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What is a Proximity Sensor?

Author: Apogeeweb
Date: 14 Apr 2021
how does a Proximity Sensor work


Ⅰ What is a Proximity Sensor

Ⅱ How does a Proximity Sensor Work

  2.1 Inductive Proximity Sensor

  2.2 Capacitive Proximity Sensor

Ⅲ Types of Proximity Sensor

  3.1 Inductive Proximity Sensor

  3.2 Capacitive Proximity Sensor

  3.3 Ultrasonic Proximity Sensor

  3.4 IR Proximity Sensor

  3.5 Photoelectric Proximity Sensor

Ⅳ The Application of Proximity Sensor

Ⅴ How to Choose a Suitable Proximity Sensor



Ⅰ What is a Proximity Sensor

The proximity sensor refers to a series of sensors used to detect the distance of an object, and their common point is that they will not touch the object during the detection process.

There are many types of proximity sensors. The commonly used proximity sensors have the same principle. They transmit electromagnetic fields or light beams and analyze the reflected changes to determine whether objects are approaching or appearing, leaving, or disappearing.


The farthest detectable distance is called "rated range". Some sensors can adjust the rated range to suit different purposes. If the rated range is adjusted within a very short distance, the proximity sensor is often used as a touch switch. Proximity sensors are generally characterized by high reliability and long functional life. This is because there is no physical contact between the sensor and the sensed object, and the damage of mechanical parts is close to zero.

Different types of proximity sensor


Different types of proximity sensors include inductive proximity sensors, capacitive proximity sensors, ultrasonic proximity sensors, photoelectric sensors, and Hall effect sensors. Different proximity sensors are suitable for detecting different types of objects. For example, capacitive sensors are suitable for detecting plastic objects, while inductive proximity sensors are used for metal targets.


Ⅱ How does a Proximity Sensor Work

The proximity sensor emits an electromagnetic or electrostatic field or an electromagnetic radiation beam (such as infrared) and waits for a return signal or a change in the field. The object being sensed is called the target of the proximity sensor.

2.1 Inductive Proximity Sensor

They have an oscillator as input and change the loss resistance by approaching a conductive medium. These sensors are the preferred metal targets.

2.2 Capacitive Proximity Sensor

They convert the change in electrostatic capacitance on both sides of the detection electrode and the ground electrode. This happens by approaching nearby objects with changes in the oscillation frequency. In order to detect nearby targets, the oscillation frequency is converted into a DC voltage and compared with a predetermined threshold. These sensors are the first choice for plastic targets.


Ⅲ Types of Proximity Sensor

The following are various common proximity sensors:

3.1 Inductive Proximity Sensor

Contactless inductive proximity sensors are used to detect only metal objects. It works on the principle of induction, with an oscillator driving a coil until a metallic object enters it.


Inductive sensors have grown in popularity in recent years, despite the fact that they are based on an old design. Inductive sensors, unlike the other technologies on this list, only operate with metallic materials. An inductive sensor creates a magnetic field and then detects changes in the magnetic field as a metallic object passes through it, similar to how a magnet rotating in a wire coil generates electricity. Any metal detector starts with this.


Inductive proximity sensor


Their detection range can be extremely limited depending on the setup, particularly in applications that count gear rotations by detecting whether or not a gear tooth is next to the sensor. Inductive sensors may be mounted inroads to detect vehicles driving over them or optimized to detect space plasma, for longer ranges.


Inductive sensors, however, tend to operate in the millimeter to meter range while working as an electronic proximity sensor. They perform best with ferrous materials, such as iron and steel, and have a smaller detection range for non-magnetic metallic materials due to their operating principles. They have extremely fast refresh rates because they are dependent on the variation of electromagnetic fields.


3.2 Capacitive Proximity Sensor

Contactless capacitive proximity sensors detect both metallic and non-metallic substances, such as liquid, powders, and granular. It works by detecting a capacitance transition.


It has an oscillator, Schmitt lever, and output switching circuit, much like inductive sensors. The only difference is that it has two charging plates for capacitation (1 internal, 1 external):

• The oscillator is attached to an internal plate.

• The sensing surface is an external plate (sensor electrodes).


Capacitive proximity sensor

When the object being sensed approaches the sensor, the object will change the dielectric constant in the capacitive sensor, and the sensor can know the distance of the object by measuring this dielectric constant.


However, the response speed of capacitive sensors is generally relatively slow, with an update frequency of only 10 to 50 Hz. However, since capacitive sensors will not be affected by dust or opaque containers, they are often used to ban optical sensors. The approximate rated range of a typical capacitive sensor is 10 mm and can detect thickness changes within 0.01 mm.


