New Prospects for Acoustic Sensors: an Overview

Abstract

Sensors employing elastic waves have been devised for detecting a wide range of non-acoustical measurands. Examples include: biological substances (antigens in a liquid); chemicals, such as styrene vapor and hydrogen sulfide; electrical and magnetic fields; temperature; and a host of mechanical measurands, such as position, force and pressure, acceleration, and material properties such as viscosity. The reason for using an acoustical sensor varies with the application. High sensitivity is attractive in the case of chemical, acceleration and temperature sensing. Near-linear response and wide dynamic range are also important considerations. Low cost and simplicity characterize several acoustical position sensing systems for use with robots and computer displays. In many of these applications, the output variable is a frequency, which is a convenient input variable for digital storage and processing circuitry.Most of the newer acoustic sensing devices employ resonators or delay-line bulk-or surface-wave oscillators whose frequency is affected by the measurand. Resonant elements that are used include conventional piezoelectric crystal resonators, such as plates and tuning forks, SAW resonators formed on crystalline piezoelectric substrates or in sputtered piezoelectric thin films, and tiny micromechanical beams and bridges formed by integrated-circuit fabrication techniques. Other sensors of current interest, particularly for position sensing and materials characterization, use measurement of timing and reflection, transmission, or attenuation made on ultrasonic waves propagating in air or in a solid.Device principles, operating characteristics, and possible limitations of a number of these acoustic sensors will be described.

Surface Acoustic Wave Devices for Harsh Environment Wireless Sensing

“…Except for a narrow range of temperatures and carefully chosen propagation directions, the surface wave velocity is temperature-dependent and the SAW device can be used as a temperature sensor [6,7] (Figure 3(Top)). Strain also influences the round-trip delay time and has been used to implement strain and pressure sensors [6,7]. …”

“…Overlayers deposited on the propagation path provide a means for chemical or biological sensing [6,7] (Figure 3(Bottom)). Some overlayers selectively absorb certain chemical or biological species, resulting in a mass change that changes the surface wave velocity.…”

Electromagnetic Excitation of Acoustic Biosensors

“…To progress further it is necessary to calculate the magnitude of the sideways force felt by the electrons in this region. This force is known as the Lorentz force, which is proportional to the current in the electrode and the applied magnetic field and is given by Equation 5:…”

“…Martin and Ricco introduced one of the first acoustic plate mode viscosity sensor using this format [4]. Similarly, White was experimenting with thin SiN waveguides that could carry Lamb wave radiation [5]. As both waves could propagate in contact with liquids, they were potentially useful for biosensor applications.…”

Wireless surface acoustic wave sensors for displacement and crack monitoring in concrete structures

“…Surface acoustic wave (SAW) devices are a class of wireless sensor that require no batteries, as they operate on wireless power. By utilising radio-frequency identification (RFID) technology, SAW sensors can provide passive measurements of strain, temperature, pressure, acceleration and chemical contamination, even in harsh environments [7][8][9][10][11]. Each device is a microelectromechanical system (MEMS), which works by monitoring the modulation of the frequency of sound waves on a millimetre-sized piezoelectric substrate.…”