Capacitive Force Sensor Research Articles

Design and FEA verification of high dimensional and multi-field smart soft material

Abstract We designed a sophisticated smart soft tissue material that simulates the properties of biological soft tissue and detects three-dimensional force. Inspired by electronic skin, this material uses capacitive pressure sensors and multi-layer sensor packaging. Finite element analysis and multi-field simulation were employed for structural design and verification. The study evaluated the theory and practicality of a flexible capacitive 3D force sensor for detecting pressure and shear force. Mechanical and electrical properties were confirmed through force and electric field simulations. The 3D force detection method proved feasible, achieving sensitivity levels of 10−4 kPa −1 in the X and Y directions, and 10−3 kPa −1 in the Z direction, demonstrating the effectiveness of our design.

A Passive Wireless Smart Washer for Locking Force Monitoring on the Orthopedic Pedicle Screw

A pedicle screw is a component for fixation in spine fusion surgery, often for patients with osteoporosis. Spine fusion condition highly depends on whether the pedicle screw is tightly fixed on spine, if not, spine fusion will not work properly. After the surgery, the first 3 months is the most crucial period, and bone healing situation cannot be shown through X-ray before the first radiologic images taken around the sixth week. Therefore, it is helpful to have a nonradiative method to monitor the locking force of the pedicle screw after surgery, especially during the early stage. Here a passive wireless force sensor is developed for monitoring the locking force of the pedicle screw, as we call it a smart washer. By integrating a capacitive ring-shape force sensor with an inductor, a passive LC sensor can be built by measuring the resonant frequency wirelessly. The smart washer is designed and calibrated to establish the relation between the locking force and resonant frequency, and then it is fixed with a screw in a porcine femur, with and without medium between the reader and inductor. When the locking force decreases from 8.3 to 0.9 N, the error is less than 0.5 N, and the maximum wireless sensing distance is 72 mm. However, the medium between the reader and the sensor inductor will affect the resonant frequency, but not the sensitivity. Therefore, the locking force variation can still be calculated by the resonant frequency shift accurately. Furthermore, by designing another five LC sensors with different operating resonant frequency ranges, it is possible to identify locking forces at different locations for more pedicle screws. To our knowledge, no LC force sensor was proposed to monitor the locking force of pedicle screws after surgery in the past.

Design and Testing of a New Microinjector With Capacitive Force Sensor for Biological Microinjection

Microinjection with force sensing plays an important role in delivering foreign materials into biological entities. In this paper, a new force-sensing microinjector with compliant mechanism is presented. It provides a high sensitivity of force sensing by offering a small stiffness in the direction of microinjection. Meanwhile, it enables a sufficient load-bearing capability thanks to a large stiffness in the lateral direction. The small stiffness is realized by the combination of a positive-stiffness mechanism and a negative-stiffness mechanism, rather than a zero-stiffness mechanism. A custom-made capacitive force sensor based on tilted micropillar array is introduced to measure the microinjection force based on the microinjector’s output displacement. Moreover, a prototype has been fabricated for experimental testing. Experimental results show that the sensitivity of the proposed design has been improved by two-fold over previous work. The developed microinjector has been applied to successfully detect the microinjection force during the injection of zebrafish larvae, which renders a promising solution for regulating the microinjection force. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —Force-assisted microinjection can provide force feedback, which is desirable to protect biological samples from excessive force and to judge successful penetration. Currently, most existing force sensor-based microinjectors can only puncture biological samples, rather than injecting materials, mainly due to the inability of bearing the weight of linker for the injection needle and actuator. In this work, a new microinjector with capacitive force sensor is presented for microinjection of zebrafish larvae. Based on the combination of a positive-stiffness mechanism and a negative-stiffness mechanism, a small-stiffness mechanism is introduced to design the microinjector. It simultaneously provides large axial sensitivity and large lateral loading ability. Moreover, a capacitive force sensor based on tilted micropillar array is devised to measure the microinjection force based on the overall displacement of the microinjector. For fixing zebrafhish larvae, a micro-array petri dish is fabricated by silicon wafer mould. Experimental results show that the microinjector is capable of delivering foreign materials and detecting the microinjection force, which is beneficial to the microinjection operation.

