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Relationship Between Photoelasticity of Polyurethane and Dielectric polymers Article Relationship between Photoelasticity of Polyurethane and Dielectric Anisotropy of Diisocyanate, and Application of High-Photoelasticity Polyurethane to Tactile Sensor for Robot Hands Masahiko Mitsuzuka 1, Yuho Kinbara 2, Mizuki Fukuhara 3, Maki Nakahara 2, Takashi Nakano 2, Jun Takarada 4, Zhongkui Wang 1, Yoshiki Mori 3, Masakazu Kageoka 2, Tsutomu Tawa 2, Sadao Kawamura 3 and Yoshiro Tajitsu 4,* 1 Research Organization of Science and Technology, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan; [email protected] (M.M.); [email protected] (Z.W.) 2 Mitsui Chemicals, Inc., Tokyo 105-7122, Japan; [email protected] (Y.K.); [email protected] (M.N.); [email protected] (T.N.); [email protected] (M.K.); [email protected] (T.T.) 3 Department of Robotics, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan; [email protected] (M.F.); [email protected] (Y.M.); [email protected] (S.K.) 4 Electrical Engineering Department, Graduate School of Science and Engineering, Kansai University, Suita, Osaka 564-8680, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-6-6368-1121 Abstract: Eight types of polyurethane were synthesized using seven types of diisocyanate. It was Citation: Mitsuzuka, M.; Kinbara, Y.; found that the elasto-optical constant depends on the concentration of diisocyanate groups in a unit Fukuhara, M.; Nakahara, M.; Nakano, volume of a polymer and the magnitude of anisotropy of the dielectric constant of diisocyanate T.; Takarada, J.; Wang, Z.; Mori, Y.; groups. It was also found that incident light scattered when bending stress was generated inside Kageoka, M.; Tawa, T.; et al. photoelastic polyurethanes. A high sensitive tactile sensor for robot hands was devised using one of Relationship between Photoelasticity the developed polyurethanes with high photoelasticity. of Polyurethane and Dielectric Anisotropy of Diisocyanate, and Keywords: photoelasticity; polyurethane; dielectric anisotropy; tactile sensor Application of High-Photoelasticity Polyurethane to Tactile Sensor for Robot Hands. Polymers 2021, 13, 143. https://doi.org/10.3390/polym 1. Introduction 13010143 1.1. History and Purpose of This Study Received: 15 November 2020 Thus far, various methods of detecting stress inside a polymer based on the bire- Accepted: 24 December 2020 fringence of light have been studied. For example, methods of irradiating a flexible and Published: 31 December 2020 transparent polymer sheet with circularly polarized light and observing the fringes in the transmitted light through a polarizing filter [1–6], and methods of measuring changes in Publisher’s Note: MDPI stays neu- refractive index by stretching the polymer [7,8] have been studied. A tactile sensor [9] for tral with regard to jurisdictional clai- stress detection based on the polarized light and the photoelasticity of transparent, flexible, ms in published maps and institutio- and robust polyurethane is expected to realize a highly sensitive robot hand tactile sensor nal affiliations. that imitates the tactile sensation of a human finger, but the performance of the sensor has not yet been fully verified. The purpose of this research was to develop a highly sensitive tactile sensor of a robot hand and use it to grasp and process lightweight, soft and brittle objects such as foodstuff. Copyright: © 2020 by the authors. Li- censee MDPI, Basel, Switzerland. It is necessary to develop a tactile sensor that can accurately measure a load of 0.1 N or less. This article is an open access article The goal of our research is to construct a versatile gripping system [10,11] at a low distributed under the terms and con- cost using a commercially available robot hand. Therefore, a cushioning function is also ditions of the Creative Commons At- required so that a soft object is not crushed when it is gripped. Most commercially available tribution (CC BY) license (https:// robot hand opening/closing operation systems do not have the function of feeding back creativecommons.org/licenses/by/ the force signal measured by the tactile sensor during operation. Therefore, a method of 4.0/). controlling the gripping force using the sensor when there is no feedback from the sensor Polymers 2021, 13, 143. https://doi.org/10.3390/polym13010143 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 21 required so that a soft object is not crushed when it is gripped. Most commercially avail- able robot hand opening/closing operation systems do not have the function of feeding Polymers 2021 13 , , 143 back the force signal measured by the tactile sensor during operation. Therefore, a method2 of 19 of controlling the gripping force using the sensor when