There are many kinds of limit switches, and their models are not clear here; limit switches are commonly divided into three types: mechanical type, electronic induction type, and photoelectric type. Although their working principles are different, their functions are exactly the same. They are all used A device for detecting the operating position of mobile electrical equipment. Let me talk about the mechanical limit switch here first. The limit switch is commonly known as the travel switch or the position switch. Its function is the same as that of the control button. It can convert the mechanical displacement signal into an electrical signal, but the action of the contact is not pressed by hand, but It is to use the mechanical displacement of some moving parts of the production equipment to bump the switch and make the contacts act to complete the control of the connection or disconnection of a certain circuit, thereby controlling the mechanical action or program execution. If the travel switch is installed at the end of the working machine to limit its travel, it is called a limit switch, or an end switch.


The limit switch can be divided into two categories according to different purposes: one is the general purpose limit switch, which is mainly used for the limit and program control of machine tools, automatic production lines and other production machinery. The commonly used models are JW2., LX19, LXK3, JLXK1, XCK , 3SE3, IXZ1, DTH, LX19, LX31, LXw5 and other series; the other is the travel switch for lifting equipment, which is mainly used to limit the travel of cranes and various metallurgical auxiliary equipment. The commonly used models are LX22, LX33, Lx36 and other series . Travel switches can be divided into three types according to the structure: button type, single-wheel rotary type and double-wheel rotary type, and their working principles are basically the same. The structure and working process of the limit switch are composed of four parts: the operating mechanism, the base, the shell, and the switch core. The operating mechanism is in contact with the iron stop to trigger the action of the switch core; the base is generally pressed by plastic, which is used to install and fix the switch core to protect the switch core from external factors; the shell has two kinds of metal and plastic, and the shell is equipped with a roller connecting rod, A pair of moving contacts and a pair of moving contacts; the switch core is the core component, which realizes the connection and breaking of the circuit according to the action of the operating mechanism.


The symbols of the travel switch moving closing contact and moving breaking contact in the circuit are shown in the lower part of the figure. Here, the structure of the LX19K travel switch is used, as shown in the figure. It belongs to the push-button limit switch, its parameters are one action on, one action off, the rated voltage of the contact is 380V, and the rated current is 5A. It has the advantages of simple structure, practical function and low price. When the external force presses the ejector rod downward, the ejector rod moves downward and compresses the spring to store a certain amount of energy. When the ejector rod moves to a certain position, the elastic force of the spring changes its direction, and the stored energy is released, forcing the contact bridge (moving contact) to bounce upward rapidly, breaking off the moving and breaking static contact, and then connecting with the moving and closing static contact , completed the jumping fast switching action. When the external force is removed, under the action of the return spring, the push rod rises, and the moving contact jumps down again to return to its original position.


Electronic sensor switches are commonly known as proximity switches, and proximity switches can be divided into the following types according to their working principles. (1) High-frequency oscillation type: used to detect various metals. (2) Capacitive type: used to detect various conductive or non-conductive liquids or solids. (3) Photoelectric type: used to detect all opaque substances. 4) Ultrasonic type: used to detect substances that do not pass through ultrasonic waves. (5) Electromagnetic induction type: used to detect magnetically conductive or non-magnetically conductive metals. The high-frequency oscillation type proximity switch is the most common type at present, and it accounts for almost more than 80% of the output of the proximity switch. It is composed of sensors, oscillators, switches, output devices, and regulated power supplies. After the electronic circuit is installed and adjusted, it is sealed with epoxy resin, which has good moisture-proof and anti-corrosion performance. It is usually made into a plug-in type, a threaded type, or an external sensor head type, etc., which can be selected according to different installation methods and usage occasions.

When working, turn on the power supply, and then the oscillator starts to vibrate. When a metal object enters the magnetic field of a high-frequency oscillator that oscillates at a certain stable frequency, due to the eddy current loss inside the metal object, there is a hysteresis loss for the ferromagnetic object, which increases the resistance of the oscillation circuit and increases the energy loss, so that the oscillation weakens. until the vibration stops. Oscillation and vibration stop are two different states. It is converted into a binary switch signal through the switch and output device connected behind the oscillation circuit, and sends out a signal that detects a metal object, and can output a corresponding control signal to control the relay or other electrical appliances. Generally, it is better to configure PLC applications. Photoelectric switch; it uses the blocking or reflection of the light beam by the object to be detected, and the circuit is turned on by the synchronous circuit to detect the presence or absence of the object. Objects are not limited to metals, all objects that can reflect light (or block light) can be detected.

Photoelectric switches have diffuse reflection type, through-beam type, etc. Diffuse reflection only needs one sensor to complete the transmission and reception, while the through-beam type needs two sensors, one for transmitting and one for receiving. Photoelectric switches also have NPN and PNP models, but in addition, photoelectric switches also have dry nodes, which can be connected to any kind of PLC. Learning these knowledge does not happen overnight, relying entirely on your own daily contact with them; this is just an introduction.