Flexible Force Sensor Research Articles

Review on Conductive Polymer/CNTs Nanocomposites Based Flexible and Stretchable Strain and Pressure Sensors.

In the last decade, significant developments of flexible and stretchable force sensors have been witnessed in order to satisfy the demand of several applications in robotic, prosthetics, wearables and structural health monitoring bringing decisive advantages due to their manifold customizability, easy integration and outstanding performance in terms of sensor properties and low-cost realization. In this paper, we review current advances in this field with a special focus on polymer/carbon nanotubes (CNTs) based sensors. Based on the electrical properties of polymer/CNTs nanocomposite, we explain underlying principles for pressure and strain sensors. We highlight the influence of the manufacturing processes on the achieved sensing properties and the manifold possibilities to realize sensors using different shapes, dimensions and measurement procedures. After an intensive review of the realized sensor performances in terms of sensitivity, stretchability, stability and durability, we describe perspectives and provide novel trends for future developments in this intriguing field.

Ionic flexible force sensors and their potential applications

Ionic flexible force sensors based on materials of different ionic electrolyte layers and flexible electrodes and their applications.

Polyethylene-Carbon Composite (Velostat®) Based Tactile Sensor.

The progress observed in ‘soft robotics’ brought some promising research in flexible tactile, pressure and force sensors, which can be based on polymeric composite materials. Therefore, in this paper, we intend to evaluate the characteristics of a force-sensitive material—polyethylene-carbon composite (Velostat®) by implementing this material into the design of the flexible tactile sensor. We have explored several possibilities to measure the electrical signal and assessed the mechanical and time-dependent properties of this tactile sensor. The response of the sensor was evaluated by performing tests in static, long-term load and cyclic modes. Experimental results of loading cycle measurements revealed the hysteresis and nonlinear properties of the sensor. The transverse resolution of the sensor was defined by measuring the response of the sensor at different distances from the loaded point. Obtained dependencies of the sensor’s sensitivity, hysteresis, response time, transversal resolution and deformation on applied compressive force promise a practical possibility to use the polyethylene-carbon composite as a sensitive material for sensors with a single electrode pair or its matrix. The results received from experimental research have defined the area of the possible implementation of the sensor based on a composite material—Velostat®.

Effect of Contacting Surface on the Performance of Thin-Film Force and Pressure Sensors

Flexible force and pressure sensors are important for assessing the wear comfort of tightly fitting apparel. Their accuracy and repeatability depend on the sensor itself and the contacting surface. Measurements of the contact pressure on soft surfaces like human skin tend to be erroneous, which could be due to incorrect sensor calibrations. This study aims to examine the effects of human body parameters such as the hardness and temperature of the contacting surface by using a custom-made calibration setup and investigating the incorporation of rigid discs on the sensor surface. Two commercial force sensors, FlexiForce and SingleTact, and one pressure sensor, Pliance X, are used in the investigation. The findings reveal that adding rigid discs on both sides of the force sensors improves their sensitivity. Systematic calibration has been performed on the surfaces with different temperatures and hardness. The results show that FlexiForce and Pliance X tend to be affected by the changes in surface temperature and surface hardness. Prolonged testing time shows that the time dependence of SingleTact and Pliance X sensor is lower, which suggests that they are more suitable for lengthier evaluations in which interface pressure is exerted on the human body. In brief, sensor attachment and proper calibration should be thoroughly considered before using sensors for applications on soft surfaces, like the human body.