Displacement amplification mechanism in passive capacitive bolt axial force sensors to suppress manufacturing variations

Abstract We present a method of minimizing manufacturing variations in passive capacitive sensors for noncontact bolt inspection using a displacement amplification mechanism. This inspection mechanism uses electromagnetic coupling to eliminate the need for contact, making it particularly suitable for inspections using unmanned aerial vehicles. Analysis by the finite element method and prototype testing verified the capability of the mechanism to amplify displacement changes due to axial force. The prototype exhibited an 80% increase in displacement relative to the bolt head, effectively halving variation effects. Despite the design challenges, the mechanism significantly improved the effectiveness of the sensor.

Design, modelling and control of a textile-based wearable actuating system with sensor feedback for therapeutic applications

This work proposes a textile-dominated wearable actuating system utilizing textile force sensor feedback. The study explores the liquid/gas phase transition behaviour of low boiling point liquids to develop a thermally driven fluidic soft actuator. The research also focuses on obtaining feedback through capacitive textile force sensors and developing a feedback control law for a single actuator as well as sequential actuation of multiple actuators. The findings demonstrate that the proposed actuators produce the desired pressure level utilized in mechanotherapy applications. Moreover, high accuracy is achieved by the capacitive textile force sensors specifically designed for detecting the applied force exerted by the textile-based actuators. The developed system constitutes a comprehensive textile-based system encompassing heating, actuation and sensing capabilities. Following the calibration of the developed system in conjunction with its sensor, a pilot-scale implementation of sequential massage application was conducted to showcase the system’s capabilities and potential. Considering its pressure and heating properties, the developed system exhibits a great potential for utilization in mechanotherapy as well as in thermotherapy applications.

A BIOCOMPATIBLE GLASS-ENCAPSULATED TRIAXIAL FORCE SENSOR FOR IMPLANTABLE TACTILE SENSING APPLICATIONS.

This paper reports a microfabricated triaxial capacitive force sensor. The sensor is fully encapsulated with inert and biocompatible glass (fused silica) material. The sensor comprises two glass plates, on which four capacitors are located. The sensor is intended for subdermal implantation in fingertips and palms and providing tactile sensing capabilities for patients with paralyzed hands. Additional electronic components, such as passives and IC chips, can also be integrated with the sensor in a hermetic glass package to achieve an implantable tactile sensing system. Through attachment to a human palm, the sensor has been shown to respond appropriately to typical hand actions, such as squeezing or picking up a bottle.

Rational Design of Flexible Mechanical Force Sensors for Healthcare and Diagnosis.

Over the past decade, there has been a significant surge in interest in flexible mechanical force sensing devices and systems. Tremendous efforts have been devoted to the development of flexible mechanical force sensors for daily healthcare and medical diagnosis, driven by the increasing demand for wearable/portable devices in long-term healthcare and precision medicine. In this review, we summarize recent advances in diverse categories of flexible mechanical force sensors, covering piezoresistive, capacitive, piezoelectric, triboelectric, magnetoelastic, and other force sensors. This review focuses on their working principles, design strategies and applications in healthcare and diagnosis, with an emphasis on the interplay among the sensor architecture, performance, and application scenario. Finally, we provide perspectives on the remaining challenges and opportunities in this field, with particular discussions on problem-driven force sensor designs, as well as developments of novel sensor architectures and intelligent mechanical force sensing systems.

Soft Three-Axis Capacitive Force Sensor for Robotic E-Skin on Curved Surfaces

Soft Three-Axis Capacitive Force Sensor for Robotic E-Skin on Curved Surfaces

Investigating capacitive force sensors with 3D printed flexible structures as dielectric layers

The utilization of a three dimension (3D) printing for the fabrication of micro-electromechanical system (MEMS) sensors presents various advantages including low cost, customization, simplicity, and speed. This study involved conducting mechanical and electrical analyses on three types of dielectric layer structures using the finite element method. Capacitive force sensors were produced through 3D printing using fused deposition modeling (FDM). The sensitivity and stiffness of the sensors were determined by measuring their capacitance and displacement changes under force using a self-made measurement system. The experimental results indicate that pre-heating the dielectric layer can reduce the hysteresis effects. In terms of more complex structures, stringing phenomena during the printing process can compromise the reliability of the sensor. A suitable dielectric layer structure can be designed based on the measurement range and sensitivity of the sensor. The results of this study demonstrate the potential for additive manufacturing to replace conventional MEMS processes in sensor production. With the advancement of FDM printing technology, it is increasingly feasible to apply 3D printing technology to sensor manufacturing in the future.

