Artificial Tactile System and Signal Processing for Haptic applications

Human have the ability to interact with the external environment through five main senses which are vision, hearing, smell, taste and touch. Most of all, the sensation like vision or hearing have been well developed and the use of various applications like TV, Camera, or artificial cochlear have been widely generalized. As the next steps, recently, the tactile sensor to mimic the tactile system of human have been attracted by many groups. Especially, after the development of Apple’s iPhone, the public interest about touch sensing applications have been increased explosively. Other researches for tactile sensing have focused on enhancing the performance of tactile sensor like the sensitivity, stability, response time and so on. As a result, there are some researches that the sensor performance of certain criteria is better than that of human tactile system. However, a human tactile system is not only very sensitive but also complex. In other words, ultimately, the tactile system mimicking the human tactile sensation should detect various parameters such as the pressure, temperature, hardness or roughness and also decide the psychological feeling like the pain by a hot material in touching or the smooth/roughness feeling in sliding the certain material. Therefore, in this thesis, it has been studied for the development of multifunctional tactile sensing system detecting various tactile parameters and deciding the kinds of psychological tactile feeling by measured stimulation. As the first step for the development of tactile system, we have studied the tactile sensor using ZnO nanowire. Therefore, in this chapter, the basic characteristics of ZnO nanowire are investigated to confirm the possibility for the tactile sensor. In addition, structural design factors of sensor units have been studied in order to enhance the sensitivity of ZnO nanowire-based tactile sensor. We have primarily demonstrated the effect of a square pattern array design in a pressure sensor using ZnO nanowires. Nanowires grown on the edge of cells can be bent easily because of growth direction, density of nanowires, and buckling effect. Since smaller square pattern arrays induce a higher circumference to cell area ratio, if one sensor unit consists of many micro-level square pattern arrays, the design enhances the piezoelectric efficiency and the sensitivity. As a result, 20um × 20um cell arrays showed three times higher pressure sensitivity than 250um × 250um cell array structures at a pressure range from 4kPa to 14kPa. The induced piezoelectric voltage with the same pressure level also increased drastically. Therefore, the smaller pattern array design is more appropriate for a high-sensitive pressure sensor than a simple one-body cell design for tactile systems, and it has the advantage of better power efficiency, which is also important for artificial tactile systems. Even if, in previous experiments, the possibility of piezoelectric materials as the tactile sensor and the method for the enhancement of pressure sensitivity are confirmed well, the tactile sensor for mimicking the human tactile sensation should measure various parameters as well as the pressure. However, many studies about ‘smooth-rough’ sensation depend on the machine learning technology with simple tactile sensors rather than developing the sensors that can measure various parameters like surface topography, hardness, quality of materials at the same time. Therefore, after the development of the pressure sensor, specific structures based on PDMS are proposed to measure and analyze above-mentioned parameters related to ‘smooth-rough’ decision, as like fingerprint of human. To find the optimized structure, three kinds of the structure shape (cone, cylinder and dome) are fabricated and the pressure sensitivity according to the shape are also measured. FEM simulation is also carried out to support the experimental result. Our tactile sensor with optimized dome structure (500um height) provides high shear force sensitivity, fast response time, stability, and durability. The high sensitivity about the shear force enables better the tactile sensor to measure the various surface information such as the pitch of pattern, the depth, the sliding velocity, the hardness and so on. In addition, after the study to measure the various surface information by dome structure, the research to measure the other surface information is also followed. In our previous study, we confirmed that the surface topography can be reconstructed by mapping the piezoelectric