Flexible Sensor Array Research Articles

Flexible GMR Sensor Array for Magnetic Flux Leakage Testing of Steel Track Ropes

This paper presents design and development of a flexible GMR sensor array for nondestructive detection of service-induced defects on the outer surface of 64 mm diameter steel track rope. The number of GMR elements and their locations within saddle-type magnetizing coils are optimized using a three dimensional finite element model. The performance of the sensor array has been evaluated by measuring the axial component of leakage flux from localized flaw (LF) and loss of metallic cross-sectional area (LMA) type defects introduced on the track rope. Studies reveal that the GMR sensor array can reliably detect both LF and LMA type defects in the track rope. The sensor array has a fast detection speed along the length of the track rope and does not require circumferential scanning. It is also possible to image defects using the array sensor for obtaining their spatial information.

Materials, Processing, and Testing of Flexible Image Sensor Arrays

Editors' note:Flexible, large-area display and sensor arrays are finding growing applications in multimedia and future smart homes. This article first analyzes and compares current flexible devices, then discusses the implementation, requirements, and testing of flexible sensor arrays.

Development of a flexible three-axial tactile sensor array for a robotic finger

In this paper, we propose a flexible three-axial tactile sensor array for measuring both normal and shear loads. The sensor array has 16 tactile sensor units based on piezoresistive strain gauge. It is constructed on a Kapton polyimide film using advanced polymer micromachining technologies. Thin metal strain gauges are embedded in polyimide to measure normal and shear loads, which are tested by applying forces from 0 to 1 N. The developed sensor unit had a hysteresis error of about 9% and repeatability error of about 1.31%. The sensor showed a good resulting image when pressed by a circle-shaped object with 10 N loads. The proposed flexible three-axial tactile sensor array can be applied in a curved or compliant surface that requires slip detection and flexibility, such as a robotic finger.

Design and Experiment of Flexible Multi-Functional Tactile Sensors for Robot Skin

Flexible multi-functional tactile sensors are designed and fabricated for intelligent robot skin to detect three-dimensional force and temperature.Using the significant piezoresistive effect of carbon black/silicone rubber,a three-dimensional force sensor with four-electrode symmetrical structure is designed.Using the significant temperature sensitive effect of carbon fiber-PDMS(polydimethylsiloxane),a temperature sensor with interdigitated electrode structure is designed. The principles of three-dimensional force and temperature detection are analyzed.In view of the themechanical/temperature sensitivity cross-interference of the two kinds of conductive composites,the flexible multi-functional tactile sensor array is structured in the way that three-dimensional force and temperature sensors are arranged in alternately concave-convex form. The experiment results prove that the flexible multi-functional tactile sensors have three-dimensional force and temperature detection functions and can be applied to robot sensitive skin.

Flexible Sensing Arrays Fabricated with Carbon Nanofiber Composite Thin Films for Posture Monitoring

Faulty posture increases joint stress and causes postural pain syndrome. In this paper, we present a portable strain sensing system with flexible sensor arrays to warn patients to correct inappropriate posture. A 3×3 flexible strain sensing array system was fabricated using patterned surface treatment and the tilted-drop process with carbon nanofiber composite solutions on polyimide substrates. Atmospheric plasma was used to enhance or reduce the surface energy in specific areas for patterned surface treatment. A scanning circuit was also developed to capture the signal from the flexible sensing array. The developed system has been used to measure the bent angle of the human neck from 15 to 60°. The results indicate that human posture can be successfully captured by analyzing the measured strains from a flexible strain sensing array.

Flexible Sensing Arrays Fabricated with Carbon Nanofiber Composite Thin Films for Posture Monitoring

Faulty posture increases joint stress and causes postural pain syndrome. In this paper, we present a portable strain sensing system with flexible sensor arrays to warn patients to correct inappropriate posture. A 3×3 flexible strain sensing array system was fabricated using patterned surface treatment and the tilted-drop process with carbon nanofiber composite solutions on polyimide substrates. Atmospheric plasma was used to enhance or reduce the surface energy in specific areas for patterned surface treatment. A scanning circuit was also developed to capture the signal from the flexible sensing array. The developed system has been used to measure the bent angle of the human neck from 15 to 60°. The results indicate that human posture can be successfully captured by analyzing the measured strains from a flexible strain sensing array.

