Journal of Sensor Science and Technology (센서학회지)

The Korean Sensors Society (한국센서학회)
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Wearable Force Sensor Using 3D-printed Mold and Liquid Metal

삼차원 프린트된 몰드와 액체 금속을 이용한 웨어러블 힘 센서 개발

  • Kim, Kyuyoung (School of Mechanical and Aerospace Engineering, Korea Advanced Institute of Science and Technology, Department of Mechanical Engineering Bldg. (N7), KAIST) ;
  • Choi, Jungrak (School of Mechanical and Aerospace Engineering, Korea Advanced Institute of Science and Technology, Department of Mechanical Engineering Bldg. (N7), KAIST) ;
  • Jeong, Yongrok (School of Mechanical and Aerospace Engineering, Korea Advanced Institute of Science and Technology, Department of Mechanical Engineering Bldg. (N7), KAIST) ;
  • Kim, Minseong (School of Mechanical and Aerospace Engineering, Korea Advanced Institute of Science and Technology, Department of Mechanical Engineering Bldg. (N7), KAIST) ;
  • Kim, Seunghwan (School of Mechanical and Aerospace Engineering, Korea Advanced Institute of Science and Technology, Department of Mechanical Engineering Bldg. (N7), KAIST) ;
  • Park, Inkyu (School of Mechanical and Aerospace Engineering, Korea Advanced Institute of Science and Technology, Department of Mechanical Engineering Bldg. (N7), KAIST)
  • 김규영 (한국과학기술원 기계항공공학부 기계공학과) ;
  • 최중락 (한국과학기술원 기계항공공학부 기계공학과) ;
  • 정용록 (한국과학기술원 기계항공공학부 기계공학과) ;
  • 김민성 (한국과학기술원 기계항공공학부 기계공학과) ;
  • 김승환 (한국과학기술원 기계항공공학부 기계공학과) ;
  • 박인규 (한국과학기술원 기계항공공학부 기계공학과)
  • Received : 2019.04.25
  • Accepted : 2019.05.29
  • Published : 2019.05.31
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Abstract

In this study, we propose a wearable force sensor using 3D printed mold and liquid metal. Liquid metal, such as Galinstan, is one of the promising functional materials in stretchable electronics known for its intrinsic mechanical and electronic properties. The proposed soft force sensor measures the external force by the resistance change caused by the cross-sectional area change. Fused deposition modeling-based 3D printing is a simple and cost-effective fabrication of resilient elastomers using liquid metal. Using a 3D printed microchannel mold, 3D multichannel Galinstan microchannels were fabricated with a serpentine structure for signal stability because it is important to maintain the sensitivity of the sensor even in various mechanical deformations. We performed various electro-mechanical tests for performance characterization and verified the signal stability while stretching and bending. The proposed sensor exhibited good signal stability under 100% longitudinal strain, and the resistance change ranged within 5% of the initial value. We attached the proposed sensor on the finger joint and evaluated the signal change during various finger movements and the application of external forces.

Keywords

HSSHBT_2019_v28n3_198_f0001.png 이미지

Fig. 1. Schematic design of wearable force sensor using 3D printed mold and liquid metal.

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Fig. 2. Fabrication process (a) 3D Printing of microchannel molds with ABS and the sensor mold with TPU. The width and the thickness of the microchannel mold are designed as 400 μm. (b) A microscopic image of 3D printed microchannel mold with serpentine pattern. (c) Schematic design of assembly of the components for the silicone elastomer casting (d) Removal of the molds for empty microchannel fabrication. Dash curves indicate the fabricated microchannels and the empty space for the reservoir. (e) Injection of the liquid metal and sealing of the holes after wiring. (Scale bar = 10 mm) (f) A micro-CT image of the fabricated sensor.

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Fig. 3. (a) Experimental setup for testing resistance change vs force (b) Loading and unloading using the force tip and the linear stage. The tip was inserted 3 mm down from the sensor surface. Resting time t1, t2 and t3 were 0, 10, and 20 seconds, respectively. (Scale bar = 10 mm) (c) Cyclic test, the number of cycle was 100 times (v = 1 mm/s displacement = 3 mm). Inset magnified the red rectangle area. (d) Input speed dependency. 1: 1 mm/s, 2: 2 mm/s, and 3: 5mm/s. Input displacement was 2 mm.

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Fig. 4. (a) Experiment setup for longitudinal strain test. (b) Resistance change from strain 0% to 100% compared with the result of the linear microchannel sensor. Blue arrows indicate the loading and unloading paths. (v=1 mm/s) (c) Experiment setup for bending test. The resistance of the sensor was monitored while the bending angle, θ was decreased. Red dash lines indicate the triangle need to obtain the bending angle, θ. Yellow arrow indicates the direction of the stage movement (v=1 mm/s) (d) Resistance change vs the bending angle, θ.

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Fig. 5. Wearable force sensor on the finger joint. (a) Various finger actions. i. Straight ii. Bending iii. Applying force while bending iv. Applying force with straight finger. Arrows indicate the force direction. (b, c) Real-time resistance monitoring during various actions. (b) is the red rectangle in (c).

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