Proximity sensors are widely used in various automated production lines, mechatronics equipment, and many industries such as petroleum, chemical, military, and scientific research. So what exactly is a proximity sensor?
proximity sensor
A proximity sensor is a general term for sensors that replace contact-based detection methods such as limit switches, aiming to detect objects without physical contact. It converts the movement and presence information of the detected object into electrical signals.
Among the detection methods that convert signals into electrical signals, there are methods that utilize eddy currents generated in the metallic body of the target object due to electromagnetic induction, methods that utilize changes in the capacitance of the electrical signal caused by the proximity of the target object, and methods using sensors and guide switches. These methods include inductive, capacitive, ultrasonic, photoelectric, and magnetic types.
A proximity sensor utilizes an alternating magnetic field generated by a vibrator. When a metallic target approaches this magnetic field and reaches the sensing distance, eddy currents are generated within the metallic target, causing vibration attenuation until the vibrator of the proximity sensor stops vibrating. The changes in vibration and cessation of the proximity sensor's vibrator are processed by a subsequent amplification circuit and converted into a switching signal, triggering the drive controller. This achieves the non-contact detection purpose of the proximity sensor. This is the operating principle of a proximity sensor.
Technological advantages
①Because it can perform detection in a non-contact manner, it will not wear out or damage the object being detected.
② Due to the use of contactless output, the lifespan is extended (except for magnetic type). The use of semiconductor output has no impact on the lifespan of the contact.
③ Unlike optical detection methods, it is suitable for use in environments with water and oil, and its detection is almost unaffected by stains, oil, and water on the object being tested. In addition, it includes Teflon-cased types and products with good chemical resistance.
④ Compared with contact switches, it can achieve high-speed response.
⑤ It can handle a wide range of temperatures.
⑥ Unaffected by the color of the object being tested: Since it detects changes in the physical properties of the object being tested, it is almost unaffected by surface color, etc.
⑦ Unlike contact sensors, they are affected by ambient temperature, surrounding objects, and similar sensors, including inductive and capacitive sensors, and there is mutual interference between sensors. Therefore, mutual interference needs to be considered when setting up sensors. In addition, the influence of surrounding metal needs to be considered in inductive sensors, while the influence of surrounding objects needs to be considered in capacitive sensors.
When a metal object approaches the sensor's sensing area, the switch can quickly issue an electrical command without contact, pressure, or sparks, accurately reflecting the position and stroke of the moving mechanism. Even when used for general stroke control, its positioning accuracy, operating frequency, service life, ease of installation and adjustment, and adaptability to harsh environments are unmatched by ordinary mechanical limit switches.
Classification of proximity sensors
Proximity sensors can be classified according to their working principle as follows: high-frequency oscillation type, capacitive type, inductive bridge type, permanent magnet type, and Hall effect type, etc.
Based on their operating principles, they can be divided into three categories: high-frequency oscillation type using electromagnetic induction, magnetic type using magnets, and capacitive type using capacitance changes.
According to the detection method, it can be divided into: general type: mainly detects ferrous metals (iron) and all metals type: detects any metal within the same detection distance.
Non-ferrous metal type: mainly detects non-ferrous metals such as aluminum. According to the structural type, it is divided into: 1. Two-wire proximity sensor: Two-wire proximity sensors are simple to install and easy to wire; they are widely used, but they have the disadvantages of residual voltage and large leakage current.
2. DC Three-Wire Type: DC three-wire proximity sensors have two output types: NPN and PNP. In the 1970s, most Japanese products were NPN output, while Western European countries used both NPN and PNP output types. PNP output proximity sensors are generally used in PLCs or computers for control commands, while NPN output proximity sensors are more often used to control DC relays. In practical applications, the output form should be selected according to the characteristics of the control circuit.
Working principles of different types of proximity sensors
The working principle of a capacitive proximity sensor: A capacitive proximity sensor consists of a high-frequency oscillator and an amplifier. A capacitor is formed between the sensor's detection surface and the ground, participating in the oscillation circuit and initially in an oscillating state. When an object approaches the sensor's detection surface, the capacitance of the circuit changes, causing the high-frequency oscillator to oscillate. The two states of oscillation and cessation are converted into electrical signals, which are then amplified into binary switching signals.
The working principle of an inductive proximity sensor: An inductive proximity sensor consists of a high-frequency oscillation, detection, amplification, triggering, and output circuit. The oscillator generates an alternating electromagnetic field at the sensor's detection surface. When a metal object approaches the sensor's detection surface, the eddy currents generated in the metal absorb the oscillator's energy, weakening or even stopping the oscillation. These two states—oscillation and cessation—are converted into electrical signals, which are then shaped, amplified, and converted into binary switching signals, and finally amplified by power before being output.
