Design and Implementation of a Multi-Function Gripper for Grasping General Objects

Long Kang; Jong-Tae Seo; Sang-Hwa Kim; Wan-Ju Kim; Byung-Ju Yi

doi:10.3390/app9245266

Kang, Long;Seo, Jong-Tae;Kim, Sang-Hwa;Kim, Wan-Ju;Yi, Byung-Ju

2019-12-04 00:00:00

applied sciences Article Design and Implementation of a Multi-Function Gripper for Grasping General Objects 1 1 2 2 2 , Long Kang , Jong-Tae Seo , Sang-Hwa Kim , Wan-Ju Kim and Byung-Ju Yi * Department of Industry-University Cooperation Foundation, Hanyang University ERICA, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Korea; hitjakie@gmail.com (L.K.); jt1000je@hanyang.ac.kr (J.-T.S.) Department of Electrical and Electronic Engineering, Hanyang University ERICA, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Korea; sonata7943@hanyang.ac.kr (S.-H.K.); wanjoo4379@naver.com (W.-J.K.) * Correspondence: bj@hanyang.ac.kr; Tel.: +82-31-400-5218 Received: 17 October 2019; Accepted: 27 November 2019; Published: 4 December 2019 Featured Application: A multi-function grasping system is developed to grasp various objects in dierent working environments, such as piece-picking in warehouses and fulﬁllment centers. Abstract: The development of a reliable pick-and-place system for industrial robotics is facing an urgent demand because many manual-labor works, such as piece-picking in warehouses and fulﬁllment centers tend toward automation. This paper presents an integrated gripper that combines a linkage-driven underactuated gripper with a suction gripping system for picking up a variety of objects in dierent working environments. The underactuated gripper consists of two ﬁngers, and each ﬁnger has three degrees of freedom that are obtained by stacking one ﬁve-bar mechanism over one double parallelogram. Furthermore, each ﬁnger is actuated by two motors, both of which can be installed at the base owing to the special architecture of the proposed robotic ﬁnger. A suction cup is used to grasp objects in narrow spaces and cluttered environments. The combination of the suction and traditional linkage-driven grippers allows stable and reliable grasping under dierent working environments. Finally, practical experiments using a wide range of objects and under dierent grasping scenarios are performed to demonstrate the grasping capability of the integrated gripper. Keywords: gripper; suction; underactuation; underactuated; robot end-eector 1. Introduction The growth in industrial automation indicates that the human–robot–environment interaction will become a common work scenario in many robot applications such as personal, service, and medical robots. In particular, robotic end-eectors such as grippers/hands are expected to frequently experience physical contact with the environment. Thus, to ensure a stable and reliable grasping, the grippers should be designed to be multi-functional. So far, a large number of robotic hands/grippers have been developed to grasp various objects. Among them, multi-ﬁngered anthropomorphic robotic hands have been proposed to attain dexterous manipulation similar to a human hand. The well-known designs include the DLR hand [1], Shadow hand [2], Nasa Robonaut 2 hand [3], and many others. These anthropomorphic hands, especially the fully actuated type, can be used to achieve dexterity similar to the human hand. However, because of multiple degrees of freedom (DOFs) and multiple actuators, the entire hand system is generally bulky and costly. Furthermore, a complicated control system is required to simultaneously operate multiple actuators located at dierent ﬁngers. Because of the aforementioned reasons, very few Appl. Sci. 2019, 9, 5266; doi:10.3390/app9245266 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 5266 2 of 21 anthropomorphic hands have been employed in the industry, and most of them are still used only in educational institutions. To overcome the disadvantages of anthropomorphic hands, many non-anthropomorphic hands have been developed. Among them, the underactuated robotic hands, which are developed to reduce the number of actuators and complexity while preserving the relatively high versatility, have received particular attention [4–17]. These hands have been widely used in the automation industry and daily-life applications. The pioneer designs include the Barret [18], Reﬂex [19], and Robotiq two-ﬁnger and three-ﬁnger hands [20,21]. For the underactuated robotic grippers/hands, passive elements such as springs, mechanical stoppers, or compliant links are generally required to automatically adapt the robotic ﬁnger to the shape of the object. The aforementioned multi-ﬁngered robotic hands/grippers can be used to safely and stably grasp objects in open space. However, in cluttered-environment applications where objects are surrounded by one another, the use of multi-ﬁngered robotic hands is not suitable because physical contacts with the objects to be grasped may occur. In comparison, suction grippers are more suitable for grasping objects in cluttered narrow spaces [22–26]. However, because the suction cup is soft, suction grasping might be not stable in relatively high-speed, high-acceleration, or high-payload applications. Furthermore, the contact area between the soft suction cup and object inﬂuences the suction-gripping force. In addition, the contact area may vary with respect to the dierent shapes and materials of the object to be grasped. For example, the suction cup may fail to grasp objects made of fabric materials or those with multiple holes. To achieve stable and reliable grasping, we develop a multi-function gripper that combines a new two-ﬁngered underactuated gripper with a vacuum grasping system. This multi-function gripper can be used to grasp general objects in dierent environments. This paper is structured as follows. Section 2 describes in detail the architecture of the multi-function gripper. Section 3 discusses the analysis of various grasping modes and grasping strategies in dierent working environments. Section 4 presents the implementation of real-world experiments using a six-DOF commercial robotic arm. Section 5 provides the discussion and conclusion. 