signals according to the location. In this research, to reduce the number of measurements from dozens to once and minimize the data loss at the empty space between adjacent sensors, the electrode array of Zig-Zag type is applied to the tactile sensor. As a result, with just one measurement, the surface topography of broad region can be successfully reconstructed by our tactile sensor as the high-resolution image. Additionally, the temperature sensor based on the resistive mechanism is fabricated between the Zig-Zag electrode lines to measures the temperature of surface materials when the tactile sensor rubs on the materials in real time. Over the development of the tactile sensing applications, the demand for an artificial system like human tactile sensation have been much more increased. Therefore, in this study, as a surrogate for human tactile sensation, we propose an artificial tactile sensing system based on the developed sensors in previous sections. For this, the piezoelectric tactile signal generated by touching and rubbing the material is transferred to DAQ system connected with our tactile sensor. First, the system decides whether the contacted material is dangerous or not. If dangerous like sharp or hot materials, the warning signal is generated by our artificial tactile system. If not, the sensor connected with the system rubs the materials and detects the roughness of the materials. Especially, the human test data related to ‘soft-rough’ detection is applied to a deep learning structure allowing personalization of the system, because tactile responses vary among humans. This approach could be applied to electronic devices with tactile emotional exchange capabilities, as well as various advanced digital experiences. In this thesis, human-like tactile sensing system based on the piezoelectric effect is successfully confirmed through various experiments. Although there are still some issues that need to be improved, this research is expected to be fundamental results for human-like tactile sensing system detecting a variety of the parameters such as the pressure, temperature, surface morphology, hardness, roughness and so on. In the future, through collaborative research with other fields like brain science, signal processing, we hope that this research can mimic psychological tactile sensations and communicate emotional exchange with external environment like real human skin.YList of Contents Abstract i List of contents iii List of tables vi List of figures vii Ⅰ. INTRODUCTION 1 1.1 Motivation 1 1.2 Various transduction mechanisms for the tactile sensor 5 1.2.1 Capacitive mechanism 5 1.2.2 Resistive mechanism 6 1.2.3 Triboelectric effect 7 1.2.4 Piezoelectric effect 9 1.3 Objectives 12 1.4 Reference 13 II. BASIC CHARACTERISTICS AND THE METHOD FOR ENHANC-ING THE PRESSURE SENSITIVITY OF THE TACTILE SENSOR BASED ON ZnO NANOWIRE 19 2.1 Introduction 19 2.2 Basic characteristics of ZnO nanowire 22 2.3 Device Fabrication 31 2.4 Morphological and Electrical characteristics 33 2.5 Pattern structure for enhanced for pressure sensitivity 38 2.6 Simulation result of piezoelectric effect for pattern structure 42 2.7 Reference 46 III. DOME STRUCTURE TO MEAUSRE THE SURFACE INFOR-MATION 52 3.1 Introduction 52 3.2 Basic characteristics of P(VDF-TrFE) 53 3.3 Device fabrication 61 3.4 Interaction mechanism between dome structure and surface material 63 3.5 Simulation and Experimental result comparing cone, cylinder, and dome structure 64 3.6 Simulation and Experimental result of the sensitivity enhancement ef-fect by dome structure 66 3.7 Depth measurement by tactile sensor with dome structure 72 3.8 Pattern of pitch by multi-array tactile sensor with dome structure 77 3.9 Hardness measurement by the tactile sensor with dome structure 79 3.10 Reference 83 IV. ZIG-ZAG ARRAYED TACTILE SENSOR BASED ON PIEZOE-LECTRIC-RESISTIVE MECHANISM TO DETECT THE SURFACE TOPOG-RAPHY AND TEMPERATURE 87 4.1 Introduction 87 4.2 Device fabrication 88 4.3 Piezoelectric characteristics of fabricated tactile sensor 90 4.4 Surface rendering method by the piezoelectric effect 95 4.5 Surface rendering result of 3D printed materials 96 4.6 Temperature sensing in sliding the high temperature material on Zig-Zag tactile sensor 99 4.7 Reference 103 V. TACTILE SENSING SYSTEM FOR PAIN AND SMOOTH/ROUGH DETECTION 105 5.1 Introduction 105 5.2 Components of the tactile sensing system 107 5.3 Artificial tactile sensing system for generating the pain warning 108 5.4 Artificial tactile sensing system for smooth/rough sensing 112 5.5 Reference 117 VⅠ. CONCLUSION 120DoctordCollectio

