ForceSticker: Wireless, Batteryless, Thin & Flexible Force Sensors

Force is a truly ubiquitous phenomenon, and is naturally exerted between any two objects in contact. For example, a bottle exerts its weight (gravitational force) onto the table that it is placed on. Similarly, our feet exert force on the ground as we walk, while simultaneously our bones exert force on each other as we perform daily activities (Fig. 1). Truly, forces are all around, and within us.

Fig. 1: Forces are all around and within us

The need for “Force Stickers”

As force is such an ubiquitous contact phenomenon, force sensors are touted to be an important technology that can bridge the gap between the physical and digital world, similar to cameras for the visual phenomenon. Thus, force sensors could be quite as important as cameras in the upcoming decades, with multiple applications in AR/VR interfaces and ubiquitous sensing. Imagine a future, where you can walk into your kitchen, take a photo with your smartphone, and in addition to the visual picture, you also get force information, how much does the milk can weigh, which groceries to refill and leftover food quantity etc (Fig. 2a).

Unlike visual phenomena captured by a camera, force is a contact phenomenon whose measurement requires the placement of dedicated sensors at the contact interfaces. In the case of weight, we already have such sensor devices namely weighing scales, that contain load cells hidden beneath them (Fig. 2b). Indeed, placing a weighing scale beneath each object is impractical since the load cells are not only bulky, and expensive but also need to be powered constantly using a battery or connection to the mains line.

Fig. 2a, Ubiquitous force sensing vision: Towards recording contact force phenomenon in addition to standard visual image, and how it can help with everyday life tasks. Fig. 2b Existing force-sensor devices (load cells)

Further, these problems are not just limited to forces exerted by static objects. Another example to highlight the limitations with bulky powered sensor-devices is seen in shoes with force-sensors. Either we have to deal with hassles of walking around with wires running till your feet (Fig. 3a), or with a bothersome coin-cell battery placed inside the shoe (Fig. 3b), that also needs to be recharged/replaced often. Even though we have force sensors which by themselves are flexible and thin (FlexiForce sensors, Fig. 3c), they need electronics, and thus batteries/wires to power these electronics, making the sensor-devices bulky. The state of today’s force sensors is well-captured by the Simpsons meme below (Fig. 3d).

Fig. 3 a) Shoe-sole sensors with wires running till the feet b) Wireless shoe sole, but at expense of a small coin cell battery inside the shoe c) A typical force sensor node of today, requiring electronics and batteries and d) a meme capturing the state of today’s force sensor technologies

Hence, in our project, we take a first step towards cutting the bulk with existing force sensors, and making them wirelessly-powered, batteryless, and hence in a thin paper-like form factor which we dub as ‘ForceStickers’!

With ForceStickers, capturing force information from a contact point of interest becomes as easy as sticking a small paper like sensor onto them, as shown in the teaser video below. Further, ForceStickers opens the door for in-vivo applications of force sensors, which were previously impractical due to the requirement of batteries/wires. For example, ForceStickers can be stuck to orthopedic implants to sense the implant’s fit and force distribution, and to surgical robots/tools to sense if the forces they are applying are within safe levels as they operate over body organs.

Teaser video introducing our research: ForceStickers

Design of ForceStickers: DIY electronics-free capacitive sensor interfaced to RFID Stickers

With the target of designing a “sticker-like”, wirelessly-powered force sensor in mind, we proceeded to try and minimize the number of electronics needed, both to minimize the amount of energy required to harvest through wireless signals, as well as reduce the real-estate occupied by such electronics and achieve the sticker like form factor. That is, goodbye to good old microcontrollers, ADCs, power-hungry Op-Amp based sensor voltage amplifiers, and wireless modules like Bluetooth/Wi-Fi. The challenge looks imposing, how do we make such a force-sticker then? Do we need to invent new technologies to achieve our forcestickers?

Turns out, no! There is a technology, in the correct form-factor (Sticker-like) and wirelessly-powered (batteryless), and that is RFIDs, which are thin-flexible barcode-like stickers, often with a squiggly looking antenna and a small RFID IC which handles the power harvesting and communication (Fig. 4a).

We realized that force-information can be piggybacked over existing RFIDs, with no additional power and requirement of any interfacing electronics, by simply interfacing a force sensitive capacitor to the RFID.

Hence, the designed force-stickers simply consist of a thin parallel-plate capacitor, smaller than a rice grain (Fig. 4b) that deforms under applied force, and is interfaced in between the RFID squiggly antenna and the RFID IC (Fig. 4c). But, how does the force-information from the capacitor get communicated via the RFID IC, without requiring any more electronics and power? The secret sauce lies in the capacitor-design, choosing the correct polymer and correct dimensions!

Fig. 4 a) Shows ForceSticker components, b.i, ii) Shows capacitor sensor compared to a rice grain, zoomed in view c) Shows how the capacitor sensor is attached to the RFID IC (peeled off in the image via tweezers) via hair-like tungsten filaments highlighted in red, can be seen more clearly in a) and b.ii)

We show that when a capacitor is interfaced in-between the RFID IC and antenna, it shows a non-linear relationship to the reflected signal’s phase. Hence, this non-linear effect can be harnessed by choosing the capacitance in the correct range, where a small change in capacitance creates a large measurable change in the reflected signal’s phase (Fig. 5).

This analog phase-change created by the capacitor is then also modulated by the RFID IC, which adds the digital on-off signature atop the phase change. Finally, this digitally on-off modulated and analog phase changed signal is reflected off from the ForceSticker, and the RFID reader first decodes the digital identity, and then estimates the phase change to read the applied force on a particular force-sticker. Please read our IMWUT paper [1] for more technical details!