there is no feedback from the sen- sor to the control circuit of the hand will be described in the following example. The con- trolto the program control of circuit a two-finger of the hand hand will is set be so described that the inclosing the following distance between example. both The fingers control isprogram 0.1 mm ofin aeach two-finger operation. hand Each is time set so the that finger the closing operation distance betweenis completed, both fingersthe ro- bot’sis 0.1 personal mm in eachcomputer operation. reads the Each gripping time the forc fingere signal closing from operationthe tactile issensor, completed, and if the the targetrobot’s gripping personal force computer has not reads yet been the gripping reached, force each signalfinger fromis further the tactile closed sensor, by 0.1 and mm. if Thethe targetoperation gripping of closing force the has hand not yet is intermittently been reached, eachrepeated finger several is further times closed until bythe 0.1 target mm. grippingThe operation force is of reached, closing theand hand when is the intermittently gripping force repeated reaches several the target times value, until the the finger target closinggripping operation force is reached,is terminated. and when the gripping force reaches the target value, the finger closingHowever, operation for isthis terminated. purpose, the tactile sensor itself must have a cushioning function. That is,However, the gripping for this surface purpose, of the the sensor tactile must sensor be itself deformed must have by up a cushioningto 0.1 mm after function. the grippingThat is, thesurface gripping comes surface in contact of the with sensor the mustobject. be This deformed function by can up reduce to 0.1 mmthe force after ex- the ertedgripping on a surfacesoft and comes brittle inobject contact to avoid with crushing the object. it. This function can reduce the force exertedOn the on aother soft andhand, brittle although object polyurethane to avoid crushing has been it. shown to exhibit photoelasticity in previousOn the studies, other hand, the mechanism although polyurethane by which it hasexhibits been photoelasticity shown to exhibit and photoelasticity the relation- shipin previous between studies, the specific the mechanism composition by whichof polyurethane it exhibits photoelasticity and the magnitude and the of relationshipphotoelas- ticitybetween have the not specific yet been composition clarified. In of this polyurethane paper, we first and propose the magnitude the mechanism of photoelasticity by which polyurethanehave not yet exhibits been clarified. photoelasticity, In this paper,then present we first experimental propose the results mechanism that support by which the validitypolyurethane of this exhibitsmechanism. photoelasticity, Then, a polyurethane then present with experimental a composition results suitable that supportfor the sen- the sorvalidity is discussed. of this mechanism.Finally, we show Then, an a example polyurethane of a tactile with sensor a composition fabricated suitable using the for pol- the yurethanesensor is discussed. obtained in Finally, this study. we show an example of a tactile sensor fabricated using the polyurethaneIn addition, obtained for hydrogels in this study. [7], polybutadiene [8], acrylic copolymers [12], nylon In addition, for hydrogels [7], polybutadiene [8], acrylic copolymers [12], nylon [13,14], [13,14], and gelatin gels [15], the magnitude of their birefringence is proportional to the and gelatin gels [15], the magnitude of their birefringence is proportional to the strain. The strain. The birefringence mechanism of these polymers is also considered to be the same birefringence mechanism of these polymers is also considered to be the same as that of as that of polyurethane. polyurethane. 1.2.1.2. Mechanism Mechanism Undelying Undelying Photoelasticity Photoelasticity SchemeScheme 1 shows how a polyurethane sheet produces photoelasticity when subjectedsubjected toto a a tensile tensile force. force. As As shown shown in in this this figure, figure, polyurethane polyurethane forms forms a a network network by by polymerizing polymerizing aa bifunctional bifunctional polyol, polyol, a a bifunctional bifunctional diisocyanate, diisocyanate, and and a a small small amount amount of of trifunctional trifunctional triol triol asas a a crosslinker crosslinker [16]. [16]. The The diisocyanate diisocyanate is is bound bound to to the the polyol polyol along along the the long long axis axis of of its its molecule,molecule, and and when when the the sheet sheet is is stretched stretched in in the the vertical vertical direction, direction, the the long long axis axis of of the the diisocyanatediisocyanate
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