Development of a Bio-inspired Soft Gripper with Claws Mingjun Li, Manjia Su, Rongzhen Xie, Yihong Zhang, Haifei Zhu, Tao Zhang and Yisheng Guan∗
Abstract— A soft gripper with claws has been proposed in this paper. This gripper is bio-inspired by inchworm’s true leg which can reliably able to adhere on roughness surface including trunks, branches,rocks and concrete. Two rows of sharp hooks are placed on two sides of a soft palm and are connected by two Shape Memory Alloy (SMA) springs. This paper covers the design concept and fabrication process. Experimental results demonstrate the grasp biting force reached 9.15 N, equivalently 0.93 kg which is 15 times its own weight. Some objects with different shapes, dimensions and roughness were also proved to be graspable for the soft gripper. Moreover, the antidumping torque, anti-slipping force and anti-loosing force were measured, warranting the grasping reliability.
I. I NTRODUCTION Evolution of creatures’ organism adapts to the environment by optimizing its structure, motion pattern and functional mechanism gradually. These special adaptability triggers bionic researchers and engineers to design intelligent robots. Among them, climbing animals (like gecko, spider, caterpillar, etc), are able to adhere on walls, rocks and trees, and to climb. Adhesion is a multidisciplinary subject about biology, mechanics, materialogy, etc. Researchers expect to design end-effectors for robots to adhere on irregular environment adaptively and reliably. Most of them focuses on applying biomimetic dry adhesive materials inspired by gecko and bettles, such as Stikybot [1], Waalbot [2] and Miniwhegs [3]. These robots utilize dry adhesive materials and are capable of adhering on flat and smooth walls. However, dry adhesive materials are lack of self-cleaning function and are extremely sensitive to dust. Therefore, they are easy to be polluted and there follows a fail in adhering. Thus, it is not suitable to apply in rough and dusty environments. In addtion to dry adhesion, there are many other adhension methods, such as pneumatic suction [4], electro-adhesion [5], magnetic adhesion [6], claw adhesion [7], etc. These adhesion methods have special suitable environments and are unfrequent used. Claw or hook-like structure of creatures widely exist in nature, showing its strong adaptability and reliability to the surroundings. For this reason, claw adhesion is a ∗ Corresponding author (Email:
[email protected]). All the authors are with the Biomimetic and Intelligent Robotics Lab (BIRL), School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, China, 510006. This work is supported in part by the NSFC-Guangdong Joint Fund (Grant No. U1401240), the Natural Science Foundation of Guangdong Province (Grant No. 2015A030308011), the National Natural Science Foundation of China (Grant No. 51605096, 51705086), the State International Science and Technology Cooperation Special Items (Grant No. 2015DFA11700), the Frontier and Key Technology Innovation Special Funds of Guangdong Province (Grant No. 2014B090919002, 2015B010917003, 2017B050506008), and the Program of Foshan Innovation Team of Science and Technology (Grant No. 2015IT100072).
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better choice in designing end-effectors. Researchers from RISE group developed a wall-climbing robot platform using claw adhesion [8]. In 2005, Kim and Asbeck developed a small climbing robot SpinyBot II, based on a study of the kinematics of American cockroaches [9]. Boston Power Company 2005 have developed a series of rising bionic clawtype wall-climbing robots [10]. In 2013, Parness designed a flexible pawl structure for RISE V2, which can stably adhere on the surface of porous meteorite and sample meteorite in LEMUR IIB [11]. In Chinese University of Hong Kong, Yangsheng Xu’s research group developed a tree climbing robot Treebot in 2012, which can be used to assist and replace workers to climb trees for inspection, maintenance, pest control and other forestry works [12]. In addition, Qingsong Xu developed a novel compliant gripper which dedicated to automated micro-assembly task by sensors [13]. Nowadays mechanisms with the bionic claw are widely applied in various occasions. However, they are all claw adhesive end-effectors with rigid mechanisms that are not able to adapt environment well. For this reason, inspired by the inchworm’s true leg, a soft gripper with claws (actuated by SMA spring) is proposed and developed, as shown in Fig. 1. The gripper’s structure, fabrication method, strong adaptability and reliability to grasp are presented detailly in this work. This soft gripper is useful for bionic inchwormlike climbing robot [14].
