액상 중합체 및 금속염을 활용한 변형 센서의 제작 및 응용

학위논문(박사) -- 서울대학교대학원 : 공과대학 기계항공공학부(멀티스케일 기계설계전공), 2022. 8. 이정훈.A variety of stretchable strain sensors have been developed for various applications in diverse fields. Based on their core function represented by the conversion of mechanical deformations into electrical signals, numerous fabrication techniques combined with miscellaneous combinations of materials have been suggested and applied for different purposes. Recently, a series of innovations in agriculture in the name of smart farming have been achieved to meet increasing needs for high-quality crops. As part of the collection of essential information for plant growth, it becomes indispensable to measure axial dimensions of trees or fruits. Although certain kinds of apparatuses were reported to show precise size measurement for the trunk of a tree or the diameter of a fruit, improved instruments categorized as dendrometers had been awaited to overcome current limitations such as bulkiness, complexities in working mechanisms, dependence on users, expensiveness, etc. In this study, I proposed a liquid polymer/metallic salt-based stretchable strain sensor. Compared to conventional strain sensors often used as wearable sensors for instant motion detection, the newly developed sensor included conductive liquid made of silver nitrate and polyethylene glycol (PEG). The introduction of this liquid polymer brought high viscosity and chemical stability while the addition of silver nitrate supplied electrolytes in the conductive liquid. The formation of the structure of the stretchable strain sensor was finalized with a mixture of distinct elastomers called polydimethylsiloxane (PDMS) and Ecoflex. After multiple experiments, the optimal mixing ratio (20:80) of these elastomers was found to reach the equilibrium between strain, stress and stickiness, which was essential to the effective monitoring of fruit growth. The performance of the stretchable strain sensor was analyzed, showing highly linear relationships between strain and resistance as well as good repeatability. The fruit monitoring test demonstrated the stability of the stretchable strain sensor at least for two weeks with increasing ratios of 1.7 to 3.9 kΩ/mm. As an alternative instrument for fruit growth measurement, this tunable band-shaped sensor would be able to show industrial potential in terms of simple fabrication, reliable measurement, and long-term evaluation. The use of the composite of silver nitrate and PEG also led to the development of an antenna-shaped biomimetic tactile sensor. The conductive liquid was selected to imitate the aqueous cavity of the hair of insects while wires connecting the conductive liquid and the measurement system of the sensor were installed to realize the tubular body, whose role is to transmit mechanical deformation-driven electric signals to the central nervous system of insects. This bio-inspired tactile sensor was designed to compensate the malfunction of visual sensors exposed to dark areas. The performance test of the tactile sensor through wall scanning experiments proved its ability to detect various geographical features expressed on three dimensional (3D)-printed walls with repeatable and linear relationships between bending angle and resistance. The working mechanism established with the conductive liquid and wires revealed that the resistance of the tactile sensor would be decided by the positioning of wires in the composite of silver nitrate and PEG. When the distance between a wall and the tactile sensor was fixed during the scanning, the bio-inspired tactile sensor could offer reliable resistance data enough to reconstruct surrounding geographical features with high accuracy. This antenna-shaped biomimetic tactile sensor was characterized by the use of novel materials compared to existing tactile sensors, the adoption of a simple fabrication process, the investigation of an alternative working mechanism, the establishment of high repeatability based on bending angle and resistance, and the presentation of a perspective of being studied further for 3D image reconstruction.