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. Proximity switches can be used for high-speed counting, detection of the presence of metal bodies, speed measurement, level control, detection of part sizes, and use as contactless buttons. At present, the widely used proximity switches can be divided into the following types based on the working principle:
High-frequency oscillation type: used to detect various metal bodies
Capacitance type: 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 transmit ultrasonic waves
Electromagnetic induction type: used to detect magnetic or non-magnetic metal
According to its shape can be divided into cylindrical shape, square type, 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.
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, and NPN output proximity switches are used to control DC relays. In practical applications, the output format of the control circuit is selected according to the characteristics of the control circuit.