Proximity switch is a position switch that can be operated without mechanical contact with moving parts. When an object approaches the sensing surface of the switch to the action distance, it can make the switch act without mechanical contact and any pressure, thus driving AC or DC appliances or providing control instructions to computer equipment. Proximity switch is a kind of switch-type sensor (i.e. contactless switch), which has the characteristics of travel switch and microswitch. It has good sensing performance, reliable action, stable performance, fast frequency response, long service life, strong anti-interference ability, waterproof, shockproof and corrosion resistance. Products include inductance, capacitance, Hall, AC and DC.
Proximity switch, also called contactless proximity switch, is an ideal electronic switch sensor. When the metal detector is close to the induction area of the switch, the switch can send out electrical instructions quickly, without contact, pressure and sparks, and accurately reflect the position and stroke of the moving mechanism. Even if it is used for general travel control, its positioning accuracy, working frequency, service life, convenience of installation and adjustment and adaptability to harsh environment are incomparable to general mechanical travel switches. Widely used in machine tools, metallurgy, chemical industry, textile, printing and other industries. It can be used as a link of limit, counting, positioning control and automatic protection in automatic control system. Proximity switch has the characteristics of long service life, reliable operation, high repeated positioning accuracy, no mechanical wear, no spark, no noise and strong anti-vibration ability. Up to now, the application scope of proximity switch is increasingly extensive, and its own development and innovation speed is also extremely rapid.
2. Main functions of proximity switch
2. 1 Check the distance
Detecting the stopping, starting and passing positions of elevators and lifting equipment; Detecting the position of the vehicle to prevent two objects from colliding; Detect the set position of the working machine and move the limit position of the machine or component; Detecting the stop position of the rotating body and the open or closed position of the valve; Detect the moving position of the piston in the cylinder or hydraulic cylinder.
2.2 Size control
Size control device for metal plate stamping and shearing; Automatically select and identify the length of metal parts; Detecting the stacking height during automatic loading and unloading; Detect the length, width, height and volume of articles.
2.3 Check whether there is a product packaging box on the production packaging line; Check the product parts.
2.4 Speed and speed control
Control the speed of the conveyor belt; Control the rotating speed of rotating machinery; Control speed and rotational speed together with various pulse generators.
2.5 Counting and control
Check the quantity of products flowing through the production line; Measurement of high-speed rotating shaft or disk revolution; Part count.
2.6 anomaly detection
Detecting the presence or absence of bottle caps; Determination of qualified and unqualified products; Detecting the lack of metal products in the packaging box; Distinguish between metal and nonmetal parts; Whether the product is labeled; Alarm in dangerous area of crane; Escalators automatically start and stop.
2.7 Metrology control
Automatic measurement of products or parts; Detect the pointer range of instruments and flowmeters to control the quantity or flow; The detection buoy controls the height and flow of the measuring surface; Detecting iron buoys in stainless steel barrels; Control of upper or lower limit of instrument range; Flow control, horizontal control.
2.8 Identifying objects
According to the code on the carrier.
2.9 information transmission
ASI (bus) connects the data of sensors at various positions on the equipment to and from the production line (50- 100m).
3. Classification and structure of proximity switch
The function of the proximity switch is to send a signal when an object approaches the proximity switch and reaches a certain distance. No external force is needed, and it is a contactless main electrical appliance. Its application has gone far beyond the travel control and limit protection of the travel switch. Proximity switches can be used for high-speed counting, detecting the presence of metal objects, measuring speed, controlling liquid level, detecting the size of parts and being used as contactless buttons. Proximity switches widely used at present can be divided into the following types according to their working principles:
High frequency oscillation type: used to detect various metal bodies.
Capacitance: used to detect various conductive or non-conductive liquids or solids.
Photoelectric type: used to detect all opaque substances.
Ultrasonic type: used to detect substances that do not penetrate ultrasonic waves.
Electromagnetic induction type: used to detect magnetic or non-magnetic metals.
According to its shape, it can be divided into cylindrical, square, groove, perforated (through) and separated types. Cylindrical type is easy to install, but its detection characteristics are the same. The groove-type detection part is in the groove for detecting the object passing through the groove. The straight-through type is rarely produced in China, but it is widely used in Japan, and can be used to assemble small parts such as screws or balls and buoys into water level detection devices.
Proximity switches can be divided into two types according to the power supply mode; DC type and AC type can be divided into DC two-wire system, DC three-wire system, DC four-wire system, AC two-wire system and AC three-wire system according to output types.
3. 1 two-wire proximity switch
The two-wire proximity switch is simple to install and convenient to wire; It is widely used, but it has the disadvantages of large residual voltage and leakage current.
3.2 DC three-wire type
The DC three-wire proximity switch has two output types: NPN and PNP. In 1970s, most Japanese products were NPN output, while Western European countries had both NPN and PNP output types. PNP output proximity switch is generally used in PLC or computer as control instruction, while NPN output proximity switch is used to control DC relay. In practical application, the output form should be selected according to the characteristics of the control circuit.
4. Selection and detection of proximity switch
4. 1. Selection of proximity switch
For different materials and different detection distances, different types of proximity switches should be selected to make them have higher cost performance in the system. Therefore, the following principles should be followed when selecting:
4. 1. 1. When the detection object is metal, the high-frequency oscillation proximity switch should be selected, which is the most sensitive for the detection of iron, nickel and A3 steel samples. For aluminum, brass and stainless steel, the detection sensitivity is low.
4. 1.2. When the test object is nonmetallic material, such as; For wood, paper, plastic, glass and water, capacitive proximity switches should be selected.
4. 1.3. When it is necessary to remotely detect and control metals and nonmetals, photoelectric proximity switch or ultrasonic proximity switch should be selected.
4. 1.4. When the detection object is metal, if the detection sensitivity is not high, low-cost magnetic proximity switch or Hall proximity switch can be selected.
4.2. Detection of technical indicators of proximity switch
4.2. 1. Action distance measurement; When the action piece approaches the sensing surface of the proximity switch from the front, the action distance of the proximity switch should be the maximum action distance of the proximity switch, and the measured data should be within the parameter range of the product.
4.2.2. Determination of release distance; When the actuating member leaves the sensing surface of the proximity switch from the front and the switch changes from action to release, measure the maximum distance between the actuating member and the sensing surface.
4.2.3. Determination of return difference H; Absolute value of the difference between the maximum action distance and the release distance.
4.2.4. Determination of action frequency; Drive bakelite disc with speed regulating motor, fix several steel plates on the disc, adjust the distance between the sensing surface of the switch and the action piece, which is about 80% of the action distance of the switch, and rotate the disc to make the action piece close to the switch in turn. The rotating speed measuring device is installed on the spindle of the disc, and the output signal of the switch is connected to the digital frequency meter after shaping. At this time, start the motor and gradually increase the speed. When the product of the speed and the action piece is equal to the frequency count, the action frequency of the switch can be directly read by the frequency meter.
4.2.5. Repetitive precision determination; Fix the action piece on the measuring tool, approach the action area of the switch from the front of the switch sensing surface, which exceeds the action distance of the switch by 120%, and control the moving speed at 0.1mm/s/s. When the switch acts, read the reading on the measuring tool, then exit the action area and close the switch. This is repeated 10 times. Finally, calculate the difference between the maximum and minimum values of 10 and the average value of 10. The biggest difference is the repeatability error.