Proximity sensors are used across a broad range of industrial and manufacturing applications. They’re used 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. This is why they’re often called non-contact sensors.
There are a number of common sensing techniques employed in proximity sensors. These techniques serve to categorize sensor types in addition to other ways such as the material to be detected or the environmental conditions best suited for that sensor type.
The most common types of proximity sensors are briefly described below :
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, which is sensed as a change.
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 sensed 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.
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 through-beam. 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 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 proximity sensors
A basic proximity sensor is used to sense the presence of objects or materials. What differentiates them from other sensors is that they don’t make physical contact with the object being sensed, and hence they’re also known as non-contact sensors.
One of the most common sensor types is the capacitive proximity sensor. As the name suggests, capacitive proximity sensors operate by noting a change in the capacitance read by the sensor. A typical capacitor consists of two conductive elements (sometimes called plates) separated by some kind of insulating material that can be one of many different types including ceramic, plastic, paper, or other materials.
The way a capacitive proximity sensor works is that one of the conductive elements, or plates, is inside the sensor itself while the other one is the object to be sensed. The internal plate is connected to an oscillator circuit that generates an electric field. The air gap between the internal plate and the external object serves as the insulator or dielectric material. When an object is present, that changes the capacitance value and registers as the presences of the object.
Capacitive proximity sensors are useful in detecting a wide range of objects. The easiest types of objects to detect are ones with a high density (such as metals) or a high dielectric constant (i.e. water). And detecting these objects doesn’t require that the sensors be fairly close to the objects to be detected, another plus if used in settings with little space to work in. Overall, good sensing targets for capacitive sensors include solids and liquids such as various metals, water, wood and plastic.
A typical sensing range for capacitive proximity sensors is from a few millimeters up to about 1 in. (or 25 mm), and some sensors have an extended range up to 2 in. Where capacitive sensors really excel, however, is in applications where they must detect objects through some kind of material such as a bag, bin, or box. They can tune out non-metallic containers and can be tuned or set to detect different levels of liquids or solid materials.
Read more : Inductive proximity sensors
[…] Ultrasonic proximity sensors are a common type of proximity sensor used in many manufacturing and automation applications. Mainly for object detection and distance measurement, they’re commonly used in food and beverage processing and various packaging applications. Ultrasonic sensors work by using sound frequencies higher than the audible limit of human hearing (around 20 kHz), which is typically in the range of 25 to 50 kHz. […]