Flexible Force Sensor Research Articles

A Novel Wearable Soft Glove for Hand Rehabilitation and Assistive Grasping.

In order to assist patients with finger rehabilitation training and grasping objects, we propose a new type of soft rehabilitation gloves (SRGs), which has both flexion/extension and abduction/adduction movement function for every finger. This paper describes the structure design of the bending actuator and rotating actuator, the fabrication process of the soft actuator, and the implementation of the soft wearable gloves based on a fabric glove. FEM simulation analysis and experiments were conducted to characterize the mechanical behavior and performance of the soft glove in terms of the angle output and force output upon pressurization. To operate this soft wearable glove, we designed the hardware system for SRGs with a flexible strain sensor and force sensor in the loop and introduced a force/position hybrid PID control algorithm to regulate the pressure inputted. Experiment evaluation focused on rehabilitation training gestures; motions and the precise grasping assistance function were executed. The rotating actuator between each finger can supply abduction/adduction motion manner for patients, which will improve rehabilitation effect. The experimental results demonstrated that the developed SRGs have the potential to improve hand movement freedom and the range of grasping successfully.

Flexible capacitive three-dimensional force sensor for hand motion capture and handwriting recognition

Handwriting recognition has been widely studied as an important signal transmission method in the field of human–computer interaction. Flexible capacitive three-dimensional (3D) force sensor is widely used in this field because of its high sensitivity, large dynamic response range and good mechanical stability. However, traditional flexible capacitive 3D force sensors based on the array structure usually only sense 3D force with equivalent forward pressure rather than the decoupling of real 3D force in space. Herein, we design a flexible capacitive 3D force sensor with petal-like electrode to capture various hand movements in 3D space and further decouple 3D force to 1D force along [Formula: see text], [Formula: see text] and [Formula: see text] directions. The sensor presents a high sensitivity (1.1 kPa[Formula: see text]at the pressures below 1 kPa), a fast response (63 ms) and excellent repeatability under continuous pressure. In addition, the electrode with eight-petal structure helps simplify the decoupling process of 3D force. Through handwriting test, the sensor presents good sensing performance for hand motion capture and handwriting recognition, which is expected to be applied in the field of human–computer interaction.

A flexible tactile sensor that uses polyimide/graphene oxide nanofiber as dielectric membrane for vertical and lateral force detection

Flexible force sensors are of great interest in the fields of healthcare, physiological signals, and aircraft smart skin applications because of their compatibility with curved surfaces. However, the simultaneous detection of multidirectional forces remains an engineering challenge, despite the great progress made in recent years. Herein, we present the development of a flexible capacitive force sensor capable of efficiently distinguishing normal and sliding shear forces. A two-layer electrospun polyimide/graphene oxide (PI/GO) nanofiber membrane is used as the dielectric layer, which is sandwiched between one top electrode and four symmetrically distributed bottom electrodes. This composite membrane has an improved dielectric constant, a reduced friction coefficient, and good compressibility, leading to superior performance that includes high sensitivity over a wide operational range with measured results of 3 MPa−1 for 0–242 kPa (0–2.2 N) and 0.92 MPa−1 for 242–550 kPa (2.2–5 N) in the normal direction; and better than 1 N−1 for 0–3 N in the x- and y-axis directions. The system also has a low detection limit of 10 Pa, fast response and recovery times of 39 ms and 13 ms, respectively, a good cyclic stability of 10,000 cycles at a pressure of 176 kPa, and promising potential for use in high-temperature environments (200 °C). Moreover, a prototype 4 × 4 sensor array has been fabricated and successfully used in a robotic system to grasp objects and operate a wireless toy car. As such, the proposed system could offer superior capabilities in simultaneous multidirectional force sensing for applications such as intelligent robots, human–machine interaction, and smart skin.

Graphene piezoresistive flexible MEMS force sensor for bi-axial micromanipulation applications

Graphene piezoresistive flexible MEMS force sensor for bi-axial micromanipulation applications

Evaluation of a 3D Printed Soft Sensor for Measuring Fingertip Interaction Forces

Current force sensors used to capture fingertip interaction forces lack compliance to the fingertip tissue resulting in the loss of touch sensation of the user. 3D printing offers the possibility to create personalized soft sensing structures. This work evaluates a 3D printed soft sensor that measures normal and shear interactions forces based on the deformations of the thumb and index fingertips of 7 subjects using an instrumented object. Due to the use of (carbon doped) thermoplastic materials, the signals provided by these sensing structures suffer from nonlinearities. Therefore, two compensation models, based on a neural network and recurrent neural network analogous to an electrical model are used to compensate for the nonlinear effects. The performance of the sensors was analysed using the normalized cross-correlation and the root-mean-square error. The output of the force sensors are highly correlated with the applied shear and normal force components. When paired with compensation models the correlation and error of the sensor output can be further improved. These results indicate that the proposed flexible fingertip interaction force sensors have a high potential for future applications.