A High-Resolution MEMS Capacitive Force Sensor With Bionic Swallow Comb Arrays for Ultralow Multiphysics Measurement

Precise force sensing is essential for the mechanical characterization and robotic micromanipulation of biological targets. In this work, a high-resolution MEMS capacitive force sensor was proposed for measuring ultralow multiphysics. A bionic swallow structure that contained multiple feathered comb arrays was designed for reducing chip dimension and eliminating undesirable mechanical cross-coupling effect. The comb structure was optimized for maximum sensitivity, linearity, and compact chip size. Utilizing a novel interconnection configuration, interferences derived from parasitic capacitance and electrostatic forces exerted negligible effects on the sensor output. The proposed bionic force sensor was fabricated following a simple three-mask process and integrated with ASIC readouts. Its measuring sensitivity was 7.151 fF/nm, 0.529 aF/nN, and 4.247 pF/g for displacement, force, and inclination measurements, respectively. The proposed sensor had a large measurement range of 1000.00 nm and 13.83 µN with a high linearity of 0.9998. The 1-σ resolution was 0.0328 nm and 0.4436 nN, and the noise floor resolution was 0.0044 nm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt {{\bm{Hz}}} $</tex-math></inline-formula> and 0.0597 nN/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt {{\bm{Hz}}} $</tex-math></inline-formula> for displacement and force measurements, respectively. The bias stability of Allan deviance was 0.0050 nm and 0.0678 nN at an integration time of 0.65 s. The proposed bionic swallow sensor exhibited considerable improvement over existing capacitive sensors and feasibility for ultralow multiphysics measurement in biomedical applications.

All-printed flexible capacitive array tactile force sensors with tunable sensitivity and low crosstalk for micro motion detection

All-printed flexible capacitive array tactile force sensors with tunable sensitivity and low crosstalk for micro motion detection

Force Sensing and Feedback System Based on Novel Triaxial Force Capacitive Sensor for Minimally Invasive Surgical Robot

AbstractMinimally invasive surgery has attracted great attention due to small trauma, light pain, and quick recovery. Currently, most minimally invasive surgical robots lack the ability to force sense, resulting in high risks. Herein, a novel minimally invasive surgical force sensing and feedback system for minimally invasive surgical robot is proposed based on a flexible triaxial force capacitive sensor array. The capacitive force sensors utilize a microstructure electrode and orthogonal triangular pyramid microstructure to tackle the trade‐off between high sensitivity and wide‐detection range, showing 0–3 N detection range for normal force and high sensitivity of 69.19% N−1. Furthermore, the triaxial capacitive force sensors are integrated into the end‐effector of the minimally invasive surgical robot to form the minimally invasive surgical force sensing system. The system can show real‐time position of gripping or touching action, and the magnitude of triaxial force on the display surface. Importantly, the sensing force can further control the movement of the clamp, thus forming a novel force sensing and feedback system. The force sensing and feedback system of this minimally invasive surgical robot lays the foundation for its application in minimally invasive surgery andis expected to improve the safety and success of robotic‐assisted minimally invasive surgery.

A high sensitivity and multi-axis fringing electric field based capacitive tactile force sensor for robot assisted surgery

This paper presents a design of multi-axis tactile force sensor using the fringe effect of an electric field between stationary patterned electrodes. The unique configuration of the electrodes consisting of four separate square-shaped sensing electrodes, with each encircled by excitation electrodes, allows to achieve enhanced fringe field effect and hence sensitivity. The proposed sensor can decouple the normal, shear and angular shear applied forces. The sensor is fabricated using low-cost rapid prototyping techniques with flexible Ecoflex 00–30 and silicone rubber RTV-528 as the elastomers for contact with the environment. An analytical model is developed that correlates the nominal capacitance of the sensor with that of the geometric dimensions of the stationary electrodes and air cavity height between the electrodes and elastomer. The force measurement ranges in the normal, shear, and angular axis are 5 N, 1.5 N, and 1 N respectively. The sensor shows a perfectly linear response, repeatability, and a low hysteresis error, thermal stability and robustness to the environmental interferences that makes it suitable to be used for force feedback in minimally invasive robotic surgery.

