A compact, slim, robust, and low-cost capacitive 6 axis force torque sensor

Abstract

A capacitive force / torque sensor utilizing two PCBs with an array of silicone pillars as the dielectric material is provided. This design enables the sensor to detect all six axes of force and torque with just two PCBs, thanks to a specially designed comb-shaped electrode pattern and the capabilities of the microcontroller being used. The microcontroller includes a capacitive-to-digital converter module, allowing firmware-controlled connections to the comb-shaped electrodes. By reconfiguring these electrical connections, the sensor can individually adjust the sensitivity of electrodes to specific subsets of force and torque inputs or combine multiple electrodes to enhance sensitivity to different subsets. This innovation allows the sensor to efficiently sense six-axis force and torque using compact, cost-effective components.

Description

[0001] A COMPACT, SLIM, ROBUST, AND LOW-COST CAPACITIVE 6 AXIS FORCE TORQUE SENSOR

[0002] FIELD OF THE INVENTION

[0003] The invention relates to capacitive force and torque sensors.

[0004] BACKGROUND OF THE INVENTION

[0005] Precise force and torque measurement is vital for robots to perform contact-rich tasks safely and effectively. Tasks such as table wiping, assembly, or palpating soft tissue require the application of force and torque within a specific range — sufficient to complete the task but not so excessive as to cause damage or waste energy. Depending on the application and interaction type, robots performing contact-rich tasks come in various forms, including robotic arms, grippers, drones, and wearable devices. Therefore, equipping these diverse robotic platforms with sensors that can accurately measure force and torque is essential.

[0006] Existing 6-axis force-torque sensors are impractical for scalable use in robots operating in real- world environments. These sensors rely on measuring precise deformations of metal bodies, which can alter characteristics with slight overloading, making them unsuitable for environments where unexpected large impulses are common, such as drones cleaning skyscraper windows, or inspecting the structural integrity of a bridge. Additionally, the high cost of commercially

[0007] S24-339 / PCT 1 / 19 available sensors, often exceeding $1000, makes them prohibitive for applications requiring multiple sensors, like multi-fingered robot hands. This invention addresses these issues by providing a compact, slim, robust, and low-cost 6-axis force-torque sensor, enabling practical and scalable force-torque sensing in robotic platforms.

[0008] SUMMARY OF THE INVENTION

[0009] Embodiments of the invention relate to a capacitive force / torque sensor utilizing two PCBs with an array of silicone pillars as the dielectric material. This design enables the sensor to detect all six axes of force and torque with just two PCBs, thanks to a specially designed comb-shaped electrode pattern and the capabilities of the microcontroller being used. The microcontroller includes a capacitive-to-digital converter (CDC) module, allowing firmware-controlled connections to the comb-shaped electrodes. By reconfiguring these electrical connections, the system can individually adjust the sensitivity of electrodes to specific subsets of force and torque inputs or combine multiple electrodes to enhance sensitivity to different subsets. This innovation allows the sensor to efficiently sense six-axis force and torque using compact, cost-effective components.

[0010] Embodiments of the invention can significantly enhance various robotic platforms by enabling safe and controlled contact. It can be used to improve the safety and efficiency of drones performing contact-based tasks, such as structural integrity inspections or attaching sensors in remote environments for environmental monitoring. Additionally, it can sensorize robotic fingers to enable precise and forceful interactions, such as harvesting produce or using tools like pliers.

[0011] S24-339 / PCT 2 / 19 Furthermore, it can enhance haptic feedback systems in surgical robots, allowing doctors to modulate force more accurately, thus improving the precision and safety of medical procedures.

[0012] Embodiments of the invention offer several advantages and improvements over existing 6-axis force / torque sensors. Traditional sensors rely on measuring precise deformations of metal bodies, which are prone to characteristic changes with slight overloading, making them unsuitable for unpredictable real-world environments. In contrast, our sensor uses a robust capacitive design with two PCBs and silicone pillars as the dielectric, providing enhanced durability and resilience to unexpected large impulses. Current market options are also prohibitively expensive, often exceeding $1000, which limits their scalability in applications requiring multiple sensors, such as multi-fingered robotic hands. Our sensor is cost-effective, utilizing affordable components while maintaining high performance, thereby reducing overall expenses and enabling broader deployment. Relevant competing technologies include traditional strain gauge-based sensors and optical force / torque sensors. While strain gauge sensors offer high precision, they are susceptible to damage from overloading and have higher costs. Optical sensors, although precise, are also expensive and complex to integrate into various robotic platforms. The capacitive sensor provides a balanced solution, combining affordability, durability, and adaptability, making it a superior choice for a wide range of robotic applications.

