IROS2019国际学术会议论文集 2132

Abstract This paper presents a soft manipulator which mimics the muscle structure of an octopus arm and its operation by the master slave method by using a wearable interface The soft manipulator consists of a rubber structure and McKibben artificial muscles arranged in axial and oblique directions This manipulator has high safety and shape adaptability and realizes smooth and continuous movement like the octopus arm such as bending and twisting For the intuitive operation of the manipulator the wearable interface using flexible strain sensors is developed It is easy to wear on the human arm with high comfortability because it consists only of soft materials and stretches easily with following the arm motion We can operate the soft manipulator by using the motions of the wrist and forearm as an operation input The soft manipulator can move following the arm motion of the operator and it shows the high possibility to conduct actual tasks I INTRODUCTION In recent years various soft actuators have attracted attention because of features such as high compliance lightweight low cost large power weight ratio and researches on robotic manipulator using them have been actively conducted Even when they collide with people or things these soft robot manipulators can absorb impacts and minimize the influence on objects due to their flexibility and high back drivability Therefore it has high safety compared with general robot manipulators mainly composed of rigid materials Particularly an octopus arm can be said an ideal soft manipulator because of its flexible structure without skeleton and multi degree of freedom movement Many soft robotic manipulators inspired by octopus arm have been developed 1 4 They have soft and lightweight mechanical elements as the actuators A McKibben artificial muscle is one of the most widely known soft pneumatic actuators 5 It has high flexibility and shape adaptability and contracts like actual muscles by applying pneumatic pressure In our research group thin McKibben artificial muscles that are several millimeters of diameter were fabricated with mass production 6 and the soft robotic manipulator mimicking the muscle arrangement of the octopus arm was fabricated by utilizing the artificial Hiroki Hagihara is with Graduate School of Natural Science and Technology Okayama University Okayama 700 8530 Japan e mail hagihara17 s okayama u ac jp Shuichi Wakimoto is with Graduate School of Natural Science and Technology Okayama University Okayama 700 8530 Japan e mail wakimoto okayama u ac jp Takefumi Kanda is with Graduate School of Natural Science and Technology Okayama University Okayama 700 8530 Japan e mail kanda t okayama u ac jp Shota Furukawa is with Graduate School of Natural Science and Technology Okayama University Okayama 700 8530 Japan e mail furukawa16 s okayama u ac jp muscles 7 The soft robotic manipulator realized not only bending contracting motions but also twisting motion without rotation mechanism such as a motor This is because the diameter of our artificial muscle can be thinner than common ones and it is flexible enough to be able to arranged spirally Because of its flexibility and safety the soft manipulator has the possibility to apply to a mechanical system working in the medical field the welfare field and the agricultural field Also they are expected to be used in a disaster site In such fields manipulator should be operated based on the operator s intention according to the task and situation unlike industrial robots conduct routine tasks with high accuracy For intuitive operation the tele operation systems for the robot arm using human body motion have been reported 8 11 They show the effectiveness of driving the robot arms based on the operators intention In addition for tele operating the bendable soft manipulator with multi degree of freedom motion the master device was developed 12 13 In this paper we develop a wearable input interface using flexible strain sensors composed of soft materials This sensor is extremely thin and highly flexible Therefore it is comfortable for wearing and no bulky external module is required By using this interface the operation of the manipulator by the master slave method is conducted and the operability is evaluated The results indicate the high possibility to apply to actual tasks II MCKIBBEN ARTIFICIAL MUSCLE The general components of the McKibben artificial muscle are a sleeve formed by knitted fibers a rubber tube as an air chamber and an air tube for supplying air to the rubber tube The McKibben artificial muscle is fabricated by covering the rubber tube with a sleeve of knitted fibers then one end is sealed and the air supply tube is attached to the other end In this study contractile type McKibben artificial muscles are used When pneumatic pressure is applied to the artificial muscle it expands in the radial direction and contracts in the axial direction with changing the knitting angle of fibers on the sleeve The contracting displacement and force are used as the actuator output The basic characteristics of the artificial muscle used in this study were measured experimentally with changing applied pressure Figure 1 shows the relation between contraction ratio and force The maximum contraction ratio and force at 400 kPa of pneumatic pressure were 22 6 N and 20 8 respectively From these characteristics it is found that the contraction force decreases with the increase of contraction displacement this property is the same as biological muscles Operation of a pneumatic soft manipulator using a wearable interface with flexible strain sensors Hiroki Hagihara Shuichi Wakimoto Takefumi Kanda Shota Furukawa 2019 IEEE RSJ International Conference on Intelligent Robots and Systems IROS Macau China November 4 8 2019 978 1 7281 4003 2 19 31 00 2019 IEEE4949 Fig 1 Relation between contraction ratio and force III SOFT MANIPULATOR A Concept and previous works Octopus arms have no skeleton and flexible