Seedless hydrothermal growth of ZnO nanorods as a promising route for flexible tactile sensors

Hydrothermal growth of ZnO nanorods has been widely used for the development of tactile sensors, with the aid of ZnO seed layers, favoring the growth of dense and vertically aligned nanorods. However, seed layers represent an additional fabrication step in the sensor design. In this study, a seedless hydrothermal growth of ZnO nanorods was carried out on Au-coated Si and polyimide substrates. The effects of both the Au morphology and the growth temperature on the characteristics of the nanorods were investigated, finding that smaller Au grains produced tilted rods, while larger grains provided vertical rods. Highly dense and high-aspect-ratio nanorods with hexagonal prismatic shape were obtained at 75 °C and 85 °C, while pyramid-like rods were grown when the temperature was set to 95 °C. Finite-element simulations demonstrated that prismatic rods produce higher voltage responses than the pyramid-shaped ones. A tactile sensor, with an active area of 1 cm2, was fabricated on flexible polyimide substrate and embedding the nanorods forest in a polydimethylsiloxane matrix as a separation layer between the bottom and the top Au electrodes. The prototype showed clear responses upon applied loads of 2–4 N and vibrations over frequencies in the range of 20–800 Hz

Development of Multifunctional E-skin Sensors

Electronic skin (e-skin) is a hot topic due to its enormous potential for health monitoring, functional prosthesis, robotics, and human-machine-interfaces (HMI). For these applications, pressure and temperature sensors and energy harvesters are essential. Their performance may be tuned by their films micro-structuring, either through expensive and time-consuming photolithography techniques or low-cost yet low-tunability approaches. This PhD thesis aimed to introduce and explore a new micro-structuring technique to the field of e-skin – laser engraving – to produce multifunctional e-skin devices able to sense pressure and temperature while being self-powered. This technique was employed to produce moulds for soft lithography, in a low-cost, fast, and highly customizable way. Several parameters of the technique were studied to evaluate their impact in the performance of the devices, such as moulds materials, laser power and speed, and design variables. Amongst the piezoresistive sensors produced, sensors suitable for blood pressure wave detection at the wrist [sensitivity of – 3.2 kPa-1 below 119 Pa, limit of detection (LOD) of 15 Pa], general health monitoring (sensitivity of 4.5 kPa-1 below 10 kPa, relaxation time of 1.4 ms, micro-structured film thickness of only 133 µm), and robotics and functional prosthesis (sensitivity of – 6.4 × 10-3 kPa-1 between 1.2 kPa and 100 kPa, stable output over 27 500 cycles) were obtained. Temperature sensors with micro-cones were achieved with a temperature coefficient of resistance (TCR) of 2.3 %/°C. Energy harvesters based on micro-structured composites of polydimethylsiloxane (PDMS) and zinc tin oxide (ZnSnO3) nanowires (NWs; 120 V and 13 µA at > 100 N) or zinc oxide (ZnO) nanorods (NRs; 6 V at 2.3 N) were produced as well. The work described herein unveils the tremendous potential of the laser engraving technique to produce different e-skin devices with adjustable performance to suit distinct applications, with a high benefit/cost ratio

Wearable pressure sensing for intelligent gesture recognition

The development of wearable sensors has become a major area of interest due to their wide range of promising applications, including health monitoring, human motion detection, human-machine interfaces, electronic skin and soft robotics. Particularly, pressure sensors have attracted considerable attention in wearable applications. However, traditional pressure sensing systems are using rigid sensors to detect the human motions. Lightweight and flexible pressure sensors are required to improve the comfortability of devices. Furthermore, in comparison with conventional sensing techniques without smart algorithm, machine learning-assisted wearable systems are capable of intelligently analysing data for classification or prediction purposes, making the system ‘smarter’ for more demanding tasks. Therefore, combining flexible pressure sensors and machine learning is a promising method to deal with human motion recognition. This thesis focuses on fabricating flexible pressure sensors and developing wearable applications to recognize human gestures. Firstly, a comprehensive literature review was conducted, including current state-of-the-art on pressure sensing techniques and machine learning algorithms. Secondly, a piezoelectric smart wristband was developed to distinguish finger typing movements. Three machine learning algorithms, K Nearest Neighbour (KNN), Decision Tree (DT) and Support Vector Machine (SVM), were used to classify the movement of different fingers. The SVM algorithm outperformed other classifiers with an overall accuracy of 98.67% and 100% when processing raw data and extracted features. Thirdly, a piezoresistive wristband was fabricated based on a flake-sphere composite configuration in which reduced graphene oxide fragments are doped with polystyrene spheres to achieve both high sensitivity and flexibility. The flexible wristband measured the pressure distribution around the wrist for accurate and comfortable hand gesture classification. The intelligent wristband was able to classify 12 hand gestures with 96.33% accuracy for five participants using a machine learning algorithm. Moreover, for demonstrating the practical applications of the proposed method, a realtime system was developed to control a robotic hand according to the classification results. Finally, this thesis also demonstrates an intelligent piezoresistive sensor to recognize different throat movements during pronunciation. The piezoresistive sensor was fabricated using two PolyDimethylsiloxane (PDMS) layers that were coated with silver nanowires and reduced graphene oxide films, where the microstructures were fabricated by the polystyrene spheres between the layers. The highly sensitive sensor was able to distinguish throat vibrations from five different spoken words with an accuracy of 96% using the artificial neural network algorithm

