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

Integrated Piezoelectric AlN Thin Film with SU-8/PDMS Supporting Layer for Flexible Sensor Array

This research focuses on the development of a flexible tactile sensor array consisting of aluminum nitride (AlN) based on micro-electro-mechanical system (MEMS) technology. A total of 2304 tactile sensors were integrated into a small area of 2.5 × 2.5 cm2. Five hundred nm thick AlN film with strong c-axis texture was sputtered on Cr/Au/Cr (50/50/5 nm) layers as the sacrificial layer coated on a Si wafer. To achieve device flexibility, polydimethylsiloxane (PDMS) polymer and SU-8 photoresist layer were used as the supporting layers after etching away a release layer. Twenty-five mM (3-mercaptopropyl) trimethoxysilane (MPTMS) improves the adhesion between metal and polymers due to formation of a self-assembled monolayer (SAM) on the surface of the top electrode. The flexible tactile sensor has 8 × 8 channels and each channel has 36 sensor elements with nine SU-8 bump blocks. The tactile sensor array was demonstrated to be flexible by bending 90 degrees. The tactile sensor array was demonstrated to show clear spatial resolution through detecting the distinct electrical response of each channel under local mechanical stimulus. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.1

Tactile Avatar: Tactile Sensing System Mimicking Human Tactile Cognition

As a surrogate for human tactile cognition, an artificial tactile perception and cognition system are proposed to produce smooth/soft and rough tactile sensations by its user's tactile feeling; and named this system as “tactile avatar”. A piezoelectric tactile sensor is developed to record dynamically various physical information such as pressure, temperature, hardness, sliding velocity, and surface topography. For artificial tactile cognition, the tactile feeling of humans to various tactile materials ranging from smooth/soft to rough are assessed and found variation among participants. Because tactile responses vary among humans, a deep learning structure is designed to allow personalization through training based on individualized histograms of human tactile cognition and recording physical tactile information. The decision error in each avatar system is less than 2% when 42 materials are used to measure the tactile data with 100 trials for each material under 1.2N of contact force with 4cm s−1 of sliding velocity. As a tactile avatar, the machine categorizes newly experienced materials based on the tactile knowledge obtained from training data. The tactile sensation showed a high correlation with the specific user's tendency. This approach can be applied to electronic devices with tactile emotional exchange capabilities, as well as advanced digital experiences. © 2021 The Authors. Advanced Science published by Wiley-VCH GmbH1

Development of Clinical Weekly-Dose Teriparatide Acetate Encapsulated Dissolving Microneedle Patch for Efficient Treatment of Osteoporosis

Teriparatide acetate (TA), which directly promotes bone formation, is subcutaneously injected to treat osteoporosis. In this study, TA with a once-weekly administration regimen was loaded on dissolving microneedles (DMNs) to effectively deliver it to the systemic circulation via the transdermal route. TA activity reduction during the drying process of various TA polymer solutions formulated with hyaluronic acid and trehalose was monitored and homogeneities were assessed. TA-DMN patches fabricated using centrifugal lithography in a two-layered structure with dried pure hyaluronic acid on the base layer and dried TA polymer solution on the top layer were evaluated for their physical properties. Rhodamine-B-loaded TA-DMNs were found to form perforations when inserted into porcine skin using a shooting device. In addition, 87.6% of TA was delivered to the porcine skin after a 5-min TA-DMN patch application. The relative bioavailability of TA via subcutaneous injection was 66.9% in rats treated with TA-DMN patches. The maximal TA concentration in rat plasma was proportional to the number of patches used. Therefore, the TA-DMN patch fabricated in this study may aid in the effective delivery of TA in a patient-friendly manner and enhance medical efficacy in osteoporosis treatment

Tactile Sensor Structure Optimized for Sliding Motion with High Resolution Recording of Surface Topography

