Types of Transducers
Contact transducers include such applications as flaw detection, thickness gauges, angle beams, and other NDT. Contact transducers are designed to be in contact with the material to be tested. This means there must be a method of coupling the ultrasonic energy from the transducer into the medium. For brief contact at lower temperatures this is usually accomplished with an ultrasonic gel. For higher temperatures (over, say, 300 °C or so), some other method of coupling is required. Typical materials would be thin sheets of soft metals such as Gold or Aluminum. Permanent contact means using a coupling material which won't evaporate or degrade. This is usually another solid material such as an unpoled piece of ceramic, graphite, or metal. Bonding is by epoxy, soldering, brazing, or spring-loaded pressure. Contact transducers are usually utilized in a pulse-echo mode, but on rare occasions can be pitch-catch.
Immersion transducers include imaging NDT, level sensing, flow measurement, object detection (Sonar), and some higher power applications like cell disruption. Immersion transducers are designed to be immersed (either their faces or their entirety) in a fluid medium of some kind. This can range from water to oils, chemicals, or even molten metals. Transducers for use with body tissues blur the line between contact and immersion, since the tissues are very close to water in their properties. Coupling is not usually an issue but proper sealing can be. Immersion transducers are usually pulse-echo but can also be pitch-catch.
Time of flight transducers (in gases, liquids, and solids) are for distance measuring, thickness measuring, and density measurements. Differential time of flight techniques can also be used to measure flow.
Doppler (flow measurement). These transducers are relatively simple devices which detect the frequency shift in a sound signal due to the velocity of a flow, which is known as the Doppler Effect. They are used in a pitch-catch mode, but most dopplers have both the transmit and receive elements in the same housing.
Power transducers such as nebulizers, heaters, HIFU devices, de-scalers, mixing or homogenizing devices, cleaners, welders, motors, and transformers are used to project significant amounts of power into something.
Actuators are for movement related applications such as sounders, fans, pumps, etc. Actuators typically either bend or simply expand up and down. Benders are made by bonding one or more Piezo elements onto a non-piezo substrate. When the Piezo expands or contracts the substrate resists and bending occurs. This can be used to make a switch, a fan (by oscillating back and forth), or a sounder (by oscillating like a drum head). Other actuators are designed to expand and contract like a piezo but usually with some kind of mechanical amplification. Piezos move very small amounts but with great force. The force can be traded mechanically by some form of lever action into large motions.