Proximity sensors are commonly used in many automation applications. They’re used to sense the presence of objects and don’t require physical contact with the target or object being sensed, which is why they’re often referred to as non-contact sensors. Common proximity sensor types include photoelectric, capacitive, and inductive sensors.
Inductive sensors operate on the basis of Faraday’s Law. One way to state Faraday’s Law is 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.
Ultrasonic 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.
The basic physical principle of ultrasonic sensing is that the sensor sends out an ultrasonic pulse and receives a pulse back. Using the time difference between the sent and received signal, the distance to the object can be determined. A common design is to build both the transmitter and the receiver into the same physical housing, although they can also be housed in separate units like certain photoelectric sensors with separate emitters and detectors. Housing the transmitter and receiver in the same unit simplifies installation and cabling.
Because ultrasonic proximity sensors use sound rather than light, they can be used where photoelectric sensors have difficulty, such as in detecting clear plastic objects and labels, highly reflective surfaces that throw off optical sensors, or even liquid levels. They’re also immune to common contaminants such as dust, moisture, and ambient light.
Depending on the application requirement, ultrasonic proximity sensors can be installed and operated in a number of different ways. In fact, because the sensing itself is based on emission of waves and their detection, the ways they can be installed parallel those of photoelectric sensors. That is, setups can include simple reflection of sound waves (as in retro-reflective modes) or they can be setup for through-beam type sensing or a diffuse mode.
For most sensing applications using ultrasonic sensors, it’s desirable to have a fairly narrow output beam in order to avoid reflections that could produce inaccurate readings. A wider beam will disperse over a greater area and may cause interference patterns that could cause inaccurate readings.
Besides beam angle, consider other parameters such as the optimal sensing mode for the application, the required measurement range, the output type (analog or switch/relay output), as well as the size, shape, and material of the housing.
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