[0013] In one embodiment, the invention can be characterized as a method for sensing six degrees of freedom of force and torque. A sensing assembly is provided that includes two printed circuit boards. The first printed circuit board (first PCB) has comb-shaped first electrodes disposed in

[0014] S24-339 / PCT 3 / 19 four quadrants on the first PCB, where the first PCB serves as a first sensing layer. The second printed circuit board (second PCB) has comb-shaped second electrodes disposed in four quadrants on the second PCB, where the second PCB serves as a second sensing layer. The first sensing layer and the second sensing layer are electrically coupled to a microcontroller, where the microcontroller is configured with a capacitance sensing module. The capacitance sensing module has two modes. The first mode, a normal mode, is used for operating the microcontroller in a first configuration. This first configuration includes (i) selecting multiple pairs of comb-shaped second electrodes in the second sensing layer, where each of the pairs is connected electrically to function as a combined second sensing electrode, (ii) connecting the comb-shaped first electrodes of the first sensing layer to an electrical ground, and (iii) acquiring first capacitance values from each of the combined second sensing electrodes to measure first force and torque components. The second mode, a shear mode, is used for reconfiguring the microcontroller to operate in a second configuration, where the second configuration includes: (j) selecting comb-shaped second electrodes of the second sensing layer to be individually assigned as individual second sensing electrodes, (jj) alternately connecting the comb-shaped first electrodes of the first sensing layer to the electrical ground and to an active shielding, and (jjj) acquiring second capacitance values from the individual sensing electrodes to measure second force and torque components. The normal mode and the shear mode are repeated, where in the repeating step the normal mode selects different multiple pairs of comb-shaped second electrodes. Then the acquired first capacitance values and the acquired second capacitance values are mapped to a reference force and torque, where the mapping outputs the six degrees of freedom of force and torque.

[0015] In one embodiment, the invention can be characterized as a sensor for sensing six degrees of

[0016] S24-339 / PCT 4 / 19 freedom of force and torque. The sensor has a sensing assembly with two printed circuit boards The first printed circuit board (first PCB) has comb-shaped first electrodes disposed in four quadrants on the first PCB, where the first PCB serves as a first sensing layer. The second printed circuit board (second PCB) has comb-shaped second electrodes disposed in four quadrants on the second PCB, where the second PCB serves as a second sensing layer. The sensor has a microcontroller that is electrically coupled to the first sensing layer and the second sensing layer. The microcontroller is configured with a capacitance sensing module having embedded therewith two computer-encoded programs for operating modes, referred to as a normal mode and a shear mode, for generating normal mode capacitance values and shear mode capacitance values, which the two computer-encoded programs correlate to the six degrees of freedom of force and torque.

[0017] For the normal mode, there are computer-encoded programs for operating the microcontroller in a first configuration, where the first configuration comprises: (i) selecting multiple pairs of combshaped second electrodes in the second sensing layer, where each of the pairs is connected electrically to function as a combined second sensing electrode, (ii) connecting the comb-shaped first electrodes of the first sensing layer to an electrical ground, and (iii) acquiring first capacitance values from each of the combined second sensing electrodes to measure first force and torque components.

[0018] For the shear mode there are computer-encoded programs for reconfiguring the microcontroller to operate in a second configuration, wherein the second configuration comprises: (j) selecting comb-shaped second electrodes of the second sensing layer to be individually assigned as

[0019] S24-339 / PCT 5 / 19 individual second sensing electrodes, (jj) alternately connecting the comb-shaped first electrodes of the first sensing layer to the electrical ground and to an active shielding, and (jjj) acquiring second capacitance values from the individual sensing electrodes to measure second force and torque components.

