IROS2019国际学术会议论文集 0282

Paper based modular origami gripper Ratchatida Phummapooti Natthanicha Jamroonpan Pongsakorn Polchankajorn Eakkachai Pengwang and Thavida Maneewarn Institute of Field Robotics King Mongkut s University of Technology Thonburi 126 Prachautid Rd Bangkok 10140 Thailand Tel 66 2 470 9339 E mail praew fibo kmutt ac th Abstract In this paper a novel method to construct a versatile gripper which can grasp various sizes and shapes of objects is proposed The design and construction of this gripper is based on a paper folding technique called modular origami This particular type of origami is constructed from multiple modules which can transform its shape by rotation The gripper is actuated by a single motor using a linkage mechanism The grasping part of this gripper is made of paper thus it provides sufficient compliance to bend itself and create multiple contact points around objects with different shapes This gripper can be used in two modes force grasping and cage grasping The proposed gripper was tested on objects with various shapes weights and sizes Furthermore how the property of the paper affects the performance of the gripper was also tested The thickness of the gripper material affects the compliance of the gripper and the range of sizes of objects which can be successfully gripped Keywords origami gripper soft gripper cage grasping paper gripper I INTRODUCTION A number of new soft gripper designs were proposed in recent years In contrast to a rigid gripper a soft gripper has sufficient compliance to allow it to grasp multiple types of objects Hirose 1 introduced the idea of a soft gripper that can conform to objects of any shape by using a pulley drive in 1976 Nowadays most soft grippers 2 3 are made of soft and bendable materials such as silicone elastomer They are usually actuated by pneumatic cable tendon or shape memory alloy There is also a universal gripper design 4 which uses a jamming technique to enclose the gripper around an object in a form closure condition Another interesting design with a two phase flexible gripper called Flexirigid 5 uses a caging technique to immobilize an object inside the gripper before then using force closure to hold it tightly in place In recent years there are also a number of researchers that employ origami or Japanese paper folding techniques for robot construction 6 7 Folding techniques allow a robot structure to be lightweight easy to construct and low cost There are some gripper designs that are inspired by the origami folding technique Firouzeh and Paik 8 introduced the origami gripper which can adjust its stiffness on different joints using layers of shape memory polymer and silicone materials Their origami gripper has two fingers each of which are tendon driven With its adjustable stiffness the proposed gripper can perform various grasping modes such as power grasping and precision grasping Also Li et al 9 showed an example of a gripper which is constructed from the fluid driven origami inspired artificial muscles that can lift and twist a water bottle II ORIGAMI GRIPPER DESIGN A Modular origami structure Modular origami is a type of origami construction technique which requires multiple modules to be assembled together to create more complex objects Modular origami usually involves insertion or folding over to hold multiple pieces together Furthermore there is a technique called fleksagonomi or flexible modular origami which is a modular origami that can rotate and bend in order to transform itself into various shapes The modular origami model fireworks was created by Yami Yamauchi in 1998 10 This model is made from 12 sheets of paper and can transform itself into various forms by rotation The design of this fireworks origami model inspires us to apply this mechanism into making a soft robotic gripper The detailed instruction of fireworks folding can be found at 10 In this proposed origami gripper design the fireworks modular origami will be used as the grasping part which will be referred to as fingers or claws This grasping part of the gripper can be made of paper with various thicknesses The thickness of paper affects the stiffness the weight bearing capability and the geometry of the gripper as will be discussed in further details in section IV B Kinematics of origami gripper This origami gripper design requires a rotation action to transform the shape of the fireworks origami from form I and II in order to open and close a gripper In the actual gripper there are twelve fingers or claws which form the circular ring of twelve points when observed from top view as shown in figure 1 The simplified side view geometry of one gripper finger is shown in figure 2 In order to open and close the gripper the tip of the finger which is defined by point A in figure 2 will rotate around point B The linkage mechanism has two attachment points on the gripper claw outer point C and inner point B The linkage mechanism will have to be designed to match the trajectory of the outer point of attachment C on the origami gripper from the open form to the close form 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 IEEE5614 a b c d Fig 1 Grasping with a flexible modular origami design a top view image when the gripper is opened b top view image when the gripper is closed Note that the closed gripper exposes different paper surfaces from the open gripper c side view image when the gripper is open d side view image when the gripper is close In this design Lopen is the maximum distance between the opposite finger when the gripper is fully open Lclose is the distance between fingertips when the gripper is closed l is the length of the gripper finger which is derived from the size of the paper Fig 2 This diagram shows the cross section view of the grasping mechanism The gray shapes represent the paper and the clear outlines above represent the linkage The four bar linkage mechanism is designed using SAM 7 0 software