Proximity Sensors: Types, Working Principles and Applications

Proximity sensors are widely utilized in automated manufacturing lines, mechatronics equipment, and a wide range of industries, including petroleum, chemical, military, and scientific research. What is the definition of a proximity sensor?

Ⅰ. What is a proximity sensor?

A proximity sensor is a general word for sensors that are designed to detect without contacting the detecting object and replace contact detection methods such as limit switches. It has the ability to convert the observed object's movement and presence information into electrical impulses.

Methods that use eddy currents generated in a metal object to be detected by electromagnetic induction, methods that capture changes in the capacity of electrical signals due to the proximity of the object to be detected, and methods that use sharp stones and guide switches are examples of detection methods that convert into electrical signals. It includes induction, electrostatic capacitance, ultrasonic, photoelectric, and magnetic types, among others.

Figure. 1

The vibrator generates an alternating magnetic field, which is used by the proximity sensor. Eddy currents form in the metal target when it approaches the magnetic field and approaches the measuring distance, causing vibration attenuation and the proximity sensor's vibrator to stop vibrating. The vibration of the proximity sensor's vibrator and the change in the stop vibration are processed by the post-amplifier circuit and transformed into a switch signal, which triggers the drive control device, fulfilling the goal of proximity sensor non-contact detection. The way proximity sensors function is as follows.

Ⅱ. What are the advantages of proximity sensors?

①The thing to be detected will not be worn or destroyed because it may be detected without touching it.

②Because the non-contact output method is used, the life-extension (save for the magnetic type) uses a semiconductor output, which has no effect on the contact point's life.

③It is suited for usage in conditions such as water and oil, and it is unaffected by the stains, oil, and water of the detection item, unlike the optical detection method. In addition, goods with  Teflon shells and high chemical resistance are included.

④ High-speed responsiveness is possible as compared to contact switches.

⑤It can withstand a wide range of temperatures.

⑥ The color of the detected object has no effect: Because the change in the physical qualities of the detected object is detected, the color of the surface has little effect.

⑦ It will be impacted by the surrounding temperature, surrounding objects, and sensors of the same type, such as inductive and electrostatic capacitance sensors, and the sensors will affect each other, unlike the contact type. As a result, mutual interference must be addressed when setting up the sensors. Furthermore, the influence of surrounding metal must be addressed in the inductive type, whereas the influence of surrounding objects must be considered in the electrostatic capacitance type.

When the metal detector is close to the sensor's sensing region, the switch can operate without touch, pressure, or spark, and send out electrical signals fast, accurately reflecting the location and stroke of the moving mechanism. Even if it is used for general stroke control, its positioning precision, operating frequency, service life, ease of installation and adjustment, and capacity to adapt to hostile conditions make it unbeatable.

Ⅲ. Classification of proximity sensors

According to the working principles:

High frequency oscillation type, capacitive type, induction bridge type, permanent magnet type, and Hall effect type, for example, are all types of proximity sensors.

According to the operating principles: 

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

According to the detection method:

Universal type: mainly detect ferrous metals (iron)

All Metal Types: Detects any metal within the same detection distance.

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

According to the structure type:

1. The two-wire proximity sensor is simple to install and connect; it is extensively used, but it suffers from high residual voltage and leakage current.

2. DC three-wire type:  DC  three-wire proximity sensors have two output types: NPN and PNP. Most Japanese items were NPN output in the 1970s, and Western European countries had both NPN and PNP output. PNP output proximity sensors are typically utilized as control instructions in PLCs or computers, while NPN output proximity sensors are typically employed to drive  DC relays. In real applications, the output form should be chosen based on the control circuit's properties.

Ⅳ. How do different types of proximity sensors work?

The capacitive proximity sensor's operating principle: the capacitive proximity sensor is made up of a high-frequency oscillator and an amplifier. The capacitance of the loop varies when an object approaches the sensor's detection surface, causing the high-frequency oscillator to vibrate. Amplifiers transform the oscillation and stop states into electrical signals, which are then turned into binary switching signals.

Figure. 2 Working Principle Diagram of Capacitive Proximity Switch

The inductive proximity sensor works on the following principle: High-frequency oscillation, detection, amplification, triggering, and output circuits make up the inductive proximity sensor. On the sensor's detection surface, the oscillator generates an alternating electromagnetic field. When a metal object comes close to the sensor's sensing surface, the eddy current formed in the metal absorbs the oscillator's energy, weakening the oscillation and stopping the vibration. The oscillator's two states of oscillation and stop are transformed into electrical signals, which are then shaped and amplified into binary switching signals before being output following power amplification.