and multi degree of freedom movements is realized by muscles arranged in multiple directions 14 Octopus arm muscles contributing the motion majorly can roughly be classified into three types according to their arrangement The Longitudinal muscles LM in the axial direction Transverse muscles TM in the radial direction Oblique muscles OM in the oblique direction are arranged around Nerve cord NC as the central axis Previously we reported the manipulator consisting of thin McKibben artificial muscles In the mechanism the artificial muscles are arranged by mimicking the arrangement of LM TM and OM around the rubber rod that is an imitation of the NC 7 By driving artificial muscles we succeeded in realizing flexible and multi degree of freedom movements such as contracting bending twisting motions and rigidity change In addition a master slave method was proposed In the control system the soft manipulator was configured with the artificial muscles corresponding to LM and OM for focusing bending and twisting motions and an interface machine with multiple wire encoders was fabricated and used for controlling the manipulator 12 B Structure of the soft manipulator Figure 2 indicates the structure of the soft manipulator 12 The rubber structure is an imitation of the NC this is just structure and no function as a nerve It has ditches for arranging the artificial muscles three line ditches are formed parallel to the axial direction at intervals of 120 from the sectional view and two spiral ditches are formed in clockwise and counterclockwise directions with the angle of 55 to the axial direction Three artificial muscles are set following the ditches These are represented in red lines in Fig 2 and correspond to LM Two artificial muscles are arranged oblique direction spirally as shown in green lines One is wound in a clockwise direction and the other is in a counterclockwise direction with following the ditches These play the role of OM Fig 2 Structure of the soft manipulator C Fabrication The rubber structure was molded using silicone rubber KE 1603 Shin Etsu Chemical Co Ltd as shown in Fig 3 and the fabricated soft manipulator is shown in Fig 4 The outer diameter is 17 mm the length is 190 mm and the diameter of each ditch for the arrangement of the artificial muscle is 3 mm Both ends of the artificial muscle were fixed to the rubber structure using the adhesive material The total weight is 43 2 g Fig 3 The molded rubber structure with ditches Fig 4 The fabricated soft manipulator D Basic characteristics In order to evaluate the basic driving characteristics of this manipulator the bending motion and twisting motion were performed and the deformation amount of each motion was measured by image analysis software respectively Figure 5 a shows the initial state and bending state of the soft manipulator It can bend in any direction by changing and or combining driving artificial muscles for LM The bending angle is defined as the angle between the tip surface in the initial state and the bending state when the manipulator is viewed from the side as shown in Fig 5 a Figure 5 b shows the twisting motion The black and white plate was attached to the tip of the manipulator for measuring the twist angle By applying pneumatic pressure to one of the artificial muscle for OM it can twist in the clockwise or counterclockwise direction The basic characteristics are shown in Fig 6 For bending motion pneumatic pressure was applied to one artificial muscle in the axial direction every 50 kPa from 0 kPa to 400 kPa The second result is shown in Fig 6 The maximum McKibben artificial muscle for LM a Over view b Cross sectional view Rubber structure McKibben artificial muscle for OM 120 55 Ditches 4950 a Bending motion b Twisting motion Fig 5 Basic motion of the soft manipulator bending angle was 75 at the pneumatic pressure of 400 kPa For twisting motion pneumatic pressure was applied to one artificial muscle in the clockwise direction every 50 kPa from 0 kPa to 400 kPa The maximum twist angle was 105 at the pneumatic pressure of 400 kPa Both characteristics have dead zone from 0 kPa to around 100 kPa and hysteresis is observed These phenomena are due to the characteristics of the artificial muscles shown in Fig 1 Fig 6 Basic characteristics of the soft manipulator IV DESIGN OF THE WEARABLE INTERFACE In order to operate the soft manipulator reflecting the operator s intention we consider that it is effective to link the arm motion of the operator to the soft manipulator motion Previously a master control system using wire encoders for drive the soft manipulator was proposed 12 This is an installation type interface due to that the main body of the encoder is heavy and bulky Therefore it was difficult to be a wearable and portable system In this report we consider that the bending and twisting motion which are the control targets motions of the soft manipulator can correspond to the bending of the wrist and the twisting of the forearm of a human arm respectively Therefore an arm cover type wearable interface was fabricated It is constituted by arranging stretchable and flexible strain sensors in axial and oblique direction on the arm cover A Flexible strain sensor The structure of the flexible strain sensor stretchable dynamic strain sensor Yamaha Corporation used in this report is shown in Fig 7 A conductive CNT carbon nanotube sheet is sandwiched between elastic materials and electrodes are attached to both ends When the strain occurs in the alignment direction of the CNT the electrical resistance value increases This variation in electrical resistance value is used as the sensor output and the generated strain can be measured It can detect strain exceeding 100 and shows linear resistance variation In addition it is thin light and flexible This sensor was applied to a wearable device and the measurements of the movement of arms and fingers were performed 15 The flexible strain sensor with a width of 2 0 mm and a gauge length