3.3 Ultrasonic Proximity Sensor


Ultrasonic proximity sensor


Ultrasonic proximity sensors sense the presence of an object or, with additional processing, the distance to the object using ultrasonic sound pulses. They work by using both a transmitter and a receiver, as well as echolocation principles.


An ultrasonic sensor can determine the distance to an object by emitting a chirp and measuring the time it takes for the chirp to bounce off a surface and return. While the transmitter and receiver are commonly seen in the configuration of being as similar to each other as possible, the concepts still apply when they are isolated. Ultrasonic transceivers are also available that combine the transmit and receive functions into a single unit.


Ultrasonic detection is incredibly precise and has a high refresh rate, allowing it to send out dozens or hundreds of pings or chirps per second. The color and transparency of an object have little effect on the readings since they are based on sound rather than electromagnetic waves.


This same property means they don't need or emit light, making them ideal for conditions that are either inherently dark or must be dark. The sound waves spread out over time, expanding the detection area - which can be beneficial or disadvantageous depending on the application. These are also very low-cost, flexible, and safe due to their simple nature.


Ultrasonic proximity sensor 2


Ultrasonic sensors, on the other hand, have their own set of disadvantages. The sensor is made up of two parts: the transmitter and the receiver, which can be combined or purchased separately. Since the speed of sound varies with air temperature, any significant temperature changes will affect accuracy. This can, however, be mitigated by using temperature measurements to update the calculations.


Since sound waves do not reflect as well on absorbent surfaces, soft materials may affect accuracy. Although ultrasonic sensors are similar to sonar in nature, they are not designed for use underwater. Finally, since there is no medium for sound transmission in a vacuum, their reliance on sound renders them useless.


3.4 IR Proximity Sensor


IR proximity sensor

IR, short for infrared, emits a beam of infrared light to detect the presence of an object. It operates in the same way as ultrasonic sensors, but instead of using sound waves, it sends out infrared signals.


Infrared proximity sensors include an IR LED that emits light and a light detector that detects reflected light. It has a signal processing circuit built in that specifies an optical spot on the PSD.


How do IR proximity sensors work? First, infrared light is emitted from the IR LED emitter. Then, the beam of light hits the object and gets reflected back at an angle. The reflected light will reach the light detector. At last, the sensor in the light detector determines the position/distance of the reflective object.


3.5 Photoelectric Proximity Sensor

The photoelectric proximity sensor is composed of a beam generator, a special beam detector, an amplifier, and a microprocessor. When the emitted light beam is reflected from an object, the photodetector will sense it, and the sensor detects the object through this method.


Photoelectric proximity sensor


The emitted light beam will be modulated to a specific frequency, and the detector also has a frequency-sensitive amplifier that will only respond to light modulated at the corresponding frequency. This prevents false detections caused by lights or sunlight. When the photoelectric proximity sensor senses a black object, the non-reflective properties of the object will prevent the sensor from working properly, and the same is true when encountering transparent or refracting objects.


Photoelectric proximity sensor sensing object


Although photoelectric proximity sensors are suitable for many industrial applications, they are also widely used in residential and commercial settings for applications such as garage door sensors and occupant counting in stores. Photoelectric sensors can be set up in a variety of ways in terms of implementation. Through-beam uses an emitter on one side and a detector on the other, with detection taking place when the beam breaks.


The emitter and detector are co-located in a retroreflective system, with a reflector on the other side bouncing the signal from the emitter back to the detector. Finally, diffused places the emitter and detector close together, but the emitted light is reflected off any surrounding surface, much like ultrasonic sensors, but without the ability to measure distance.


Due to the lack of moving parts, photoelectric sensors have a long lifetime and can detect a wide range of materials, while transparent materials and water can pose a problem. Long sensing ranges and quick response times are given by the through-beam and retroreflective setups. Small objects can be detected by diffuse-style setups, which can also be mobile detectors.


These are all tolerant of dirty conditions found in industrial applications as long as the lens does not get polluted. However, their ability to measure the distance to an object is severely restricted, and object color and reflectivity can cause problems. The device installation can be complicated in busy environments because the through-beam and retroreflective must be installed and aligned.


Ⅳ The Application of Proximity Sensor


Application of inductive proximity sensor


Inductive sensors are found in machine tools, machines for the textile industry, the automotive industry, assembly lines, etc. They are used for the detection of metal parts in harsh environments and when it is necessary to check parts that move quickly.

Capacitive sensors are found on packaging lines, packaging installations and when filling levels are measured through plastic or glass walls.

