WiForceSticker: Batteryless, Thin Sticker-like Flexible Force Sensor

Abstract

Any two objects in contact with each other exert a force that could be simply due to gravity or mechanical contact, such as a robotic arm gripping an object or even the contact between two bones at our knee joints. The ability to naturally measure and monitor these contact forces allows a plethora of applications from warehouse management (detect faulty packages based on weights) to robotics (making a robotic arms' grip as sensitive as human skin) and healthcare (knee-implants). It is challenging to design a ubiquitous force sensor that can be used naturally for all these applications. First, the sensor should be small enough to fit in narrow joints or not make the robotic arm bulky. Next, we don't want to lay long, cumbersome cables to read the force values from the sensor to the monitoring device. And finally, even current wireless technology requires a battery that could be hazardous for in-body applications. We develop WiForceSticker, a wireless, battery-free, sticker-like force sensor that can be ubiquitously deployed on any surface, such as all warehouse packages, robotic arms, and knee joints. WiForceSticker first designs a tiny 4 mm × 2 mm × 0.4 mm capacitative sensor design equipped with a 10 mm × 10 mm antenna designed on a flexible PCB substrate. Secondly, it introduces a new mechanism to transduce the force information on ambient RF radiations that can be read by a remotely located reader wirelessly without requiring any battery or active components at the force sensor, by interfacing the sensors with COTS RFID systems. The sensor can detect forces in the range of 0-6 N with sensing accuracy of < 0.5 N across multiple testing environments and evaluated with over 10, 000 varying force level presses on the sensor. We also showcase two application case studies with our designed sensors, weighing warehouse packages and sensing forces applied by bone joints.