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Proximity sensors are widely used in a variety of automatic production lines, mechatronics equipment and petroleum, chemical, military, scientific research and other industries, so what is proximity sensors?

Proximity sensor refers to the general name of the sensor which takes the place of the limit switch and other contact detection methods for the purpose of detection without contact with the detection object. It can convert the movement information and existence information of the detected object into electrical signals.

In the detection mode of converting to electrical signal, it includes the mode of eddy current generated in the metal body of the detection object caused by electromagnetic induction, the change of the capacity of electrical signal caused by the proximity of the capture body, the way of stone and the way of guiding switch. It is composed of inductive type, electrostatic capacity type, ultrasonic type, photoelectric type, magnetic type and so on.

The proximity sensor is an alternating magnetic field generated by the vibrator. when the metal target approaches the magnetic field and reaches the induction distance, eddy current occurs in the metal target, which leads to vibration attenuation and even the vibration of the vibrator close to the sensor stops. The vibration and stop vibration changes of the vibrator close to the sensor are processed by the post-stage amplification circuit and converted into a switch signal to trigger the drive control device, so as to achieve the purpose of non-contact detection of the proximity sensor. This is how proximity sensors work.

Technical advantages ① because it can detect in a non-contact way, so it will not wear and damage the tested object.

Due to the use of contactless output mode, ② uses semiconductor output to extend its life (except magnetic type), which has no effect on the life of contacts.

Different from the light detection method, ③ is suitable for detection in water and oil environment, which is almost not affected by the stains, oil and water of the detection object. In addition, it also includes Teflon shell type and good drug resistance products.

Compared with the contact switch, ④ can achieve high-speed response.

⑤ can correspond to a wide temperature range.

⑥ is not affected by the color of the detected object: it detects the changes of the physical properties of the detected object, so it is almost not affected by the surface color.

Different from the contact type, ⑦ is affected by the surrounding temperature, surrounding objects and similar sensors, including inductive type and electrostatic capacity type, and the sensors influence each other. Therefore, for the setting of the sensor, mutual interference needs to be considered. In addition, in the inductive type, the influence of the surrounding metal needs to be considered, while in the electrostatic capacity type, the influence of the surrounding object needs to be considered.

When the metal detector is close to the induction area of the sensor, the switch can send out electrical instructions without contact, no pressure, no spark, and accurately reflect the position and stroke of the moving mechanism, even if it is used for general stroke control, its positioning accuracy, operating frequency, service life, convenience of installation and adjustment and adaptability to harsh environment can not be compared with the general mechanical stroke switch.

Classification of proximity sensors proximity sensors are divided according to their working principle:

High frequency oscillation type, capacitive type, induction bridge type, permanent magnet type and Hall effect type and so on.

According to the principle of operation, it can be divided into three categories:

The high frequency oscillation type of electromagnetic induction, the magnetic type of magnet and the capacitance type of capacitance change are used.

According to the detection method:

General type: mainly detect ferrous metals (iron)

All metal molds: detect any metal within the same detection distance.

Non-ferrous metal type: mainly detect non-ferrous metals such as aluminum

According to the type of structure:

1. Two-wire proximity sensor:

The two-wire proximity sensor has the advantages of simple installation and convenient wiring, and has a wide range of applications, but it has the disadvantages of large residual voltage and leakage current.

2. DC three-wire type:

There are two output types of DC three-wire proximity sensor: NPN and PNP. The vast majority of Japanese products in the 1970s were NPN output, while Western European countries had both NPN and PNP output. PNP output proximity sensor is generally used in PLC or computer as control instructions, while NPN output proximity sensor is often used to control DC relays. In practical application, its output form should be selected according to the characteristics of the control circuit.

The working principle of different types of proximity sensors the working principle of capacitive proximity sensors: capacitive proximity sensors are composed of high-frequency oscillators and amplifiers, and a capacitor is formed between the detection surface of the sensor and the earth. Participate in the work of the oscillation circuit, start in the state of oscillation. When the object approaches the detection surface of the sensor, the capacitance of the loop changes, which makes the high-frequency oscillator oscillate. The two states of oscillation and shutdown are converted into electrical signals which are converted into binary switching signals by amplifiers.

The working principle of inductive proximity sensor: inductive proximity sensor is composed of high frequency oscillation, detection, amplification, trigger and output circuit. The oscillator generates an alternating electromagnetic field on the sensor detection surface. when the metal object approaches the sensor detection surface, the eddy current generated in the metal absorbs the energy of the oscillator, weakening the oscillation and even stopping the vibration. The oscillating and stopping states of the oscillator are converted into binary switching signals by shaping and amplifying, and then output after power amplification.

The working principle of the high-frequency oscillatory proximity sensor: it is composed of the LC high-frequency oscillator and the amplifier processor circuit. When the metal object approaches the oscillation sensor head, it will produce eddy current, so that the oscillation ability of the proximity sensor attenuates, and the parameters of the internal circuit change, so as to identify whether there is a metal object approaching, and then control the switch on or off.