The linkage mechanism is made of PLA using a 3D printer Three linkage mechanisms are attached to the same actuation rod at 120 degrees apart The circular support is also attached to these three linkages inside the origami gripper grasping part as shown in figure 4 In order to open and close the origami gripper this gripper is driven by a DC servo motor Robotis MX 28 through a rack and pinion mechanism of an actuation rod which connects the three linkage mechanisms together so that they are all driven by a single actuator The four bar linkage is attached with origami paper by inserting the thin link inside the squash fold of module numbers 1 5 9 at 120 degrees apart a b Fig 3 Modular origami gripper with linkage mechanism a 3D CAD drawing of the linkage mechanism b the prototype of the gripper with the linkage mechanism attached a b Fig 4 a The thin linkage of the mechanism in inserted inside the squash fold of the paper origami grasping part b 3D spring model of the origami module in one section between two rigid linkage mechanisms The fireworks origami grasping part is a closed circular shape which comprises of 12 modules that are connected together Figure 5 shows a single module in which its folded structure creates a spring like mechanism k1 and k2 represent the stiffness of the lock folding pattern that locks the consecutive modules together at the top and bottom parts k3 represents the stiffness of the squash folding pattern at the backbone of each module These three stiffnesses depend upon the stiffness and the thickness of the paper and can be modeled as a set of parallel springs as shown in figure 4 and 5 These stiffnesses affect the grasping force as well as constrain the maximum opening Lopen and the minimum closing distance Lclose of the origami grasping part Fig 5 A single module of the modular fireworks origami can be represented by a parallel spring with stiffness k1 k2 and k3 5615 Fig 6 The simplified kinematic model of one 120 degree section which is comprised of four origami modules top view Figure 6 represents one of three 120 degree sections of the gripper Each section encompasses 4 of the 12 modules in the origami gripper Fg represents the force exerted by a rigid driving linkage Fspi is the total compression force of origami gripper module i th that is generated by the stiffness of a single module which can be modeled as three parallel springs k1i k2i k3i are the stiffness of the paper module i th as shown in figure 5 Fspi k1ix1i k2ix2i k3ix3i 1 These connected modules within each section create a compliance under actuation mechanism Force exerted at each finger s grasping point can be written as a function of the stiffness of these connected elements and the configuration of the modules Fig 7 The free body diagram of the four bar linkage mechanism and the origami grasping module red dotted line The driving linkage mechanism can be described as slider crank and four bar linkage in serial The actuation torque Ta is generated by a sliding force via the slider crank mechanism and can be calculated with equation 2 The sliding force Fs is generated by a servo motor with the maximum driving torque of 3 1 Nm The grasping torque at the rigid linkage mechanism Tg can be calculated from the geometric relationship between the actuation torque Ta and the linkage parameters in figure 7 The grasping force Fg is applied through a compliant origami link which can be described by equation 3 k is the torsional stiffness of the origami link which can be derived from the estimated Young modulus cross section inertia EI of the origami paper link in equation 4 and is the angle of deflection caused by applying a force at the tip of the finger Ta Fs 3 l2sin acos l2 2 l 1 2 d2 2l2d 2 Tg k Fgl9 3 k EI l9 4 The grasping angle of the origami part is defined as g in figure 7 The transmission ratio of the grasping force from the sliding force at the actuation rod can be calculated from the parameters of the four bar linkage mechanism The relationship between the grasping force ratio and the grasping angle g from 10 to 45 is shown in figure 8 Fig 8 Relationship between the grasping force ratio and the grasping angle g This origami gripper prototype is called OriGrip as shown in figure 9 For experimental purposes the OriGrip is attached to a gantry mechanism which can be controlled to move up and down at the specified height using a stepper motor Fig 9 The OriGrip prototype and the experimental testbed 5616 III GRASPING The origami grasping part of the gripper provides twelve fingers that will wrap around an object In theory the gripper should be able to grasp an object that has an effective diameter which is smaller than the opening distance Lopen of the gripper However the opening and closing distance of the gripper is affected by the type of paper which the gripper is constructed from This factor will affect the range of sizes and weights of objects that the gripper can grasp and will be further discussed in section IV Since the grasping part is made of paper the fingers are bendable and thus it can accommodate different object shapes Furthermore the gripper will not damage soft objects such as fruits and vegetables since it will not apply an excessive amount of force at the contact point The proposed origami gripper can grasp objects in two different grasping modes The force grasping mode requires sufficient grasping force at the contact points such that an object does not slip out of grasp In the cage grasping mode the gripper surrounds an object with a small opening so that an object can not escape from the grasp These two grasping modes will be discussed in section A and B A Force grasping Force grasping is the basic mode of grasping used by most industrial grippers Most soft grippers 3 also grasp objects with force grasping In the origami gripper that was proposed by Firouzeh and Paik 8 the stiffness of the joint in its fingers can be adjusted so that the gripper can