[0020] The microcontroller further includes computer-encoded programs for repeating the normal mode and the shear mode, where in the repeating step the normal mode includes computer code for selecting different multiple pairs of comb-shaped second electrodes. The microcontroller further includes computer-encoded programs for mapping the acquired first capacitance values and the acquired second capacitance values to a reference force and torque, where the mapping outputs the six degrees of freedom of force and torque.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 shows according to an exemplary embodiment of the invention a design of the capacitive sensor referred to as CoinFT. CoinFT is approximately the size of a U.S. quarter-dollar coin. Shown is an exploded view of CoinFT where has of 2 rigid PCBs (first sensing layer (aka as top layer) and second sensing layer (aka middle layer)) connected with an array of silicone rubber pillars (aka as pillar layer). The fPCB bottom layer provides passive shielding (bottom layer aka as passive shielding, which is an optional layer).

[0023] FIG. 2 shows according to an exemplary embodiment of the invention dimensions of each layer of CoinFT. The overall thickness for this embodiment is approximately 2mm.

[0024] S24-339 / PCT 6 / 19 Noted is that 250, which is a passive shielding layer is an optional layer.

[0025] FIGs. 3A-B show according to exemplary embodiments of the invention the working principle of CoinFT. The microcontroller firmware is programmed to switch the pair of rigid PCBs between two different configurations - normal layer and shear layer. The capacitance signals collected from these two configurations form a unique pattern under different force and torque inputs.

[0026] FIGs. 4A-J show according to exemplary embodiments of the invention the fabrication process of CoinFT. (FIG. 4A) Fresh uncured silicone is spread on a UV laser cut mask that is placed on an acrylic plate with alignment pins. (FIG. 4B) A primed top layer PCB and an acrylic plate for equal pressure distribution is stacked with a 3.1kg weight. (FIG. 4C) With a 0.1kg weight, the assembly is cured inside a pressurized chamber. (FIGs. 4D-4E) Once the mask and acrylic plates are removed, the pillar layer is complete. (FIG. 4F) On a primed middle layer PCB, fresh uncured silicone is spread. (FIG. 4G) The silicone layer is made thin and uniform through spincoating. (FIG. 4H) The pillar layer is assembled with the middle layer PCB through precise distance control by adding spacers in between. A 0.1kg weight and an acrylic plate for pressure distribution is added. (FIG. 41) The bottom layer is attached using an adhesive. (FIG. 4J) The cross section of the pillar layer shows desirable bonding of the two layers. Grey scale shown below FIGs. 4E and 4J apply to all FIGs. 4A-J.

[0027] FIGs. 5A-I show according to exemplary embodiments of the invention CoinFT characterization using FEA. Raw capacitance response of a half of CoinFT (FIG. 5A) under normal force, (FIG. 5B) shear force, and (FIG. 5C) torsion. Full CoinFT response with varied pillar width under (FIG. 5D) normal force (real samples and

[0028] S24-339 / PCT 7 / 19 FEA), (FIG. 5E) shear force (FEA), (FIG. 5F) and torsion (FEA). (FIG. 5G) Full CoinFT response with compressive displacement (FEA). (FIG. 5H) Structural modeling in FEA. (FIG. 51) Electrostatics modeling in FEA.

[0029] DETAILED DESCRIPTION

[0030] The sensor of the present invention is referred to CoinFT, which is a capacitive 6-axis force / torque sensor. In an exemplary embodiment its circular sensing area is 20mm in diameter and approximately 2mm in thickness (FIG. 2), having a comparable form-factor to a quarter-dollar coin in the United States currency (not shown). With a total weight of 2g, it has two 0.8mm thick rigid PCBs (Printed Circuit Boards) connected by an array of 127 pm thick cylindrical pillars cast from silicone rubber (TAP Silicone RTV Mold-Making System). The radial density of pillars increases towards the edge of the PCB for improved strength against delamination. The inventors employed a dense array of microns-wide pillars despite the added challenges in fabrication to retain sensitivity. Wider structures will increase the effective stiffness of the pillar layer, shown by equation 1 :

