Proximity sensor selection and detection

- Oct 13, 2019-

Classification and structure of proximity sensors

Two-wire proximity sensor


The two-wire proximity sensor is simple to install and convenient to wire; it is widely used, but has the disadvantage of large residual voltage and leakage current.


DC three-wire


The output types of DC three-wire proximity sensors are NPN and PNP. Most of the Japanese products in the 1970s were NPN outputs, and NPN and PNP are available in Western European countries. PNP output proximity sensors are generally used in PLCs or computers as control commands. NPN output proximity sensors are used to control DC relays. In practical applications, the output form should be selected according to the characteristics of the control circuit.


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Proximity sensor selection and detection:

For different material samples and different detection distances, different types of proximity sensors should be used to make them have high performance-price ratio in the system. For this reason, the following principles should be followed in the selection:


1. When the detector is made of metal material: the high-frequency oscillation type proximity sensor should be used. This type of proximity sensor is most sensitive to the detection of iron-nickel and A3 steel detectors. For aluminum, brass and stainless steel samples, the detection sensitivity is low.


2. When the test object is a non-metallic material: Capacitive proximity sensors such as wood, paper, plastic, glass and water should be used.


3. For long-distance detection and control of metal and non-metal: Photoelectric proximity sensor or ultrasonic proximity sensor should be used.


4. When detecting body metal but the sensitivity is not high: use a low-cost magnetic proximity sensor or a Hall-type proximity sensor.


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Elements of proximity sensor selection:

1 Detection type: amplifier built-in type, amplifier separated type; 2 shape: round, square, groove type; 3 detection distance: in mm; 4 detection object: iron, steel, copper, aluminum, plastic, water, Paper, etc.; 5 working power: DC, AC, AC and DC universal; 6 Output form: normally open (NO), normally closed (NC); 7 output mode: two-wire, three-wire (NPN, PNP); 8 shielding, Unshielded; 9 wire lead-out type, connector type, connector relay type; 10 response frequency: several objects can be detected in one second


For different material samples and different detection distances, different types of proximity sensors should be used to make them have high performance-price ratio in the system. For this reason, the following principles should be followed in the selection:


When the detector is a metal material, a high-frequency oscillating proximity sensor should be used. This type of proximity sensor is most sensitive to the detection of iron-nickel and A3 steel detectors. For aluminum, brass and stainless steel samples, the detection sensitivity is low.


When the test object is a non-metallic material, such as wood, paper, plastic, glass and water, a capacitive proximity sensor should be used.


For long-distance detection and control of metal and non-metal, photoelectric proximity sensors or ultrasonic proximity sensors should be used.


When the detection body is made of metal, if the detection sensitivity is not high, a magnetic proximity sensor or a Hall-type proximity sensor can be selected.


Determination of the release distance: When the action piece leaves the sensing surface of the proximity sensor from the front side, and the switch is turned from the action to the release, the maximum distance of the action piece from the sensing surface is measured.


Measurement of the hysteresis H: the absolute value of the difference between the maximum operating distance and the releasing distance.


Action frequency measurement: drive the bakelite disc with a speed-regulating motor, fix a number of steel sheets on the disc, adjust the distance between the switch sensing surface and the action piece, about 80% of the switch action distance, rotate the disc, and then make the action The piece is close to the proximity sensor, and a speed measuring device is mounted on the spindle of the disk, and the switch output signal is shaped and connected to the digital frequency meter. At this time, the motor is started, and the rotation speed is gradually increased. Under the condition that the product of the rotation speed and the action piece is equal to the frequency count, the frequency of the switch can be directly read by the frequency meter.


Repeating accuracy measurement: The action piece is fixed on the measuring tool, and the moving speed is controlled at 0.1 mm/s from the action area of the switch sensing surface facing the switch, except for 120% of the switching action distance. When the switch is activated, read the reading on the gauge and then exit the action zone to open the switch. This is repeated 10 times, and finally the difference between the maximum value and the minimum value of 10 measured values and the 10 times average value is calculated, and the difference is the repeat precision error.