grasp an object with different amounts of compliance and force Generally a gripper has to create a sufficient amount of force at the contact points in a direction that creates a stable constraint on the object In force grasping the number of contact points also affect the stability of the grasp Since the contact points between the fingers of the origami gripper and the object varies upon object geometry and the stiffness of the paper fingers a higher number of contact points may result in a higher likelihood of grasping stability Figure 10 shows the force diagram at a finger tip during the force grasping mode The static friction force Ff is a function of the normal grasping force Fg applied by the gripper and the coefficient of friction 5 6 Flift the lifting force from the gripper at the contact point Fg the normal contact force applied by gripper at the contact point perpendicular to the surface of an object the contact angle At each point of contact the lifting force must be greater than the gravitational force divided by the number of contact points to prevent the object slipping out In theory the spherical ball would create 12 contact points as shown in figure 10 However in real experiments the compliance of the paper finger grasping module and the orientation of the gripper when approaching an object may create fewer contact points According to 11 in order to create a stable force grasping condition in three dimensions at least four contact points must be established a b Fig 10 a Cross section view The force diagram in force grasping mode at the contact point b Top view The gripper applies force to the object in force grasping mode B Cage grasping This origami gripper can also grasp an object by surrounding its claws around an object without applying grasping force This type of grasping is called caging or cage grasping Three dimensional caging is created by forming a three dimensional geometric constraints that traps an object within a given space such that an object can not escape Caging has been used to manipulate and to transport objects by a gripper or by multiple robots 12 Maeda et al 13 have proposed a method to analyze caging based grasping for a robot hand that has both rigid and soft parts Fig 11 Front view the diagram of the gripper in cage grasping mode in which the object is enclosed inside the gripper when the gripper is closed underneath the object With the proposed origami gripper cage grasping can be performed when the fingertips are closed under an object as shown in figure 11 In order to satisfy this condition an object must be smaller than the gripper opening distance Lopen When the gripper is closed the object should be larger than the opening of the gripper in the closed configuration so that it can not escape from the gripper as shown in figure 12 Paper stiffness and paper thickness of the grasping part affect the distance Lclose because of the geometrical and force constraints imposed by the folding structure Since the gripper always encloses around an object using envelope type caging the sufficient condition for cage grasping can be defined as Lclose Dobj 7 Ff Fg Flift Fgsin Fgcos 5617 Lclose the distance between fingertips when the gripper is closed Dobj the diameter of a spherical bounding box around an object a b Fig 12 a Front view The cage grasping mode The cage grasping condition where Lclose D obj b Top view The cage grasping mode The cage grasping condition where Lclose Dobj IV EXPERIMENT In order to examine the design parameters that affect the ability of the proposed gripper different types of paper were used to construct the origami grasping part of the gripper In the second and third experiment force grasping and cage grasping were tested on objects with different shape and size A PAPER STRUCTURE The physical properties of the paper can affect the ability of the origami gripper Paper has multiple key inherent properties including stiffness thickness compressibility density and folding endurance When the paper is thicker the thickness of the paper directly causes the fold to be thicker and increases the difficulty of folding In this experiment 15 objects with different shapes cube rectangular prism triangular prism cylinder and pyramid and sizes as shown in Table I were tested with both force grasping and cage grasping modes These objects were made of PLA using a 3D printer Four different types of paper art paper with various weight and thickness were used to construct the gripper twelve pieces of 170 x170 mm2 120 160 200 220 gsm The results of the experiment are summarized in table II TABLE I DIMENSION OF TEST OBJECTS Shapemm3mm3mm3 rectangular prism 15x15x3020 x20 x4025x25x50 cube30 x30 x3040 x40 x4050 x50 x50 triangular prism 30 x30 x3040 x40 x4050 x50 x50 cylinder 30 x30 40 x40 50 x50 pyramid30 x30 x3040 x40 x4050 x50 x50 TABLE II EFFECTIVE PARAMETERS OF THE GRIPPER WHEN IT IS MADE FROM DIFFERENT TYPES OF PAPER Paper weight gsm Paper thickness mm Lclose mm Lopen mm 1200 1052081 1600 1353177 2000 173273 2200 213270 Since our gripper design is based on modular origami thicker paper increases the thickness of each fold and also increases the stiffness at each folding joint and the overall stiffness of the assembled part Therefore thicker paper results in a larger opening of the fingertip when the gripper is closed Lclose and a smaller opening when the gripper is opened The larger distance of the fingertip when the gripper is closed directly affects the cage grasping condition which is described in equation 7 Therefore the range of objects that can be grasped by cage grasping is reduced when the gripper is made from thicker paper However the thickness of paper may has an advantage of increasing the weight bearing capability of the paper finger B FORCE GRASPING In order to test the capability of the origami gripper in force grasping mode an object is place on the ground The gripper is controlled to grasp the object when the finge