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.
Table Of Contents
List of Contents
Abstract i List of contents iii List of tables vi List of figures vii
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