【Overview of Proximity Switch】
A proximity switch is a type of position switch that operates without direct mechanical contact with the moving parts. When an object approaches the sensing surface of the switch to the operating distance, the switch can actuate without the need for mechanical contact or applying any pressure, thereby driving the DC device or providing control instructions to the computer (PLC) device. The proximity switch is a type of switch sensor (i.e., non-contact switch), which has the characteristics of limit switches and microswitches, and also has sensing capabilities. It is reliable in operation, stable in performance, fast in frequency response, long in application life, and strong in anti-interference ability. It also has characteristics such as waterproof, shockproof, and corrosion-resistant. The products include inductive, capacitive, Hall-effect, and AC/DC types. The proximity switch is also known as a non-contact proximity switch and is an ideal electronic switch quantity sensor. When a metal sensing body approaches the sensing area of the switch, the switch can actuate without contact, pressure, or spark, quickly sending out electrical signals to accurately reflect the position and stroke of the moving mechanism. It is suitable for general stroke control, and its positioning accuracy, operating frequency, service life, ease of installation and adjustment, and applicability to harsh environments cannot be compared with general mechanical stroke switches. It is widely used in industries such as machine tools, metallurgy, chemical industry, light textile, and printing. It can be used as a limit, counting, positioning control, and automatic protection link in automatic control systems.
【Performance Characteristics of Proximity Switch】
Among various switches, there is a component that has the "perception" ability to approach other objects: a displacement sensor. The proximity switch is achieved by using the sensitive characteristics of the displacement sensor to approach the object to control the switch on or off, and the switch only operates when an object moves towards and approaches a certain distance. This distance is usually called the "detection distance." However, different proximity switches have different detection distances. Sometimes the detected test object moves toward the proximity switch at certain time intervals, one by one, and leaves one by one, repeatedly. Different proximity switches have different response capabilities to the detected object. This response characteristic is called the "response frequency."
【Types of Proximity Switch】
Because displacement sensors can be made according to different principles and methods, and different displacement sensors have different methods of "sensing" objects, common proximity switches include the following:
1. Passive proximity switch: This switch does not require power and controls the switch's closed state through magnetic induction. When the magnetic or ferrous trigger approaches the switch's magnetic field, the internal magnetic action of the switch controls the closure. Features: no power supply required, non-contact, maintenance-free, and environmentally friendly.
2. Eddy current proximity switch: This switch is sometimes also called an inductive proximity switch. It uses the fact that when a conductive object approaches the switch, it generates eddy currents in the object. This eddy current counteracts the proximity switch, causing changes in the internal circuit parameters of the switch, thereby identifying whether there is a conductive object approaching and controlling the switch on or off. This proximity switch can only detect conductive objects. 2.1 Principle: It consists of an inductive coil, capacitor, and transistor that form an oscillator and produce an alternating magnetic field. When a metal object approaches this magnetic field, eddy currents will be generated in the metal object, causing the oscillator to stop. This change is processed by the rear pole amplifier and converted into a transistor switch signal output. 2.2 Features: A. Good anti-interference performance, high switching frequency, greater than 200HZ; B. Can only sense metal; 2.3 Applied to position detection, count signal acquisition, etc., on various mechanical devices.
3. Capacitive proximity switch: The measurement of this switch usually consists of one pole of a capacitor, and the other pole is the switch's shell. This shell is usually grounded or connected to the equipment's casing during the measurement process. When an object moves toward the proximity switch, regardless of whether it is a conductor, its approach will change the dielectric constant of the capacitor, thereby changing the capacitance and causing the circuit state connected to the measuring head to change accordingly, which can control the switch's on or off. This type of proximity switch can detect not only conductors but also insulating liquids or powders.
4. Hall effect proximity switch: The Hall element is a magnetic-sensitive element. A switch made using the Hall element is called a Hall switch. When a magnetic object approaches the Hall switch, the Hall element on the switch detection surface undergoes the Hall effect, causing changes in the internal circuit state of the switch, thereby identifying the presence of a magnetic object nearby and controlling the switch on or off. This type of proximity switch detects only magnetic objects.
5. Photoelectric proximity switch: A switch made using the photoelectric effect is called a photoelectric switch. The emitter and the photoelectric device are installed in the same detection head in a certain direction. When a reflective surface (the detected object) approaches, the photoelectric device receives the reflected light and outputs a signal, thereby "sensing" the approach of an object.
6. Other forms: When the distance from the observer or system to the wave source changes, the frequency of the approaching wave will shift, which is called the Doppler effect. Sonar and radar are made using this principle. The Doppler effect can be used to make ultrasonic proximity switches, microwave proximity switches, etc. When an object approaches, the proximity switch receives the reflected signal, which produces a Doppler frequency shift, thereby identifying whether there is an object nearby.
