Different applications require different types of proximity sensors, with each sensor being designed to serve a specific purpose. Other factors, such as target type and operating range or response time, also dictate the type of sensor to use. With that in mind, the information in this article will help you find the proximity sensor types that will work best for your project.
About Proximity Sensor
The proximity sensor refers to a type of contactless sensor that emits a field of energy to detect the presence—or absence—of object. This can be a proximity sensor for mobile, proximity sensor for security systems, or the different types of proximity sensors used in industrial automation.
Because of their non-contact nature, proximity sensors have many advantages over contact sensors. They are reliable, durable and require low maintenance. They also do not produce any physical movement or heat transfer to the target object, and can be used in harsh environments.
A proximity sensor may use electromagnetic waves, light, or ultrasound to detect object. Some only detect metals, while others can pick both metal and non-metal targets.
Still, some types of proximity sensors offer longer detection ranges than others, and so on. The proximity sensor and its types in today’s world are discussed in the next section.
Types of Proximity Sensors
Based on the different forms of detection technologies, proximity sensors are classified into five categories: Inductive Proximity Sensor; Capacitive Proximity Sensor; Ultrasonic Proximity Sensor; Magnetic Proximity Sensor; and Optical Proximity Sensor. Learn about these types of proximity sensors, including their different uses, below.
1. Inductive Proximity Sensor
The inductive proximity sensor is called so for using inductance principles to detect the presence of a metal target, without making any physical contact. One of the most common types of this sensor is the eddy current proximity sensor.
These sensors primarily consist of an oscillator, a coil around a ferrite core, and a Schmitt trigger. Here’s now see how an inductive proximity sensor works:
- During operation, the oscillator generates an alternating current that produces an alternating electromagnetic field around the coil.
- This field radiates from the coil to form the detection zone.
- If a metallic object enters this detection zone, the oscillating magnetic field induces electric currents in its body. These are called eddy currents.
- The eddy currents then start producing an alternating magnetic field, interfering with the original oscillating field from the sensor and changing its properties.
- This change activates the Schmitt trigger and the sensor is able to detect.
- Note that these types of proximity sensors cannot detect non-metallic objects as such materials will not produce Eddy currents.
Application: One of the benefits of inductive sensors is their ability to operate in contaminated environments: they’re resistant to the presence of oil, dirt, and even moisture. Inductive proximity sensors are, therefore, widely used in industrial, automotive, and machine tool applications.
2. Capacitive Proximity Sensor
The capacitive proximity sensor uses an electric field to detect the presence of a target object. It’s basically an open capacitor whose other plate is replaced by the target, while the air between the sensor plate and target forms the dielectric. Here is how a capacitive proximity sensor works:
- When the target enters the sensor’s range, it forms a capacitance with its sensing plate, which increases as the object draws nearer.
- This action changes the capacitance value of the circuit, which in turn produces an electrical signal used to detect presence.
- The capacitive sensor can detect both metals and nonmetals. These can be powders, granules and liquids or even solid objects.
- Because the working principle of capacitive proximity sensor is based on the gradual increase of the capacitance, its sensing speed is generally lower than that of inductive sensors.
Application: Capacitive proximity sensors are used in a wide range of applications, including food and beverage production processes, level detection, material handling, automation control systems and other industrial environments. In the world of electronics, this is the type of proximity sensor for mobile phone or tablet sensing applications.
3. Ultrasonic Proximity Sensor
The ultrasonic sensor is somewhat different from inductive and capacitive sensors. These types of proximity sensors work by emitting ultrasonic waves—or sound waves with a frequency higher than the upper limit of human hearing, which is around 20 kHz. The working of ultrasonic proximity sensor is described below:
- The ultrasonic sensor consists of these parts: transmitter, receiver, signal processor, amplifier and power supply module, and works by sending out high-frequency sound pulses.
- When the sound waves encounter an obstacle, they will bounce back to the receiver.
- The receiver then uses this information to determine the presence as well as distance between the object and the sensor.
- Ultrasonic proximity sensors offer fast detection speed, even for small objects, and have a wide sensing range. They can also detect both solid and liquid targets in their sensing zone.
Application: Ultrasonic proximity sensors are mainly used in robotics, obstacle detection and avoidance systems, industrial automation, parking sensors, etc. In addition, these types of sensors can also pick up vibrations, making them suitable for vibration monitoring applications.
4. Optical Proximity Sensor
The optical proximity sensor works on the principle of light reflection. The light is either the visible or infrared spectrum. The sensor emits light towards a target object and measures the amount of reflected light back to it.
- In most cases, the optical proximity sensors are used in conjunction with an infrared LED or laser diode. When a target object is near enough to the sensor, it reflects some of the light energy back to the detector.
- This will then be amplified and used as an electrical signal to detect the presence of the object.
- The optical proximity sensors are usually not affected by dust, dirt or moisture.
- They also have a high resolution and can detect even very small objects in their sensing range with ease.
Application: Optical proximity sensors are widely used for level detection in liquids, position sensing in machines and automation processes. They are also used as metal detectors in security systems and access control devices. These types of proximity sensors have also found application in navigation systems for cars or drones.
5. Magnetic Proximity Sensor
The magnetic proximity sensor works using the attraction between the magnet and target object to detect presence of an object. One of the advantages of these sensors is that they can detect magnetic targets through non-metallic materials, such as plastics and wood. They also have a fairly long sensing range.
So, how does a magnetic proximity sensor work? This sensor can be different types, and its working is based on the type of technology used. The types of magnetic proximity sensors include: reluctance based, reed switch type, magneto-resistive, Hall Effect sensor, and GMR (giant magneto resistive).
- Variable reluctance sensor: this sensor consists of a permanent magnet and a pickup coil around a magnetic pole piece, and works by measuring the variations in reluctance.
- Reed switch sensor: the magnetic reed proximity sensor consists of two ferromagnetic contacts housed in a sealed glass envelope. When a magnet is brought close to the sensor, it causes the reeds close and completes a circuit.
- Hall-effect sensor: this type of sensor works by measuring the changes resistance in a ferromagnetic material when a magnetic field is applied to it.
- Magneto-resistive sensor: these sensors work by sensing the variations in electrical resistance caused by the presence of a magnet close to it.
- GMR (giant magneto resistive) sensor: this type of sensor primarily comprises ferromagnetic plates separated by a non-magnetic spacer. When a magnet is brought close to the sensor, it causes a change in resistance that triggers a circuit.
Application: Magnetic proximity sensors are commonly used as position sensing devices in industrial machinery, automotive components such as the crankshaft, and other machinery. Other applications include robotics and safety systems. These types of proximity sensor offer easy operation, and can be used in tough environments such as contaminated or high vibration situations.
Different types of proximity sensors are built to accommodate different applications. The choice of the type of proximity sensor depends on the specific requirements of the project. Based on the application, one can select a suitable sensor with the right accuracy and range. Inductive and capacitive sensors are the most used types, so your choice will most likely be one of these.