Flexible Capacitive Tactile Sensor Research Articles

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.

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.

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 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%.

A flexible PDMS capacitive tactile sensor with adjustable measurement range for plantar pressure measurement

A flexible capacitive tactile sensor with adjustable characteristics, i.e., measurement range and sensitivity, has been developed. The proposed sensor is designed for large pressure measurement; therefore, polydimethylsiloxane (PDMS) material is selected as the material of the dielectric layer between the parallel plate electrodes of the sensor. Since the elasticity of the PDMS material can be adjusted by the mixing ratio of PDMS pre-polymer and curing agent during formation, sensors in different measurement ranges, i.e., 240–1,000 and 400–3,000 kPa, and corresponding sensitivities, i.e., 2.24 and 0.28 %/MPa, were respectively constructed and demonstrated. These measurement ranges are suitable for most of the biomechanical applications, especially for plantar pressure measurement. Moreover, because the output of the sensor, i.e., capacitance, is highly influenced by the dimension of the sensor structure, each sensor consists of four independent capacitance elements. The output of each sensor is averaged by four capacitances for single force measurement. This could improve the measurement accuracy in practical situation. Also, linearity of the measurement response could be enhanced and it was shown by the R-squared values in two measurement ranges, i.e., 0.9751 and 0.9881, respectively. The proposed sensor is flexible and miniaturized and has the potential to be applied to biomechanical applications.

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.

Simple and Low-Cost Fabrication of Flexible Capacitive Tactile Sensors

We fabricated novel flexible capacitive tactile sensors that can detect both the direction and distribution of an applied force. The sensor consists of a conductive silicone rubber as a movable electrode and fixed electrodes formed on a poly(ethylene terephthalate) (PET) film with silver paste. The dimensions of sensor head are 10.4 ×10.4 ×0.8 mm3. We developed the tactile sensor using screen-printing techniques and atmospheric-pressure plasma pretreatments. We experimentally evaluated the tactile responses to the applied force. The sensor detected the input force-distribution with a force ranging up to 10 N. The flexible thin PET film substrate enables fitting to the tight space and curved surface, and this capacitive tactile sensor is applicable for low-cost controllers of cellular phones and personal computers.

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

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

Simple and Low-Cost Fabrication of Flexible Capacitive Tactile Sensors

Simple and Low-Cost Fabrication of Flexible Capacitive Tactile Sensors

Capacitive tactile sensor array for touch screen application

In this paper, we propose and demonstrate a transparent and flexible capacitive tactile sensor which is designed for multi-touch screen application with force sensing. A sensor module is composed of 2D array tactile cells with a spatial resolution of 2 mm to measure the touch force at multiple positions. The device is fabricated by using transparent materials on a transparent plastic substrate. The optical transmittance of the fabricated tactile sensor is approximately 86% in the visible wavelength region, and the maximum bending radius is approximately 30 mm. The cell size is 1 mm × 1 mm, and the initial capacitance of each cell is approximately 900 fF. The tactile response of a cell is measured with a commercial force gauge having a resolution of 1 mN. The sensitivity of a cell is 4%/mN within the full scale range of 0.3 N.

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.