Choosing the Right Proximity Sensor

When choosing a proximity sensor, it is helpful to ask yourself the following questions:

• What is the nature of the object to be detected: solid, liquid, granular, metallic, etc.?
• What is the distance between the sensor and the object?
• What is the shape of the object to be detected?

Inductive proximity sensors have the largest sales volumes. They include an oscillating circuit that generates an electromagnetic field. Any metal part that approaches it is detected because it becomes the seat of induced currents and then reduces oscillations, which the detector notices.

The main characteristics of inductive sensors are:

• Can only be used for metal parts
• Relatively limited range: up to 80 mm, varying depending on the nature of the alloy
• Low cost: half the price of a photoelectric detector
• Robust and resistant to harsh environments, not sensitive to impact, vibrations, dust, etc.
• Relatively high switching frequency (several kHz), which enables parts passing at a high speed to be checked, even in rotation.
• Contains no moving parts subject to wear

WHAT CAN IT BE USED FOR?

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.

The operating principle of capacitive proximity sensors is similar to that of inductive sensors. An electromagnetic field is generated by a capacitor located on the main side of the sensor. A nearby part changes the intensity and frequency of oscillations. Unlike inductive sensors, capacitive sensors do not only detect metal parts, but parts of all shapes and materials (solid, liquid, viscous, powdery, etc.).

The main characteristics of capacitive sensors are:

• Low range: < 60 mm
• Slightly more expensive than inductive sensors
• Used for all types of parts made from all different materials
• Can detect objects through non-metallic walls
• Sensitive to moisture and dense vapors
• Used extensively in level detection (i.e. through plastic vials) and detection of transparent materials within a short range
• No mechanical wear, long service life independent of use
• Adapted to industrial environments (polluting atmosphere)
• High throughput

WHAT CAN IT BE USED FOR?

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

Also known as a Hall effect sensor, this sensor operates on a principle similar to that of inductive sensors. Magnetic proximity sensors additionally incorporate a glass and metal blade that magnetizes very quickly when it is in the presence of a magnet and demagnetizes just as quickly when there is no magnet present. Magnetic detectors thus have a large range, given their relatively small dimensions. The part to be detected must contain a magnet or be magnetized.

The main characteristics of magnetic sensors are:

• Use for parts that are magnetized or contain a magnet
• Low cost
• Enables detection through non-ferromagnetic walls
• Not sensitive to vibrations and dirt
• No wear
• Galvanic isolation between the object measured and the circuit can be considered.

The operating principle of ultrasonic proximity sensors is based on the emission and reception of high-frequency ultrasonic waves (around 200 kHz). The return of the wave makes it possible to detect the presence of a part and measure its distance from the sensor (by measuring the time the wave takes to come back). Ultrasonic sensors can use transmitted or reflected ultrasonic waves.

The main characteristics of ultrasound sensors are:

• Detection of all types of parts (powder, metal, solid, liquid, translucent glass, plastic, cardboard, wood, etc.)
• Range of several meters (15 m)
• Very little sensitivity to the environment
• Response time limited by the speed of sound propagation in the air
• Relatively high cost (€200-€1,000).
• Sensitive to drafts and temperature (-10°C to 50°C)
• Can’t detect sound absorbing materials (cotton wool, foam, etc.)

WHAT CAN IT BE USED FOR?

This type of sensor is designed for very specific applications: long-distance detection in harsh environments, detection of transparent or highly reflective objects, etc.
For example, 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).

This kind of detector has a significant market share. It is based on an optical principle. It detects the object when the light beam is either reduced or interrupted by the object passing through it. Depending on the object passing through the light beam and the detection distance, different configurations are possible:

• Thru beam: the emitter and receiver are separate.
• Retroreflective: the light emitted is reflected back by a reflector
• Direct reflection: the light is simply reflected directly off the object

While they are sensitive to environmental contamination, photoelectric proximity sensors have significant advantages:

• Detect all types of parts (including transparent materials)
• Among the most efficient in terms of detection distance: they detect objects up to 200 m away

WHAT CAN IT BE USED FOR?

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.