Robot Skin Research Articles

Large-Scale and Flexible Self-Powered Triboelectric Tactile Sensing Array for Sensitive Robot Skin.

Advances in flexible and multifunctional electronic devices have enabled the realization of sophisticated skin for robotics applications. In this paper, a large-scale, flexible and self-powered tactile sensing array (TSA) for sensitive robot skin is demonstrated based on the triboelectric effect. The device, with 4 × 4 sensing units, was composed of a top triboelectric polyethylene terephthalate (PET) layer, a bottom triboelectric copper (Cu) layer and a bottom PET substrate. A low-cost roll-to-roll ultraviolet embossing fabrication process was induced to pattern the large-scale top PET film with microstructures for high-output performance. The working mechanism and output performance of the triboelectric TSA were demonstrated and characterized, exhibiting good stability and high sensitivity. By integrating a tactile feedback system, the large-scale TSA, acting as intelligent skin for an industrial robot, was able to realize emergency avoidance and safety stop for various unknown obstacles under various working conditions. The system also has good real-time performance. By using a large-scale roll-to-roll fabrication method, this work pushes forward a significant step to self-powered triboelectric TSA and its potential applications in intelligent robot skin.

Paper/Carbon Nanotube-Based Wearable Pressure Sensor for Physiological Signal Acquisition and Soft Robotic Skin.

A wearable and flexible pressure sensor is essential to the realization of personalized medicine through continuously monitoring an individual's state of health and also the development of a highly intelligent robot. A flexible, wearable pressure sensor is fabricated based on novel single-wall carbon nanotube /tissue paper through a low-cost and scalable approach. The flexible, wearable sensor showed superior performance with concurrence of several merits, including high sensitivity for a broad pressure range and an ultralow energy consumption level of 10-6 W. Benefited from the excellent performance and the ultraconformal contact of the sensor with an uneven surface, vital human physiological signals (such as radial arterial pulse and muscle activity at various positions) can be monitored in real time and in situ. In addition, the pressure sensors could also be integrated onto robots as the artificial skin that could sense the force/pressure and also the distribution of force/pressure on the artificial skin.

Octopus-inspired robot skin goes 3-D

Engineers have used the fluid and flexible nature of the octopus as inspiration for designing soft-bodied robots and artificial skins that change color or glow. Now, researchers have captured another facet of octopus camouflage: three-dimensional skin texturing. Cephalopods such as octopuses and cuttlefish raise specialized bumps on their skin, called papillae, to blend into their surroundings. One papilla can form a variety of shapes, such as a cone or trilobe, depending on how the muscles within it are arranged. To create an artificial skin that mimics this 3-D camouflage strategy, a team led by Robert F. Shepherd of Cornell University embedded laser-cut fiber mesh into a mold containing a silicone liquid. The mesh—a common nonwoven nylon embroidery stabilizer—provides structural support to cured silicone when the material is inflated and, depending on the laser-cut pattern, helps create a targeted shape (Science 2017, DOI: 10.1126/science.aan5627). The engineers used an algorithm, developed by

Highly sensitive, self-powered and wearable electronic skin based on pressure-sensitive nanofiber woven fabric sensor

The wearable electronic skin with high sensitivity and self-power has shown increasing prospects for applications such as human health monitoring, robotic skin, and intelligent electronic products. In this work, we introduced and demonstrated a design of highly sensitive, self-powered, and wearable electronic skin based on a pressure-sensitive nanofiber woven fabric sensor fabricated by weaving PVDF electrospun yarns of nanofibers coated with PEDOT. Particularly, the nanofiber woven fabric sensor with multi-leveled hierarchical structure, which significantly induced the change in contact area under ultra-low load, showed combined superiority of high sensitivity (18.376 kPa−1, at ~100 Pa), wide pressure range (0.002–10 kPa), fast response time (15 ms) and better durability (7500 cycles). More importantly, an open-circuit voltage signal of the PPNWF pressure sensor was obtained through applying periodic pressure of 10 kPa, and the output open-circuit voltage exhibited a distinct switching behavior to the applied pressure, indicating the wearable nanofiber woven fabric sensor could be self-powered under an applied pressure. Furthermore, we demonstrated the potential application of this wearable nanofiber woven fabric sensor in electronic skin for health monitoring, human motion detection, and muscle tremor detection.

