Capacitive Tactile Sensor Research Articles

Capacitive wearable tactile sensor based on smart textile substrate with carbon black /silicone rubber composite dielectric

To achieve the wearable comfort of electronic skin (e-skin), a capacitive sensor printed on a flexible textile substrate with a carbon black (CB)/silicone rubber (SR) composite dielectric was demonstrated in this paper. Organo-silicone conductive silver adhesive serves as a flexible electrodes/shielding layer. The structure design, sensing mechanism and the influence of the conductive filler content and temperature variations on the sensor performance were investigated. The proposed device can effectively enhance the flexibility and comfort of wearing the device asthe sensing element has achieved a sensitivity of 0.02536%/KPa, a hysteresis error of 5.6%, and a dynamic response time of ~89 ms at the range of 0–700 KPa. The drift induced by temperature variations has been calibrated by presenting the temperature compensation model. The research on the time–space distribution of plantar pressure information and the experiment of the manipulator soft-grasping were implemented with the introduced device, and the experimental results indicate that the capacitive flexible textile tactile sensor has good stability and tactile perception capacity. This study provides a good candidate for wearable artificial skin.

An analytical model for studying the structural effects and optimization of a capacitive tactile sensor array

This paper presents an analytical model to study the structural effects of a capacitive tactile sensor array on its capacitance changes and sensitivities. The tactile sensor array has 8 × 8 sensor units, and each unit utilizes the truncated polydimethylsiloxane (PDMS) pyramid array structure as the dielectric layer to enhance the sensing performance. To predict the capacitance changes of the sensor unit, it is simplified into a two-layered structure: upper polyethylene terephthalate (PET) film and bottom truncated PDMS pyramid array. The upper PET is modeled by a displacement field function, while each of the truncated pyramids is analyzed to obtain its stress–strain relation. Using the Ritz method, the displacement field functions are solved. The deformation of the upper electrodes and the capacitance changes of the sensor unit can then be calculated. Using the developed model, the structural effects of the truncated PDMS pyramid array and the PDMS bump on the capacitance changes and sensitivities are studied. To achieve the largest capacitance changes, the dimensions have been optimized for the sensor unit. To verify the developed model, we have fabricated the sensor array, and the average sensitivities of the sensor unit to the x-, y-, and z-axes force are 0.49, 0.50, and 0.32% mN−1, respectively, while the model predicted values are 0.54, 0.54, and 0.35% mN−1. Results demonstrate that the developed model can accurately predict the sensing performance of the sensor array and could be utilized for structural optimization.

Surface-mountable capacitive tactile sensors with flipped CMOS-diaphragm on a flexible and stretchable bus line

This paper describes a MEMS-CMOS integrated tactile sensor for surface mounting on a flexible and stretchable bus line. The sensor is featured by the following configurations; (1) A sensing diaphragm is formed on a CMOS substrate by backside etching, and (2) the CMOS substrate is flip-bonded to a special low temperature cofired ceramic (LTCC) substrate with Au through vias. The flipped CMOS substrate and the LTCC substrate were electrically and mechanically connected using Au–Au bonding, which also formed differential capacitive gaps. The structure and fabrication process are simple compared with the existing surface-mountable MEMS-CMOS integrated sensors. Bendable and stretchable bus lines were fabricated by etching metal, which is deposited on a polyimide substrate and the outer shape is then determined by laser cutting. The bus line with a silicone protection coat is capable of stretching up to 50%. The sensors covered with silicone worked even under 10% stretching of the bus line. The tactile sensors mounted on the surface of the flexible bus line were characterized in terms of force sensitivity. Also, smart functions such as threshold and adaptation operations and the configuration of sensor parameters were demonstrated.

High precision intelligent flexible grasping front-end with CMOS interface for robots application

This paper presents a high-precision intelligent flexible robot grasping front-end with an integrated capacitive tactile sensor array and a conditioning chip. The capacitive tactile sensor is the primary part of the front-end, it determines the overall performance. The micro-needle array sandwich structure in the tactile sensor increases the repeatability and stability, and ensures the sensitivity. The assembled sensor exhibits a saturation at 10.53 N (421 kPa) with a sensitivity of 1.9%/kPa. Furthermore, a conditioning chip is utilized in a custom readout interface to achieve better performance by reducing signal attenuation, and to increase the compatibility of the front-end. The chip is optimized for the parasitic shunt capacitance in the capacitor array. A dual bidirectional charge-discharge conversion method and a two-port detection method are matched to achieve the goal of reducing the shunting influence, and attenuating the offset voltage or the noise input effects. A prototype of the interface has been fabricated using 180-nm CMOS technology. Sensor with the value of 0.5 pF shunted by capacitors of 47 pF has been detected with an error of 1% within 100 us.