Aerosol-Jet Printed Conformable Microfluidic Force Sensors

Force sensors that are thin, low cost, flexible and compatible with commercial microelectronic chips are of great interest for use in biomedical sensing, precision surgery, robotics and in various “smart control” technologies. By leveraging a novel combination of microfluidics and capacitive sensing we have developed 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 that are coated with a thin polymer layer for insulation. When a force is applied to a fluid reservoir at the base of the channel, the fluid is displaced along the channel, passing over the electrodes and thus inducing a capacitance change, which is proportional to the amount of force applied. The microfluidic molds themselves are made of low-cost sacrificial material inks deposited via aerosol-jet printing, which is also used to print the electrode layer. The sensors exhibit a nearly linear change in capacitance in response to external load, with a sensitivity of up to 3.8 pF/N and a force range of up to 9 N achieved with the present design. We quantify the effects of changing the electrode morphology, liquid permittivity, sensor thickness and geometry, as well as the thickness of the insulation layer over the electrodes, on sensor range and sensitivity, using a combination of experiments and finite element analysis simulations. We further demonstrate how the sensor can be used to remotely control a robotic clamp. The conformable force sensors developed here are thin, flexible and facilitate easy signal processing. The fabrication processes are low-cost, amenable to fast prototyping and mass manufacturing. We envisage a large range of industrial and biomedical applications for this novel force sensor.

A Smart System for Sitting Posture Detection Based on Force Sensors and Mobile Application

The employees’ health and well-being are an actual topic in our fast-moving world. Employers lose money when their employees suffer from different health problems and cannot work. The major problem is the spinal pain caused by the poor sitting posture on the office chair. This paper deals with the proposal and realization of the system for the detection of incorrect sitting positions. The smart chair has six flexible force sensors. The Internet of Things (IoT) node based on Arduino connects these sensors into the system. The system detects wrong seating positions and notifies the users. In advance, we develop a mobile application to receive those notifications. The user gets feedback about sitting posture and additional statistical data. We defined simple rules for processing the sensor data for recognizing wrong sitting postures. The data from smart chairs are collected by a private cloud solution from QNAP and are stored in the MongoDB database. We used the Node-RED application for the whole logic implementation.

Low-Power Wireless System to Monitor Tongue Strength Against the Palate

The study of the tongue - especially the analysis of its strength against teeth and palate - provides important information on health conditions or treatment outcomes. In this work, an easy-to-use intra-oral device was designed and fabricated to monitor position and strength of the tongue in real-time, without using cables or tubes. The proposed intra-oral device is based on tailored-made capacitive flexible force sensors. Measured data are collected and transmitted via Bluetooth Low Energy (BLE) to a custom-made iOS application. The proposed fabrication process offers a custom-fit device made of materials and components commonly used in dentistry, ensuring biocompatibility. A preliminary device evaluation shows positive results in terms of battery duration and quality of the measured data also after its immersion in water. The 50-mAh battery allows the proposed system to be used for multiple rehabilitation sessions before recharging it. The non-linear behavior and the hysteresis of the capacitive sensors allow to detect five levels from 0 N to 1.11 N of applied force against the palate.

Development of a Soft Pneumatic Actuator System Based on Flexible Force Sensor for Lower Limb Compression Therapy

The use of a soft pneumatic actuator (SPA) in compression therapy to enhance blood circulation shows potential because of its unique proactive bio-mechanical force delivery mode and wide range of pneumatic pressure control. This work presented new insights into the model-experiment method using mathematical simulation and flexible force sensor array (FFSA) to explore the bio-mechanical force delivery mechanism of a newly developed SPA and offer accurate pressure for lower limb compression therapy. Multichambered bladders were incorporated in the SPA system as key components to exert pressure on the lower limb, and the FFSA visualized the skin pressure on a software interface. Pressure signals were transiently collected by differential sensors and FFSA using the data acquisition unit and then transmitted to a computer software via Bluetooth module to build the interface pressure feedback and control subsystem. According to the interface pressure variation, the SPA system could automatically adjust the air pressure in bladders to generate suitable bio-mechanical force for compression therapy. Results revealed that the sensors-based SPA system could generate rhythmic (intermittent) bio-mechanical force, which could be transformed by FFSA and visualized in a morphological interface pressure mapping from quantitative and qualitative perspectives. This study provided an evidence-based technical guidance for a new SPA system design which could delivered controllable and precise dynamic pressure to affect the lower limb during compression therapy.

