Proximity switches include inductive, capacitive, Hall, AC, and DC types. Proximity switch, also known as non-contact proximity switch, is an ideal electronic switch sensor.
(A) The main function
1. Detection distance
Detects the stop, start, and passing positions of elevators and lifting equipment; detects the position of vehicles to prevent collision detection of two objects; detects the setting position of the working machine, moves the limit position of the machine or component; detects the stopping position of the rotating body, and the valve Open or close position;
Detects the piston movement position in the cylinder or cylinder.
2. Size control
Metal plate punching size control device; automatic selection, identification of metal length; detection of automatic loading and unloading height; detection of the length, width, height and volume.
3. Check whether the presence of objects exists. Check if there are any product packaging boxes on the production packaging line; check for product parts.
4. Speed and speed control
Control the speed of the conveyor belt; control the rotational speed of the rotating machine; control the speed and the number of revolutions with various pulse generators.
5. Counting and Control
Detect the number of products flowing on the production line; measure the number of revolutions of a high-speed rotary shaft or disk; count parts.
6. Detection of abnormal
Check the presence or absence of bottle caps; judge whether products are qualified or not; detect the lack of metal products in the box; distinguish between metal and non-metal parts; whether or not the product has been tested for signs; the danger zone of the crane is on alarm; and the escalator is automatically started and stopped.
7. Measurement control
Automatic metering of products or parts; detection of meter or meter pointer range to control number or flow; detection of buoys to control surface height, flow; detection of iron buoys in stainless steel barrels; control of upper or lower meter range; flow control, level control.
8. Identifying objects
The recognition is based on the code on the carrier.
9. Information transmission
ASI (Bus) connects the sensors in various positions on the equipment in the production line (50-100 meters) for data transmission back and forth.
(B) Classification and Structure
The function of the proximity switch is to send a signal when an object approaches the proximity switch and reaches a certain distance. It does not require the application of external force, is a non-contact main commander. Its use has far exceeded the trip control and limit protection provided by the trip switch.
1. The proximity switch can be used for high-speed counting, detecting the presence of a metal body, speed measurement, level control, detection of part size, and use as a contactless button. At present, the widely used proximity switches can be divided into the following types based on the working principle:
(1) High-frequency oscillation type: used to detect various metal bodies
(2) Capacitive type: 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 transmit ultrasonic waves
(5) Electromagnetic induction type: used to detect magnetic or non-magnetic metal
2. According to its shape can be divided into cylindrical shape, square, groove type, perforation (through) type and separation type. Cylindrical type is easy to install, but its detection characteristics are the same. The groove type detection part is inside the tank. It is used to detect the objects passing through the tank. The penetration type is rarely produced in our country, while in Japan it is more common and can be used. Small parts such as small screws or balls and buoys are assembled into water level detection devices.
3. Proximity switch can be divided according to the power supply mode; DC type and AC type, according to the output 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.
(1) Two-wire proximity switch
The two-wire proximity switch is simple to install, convenient for wiring, and widely used, but has disadvantages of large residual voltage and leakage current.
(2) DC three-wire type
The output type of the DC three-wire proximity switch is NPN and PNP. In the 70’s, the vast majority of Japanese products were NPN output, and both NPN and PNP output models in Western Europe were available. PNP output proximity switches are generally used in PLCs or computers as control commands.
The NPN output proximity switch is used to control a large number of DC relays. In practical applications, the output format of the control circuit is selected according to the characteristics of the control circuit.
(III) Type selection and detection
1. Selection of proximity switches
For different material samples and different detection distances, different types of proximity switches should be selected so that they have a high performance-to-price ratio in the system. Therefore, the following principles should be followed in the selection:
2. When the detection body is a metal material, a high-frequency oscillation proximity switch should be selected. This type of proximity switch is the most sensitive to the detection of iron-nickel and A3 steels. For aluminum, brass and stainless steel samples, the detection sensitivity is low.
(1) Capacitive proximity switches should be used when the detector is non-metallic, such as wood, paper, plastic, glass, and water.
(2) When the metal body and the nonmetal are to be detected and controlled at a long distance, photoelectric proximity switches or ultrasonic proximity switches should be used.
(3) When the detection body is metal, if the detection sensitivity is not high, an inexpensive magnetic proximity switch or Hall proximity switch can be selected.
3. Proximity switch technology indicator detection
(1) Determination of the operating distance; When the action piece is moved from the front face to the proximity face of the proximity switch, the distance from the proximity switch is the maximum operating distance of the proximity switch. The measured data should be within the parameters of the product.
(2) Determination of the release distance; When the action piece moves from the front side away from the sensing surface of the proximity switch, and the switch is turned from motion to release, the maximum distance from the action surface of the action piece is measured.
(3) The measurement of the hysteresis H; the absolute value of the difference between the maximum movement distance and the release distance.
(4) Determination of the operating frequency; using a speed-regulating motor to drive the bakelite disc, fixing a number of steel plates on the disc, adjusting the distance between the switch sensing surface and the action film, about 80% of the switching distance, rotating the disc, In order to move the action piece closer to the proximity switch,
A speed measuring device is installed on the spindle of the disc. The output signal of the switch is shaped and connected to a digital frequency meter. Start the motor at this time, and gradually increase the speed. Under the condition that the product of the speed and the action piece is equal to the frequency count, the frequency of the switch can be read directly by the frequency meter.
(5). Repeated accuracy measurement; The action piece is fixed on the measuring tool, and the motion speed is controlled to be 0.1 mm/s from the switch action distance of 120% of the switching distance, from the front of the switch sensing surface to the action area of the switch. When the switch is activated, read the reading on the gage and then exit the action zone to turn the switch off. This is repeated 10 times. Finally, the difference between the maximum and minimum values of the 10 measured values and the 10 average values is calculated, and the difference is the repeat accuracy error.
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