Flexible tensile strain-pressure sensor with an off-axis deformation-insensitivity

The high demand for flexible force sensors with both strain and pressure sensing has attracted considerable attention for various application scenarios, such as electronic skins and smart prostheses. However, successful application of these sensors in real-world is challenging because the performance of the sensors can be severely degraded under applied off-axial deformations (e.g., bending and twisting) and it is also difficult to successfully decouple these signals due to electromechanical crosstalk. Here, we developed an integrated sensor patch (ISP) consisting of a strain sensor insensitive to pressure, bending and twisting, coupled with a pressure sensor insensitive to tension, bending and twisting. Benefiting from the serpentine structure and bionic design of strain sensor material, as well as the inherent rigidity properties of the piezoelectric ceramic, the resulting patch exhibits insensitivity to off-axis sensing with independent tensile strain and out-of-plane pressure sensing capabilities. The patch achieves a wide range of tensile strains (up to 160% with an apparent pressure coefficient of over 1.23) and a wide range of pressures (1 Pa to 100 kPa). We demonstrate that the wearable ISP can interact human gestures with the robotic hand in real time through a fully soft integrated glove and further continuously record wrist activity to verify tensile strain sensing independent of pressure mode. In addition, the ISP successfully captures the subtle strain of less than 20% of eye blinks and the ultra-low pressure of less than 1.2 kPa of unobtrusive exhalation force with low off-axis interference. Moving forward, this strategy of endowing sensors with off-axis sensing insensitivity and decoupled strain and pressure sensing has great potential in human-machine interface (HMI), virtual reality (VR)/augmented reality (AR) user interfaces, and other soft electronics fields.

The behaviour of different design of flexible force sensor based velostat during implementation of static load with different contact area

Great attention has been given to flexible force sensors and the materials that have been used as a sensing material by studying their properties and evaluate their behaviours with different types of loads. Velostat is one of the most promising sensing materials that has been widely studied and evaluated for different applications due to their distinct traits of flexibility, suitable cost as well as their ability to cover the wearable applications. This work makes focus on the behaviour of Velostat flexible force sensor by measuring the resistivity of the sensor while applying loads with different contact area. Two different designs of sensor have been studied, which are the single output flexible force sensor and multioutput matrix flexible force sensor, by applying loads starting from 0.98 (N) to 9.8 (N) with four compression disc having different diameters. It has been found that the two design behave in opposite manner. The resistance of the single output design decreased as the contact area increased, while the matrix design showed an increasing in the resistance as the contact area increased.

Flexible six-dimensional force sensor inspired by the tenon-and-mortise structure of ancient Chinese architecture for orthodontics

Precise multi-axis operation is essentially required in orthodontics for tooth movement. Despite the development of flexible multi-dimensional force sensors that have effectively perceived multi-dimensional forces, they still face the challenge of simultaneously 3D force and moment in a single flexible force sensor. Six-dimensional force perception can successfully operate objects that critically rely on directional tracking and accurate monitoring of complex multi-axis stimuli. To realize the integration of sensing units with the perception of six-dimensional force under even a soft touch, we design a flexible six-dimensional force sensor with tenon-and-mortise interlocking structures inspired by traditional Chinese ancient architecture. This unique structure enables conjunction of deformation, which can be studied and decoded by Deep Neural Networks (DNN) with six-dimensional force, including forces and rotating moments in the x, y, and z directions. This soft sensor with minimal size (7 × 7 × 7 mm3) and high detection accuracy (the DNN error is below 10−4) can be used in orthodontic treatment for precise correction with a full collection of orthodontic force. This unique flexible six-dimensional force sensor provides a new strategy for the design of multi-dimensional force sensors, paving the way for the development of intelligent robotics, interactive human-machine interfacing, and advanced prosthetics.