The working principle of a high-frequency oscillation proximity sensor: It consists of an LC high-frequency oscillator and an amplifier/processor circuit. When a metal object approaches the oscillating sensor head, eddy currents are generated, causing the proximity sensor's oscillation capability to attenuate. This changes the parameters of the internal circuit, thereby detecting the presence or absence of a metal object and controlling the switch to open or close. The working principle of all metal-type sensors: All metal-type sensors are essentially high-frequency oscillation sensors. Like ordinary sensors, they also have an oscillation circuit. Energy loss caused by the induced current flowing within the target object affects the oscillation frequency. When a target object approaches the sensor, regardless of the type of metal, the oscillation frequency increases. The sensor detects this change and outputs a detection signal.
The working principle of non-ferrous metal sensors: Non-ferrous metal sensors are basically high-frequency oscillation type. They have an oscillation circuit, where energy loss caused by the flow of induced current within the target object affects the oscillation frequency. When a non-ferrous metal target object such as aluminum or copper approaches the sensor, the oscillation frequency increases; when a ferrous metal target object 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 a general-purpose proximity sensor: A coil L in the oscillation circuit generates a high-frequency magnetic field. When a target object approaches the magnetic field, an induced current (eddy current) is generated in the target object due to electromagnetic induction. As the target object approaches the sensor, the induced current increases, causing an increase in the load on the oscillation circuit. Then, the oscillation weakens until it stops. The sensor uses an amplitude detection circuit to detect the change in the oscillation state and outputs a detection signal.
Proximity sensor selection and detection
Proximity sensor selection:
Different types of proximity sensors should be selected for different materials and different detection distances to ensure a high performance-price ratio within the system. Therefore, the following principles should be followed during the selection process:
1. When the object to be detected is a metallic material: a high-frequency oscillation proximity sensor should be selected. This type of proximity sensor is most sensitive to iron-nickel and A3 steel objects. Its sensitivity is lower for aluminum, brass, and stainless steel objects.
2. When the object to be detected is a non-metallic material: a capacitive proximity sensor should be selected, such as for wood, paper, plastic, glass and water.
3. When remote detection and control of metallic and non-metallic objects are required: photoelectric proximity sensors or ultrasonic proximity sensors should be selected.
4. When detecting metal but not requiring high sensitivity: inexpensive magnetic proximity sensors or Hall effect proximity sensors can be used.
Factors to consider when selecting a proximity sensor:
①Detection type: Amplifier built-in type, amplifier separate type;
② Shape: round, square, or concave;
③ Detection distance: in mm;
④ Detectable objects: iron, steel, copper, aluminum, plastic, water, paper, etc.;
⑤ Power supply: DC, AC, and AC/DC compatible;
⑥ Output state: Normally open (NO), normally closed (NC);
⑦ Output method: Two-wire, three-wire (NPN, PNP);
⑧ Shielded, unshielded;
⑨ Lead wire type, connector type, connector repeater type;
⑩ Response frequency: How many objects can be detected per second?
Proximity sensor detection: Determination of release distance: When the actuating piece leaves the sensing surface of the proximity sensor from the front, and the switch changes from actuation to release, the maximum distance the actuating piece leaves the sensing surface is measured.
Measurement of hysteresis H: The absolute value of the difference between the maximum action distance and the release distance.
Operating frequency measurement: A variable-speed motor drives a bakelite disc, on which several steel plates are fixed. The distance between the switch sensing surface and the actuating plates is adjusted to approximately 80% of the switch's operating distance. The disc is rotated, bringing the actuating plates closer to the proximity sensor in sequence. A speed measuring device is mounted on the disc's main shaft. The switch output signal is shaped and connected to a digital frequency meter. The motor is then started, and the speed is gradually increased. When the product of the speed and the actuating plate equals the frequency count, the switch's operating frequency can be directly read from the frequency meter.
Repeatability measurement: Fix the actuator plate on the measuring instrument. From 120% of the switch's actuation distance, approach the switch's actuation zone from the front of the sensing surface, maintaining a speed of 0.1 mm/s. When the switch actuates, read the value on the measuring instrument, then withdraw from the actuation zone to disconnect the switch. Repeat this process 10 times. Finally, calculate the difference between the maximum and minimum values of the 10 measurements and the average value of the 10 measurements. The larger difference is the repeatability error.
Troubleshooting common proximity sensor problems
① A stable power supply provides a separate power source for the proximity sensor;
② The response frequency is within the rated range; ③ There is jitter during object detection, causing it to exceed the detection area; ④ Multiple probes are installed closely and interfere with each other; ⑤ There are other objects being measured in the detection area around the sensor probe; ⑥ There are high-power devices near the sensor, causing electrical interference.
Proximity sensors are widely used in industries such as machine tools, metallurgy, chemical engineering, textiles, and printing. In automatic control systems, they serve as limit switches, counters, positioning controls, and automatic protection components. Proximity sensors are characterized by long service life, reliable operation, high repeatability, no mechanical wear, no sparks, no noise, and strong vibration resistance. Currently, the application range of proximity sensors is increasingly wide, and their development and innovation are extremely rapid.