Performance Evaluation of Capacitive Based Force Sensor for Electroencephalography Head Caps

Accurate electrode signal measurement using EEG head caps can only be achieved through sufficient contact or force. A flexible force sensor is required to obtain accurate force measurement underneath EEG head caps. In this study, we evaluate the performance of a capacitive based sensor including its accuracy, repeatability, hysteresis, and stability. The result shows that accuracy error and repeatability error were 3.03±2.8 % and 3.84±2.92 %, respectively. The stability errors were 2.37±0.15 %(10 gram), 2.54±0.00 % (50 gram), 2.37±0.15 % (100 gram), 5.07±1.16 % (150 gram), 7.27±0.39 % (200 gram). The hysteresis error of the sensor was 4.48±0.47 %. Based on the results, the capacitive based force sensor provides sufficiently low errors in accuracy, repeatability, stability, and hysteresis and is thus suitable for measuring adduction force in EEG cap applications

Identifying human body states by using a flexible integrated sensor

Flexible sensors are required to be lightweight, compatible with the skin, sufficiently sensitive, and easily integrated to extract various kinds of body vital signs during continuous healthcare monitoring in daily life. For this, a simple and low-cost flexible temperature and force sensor that uses only two carbon fiber beams as the sensing layer is reported in this work. This simple, flexible sensor can not only monitor skin temperature changes in real time but can also extract most pulse waves, including venous waves, from most parts of the human body. A pulse diagnostic glove containing three such flexible sensors was designed to simulate pulse diagnostic methods used in traditional Chinese medicine. Wearable equipment was also designed in which four flexible sensors were fixed onto different body parts (neck, chest, armpit, and fingertip) to simultaneously monitor body temperature, carotid pulse, fingertip artery pulse, and respiratory rate. Four important physiological indicators—body temperature (BT), blood pressure (BP), heart rate (HR), and respiratory rate (RR)—were extracted by the wearable equipment and analyzed to identify exercise, excited, tired, angry, and frightened body states.

Self-Synchronized, Continuous Body Weight Monitoring Using Flexible Force Sensors and Ground Reaction Force Signal Processing

Change patterns in body weight are significant indicators of a variety of medical conditions and in some instances, such as congestive heart failure, strong predictors of critical care interventions. The continuous monitoring of body weight can therefore help care givers and patients reduce the risks that are inherent in ignoring body weight changes or in measuring them infrequently. One possible approach to continuous body weight monitoring is to use shoe-integrated sensors. Such approach is however hampered by the lack of patient’s adherence to medical protocols and by the detrimental impact of motion artifacts on weight measurements during body movement. The objective of this paper is to show that both problems can be readily addressed by pushing body weight measurement into the background of the patient’s daily routine and by adapting piezoresistive flexible force sensors to the specific requirements of accurate body weight measurement during motion. The proposed solution is enabled by two major innovations. The first is a sensor array architecture that helps in collecting the total vertical GRF on each foot during the walk cycle. The second is based on extracting unique features from the waveforms of ground reaction forces (GRFs) as measured by the flexible force sensor array. This paper is dedicated to a full and detailed description of this second innovation that includes the extraction of GRF waveform features using real-time signal processing and the elimination of motion artifacts using accurate estimates of the vertical acceleration of the body’s center of mass. A prototype demo system of the proposed solution is described along with testing results using a human subject in various walk conditions. The prototype is comprised of three subsystems: the shoe-integrated flexible sensor array, a sentinel subsystem implemented as a wearable smart watch, and a communication subsystem between sensors and sentinel. The prototype is distinguished with several unique features that ensures its wearability and low cost, including its self synchronizing data collection and the absence of any inertial motion units.

A Wireless-controlled 3D printed Robotic Hand Motion System with Flex Force Sensors

Hand gesture recognition is an emerging field of technology in robotics and human-computer interaction. It has tremendous applications in daily life activities and intelligent workplaces. In this study, a system which could help people to work and operate without directly using hands or contacting by hands, is proposed and demonstrated. This system composed of a glove with flexible force sensors and a 3D printed robotic forearm. The user wearing the glove could control the action of the 3D printed robotic forearm. The 3D printed forearm simultaneously acted following the motion of the glove. The 3D printed forearm was composed of 46 individual parts that were printed with white biodegradable polylactic acid (PLA). Electronic components in the system are five flex sensors, a master Arduino Nano, a slave Arduino Nano, a wireless NRF24L01 transmitter module banding on the glove, a second wireless NRF24L01 receiver module in the forearm and five motors. The five flex sensors on the fingers of the glove detected and collected the signals reflecting the movements of the hands. The Arduino Nano processed the signals from the flex sensors and sent them through the wireless transmitter module to the slave Arduino Nano. In order to control the action of the robotic forearm, it was embedded with a slave Arduino Nano as a control kernel, a wireless NRF24L01 receiver module and five actuators. The slave Arduino Nano received and processed the signals through the wireless receiver module. After that, the signals were sent to the actuators- servo motors. The fingers’ action in the robotics arm was executed with the actuators. After carefully testing the system, the robotic arm followed the action correctly with a maximum 0.133 ms time delay all the time. This system could be really useful for the users who work in dangerous conditions, hazardous environment or require remote operation for safety reasons.

