Understanding Ultrasonics Tutorial

Ultrasonics — How it Works Rev. 8.03

Understanding Ultrasonics ultrasonic sensing

Ultrasonic sensors measure distance by using a transducer to send out ultrasonic bursts. Each burst contains a series of pulsed sound waves. The ultrasonic burst emits in the shape of a cone, reflects off the detected target, and is received by the transducer. The time required for this burst to travel to and from the target is measured and converted into a distance measurement by the sensor. Ultrasonic sensing is affected by several factors including the target’s surface, size, angle and the distance from the sensor. Environmental conditions such as temperature, humidity, gases, and pressure may also affect the measurement. Sensors are equipped with manual or automatic adjustments to compensate for most of these varied conditions. The following considerations will help ensure the best possible sensing conditions.

Surface The ideal target surface is hard and smooth. This surface will reflect a greater amount of signal than a soft, rough surface. A week echo, which is the result of a small or soft object, will reduce the operating distance of the sensor and decrease its accuracy. Distance The shorter the distance from the sensor to an object, the stronger the returning echo will be. Therefore, as the distance increases, the object requires better reflective characteristics to return a sufficient echo. Size A large object will have more surface to reflect the signal than a small one. The surface area recognized as the target is generally the portion closest to the sensor. Angle The inclination of the objects’ surface facing the ultrasonic sensor affects the reflectivity of the object. The portion perpendicular to the sensor returns the echo. If the entire surface is a great enough angle, the signal will be reflected away from the sensor and no echo will be detected.

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Ultrasonics — How it Works

ultrasonic sensing

Ultrasonic Beam Pattern Produced by the Ultrasonic Sensor The beam pattern produced by the ultrasonic sensor is expressed as the number of degrees the beam will spread from the center line of the sensor. The beam spreads in a conical pattern from the transducer. Although the transmitted ultrasonic beam continues to spread, the detection area of the sensor begins to narrow beyond the published sensor range. The detection area is affected by the number of pulses transmitted by the sensor and the sensitivity level. At high sensitivity and pulse levels, the detection area is wider and longer than at low settings. The following drawing illustrates this principle.

low sensitivity and pulses

detection area high sensitivity and pulses

beam spread

Figuring the beam spread is simply trigonometry using the following formula: a = b * tanA Where a = b spread off axis b = distance A = degree of beam spread off axis

a

b

Tangent of common sensor angles: 7° = 0.12278 8° = 0.14054 9° = 0.15838

Ideally, the target area should always exceed the diameter of the beam spread at the desired sensing range.

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STI Automation Products Group www.stiapg.com

USA APG Headquarters

UK Sales Office

European Sales Office

Asian Sales Office

Logan, UT Tel. 1/435/753-7300 1/888/525-7300

Gloucestershire Tel. +44- (0)1453-544086

Freiburg, Germany Tel. +49-761-455 26 0

Singapore Tel. +65 64844001