Fig. 1: Soft gripper with claw (SGC)
II. B IO -I NSPIRED D ESIGN A. Mechanism Of Inchworm’s True Leg Inchworm is an extremely excellent climbing expert in nature. In addition to its flexible slim body, its biped legs with claws are able to grasp or adhere on a branch, leaf edge and face, as shown in Fig. 2. Inchworm’s great adhesive ability on different objects, which are of different sizes, shapes, and roughness, is on account of its leg’s structure. Inchworm has two kinds of legs, which are the true leg on the head and the proleg on the bottom. Actually, the microscopic mechanisms are different between the true leg and the proleg. The proleg is better at supporting the whole body, while the true leg is able not only to support the body but also to perform operation tasks as well. The true leg has six claws which are distributed in the two sides of the head. Each claw can be actuated by muscular tissue independently, which allows the true leg to adapt to objects with different sizes and shapes. Moreover, the true leg can plant spiky claws on objects, so that it is able to adhere on surfaces with different roughness. Therefore, a soft gripper with claws is designed to imitate the inchworm’s outstanding adaptability and reliability.
compliant deformation leads to adaptive grasping. Obviously, the compliant adapting motion and compact structure of soft gripper are unique advantages when compared to a rigid gripper.
(a) Active openning motion
(b) Active motion
closing
(c) Passive adapting motion
Fig. 3: Bio-mechanism of inchworm’s true leg
Fig. 2: Inchworm’s leg with claws
B. Mechanical System The mechanical features of inchworm’s true leg can be obtained by analyzing its structure, motion and deformation. The motion of inchworm’s true leg can be briefly divided into active motion and passive motion. Inchworm opens or closes its claws actively to grasp objects, and adapts to objects with different sizes or shapes passively by the soft palm. As shown in Fig. 3, artificial muscles are designed to simulate muscles around each claw of inchworm’s leg. Closing motion occurs when muscles 1 and 2 contract while muscles 3 and 4 relax. On the contrary, opening motion occurs when muscles 3 and 4 contract while muscles 1 and 2 relax. Usually, closing force should be much larger than opening force so that grasping or adhering operations are reliable. In addition to the two former active motions, inchworm’s true leg and soft head bring another passive motion. The adaptive deformation motion of the soft head occurs if the true leg grasps an irregularly shaped object. Based on this phenomenon, a soft palm is derived so that
Based on the former analysis, the soft gripper with claws is designed as Fig. 4. It consists of five mechanical components: two row of hooks, two SMA springs, a silicon sheet, a spring sheet and a Ecoflex soft bounding volume. The hooks are embedded inside the silicon sheet which is boxed as a loop. The silicon sheet works together with the spring sheet as muscles 3 and 4 for opening motion. Two paralleled SMA coils connect the hooks, acting as muscles 1 and 2. Ecoflex rubber is used to bound the whole inner structure, making SGC compact, soft and delicate. If SMA springs are heated simultaneously, the two rows of hook get closed and bite target object. If SMA coils stop being heated, the bent silicon sheet and spring sheet will spread along with SMA springs’ getting cold. Since SGC is designed to be the base of an inchwormlike robot, its target biting force, payload and dead weight for reliable grasping and supporting operations are required to be appropriate. According to our previous work about the body of the inchworm-like robot: target biting force need be more than 5 N; payload that SGC grasps on a surface with different roughnesses should be more than 250 g and selfweight should be less than 80 g. In addition, target diameter of graspable branches varies from 10 mm to 60 mm. Hence, more design parameters of SGC are determinated for a better base of inchworm-like robot, as shown in TABLE I. III. FABRICATION Different from traditional multi-rigid mechanical system, SGC consists of quantities of soft material and few of rigid parts. The soft and rigid parts distribute inside SGC
designed. Since SMA spring can be heated by electrical power, its resistance and phase change temperature are chosen as well after a power calculation of curcuit. Ecoflex rubbers are platinum-catalyzed silicones that are mixed 1A:1B by weight or volume and cure at room temperature with negligible shrinkage. Low viscosity ensures easy mixing and de-airing without special equipment. Cured rubber is very soft, very strong and very ”stretchy”, stretching many times its original size without tearing and will rebound to its original form without distortion. TABLE II: MATERIAL PROPERTIES Material
(a) Front view
Sillicon sheet
Spring sheet
SMA spring
(b) Side view
Fig. 4: Mechanical system
Ecoflex rubber
Property Young’s modulus E Thickness Temperature range Young’s modulus E Length With Thickness Phase change temperature Diameter Wire diameter Length Resistance per meter Specific Gravity Young’s modulus E Mix Ratio By Volume Mix Ratio By Weight Cure Time Temperature range
Value 1.2 GPa 1.5 mm -65 to 250 ◦ C 1.97 GPa 50 mm 10 mm 0.2 mm 70 ◦ C 1.5 mm 0.6 mm 50 mm 0.88 Ω 1.07 g/cm3 0.06 MPa 1A : 1B 1A : 1B 3h -65 to 450 ◦ C
TABLE I: DESIGN PARAMETERS Parameters Length Width Thickness Min target object size Max target object size Target biting force Target payload Self-weight Close motion period Open motion period
Values 70 mm 40 mm 21 mm 10 mm 60 mm > 5.0 N > 250 g < 80 g
