Passive Wireless Sensors with Piezoelectric MEMS Resonators: Tiny, Mighty Machines

December 3, 2015

Sensors — devices that detect and respond to some stimulus (e.g., light, heat, motion) — are part of our everyday lives. Take a smartphone, for instance. This one small device carries a plethora of sensors that gather data for apps, such as the battery’s temperature. They use a technology called microelectromechanical systems, or MEMS.

MEMS are mini-machines, comprised of mechanical parts (e.g., levers, springs, and vibrating structures) and electrical parts (e.g., resistors, capacitors, and inductors) which measure between 1 and 100 micrometers in size. How small is that, exactly? In comparison, the diameter of a human hair is 25 micrometers and the size of a grain of salt is 100 micrometers.

These two types of parts work together to gather data and/or physically interact with the surrounding environment. These devices are currently found in items such as car airbag systems, small cameras, microphones, fitness watches like Jawbone and Fitbit, and toasters. This graphic from the MEMS Industry Group shows how we use MEMS devices on a routine basis. Yet even with the ubiquity of this technology, there are still limitations with where these sensors can be placed.

rotating jet engineOne new type of sensor is a passive wireless sensor system that utilizes piezoelectric MEMS resonators created by Reza Abdolvand, Ph.D., who is also our featured researcher for the month of December. This invention is comprised of a few different parts, working in concert.

Unlike wired sensors, which require a power source such as a battery, these new sensors scavenge energy from the signal that is transmitted from a transreceiver to a tiny antenna in the system. This makes the sensor wireless because it passively receives energy.

Piezoelectric (the prefix piezo- is Greek for “press” or “squeeze”) sensors measure changes in pressure, temperature, strain, or force by converting them into electrical charges. MEMS resonators are silicon-based, miniature structures that vibrate at high frequencies. Together, piezoelectric MEMS resonators use the best of both worlds: the electromechanical strength of the piezoelectric material and the stability of the MEMS structure are combined to create a reliable, cost-effective sensor system.

One of the future innovations that Abdolvand and his group is working on involves creating a smaller antenna, effectively reducing the device’s size and opening the field to new applications that otherwise wouldn’t be available or useful. This could go well beyond the sensors found in a smartphone. It may mean, for example, the ability to access biometric data, on demand, from within the human body through sensors placed just under the skin. These sensors could be placed in many areas that were previously inaccessible (e.g., the moving parts of an engine, inside rotating objects like tires). Additionally, wherever there is a wired sensor, Abdolvand’s invention could possibly replace it.

These lower cost, tiny and mighty devices can be mass-produced and can even become disposable (e.g., ensuring food safety by measuring temperature on packaged food). Soon, the future will fit on the head of a pin, offering an abundance of information to improve our everyday lives.

There are a myriad of possibilities with this new sensor system. If you’re interested in exploring them, contact Raju Nagaiah, Ph.D. for more information.