What are Proximity Sensors ?

Proximity sensors

Proximity sensors find extensive use in automated manufacturing lines, mechatronics equipment, and diverse industries such as petroleum, chemical, military, and scientific research. How do we define a proximity sensor?

Ⅰ. Understanding Proximity Sensors

A proximity sensor serves as a comprehensive term encompassing sensors engineered to detect objects without physical contact, thereby substituting traditional contact detection techniques like limit switches. This type of sensor possesses the capability to transform the movement and presence information of the observed object into electrical impulses.

Various detection methods fall under the umbrella of proximity sensors, including those utilizing eddy currents induced in a metal object through electromagnetic induction, methods capturing changes in the electrical signal's capacitance caused by the proximity of the object, and techniques employing sharp stones and guide switches. The categories of proximity sensors encompass induction, electrostatic capacitance, ultrasonic, photoelectric, magnetic, and others.

The proximity sensor utilizes a vibrator to produce an oscillating magnetic field. As the metal target nears the measuring distance, eddy currents are induced in the metal, leading to vibration attenuation and eventually causing the proximity sensor's vibrator to cease its oscillations. The post-amplifier circuit processes both the vibration of the proximity sensor's vibrator and the alterations in its cessation, converting them into a switch signal. This signal, in turn, activates the drive control device, thereby achieving the objective of non-contact detection with proximity sensors. This encapsulates the operational process of proximity sensors.

Ⅱ. Advantages of Proximity Sensors

Preservation of Detected Objects:

The detection process avoids physical contact, ensuring the preservation and integrity of the detected object.

Extended Lifespan:

Non-contact output methods, especially those utilizing semiconductor outputs (except for magnetic types), contribute to extended device life without impacting the contact point's durability.

Versatility in Challenging Environments:

Suited for usage in conditions involving water and oil, proximity sensors are unaffected by stains, oil, and water on the detection item, unlike optical detection methods. Additionally, they are compatible with items featuring Teflon shells and high chemical resistance.

High-Speed Responsiveness:

Proximity sensors exhibit high-speed responsiveness, outperforming contact switches in terms of speed.

Wide Temperature Range Tolerance:

Proximity sensors can operate effectively across a broad temperature range.

Color Independence:

The color of the detected object has minimal impact, as the sensors detect changes in the physical qualities of the object's surface.

Minimal Interference:

Unlike contact types, proximity sensors are less impacted by surrounding temperature, objects, and sensors of the same type. However, mutual interference should be considered during sensor setup.

Ⅲ. Classification of Proximity Sensors

Principles:

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

Operating Principles:

High-frequency oscillation using electromagnetic induction, magnetic type using magnets, and capacitive type using capacitance change.

Detection Methods:

Universal type (ferrous metals), All Metal Types (detects any metal), Non-ferrous metal type (non-ferrous metals such as aluminum).

Structure Types:

Two-wire proximity sensor: Simple installation but may experience high residual voltage and leakage current.

DC three-wire type: NPN and PNP output options, chosen based on the control circuit's properties in real applications.

Ⅳ. Operation of Various Proximity Sensor Types

Capacitive Proximity Sensor:

Operating Principle: Comprising a high-frequency oscillator and an amplifier, the sensor detects variations in capacitance as an object approaches, causing the oscillator to vibrate. Amplifiers transform these oscillations into electrical signals, ultimately converted into binary switching signals.

The working principle of the inductive proximity sensor involves a sequence of steps: High-frequency oscillation, detection, amplification, triggering, and output circuits collectively constitute the inductive proximity sensor. The sensor's detection surface hosts an oscillator generating an alternating electromagnetic field. As a metal object approaches the sensor's sensing surface, eddy currents induced in the metal absorb the oscillator's energy, leading to a reduction in oscillation and cessation of vibration. The two states of the oscillator, oscillation, and stop are converted into electrical signals. These signals undergo shaping and amplification, resulting in binary switching signals that are subsequently output after power amplification.

The operational principle of the high-frequency oscillation proximity sensor involves an LC high-frequency oscillator and an amplifying processor circuit. As a metal object nears the oscillating induction head, the generation of eddy currents diminishes the proximity sensor's oscillation capability, causing alterations in internal circuit parameters. This mechanism detects the proximity of a metal object and regulates the switch's on or off state accordingly.

