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Article Excerpt
Ultrasonic sensors, which rely on high-frequency sound waves above the level of human hearing (or above about 20,000 hertz) to sense such parameters as proximity, presence/absence, distance, level, flow, or thickness, can offer key benefits for sensing proximity or level in difficult or challenging environments and in applications where competing sensing technologies have difficulty providing accurate, repeatable detection.
Ultrasonic proximity and level sensors utilize sound waves above 20,000 hertz to detect the presence/absence of objects; the level of liquids, solids, or powders in containers, or to measure the distance of an object from the face of the sensor. Ultrasonic devices used in level and proximity sensing are often air-based systems, where the sound waves travel through air to detect the target.
There are two major types of air-based ultrasonic transducers: piezoelectric and electrostatic. Piezoelectric transducers, which are commonly used in proximity and level sensing ultrasonic devices, use a piezoelectric disc (which is often a composite ceramic attached to an acoustic impedance matching material). The coupling layer can be rubber, glass epoxy, or very thin sheet of steel. When the piezo element is stimulated with a high voltage source, it begins to oscillate. This phenomenon causes the coupling device to also oscillate and passes a pulse of sound through the air. The pulse can be bounced off the target back to the sensor where a receiving element, tuned to the same frequency, receives the returning pulse.
The microprocessor in the system starts a timer when the pulse is transmitted. The processor will use a time-of-flight algorithm to determine where the return echo came from in the sensing path. Due to its ability to repeatably measure the flight time of the echo and to use temperature compensation to adjust for the variance of the speed of sound at different temperatures, ultrasonic technology allows the user to set a defined window in the sensing path.
The sensor can be programmed through software or a push-button to trigger a change in its output state when it detects a target within a user-defined window. This capability enables the user to define a window in space and only trigger on objects within the window. Objects that pass through the sensing path but are outside that window will be detected, but will not trigger a change in the output state. The ability to define a window allows the ultrasonic sensor to provide true foreground and background suppression in sensing applications where this capability is required.
Ultrasonic sensors are not susceptible to error as a result of the target material's color, shape, or composition (e.g., transparent or opaque, liquid or solid). Ultrasonic sensing technology is a beneficial solution when, for example, the user must sense a non-metallic object in an environment where there is systematic, heavy wash down, liquid, dust, heavy spray, food, ink, or other environmental hazards. Since sound energy is used for detection, the reflecting object does not have be metal, but can be glass, plastic, metal, or even paper.
In harsh environmental conditions, photoelectric sensors are susceptible to the accumulation of dust, food, or ink, or other objects on...
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