Polymer Transducers

In 1969, Kawai69 discovered that polyvinylidene difluoride (PVDF, PVF2), which is an organic ferroelectric polymer, displayed strong piezoelectric properties. In the last decade, an organic copolymer known as polyvinylidene difluoride trifluoroethylene P(VDF-TrFE) with a higher coupling coefficient has become available. These polymers are used in hydrophones, audio microphones, and robotic tactile transducers. In addition, they find application in wave-propagation transducers for NDT, medical imaging, hi-fi stereophones, and tweeters. Table shows the piezoelectric and related properties of these copolymers.

A hydrophone is a very sensitive pressure transducer that is used to map the temporal and spatial acoustic pressure field of another transducer that is propagating acoustic energy through a fluid.

Hydrophones have to satisfy many requirements: low acoustic impedance to optimally match the fluid impedance and cause the least disturbance to the field being measured, small sensing spot size to obtain good spatial resolution, large bandwidth and flat frequency response to respond to harmonics of the measured signal, and good linearity to handle the wide dynamic range of pressures.

Two types of hydrophones most commonly used are the membrane hydrophone and the Lewin-type needle hydrophone.

In the membrane hydrophone, a metallized 250- to 500-μm-diameter circular dot is vacuum-deposited on either side of a poled copolymer film using a simple mask. This results in a single-element dot transducer. Suitable connections are made to the dots by vacuum-deposited metallic traces. The copolymer is then stretched and held in position over a circular hoop. The pressure of the acoustic field is measured as the electrical output of the dot transducer.

The needle hydrophone consists of a poled, 0.5- to 1.0-mm-diameter, PVDF or copolymer film transducer which is bonded to the flattened end of a hypodermic needle but electrically insulated from it. The electrical output of the transducer gives a measure of the acoustic field pressure.

Angle-Beam Transducers

Angle-beam transducers are used in nondestructive evaluation of castings and riveted steel connections and in the inspection of welded structural elements by the pulse-echo technique. This technique requires the ultrasonic beam to travel at a small angle to the surface of the structure Angle-beam transducers are based on the principle that a longitudinal wave incident on a solid 1-solid 2 interface is mode converted into a refracted shear wave and a refracted longitudinal wave, propagating in solid 2 as shown in Fig. 5.24. 

Fig 5.24 Angle-beam transducer.

The directions of the refracted waves are dictated by Snell’s law.These waves are used to selectively investigate welded joints,cracks, and other structural faults. According to Snell’s law, as _L1 is increased, _L2 and _S2 also increase. Corresponding to _L1 (crit.), _L2 becomes 90° and VL2 ceases to exist, and only VS2 propagates in solid 2. If _L1 is increased much beyond _L1 (crit.), VS2 also disappears. In this situation a SAW propagates on the surface of solid 2. SAWs are used in NDT to detect surface cracks.

Inductive Transducers

In these transducers, the transduction mechanism is one where the self-inductance of a single coil or the mutual inductance between two coils is changed by a measurand. In general, the measurand could be a linear or rotary displacement, pressure, force, torque, vibration velocity, and acceleration.
The inductance changes are brought about by the movement of a concentric magnetic core.

The inductance of a single coil increases as the core is inserted in the coil and reaches a maximum value when it is centered on the coil length. Similarly, two separate coils L1 and L2 wound on the same bobbin can also be used as a displacement transducer. Any measurand that moves the core directly through a summing device will produce a change in the impedance of the coils that is proportional to the magnitude of the measurand. The coils can be used as the adjacent arms of an impedance bridge. Since L1 increases by the same amount that L2 decreases, or vice versa, the bridge output will be doubled.

A variation of the inductive transducer, shown schematically in Fig. 5.6a, is known as the linear variable differential transformer (LVDT).This transducer consists of a primary coil L1, two interconnected coils L2, L3, and a common magnetic core. The coils are wound on a hollow nonmagnetic glass-filled nylon tube and the core slides coaxially inside the tube. The excitation frequency for coil L1 ranges from 1 to 10 kHz. Coils L2 and L3 are wound in phase opposition in a way that the voltages induced in them by coil L1 are 180° out of phase.

Fig 5.6 Inductive transducers. (a) Linear variable differential transformer (LVDT) (b) LVDT accelerometer.

Consequently, the voltage at terminals c-d is zero when the core is centered inside the tube between coils L2 and L3. When the core is moved away from the null position, the voltage at terminals c-d changes in amplitude and phase (polarity). This change, when brought about by a measurand, is proportional to the magnitude of the measurand. LVDTs are available in linear stroke lengths of ±1 to ±300 mm and sensitivities of 1.7 to 250 mV/V/mm, depending on the stroke length.

The same transduction mechanism is also used in a rotary variable differential transformer (RVDT) to measure angular displacements and torque.To achieve good linearity, the angle of rotation is limited to ±40°. The LVDT can be used with Bourdon tubes, bellows, and proving rings to measure force and pressure.and the leaf springs provide the restoring force. This is an open-loop accelerometer.