Several sensor switches commonly used in electrical equipment
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I. Overview In all types of sensors, there is a component that has "feel" ability to approach its object - displacement sensor. This kind of sensor does not need to touch the detected object. When an object moves to the displacement sensor and approaches a certain distance, the displacement sensor has “perceivedâ€. Usually this distance is called “detection distanceâ€. The proximity switch is a switch that is made by using a displacement sensor to sense the proximity of an object.
Different sensors detect different distances. Sometimes the objects to be inspected are moved one by one at certain time intervals and one by one, so that they repeat continuously. Different sensor elements have different response capabilities to detection objects. This response characteristic is called "response frequency".
Second, the sensor classification Because the displacement sensor can be made according to different principles and different methods, and different displacement sensors on the object's "perception" method is also different, so the common sensors are the following:
1. Inductive Sensors Inductive sensors, also called eddy current sensors, consist of three major components: an oscillator, a switch circuit, and an amplifier output circuit. The oscillator generates an alternating magnetic field. When the metal target approaches this magnetic field and reaches the sensing distance, eddy currents are generated in the metal target, which causes the oscillation to attenuate and even stop vibration. Oscillator oscillation and stop changes are processed by the post-amplifier circuit and converted into a switching signal to trigger the drive control device to achieve non-contact detection purposes. Thus, the object that can be detected by this proximity switch must be a conductor.
2. Capacitive Sensors The measurement of such a sensor is usually a plate where the sensor is fixed to form a capacitor, while the other plate is usually grounded or connected to the housing of the device during the measurement process. When an object moves to a sensor, whether it is a conductor or not, because of its proximity, the dielectric constant between the two plates of the capacitor will always change, so that the capacitance of the capacitor will change, making the state of the circuit connected to the measuring head. It also changes along with it, thereby achieving non-contact detection. The object to be detected by this proximity switch is not limited to a conductor, an insulating liquid, a powder, or the like.
3. Hall-effect sensor When a metal or semiconductor sheet with a current is placed vertically in a magnetic field, potential differences are generated at both ends of the sheet. This phenomenon is called the Hall effect. The potential difference between the two ends is called the Hall potential U, and its expression is U=K·I·B/d, where K is the Hall coefficient, I is the passing current in the slice, and B is the applied magnetic field (Loren The magnetic induction intensity of L. C. Lorrentz), d is the thickness of the slice.
It can be seen that the sensitivity of the Hall effect is directly proportional to the magnetic induction intensity of the applied magnetic field. The Hall element belongs to such an active magnetoelectric conversion device and is a magnetic sensor. It is based on the Hall effect principle, using the integrated packaging and assembly process, it can easily convert the magnetic input signal into the actual application of electrical signals, but also has the practical application of industrial applications, easy operation and reliability The request. The Hall switch is made using this feature of the Hall element. Its input is represented by the magnetic induction B. When the B value reaches a certain level (such as B1), the trigger inside the Hall switch is turned over. The state of the output level of the Hall switch is also reversed. Transistor output is generally used at the output end. Similar to other sensors, there are NPN, PNP, normally open, normally closed, latched (bipolar), and dual signal outputs. Hall switch has the characteristics of no electric shock, low power consumption, long service life, high response frequency, etc. The internal use of epoxy sealing and integration, so it can work reliably in all kinds of harsh environments.
When the magnetic object moves close to the Hall switch, the Hall element on the detection surface of the switch changes the internal circuit state of the switch due to the generation of the Hall effect, thereby recognizing the presence of a magnetic object in the vicinity, thereby controlling the on or off of the switch. The proximity switch must be a magnetic object.
4, photoelectric sensor photoelectric sensor uses the photoelectric effect. The light emitting device and the optoelectronic device are mounted in the same test head in a certain direction. When there is a reflective surface (detected object) approaching, the optoelectronic device receives the reflected light and then outputs the signal, thereby "sensing" the approach of the object.