A Tactile-Based Framework for Active Object Learning and Discrimination using Multimodal Robotic Skin

In this letter, we propose a complete probabilistic tactile-based framework to enable robots to autonomously explore unknown workspaces and recognize objects based on their physical properties. Our framework consists of three components: 1) an active pretouch strategy to efficiently explore unknown workspaces; 2) an active touch learning method to learn about unknown objects based on their physical properties (surface texture, stiffness, and thermal conductivity) with the least number of training samples; and 3) an active touch algorithm for object discrimination, which selects the most informative exploratory action to apply to the object, so that the robot can efficiently distinguish between objects with a few number of actions. Our proposed framework was experimentally evaluated using a robotic arm equipped with multimodal artificial skin. The robot with the active pretouch method reduced the uncertainty of the workspace up to 30% and 70% compared to uniform and random strategies, respectively. By means of the active touch learning algorithm, the robot used 50% fewer samples to achieve the same learning accuracy than the baseline methods. By taking advantage of the prior knowledge obtained during the learning process, the robot actively discriminated objects with an improvement of 10% recognition accuracy compare to the random action selection approach.

A flexible dual-mode proximity sensor based on cooperative sensing for robot skin applications.

A flexible dual-mode proximity sensor has been designed and implemented, which is capable of combining capacitive-resistive detection in this paper. The capacitive type proximity sensor detecting is defined as mode-C, and the resistive type proximity sensor detecting is defined as mode-R. The characteristics of the proximity sensor are as follows: (1) the theoretical mode is developed which indicates that this proximity sensor can reflect proximity information accurately; (2) both sensing modes are vertically integrated into a sandwich-like chip with an 8 mm × 12 mm unit area. The thickness of a mode-R sensing material (graphene nanoplatelets) and mode-C dielectric (the mixture of carbon black and silicone rubber) is 1 mm and 2.5 mm, respectively; (3) for mode-R, the linearity of temperature-resistance curve can achieve 0.998 in the temperature range from 25 °C to 65 °C. And for mode-C, various materials can be successfully detected with fast response and high reversibility. Meanwhile, the study compensated the influence of object temperature to ensure mode-C properly works. A cooperative sensing test shows that R-C dual modes sense effectively which can enlarge the sensing distance compared with the single mode proximity sensor. The fabrication of this sensor is convenient, and the integrity of a flexible sandwich-like structure based on dual modes is beneficial to form arrays, which is suitable to be used in skin-like sensing applications.

A force and thermal sensing skin for robots in human environments

Working together, heated and unheated temperature sensors can recognize contact with different materials and contact with the human body. As such, distributing these sensors across a robot’s body could be beneficial for operation in human environments. We present a stretchable fabric-based skin with force and thermal sensors that is suitable for covering areas of a robot’s body, including curved surfaces. It also adds a layer of compliance that conforms to manipulated objects, improving thermal sensing. Our open hardware design addresses thermal sensing challenges, such as the time to heat the sensors, the efficiency of sensing, and the distribution of sensors across the skin. It incorporates small self-heated temperature sensors on the surface of the skin that directly make contact with objects, improving the sensors’ response times. Our approach seeks to fully cover the robot’s body with large force sensors, but treats temperature sensors as small, point-like sensors sparsely distributed across the skin. We present a mathematical model to help predict how many of these point-like temperature sensors should be used in order to increase the likelihood of them making contact with an object. To evaluate our design, we conducted tests in which a robot arm used a cylindrical end effector covered with skin to slide objects and press on objects made from four different materials. After assessing the safety of our design, we also had the robot make contact with the forearms and clothed shoulders of 10 human participants. With 2.0 s of contact, the actively-heated temperature sensors enabled binary classification accuracy over 90% for the majority of material pairs. The system could more rapidly distinguish between materials with large differences in their thermal effusivities (e.g., 90% accuracy for pine wood vs. aluminum with 0.5 s of contact). For discrimination between humans vs. the four materials, the skin’s force and thermal sensing modalities achieved 93% classification accuracy with 0.5 s of contact. Overall, our results suggest that our skin design could enable robots to recognize contact with distinct task-relevant materials and humans while performing manipulation tasks in human environments.