Fig. 5, When a capacitor is interfaced to an antenna, it shows a non-linear relationship between the frequency, capacitance and the phase change. We find that when capacitor is designed between 1–10 pF range, the phase change effect is sensitive to RFID frequency of 900 MHz

There have been prior attempts to achieve something similar: carry over analog phase changes over a digitally modulated signal. However, either they utilized a bulky analog force-sensor (our own past work WiForce [2] and some strain sensors [3]), and/or required a new technology for digital modulation (freq. shift based platforms, RF switches [2,4]), which are not in a sticker-like form factor currently. Again, the secret sauce here was the humble capacitor. With the non-linear phase change model from the capacitor, we were able to reduce the size of previously shown phase change force sensors by 1000x times as compared to WiForce (Fig. 6), and further, can be directly interfaced to RFIDs without needing any extra power and electronics.

Fig. 6, Because of the new capacitive transduction, ForceSticker sensor is 1000x smaller (in terms of volume) compared to past work on batteryless force sensors, while keeping the same sensitivity and error performance

Fun Experiments with ForceStickers

Once we finished some boring verification experiments to confirm our capacitive phase change model, we noticed consistency with simulations, that further strengthened confidence in our sensor. There onwards, we created a bunch of fun experiments with our “ForceStickers”.

Package Consistency checks with ForceStickers: I’m pretty sure at some point in your life, you would’ve received an empty package in your mailbox. Or, it could be, you are a systems PhD student rushing for a deadline, needing 3 RPis for your experiment but received only one in your package. Won’t it be great if the barcodes on the packages can read the weight of the package and use it to notify warehouses of incorrectly packed objects? This is exactly a real-life application which ForceSticker can enable (Fig. 7)! We realized that by sticking ForceStickers below packages, we can with almost 100% accuracy tell if a package is empty or not, and with >95% accuracy even give a count of number of objects.

Fig. 7, We show that by sticking ForceStickers to bottom of packages, we can sense weight and determine number of items kept inside, for consistency checks in warehouse settings. We take 160 measurements with different number of items (RPis placed in the package) and obtain classification confusion matrix.

Sensing knee-impact forces: Moving beyond these routine applications, Forcestickers can enable several in-vivo applications of force-sensing, since it is batteryless. We motivate one such example by placing a force-sticker in dummy knee-joint model to try and sense the knee-impact forces from the stickers. We ran into a problem, even the smallest of RFIDs available today, were not small enough to fit inside the knee joint. So, we decided to make our own ‘small-enough’ RFID sticker, on a flexible PCB substrate, consisting of a funny looking spiral antenna, interfaced to the capacitor and RFID IC via a co-planar waveguide to match the impedences (Fig. 8). This resulted in a smaller version of ForceSticker, which could snugly fit inside the knee-joint and we could record the joint forces as we pressed down on the dummy knee model!

Fig. 8, We show how ForceSticker can be prototyped with a custom Flexible PCB RFID, that allows us to make a 2x smaller ForceSticker, and makes it small enough to allow sensing forces from a knee-joint

This showed that ForceSticker can even be used in scenarios like knee-implants which are irregularly shaped and massively space constraint. Further, by monitoring the knee joint forces over time, the implant’s fit and function can be evaluated, as well as the implant health, if the implant starts wearing down then the sensed force will also change.

Cyclic testing of ForceSticker uptill 10,000 force presses: Once all the dust was settled, and all the required results for the paper generated, we decided to see how robust is the sensor by applying forces on it using an in-house developed automated actuator-based cyclic test setup (Fig. 9), until the sensor breaks :P

Having no high expectations, and almost prepared for a negative result which will be improved in future work, we first gave the sensor about 1000 repeated force-presses, and surprisingly the sensor was consistent with no visible changes in error performance. Then, we extended it to 2000, 3000 and kept on going till 10,000, at which point, the actuator setup got heated up to dangerous level and we called the experiment off, with the sensor showing only slight degradation of performance, and generating a pleasant results that called it a day for our current paper!

Fig. 9, The cyclical testing setup where an actuator applies forces on the sensor, and we see that even after 10,000 force presses by the actuator, the sensor’s error performance remains about the same (0.3N)

Future Work

Even though the sensors we designed were thin sticker-like, enabled first demonstrations of bunch of interesting applications and even turned out to be robust, ForceSticker is still research in motion and there is a lot of ground to cover before we could achieve the force vision showed earlier in Fig. 2a and the teaser video.

The biggest roadblock lies in enabling such ForceStickers to work with smartphones, since the existing ForceStickers only work with dedicated RFID readers, which may not be always available. Further, we need to realize mass fabrication of these sensors (right now it is mostly manual and DIY assembly), explore commercially-viable materials for everyday life applications, and bio-degradable hermetically sealed sensors for in-vivo applications. We also need to improve the error performance of the sensor in more challenging environments, and scaling the performance to a large number of sensors.

We are already working towards many improvements for ForceStickersV2, but would gladly explore a collaboration, or help anyone interested in pursuing an extension of our work. Please do read our detailed paper on ForceSticker, and feel free to email me if there are any questions on our work. Thank you for reading our blog, and being one with the Force(Stickers)!

Fig. 10, A cute baby-Yoda meme for all the readers who stayed till the end :)

References:
[1] “ForceSticker: Wireless, Batteryless, Thin & Flexible Force Sensors”, Gupta el al. IMWUT Issue 1 2023
[2] “WiForce: Wireless Sensing and Localization of Contact Forces on a Space Continuum”, Gupta et al. NSDI’21
[3] “Soft Radio-Frequency Identification Sensors: Wireless Long-Range Strain Sensors Using Radio-Frequency Identification”, Teng at al. SORO’19
[4] “MARS: Nano-Power Battery-free Wireless Interfaces for Touch, Swipe and Speech Input”, Arora et al UIST’21