A proximity sensor is a sensor device capable of detecting the presence of objects nearby without any physical contact.
It often emits an electromagnetic/electrostatic field or a beam of electromagnetic radiation (infrared, for instance) and looks for changes in the field or return signal. The object being sensed is often referred to as the proximity sensor’s target.
Unlike other sensors, such as limit switches, which detect objects by physically contacting objects, proximity sensors perform non-contact detection of target objects close by or within the sensor’s general vicinity by converting information on the movement or presence of an object into an electrical signal.
Proximity sensors vary depending on which object material is to be detected or which special application is to be solved. The common types of proximity sensors used for industrial applications include inductive, capacitive, ultrasonic, photoelectric, magnetic, etc. These sensors are best suited to detect motion and track objects in real-time to offset industrial manufacturing issues.
The sensing range Sn of a proximity sensor signifies the distance at which a signal change is triggered when the target, i.e., the object to be detected, approaches the active surface axially.
- Unlike devices such as limit switches, proximity sensors detect objects without touching them, and they, therefore, do not cause abrasion or damage to the objects.
- The sensor has a longer service life since no contact is used for output.
- Unlike optical detection methods, proximity sensors are best suited for use in locations where water or oil is used. They have excellent chemical resistance. They are not affected by colors.
- Proximity sensors provide a high-speed response and can be used in a wide temperature range from −40 to 200°C.
- They are affected by ambient temperatures, surrounding objects, and other sensors. Therefore, care must be taken when installing them to prevent mutual interference.
Types of proximity sensors
1. Inductive proximity sensors
Millions of inductive sensors are used today in virtually all industries. They detect the presence of metal objects such as steel, aluminum, brass, and copper, which come within range of the oscillating field, and provide target detection to “zero speed.” They have a long service life and extreme ruggedness.
An inductive proximity sensor consists of an LC circuit – i.e., a resonant electrical circuit comprising a coil (L) and a capacitor (C) – a signal evaluator and a switching amplifier. To detect metallic objects, the resonant circuit’s coil generates a high-frequency electromagnetic alternating field that emerges from the active area of the sensor.
When a metallic object enters this field, eddy currents are induced into the object. When the metal moves closer to the sensor, these eddy currents increase and result in the absorption of energy from the coil, which dampens the oscillator amplitude until it stops. A signal evaluator detects this change and translates this into a switching signal. The size of the field is often defined by the size of the ferrite core and the coil.
2. Capacitive proximity sensors
Used for non-contact detection of virtually all materials – even behind a wall or in packaging, including powders, granules, wood, paper, and liquid like water and acid, a capacitive proximity sensor uses the principle of an open capacitor. It contains an active electrode from which an electrostatic field develops.
When an object enters the electrical field, the resonant circuit changes based on its dielectric properties, and there is a change in amplitude. A signal evaluator detects this change in amplitude and the switching amplifier signals to switch. The closer the object gets to the active electrode, the greater the change in capacitance.
Special features of capacitive proximity sensors:
- Detection through the material – Wall thicknesses of 10 to 20 mm in the case of plastic or glass containers.
- Tough/rough ambient conditions – Highly resistant to vibrations, dust, dirt, etc.
- Harsh environment – Highly resistant to vibrations, dust, dirt, etc.
- Detecting of all materials possible – Wood, paper, metal, plastic, liquids, granules.
- Relatively large sensing ranges with a compact design – Up to Sn = 25 mm (CM30/CQ35).
- High electromagnetic and electrostatic compatibility
3. Magnetic proximity sensors
Used for non-contact detection of permanent magnets, magnetic proximity sensors always respond to a magnet (magnetic encoder). Magnetic fields penetrate all non-ferromagnetic materials such as stainless steel, aluminum, plastic, or wood. Therefore, magnets can detect through these walls without affecting the sensing range. These sensors are resistant to dust, heat, and vibration, making them ideal for harsh environments and high-temperature applications.
4. Ultrasonic proximity sensors
Ultrasonic proximity sensors use a transducer to send and receive high-frequency sound signals. When a target enters the beam, the sound is reflected back to the switch, causing it to energize or de-energize the output circuit.
A piezoelectric ceramic disk is mounted on the sensor surface, and it can transmit and receive high-frequency pulses. A high-frequency voltage is applied to the disk, causing it to vibrate at the same frequency. The vibrating disk produces high-frequency sound waves. When transmitted pulses strike a sound-reflecting object, echoes are produced.
The duration of the reflected pulse is evaluated at the transducer. When the target enters the operating range, the output of the switch changes state. When the target leaves the operating range, the output returns to its original state.