The All Metal type sensors operate on a similar principle, constituting high-frequency oscillation sensors with an oscillation circuit. The induced current's energy loss in the target influences the oscillation frequency, increasing as the target approaches the sensor, irrespective of the metal type.

In the case of non-ferrous metal sensors, the high-frequency oscillation principle applies. The oscillation circuit, affected by energy loss from induced current in the target, leads to a frequency change. The frequency increases with the approach of a non-ferrous metal like aluminum or copper, while it decreases with the approach of a ferrous metal like iron. A detection signal is generated if the oscillation frequency surpasses the reference frequency.

The general-purpose proximity sensor operates by generating a high-frequency magnetic field through the coil L in the oscillating circuit. Electromagnetic induction occurs as an object approaches the magnetic field, generating an induced current (eddy current) in the object. The induced current intensifies as the target nears the sensor, causing an increase in the load on the oscillator circuit. Subsequently, the oscillations fade until they cease. The sensor's amplitude detection circuit identifies the change in the oscillation state and outputs the detection signal.

Ⅴ. Choosing and Evaluating Proximity Sensors

Selecting a proximity sensor:

To ensure an optimal performance-price ratio in the system, different types of proximity sensors should be chosen based on the materials being detected and the required detecting distances. Following these principles is crucial in the selection process:

When the detected object is metallic, prioritize the use of the high-frequency oscillation type proximity sensor. This sensor type exhibits high sensitivity in detecting iron-nickel and A3 steel. However, it is worth noting that the detection sensitivity is relatively low for aluminum, brass, and stainless steel objects.

When dealing with a detection body made of non-metallic materials like wood, paper, plastic, glass, or water, it is recommended to employ capacitive proximity sensors.

For the distant detection and control of both metal and non-metal bodies, optoelectronic proximity sensors or ultrasonic proximity sensors should be opted for.

In scenarios where the detection body is metal but sensitivity requirements are not stringent, a cost-effective magnetic proximity sensor or a Hall-type proximity sensor can be chosen.

Aspects of proximity sensor selection:

① Detection type: integrated amplifier, separate amplifier;

② Shape: circular, square, or groove types;

③ Detection distance: in millimeters;

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

⑤ Power supply: DC, AC, universal AC and DC;

⑥ Output form: normally open (NO), normally closed (NC);

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

⑧ Shielded and unshielded;

⑨ Lead-out type, connector type, connector relay type;

⑩ Response frequency: the number of objects detectable in one second.

Proximity sensor detection:

Determination of release distance: Measure the maximum distance the actuating part departs from the sensing surface when transitioning from actuation to release.

Determination of hysteresis H: Calculate the absolute value of the difference between the maximum actuation distance and the release distance.

Action frequency measurement: Utilize a speed-regulated motor to drive a bakelite disc, place several steel sheets on the disc, adjust the distance between the switch detecting surface and the actuating sheet to approximately 80% of the switch's actuation distance, and rotate the disc. By observing the output signal on a digital frequency meter connected to the main shaft's speed measuring device, the operating frequency of the switch can be directly read.

Repeat accuracy measurement: Approach the switch's actuation area beyond 120% of the switch's actuation distance, maintaining a movement speed of 0.1 mm/s. After activation, measure the reading on the measurement tool before exiting the actuation area to deactivate the switch. Repeat this process 10 times, then calculate the difference between the highest and lowest values and the average of the 10 measurements.

Ⅵ. Standard Troubleshooting for Proximity Sensors

① Ensure a stable power supply is provided separately to the proximity sensor;

② Verify that the response frequency falls within the rated range;

③ Address any jitter during the object detection process, preventing it from exceeding the detection area;

④ Avoid installing multiple probes in close proximity to avoid interference;

⑤ Ensure no other measured objects are present in the detection area around the sensor probe;

⑥ Check for high-power devices around the proximity sensor that may cause electrical interference.

Proximity sensors find applications in machine tools, metallurgy, chemicals, textiles, and printing industries. They serve various functions such as limit sensing, counting, positioning control, and automatic protection in automatic control systems. Noteworthy characteristics include a long service life, dependable operation, high repeat positioning accuracy, no mechanical wear, absence of sparks and noise, and exceptional anti-vibration capabilities. The applications for proximity sensors are continually expanding, with ongoing research and innovation accelerating their evolution.

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