Figure. 3

The high-frequency oscillation proximity sensor's working principle is that it is made up of an LC high-frequency oscillator and an amplifying processor circuit.When a metal object approaches the oscillating induction head, an eddy current is generated, which reduces the proximity sensor's oscillation ability and changes the internal circuit parameters. This detects whether a metal object is approaching and regulates whether the switch is turned on or off.

All metal type sensors function on the same principle: All metal sensors are essentially high frequency oscillation sensors  . It has an oscillation circuit, just like the standard type, in which the energy loss caused by the induced current flowing in the target influences the oscillation frequency. The oscillation frequency increases as the target approaches the sensor, regardless of the target metal type.

The principle of operation of a non-ferrous metal sensor is that it is a high frequency oscillation type. It has an oscillating circuit in which the energy loss caused by the induced current passing through the target influences the oscillation frequency change. The oscillation frequency increases when a non-ferrous metal object, such as aluminum or copper, approaches the sensor; when a ferrous metal object, such as iron, approaches the sensor, the oscillation frequency lowers. The sensor generates a signal if the oscillation frequency exceeds the reference frequency.

The coil L in the oscillating circuit generates a high-frequency magnetic field, which is how the general-purpose proximity sensor works. Due to electromagnetic induction, when an object approaches a magnetic field, an induced current (eddy current) is generated in the object. The induced current grows as the target approaches the sensor, increasing the load on the oscillator circuit. The oscillations then fade away until they stop. The amplitude detection circuit of the sensor detects the change in oscillation state and outputs the detection signal.

Ⅴ. How to select and detect proximity sensors?

Selecting a proximity sensor:

Different types of proximity sensors should be chosen for detectors of various materials and detecting distances in order to get a high performance-price ratio in the system. For this reason, the following principles should be followed in the selection:

1. When the detecting body is made of metal, the high-frequency oscillation type proximity sensor should be used, as it is the most sensitive for iron-nickel and A3 steel detection. The detection sensitivity of aluminum, brass, and stainless steel detector bodies is low.

Figure. 4

2. When the detection body is made of a non-metallic material, such as wood, paper, plastic, glass, or water, capacitive proximity sensors should be employed.

3. Optoelectronic proximity sensors or ultrasonic proximity sensors should be utilized to detect and control metal bodies and non-metals from a distance.

4. A low-cost magnetic proximity sensor or a Hall-type proximity sensor can be used when the detector body is metal but the sensitivity requirements are not high.

Elements of proximity sensor selection:

① Detection type: built-in amplifier, separate amplifier;

② Shape: round, square, groove type;

③ Detection distance: in mm;

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

⑤ Working power supply: 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 Shielded and unshielded;

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

⑩ Response frequency: several objects can be detected in one second

Proximity sensor detection:

Determination of release distance: When the action piece leaves the proximity sensor's sensing surface from the front and the switch changes from action to release, measure the maximum distance the action piece departs the sensing surface.

Determination of hysteresis H: The absolute value of the difference between the maximum action distance and the release distance.

Action frequency measurement: use a speed-regulating motor to drive a bakelite disc, place a number of steel sheets on the disc, adjust the distance between the switch detecting surface and the action sheet to roughly 80% of the switch action distance, rotate the disc, and perform the action in turn. The chip is close to the proximity sensor, the main shaft of the disc has a speed measuring device, and the switch's output signal is formed and connected to the digital frequency meter. Start the motor at this point and gradually increase the speed. The operating frequency of the switch can be directly read out by the frequency meter if the product of the speed and the action film equals the frequency count.

Repeat the accuracy measurement by mounting the action piece on the measuring tool and approaching the switch action area from the front of the switch sensing surface beyond 120 percent of the switch action distance, while keeping the movement speed at 0.1mm/s. After the switch has been activated, read the reading on the measurement tool before exiting the action area to turn off the switch. Repeat this procedure 10 times, then calculate the difference between the highest and minimum values of the 10 measured values and the average value of the 10 times; the wider the difference, the greater the repeatability error.

Ⅵ. Common troubleshooting for proximity sensors

① The stable power supply supplies power to the proximity sensor separately;

② The response frequency is within the rated range;

③ There is jitter in the object detection process, resulting in exceeding the detection area;

④ Multiple probes are installed closely to interfere with each other;

⑤ There are other measured objects in the detection area around the sensor probe;

⑥ There are high-power devices around the proximity sensor, and there is electrical interference.

Machine tool, metallurgical, chemical, textile, and printing industries all employ proximity sensors. It can be utilized as a limit, counting, positioning control, and automatic protection link in an automatic control system. Long service life, dependable operation, high repeat positioning accuracy, no mechanical wear, no spark, no noise, and excellent anti-vibration ability are all features of the proximity sensor. The spectrum of applications for proximity sensors is expanding all the time, and the rate of research and innovation is also accelerating.