A Comparative Study of Compressible and Conductive Vertically Aligned Carbon Nanotube Forest in Different Polymer Matrixes for High-Performance Piezoresistive Force Sensors.

In the present scenario, conducting and lightweight flexible polymer nanocomposites rival metallic and inorganic semiconducting materials as highly sensitive piezoresistive force sensors. Herein, we explore the feasibility of vertically aligned carbon nanotube (VACNT) nanocomposites impregnated in different polymer matrixes, envisioned as highly efficient piezoresistors in sensor applications. Polymer nanocomposites are selectively designed and fabricated using three different polymer matrixes, i.e., polydimethylsiloxane (PDMS), polyurethane (PU), and epoxy resins with ideal reinforcement of VACNTs to enhance the thermal stability, conductivity, compressibility, piezoresistivity, and sensitivity of these nanocomposites. To predict the best piezoresistive force sensor, we evaluated the structural, optical, thermal, electrical, mechanical, and piezoresistive properties of the nanocomposites using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA), I-V measurements, compressive stress-strain measurements, hysteresis, sensitivity, and force studies. The results demonstrate that the PDMS/VACNT nanocomposite is capable of sustaining large force with almost complete recovery and enhanced sensitivity, thereby fulfilling the desirable need for a highly efficient conductive and flexible force sensor as compared to PU/VACNT and epoxy/VACNT nanocomposites.

A Polyimide Based Force Sensor Fabricated Using Additive Screen-Printing Process for Flexible Electronics

A flexible capacitive force sensor was developed for force sensing applications. The force sensor consists of two electrodes and a dielectric layer. The electrodes were fabricated by depositing conductive silver (Ag) ink on a flexible polyimide platform using screen-printing process. The dielectric layer was prepared by mixing the contents of polydimethylsiloxane (PDMS) (pre-polymer and curing agent) in a 16:1 ratio. Then the PDMS dielectric layer was sandwiched between the screen-printed Ag electrodes. The capability of the fabricated force sensor was investigated by recording its capacitive response for varying applied forces of 100 N. It was observed that the capacitance increased from 5.83 pF to 6.36 pF as the force was increased from 0 N (no load) to 100 N. A sensitivity and correlation coefficient of 0.081%N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 0.998 were calculated for the force sensor. In addition, a response time and recovery time of 3.7 seconds and 5.7 seconds was measured for the fabricated force sensor. The relative humidity (% RH) tests performed from 20% RH to 80% RH, in steps of 20% RH, revealed that there was a minimal effect of RH on the base capacitance of the force sensor at room temperature.

A Force Measurement System for HTS Maglev Studies

An improved force measurement system was developed for investigating high-temperature superconducting (HTS) maglev properties. The measurement system consists of assorted sensors, a 3-D sliding platform, a mechanical drive with automatic control software, and a data collecting and processing software. This system features high precision, instant measurement at the movement of the targeted HTS bulk, synchronous measurement of levitation force and guidance force, and the magnetic field. This system can be used for studying the interaction behavior between an HTS bulk and a permanent magnet guideway (PMG) or an electromagnetic guideway (EMG) of different shapes and sizes. It can realize arbitrary 3-D complex curvilinear motions. This system was built with a flexible platform and high precision force and distance sensors. As a result, the magnetic levitation characteristics of an HTS bulk above a magnet guideway can be precisely investigated with this system. In general, the aforementioned functions all have been verified by experiments.