Large area and flexible micro-porous piezoelectric materials for soft robotic skin

The need for flexible, highly sensitive tactile sensors that can fit onto curved surfaces is driving the conformable sensor materials research in the field of human–machine interactions. Here we report a new type of compliant piezoelectric active composite, a micro-porous polyurethane-PZT material, capable of generating a voltage output upon touch. The composites are synthesized with the aim of maximizing the piezoelectric sensitivity of particulate composite sensor materials. The goal is to reduce the dielectric constant of the polymer matrix and improve flexibility of conventional bulk piezo-composites, consisting of ceramic particles in a dense polymeric matrix, by adding a third (gaseous) phase to the system in the form of uniformly sized pores. The presence of the gaseous component in the polymer matrix in the form of well-distributed spherical inclusions effectively decreases the polymer dielectric permittivity, which increases the piezoelectric voltage sensitivity (g33) of the composite sensors significantly. The unique combination of dielectrophoretic structuring of PZT particles and the addition of a gaseous phase to the polymer resin results in the highest performance of the particulate composite sensors reported in the literature so far. The newly developed micro-porous composites show g33 value of 165mVm/N that is twice that of the structured PZT-bulk PU composites (80mVm/N) and more than five times the g33 value of bulk PZT ceramics (24–28mVm/N). The capability of the flexible freestanding sensors for application in touch sensing devices for soft robotics is demonstrated.

A novel flexible tactile sensor based on Ce-doped BaTiO3 nanofibers

The performance of a robotic hand is severely limited by the tactile feedback information similar to a human hand. Hence, a novel and robust tactile sensor has been developed to cope with the challenge of robotic hand technology. Piezoelectric material is proposed as a suitable candidate for a new efficient tactile sensor due to its excellent piezoelectric properties. In this paper, a novel flexible tactile sensor based on Ce-doped BTO nanofibers was developed. The doping mechanism of cerium ions and the working process of the sensor were analysed. The results showed that sheer stress had no contribution to the sensor, this indicated that the sensor was easy to control according to the individual’s wish. The output voltage of the sensor could reach up to 0.078 V which showed great potential for the future of intelligent robot skin application.

On the Polygon Containment Problem on an Isometric Grid

This paper addresses the issue of placing a simple polygon (upon translation and rotation) on an isometric triangular grid such that the polygon contains the maximum number of triangles in its closure. This solves the problem left open in two recent papers titled “On the problem of the automated design of large-scale robot skin” and “An improved algorithm for the automated design of large scale robot skin” published in the IEEE Transactions on Automation Science and Engineering . Based on the properties of the grid, an improved algorithm is also presented. We also present some experimental results describing the use of this algorithm.

Development of a Hybrid Piezo Natural Rubber Piezoelectricity and Piezoresistivity Sensor with Magnetic Clusters Made by Electric and Magnetic Field Assistance and Filling with Magnetic Compound Fluid.

