CoinFT: A Coin-Sized, Capacitive 6-Axis Force Torque Sensor for Robotic Applications

TL;DR

CoinFT presents a coin-sized capacitive 6-axis force/torque sensor built from two PCB layers connected by silicone pillars, achieving multi-axis sensing via mode-switching electrode configurations. Its design emphasizes compactness, low cost, robustness, and reasonable bandwidth, validated through calibration against a reference Gamma sensor and two demonstrations: fingertip-scale manipulation and drone-based contact force control with payload deployment. The authors provide comprehensive fabrication details, open-source firmware and models, and show that CoinFT can operate in real-world, contact-rich environments where traditional sensors are too bulky or fragile. Overall, CoinFT democratizes 6-axis F/T sensing by delivering a slim, inexpensive, and robust solution suitable for drones, wearable haptics, and dexterous manipulation, with open access to design resources.

Abstract

We introduce CoinFT, a capacitive 6-axis force/torque (F/T) sensor that is compact, light, low-cost, and robust with an average root-mean-squared error of 0.16N for force and 1.08mNm for moment when the input ranges from 0~14N and 0~5N in normal and shear directions, respectively. CoinFT is a stack of two rigid PCBs with comb-shaped electrodes connected by an array of silicone rubber pillars. The microcontroller interrogates the electrodes in different subsets in order to enhance sensitivity for measuring 6-axis F/T. The combination of features of CoinFT enables various contact-rich robot interactions across different embodiment domains including drones, robot end-effectors, and wearable haptic devices. We demonstrate the utility of CoinFT through two representative applications: a multi-axial contact-probing experiment in which a CoinFT mounted beneath a hemispherical fingertip measures 6-axes of force and torque representative of manipulation scenarios, and an attitude-based force-control task on a drone. The design, fabrication, and firmware of CoinFT are open-sourced at https://coin-ft.github.io/.

Figures (13)

• Figure 1: (a) CoinFT is approximately the size of a U.S. quarter-dollar coin. (b) Exploded view of CoinFT. It consists of two rigid PCBs (upper and lower sensing layers) connected with an array of silicone rubber pillars. The fPCB top shield layer provides passive shielding. The alignment tabs are removed after assembly. (c) Dimensions of each layer. The overall thickness is approximately 2 mm.
• Figure 2: CoinFT working principle. The microcontroller firmware is programmed to switch the pair of rigid PCBs between two different electrode configurations - normal mode and shear mode. The capacitance signals collected from these two configurations produce unique patterns under different force and torque inputs.
• Figure 3: The fabrication process of CoinFT. (a) Fresh uncured silicone is spread on a UV laser cut mask that is placed on an acrylic plate with alignment pins. (b) A primed lower sensing layer PCB and an acrylic plate for equal pressure distribution is stacked with a 3.1 kg weight. (c) With a 0.1 kg weight, the assembly is cured inside a pressurized chamber. (d),(e) Once the mask and acrylic plates are removed, the pillar layer is complete. (f) On a primed upper sensing layer PCB, fresh uncured silicone is spread. (g) The silicone layer is made thin and uniform through spin-coating. (h) The lower sensing layer with pillars is assembled with the upper sensing layer through precise distance control by adding spacers in between. A 0.1 kg weight and an acrylic plate for pressure distribution are added. (i) The top shield layer is attached using an adhesive. (j) A horizontal cross section of the pillar layer shows desirable bonding of the two PCB layers.
• Figure 4: CoinFT characterization using FEA. Raw capacitance response of a half of CoinFT (a) under normal force, (b) shear force, and (c) torsion. Full CoinFT response with varied pillar diameter under (d) normal force (real samples & FEA), (e) shear force (FEA), (f) and torsion (FEA). (g) Raw capacitance change with displacement and corresponding compressive force. (h) Structural modeling in FEA. (i) Electrostatics modeling in FEA.
• Figure 5: Comparison of sensor readings between CoinFT and Gamma (ATI Industrial Automation) in (a) Fx, (b) Fy, (c) Fz, (d) Mx, (e) My, (f) Mz.