Silver Nanowire Transparent Electrodes for Soft Optoelectronic and Electronic Devices

School of Energy and Chemical Engineering (Energy Engineering)Recently, with an increasing importance of human-machine interface along with the rapid growth of Internet of Things (IoT), various flexible and stretchable electronic and optoelectronic devices have been developed for the wide range of multifunctional and wearable applications such as touch screen panels, organic solar cells, organic light-emitting diodes, thin-film loudspeakers, microphones, interactive displays, and electronic skins. High mechanical flexibility/stretchability, optical transparency, and electrical conductivity are the critical properties that transparent conductive electrodes (TCEs) should possess for the realization of high-performance flexible/stretchable electronics and optoelectronics. While indium tin oxide (ITO) has been widely used in commercial TCEs, the further development and application of ITO have been limited by the high cost and inherent brittleness of the material. One promising alternative to ITO as a TCE material is silver nanowire (AgNW) networks having good flexibility and stretchability, which can provide lower sheet resistance (Rs) and higher optical transmittance (T) than other TCE candidates such as carbon nanotubes, graphene, and conducting polymers. Moreover, AgNW networks can be readily prepared by low-cost solution-based process, enabling the mass production of next-generation optoelectronic and electronic applications. The integration of AgNW networks with the flexible/stretchable substrates can provide powerful platforms to realize highly stable and high-performance soft optoelectronic and electronic devices with the superior transparency and stable supply of electrical conductivity during mechanical deformations. This thesis covers our recent studies about flexible/stretchable AgNW TCEs and their applications in various soft optoelectronic and functional electronic devices. First, chapter 1 introduces research trends in flexible/stretchable transparent electrodes and several issues of AgNW networks that should be carefully considered for their future soft optoelectronic and electronic device applications. In chapter 2, we demonstrated a simple and efficient assembly strategy for the large-area, highly cross-aligned AgNW arrays for TCE applications through a modified bar-coating assembly. As opposed to conventional solvent-evaporation-induced assemblies, which are slow and produce nonuniform conductive networks, our modified bar-coating strategy enables fast, efficient, and uniform alignment of AgNWs in a large-area by simply dragging the Meyer rod over the AgNW solution on the target substrates. For the potential applications, we demonstrated large-scale, flexible, and transparent resistive-type touch screens and force-sensitive mechanochromic touch screens using cross-aligned AgNW transparent electrodes which exhibited highly uniform and precise touch sensing performance across the entire region. In chapter 3, we introduced ultrathin, transparent, and conductive hybrid nanomembranes (NMs) with nanoscale thickness, consisting of the orthogonal AgNW arrays embedded in a polymer matrix. Here, we present a skin-attachable NM loudspeaker and wearable transparent NM microphone, which can emit thermoacoustic sound and can provide excellent acoustic sensing capabilities. In chapter 4, solution-processable, high-performance flexible alternating-current electroluminescent (ACEL) devices are developed based on high-k nanodielectrics and cross-aligned AgNW transparent electrodes. The solution-processed La-doped barium titanate (BTO:La) nanocuboids are fabricated as high dielectric constant nanodielectrics, which can enhance the dielectric constant of an ACEL devices, enabling the fabrication of high-performance flexible ACEL devices with a lower operating voltage as well as higher brightness. In chapter 5, we fabricated transparent, flexible, and self-healable thermoacoustic loudspeakers based on AgNW/poly(urethane-hindered urea) (PUHU) conductive electrodes. Our self-healable AgNW/PUHU electrodes exhibit a great self-healing property for the surface damages by means of the dynamic reconstruction of reversible bulky urea bonds in PUHU. In chapter 6, synesthetic bimodal generation of sound and color is demonstrated by stretchable sound-in-display devices consisting of strain-insensitive stretchable AgNW electrodes and field-induced inorganic EL phosphor emissive layers. The stretchable sound-in-display devices show highly robust and reliable EL and sound generating performances that can be repeatedly stretched and released without severe performance degradation because of the use of strain-insensitive AgNW electrodes. Finally, in chapter 7, we summarize this thesis along with the future perspective of flexible/stretchable transparent electrodes that should be considered for next-generation soft electronic and optoelectronic device applications. In this thesis, studies on flexible/stretchable AgNW transparent electrodes and their device applications could be further expanded for diverse soft and wearable optoelectronic and electronic applications such as wearable sensors, healthcare monitoring devices, and human-machine interfaces with better convenience, appearance, and reusability.ope