Direct ink write multi-material printing of PDMS-BTO composites with MWCNT electrodes for flexible force sensors

With recent advances of additive manufacturing technology, direct ink write (DIW) printing has allowed to incorporate multi-material printing of various materials with freedom of design and complex geometric shapes to complete functional sensors in a one-step fabrication. This paper introduces the use of DIW 3D printing of polydimethylsiloxane (PDMS) with barium titanate (BTO) filler as stretchable composites with tunable piezoelectric properties that can be used for force sensors applications. To improve the bonding between stretchable piezoelectric composites and electrodes, multi-walled carbon nanotubes was included in the fabrication of electrodes at a fixed ratio of 11 wt. %. The alignment of the BTO dipoles was achieved through corona poling method, which applies an electric charge on the surface layer of the functional material, aligning the dipoles in the desired direction and thus gaining the piezoelectricity. Different BTO mixing ratios (10–50 wt. %) were evaluated in order to obtain tunable piezoelectric properties and compare the sensitivity with respect their elastic properties. Tensile testing and piezoelectric testing were carried out to characterize mechanical and piezoelectric properties. Results showed that fabricated PDMS with 50 wt. % BTO gave the highest piezoelectric coefficient (d 33) of 11.5 pC N−1 and with an output voltage of 385 mV under compression loading of >200 lbF. This demonstrates feasibility of using multi-material DIW printing to fabricate piezoelectric force sensors with integrated electrodes in one-step without compromising the flexibility of the material.

Fully Printed Flexible Capacitive Array Tactile Force Sensors with Tunable Sensitivity and Low Crosstalk for Monitoring Human Motion and Sound Signals

Fully Printed Flexible Capacitive Array Tactile Force Sensors with Tunable Sensitivity and Low Crosstalk for Monitoring Human Motion and Sound Signals

Contact Force Measurement Approach for Measuring Glove-Skin Interfacial Pressure

Pressure garments for medical had been widely used to enhance the recovery after an operation, for treatment of arthritis, deep vein thrombosis, leg ulcers, varicose veins or hypertrophic scars. This research aims to develop a contact force measuring approach to measure glove-skin interfacial pressure. With the aid of a force measuring sensors system, it is easier to study the efficiency of pressure garments. Besides, occupational therapists can also design a user-friendly pressure garment that is comfortable and easy to donning and doffing, especially for children. Two types of flexible force sensors were used to measure the glove-skin interfacial pressure using two types of pressure therapy gloves. Pressure therapy glove is selected to represent pressure garments. The measuring sensors were placed at three different locations on the dorsal side of the hand, and the glove-skin interfacial pressure was measured in two different postures, which is full fist and tabletop posture. It was found that different postures had led to different interfacial pressure. The glove-skin interfacial pressure was found to be greater during full fist posture compared to tabletop posture. This show that the applied glove-skin interfacial pressure is affected by postures and movement.

Forces and translation distance during an inferior glide of the shoulder in asymptomatic individuals measured with the novel pliance glove and ultrasound imaging

BackgroundThere has been limited research on how the variance of force affects manual therapy outcomes and what the best practices should be. No specific force threshold necessary to achieve a predetermined translational distance within the joint has been quantified within the literature. PurposeTo quantify the amount of force necessary to perform an inferior glide to the glenohumeral joint and reach to end range. A secondary aim was to determine the impact of co-variables, such as gender, height, weight, and age, on the amount of force required to translate the humeral head within the glenohumeral joint. MethodsA convenience sample of 64 healthy subjects were recruited. Musculoskeletal ultrasound imaging using the GE LogiQe was used to measure the translation of the humeral head. The manipulation force was measured using the novel pliance glove device and software. The ANOVA was used to determine if there was a difference in translation distance and force between trials. The Pearson's correlation was used to correlate translation and force and between covariables. ResultsThere was no significant difference in translation distance between trials (p = .14). There was no significant difference in the mean force for this translation (p = .45). There was a poor correlation between age and force (r = 0.28) and weight and force (r = 0.12). ConclusionAn average force of 14.27 N (n = 61) was needed to displace the humeral head to reach end range. This was the first study using the combination of a flexible force sensor technology and real-time ultrasound imaging to measure humeral head translation.