Experimental and numerical estimations into the force distribution on an occlusal surface utilizing a flexible force sensor array

This study presents a novel flexible force sensor array for measuring the distribution of the force distribution over the first molar. The developed force sensor array is composed of a flexible polyimide electrode and barium-titanate-based multilayer ceramic capacitors (MLCCs). The piezoelectric and material properties of industrial-grade MLCCs are ideal for measuring large-force loadings. The sensors are cheap and easy to integrate with automated manufacturing processes. Prior to experimental measurements, the force responses for the MLCC sensor cells were systematically measured and evaluated, confirming their high fracture strength and good sensing properties. Finite element (FE) simulations were used to calculate the force distribution over the tooth crown from the measurement results of the 3×3 force sensor array. Results indicate that the sensor has great sensitivity and linearity under a high-speed cycle loading of 500 N/s conducted to simulate normal chewing. The total force measured using the developed sensor array within the artificial tooth had an error of less than 5%. In addition, the force distributions over the molar crown obtained using a numerical method of FE analysis agree well with those obtained from experiments. The developed flexible force sensor array thus has potential for in-situ bite force measurements that are low-cost and reliable.

Real-time measurement of the three-axis contact force distribution using a flexible capacitive polymer tactile sensor

In this paper, we report real-time measurement results of various contact forces exerted on a new flexible capacitive three-axis tactile sensor array based on polydimethylsiloxane (PDMS). A unit sensor consists of two thick PDMS layers with embedded copper electrodes, a spacer layer, an insulation layer and a bump layer. There are four capacitors in a unit sensor to decompose a contact force into its normal and shear components. They are separated by a wall-type spacer to improve the mechanical response time. Four capacitors are arranged in a square form. The whole sensor is an 8 × 8 array of unit sensors and each unit sensor responds to forces in all three axes. Measurement results show that the full-scale range of detectable force is around 0–20 mN (250 kPa) for all three axes. The estimated sensitivities of a unit sensor with the current setup are 1.3, 1.2 and 1.2%/mN for the x-, y- and z-axes, respectively. A simple mechanical model has been established to calculate each axial force component from the measured capacitance value. Normal and shear force distribution images are captured from the fabricated sensor using a real-time measurement system. The mechanical response time of a unit sensor has been estimated to be less than 160 ms. The flexibility of the sensor has also been demonstrated by operating the sensor on a curved surface of 4 mm radius of curvature.

Direction of Arrival Estimation and Localization Using Acoustic Sensor Arrays

Sound source localization has numerous applications such as detection and localization of mechanical or structural failures in vehicles and buildings or bridges, security systems, collision avoidance, and robotic vision. The paper presents the design of an anechoic chamber, sensor arrays and an analysis of how the data acquired from the sensors could be used for sound source localization and object detection. An anechoic chamber is designed to create a clean environment which isolates the experiment from external noises and reverberation echoes. An FPGA based data acquisition system is developed for a flexible acoustic sensor array platform. Using this sensor platform, we investigate direction of arrival estimation and source localization experiments with different geometries and with different numbers of sensors. We further present a discussion of parameters that influence the sensitivity and accuracy of the results of these experiments.

Design and fabrication of a flexible three-axial tactile sensor array based on polyimide micromachining

This paper describes the design and fabrication of a flexible three-axial tactile sensor array using advanced polyimide micromachining technologies. The tactile sensor array is comprised of sixteen micro force sensors and it measures 13 mm × 18 mm. Each micro force sensor has a square membrane and four strain gauges, and its force capacity is 0.6 N in the three-axial directions. The optimal positions of the strain gauges are determined by the strain distribution obtained form finite element analysis (FEA). The normal and shear forces are detected by combining responses from four thin-film metal strain gauges embedded in a polyimide membrane. In order to acquire force signals from individual micro force sensors, we fabricated a PCB based on a multiplexer, operational amplifier and microprocessor with CAN network function. The sensor array is tested from the evaluation system with a three-component load cell. The developed sensor array can be applied in robots’ fingertips, as well as to other electronic applications with three-axial force measurement and flexibility keyword requirements.

Flexible strain sensors fabricated with carbon nano-tube and carbon nano-fiber composite thin films

This paper presents the fabrication of 3 × 3 flexible strain sensor arrays using conductive polymer solutions with fillers including carbon nano-fibers and multi-walled carbon nano-tubes. The strain sensor arrays were made on polyurethane substrates using patterned surface treatment and the tilted-drop process. Atmospheric plasma was used to enhance or reduce the surface energy in specific areas for patterned surface treatment. The performance of fabricated strain sensors made using conductive polymer solutions with different ingredients was investigated. The measured gauge factors were in the 0.34 to 7.98 range for the strain of 3–7%. Under a bending test exceeding 50 times at a 150° angle, sensor damage was not observed. The demonstrated fabrication method is capable of producing conductive polymer sensors with complex designs, high reliability and is suitable for mass production.