Structure-property relations in Sr, Nb, Ba doped lead zirconate titanate.

rhombohedral or tetragonal forms or as mixture of the two (MPB), depending on Zi:Ti ratio. Zr:Ti ratio strongly affected d sub 3 sub 3 , which was maximised in the tetragonal phase close to, but not at, the MPB. Sr sup 2 sup + substitution on the A-site promoted tetragonality in PZT, greatly reducing T sub C , and broadening the dielectric maximum. As the Sr sup 2 sup + content was increased, Zr:Ti ratio was adjusted to maximise d sub 3 sub 3 and the optimised d sub 3 sub 3 values increased from 410 pC/N (Sr sup 2 sup + = 0) to 640 pC/N (Sr sup 2 sup + = 0.12), commensurate with a decrease in the T sub C. However, for ceramics where Sr sup 2 sup + > 0.12, optimised d sub 3 sub 3 decreased with respect to the values for ceramics where Sr sup 2 sup + = 0.12 even though T sub C was lowered. Electron diffraction patterns revealed superlattice reflections occurring at 1/2 left brace hkl right brace positions associated with rotations of oxygen octahedra in anti-phase. It was suggested that Sr sup 2 sup + substitution on the A-site decreased the tolerance factor t, resulting in the onset of oxygen octahedral tilting. Co-doping PZT with Sr sup 2 sup + and Ba sup 2 sup + on the A-site resulted in the disappearance of the 1/2 left brace hkl right brace superlattice reflections. However, the d sub 3 sub 3 was not improved. Evidence of relaxor behaviour revealed by TEM in Sr, Ba co-doped PZT was thought to be responsible for the deterioration in piezoelectric properties. Effect of sintering temperature on the decomposition of perovskite phase was also examined. PbO loss was detected in Sr-doped PZT (PSZT) at the sample surface >= 1170 deg C, which was accompanied by the formation of a second phase. The second phase was identified as monoclinic ZrO sub 2. An increase in degree of tetragonality was also observed in the perovskite matrix. Lead zirconate titanate (PZT) ceramics have been utilised for several decades to fabricate electromechanical sensors and actuators. Compositional modifications have led to the development of 'hard' and 'soft' PZT's. Soft PZT's are used in applications such as receiving transducers requiring high sensitivity, pulsed transmitting transducers with high acoustic outputs, high sensitivity receivers and actuators with large displacements. In this investigation a systematic study was performed on a range of soft PZT materials. Their structures, microstructures and domain structures were characterised using X-ray diffraction, scanning and transmission electron microscopy. This data was then used to interpret dielectric, ferroelectric and piezoelectric properties. The relative importance of three known softening methods in PZT was assessed: i) donor doping using Nb sup 5 sup + on the B-site, ii) proximity to a morphotropic phase boundary (MPB), and iii) lowering the paraelectric-ferroelectric phase transition temperature (T sub C) by substitution of Sr sup 2 sup + or Ba sup 2 sup + on the A-site. Nb sup 5 sup + substitution onto the B-site reduced the grain size although the domain structure (approx 100 nm wide) remained the same except finer-scale domains (approx 20 nm wide) and higher dislocations were present in ceramics with 7.2 mol% Nb sup 5 sup +. T sub C was lowered and the piezoelectric coefficient (d sub 3 sub 3) increased. Nb-doped PZT could be stabilised either i