MEMS Shielded Capacitive Pressure and Force Sensors with Excellent Thermal Stability and High Operating Temperature.

In this paper, we present an innovative manufacturing process for the production of capacitive pressure and force sensors with excellent thermal stability for high-temperature applications. The sensors, which are manufactured from a stack of two silicon chips mounted via with gold-silicon (Au-Si) or aluminum-silicon (Al-Si) eutectic bonding, are shielded, miniaturized, and allow an operating temperature of up to 500 °C. Compared to conventional methods, the greatest benefit of the manufacturing process is that different sensor dimensions can be produced in the same batch for a wide measuring range, from mN to kN. The characterization of the realized sensors shows a high linearity and a low temperature drift of 99.992% FS and -0.001% FS/K at 350 °C, as well as a nonlinearity of 0.035% FS and a temperature drift of -0.0027% FS/K at 500 °C.

A Calculation Method for the Optimal Initial Polar Distance of Capacitive Force Sensors

For capacitive sensors, the initial polar distance of capacitors has a vital influence on the performances of capacitive sensors; however, there are few current studies on the method of calculating the initial polar distance of capacitors, leading to a lack of corresponding theoretical basis for the design of capacitive sensors. In this article, a calculation method for the optimal initial polar distance based on minimum error is proposed. The total error expression is derived from error analysis, which clarifies the complex relationship between the total error and the initial polar distance, and the optimal initial polar distance is solved by Newton’s method. A capacitive torque sensor is used in the experiment to verify the calculation method for the optimal initial polar distance, the theoretical error and experimental error of the sensor are compared, and the results show that the experimental error is consistent with the theoretical error curve. The optimal initial polar distance obtained by the calculation method is 0.8479 mm, which is within the range of 0.8–0.9 mm determined by the experiment. The theoretical minimum total error is 0.211% and the experimental minimum total error is 0.208%. The total error expression proposed in this article is accurate, and the calculation method of the initial polar distance is effective. In addition, theoretical analysis reveals that the differential parallel plate capacitor has the characteristic that any small deformation of the moving plate can be equivalent to a translation whose magnitude is the mean value of the deformation.

ForceSticker

Any two objects in contact with each other exert a force that could be simply due to gravity or mechanical contact, such as any ubiquitous object exerting weight on a platform or the contact between two bones at our knee joints. The most ideal way of capturing these contact forces is to have a flexible force sensor which can conform well to the contact surface. Further, the sensor should be thin enough to not affect the contact physics between the two objects. In this paper, we showcase the design of such thin, flexible sticker-like force sensors dubbed as 'ForceStickers', ushering into a new era of miniaturized force sensors. ForceSticker achieves this miniaturization by creating new class of capacitive force sensors which avoid both batteries, as well as wires. The wireless and batteryless readout is enabled via hybrid analog-digital backscatter, by piggybacking analog sensor data onto a digitally identified RFID link. Hence, ForceSticker finds natural applications in space and battery-constraint in-vivo usecases, like force-sensor backed orthopaedic implants, surgical robots. Further, ForceSticker finds applications in ubiquiti-constraint scenarios. For example, these force-stickers enable cheap, digitally readable barcodes that can provide weight information, with possible usecases in warehouse integrity checks. To meet these varied application scenarios, we showcase the general framework behind design of ForceSticker. With ForceSticker framework, we design 4mm*2mm sensor prototypes, with two different polymer layers of ecoflex and neoprene rubber, having force ranges of 0-6N and 0-40N respectively, with readout errors of 0.25, 1.6 N error each (&lt;5% of max. force). Further, we stress test ForceSticker by &gt;10,000 force applications without significant error degradation. We also showcase two case-studies onto the possible applications of ForceSticker: sensing forces from a toy knee-joint model and integrity checks of warehouse packaging.