Optical proximity switches made with photoelectric sensors can detect a variety of substances, but have a large error in the detection of fluids.
5, pyroelectric sensor pyroelectric sensor can sense the temperature changes, the pyroelectric device is installed on the switch detection surface, when there is a different temperature and close to the object, the pyroelectric sensor output signal changes The proximity of the object can be detected by converting the sensor output signal.
6. Linear proximity sensor The linear proximity sensor is a kind of linear device that belongs to metal induction. After the power is turned on, an alternating magnetic field will be generated on the sensing surface of the sensor. When the metal object approaches this sensing surface, the metal will generate eddy currents. The energy of the oscillator is absorbed, and the amplitude of the oscillator output is linearly attenuated, and then the object of non-contact detection of the object is completed according to the change of attenuation.
The proximity sensor has no sliding contact, is not affected by dust and other non-metallic factors during operation, and has low power consumption, long life, and can be used in various harsh conditions. The linear sensor is mainly applied to the intelligent control of analog quantity in the automation equipment production line.
Third, characteristics 1, non-contact detection, to avoid damage to the sensor itself and the target.
2, no contact output, long operating life.
3. Stable detection even in harsh environments with water or oil splashes.
4, the reaction speed.
5, a small sensor head, flexible installation.
Fourth, the main purpose of these proximity switch sensors in aerospace, aerospace technology and industrial production in a very wide range of applications; in daily life, such as hotels, restaurants, garages automatic doors, automatic hot air have applications; in terms of security, such as information Archives, accounting, finance, museums, treasury, and other places are usually equipped with anti-theft devices consisting of various proximity switches. In measurement technology, such as length, position measurement; in the control technology, such as displacement, velocity, acceleration measurement and control, also have a lot of use. The method used is usually made into a proximity switch.
V. Application Cases Proximity Switch Sensors Applied to Metro Screen Doors With the rapid development of the domestic subway industry and the frequent occurrence of subway traffic accidents, subway screen doors will be used more and more by subway stations. Sensors are the best instruments for automated intelligent devices. Proximity switch sensors will be heavily used in the technical application of screen doors. Subway accidents will be minimized in the application of sensors.
Proximity switch sensor is a common scheme used by many screen door systems to detect door opening and closing. Generally, two door switches are used to detect door opening and closing. In addition, the amount of photoelectric switches used in the system is also very large, and all screen doors will use more sensor technologies. It is not difficult to see who has applied many sensor technologies and who will be the strongest in the market. Intelligentization and automation will rely on sensors to be closely linked.
6. Selection Principles In general industrial production sites, eddy current proximity switches and capacitive proximity switches are generally used. Because these two kinds of proximity switches require less environmental conditions.
When the object to be measured is a conductive object or an object that can be fixed on a metal object, an eddy current proximity switch is generally selected because of its high response frequency, good resistance to environmental interference, wide application range, and low price.
If the measured object is non-metal (or metal), liquid level, powder height, plastic, tobacco and so on. Capacitive proximity switches should be used. This switch has a low response frequency but good stability. The environmental factors should be considered when installing.
If the object is magnetically permeable or immersed in the object to be measured in order to distinguish it from objects that move with it, the Hall proximity switch should be used, which has the lowest price.
In the case of better environmental conditions and no dust pollution, photoelectric proximity switches can be used. When the photoelectric proximity switch works, it has almost no influence on the measured object. Therefore, on demanding fax machines, automatic machines, money detectors, power take-off switches and the like are widely used in tobacco machinery.
In anti-theft systems, automatic doors typically use pyroelectric proximity switches, ultrasonic proximity switches, and microwave proximity switches. Sometimes, in order to improve the reliability of recognition, the above-mentioned types of proximity switches are often used in combination.
No matter which kind of proximity switch is chosen, attention should be paid to the requirements for various indicators such as working voltage, load current, response frequency, and detection distance.