Ultra-Thin Silicon based Piezoelectric Capacitive Tactile Sensor

This paper presents an ultra-thin bendable silicon based tactile sensor, in a piezoelectric capacitor configuration, realized by wet anisotropic etching as post-processing steps. The device is fabricated over bulk silicon, which is thinned down to 35 μm from an original thickness of 636 μm. Dicing of thin membrane is achieved by low cost novel technique of Dicing before Etching. The piezoelectric capacitor is composed of polyvinylidene fluoride trifluoroethylene (PVDF-TrFE), which present an attractive avenue for tactile sensing as they respond to dynamic contact events (which is critical for robotic tasks), easy to fabricate at low cost and are inherently flexible. The sensor exhibits enhanced piezoelectric properties, thanks to the optimization of the poling procedure. The sensor capacitive behaviour is confirmed using impedance analysis and the electro-mechanical characterization is done using TIRA shaker setup.

Capacitive tactile sensor for angle detection and its accuracy study

This paper proposes a capacitive tactile sensor for applications in normal force, shear force, and xy -plane shear angle sensing. This tactile sensor enabled shear force angle detection with less than 3° tolerance to be realized for the first time. The average sensitivity when applying the normal force was 4.43 pF/N, and the average sensitivity when applying the shear force ranged between 0.13 and 0.85 pF/N, depending on the angle. Angle detection errors caused by planar-, rotational-, and vertical-pattern shifts during the lamination process and operation are detailed. The results of an analysis indicated that both the planar and rotational shift greatly contributed to detection errors, whereas the contribution of the vertical shift was negligible. In addition to the experiment and analysis, simulations were performed to prove that the angle detection methodology yields consistent results in theory and in practice.

Flexible Capacitive Tactile Sensor Array With Truncated Pyramids as Dielectric Layer for Three-Axis Force Measurement

This paper presents a flexible capacitive tactile sensor array embedded with a truncated polydimethylsiloxane pyramid array as a dielectric layer. The proposed sensor array has been fabricated with $4\times 4$ sensor units. The measurement ranges of forces in the $x$ -axis, $y$ -axis, and $z$ -axis are 0–0.5, 0–0.5, and 0–4 N, respectively. In the range of 0–0.5 N, the sensitivities of the sensor unit are 58.3%/N, 57.4%/N, and 67.2%/N in the $x$ -axis, $y$ -axis, and $z$ -axis, respectively. In the range of 0.5–4 N, the sensitivity in the $z$ -axis is 7.7%/N. Three-axis force measurement has been conducted for all the sensor units. The average errors between the applied and calculated forces are 11.8% ± 6.4%. The sensor array has been mounted on a prosthetic hand. A paper cup and a cube are grasped by the prosthetic hand and the three-axis contact force is measured in real time by the sensor array. Results show that the sensor can capture the three-axis contact force image both in light and tight grasping. The proposed capacitive tactile sensor array can be utilized in robotics and prosthetic hand applications. [2014-0350]

Modeling and Analysis of a Novel Flexible Capacitive-Based Tactile Sensor

In recent years, autonomous robots have been increasingly deployed in unstructured and unknown environments. In order to survive in theses environments, robots are equipped with sensors. One of the main sensors is tactile sensor which provides the robots with tactile information like texture, stiffness, temperature, vibration and normal and shear forces. In this paper, we propose a flexible capacitive tactile sensor which is designed for measuring both normal and shear forces. The tactile sensing unit consists of five layers, a bottom layer of Polyethylene Terephthalate (PET) with a pillar, two copper electrodes embedded into a Polydimethylsiloxane (PDMS) film, a spacer, a Polyimide (PI) film and finally a top PI bump. The bump and the pillar structure play a significant role in producing a torque for shear force measurement. Finite element modeling (FEM) is conducted to analyze the deformation of the sensing unit and simulated using COMSOL Multiphysics. The change of capacitance verse normal and shear forces are obtained, a comparison between the proposed sensor and other pervious sensor is conducted. The sensitivity of a cell is 0.22%/N within the full scale range of 10 N for normal force and 4%/N within the full scale range of 10 N for shear force.