How all metal sensors work: all metal sensors basically belong to high frequency oscillation. Like the ordinary type, it also has an oscillating circuit in which the energy loss caused by the induced current flowing in the target affects the oscillating frequency. When the target approaches the sensor, the oscillation frequency will increase regardless of the type of metal of the target. The sensor detects the change and outputs the detection signal.

The working principle of the non-ferrous metal sensor: the non-ferrous metal sensor basically belongs to the high-frequency oscillation type. It has an oscillating circuit in which the energy loss caused by the flow of the induced current in the target affects the change of the oscillating frequency. When a non-ferrous metal target such as aluminum or copper approaches the sensor, the oscillation frequency increases; when a ferrous metal target such as iron approaches the sensor, the oscillation frequency decreases. If the oscillation frequency is higher than the reference frequency, the sensor outputs a signal.

The working principle of the universal proximity sensor: the coil L in the oscillating circuit produces a high frequency magnetic field. When the target is close to the magnetic field, an induced current (eddy current) is generated in the target due to electromagnetic induction. As the target approaches the sensor, the induced current increases, resulting in an increase in the load in the oscillating circuit. Then the oscillation weakens until it stops. The sensor uses the amplitude detection circuit to detect the change of the oscillation state and outputs the detection signal.

Selection of proximity sensor and detection of proximity sensor selection:

For the detection bodies of different materials and different detection distances, different types of proximity sensors should be selected to make them have a high ratio of performance to price in the system, so the following principles should be followed in type selection:

1. When the detection body is metal material: high-frequency oscillatory proximity sensor should be selected. This type of proximity sensor is the most sensitive to iron, nickel and A3 steel detection. For aluminum, brass and stainless steel, the detection sensitivity is low.

two。 When the detection body is a non-metallic material: capacitive proximity sensors should be selected, such as wood, paper, plastic, glass and water.

3. When metal and non-metal are to be detected and controlled over a long distance, photoelectric proximity sensor or ultrasonic proximity sensor should be selected.

4. When the body metal is detected but the sensitivity requirement is not high: low-cost magnetic proximity sensor or Hall proximity sensor can be selected.

Close to the elements of sensor selection:

① detection type: amplifier built-in type, amplifier separate type

② shape: round, square, groove

③ detection distance: in mm

④ testing objects: iron, steel, copper, aluminum, plastic, water, paper, etc.

⑤ working power supply: DC, AC, AC DC

⑥ output form: normally open (NO), normally closed (NC)

⑦ output mode: two-wire, three-wire (NPN, PNP)

⑧ shielded, unshielded

⑨ lead-out type, connector type, connector relay type

⑩ response frequency: can detect several objects in a second

Proximity sensor detection:

Determination of the release distance: when the action piece leaves the sensor surface from the front and the switch changes from action to release, the maximum distance between the action piece and the sensor surface is measured.

Determination of backlash H: the absolute value of the difference between the maximum operating distance and the release distance.

Action frequency measurement: drive the Bakelite disc with a speed regulation motor, fix a number of steel sheets on the disc, adjust the distance between the switch induction surface and the action sheet, about 80% of the switch action distance, rotate the disc, and in turn make the action sheet close to the sensor. A speed measuring device is installed on the disc spindle, and the switch output signal is shaped and connected to the digital frequency meter. At this time, start the motor and gradually increase the rotational speed. Under the condition that the product of the rotational speed and the action piece is equal to the frequency count, the operation frequency of the switch can be read directly by the frequency meter.

Measurement of repetition accuracy: the action piece is fixed on the measuring tool. Beyond 120% of the switch action distance, the action area of the switch is close to the front of the switch sensor surface, and the movement speed is controlled at 0.1mm / s. When the switch operates, read out the reading on the measuring tool, and then exit the action area to turn off the switch. If it is repeated for 10 times, the difference between the maximum and minimum values of the measured values and the average value of 10 times will be calculated. The larger the difference is the error of repetition accuracy.

Common troubleshooting of proximity sensors ① stable power supply supplies separate power to proximity sensors

The ② response frequency is within the rated range.

There is jitter in the process of ③ object detection, which causes it to exceed the detection area.

Multiple ④ probes are tightly installed to interfere with each other

There are other tested objects in the detection area around the probe of the ⑤ sensor.

⑥ proximity sensor is surrounded by high-power equipment and electrical interference.

Proximity sensors are widely used in machine tools, metallurgy, chemical industry, textile, printing and other industries. It can be used as position limit, counting, positioning control and automatic protection in the automatic control system. The proximity sensor has the characteristics of long service life, reliable operation, high repetitive positioning accuracy, no mechanical wear, no spark, no noise, strong anti-vibration ability and so on. At present, the application range of proximity sensor is becoming more and more extensive, and its own development and innovation speed is extremely rapid.

This article is from the official account of Wechat: sensor Technology (ID:WW_CGQJS). Author: Faraday.

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