnetic field, eddy currents are induced within the object. This eddy current acts on the proximity switch, causing changes in the circuit parameters within the switch; this allows detection of whether a conductive object is approaching, thereby controlling whether the switch is turned on or off. The object that this proximity switch can detect must be a conductor. 2. Capacitive proximity switch: In this type of switch, one plate forms part of the capacitor, while the other plate is the enclosure of the switch. During measurement, this enclosure is usually grounded or connected to the device’s casing. When an object moves toward the proximity switch, whether it is a conductor or not, its approach always causes a change in the dielectric constant of the capacitor, which in turn leads to a change in the capacitance. This results in a change in the state of the circuit connected to the sensing element, and thus it becomes possible to control whether the switch is turned on or off. The objects detected by this proximity switch are not limited to conductors; insulating liquids or powders, etc., can also be detected. 3. Hall proximity switch: The Hall element is a type of magnetosensitive component. A switch made using a Hall element is called a Hall switch. When a magnetic object approaches the Hall switch, the Hall element on the switch’s detection surface generates a Hall effect, which causes a change in the state of the switch’s internal circuit. This allows detection of the presence of a magnetic object in the vicinity, thereby enabling control over whether the switch is turned on or off. The object to be detected by this proximity switch must be a magnetic object. 4. A proximity switch that utilizes the photoelectric effect is called a photoelectric switch. The light-emitting devices and optoelectronic devices are installed in the same detection head in a specific direction. When a reflective surface (the object being detected) approaches, the photoelectric device receives the reflected light and outputs a signal, thereby allowing it to \"detect\" the approach of an object. 5. A pyroelectric proximity switch is a switch made using a component that can detect temperature changes. In this type of switch, a pyroelectric device is installed on the sensing surface of the switch; when an object with a temperature different from that of the environment approaches, the output of the pyroelectric device changes, thereby allowing detection of the approach of an object. 6. In other types of proximity switches, when the distance between the observer or the system and the wave source changes, the frequency of the waves reaching them shifts; this phenomenon is known as the Doppler effect. Sonar and radar are built on the principle of this effect. By utilizing the Doppler effect, ultrasonic proximity switches, microwave proximity switches, and similar devices can be created. When an object approaches, the reflected signal received by the proximity switch undergoes a Doppler shift, which allows detection of whether an object is approaching or not. III. Main Applications Proximity switches are widely used in aviation, aerospace technology, as well as industrial production. In daily life, it is used in automatic doors of hotels, restaurants, and garages, as well as in automatic heat blowers. In terms of security against theft, in sensitive areas such as data archives, accounting departments, financial institutions, museums, and vaults, security devices composed of various proximity switches are usually installed. In measurement techniques, such as the measurement of length and position ； In control technology, such as the measurement and control of displacement, velocity, and acceleration, a large number of proximity switches are also used. IV. Considerations for Selection In typical industrial production environments, eddy current proximity switches and capacitive proximity switches are usually chosen. Because these two proximity switches have low requirements regarding the environment. When the object to be measured is a conductive object or one that can be fixed to a metal surface, an eddy current proximity switch is generally chosen, as it features a high response frequency, good resistance to environmental interference, a wide range of applications, and a low cost. If the object being measured is a non-metal (or metal), liquid level, powder level, plastic, tobacco, etc. In that case, a capacitive proximity switch should be used. This type of switch has a low response frequency, but good stability. The impact of environmental factors should be taken into account during installation. If the object being measured is a ferromagnetic material, or if magnetic rods are embedded within it in order to distinguish it from other objects moving together with it, a Hall proximity switch should be used, as it is the least expensive option. In environments with good conditions and no dust pollution, photoelectric proximity switches can be used. A photoelectric proximity switch has almost no impact on the object being measured when it is in operation. Therefore, it is widely used in high-performance fax machines as well as in tobacco machinery. In security systems, automatic doors typically use pyroelectric proximity switches, ultrasonic proximity switches, and microwave proximity switches. Sometimes, to improve the reliability of recognition, the aforementioned types of proximity switches are used in combination. Regardless of which proximity switch is chosen, attention should be paid to the requirements for parameters such as operating voltage, load current, response frequency, and detection distance.