Platelet retraction force measurements using flexible post force sensors

Platelets play an important role in hemostasis by forming a thrombotic plug that seals the vessel wall and promotes vascular healing. After platelets adhere and aggregate at the wound site, their next step is to generate contractile forces through the coordination of physicochemical interactions between actin, myosin, and alpha(IIb)beta(3) integrin receptors that retract the thrombus' size and strengthen its adhesion to the exposed matrix. Although platelet contractile forces (PCF) are a definitive feature of hemostasis and thrombosis, there are few approaches that can directly measure them. In this study, we describe the development of an approach to measure PCF in microthrombi using a microscopic flexible post force sensor array. Quasi-static measurements and live microscopic imaging of thrombin-activated platelets on the posts were conducted to assay the development of PCF to various hemostatic conditions. Microthrombi were observed to produce forces that monotonically increased with thrombin concentration and activation time, but forces subsided when thrombin was removed. PCF results were statistically similar on arrays of posts printed with fibronectin or fibrinogen. PCF measurements were combined with clot volume measurements to determine that the average force per platelet was 2.1 +/- 0.1 nN after 60 min, which is significantly higher than what has been measured with previous approaches. Overall, the flexible post arrays for PCF measurements are a promising approach for evaluating platelet functionality, platelet physiology and pathology, the impacts of different matrices or agonists on hemostatic responses, and in providing critical information regarding platelet activity that can guide new hemostatic or thrombotic strategies.

Development of wireless compressive/relaxation-stress measurement system integrated with pressure-sensitive carbon black-filled silicone rubber-based sensors

In order to detect the installation compressive stress and monitor the stress relaxation between two bending surfaces on a defensive furnishment, a wireless compressive-stress/relaxation-stress measurement system based on pressure-sensitive sensors is developed. The flexible pressure-sensitive stress sensor array is fabricated by using carbon black-filled silicone rubber-based composite. The wireless stress measurement system integrated with this sensor array is tested with compressive stress in the range from 0 MPa to 3 MPa for performance evaluation. Experimental results indicate that the fractional change in electrical resistance of the pressure-sensitive stress sensor changes linearly and reversibly with the compressive stress, and its fractional change goes up to 355% under uniaxial compression; the change rate of the electrical resistance can track the relaxation stress and give out a credible measurement in the process of stress relaxation. The relationship between input (compressive stress) and output (the fractional change in electrical resistance) of the pressure-sensitive sensor is Δ R/ R 0 = σ × 1.2 MPa −1. The wireless compressive stress measurement system can be used to achieve sensitivity of 1.33 V/MPa to the stress at stress resolution of 920.3 Pa. The newly developed wireless stress measurement system integrated with pressure-sensitive carbon black-filled silicone rubber-based sensors has advantages such as high sensitivity to stress, high stress resolution, simple circuit and low energy consumption.

Novel lamination and interconnection technologies demonstrated in a flexible modular optical sensor array for wound monitoring

Flexible electronics is an emerging technology with all-printed cost effective, smart electronic products that will find a wide range of applications in large quantities in our society. Such products will be based on low-cost substrate materials like PEN or PET foils. Building of these printed electronic products is preferably done by combining foils with different functionalities through lamination and interconnection technology. This allows separate functional foils, made using mutually incompatible processing steps, to be combined and also allows independent optimisation of the foils. The current paper describes a novel lamination and interconnection process which uses low-cost solid state adhesive films for the structural bonding and isotropic conductive adhesives for the electrical interconnects. It is shown that the process allows the manufacture of interconnected foils which have a good flexural reliability we demonstrate the process by manufacturing a flexible modular optical sensor for wound monitoring.

Characterization and patterning of novel high-TCR Ta–Si–N thin films for sensor application

Temperature coefficient of resistance (TCR) and resistivity of films are crucial factors for thermal resistive sensors and concerned with microstructure. In this paper, a Ta–Si–N thin film with high negative TCR of −6250 ppm/°C at 30–100 °C has been fabricated on the silicon substrates by reactive co-sputtering at high nitrogen flow ratio (FN 2% = FN 2/(FN 2 + FAr) × 100%). It is larger than conventional materials, e.g. Pt, Cu, Ni and NiO x for thermal or flow sensors with TCR in the range of 2000–4540 ppm/°C. The microstructure and crystallinity of Ta–Si–N films were examined by X-ray diffraction. The resistivity and TCR were measured by the four-point probe and Keithley 2400 multimeter. The resistivity decreases with increasing temperature for the nature of negative TCR. The magnitude of both resistivity and TCR increases with increasing FN 2%. The patterning of high-TCR Ta–Si–N film on a flexible material has been performed by IC compatible processes, therefore it will be suitable for the integration with circuit design for the flexible sensor arrays in future.

