Five kinds of proximity switch principle and application range

Proximity Switch, also known as Proximity Switch, can detect the presence or absence of an object so that the controller can know the presence or absence of the current mechanism, or it can also be designed to count sensing and calculate items on the conveyor belt. The number of passes, its use is very wide, and its characteristics are also very good, such as: fast cycle, non-contact sensing, miniaturized design, environmental resistance, low price, etc. Many manufacturers are actively engaged in research and development, production and types There are many types of square, square, cylindrical, round, groove, multi-point, etc. The action modes are divided into normally open (NO) and normally closed (NC). Generally, the proximity switches are non-contact type. The principle can be divided into the following five types:

1. High-frequency oscillation type proximity switch (Inductive sensor):

The high-frequency oscillation type sensor is composed of a detecting element, a detecting unit, an amplifying unit, an integer unit, and an output unit. The detecting component installed inside the sensor is composed of a detecting coil and a high-frequency oscillator. After the power is applied, the detecting coil generates an alternating electromagnetic field. When the metal object approaches the electromagnetic field, the magnetic flux density of the metal surface changes to generate an induced current. - Eddy current, the magnetic flux generated by the eddy current is always opposite to the magnetic flux of the detecting coil. Due to the action of the eddy current, the energy consumption of the detection coil is increased, the quality coefficient is lowered, the amplitude is lowered, and the oscillator is stopped. Conversely, when the metal object is away from this active area, the oscillator begins to oscillate again. After detecting the state change of the amplitude device, the detection circuit converts into a switching signal.

Inductive proximity sensors are only sensitive to metal objects, so inductive proximity sensors cannot be used for non-metallic object inspection. At the same time, the alternating magnetic field generated by the high-frequency oscillating coil is scattered, so that when the metal object keeps approaching the front end of the sensor, it will trigger the change of the state of the sensor, and the sense of metal objects appear around the sensor. The detector will also signal. Shielded inductive proximity sensors are required for applications where high accuracy is required or where metal objects are placed around the sensor, as this type of sensor has previously shielded the magnetic field around the oscillating coil. The change in sensor state is triggered only when the metal object is at the front end of the sensor.

In addition, the detection distance of the inductive proximity sensor may vary depending on the size of the object to be tested, the metal material, and even the type and thickness of the coating on the surface of the metal material; therefore, the relevant reference manual should be consulted when selecting.

For testing: metal

2. Capacitive proximity sensor:

The capacitive proximity switch takes a part of the high-frequency oscillation circuit of several hundred kHz to several MHz to the detection electrode plate, and the high-frequency magnetic field is generated by the electrode plate. If an object approaches, the surface of the object and the surface of the detection electrode plate The polarization phenomenon increases or decreases the overall capacitance. Therefore, the change in capacitance can indirectly estimate the amount of motion or position change of the moving object or the sensed object. The type of sensing object can be metal, plastic, liquid, wood, etc. .

For testing: to detect various conductive or non-conductive liquids or solids

3. Photoelectric proximity sensor (Photoelectric sensor):

Widely defined as photo-inductor, it is quite a variety of types. The diffuse-reflective photoelectric proximity is the same as the general proximity structure. The sensor has a built-in light source circuit, and the intensity of the light reflected from the surface of the object by the light emitter is To determine whether there is an object, this type of sensor does not need a reflector, although the detection distance setting is easy, but the disadvantages are as follows: short sensing distance, different detection accuracy, sensing object background needs, detection sensitivity adjustment The two photoelectric proximity devices are too close to cause interference and malfunction.

For detection: used to detect opaque objects

4. Magnetic gas proximity switch (Magnetic sensor):

Its principle is the same as that of the reed switch. The reed switch in the sensor is composed of two pieces of iron reeds placed in parallel to form a gap. These two elongated flat reeds are plated with precious metals to ensure their best. Function, the precious metal is generally made of bismuth or gold. The two reeds are completely sealed on a glass tube filled with inert gas. When there is a magnetic field close (usually a permanent magnet), the magnetic force will be large enough. At the same time, the contact will be from OFF to ON. There are no mechanical parts or electronic parts on the structure of the reed switch, so there will be no jamming or circuit damage, and the contact life can be accurate and high-speed for hundreds of times. Millions of times, this type of sensor is usually a group (sensor and magnet), most commonly used in door and window switch detection.

For testing: for detecting magnetic or non-magnetic metals

5. Hall proximity switch

The Hall element is a magnetic sensing element. A switch made of a Hall element is called a Hall switch. When the magnetic object moves closer to the Hall switch, the Hall element on the switch detection surface changes the internal circuit state of the switch due to the Hall effect, thereby identifying the presence of a magnetic object nearby, thereby controlling the on or off of the switch. The detection object of such a proximity switch must be a magnetic object.

For testing: for detecting substances that do not transmit ultrasonic waves

The proximity switch can be divided into two types according to the power supply mode: DC type and AC type. According to the output type, it 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.

Even if the circuit is short-circuited, the two-wire proximity switch is safe. Internal electrical components failure, wire breakage, trauma, power surges, etc. may cause internal circuit short-circuit, and the DC two-wire switch connected to the power supply through the load will not The current above the load current passes, and unlike the DC three-wire proximity switch, there is no danger of fire.