Ultrasonic Transducers Information
Ultrasonic transducers send and receive waves for many types of sensing. They convert energy into high frequency sound waves in the ultrasonic range. These sound waves are then focused on a target and the echo is read by sensors to determine a specific variable such as distance, proximity, speed, or level in applications like nondestructive evaluation, web break detection, counting, security applications, and cutting and welding. Ultrasonic transducers typically operate at their resonant frequency with various construction options, beam patterns, and power levels.
Ultrasonic Transducer Specifications
Ultrasonic transducers have many critical specifications. Transmitting frequency is the usable frequency range of the device. Bandwidth is the difference between low and high operational frequency limits. Rated signal power available from the transducer is another important specification. Transmit sensitivity is the ratio of sound pressure produced to input voltage. Receive sensitivity is the ratio of output voltage produced over sound pressure sensed. The beam angle is the total included angle of ultrasonic beam. In general, a high frequency ultrasonic transducer will produce a narrow beam and a lower frequency transducer a wider beam. The beam angle can be influenced somewhat by the transducer housing construction.
Types of Ultrasonic Transducers
Angle beam ultrasonic transducers include mounted transparent angle blocks; often used for weld inspection and flaw detection. They typically utilize refracted shear waves to detect flaws throughout the depth of welded areas. Times of flight diffracted wave (TOFD) ultrasonic transducers are used with a wedge and emit a highly damped longitudinal wave to produce lateral waves for crack detection and measurement. A TOFD image is constructed using four different ultrasonic wave types: (1) longitudinal wave, (2) lateral wave on the surface between the receiver and transmitter, (3) reflected longitudinal wave and (4) shear wave generated at the cracks, defects or discontinuities.
Wheel probes or dry couplant continuous test rolling probes have contact and roll across the surface to be evaluated. Originally, the dry couplant rollers or wheels were oil filled rubber tires containing one immersion ultrasonic transducer. Current wheel probes use multiple ultrasonic transducers in the stator or axle. Applications include flaw detection and thickness measurements.
Electromagnetic acoustic transducers (EMAT) use a magnetorestrictive or eddy current effect to transmit and receive ultrasonic signals. EMAT has several advantage over conventional ultrasonic transducers such as no couplant requirement; usable on sample with scale, coatings or heavy oxide layers; functional at high temperatures; and usable in contact with or at a small distance from surface; and even small diameter pipes can be inspected. The non-contact and no couplant features allow moving materials to be inspected during production processing.
Delay line ultrasonic transducers are versatile, often with replaceable head options such as membranes and wear caps. They are used to gage or detect flaws such as delaminations in thin materials. Shear wave transducers introduce shear waves into material without using an angle beam wedge. The ratio of shear wave components to longitudinal components can exceed 30dB. Medical style transducers and housings are designed for specific medical applications.
Features of Ultrasonic Transducers
Ultrasonic transducers come in different styles for different applications. Plain general-purpose transducers, including air transducers, are available with no specialized features. More specialized styles are common as well, such as contact transducers for placing directly on the surface to be measured. Dual element transducers have two elements in the transducer housing and allow the transmitter and receiver to operate independently. The elements are angled toward each other to create a reflective transmit/receive pathway. An angle beam ultrasonic transducer includes a mounted transparent angle block and is often used for weld inspection and flaw detection. It typically utilizes refracted shear waves to detect flaws throughout the depth of welded areas. Immersible ultrasonic transducers are designed to be totally submerged in a liquid medium, most often fresh water. The protected element style has the transducer element protected for use on rough surfaces.
Common features available are ultrasonic transducer array configurations for connecting more than one transducer in series or parallel; temperature compensation circuitry that compensates for sensitivities changing with ambient temperature; and optional analog output. Most transducers output analog voltage, but may have provisions for current loop output, etc.
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