Piezoelements used in robotics require large elasticity and extensibility to be installed in an artificial robot skin. However, the piezoelements used until recently are vulnerable to large forces because of the thin solid materials employed. To resolve this issue, we utilized a natural rubber and applied our proposed new method of aiding with magnetic and electric fields as well as filling with magnetic compound fluid (MCF) and doping. We have verified the piezoproperties of the resulting MCF rubber. The effect of the created magnetic clusters is featured in a new two types of multilayered structures of the piezoelement. By measuring the piezoelectricity response to pressure, the synergetic effects of the magnetic clusters, the doping and the electric polymerization on the piezoelectric effect were clarified. In addition, by examining the relation between the piezoelectricity and the piezoresistivity created in the MCF piezo element, we propose a hybrid piezoelement.

Tactile Sensors: Rapid Manufacturing of Mechanoreceptive Skins for Slip Detection in Robotic Grasping (Adv. Mater. Technol. 1/2017)

In article number 1600188, Alexi Charalambides and Sarah Bergbreiter present a rapid manufacturing process using computerized numerical control (CNC) milling to create a “robot skin” with normal and shear force tactile sensing. The robot skin is made entirely of elastomer and contains tactile pixels with microscale features distributed over a large area. Using this approach, the robot skin is integrated with a 1 degree-off-reedom gripper for closed-loop grasping and slip detection.

Stretchable electronic skin based on silver nanowire composite fiber electrodes for sensing pressure, proximity, and multidirectional strain.

Electronic skin (E-skin) has been attracting great research interest and effort due to its potential applications in wearable health monitoring, smart prosthetics, robot skins and so on. To expand its applications, two key challenges lie in the realization of device stretchability, and independent sensing of pressure and multidirectional lateral strain. Here we made a combination of rational device structure and artfully engineered sensing materials to fulfill the mentioned demands. The as-prepared E-skin took a simple orthogonal configuration to enable both capacitive mode for pressure sensing and resistive mode for multidirectional strain sensing, independently. Pre-cracked silver nanowire based fibers with helical microstructures were utilized as basic electrodes to endow the E-skin with intrinsic stretchability and strain sensing capability. Through dielectric layer optimization, the pressure sensing sensitivity was greatly enhanced, with a detection limit of 1.5 Pa. For application demonstrations, we utilized the E-skin as both flat and curved platforms for pressure mapping, and also as human motion sensors, such as palm and thumb bending.

Flexible Pressure Sensor with Ag Wrinkled Electrodes Based on PDMS Substrate.

Flexible pressure sensors are essential components of electronic skins for future attractive applications ranging from human healthcare monitoring to biomedical diagnostics, robotic skins, and prosthetic limbs. Here we report a new kind of flexible pressure sensor. The sensors are capacitive, and composed of two Ag wrinkled electrodes separated by a carbon nanotubes (CNTs)/polydimethylsiloxane (PDMS) composite deformable dielectric layer. Ag wrinkled electrodes were formed by vacuum deposition on top of pre-strained and relaxed PDMS substrates which were treated using an O2 plasma, a surface functionalization process, and a magnetron sputtering process. Ultimately, the developed sensor exhibits a maximum sensitivity of 19.80% kPa−1 to capacitance, great durability over 500 cycles, and rapid mechanical responses (<200 ms). We also demonstrate that our sensor can be used to effectively detect the location and distribution of finger pressure.

Skinning a Robot: Design Methodologies for Large-Scale Robot Skin

Providing a robot with large-scale tactile-sensing capabilities requires the use of design tools, bridging the gap between user requirements and technical solutions. Given a set of functional requirements (e.g., minimum spatial sensitivity or minimum detectable force), two prerequisites must be considered: 1) the capability of the chosen tactile technology to satisfy these requirements from a technical standpoint and 2) the ability of the customization process to find a tradeoff among different design parameters, such as (in case of robot skins based on the capacitive principle) dielectric thickness, diameter of sensing points, and weight. The contribution of this article is twofold: 1) a description of the possibilities offered by a design toolbox for large-scale robot skin based on finite element analysis and optimization principles, which provides a designer with insights and alternative choices to obtain a given tactile performance according to the scenario at hand and 2) a discussion about the intrinsic limitations in simulating robot skin.