(Keynote) Heterogeneous Integration of MEMS by Adhesive Bonding

Technology called microsystems or MEMS (micro electro mechanical systems) has been developed and applied as value added devices for systems. It is based on semiconductor microfabrication integrating not only circuits but also sensors, actuators and microstructures. MEMS as switches and filters fabricated on CMOS LSI are needed for future multi-band wireless systems, in which good mechanical properties or piezoelectric materials are required for the MEMS and state of the art for the LSI. Such heterogeneous integration can be performed by transferring MEMS devices fabricated on a carrier wafer to a LSI wafer by adhesive bonding (Figure). Wafer level selective transfer from one carrier wafer to multiple target wafers are needed for cost-effective integration of different size chips. The selective transfer technology was applied to multiple SAW (surface acoustic wave) resonators on LSI and tunable SAW filter having variable ferroelectric capacitor. Distributed tactile sensors are needed on the skin of robots to ensure their safety in nursing care robots. A tactile sensor network acquires sensing data from each tactile sensor element by autonomous data transmission (event driven). Capacitive tactile force sensors were formed on a communication LSI by adhesive bonding. An electron emitter array integrated with a 100×100 active-matrix driving LSI has been developed using the adhesive bonding. Massive parallel EB (electron beam) exposure systems for maskless lithography (digital fabrication of LSI) are under development. A nc (nanocrystalline)-Si emitter consists of cascaded tunnel junctions can be controlled at low voltage (10V). Resist patterning by the emitted electron was successfully confirmed. Amperometric biosensor in which 20×20 boron doped diamond electrode array are formed on LSI using the adhesive bonding. Distribution of chemicals like neurotransmitter can be detected owing to a wide potential window of the diamond electrode. Cost effective development is required for micro systems because they are versatile and not standardized. A hands-on access fabrication facility in our University is reuse of a production line for power transistor and donated equipments. Companies can easily access and utilize for their prototyping or small-volume production. It is equipped with 4 and 6 inch facilities. The users are charged depending on their amount of usage. They can access a lot of technology and know-how accumulated and can be assisted by skilled engineers. More than 150 companies are using this facility and it is allowed to sell produced small volume devices. Figure 1

Large deformation mechanics of a soft elastomeric layer under compressive loading for a MEMS tactile sensor application

The study is motivated by the need to develop highly sensitive tactile sensors for both robotic and bionic applications. The ability to predict the response of an elastomeric layer under severe pressure conditions is key to the development of highly sensitive capacitive tactile sensors capable of detecting the location and magnitude of applied forces over a broad range of contact severity and layer depression. Thus, in this work, a large deformation Mooney–Rivlin material model is employed in establishing the non-linear mechanics of an elastomeric layer of finite thickness, subjected to uniform displacement of controlled compression. Thus, an analytical non-linear model for the above described problem which is validated numerically via the method of finite elements is developed. Two dimensional, plane strain conditions of an infinitely long and of finite thickness elastomeric layer are assumed. The layer is subjected to a uniform vertical large displacement with symmetry conditions applied at the contact center. Cauchy normal and shear stress profiles as well as displacement profiles are established over a broad range of a layer compression including up to 40% of layer thinning. The model allows for the determination of the non-linear relationship between the relative separation of embedded conducting electrodes and thus the sensor capacitance during touch, to the force magnitude of the force concentrated at the symmetry plane or sensor center. The current model is expected to further improve the sensitivity and range of polymeric tactile sensors currently under development (Charalambides and Bergbreiter, 2013) [1]. As shown elsewhere (Kalayeh et al., 2015) [2], capacitance–force model predictions are found to be in remarkable agreement with experimental measurements for a broad family of self-similar pressure sensors.