Flexible three-dimensional force sensor of high sensing stability with bonding and supporting composite structure for smart devices

Characteristics, such as large measurement range, good repeatability and stability, are the guarantee for a wide application of flexible three-dimensional force sensors in robots and other smart devices. In this research, a flexible three-dimensional force sensor is presented, whose structure includes force-sensitive cells, fixed under the bump and a support layer, filled around the force-sensitive cells. This setup allows the sensor to quickly recover, after the force is released, to ensure its excellent performance. The production method is very simple and has the advantage of large-scale and low-cost manufacturing. The sensor exhibited good repeatability and stability, after 25 000 cycles of loading and releasing, under 8 N force. Meanwhile, it has a large measuring range, which is 0–15 N in the direction of normal force and 0–5.5 N in the direction of shear force. The sensitivity of the sensor in the x, y, and z directions is 1.5321 V−1 N−1, 1.501 29 V−1 N−1, and 0.936 87 V−1 N−1, respectively. The load response time of the sensor is 33 ms and the unload recovery time of 18 ms. Regarding practical applications, the sensor shows good prospects. For example, the sensor and its array can be installed on the manipulator to sense the magnitude and location of the three-dimensional force; in the case of the flexible smart keyboard, the presented sensor array can save space significantly and recognize multiple specific input features, in order to enhance information security; while as a car controller, it is multi-functional integrated and portable, thus showing great potential as replacement to the traditional operating methods.

Novel synthetic iron (III) oxide-based force sensor

Recent progress observed in 'powdered composites', considered as advanced electrical inductance materials, brought some promising applications in flexible tactile, pressure and force sensors, which can be based on compressed metal oxide powder mass materials. Therefore, in this paper the evaluation of the resistance of force-sensitive material – synthetic iron (III) oxide powder – by implementing this material into the design of two force sensors is described. Two types of sensors with different external (height, diameter, construction) parameters were investigated. Sensor responses were evaluated by performing tests in static, long-term load and cyclic modes. Experimental results of loading cycle measurements revealed the hysteresis and nonlinearity of the sensors. Obtained sensors sensitivity, hysteresis, response time, and deformation dependencies on the applied compressive force promises a practical possibility to use the synthetic iron (III) oxide powder composite as sensitive material for force and pressure sensors. Developed sensors have simple construction, very high mechanical resistance, are very resistant to wear and environmental impact and can operate at high temperatures. The results of experimental research have defined the area of the possible implementation of the sensors based on iron (III) oxide powders. The active substance in the form of a powder is especially relevant in mechanical systems and robotics. Size and shape of powders can be easily adjusted to the working area parameters, the operating temperature range (can be up to 600 °C), and other industrial needs.

Flexible piezoresistive three-dimensional force sensor based on interlocked structures

Three-dimensional force sensing has attracted increasing attention due to the rising application requirements from smart robots. Herein, we report a flexible three-dimensional force sensor with an interlocked structure as bionic human skin, which consists of flexible silicone substrate and polymer composite materials. Surface piezoresistive layer is patterned through electrospray and laser direct writing. The sensing mechanism is based on the contact resistance changes from three interlocked hemispheres units from either the normal force or the shear force. Shear force directional detection is rendered through three-axis structural arrangement in-plane. As a result, the sensor displays a normal force sensitivity of 6.33 kPa−1, response time of 59 ms, and shear force angular resolution of below 30°. The manufacturing method is amenable for minimized sensor array fabrication. Further application is demonstrated with a mechanical arm for gripping objects, where the sensor provides in-time feedback for precise grasping.

Development and Grasp Stability Estimation of Sensorized Soft Robotic Hand.

This paper introduces the development of an anthropomorphic soft robotic hand integrated with multiple flexible force sensors in the fingers. By leveraging on the integrated force sensing mechanism, grip state estimation networks have been developed. The robotic hand was tasked to hold the given object on the table for 1.5 s and lift it up within 1 s. The object manipulation experiment of grasping and lifting the given objects were conducted with various pneumatic pressure (50, 80, and 120 kPa). Learning networks were developed to estimate occurrence of object instability and slippage due to acceleration of the robot or insufficient grasp strength. Hence the grip state estimation network can potentially feedback object stability status to the pneumatic control system. This would allow the pneumatic system to use suitable pneumatic pressure to efficiently handle different objects, i.e., lower pneumatic pressure (50 kPa) for lightweight objects which do not require high grasping strength. The learning process of the soft hand is made challenging by curating a diverse selection of daily objects, some of which displays dynamic change in shape upon grasping. To address the cost of collecting extensive training datasets, we adopted one-shot learning (OSL) technique with a long short-term memory (LSTM) recurrent neural network. OSL aims to allow the networks to learn based on limited training data. It also promotes the scalability of the network to accommodate more grasping objects in the future. Three types of LSTM-based networks have been developed and their performance has been evaluated in this study. Among the three LSTM networks, triplet network achieved overall stability estimation accuracy at 89.96%, followed by LSTM network with 88.00% and Siamese LSTM network with 85.16%.