Ultrasonic sensors can be found on conveyors to detect bottles or packaging. They can also be used to detect the level of a liquid (in vials) or pellets (in hoppers).

Photoelectric sensors are used for part detection in the textile, robotics, elevator and general construction sectors. They are also found in the fields of handling and conveying as well as for applications requiring the detection of people, vehicles or animals.


Application of photoelectric proximity sensor


Proximity sensors can also be used to monitor changes in machine vibration to measure the distance between the shaft and its supporting bearings. Large steam turbines, compressors, and motors using sleeve bearings are common applications.


Besides, when a row of proximity sensors is arranged in a matrix, the flatness of the surface of the object can be detected. Using some known flat objects, after validating each proximity sensor, the matrix can automatically detect whether the flatness of the ex-factory objects is within an acceptable range.


The most common example is the proximity sensor on a mobile phone. The function of this sensor is to prevent the user from accidentally touching the display screen when the user uses the call function to make an erroneous operation. When the sensor senses the proximity of an object, it will order the display to close to prevent accidental touch.


Ⅴ How to Choose a Suitable Proximity Sensor

Now, to help you in choosing the best one out of the first four, I've mentioned some parameters to consider when choosing a proximity sensor. However, you must first consider your intended purpose; what are you trying to use in the first place.


Proximity Sensor Criteria How to select Sensor Suitability
Object requirements

Consider the following factors when looking at the object you choose to use a proximity sensor on:

     Color of the object

  The object's shape

  Material of the object

Objects with complex specifications are best served by:

  IR proximity sensor

Not suitable for the design of complex objects:

  Ultrasonic proximity sensor

Environment of sensing

Take a look around the area where you'll be detecting your object.

Take into consideration the following:




Suitable for use in a harsh environment:

  Capacitive (most-suited)



Incompatible with a harsh environment:

  IR proximity sensor

Sensing Range/Distance

Examine if the object would be positioned near the sensor face.

Take into consideration the following:

  The distance between the positioned object and the sensor (Far or Close)

Suitable for sensing at close ranges:

  Inductive and Capacitive proximity sensors

Suitable for sensing long-range :

  Ultrasonic and IR proximity sensors




1. What does a proximity sensor do?

Proximity sensors are suitable for damp conditions and wide temperature range usage, unlike your traditional optical detection. Proximity sensors are also applicable in phones as well, be it your Andriod or IOS devices. It consists of simple IR technology that switches on and off display accordingly to your usage.


2. How accurate are proximity sensors?

Today's quality inductive proximity sensors can have trigger points that are repeatable to 0.0001 in. To obtain such precision, though, the detectable object must be moved the reset distance away from the sensor after each time the sensor is triggered.


3. How do I choose a proximity sensor?

When detecting metals, the inductive proximity sensor should be selected in priority. When detecting nonmetallic materials, the capacitive proximity sensor should be selected in priority. When detecting magnetic signals, the magnetic induction proximity sensor should be selected as a priority.


4. What is the range of the proximity sensor?

The sensor can also be used to detect a wide variety of non-metallic and metallic objects and typically operate over a range of 3 to 30 mm.


5. What are the different types of inductive sensors?

Inductive Proximity Sensors are roughly classified into the following three types according to the operating principle: the high-frequency oscillation type using electromagnetic induction, the magnetic type using a magnet, and the capacitance type using the change in capacitance.


6. What is an inductive proximity sensor?

An inductive proximity sensor is a non-contact electronic proximity sensor. It is used for the positioning and detection of metal objects. The sensing range of an inductive switch is dependent on the type of metal being detected. The sensor consists of an induction loop or detector coil.


7. Where are inductive proximity sensors used?

Other applications for inductive proximity sensors include use on forklift trucks, monitoring component position within hydraulic machines, and for industries that mold plastic parts.


8. How do capacitive proximity sensors work?

As the name suggests, capacitive proximity sensors operate by noting a change in the capacitance read by the sensor. When an object is present, that changes the capacitance value and registers as the presence of the object. Capacitive proximity sensors are useful in detecting a wide range of objects.


9. What do capacitive sensors detect?

Many types of sensors use capacitive sensing, including sensors to detect and measure proximity, pressure, position and displacement, force, humidity, fluid level, and acceleration. Human interface devices based on capacitive sensings, such as trackpads, can replace the computer mouse.


10. What is a phone proximity sensor?

In Android, the proximity sensor is primarily used to detect when the user's face is close to the screen. This is how the phone screen seems to know to switch off when you hold it up to your ear during phone calls, preventing any errant button presses.


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