The demand for tactile devices with human-like exquisiteness has recently been increasing in various fields. Among the various parameters that humans feel through the tactile system, temperature and surface topography are the most important parameters to achieve artificial tactile devices with human level precision. Here, we present a new tactile sensor with high resolution surface topography recording and temperature measurement. Our tactile sensor was designed to have a surface topography sensing part driven by P(VDF-TrFE) and a thermistor part for temperature sensing. Even though the sensor touched the same temperature object, the sliding condition showed different resistance change values. Therefore, the correlation factors of the sliding velocity to temperature were defined with a simple relation function. To achieve high resolution recording, ‘zig-zag’ arrayed tactile sensor can overcome the limitations of simple matrix cell design. The design eliminates empty spaces between sensor cells. By using these systems, a resolution of ~500μm to x-direction was achieved. This tactile sensor has linear sensitivity to pressure with a superior response time of 10ms for dynamic sensing. By integrating the dynamic piezoelectric signals during the sliding motion, we can reconstruct the surface topography of various objects. IEEEFALS

Deep Neural Network Classification of Tactile Materials Explored by Tactile Sensor Array With Various Active-Cell Formations

Reducing the input data of tactile sensory systems brings a large degree of freedom to real-world implementations from the perspectives of bandwidth and computational complexity. For this, in this letter, we suggest efficient active-cell formations with a high classification accuracy of tactile materials. By revealing that averaged Kullback-Leibler-divergence and common frequency component power to variance ratio are proportional to the classification accuracy, we showed that those methods can be useful in estimating valid active-cell formations.1

Artificial Tactile Sensor with Pin-type Module for Depth Profile and Surface Topography Detection

Haptic sensors based on piezoelectric sensor arrays with pin-type modules which have high responses and dynamic sensing capabilities were designed and studied for surface topography measurements. Unlike the human finger, most flexible tactile sensor designs do not detect well the depth information of surfaces which change at the ~mm level despite the fact that they have a good sensitivity for pressure or force. To enhance the ability to detect depth information of the surfaces of objects, a piezoelectric sensor combining a pin-type module with excellent monitoring in the depth direction by spring is developed here. Because spike types of piezoelectric signals do not match directly a specific surface topography, a signal processing method that reconstructs the surface topography was studied considering the piezoelectric working principle and spring dynamics. According to the sensor design, it can detect 3mm depth changes with a two-dimensional plane structure at mm-level resolutions. The results showed that that the proposed sensor could measure various shapes and depth profiles precisely via a sliding motion, and the surface topography was reconstructed through highly accurate measurement results, similar to a human.1

Fundamental insights into the electrical signals of a piezoelectric sensor in a sliding condition

The piezoelectric mechanism represents a promising approach for advanced sensors that measure physical factors such as pressures, strain levels and even temperature responses to various stimuli. However, a conventional analysis of piezoelectric signals is mainly related to the peak output voltage for normal force, meaning that there remain numerous challenges to overcome when analyzing piezoelectric signals over time corresponding to complex or dynamic situations. Here, a fundamental analysis of piezoelectric signals for a dynamic change situation induced by sliding motion and resulting in the partial deformation of a piezoelectric material is introduced. Given that a piezoelectric voltage at a certain time represents the sum value of diploe moments in all piezoelectric material segments, in this respect, some parts are compressed and others are released, and we compared the electric signals between small segments and one unit cell to obtain a clue about the signal process and to confirm the mathematical formula. Based on the results of piezoelectric signal fitting with the exponential and error function, general solutions that can model piezoelectric signals were proposed to create artificial piezoelectric signals which corresponded to each small segment of the piezoelectric material. By comparing the artificial signals and actually measured signals, the general solution was optimized and the induced artificial piezoelectric signals were found to have good reliability and reproducibility. Various depth profiles with sliding motion could be calculated from the combination of artificial signals in the 0.6 mm, 0.9 mm, and 1.2 mm pressing conditions using piezoelectric integral values. In addition, the calculated depth profiles had a resolution of approximately 100 µm, and simultaneously measurable depth profiles were improved with more artificial signals. © 2022 Elsevier LtdFALS

Psychological tactile sensor structure based on piezoelectric nanowire cell arrays

To produce artificial psychological feeling, especially 'pain', an electrical structure that mimics human skin has been studied. Touch by a sharp and a blunt object induces different deformations of the skin of human fingers. In order to imitate this phenomenon, a skin-mimicking device employing piezoelectric nanowire sensor arrays is used to generate an electrical 'pain' signal with signal processing when a pen cap presses the device. The electrical 'pain' signal is expected to enhance the protection mechanism of an android robot or mobile phone against harsh environments. © 2015 The Royal Society of Chemistry.1