Structural design and performance analysis of elastomer for robot wrist force sensor

The robot wrist force sensor is a force sensor installed in the junction of robot hand and arm,is one of the important sensor in intelligent robot system. As a carrier of strain type elastic paste wrist force sensor of strain gauge,its importance is self-evident. For an industry to design a flexible cantilever beam wrist force sensor body structure series robot,the selection of materials are described,has carried on the modeling,set constraints and load using finite element analysis software ANSYS,finally inspected the strain diagram of the elastomer and has carried an the strength and rigidity check. The analysis results show that the elastic structure has good mechanical properties,and the strain gauge is large,which is in line with with the requirements of the robot.

Flexible Force Sensors Based on Permeability Change in Ultra-Soft Amorphous Wires

A prototype of robust, durable, and reliable force sensor is proposed. The proposed sensor consists of a cylindrical elastic tube, which changes its transversal cross section from circular to elliptical under the influence of the applied force, and a Co–Fe–Si–B ultra-soft magnetic wire-based sensitive element. The sensing element is placed onto the circumference of the elastic tube, being able to detect and measure the deformation produced by an applied force. The impedance of the sensitive element changes when it bends, due to the permeability change when the amorphous wire is subjected to mechanical stress. A voltage proportional to the applied force is picked up at the sensor output by using adapted electronics. The relation between the force and the electrical output signal is determined through calibration. Three different configurations of the sensitive elements distributed around the elastic tube have been analyzed. The developed sensor has an excellent adaptability and an important application potential for medical devices, instrumentation, or other devices which require force feedback.

High‐Performance Flexible Tactile Sensor Enabling Intelligent Haptic Perception for a Soft Prosthetic Hand

AbstractCompared to the traditional motor‐driven prosthetic hand, a soft prosthetic hand (SPD) has intrisic advantages such as kinematics dexterity and friendly external interaction. In order for an SPD to realize the function of a real hand, the integration of flexible high‐performance tactile sensors is essential. Current research on flexible tactile sensors is mainly focused on increasing sensitivity, but ignores other issues, such as the complexity of arraying and compatibility with soft actuators. A high‐performance flexible force sensor enabling intelligent tactile perception for an SPD is reported. With the aid of reasonable arrangement of functional material and effective 3D structure design, the force sensor exhibits good linearity (R2 = 0.982), accuracy and repeatability, very low hysteresis, fast dynamical response (&lt;1 ms), high stability (30000 loading–unloading cycles), stable performance under high frequency (&gt;50 Hz), and can easily be arrayed and integrated with an SPD. With such a sensor mounted on its surface, an SPD achieves various complicated tasks in the same manner as a real hand, including feeling the softness of objects and imitating the process of traditional Chinese medicine pulse diagnosis to monitor a pulse wave in the radial artery. Finally, through use of a force‐sensor array, an SPD provides real‐time spatial distribution of pressure during the grabbing of objects.

Development of a Virtual Force Sensor for a Low-Cost Collaborative Robot and Applications to Safety Control.

To protect operators and conform to safety standards for human–machine interactions, the design of collaborative robot arms often incorporates flexible mechanisms and force sensors to detect and absorb external impact forces. However, this approach increases production costs, making the introduction of such robot arms into low-cost service applications difficult. This study proposes a low-cost, sensorless rigid robot arm design that employs a virtual force sensor and stiffness control to enable the safety collision detection and low-precision force control of robot arms. In this design, when a robot arm is subjected to an external force while in motion, the contact force observer estimates the external torques on each joint according to the motor electric current and calculation errors of the system model, which are then used to estimate the external contact force exerted on the robot arm’s end-effector. Additionally, a torque saturation limiter is added to the servo drive for each axis to enable the real-time adjustment of joint torque output according to the estimated external force, regulation of system stiffness, and achievement of impedance control that can be applied in safety measures and force control. The design this study developed is a departure from the conventional multisensor flexible mechanism approach. Moreover, it is a low-cost and sensorless design that relies on model-based control for stiffness regulation, thereby improving the safety and force control in robot arm applications.

A flexible capacitive force sensor for cardiopulmonary resuscitation

A flexible capacitive force sensor for cardiopulmonary resuscitation