Polymer-based flexible tactile sensor up to 32 × 32 arrays integrated with interconnection terminals

A polymer-based flexible tactile sensor array integrated with interconnection terminals was fabricated with polymer MEMS technologies and electroplating. The tactile sensing arrays were made as 4 × 4, 8 × 8, 16 × 16 and 32 × 32 sensing elements connected with a pluggable connector. Both the tactile sensor arrays and the interconnection terminals were fabricated together on the same polymer substrate in order to integrate them as a sensor module. The tactile sensor array with FFC (flexible flat cable) can be easily connected with the sensor array mounting PCB board by inserting the flexible cable into the pluggable connector socket on the board. The number and pitch of the fabricated interconnection terminals for 32 × 32 arrays were 68 and 500 μm, respectively. Continuous normal force ranging 0–1 N was applied to a unit tactile sensor for evaluating the tactile sensor characteristics through the integrated interconnection terminals. The measured resistance increased linearly with normal force in the range of 0–0.6 N but the resistance variation rate decreased above 0.6 N. The variation rate of the resistance is about 2.0%/N in the range of 0–0.6 N and 1%/N in the range of 0.6–1 N. The characterization of the tactile sensor array for the rod bar and a fingertip was successfully evaluated using the custom designed controller and the window-based 3D display software.

Thin Flexible Pressure Sensor Array Based on Carbon Black/Silicone Rubber Nanocomposite

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, the key technologies for the development of the thin flexible pressure sensor array based on carbon black/silicone rubber nanocomposite are reported. The piezoresistive mechanism of the nanocomposite is explained by analyzing the changes in effective conductive paths. The technical data of the sensor system are given. With the measurement range of 0–1 MPa, the maximum measurement deviation is less than 30 kPa. </para>

A 32 × 32 temperature and tactile sensing array using PI-copper films

The development of a flexible 32 × 32 temperature and tactile sensing array, which will serve as the artificial skin for robot applications, is presented in this work. Pressure conductive rubber is employed as the tactile sensing material, and discrete temperature sensor chips are employed as the temperature sensing cells. Small disks of pressure conductive rubber are bonded on predefined interdigital copper electrode pairs which are patterned on a flexible copper–polyimide substrate which is fabricated by micromachining techniques. This approach can effectively reduce the crosstalk between each tactile sensing element. The mechanical and electrical properties of tactile sensing elements are measured. Also, the corresponding scanning circuits are designed and implemented. The temperature and tactile sensing elements are heterogeneously integrated on the flexible substrate. By using the integrated 32 × 32 sensing arrays, temperature and tactile images induced by the heaters/stamps of different shapes have been successfully measured. The flexible sensor arrays are bendable down to a 4-mm radius without any degradation in functionality.

Design and Fabrication of Stretchable Multilayer Self-Aligned Interconnects for Flexible Electronics and Large-Area Sensor Arrays Using Excimer Laser Photoablation

Stretchable interconnects are fabricated on polymer substrates using metal patterns both as functional interconnect layers and as in situ masks for excimer laser photoablation. Single-layer and multilayer interconnects of various designs (rectilinear and “meandering”) have been fabricated, and certain “meandering” interconnect designs can be stretched up to 50% uniaxially while maintaining good electrical conductivity and structural integrity. This approach eliminates masks and microfabrication processing steps as compared to traditional fabrication approaches. Furthermore, this technology is scalable for large-area sensor arrays and electronic circuits, adaptable for a variety of materials and interconnects designs, and compatible with MEMS-based capacitive sensor technology.

Normal and Shear Force Measurement Using a Flexible Polymer Tactile Sensor With Embedded Multiple Capacitors

In this paper, we report a new flexible capacitive tactile sensor array with the capability of measuring both normal and shear force distribution using polydimethylsiloxane (PDMS) as a base material. A tactile cell consists of two thick PDMS layers with embedded electrodes, an air gap, and a pillar structure. The pillar structure is formed at the center of each tactile cell between the air gap under a large bump. There are four capacitors in a cell to decompose the contact force into normal and shear components. Four capacitors are arranged in a square form. If a normal force is applied on the bump, the PDMS layer on the pillar structure is compressed, and the air gap between the top and bottom electrodes decreases, resulting in the increase in all four capacitances. If a shear force is applied, a torque is induced around the pillar. Therefore, the capacitance of the two capacitors increases, whereas that of the other two decreases. The bump and the pillar structure play a critical role to generate a torque for shear force measurement. The sensor has been realized in an 8 x 8 array of unit sensors, and each unit sensor responds to normal and shear stresses in all three axes, respectively. Measurement of a single sensor shows that the full-scale range of detectable force is about 10 mN, which corresponds to 131 kPa in three directions. The sensitivities of a cell measured with a current setup are 2.5%/mN, 3.0%/mN, and 2.9 %/mN for the x-, y-, and z-directions, respectively. Normal and shear force images are also captured from a 4 x 4 array of the fabricated sensor. Distinctive characteristic patterns appear when a shear force is applied to the sensor.