Sensors measure distance, detect objects

This new family of safety sensors from Pepperl+Fuchs are TÜV-certified in accordance with the Machinery Directive (EN 13849) Performance Level PLd, Category 2, and SIL 2. They are used to safeguard machines and plant components as well as for reliable position detection within these environments.

Power transmission systems use a number of different types of sensors for various purposes. For instance, rotary and linear encoders for position feedback are perhaps the most common. But others include tachometers for speed sensing and proximity sensors for a variety of functions such as object detection and distance measurement.

Many industrial and manufacturing applications use proximity sensors to sense the presence of objects or materials and then either initiate some action or simply flag their presence or absence. Key to their operation is that they don’t require physical contact with the target or object being sensed, and why they’re often called non-contact sensors.

There are a number of common sensing techniques employed in proximity sensors that serve to categorize sensor types. The most common are photoelectric, ultrasonic, capacitive and inductive.

Photoelectric – these sensors operate on the basis of light, dependent on a change in the amount of light available to a detector in the sensor. There are two basic types of photoelectric sensor; reflective, and throughbeam. Reflective sensors work by emitting a beam of light that strikes the object and is reflected back to the detector, usually in the same physical housing as the emitter beam. Through-beam sensors, on the other hand, have two separate units, an emitter or source of light and a separate receiver or detector. When an object breaks the light beam, the detector registers this as a break.

Ultrasonic – these sensors use sound waves to detect objects. They emit a high frequency sound wave (higher than human ears can detect) and when it strikes an object it’s reflected back to the sensor where the distance of the object can be calculated based on the time required for it to return. They’re used in applications to measure distance of objects, such as in automotive park-assist functions, and in bottling and filling applications to detect fluid levels.

Capacitive – as the name indicates, these sensors operate by noting a change in the capacitance, capacitance being a function of both electrical charge and voltage between two surfaces with either an air gap or some other material between them, which is the dielectric constant. When an object to be detected enters the field of the sensor, it effects the dielectric and thus changes the capacitance.

Inductive – these types of sensors are based on changing inductance, which is a measure of the ability of inducing a voltage in a conductor as a result of a changing current in a different conductor. Inductive sensors work with metallic objects because these have inductive properties, so can’t be used to detect plastic, for instance. Also, the type of material will influence the sensing distance. For example, ferromagnetic materials like steel generally have the longest sensing distances, whereas other metals such as aluminum or copper have much shorter sensing distances. The inductive type of proximity sensor is one of the more common types in motion and automation systems, partly because it’s one of the oldest types but also because of its long useful life and versatility and durability.

Inductive sensors operate on the basis of Faraday’s Law that states that a change in magnetic flux in a coil of wire will induce a voltage in a nearby coil. This is applied in inductive proximity sensors in the following way: The sensor itself contains an oscillator circuit and a coil from which an electromagnetic field radiates out and induces eddy currents in any nearby metallic objects. The eddy currents have the effect of attenuating the oscillations from the amplifier. This reduction in oscillations is registered as the presence of a metallic object.

Because only metallic objects have inductive properties, inductive sensors can’t be used to detect plastic or cardboard or other non-metallic objects. However, different metals have different inductive properties and the type of metal being sensed will influence the sensing distance. For instance, ferromagnetic materials like steel generally have the longest sensing distance, while non-ferrous metals such as aluminum or copper have much shorter sensing distances. In general, inductive proximity sensors are well suited to shorter-range applications as the inductive effect wears off with growing distance between the sensor and object to be detected.

Inductive proximity sensors hold up well in dirty environments where contaminants don’t interfere with the sensor’s ability to detect metallic objects. For example, they’re resistant to dirt, dust, and smoke in the environment between the sensor and the object to be detected. As for build-up of contaminants on the sensor face such as dirt and dust, oil, grease or soot, these don’t effect the inductive sensing. However, metallic contaminants such as metal chips in machining applications will impact sensor operation. The key is to be sure to understand what type of contaminants an application contains in order to select the correct type of sensor that can handle them and operate effectively.