Flexible Capacitive Tactile Sensors Based on Carbon Nanotube Thin Films

Carbon nanotubes (CNTs) are an interesting material for mechanically flexible electronic devices. The possibility to fabricate CNT thin films from solution by scalable coating and printing techniques has paved the way for large-area and low-cost sensor applications. In this paper, we demonstrate mechanically flexible capacitive tactile sensors utilizing spray deposited CNT thin-films and microstructured PDMS spacers. The capacitance is monitored during sensor operation and forces smaller than 10 mN could be detected and changes of up to 20% are achieved for 1 N of applied force. Sensor response can be tailored through an appropriate design of the dielectric spacer. The long-term operational stability of the proposed sensors is demonstrated. Further, a theoretical model is proposed and fitted to experimental data, providing a powerful tool to facilitate the design process.

Woven flexible textile structure for wearable power-generating tactile sensor array

In this paper, we propose and demonstrate a power-generating tactile sensor array in which an energy harvester and an array of tactile sensors are integrated in a single device. The device consists of rows and columns of piezoelectric straps woven on a mesh structure of elastic hollow tubes. The fabricated device, which includes 5 × 5 capacitive tactile sensors in an area of 9 × 9 cm2, is highly flexible and stretchable. When the device was stretched in a lateral direction, the maximum output voltage and power were measured as 51 V and 850 μW, respectively. In addition, the capacitance value employed as a signal for the tactile sensor operation was measured while applying a force vertically to the surface using a force gauge. The initial capacitance and sensitivity of a single cell employed as a tactile sensor were approximately 1.727 pF and 40 fF/N, respectively, within a force range of 2 N.

Elastomeric Electronic Skin for Prosthetic Tactile Sensation

This report demonstrates a wearable elastomer‐based electronic skin including resistive sensors for monitoring finger articulation and capacitive tactile pressure sensors that register distributed pressure along the entire length of the finger. Pressure sensitivity in the order of 0.001 to 0.01 kPa−1 for pressures from 5 to 405 kPa, which includes much of the range of human physiological sensing, is achieved by implementing soft, compressible silicone foam as the dielectric and stretchable thin‐metal films. Integrating these sensors in a textile glove allows the decoupling of the strain and pressure cross‐sensitivity of the tactile sensors, enabling precise grasp analysis. The sensorized glove is implemented in a human‐in‐the‐loop system for controlling the grasp of objects, a critical step toward hand prosthesis with integrated sensing capabilities.

Soft dielectrics for capacitive sensing in robot skins: Performance of different elastomer types

In a capacitive tactile sensor the dielectric layer plays an important role: both its electrical and mechanical properties affect the capacitance variation and the sensor response in terms of sensitivity, since the deformation ability of the dielectric layer allows for the detection of small pressures, and spatial resolution, since the dielectric layer acts as a low-pass spatial filter decreasing the spatial resolution. In this work we comparatively evaluate the effect of different elastomers on the performance of a capacitive sensor designed to cover large areas of a robot body. In particular, we compare the sensitivity, the spatial resolution and the durability of the sensor using foams, bulk elastomers and high permittivity composites.

A modified analytical model to study the sensing performance of a flexible capacitive tactile sensor array

This paper presents a modified analytical model to study the sensing performance of a flexible capacitive tactile sensor array, which utilizes solid polydimethylsiloxane (PDMS) film as the dielectric layer. To predict the deformation of the sensing unit and capacitance changes, each sensing unit is simplified into a three-layer plate structure and divided into central, edge and corner regions. The plate structure and the three regions are studied by the general and modified models, respectively. For experimental validation, the capacitive tactile sensor array with 8 × 8 (= 64) sensing units is fabricated. Experiments are conducted by measuring the capacitance changes versus applied external forces and compared with the general and modified models’ predictions. For the developed tactile sensor array, the sensitivity predicted by the modified analytical model is 1.25%/N, only 0.8% discrepancy from the experimental measurement. Results demonstrate that the modified analytical model can accurately predict the sensing performance of the sensor array and could be utilized for model-based optimal capacitive tactile sensor array design.