【Main Functions of Proximity Switch】
1. Inspection distance: Detect the stop, start, and passing position of elevators and lifting equipment; prevent collision detection between two objects; detect the set position of working machinery and the limit position of moving machines or components; detect the stop position of the rotary body, and the open or closed position of the valve.
2. Size control: Control device for the size of metal sheet punching and shearing; automatic selection and identification of the length of metal parts; detection of the height of stacked goods during automatic loading and unloading; detection of the length, width, height, and volume of items.
3. Detection of object presence: Detect whether there are product packaging boxes on the production packaging line; detect the presence of product parts.
4. Speed and velocity control: Control the speed of conveyors; control the speed of rotating machinery; control speed and number with various pulse generators.
5. Counting and control: Count the number of products flowing through the production line; measure the rotation speed of high-speed rotating shafts or discs; count parts.
6. Detect anomalies: Detect the presence of bottle caps; judge whether products are qualified or not; detect the absence of metal products in the packaging box; distinguish between metal and non-metal parts; detect product labels; crane danger zone alarm; escalator automatic start and stop.
7. Measurement control: Automatic measurement of products or parts; control numbers or flow rates based on the pointer range of measuring instruments and instruments; control float levels and flow rates by detecting floating levels; control the upper or lower limits of instrument ranges; flow control; level control.
8. Object recognition: Identification of yes or no based on codes on carriers.
9. Information transmission: ASI (bus) connects sensors at various positions on the equipment for data exchange in the production line (50-100 meters), etc.
【Wiring Diagram of Proximity Switch】
1) There is a difference between the two-wire and three-wire proximity switches, and the three-wire proximity switches are further divided into NPN and PNP types, with different wiring methods.
2) The wiring of the two-wire proximity switch is relatively simple, and the proximity switch is connected to the power supply in series with the load.
3) Wiring of the three-wire proximity switch: the red (brown) wire is connected to the positive terminal of the power supply; the blue wire is connected to the 0V terminal of the power supply; the yellow (black) wire is the signal, should be connected to the load. The other end of the load is connected as follows: for NPN type proximity switches, it should be connected to the positive terminal of the power supply; for PNP type proximity switches, it should be connected to the 0V terminal of the power supply.
4) The load of the proximity switch can be a signal light, relay coil, or digital quantity input module of a programmable controller PLC.
5) Pay special attention to the selection of the type of three-wire proximity switch connected to the PLC digital input module. PLC digital input modules are generally divided into two types: one type has the common input terminal as the power supply 0V, and the current flows out of the input module (Japanese model), in this case, NPN type proximity switches must be used; the other type has the common input terminal as the positive terminal of the power supply, and the current flows into the input module, known as the source input (European model), in this case, PNP type proximity switches must be used. Be sure not to choose the wrong type.
6) Although the two-wire proximity switch has fewer lines, it is limited by working conditions. When it is conducting, the switch itself generates a certain voltage drop, and when it is cut off, there is a certain residual current passing through. Consideration should be given when selecting. The three-wire proximity switch, although it has one more line, is not troubled by adverse factors such as residual current and works more reliably.
7) Some manufacturers simultaneously lead out the "normally open" and "normally closed" signals of the proximity switch or add other functions. In this case, please refer to the product manual for specific wiring.
8) Wiring instructions: 1. The three wires of the DC three-wire proximity switch are: brown wire---positive pole of the power supply; blue wire---negative pole of the power supply; black wire---output signal; 2. AC two-wire proximity switch: connect the load and the proximity switch in series and then connect it to the AC power supply terminal.
【Precautions for Proximity Switch】
1) In general industrial production sites, eddy current proximity switches and capacitive proximity switches are usually selected because these two types of proximity switches have lower environmental requirements.
2) When the measured object is a conductive object or an object that can be fixed on a metal, eddy current proximity switches are generally selected because of their high response frequency, good environmental interference resistance, wide application range, and low price.
3) If the measured object is non-metal (or metal), liquid level, powder height, plastic, tobacco, etc., a capacitive proximity switch should be used. This type of switch has a low response frequency but good stability. Environmental factors should be considered during installation.
4) If the measured object is a magnetic material or if a magnetic steel is embedded in the measured object to distinguish it from other objects moving together, a Hall effect proximity switch should be used, which is the most economical.
5) In places with relatively good environmental conditions and no dust pollution, photoelectric proximity switches can be used. The photoelectric proximity switch has almost no effect on the object being tested during operation. Therefore, it is widely used in fax machines and tobacco machinery that require higher requirements.
6) In the anti-theft system, automatic doors usually use pyroelectric proximity switches, ultrasonic proximity switches, and microwave proximity switches. Sometimes, to improve the reliability of identification, several proximity switches are often used in combination. Regardless of the type of proximity switch chosen, attention should be paid to the requirements of various indicators such as operating voltage, load current, response frequency, and detection distance.
