Proximity sensors for marine application
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.
Automated processes require sensors for supplying information. They provide signals about positions, limits, levels or serve as pulse pick-ups. Without reliable sensors even the best controller is not able to control processes.
In general, all these sensors consist of two parts: The first registers the change in the physical conditions (basic sensor), the second converts the signals of the basic sensor into electrical output signals (signal processing). In general, a distinction is made between binary sensors which provide a defined high-low signal and analogue sensors which are preferably used for temperature, distance, pressure, force measurement, etc. The sensor supplies an analogue signal which is further analysed for measurement and control.
In the figure above you see the general diagram which basically applies to every sensor. The sensors only differ in some details, e.g. individual components are not used or cannot be used separately. Sometimes the basic sensor is also called just sensor. In this case it must be seen from the context whether the whole unit or the basic sensor is meant. Some units consist of separate components, e.g. NAMUR sensors or often also temperature sensors.
Here the transducer is connected to a separate evaluation unit or amplifier. In Figure above the characteristic feature of the intelligent sensor is its communication capability. But this term is also used in a different sense. A sensor which only supplies the binary information object detected or object not detected is in general not called intelligent. But a sensor which is able to supply additional information, e.g. object reliably detected or object unreliably detected is considered to be intelligent.
In order to avoid misunderstandings the difference is briefly explained. Binary means “two states” (on/off). An analogue signal which can have any value within certain limits is often digitized today so that it can be processed in electronic controllers. This is done using an A/D converter (analogue to digital). It divides the analogue signal into steps. The number of steps results from the number of the bits used. Whereas one bit can only take two values, 8 bits can take 256 and 10 bits 1024. This is also called resolution. Less than 8 bits are seldom used because the resolution would then be too coarse. More than 12 bits are also seldom used because it does not make sense if the resolution is much higher than the measurement accuracy. Encoders are an exception. They provide digital signals straight away.
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