Endoluminal Motion Recognition of a Magnetically-Guided Capsule Endoscope Based on Capsule-Tissue Interaction Force.

A magnetically-guided capsule endoscope, embedding flexible force sensors, is designed to measure the capsule-tissue interaction force. The flexible force sensor is composed of eight force-sensitive elements surrounding the internal permanent magnet (IPM). The control of interaction force acting on the intestinal wall can reduce patient’s discomfort and maintain the magnetic coupling between the external permanent magnet (EPM) and the IPM during capsule navigation. A flexible force sensor can achieve this control. In particular, by analyzing the signals of the force sensitive elements, we propose a method to recognize the status of the motion of the magnetic capsule, and provide corresponding formulas to evaluate whether the magnetic capsule follows the motion of the external driving magnet. Accuracy of the motion recognition in Ex Vivo tests reached 94% when the EPM was translated along the longitudinal axis. In addition, a method is proposed to realign the EPM and the IPM before the loss of their magnetic coupling. Its translational error, rotational error, and runtime are 7.04 ± 0.71 mm, 3.13 ± 0.47, and 11.4 ± 0.39 s, respectively. Finally, a control strategy is proposed to prevent the magnetic capsule endoscope from losing control during the magnetically-guided capsule colonoscopy.

Aerosol-jet-printed, conformable microfluidic force sensors

SummaryForce sensors that are thin, low-cost, flexible, and compatible with commercial microelectronic chips are of great interest for use in biomedical sensing, precision surgery, and robotics. By leveraging a combination of microfluidics and capacitive sensing, we develop a thin, flexible force sensor that is conformable and robust. The sensor consists of a partially filled microfluidic channel made from a deformable material, with the channel overlaying a series of interdigitated electrodes coated with a thin, insulating polymer layer. When a force is applied to the microfluidic channel reservoir, the fluid is displaced along the channel over the electrodes, thus inducing a capacitance change proportional to the applied force. The microfluidic molds themselves are made of low-cost sacrificial materials deposited via aerosol-jet printing, which is also used to print the electrode layer. We envisage a large range of industrial and biomedical applications for this force sensor.

Electromechanical sorting method for improving the sensitivity of micropyramid carbon nanotube film flexible force sensor

As one of the most concerned sensitive materials, carbon nanotubes (CNTs) are widely used in flexible sensors for wearable electronic devices. Here, we present an electromechanical sorting method that applies to micropyramid single-walled CNT (SWNT)-film flexible force sensor to further enhance its sensitivity. The principles of the method are as follows. When the micropyramid structure is under pressure, stress concentration will occur at its tip, and the resistivity of the SWNT film coated on the surface of the micropyramid structure will change remarkably, which leads to changes in the resistance of the SWNTs. The resistance of quasimetallic SWNTs increases greatly under pressure, whereas that of metallic SWNTs increases minimally (band theory). Therefore, when the threshold voltage was applied to SWNTs, the metallic SWNTs exposed in the air were oxidized and failed when the current induced defects in the graphite layer, whereas the quasimetallic SWNTs with a high gauge factor remained functional. The experimental results showed that the sensitivity of the sorted SWNTs flexible force sensor was 3.4891 kPa−1, which denotes an improvement of 56.86% compared with the initial unsorted devices. In addition, the flexible force sensor exhibited low power consumption, which is less than a microwatt, and good repeatability and stability. Finally, the sensor was successfully used in the knuckles of a person's fingers to monitor finger motions.

A Review on 3D printed force sensors

A force sensor plays major role in robotics, wearable electronics and meteorological applications. Three dimensional (3D) printing is an exceptional technology and provides easy and economical fabrication process for the force sensors. Some salient features of 3D printing and development of force sensors in recent times have been represented in this paper. All possible forms of force sensors, for instance flexible, transparent, stretchable and highly sensitive can be developed using 3D printing technologies in an economical, quick and environmental-friendly approach. Over the years, fused deposition modeling (FDM) is gaining attention for the fabrication of force sensors. Also, due to advancements in materials and fabrication process, these sensors are being developed with enormous benefits. Some recent developments include flexible capacitive force sensor, wearable sensor and tactile sensor that are popular with 3D printing.