Chemical and Environmental Transducers.

In chemical SAW transducers, the pairs of IDTs are formed on the same side of the substrate as shown in Fig. contrast to the physical SAW transducers described earlier.

In the intervening space L of the sensing oscillator, a (bio)chemical interface layer is deposited. The second oscillator serves as a reference. Mass loading caused by the interface layer will change the sensing oscillator frequency.

Temperature and stress affect both oscillators equally, and they represent common mode signals. The difference in frequency (Fs-FR) will then be proportional to the magnitude of the measurand, to the extent that it perturbs the interface layer.

The transduction mechanisms are otherwise identical to those described for physical transducers. The magnitude of the change in frequency caused by mass loading of a polymeric interface layer is given by

In chemical, environmental, and biochemical transducers, the changes in the frequency Fs are primarily brought about by the mass loading effect of the interface layer. A biochemical transducer uses the selectivity of enzymes and antibodies to mass load the layer, whereas a chemical sensor depends on the adsorption and chemisorption of the analyte gases.

These changes alter the SAW velocity. The transduction takes place when the velocity changes cause changes in the total electrical phase shift _T around the oscillator loop and corresponding changes in the oscillator frequency proportional to the mass loading of the interface layer.

In addition to frequency changes caused by the interface layer, the SAW attenuates as it propagates between the IDTs. Viscoelastic polymer interface layers, on absorption of volatile organic species, become soft because of plasticity effects and the SAW propagation losses increase because of softening of the polymer. The attenuation of the SAW represents yet another transduction mechanism in a chemical sensor which has been used to identify several chemical species for a particular chemical environment.

Doppler Effect Transducers

When a sound wave at a given frequency is reflected from a moving target, the frequency of the reflected or backscattered sound is different. The shift in frequency is caused by the Doppler effect. The frequency is up-shifted if the target is moving toward the observer and down-shifted if it’s moving away. The Doppler shift in frequency is proportional to the velocity of the moving target and is given by 

Two techniques are used in the measurement of fluid flow velocity by the Doppler technique. The continuous-wave (CW) method, as used to determine the flow velocity of slurries in a pipe, is illustrated by Fig. 5.25a.52 The transmitted CW

Figure 5.25 Measurement of velocity by Doppler shift. (a) Continuous-wave (CW) method. (b) Pulse-wave (PW) method gives the peak velocity.

signal is partly reflected by the suspended particles or gas bubbles in the slurry. This backscattered signal is received by a second transducer and its output is compared with the transmitted signal. The Doppler shifted signal fD is given by Eq. 5.21. Knowing _ and f0, the velocity V can be obtained.

The second method, as used in a medical diagnostic application, is illustrated by Fig. 5.25b. A pulsed-wave (PW) signal is used to measure the blood flow velocity in a small blood sample volume or range cell localized in the bloodstream of a coronary artery. The device is constructed from a 0.45-mm-diameter, flexible and steerable guide wire with a 12-MHz transducer integrated into its tip.

The transducer transmits a sequence of 0.83-μs-duration pulses at a pulse repetition frequency of 40 kHz into the bloodstream. The range cell is located by time (range) gating the Doppler shifted backscattered signal generated by the red blood cells and received by the same transducer. This signal is compared with the transmitted signal, where as before the velocity V can be calculated using Eq. 5.21. In this case, cos ө=1.

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.

Resistive Transducer (Potentiometric transducers)

Resistive transducers have many and varied applications in the transduction of measurands such as displacements, mechanical strain, pressure, force and load, temperature, and fluid velocity into electrical outputs. The transduction mechanisms are based on the change in resistance brought about by the measured.

 Potentiometric transducers

A potentiometric transducer is a mechanically driven variable resistor. It consists of a wire-wound fixed resistor and a wiper arm that slides over it and in so doing taps a different segment of the resistor, as shown diagrammatically in Fig. 5.8a and b, where K represents a fraction of the resistor that is tapped.

The displacement to be measured is linked by a shaft to the wiper arm, and a measure of the displacement is the fractional resistance KR or the fractional voltage KV.

This is the transduction mechanism. The resolution that one can achieve with this transducer depends on the gage of the nickel alloy or platinum wire used. For extremely fine resolution, the wire is replaced by a metallized ceramic or a film resistor. If the resistance wire is wound on a doughnut-shaped tube, the wiper will measure angular displacements. The output voltage corresponding to a displacement, force, or pressure is a fraction of the external voltage V, and therefore it does not need any amplification to activate external circuitry.

Fig 5.8 Potentiometric displacement transducers. (a) Resistance proportional to displacement or position. (b) Voltage proportional to the same measurands. (c) Displacement is measured around a null position and the output voltage is ±K0V0. K0 is referenced to the center of the resistor.

Carbon granules, packed in a small volume in the shape of a button and connected in series with a voltage source and a load resistor, have been used in the past as microphones. Using this transduction mechanism, carbon-strip strain gages were developed in the early 1930s. These were, in turn, followed by unbonded and bonded wire strain gages, foil strain gages, and semiconductor strain gages.