RFID force measurement stickers redefine precise measurement

Any two objects in contact will exert a certain force on each other, which may be due to gravity or mechanical contact, such as the weight of the object on the platform or the contact of two bones at the human knee joint. In order to measure this force more effectively and conveniently, a research team at the University of California, San Diego developed an ultra-thin RFID force measurement "sticker" to assist in measuring these phenomena.

ForceSticker was developed from the integration of two main components: a tiny capacitor just a few millimeters thick and about the size of a grain of rice, and a commercial 900MHz ultra-high frequency RFID tag. The researchers integrated the two components so that they could measure the applied force and wirelessly transmit the information to a standard RFID Reader.

A thin layer of flexible polymer is placed between two conductive copper strips of the capacitor to form the capacitor. When an external force acts on the polymer, it compresses, causing the copper strips to move closer together, increasing the charge within the capacitor.

The design of this force-measuring sticker was inspired by keen observation of changes in capacitance. When an external force is applied, the polymer compresses, pulling the copper strips closer together, thereby increasing capacitance. With this design, researchers can evaluate the sensor's switching capabilities based on an optimized capacitance range design derived from mathematical RF modeling and perform multiphysics simulations in COMSOL.

In the actual application of the ForceSticker, the researchers used two different 4×2 mm sensor implementations with different layers of Ecoflex polymer (a biodegradable platinum-catalyzed silicon-based polymer) and neoprene covering 0 to 6 N and 0 to 40 N ranges, the reading errors are 0.25 N and 1.6 N respectively. Additionally, they stress-tested the ForceSticker over 10,000 times and found no significant error reduction.

This passive RFID tag uses backscatter for power and data transmission. It receives the incoming radio signal from the RFID reader, modifies the signal through electrical changes induced by the capacitor, and then reflects the modified signal back to the RFID reader, which interprets and converts it into force. This method directly inserts the analog RF phase transformation generated by the sensor into the wireless channel path of the RFID electronic tag, creating an analog-to-digital backscatter link.

In the process of achieving sensor integration, a key challenge is the design of the sensor interface. To enable sensor integration without losing signal fidelity, the researchers used a matched-impedance coplanar waveguide approach. Furthermore, in order to obtain this sensitivity tuning, the capacitor must have a properly designed "nominal value" at zero force. This is determined by various nonlinear equations that model this situation, taking into account the impedance and reflection coefficient of the transmission line.

When simulating the interface between capacitive sensors and digital identification RFID, the researchers did so by inserting the sensor between an antenna and an RFID tag in parallel with the two. However, the researchers note that there are two so-called "degenerate" solutions (meaning at least one fundamental variable is zero). One of the solutions assumes that all phase changes are reflected directly from the sensor and no signal reaches the RFID Module. Another solution assumes that the sensor's capacitive switching mode actually operates. Both solutions provide guidance for further optimization of the technology.

Overall, this team at the University of California, San Diego (UCSD) has demonstrated what is possible in Engineering breakthroughs by developing the ForceSticker, an innovative force-measuring sticker. By integrating microcapacitors and commercial RFID tags, they created a device that measures applied force and transmits the information wirelessly.

"Humans are born with an inherent ability to sense force," Dinesh Bharadia, a professor at UC San Diego's School of Engineering, said in a statement from the school. "This gives us the ability to interact seamlessly with our surroundings and allows clinicians to perform delicate surgical procedures. .Bringing this ability to sense forces into electronic devices and medical implants could revolutionize many industries."

And this technology not only has the potential for medical and industrial applications, but can also be used to measure the weight of the bottom of warehouse packages. Through continued research and innovation, we have reason to believe that there will be more breakthroughs like this to improve our lives and work in the future.