It is not surprising that acoustic sensors are increasingly being used in measurement tasks. Everything around us produces vibrations and can therefore be measured acoustically. Acoustic sensors are versatile and yet still in the early stages of their development.
Rob Pieters has been following their evolution for years. Below you can read the main highlights from his analysis.
Why acoustic wave sensors?
Acoustic wave sensors are enormously adaptable sensors whose commercial potential is only just beginning to emerge. They are cost-effective, robust, sensitive and inherently reliable. In addition, they can be applied passively and wirelessly. Wireless sensors are useful for monitoring parameters on moving objects, such as vehicle tire pressures or torque on axles (for predictive maintenance).
Sensors that do not require a power supply are essential for remote monitoring of chemical vapors, moisture and temperature. Other applications include measuring force, acceleration, shocks, angular velocity, viscosity, displacement and flow. The sensors also have acoustic-electric sensitivity, allowing the detection of pH levels, ionic contaminants and electric fields.
Acoustic surface wave sensors in general have proven to be the most sensitive due to their high energy density at the surface. For liquid detection, a special class of shear-horizontal acoustic surface wave sensors called "Love Wave sensors" proved to be the most sensitive. Much work remains to be done in developing these sensors for future applications.
9 acoustic sensor measurement types
Acoustic sensors can measure various physical quantities by detecting sound waves or vibrations. Here are 9 examples of what they can measure:
- Distance
Acoustic sensors measure the time it takes for a sound wave to return after reflecting off an object. This is similar to echolocation. - Force
They measure the force exerted on a surface by analyzing how sound waves propagate through the material. - Displacement
Vibration or displacement of an object can be measured by changes in the sound waves passing through the object. - Temperature
Acoustic sensors detect temperature changes by measuring the speed of sound waves in different materials. - Liquid levels
By measuring the time it takes for sound to travel from the sensor to the liquid surface and back again, they can determine the level of liquid in tanks or pipes. - Shock and acceleration
Detect the speed and direction of impact or acceleration by observing how sound waves respond to motion. - Humidity
Acoustic sensors measure changes in humidity by detecting the effect of water vapor on the sound signal. - Chemicals
Some sensors can detect chemicals and contaminants by analyzing how sound waves interact with molecules in the air or on surfaces. - Viscosity
Acoustic sensors measure the viscosity of liquids by observing how the sound waves change in response to the liquid.
How do acoustic wave sensors work?
Acoustic wave sensors use a mechanical or acoustic wave as the sensing mechanism. When an acoustic wave propagates through or on the surface of a material, changes in the propagation path affect the wave's velocity and/or amplitude. These changes in velocity are detected by measuring the frequency or phase characteristics of the sensor and correlating them with the measured physical quantity.
Acoustic sensors can be used for predictive maintenance. For example, they can detect abnormal noise from conveyor belts that may indicate wear and tear. This reduces the risk of unexpected breakdowns.
From piezoelectric substrate to sensor
Production of these sensors begins with the careful polishing and cleaning of a piezoelectric substrate, such as quartz, lithium tantalate or lithium niobate. These materials are chosen because of their specific properties, including cost, temperature dependence and propagation speed. The manufacturing process involves applying a metal layer, usually aluminum, and using photolithographic techniques to form an interdigital transducer (IDT).
Bulk wave versus surface wave
Acoustic wave sensors are distinguished by their propagation modes, such as bulk wave and surface wave. The most commonly used bulk acoustic wave (BAW) devices are the thickness-shear mode (TSM) resonator and the shear-horizontal acoustic plate mode (SH-APM) sensor. Surface wave devices such as the surface acoustic wave sensor and the shear-horizontal surface acoustic wave sensor (SH-SAW) are also popular. The choice of device depends on the specific application and required sensitivity.
The versatility of acoustic sensors: from automotive to medical industry
Acoustic wave sensors are used in a variety of industries. In the automotive industry, they are used for torque and tire pressure sensors. In the medical sector, we find them in use as chemical sensors. They can also be deployed in industrial and commercial applications as vapor, humidity, temperature and mass sensors. Thanks to their competitive price, robustness, high sensitivity and reliability, these sensors are rapidly gaining in popularity. In addition, some sensors can be read passively and wirelessly, offering additional advantages in certain applications.
The future of acoustic wave sensors
Recent developments in acoustic wave technology include the creation of sensors with higher frequencies and sensitivities using advanced materials and micro-manufacturing technologies. These innovations are opening doors to new applications and improvements in sensor performance. The focus is on increasing sensitivity, reducing costs and broadening the scope of use.
Acoustic wave sensors are on the threshold of a new wave of technological innovations and applications. With their versatility, cost-effectiveness, robustness and high sensitivity, they offer promising opportunities for a wide range of industries. Whether monitoring tire pressure in moving vehicles, detecting chemical vapors at a distance, or measuring force and acceleration, acoustic wave sensors will greatly advance the way we try to understand our environment.
Stay ahead in sensor innovation
In a world where technology is developing at lightning speed, it is difficult to stay up-to-date. Want to keep up with the latest developments in sensor technology? Our newsletter gives you a head start.
Through our monthly newsletter, you will receive technology blogs, news, trends and background stories in your mailbox. Sign up now and discover what sensor technology has in store for the future!