A New Force‐Decoupling Triaxial Tactile Sensor Based on Elastic Microcones for Accurately Grasping Feedback

Tactile sensor capable of detecting both normal and tangential forces is essential for the next‐generation robotic electronics. However, designing a reliable triaxial tactile sensor to decouple the normal and tangential forces is still a challenge for current research. Herein, a capacitive triaxial force sensor based on a microcone dielectric layer together with the force decoupling method is proposed. The lateral and vertical displacement of the top electrode relative to the bottom electrodes under the action of external forces can be perfectly detected according to the capacitance variations. The microcone‐structured sensor combined with the new decoupling method leads to a high sensitivity of 3.5 kPa−1 for normal force in a low‐pressure range of 0––50 Pa and 0.134 N−1 for tangential force in 0–0.5 N, and a fast response time of 26 ms. The sensors mounted on the fingers of a robotic arm can provide effective tactile information during the grasping task. Finally, a conceptual 3 × 3 tactile sensor array is fabricated and verified for spatial pressure mapping. This work provides new insight into piezocapacitive tactile sensor design and shows its great potential in robotic applications.

A Viscoelastic Compensator for Force Sensors With Soft Materials

Model-based compensators have been widely applied in force sensors with soft materials to eliminate viscoelasticity that causes creep output, slow dynamic response, and limits the performance of the sensors. However, the lack of accurate characterization of high-order viscoelasticity and the high computational complexity restricts the performance and application of existing methods. For the above problem, we propose a first-order Volterra grey box model-based viscoelastic compensator for force sensors with soft materials. First, the measurement mechanism of the soft-material-based force sensor is analyzed, and a grey box model for accurately characterizing the high-order linear viscoelasticity by the first-order Volterra series is proposed, without needing high-order spring-damping parameters and their exponential operations. Then, the high-order viscoelasticity compensator is designed and directly linearly fitted with the variation of the historical output, which avoids the computational complexity of the high-order creep compliance and its exponential parameters fitting. Finally, the proposed method is verified with a self-developed soft-material-based capacitive force sensor. The real-time experimental results show that, compared with existing methods, the proposed compensator not only has better compensation performance, but also has a shorter calculation time for force sensors with soft materials.

Indirect measurement of cutting forces during robotic milling using multiple sensors and a machine learning-based system identifier

Robotic machining has attracted great interest in the fields of aerospace and automotive manufacturing because of its advantages over conventional computer numerical controlled (CNC) machine tools; robotic machining is flexible and can fabricate components with complex (sculpted) geometries and of large dimensions. The principal challenge, however, is the poor structural rigidity of robots, leading to high-level vibration and chatter during machining. The cutting force must be monitored to detect and suppress chatter as well as optimize machining. A practical method is required to measure the cutting force during machining without impeding robot movement. This study presents an indirect method that accurately measures the milling force using an accelerometer located on the spindle and a flange-mounted capacitive force sensor. A Kalman filter was used to compensate for the dynamics between the robot tool center point (TCP) and the location of the accelerometer. In the context of pose-dependent modeling of the dynamics of the robot, a novel machine-learning (ML)-based identifier is presented that rapidly identifies the relevant dynamics (i.e., the modal parameters) without any initial human guesswork. Data from dynamic simulations and experiments were used to train and validate the ML-based identifier. The validation results showed that the identifier rapidly predicted the behaviour of physical systems with an average accuracy of 98.49 % compared to the modal analysis method. Next, the cutting forces were indirectly measured using the acceleration signals and a Kalman filter, developed based on the modal parameters. The forces measured by the proposed method were in good agreement with the reference forces, measured using a table dynamometer; the average peak-to-peak accuracy was 90.17 % for the AC component of the cutting force, and the average time-domain accuracy was 95.27 % when the DC component was also considered.

Printed, wearable e-skin force sensor array

The need to develop electronic skin (e-skin) for robotics is essential to provide a sense of touch on a large area, the same as human skin. This paper reports the design and development of printed e-skin force sensor arrays. The performance analysis of these capacitive force sensor arrays showed a resolution of 2.5 sq. mm, localization accuracy of 99% and a sensitivity of 0.53 pF/N. The sensor patches provided significant force sensitivity and stable loading and unloading for static force. A graphical user interface is developed in open-source python software. An unsupervised machine learning algorithm trains the tactile e-skin system for different user styles to distinguish the cluster of lower and higher touch angles for improving accuracy. The proposed e-skin sensor finds application in large-area sensors and gripper pads for prosthetics in biomedical devices where the sense of touch and high resolution is needed.