다양한 분야에서 사용 목적에 따라 수많은 종류의 변형 센서들이 개발되어 왔다. 기계적 변형을 전기적 신호로 바꾸어 주는 본연의 기능에 기초하여 여러 물질들을 활용한 변형 센서 제작 방법이 소개되었다. 최근 스마트 농장이라는 이름으로 농업에 일련의 혁신이 일어나고 있는 가운데 고품질 작물을 얻기 위한 수요가 꾸준히 증가하고 있다. 식물 생장에 필수적인 정보를 획득하기 위한 방편 중 하나로 나무 줄기나 과실 크기를 측정하는 것이 중요해지고 있다. 이러한 목적의 도구들이 이미 보고된 바 있으나 기존 제품에 비하여 발전된 형태의 측수기(測樹器)에 대한 기대에 못 미친 것이 사실이다. 즉, 크기를 줄이고 작동 원리를 단순화하며 사용자 숙련도에 대한 의존성을 줄이는 동시에 가격적으로 경쟁력 있는 제품에 대한 필요성이 제기되어 왔다. 본 연구에서 액상 중합체 및 금속염(金屬鹽) 기반 신축성 변형 센서를 제안하였다. 신체 등의 즉각적인 움직임을 감지하기 위한 착용형(着用型) 센서로 쓰이는 종래의 변형 센서들에 비하여, 새로이 개발된 센서에는 전도성(傳導性) 액체로 질산은과 폴리에틸렌글리콜(PEG)의 혼합물이 적용되었다. 이 액상 중합체를 통하여 높은 점도와 화학적 안정성을 도모하면서 질산은을 이용하여 전해질을 전도성 액체에 공급할 수 있다. 신축성 변형 센서의 골격은 탄성 중합체인 폴리디메틸실록산(PDMS)과 에코플렉스(Ecoflex)로 만들었다. 반복적인 실험을 거쳐 최적의 배합 비율(PDMS 20:80 Ecoflex)을 찾음으로써 효과적인 과실 생장 관찰에 필요한 변수인 변형, 응력(應力), 마찰의 적절한 타협점을 알아낼 수 있었다. 신축성 변형 센서의 성능 분석 결과, 변형과 저항 사이에 높은 선형성을 보이는 것이 확인되었다. 과실 생장 관찰을 통하여 2주 동안 약 1.7 ~ 3.9 kΩ/mm의 범위 내에서 신축성 변형 센서가 안정적으로 작동하는 것을 증명하였다. 과실 생장 측정을 위한 대안적인 도구로서, 이번에 개발된 조정 가능한 밴드형 센서는 제작 공정이 단순하고 신뢰성 있는 측정이 가능하며 장기간 평가에 적합하다는 면에서 우수한 잠재성을 가지고 있다고 말할 수 있을 것이다. 질산은과 PEG의 조합을 이용하여 더듬이 형태의 생체모방 촉각 센서도 선보였다. 이 전도성 액체는 곤충의 더듬이 내에서 림프로 이루어진 부분(lymph space)을, 금속 전선은 관 모양의 구조물(tubular body)을 구현하는 데 활용되었다. 특히 이 관 모양의 구조물은 곤충의 촉각에서 기계적 변형을 전기적 신호로 바꾸어 중추 신경계로 전달하는 역할을 한다. 이 생체모방 촉각 센서는 어두운 공간에 노출된 시각 센서의 오작동에서 비롯되는 문제를 보완하기 위하여 고안되었다. 3D 프린터로 제작된 요철이 있는 벽면을 가지고 스캔 실험을 진행한 결과 촉각 센서가 굽히는 각도와 저항 사이에서 반복적이면서 선형적인 관계가 형성된다는 것을 증명하였다. 아울러 금속 전선이 전도성 액체에 놓이는 위치에 따라 촉각 센서의 저항이 결정된다는 사실도 밝혔다. 그리고 촉각 센서와 벽면 사이 거리가 고정되어 있다는 가정하에 스캔 실험을 실시한 결과 벽면의 요철을 높은 정확도로 재구성할 만큼 신뢰성 있는 값을 제공한다는 것을 알 수 있었다. 이 더듬이 형태의 생체모방 촉각 센서는 기존의 촉각 센서와 비교하였을 때 새로운 물질이 적용되었고 상대적으로 단순한 제작 공정을 통하여 제작되었다. 기존 촉각 센서와 다른 작동 원리에 대한 이해를 높이는 동시에 센서가 굽히는 각도와 저항 사이의 높은 반복성을 확립함으로써 후속 연구를 통하여 동일 센서를 활용한 3차원 이미지 재구성에 대한 가능성도 엿볼 수 있었다.Chapter 1. Introduction 1 1.1 Conventional strain sensors 1 1.2 Strain sensors in agricultural engineering 2 1.3 Current tactile sensors 3 1.4 Tactile sensors in military industries 4 1.5 Research objectives and contributions 5 Chapter 2. Fabrication of a stretchable strain sensor 7 2.1 Synthesis of polyethylene glycol (PEG)/silver nitrate composites 7 2.2 Fabrication of a strain sensor with the liquid composites 8 2.3 Preparation of a flexible band for the incorporation of the strain sensor 9 2.4 Encapsulation of the strain sensor into the flexible band 9 Chapter 3. Methods for the stretchable strain sensor 10 3.1 Measurement of strain and resistance 10 3.2 Tensile strength measurement 10 3.3 Ultraviolet-visible (UV-Vis) spectroscopy 11 3.4 Field emission-scanning electron microscopy (FE-SEM) and elemental analysis 11 3.5 Fruit model simulation 11 3.6 Performance test as a dendrometer using real fruits 12 Chapter 4. Analysis of the stretchable strain sensor 13 4.1 Formation of PEG/silver nitrate composites 13 4.2 Sealing process of the strain sensor 15 4.3 Correlation between strain and resistance 17 4.4 Comparison between theoretical calculations and experiments 21 4.5 Optimization of elasticity for large strain, low stress and high sensitivity 24 4.6 Characterization of silver nanoparticles in the PEG/silver nitrate composites 27 4.7 Reliability test of the strain sensor through fruit model simulation 30 4.8 Continuous