of 90 mm was used Figure 8 a shows the strain resistance characteristics of this sensor In the experiment the sensor was stretched with a speed of 1 0 mm s namely quasi static characteristics The dead zone from 0 to 5 strain was observed and the linearity of resistance variation over 5 strain was high Figure 8 b shows the step response of the sensor High responsiveness can be confirmed On the other hand slight drift was observed however the soft manipulator characterized by high safety owing to flexibility and considering that it is not aiming for high precision positioning the characteristics of this sensor are sufficient Fig 7 Structure of the flexible strain sensor Conductive paste Elastic material Lead wire Elastic assist layer Pad electrode CNT sheet Sealing layer Bending state Camera Twisting state View from bottom side Black and white plate 4951 a Relation between strain and electrical resistance b Step responce Fig 8 Characteristics of the flexible strain sensor B Fabrication of the wearable interface The wearable interface was developed by attaching the flexible strain sensors to an arm cloth cover with high elasticity as shown in Fig 9 In order to detect the bending motion of the wrist three flexible strain sensors were arranged at equal intervals along the axis of the arm on the wrist part In addition to detect the twisting motion of the forearm two flexible strain sensors were arranged with crossing on the forearm part For avoiding the influence of the dead zone of the flexible strain sensor pre strain of 10 was applied An operator wears this interface and bends the wrist and or twists the forearm Then the sensor corresponding to each motion stretches and the electrical resistance value changes This variation of the sensors is used as a drive input signal of the soft manipulator Fig 9 The wearable interface V OPERATION AND APPLICATION A Operation of the soft manipulator Using the developed wearable interface bending and twisting motion of the soft manipulator were controlled based on electrical resistance variation of the sensor caused by bending of the wrist and twisting of the forearm The applied pneumatic pressure to the corresponding artificial muscle is calculated by eq 1 Pi ki ri Ri i 1 2 5 where Pi is the applied pneumatic pressure to artificial muscle i if Pi 0 ri is resistance value of the sensor and Ri is the initial resistance value of the sensor with 10 pre strain As shown in Fig 10 artificial muscle i and sensor i correspond each other by one to one When the sensor i is stretched the artificial muscle i contracts The gain ki was determined so that the applied pneumatic pressure was the maximum value 400 kPa at the maximum bending of the wrist and twisting of the forearm As a result it was possible to bend and twist in any directions accompanying the movement of the arm Figure 11 is an example state during operation please refer to the attached movie The bending angle of the soft manipulator and the wrist in the bending operation in certain direction were measured also the twisting angle of the soft manipulator and the forearm were measured by the motion capture system A comparison between the measurement results and the sensor output by step like and wave like motion are shown in Fig 12 Figure 12 a and b show the results of the bending motion In the experiments the wrist was bent so that sensor 1 was stretched and then sensor 2 and sensor 3 were contracts Therefore artificial muscle 1 contracted and artificial muscle 2 and 3 were not driven Figure 12 c and d show the twisting motion In these graphs the forearm was twisted so that sensor 4 is extended and actuator 4 was driven From Fig 12 it is found that the soft manipulator can follow the wrist and forearm motion of the operator although small drift motion coming from sensor property occurred This system is open loop control system and the motion range of the operator s arm and the manipulator differ therefore amount of the angles between them does not agree each other in principle Fig 10 Correspondence between artificial muscles and sensors Fig 11 An example state during operation Flexible strain sensor for bending Flexible strain sensor for twisting 1 2 3 4 5 Writ part Forearm part Front side Back side Arm cover Soft manipulator Operator s arm sensor Artificial muscle 4952 a Step like motion in bending motion b Wave like motion in bending motion c Step like motion in twisting motion d Wave like motion in twisting motion Fig 12 Comparison between operator s motion soft manipulator s motion and sensor outputs B Application In order to realize handling an object the soft gripper using the McKibben artificial muscles was developed and attached on the tip of the soft manipulator The claw was manufactured by fixing the artificial muscle on one surface of a rubber plate which is length of 60 mm the width of 15 mm and the thickness of 3 mm as shown in Fig 13 Additionally the spring was arranged on the other surface of the rubber plate The claw can bend by contraction motion of the artificial muscle and return the initial state by recovering force by the spring Three claws were utilized to constructs the soft gripper The developed soft gripper is shown in Fig 14 As the artificial muscle contracts the gripping motion can be performed Motion of the soft gripper is controlled by using an attachment device which combines a rubber plate and the flexible strain sensor and is mounted on the forefinger as shown in Fig 15 Because the soft gripper has high shape adaptability and safety owing to its flexibility ON OFF control was adopted to drive the soft gripper in this report as the simple method When the sensor is extended due to bending of the forefinger and the electrical resistance value exceeds a threshold value the soft gripper is driven As a result of combining the bending twisting motion of the soft manipulator and the gripping motion of the soft gripper the opening of the PET bottle cap was realized as shown in Fig 16 please