Soft RFID sensors: Wireless Long-range Strain Sensors Using Radio-frequency Identification

Increasing amounts of attention are being paid to the study of Soft Sensors and Soft Systems. Soft Robotic Systems require input from advances in the field of Soft Sensors. Soft sensors can help a soft robot to perceive and to act upon its immediate environment. The concept of integrating sensing capabilities into soft robotic systems is becoming increasingly important. One challenge is that most
of the existing soft sensors have a requirement to be hardwired to power supplies, or external dataprocessing equipment. This requirement hinders the ability of a system designer to integrate softsensors into soft robotic systems.

In this paper we design, fabricate, and characterise a new soft sensor which benefits from a combination of RFID tag design and microfluidic sensor fabrication technologies. We designed this sensor using the working principle of an RFID transporter antenna, but one whose resonant frequency changes in response to an applied strain. This new microfluidic sensor is intrinsically stretchable and can be reversibly strained. This sensor is a passive and wireless device and as such, it does not require a power supply, and is capable of transporting data without a wired connection.

This strain sensor is best understood as an RFID tag antenna; it shows a resonant frequency change from 860MHz to 800MHz upon an applied strain change from 0% to 50%. Within the operating frequency, the sensor shows a stand-off reading range of over 7.5 metres (at the resonant frequency). We characterise, experimentally, the electrical performance and the reliability of the fabrication process. We demonstrate a pneumatic soft robot that has four microfluidic sensors embedded in four of its legs, and we describe the implementation circuit to show that we can obtain movement information from the soft robot using our wireless soft sensors.