[0031] EeE 1 -J?”2) equation 1

[0032] Eeis the effective Young’s Modulus, E is the Young’s Modulus of the material, and r is the ratio between the height and radius of the pillars. Each rigid PCB is a multi-layered, with four quadrants of a pair of comb-shaped electrodes on one side and a plane of electrode for passive shielding on the other side (FIG. 2). The first sensing layer has two physical electrodes while the second

[0033] S24-339 / PCT 8 / 19 sensing layer houses eight physical electrodes. It is by leveraging this design with the unique capabilities of the microcontroller (Cypress, PSoCTM 4100S) on CoinFT, that a pair of PCBs become sensitive to 6-axis F / T, which will be discussed in detail infra. The (optional) passive shielding layer made of a 127um thick fPCB (flexible Printed Circuit Board) can then be added to provide additional passive shielding, as the via holes in the middle layer can introduce noise from stray capacitance in the environment (FIG. 2). Each PCB layer has structures for alignment on its sides, which is later removed during the final stages of fabrication.

[0034] A tab (10mm x Hmm) attached to the side of the circular sensing area houses the electronics necessary for capacitance sensing (FIG. 1). A microcontroller with external capacitors measures and converts capacitance into digital counts using its native capacitance-to-digital conversion (CDC) module at 360Hz. Signals can be acquired by other devices through direct serial communication or USB ports through a serial to USB converter, such as a PSoCTM KitProg2, with a latency of 2ms on average.

[0035] Working Principle

[0036] CoinFT measures 6-axis force and torque by observing the changes in capacitance signal patterns between the top layer and the middle layer (FIG. 1), which we now refer to as sensing layer. Capacitance is expressed as equation 2:

[0037] Equation 2 where C is capacitance, 8 is the dielectric constant, A is the overlapping area of the electrodes,

[0038] S24-339 / PCT 9 / 19 and d is the distance between the electrodes. During each sampling sequence, the sensing layer alternates between two configurations: a normal mode and a shear mode.

[0039] This reconfiguration is achieved by modifying the internal electronic switches in the microcontroller, which either connect or disconnect the electrodes from the microcontroller’s capacitance-to-digital conversion (CDC) module.

[0040] In the normal layer configuration, the pair of electrodes in each quadrant in the middle layer PCB are internally connected to function as a single electrode, effectively reducing the eight physical electrodes to four: Nl, N2, N3, and N4 (FIGs. 3A-B). The two electrodes in the top layer PCB are reconfigured to connect to ground. This design allows the normal layer to be sensitive to inputs that changes d, such as Fz, Mx, and My, but not to changes in A, which results from Fx, Fy, and Mz. Under Fz, all normal mode electrodes (Ni, N2, N3, and N4) show a rise in signal, while under Mxor My, a differential pattern between pairs of normal mode electrodes is created (FIGs. 3A-B).

[0041] In the shear mode configuration, each electrode in the middle layer PCB are kept as independent sensing electrodes (XI, X2, X3, X4, Yl, Y2, Y3, and Y4), while the electrodes in the top layer are configured such that the ground and shield electrodes form an alternating pattern (FIGs. 3A- B). The shield electrode actively mitigates interference from stray capacitance by being excited to the same electrical potential as the sensing electrodes, an innate capability of PSoCTM 4100S. This design allows the sensing layer to be sensitive primarily to shear inputs such as Fx, Fy, and Tz. Under a shear pressure, the relative position of the top layer and the middle layer shifts laterally, changing A between a sensing electrode and ground electrode. Due to the negligible capacitance formation between a sensing electrode and a shield electrode, the relative position

[0042] S24-339 / PCT 10 / 19 change results in a differential signal pattern in the shear layer. The comb shaped electrodes are populated in a way that allows the first and third quadrant of the shear layer is sensitive to loads in the X direction, while the second and fourth quadrants are sensitive to the Y direction. Under torsion (Tz), all four quadrants in the shear layer show a differential signal pattern. Unlike the normal layer configuration where there is negligible cross-modal sensitivity by design, there is cross-coupling in the shear layer configuration. The unique signal pattern generated from the normal and shear layer configurations upon loads in different axes makes CoinFT, a pair of PCBs, a 6-axis F / T sensor.