A Flexible Capacitive Tactile Sensor Array With CMOS Readout Circuits for Pulse Diagnosis

Pulse diagnosis is one of the major methods used in traditional Chinese medicine to evaluate a patient's health conditions. Determination of pulse patterns is, however, a matter that depends on technical skill and subjective experience of a doctor. Development of a pulse diagnosis apparatus can lead to modernization and advancement of the field by providing more objective and accurate evaluations. This paper aims at demonstrating real-time pulse measurement using the developed flexible capacitive tactile sensors. A $5\times 5$ sensor array of 0.75 cm $\times $ 1 cm in size was implemented using flexible printed circuit boards and integrated with CMOS switched-capacitor readout circuits. The measured average output sensitivity was 1.57 mV/mmHg and minimum detectable pressure and capacitance were at the sub-mmHg and sub-fF levels, respectively. The cross sensitivity was measured to be 7.8%. Real-time pulse measurement has been successfully demonstrated using the developed flexible sensor array.

Modeling and Analysis of a Flexible Capacitive Tactile Sensor Array for Normal Force Measurement

This paper presents an analytical model to analyze the flexible capacitive tactile sensor array for normal force measurement. The tactile sensor array consists of six layers, including a bottom polyethylene terephthalate film, a lower copper plate, a polydimethylsiloxane film, an upper copper plate, a polyimide (PI) film, and a top PI bump, respectively. By simplifying the sensing unit into a two-layer structure, an analytical model is developed to study the normal force prediction. Finite element modeling (FEM) is conducted to analyze the deformation of the sensing unit, and is compared with the analytical model. Results show that analytical model predicted deformation matches well with the FEM analysis. The change of capacitance verse normal force is obtained, and can be utilized for accurate normal force measurement. Based on the established analytical model, the behaviors of the proposed sensor are analyzed by applying uneven force and shear force.

A CMOS micromachined capacitive tactile sensor with integrated readout circuits and compensation of process variations.

This paper presents a capacitive tactile sensor fabricated in a standard CMOS process. Both of the sensor and readout circuits are integrated on a single chip by a TSMC 0.35 μm CMOS MEMS technology. In order to improve the sensitivity, a T-shaped protrusion is proposed and implemented. This sensor comprises the metal layer and the dielectric layer without extra thin film deposition, and can be completed with few post-processing steps. By a nano-indenter, the measured spring constant of the T-shaped structure is 2.19 kNewton/m. Fully differential correlated double sampling capacitor-to-voltage converter (CDS-CVC) and reference capacitor correction are utilized to compensate process variations and improve the accuracy of the readout circuits. The measured displacement-to-voltage transductance is 7.15 mV/nm, and the sensitivity is 3.26 mV/μNewton. The overall power dissipation is 132.8 μW.

A flexible capacitive tactile sensor array with micro structure for robotic application

A flexible capacitive tactile sensor array with micro needle structure is proposed in this paper for robotic application. Micro needle layer made of polydimethylsiloxane (PDMS) is sandwiched between the upper electrode layer made of PDMS and the bottom electrode layer fabricated on polyester (PET) film. The PDMS material renders the device adequate flexibility as it can be rolled into a cylinder. The single cell size in the fabricated 4 × 4 sensors array is 0.7 × 0.5 cm2 and the initial capacitance of each cell is 0.86 pF. The fabricated cell shows a sensitivity of 3.26%/mN within the full scale range of 1 kPa. The micro needle structure gives better repeatability and stability. The maximum error during each measurement is about 3.2%, while the minimum error is about 1.2%.

OBJECT-SHAPE RECOGNITION BY TACTILE IMAGE ANALYSIS USING SUPPORT VECTOR MACHINE

The sense of touch is important to human to understand shape, texture, and hardness of the objects. An object under grip, i.e. object exploration by enclosure, provides a unique pressure distribution on the different regions of palm depending on its shape. This paper utilizes the above experience for recognition of object shapes by tactile image analysis. The high pressure regions (HPRs) are segmented and analyzed for object shape recognition rather than analyzing the entire image. Tactile images are acquired by capacitive tactile sensor while grasping a particular object. Geometrical features are extracted from the chain codes obtained by polygon approximation of the contours of the segmented HPRs. Two-level classification scheme using linear support vector machine (LSVM) is employed to classify the input feature vector in respective object shape classes with an average classification accuracy of 93.46% and computational time of 1.19 s for 12 different object shape classes. Our proposed two-level LSVM reduces the misclassification rates, thus efficiently recognizes various object shapes from the tactile images.