The Design and Characterization of a Flexible Tactile Sensing Array for Robot Skin.

In this study, a flexible tactile sensing array based on a capacitive mechanism was designed, fabricated, and characterized for sensitive robot skin. A device with 8 × 8 sensing units was composed of top and bottom flexible polyethyleneterephthalate (PET) substrates with copper (Cu) electrodes, a polydimethylsiloxane (PDMS) dielectric layer, and a bump contact layer. Four types of microstructures (i.e., pyramids and V-shape grooves) atop a PDMS dielectric layer were well-designed and fabricated to enhance tactile sensitivity. The optimal sensing unit achieved a high sensitivity of 35.9%/N in a force range of 0–1 N. By incorporating a tactile feedback control system, the flexible sensing array as the sensitive skin of a robotic manipulator demonstrated a potential capability of robotic obstacle avoidance.

Stretchable Triboelectric Fiber for Self-powered Kinematic Sensing Textile.

Stretchable fiber and yarn triboelectric nanogenerator are sought for such applications as wearable sensing system such as cloth communication devices, electronic textiles, and robotic sensory skin. Unfortunately, previously reported triboelectric fiber and yarn are difficult to have stretchable property. We introduce here a new type of stretchable and weavable triboelectric fibers with microdiameter dimensions. The stretchable triboelectric fibers can be reversibly stretched up to 50% in tensile direction while generating voltage output proportional to the applied tensile strain. The reversible distance change induced by the Poisson’s ratio difference between the core fiber (silver-coated nylon/polyurethane) and the shell (wrinkled polyvinylidene fluoride-co-trifluoroethylene/carbon nanotube layer) during tensile deformation is the key working principle for electrical generation. Owing to exceptional structural stability, the stretchable triboelectric fibers show high performance retention after 10,000 times repeated stretching/releasing cycle. Furthermore, the stretchable triboelectric fibers are mechanically strong to be woven into a commercial textile for textile based sensors, which can detect magnitude as well as direction of the motion.

Three-Dimensional Arrays of Lead Halide Perovskite Nanowire Based Image Sensors

Lead halide perovskite materials are emerging as highly promising materials for high performance optoelectronic devices thus triggered broad attention globally. In this work, we report for the first time a chemical vapor deposition (CVD) process to grow ordered three-dimensional (3-D) MAPbI3 NW arrays in nanoengineering templates. This unique CVD process utilizes Pb metal nanoclusters at the bottom of vertical nanochannels to initiate high quality MAPbI3 NW growth. As the nanochannels have largely controllable geometrical factors, namely, periodicity, diameter and depth, NW geometry can also be precisely nanoengineered. As the result, the ordered 3-D NW arrays can achieve ultra-high NW density in the range of 4×108/cm2~109/cm2 at a sizable scale of ~9 cm2. The 3-D NW arrays are conspicuously promising for 3-D integrated nano-electronics/optoelectronics. To further demonstrate the technological potency of the perovskite NW arrays, they have been fabricated into proof-of-concept image sensors. Each image sensor consists of 1,024 photodiode pixels made of vertical perovskite NWs, and the imaging functionality has been verified by recognizing various optical patterns projected on the sensor. It was found that the NW sensors can respond to dynamic optical input with reasonable speed, thus video capturing function of the NW image sensor was also successfully demonstrated. As the diameter of each NW can be as small as hundreds of nanometers and each NW can serve as one sensor pixel, this unique image sensor design can potentially lead to extremely high resolution approaching optical diffraction limit. In addition, fabrication and characterization of flexible image sensors have also been demonstrated in this work. The flexible optical sensor arrays can find broad applications for wearable electronics, electronic eyes, multifunctional robotic and artificial skins, etc. Lead halide perovskite materials are emerging as highly promising materials for high performance optoelectronic devices thus triggered broad attention globally. In this work, we report for the first time a chemical vapor deposition (CVD) process to grow ordered three-dimensional (3-D) MAPbI3 NW arrays in nanoengineering templates. This unique CVD process utilizes Pb metal nanoclusters at the bottom of vertical nanochannels to initiate high quality MAPbI3 NW growth. As the nanochannels have largely controllable geometrical factors, namely, periodicity, diameter and depth, NW geometry can also be precisely nanoengineered. As the result, the ordered 3-D NW arrays can achieve ultra-high NW density in the range of 4×108/cm2~109/cm2 at a sizable scale of ~9 cm2. The 3-D NW arrays are conspicuously promising for 3-D integrated nano-electronics/optoelectronics. To further demonstrate the technological potency of the perovskite NW arrays, they have been fabricated into proof-of-concept image sensors. Each image sensor consists of 1,024 photodiode pixels made of vertical perovskite NWs, and the imaging functionality has been verified by recognizing various optical patterns projected on the sensor. It was found that the NW sensors can respond to dynamic optical input with reasonable speed, thus video capturing function of the NW image sensor was also successfully demonstrated. As the diameter of each NW can be as small as hundreds of nanometers and each NW can serve as one sensor pixel, this unique image sensor design can potentially lead to extremely high resolution approaching optical diffraction limit. In addition, fabrication and characterization of flexible image sensors have also been demonstrated in this work. The flexible optical sensor arrays can find broad applications for wearable electronics, electronic eyes, multifunctional robotic and artificial skins,etc.

