1. Overview
Proximity sensor, also called non-contact proximity sensor, is an ideal electronic switch sensor. When the metal detector is close to the sensing area of the sensor, 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 sensor has the characteristics of long service life, reliable operation, high repetitive positioning accuracy, no mechanical wear, no spark, no noise and strong anti-vibration ability. Up to now, the application range of proximity sensor is increasingly extensive, and its own development and innovation speed is also extremely rapid.
2. Working principle
2. 1 Working principle of inductive proximity switch
Inductive proximity sensor is a position sensor with switch output, which consists of LC high-frequency oscillator and amplification processing circuit. When a metal object approaches the oscillating induction head which can generate electromagnetic field, eddy current will be generated inside the object. This eddy current acts on the proximity switch, which weakens the oscillation ability of the proximity switch and changes the parameters of the internal circuit, so as to identify whether there is a metal object approaching, and then control the on-off of the switch. The object that this proximity switch can detect must be a metal object.
2.2 capacitive proximity switch series
Capacitive proximity sensor also belongs to position sensor with on-off output. Its measuring head is usually one plate of the capacitor, and the other plate is the object itself. When the object moves to the proximity switch, the dielectric constant of the object and the proximity switch changes, which changes the state of the circuit connected to the probe, thus controlling the on-off of the switch. The detection object of the proximity switch is not limited to metal conductors, but also insulating liquid or powdery objects. When detecting objects with low dielectric constant ε, the multi-turn potentiometer (located on the back of the switch) can be adjusted clockwise to improve the induction sensitivity. In general, adjust the potentiometer to make the capacitive proximity switch work at the position of 0.7-0.8Sn.
2.3 the working principle of hall switch
2.3. 1 Brief introduction of principle
When a metal or semiconductor sheet with current is placed vertically in a magnetic field, a potential difference will be generated at both ends of the sheet, which is called Hall effect. The potential difference between the two ends is called Hall potential U, and its expression is
U=K？ 6? 1I？ 6? 1B/d
Where k is the Hall coefficient, i is the current passing through the sheet, b is the magnetic induction intensity (Lorentz force) of the external magnetic field, and d is the thickness of the sheet.
It can be seen that the sensitivity of Hall effect is directly proportional to the magnetic induction intensity of the external magnetic field.
The input of the Hall switch is characterized by the magnetic induction intensity B. When the value of B reaches a certain level (such as B 1), the trigger inside the Hall switch flips, and the output level state of the Hall switch also flips. The output terminal generally adopts transistor output, similar to proximity switch, including NPN, PNP, normally open, normally closed, latched (bipolar) and dual signal output.
3. Classification and structure of proximity sensors
3. 1 Two-wire proximity sensor
The two-wire proximity sensor 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 sensor 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 sensor is generally used in PLC or computer as control instruction, while NPN output proximity sensor 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 sensors
4. 1 Different types of proximity sensors should be selected for different materials and different detection distances, so that they have higher cost performance in the system. Therefore, the following principles should be followed when selecting:
4. 1.65438+ 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 sensors should be selected.
4. 1.3 When detecting and controlling metals and nonmetals remotely, photoelectric proximity sensors or ultrasonic proximity sensors should be selected.
4. 1.4 When the detection object is metal, if the detection sensitivity is not high, low-cost magnetic proximity sensor or Hall proximity sensor can be selected.
4.2 Detection of technical indicators of proximity sensors
4.2. 1 Action distance measurement; When the acting part approaches the sensing surface of the proximity sensor from the front, the acting distance of the proximity sensor should be the maximum acting distance of the proximity sensor, and the measured data should be within the parameter range of the product.
4.2.2 Determination of release distance; When the action piece leaves the sensing surface of the proximity sensor from the front and the switch changes from action to release, measure the maximum distance between the action piece 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 the bakelite disc with a 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 sensor 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 Determination of repeatability accuracy; 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.