[0043] Sensor Fabrication

[0044] CoinFT is fabricated using the process illustrated in FIGs. 4A-J. A 127pm polyimide film pillar mask cut by a UV laser cutter (DPSS Lasers Inc., Samurai UV Marking System) is placed on an acrylic plate using dowel pins for alignment. Vacuum degassed uncured silicone rubber (TAP Silicone RTV Mold-Making System) is smeared on the surface of the mask to fill the cavities for pillars (FIG. 4A). Then, the top layer PCB primed using DOWSILTM PR- 1200 RTV Primer for enhanced adhesion is aligned and placed on the pillar mask. Another acrylic plate is placed on the assembly for equal pressure distribution and a 3.1kg mass is added for 3 minutes to minimize the thickness of the base layer of the silicone pillars (FIG. 4B). The 3kg mass is removed, and the pillar assembly, with a 100g mass added, is cast inside a pressure chamber for at least 12 hours at approximately 60psi to compress any remaining micro-bubbles inside the silicone rubber (FIG. 4C). Once the acrylic plates and the pillar mask are removed, the pillar layer is complete (FIGs.

[0045] 4D-E)

[0046] On the middle layer PCB, after the microcontroller and the electrical components are hand-

[0047] S24-339 / PCT 11 / 19 soldered, the sensing area is primed and applied with vacuum degassed uncured silicone rubber (FIG. 4F). Then, the silicone layer is made uniform and thin inside a spin-coater at 5000rpm for 75 seconds (FIG. 4G) The pillar layer is attached to the middle layer PCB with a 203um polyimide film spacer in between, to control the distance between the two PCBs. A 100g mass and an acrylic plate is placed on top of the assembly to apply equally distributed pressure, ensuring that the pillars and the spun coat silicone layer are in contact (FIG. 4H). The assembly is then cured inside a 65°C oven for 72 hours. After the alignment pins and spacers are removed, the bottom layer fPCB is adhered to the other side of the middle layer PCB using an adhesive (Henkel Corporation, Loctite 401 Instant Adhesive) (FIG. 41). The cross-section of the pillar assembly under a microscope is shown in FIG. 4 J). The total cost of all components is less than $10.

[0048] Sensor Characterization

[0049] A quantitative analysis using finite element analysis (FEA) and real sample testing has been conducted to understand the behavior of CoinFT. The FEA involved a structural analysis using ANSYS (FIG. 5H) to investigate the deformation of the pillar array under different loading conditions, and an electrostatics analysis in COMSOL (FIG. 51), to study the change in capacitance in the sensing electrodes resulting from pillar deformation. The pillar mechanics was modelled using a linear elastic model with a high deformation option to account for the geometric nonlinearities. The Gent model (Gent, “On the relation between indentation hardness and young’s modulus,” Rubber Chemistry and Technology, vol. 31, no. 4, pp. 896-906, 1958) was applied to convert the shore hardness of the silicone rubber (TAP Silicone RTV Mold-Making System) into Young’s modulus. With the assumption that the silicone rubber is incompressible, the effective Young’s Modulus of each pillar was derived using a disk model (Haddow et al. “Compression of bonded elastic bodies,” Journal of the Mechanics and Physics of Solids, vol. 36, no. 5, p. 551—

[0050] S24-339 / PCT 12 / 19 579, 1988) of an elastomer bonded between two plates, as shown in Equation 1. The dielectric constant of the silicone rubber (TAP Silicone RTV Mold-Making System) was assumed to be 3.0, a common value for silicone rubber, while the dielectric constant of air was taken as 1.0. To verify the reliability of our FEA model, the inventors compared the real and simulated response of CoinFT to normal force (FIG. 5D), where the load for the real sample was given up to ION at 0.1Hz. The FEA model closely matched the actual sample response, accurately capturing the stiffening effects due to pillar deformation at higher loads, and effectively representing the true behavior of CoinFT.