A flexible touch-pressure sensor array with wireless transmission system for robotic skin.

Human skin contains multiple receptors and is able to sense various stimuli such as temperature, touch, pressure, and deformation, with high sensitivity and resolution. The development of skin-like sensors capable of sensing these stimuli is of great importance for various applications such as robots, touch detection, temperature monitoring, and strain gauges. Great efforts have been made to develop high performance touch sensor and pressure sensor. Compared with general sensor, the touch-pressure sensor which is reported in this paper not only can measure large pressure but also has a high resolution in the small range so that it can feel slight touch. The sensor has a vertical structure. The upper layer is made of silicone rubber as the capacitive layer and the lower layer employs multiwall carbon nanotubes and carbon black filled silicone rubber as the resistive layer. The electrodes are made by conductive silver adhesives. In addition, the electrodes are connected to the pads on the top surface of the flexible printed circuit board by enamelled wires which made it easier to fabricate sensor array. The resolution of the touch-pressure sensor in the range of 0-10 N and 10-100 N are 0.1 N and 1 N, respectively. The experimental data of the sensor are sent by ZigBee wireless technology which reduces the complexity of the wiring and provides a convenient way to apply and maintain the sensor array.

Stretchable Strain Sensor With Anisotropy and Application for Joint Angle Measurement

Flexible and stretchable strain sensors are expected to significantly contribute to new technologies and be applied in various applications, including human interfaces, smart clothes, and robotic skin. Hence, we have proposed a flexible film-shaped strain sensor that is composed of three elastomer films and two carbon-nanotube electrodes. It is thin, lightweight, and has low elasticity. Strain measurements taken by the sensor are based on changes in capacitance, which is proportional to the square of the area of the sensor’s sensing part. Consequently, the capacitance of the strain sensor does not show the direction of the strain. As some applications require the measurement of strain in a fixed direction, this paper proposes an anisotropic strain sensor. This anisotropic strain sensor decreases the effect of the strain in the fixed direction to less than 10%. The model of the anisotropic strain sensor and experimental results show that its capacitance is proportional to the square of the length of the sensing part in the stretch direction. To check the anisotropic strain sensor’s practicality, it is applied to estimate the angle of a wrist joint. The results show that the anisotropy decreases the root-mean-square error of the estimated angle to less than 3°.