monitoring of real fruits with the strain sensor 32 Chapter 5. Fabrication of a bio-inspired tactile sensor and its methods 35 5.1 Fabrication of a bio-inspired tactile sensor 35 5.2 Compatibility test using PEG, silver nitrate and sodium chloride 36 5.3 Fourier transform infrared (FTIR) spectrum analysis 37 5.4 Measurement of silver particles in the liquid composites 37 5.5 Simulations for the bio-inspired tactile sensor 38 5.6 Wall scanning tests with bio-inspired tactile sensors 38 Chapter 6. Analysis of the bio-inspired tactile sensor 39 6.1 Compatibility test between PEG and silver nitrate 39 6.2 Chemical properties of PEG/silver nitrate composites 41 6.3 Simulations of mechanical and electrical properties 43 6.4 Wall scanning test with the bio-inspired tactile sensor 45 6.5 Wall scanning test with multiple walls and multiple sensors 48 6.6 Wall scanning test with a single wall and multiple sensors 49 6.7 Geographical reconstruction 50 Chapter 7. Discussion 53 7.1 Temperature compensation of the stretchable strain sensor 53 7.2 Portable power supplier for the stretchable strain sensor and the bio-inspired tactile sensor 54 7.3 Future work 55 Chapter 8. Conclusion 57 Bibliography 59 Appendix 67 A. Statistical analysis of the relationship between strain and resistance 67 B. Dendrometer requirements and specifications 68 Abstract in Korean 71박

An insect-inspired bionic sensor for tactile localization and material classification with state-dependent modulation

Patanè L, Hellbach S, Krause AF, Arena P, Dürr V. An insect-inspired bionic sensor for tactile localization and material classification with state-dependent modulation. Frontiers in Neurorobotics. 2012;6:1-18.Insects carry a pair of antennae on their head: multimodal sensory organs that serve a wide range of sensory-guided behaviors. During locomotion, antennae are involved in near-range orientation, for example in detecting, localizing, probing, and negotiating obstacles. Here we present a bionic, active tactile sensing system inspired by insect antennae. It comprises an actuated elastic rod equipped with a terminal acceleration sensor. The measurement principle is based on the analysis of damped harmonic oscillations registered upon contact with an object.The dominant frequency of the oscillation is extracted to determine the distance of the contact point along the probe and basal angular encoders allow tactile localization in a polar coordinate system. Finally, the damping behavior of the registered signalis exploited to determine the most likely material. The tactile sensor is tested in four approaches with increasing neural plausibility: first, we show that peak extraction from the Fourier spectrum is sufficient for tactile localization with position errors below 1%. Also,the damping property of the extracted frequency isused for material classification. Second, we show that the Fourier spectrum can be analysed by an Artificial Neural Network (ANN) which can be trained to decode contact distance and to classify contact materials.Thirdly, we show how efficiency can be improved by band-pass filtering the Fourier spectrum by application of non-negative matrix factorization. This reduces the input dimension by 95% while reducing classification performance by 8% only. Finally, we replace the FFT by an array of spiking neurons with gradually differing resonance properties, such that their spike rate is a function of the input frequency. We show that this network can be applied to detect tactile contact events of a wheeled robot, and how detrimental effects of robot velocity on antennal dynamics can be suppressed by state-dependent modulation of the input signals