[0051] S24-339 / PCT 13 / 19

Claims

CLAIMSWhat is claimed is:

1. A method for sensing six degrees of freedom of force and torque, comprising:(a) providing a sensing assembly, wherein the sensing assembly comprises:(i) a first printed circuit board (first PCB) comprising comb-shaped first electrodes disposed in four quadrants on the first PCB, wherein the first PCB serves as a first sensing layer, and(ii) a second printed circuit board (second PCB) comprising comb-shaped second electrodes disposed in four quadrants on the second PCB, wherein the second PCB serves as a second sensing layer;(b) electrically coupling the first sensing layer and the second sensing layer to a microcontroller, wherein the microcontroller is configured with a capacitance sensing module, wherein the capacitance sensing module has two modes:(j) a normal mode comprising operating the microcontroller in a first configuration, wherein the first configuration comprises:(k) selecting multiple pairs of comb-shaped second electrodes in the second sensing layer, wherein each of the pairs is connected electrically to function as a combined second sensing electrode,(kk) connecting the comb-shaped first electrodes of the first sensing layer to an electrical ground, andS24-339 / PCT 14 / 19(kkk) acquiring first capacitance values from each of the combined second sensing electrodes to measure first force and torque components,(jj) a shear mode comprising reconfiguring the microcontroller to operate in a second configuration, wherein the second configuration comprises:(I) selecting comb-shaped second electrodes of the second sensing layer to be individually assigned as individual second sensing electrodes,(II) alternately connecting the comb-shaped first electrodes of the first sensing layer to the electrical ground and to an active shielding, and (111) acquiring second capacitance values from the individual sensing electrodes to measure second force and torque components;(c) repeating the normal mode and the shear mode, wherein in the repeating step the normal mode comprises selecting different multiple pairs of comb-shaped second electrodes; and (d) mapping the acquired first capacitance values and the acquired second capacitance values to a reference force and torque, wherein the mapping outputs the six degrees of freedom of force and torque.S24-339 / PCT 15 / 192. A sensor for sensing six degrees of freedom of force and torque, comprising:(a) a sensing assembly, wherein the sensing assembly comprises:(i) a first printed circuit board (first PCB) comprising comb-shaped first electrodes disposed in four quadrants on the first PCB, wherein the first PCB serves as a first sensing layer, and(ii) a second printed circuit board (second PCB) comprising comb-shaped second electrodes disposed in four quadrants on the second PCB, wherein the second PCB serves as a second sensing layer; and(b) a microcontroller electrically coupled to the first sensing layer and the second sensing layer, wherein the microcontroller is configured with a capacitance sensing module having embedded therewith two computer-encoded programs for operating modes, referred to as a normal mode and a shear mode, for generating normal mode capacitance values and shear mode capacitance values, which the two computer- encoded programs correlate to the six degrees of freedom of force and torque.

3. The sensor as set forth in claim 2, wherein the normal mode comprising computer- encoded programs for operating the microcontroller in a first configuration, wherein the first configuration comprises:(i) selecting multiple pairs of comb-shaped second electrodes in the second sensing layer, wherein each of the pairs is connected electrically to function as a combined second sensing electrode,S24-339 / PCT 16 / 19(ii) connecting the comb-shaped first electrodes of the first sensing layer to an electrical ground, and(iii) acquiring first capacitance values from each of the combined second sensing electrodes to measure first force and torque components.

4. The sensor as set forth in claim 2, wherein the shear mode comprising computer- encoded programs for reconfiguring the microcontroller to operate in a second configuration, wherein the second configuration comprises:(j) selecting comb-shaped second electrodes of the second sensing layer to be individually assigned as individual second sensing electrodes,(jj) alternately connecting the comb-shaped first electrodes of the first sensing layer to the electrical ground and to an active shielding, and(jjj) acquiring second capacitance values from the individual sensing electrodes to measure second force and torque components.

5. The sensor as set forth in claims 3 and 4, wherein the microcontroller further comprises:(a) computer-encoded programs for repeating the normal mode and the shear mode, wherein in the repeating step the normal mode comprises selecting different multiple pairs of comb-shaped second electrodes, andS24-339 / PCT 17 / 19(b) computer-encoded programs for mapping the acquired first capacitance values and the acquired second capacitance values to a reference force and torque, wherein the mapping outputs the six degrees of freedom of force and torque.S24-339 / PCT 18 / 19

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