An accelerometer is a device that measures acceleration. Accelerometers are used in many systems, from navigation modules of planes and submarines to smartphones and other gadgets. In the first accelerometers, acceleration was measured based on the compression of the spring with a load attached to it. The same principle of movable mass is used in modern-day accelerometers but on a smaller scale. However, many devices (such as industrial robots) require navigation systems that are not only small in size but also resistant to impact, vibration, and high acceleration. Accelerometers based on surface acoustic waves can provide accurate data even in these conditions. Surface acoustic waves spread across the surfaces of solid bodies and can be registered in piezoelectric materials (i.e. the materials that have electric fields reacting to mechanical impact). To do so, a piezoelectric membrane is connected to devices that transform mechanical waves into acoustic ones.
In modern-day accelerometers, membranes are usually made of quartz and lithium niobate. Although effective to some extent, these materials are still not the best: quartz isn’t sensitive enough, and lithium niobate becomes unstable when the temperature changes. To find an alternative solution, a team of physicists from LETI modeled the sensitive elements of accelerometers from aluminum nitride using the COMSOL Multiphysics software package. This tool allows one to set up various mechanical and electric properties of models and test them in different conditions. The team modeled round membranes surrounded by ring transducers and encased in thick aluminum nitride frames. For comparison, they also created models of similar structures from quartz and lithium niobate. The sensitive elements had 3 mm in diameter, and the membranes were only 0.22 mm in thickness.
First, the researchers tested different ways of fixing a membrane in a frame. According to the model, the best option was to use a thin layer of silicone adhesive. If a membrane is simply inserted into a frame, it can deform at fixation points under load thus reducing the sensitivity of the accelerometer. In the following tests, the team considered a model with adhesive fixtures. The researchers modeled the device’s behavior at acceleration tens, hundreds, and thousands of times higher than the standard acceleration of free fall (g=9.81 m/c2). Regardless of the acceleration value, the aluminum nitride membrane moved less than the quartz or lithium niobate one. It means that a meter with it would work more effectively. Another considerable advantage of this material was its relatively small energy loss. At the same time, the accelerometer with the membrane made of aluminum nitride was more sensitive to temperatures than the device with the quartz membrane.
“Based on the results of computer modeling, we can conclude that aluminum nitride is a promising material for acoustic accelerometer sensors, especially for measuring high levels of acceleration. Its resistance to mechanical deformation is two times higher than that of quartz which increases the sensitivity 1.5 times. Like in the case of lithium niobate, the main issue that can limit the use of aluminum nitride is its sensitivity to temperature changes,” said Sergei Shevchenko, Associate Professor of the Department of Laser Measuring and Navigation Systems at LETI.