2. Architecture Description of the Multi-Function Gripper In this section, the architecture of the proposed three-DOF robotic ﬁnger and the two-ﬁngered underactuated gripper is presented in detail. The suction system and its integration into the gripper are introduced. As we mentioned earlier, underactuation can be achieved through using passive elements such as springs and mechanical stoppers. First, a demonstration of the closing sequence of a two-phalanx robotic ﬁnger is shown in Figure 1 to clearly understand the working principle of underactuation. This ﬁnger is actuated by the lower link indicated by the arrow. This ﬁnger has two phalanxes, whereas only one actuator is used for control. The spring and mechanical stopper are used to constrain the relative motion between the two phalanxes. First, before the proximal phalanx makes physical contact with the object to be grasped, the whole ﬁnger moves as a single rigid body, as shown in Figure 1a,b. Second, when the proximal phalanx makes physical contact with the object, it stops its movement. In this case, the actuation toque overcomes the preloading of the spring, and the distal phalanx continues to rotate [as shown in Figure 1c] relative to the proximal phalanx until it also makes physical contact with the object [as shown in Figure 1d]. It is noted that the ﬁnger closing sequence is automatically generated by continuous actuation of the lower link, as indicated by the arrow. Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 21 Appl. Sci. 2019, 9, 5266 3 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 21 Figure 1. The closing sequence of a two-phalanx underactuated robotic finger. Similarly, the closing sequence of the traditional three-phalanx robotic finger is shown in Figure 2, which is an underactuated finger that can be controlled using only one motor. Robotiq three-finger adaptive gripper [21] is based on the modification of this principle. The springs and mechanical stoppers are installed at the pivot joint between each phalanx. Actuating the lower link (indicated by the arrow) can automatically generate this closing sequence. The three phalanxes will Figure 1. The closing sequence of a two-phalanx underactuated robotic ﬁnger. make contact Figure 1. with the obj The closing sequence of a ect in sequence. However, two-phalanx thi usn undera deractuated robot ctuated robotic fi ic finger. nger can only be Similarly, the closing sequence of the traditional three-phalanx robotic ﬁnger is shown in Figure 2, used to perform shape-adaptive grasping. In a real world scenario, we expect the robotic finger to whichSiis mila anrly, the cl underactuated osing sequence of ﬁnger that can the tra be contr diolled tional three-phalanx using only one robotic motor. Robotiq finger is thrshown ee-ﬁnger in have multiple grasping modes such as parallel and shape-adaptive grasping. In this case, special adaptive Figure 2, w gripper hich i[s21 an ] u is n based deracton uatth ed e fmodiﬁcation inger that can of bthis e coprinciple. ntrolled usThe ing o springs nly oneand motmechanical or. Robotiq design architecture of the three-phalanx robotic finger needs to be developed. Moreover, sometimes, stoppers three-finger are installed adaptive at gri the pper [21 pivot joint ] is b between ased on th each e m phalanx. odificatiActuating on of this princ the lower iple. The link (indicated springs and by we need to fully actuate a linkage-driven robotic finger to achieve high dexterity. With respect to the mechanic arrow)al can stop automatically pers are instal generate led at ththis e piv closing ot joint sequence. between each p The thr hee alanx. A phalanxes ctuatwill ing tmake he low contact er link the three-phalanx robotic finger shown in Figure 2, we cannot install all actuators at the base. with (indithe cated by the object in sequence. arrow) can However automati , this cally genera underactuated te this c rlobotic osing sequen ﬁnger can ce. The onlythree phalanx be used to perform es will Floating actuators will generate large moving inertia, and the size of the floating actuator is shape-adaptive make contact with grasping. the obje In ct in a r se ealquen world ce. scenario, However,we thiexpect s undera the ctua robotic ted robotic fi ﬁnger to nger ca have n multiple only be confined to the mechanical dimension of the finger. grasping used to p modes erform such shap as e-parallel adaptive and grasp shape-adaptive ing. In a realgrasping. world scenario In this, we expect th case, special design e robotic architectur finger to e Thus, the initial motivation of this research is to propose a new three-phalanx robotic finger of hav the e m thr uee-phalanx ltiple graspirng m obotic odes such ﬁnger needs as pto ara be lledeveloped. l and shape-Mor adap eover tive grasp , sometimes, ing. In t we his case need, sp to fully ecial that can achieve multiple grasping modes. To fully actuate this three-phalanx robotic finger, actuate design arch a linkage-driven itecture of the t robotic hree-phalanx ﬁnger torobotic fi achieve nge high r need dexterity s to be . W dith eveloped. respect Mo to the reover, three-phalanx sometimes, another design goal is to install all the actuators at the base. Furthermore, the motion of the distal rwe need to ful obotic ﬁnger shown ly actua inte Figur a lin eka 2,ge- we dri cannot ven roboti install c fi all ngactuators er to achiat eve high d the base.exter Floating ity. W actuators ith respect to will phalanx is expected to be decoupled from the proximal and intermediate phalanxes to reduce generate the three-p larh ge alanx moving roboinertia, tic finge and r shown the size in of Fig the ure 2 ﬂoating , we cactuator annot inst isal conﬁned l all actu to ators the at mechanical the base. control complexity and increase dexterity. Finally, a suction-gripping system is integrated to grasp Floating actuators will generate large moving inertia, and the size of the floating actuator is dimension of the ﬁnger. objects in cluttered, narrow spaces. confined to the mechanical dimension of the finger. Thus, the initial motivation of this research is to propose a new three-phalanx robotic finger that can achieve multiple grasping modes. To fully actuate this three-phalanx robotic finger, another design goal is to install all the actuators at the base. Furthermore, the motion of the distal phalanx is expected to be decoupled from the proximal and intermediate phalanxes to reduce control complexity and increase dexterity. Finally, a suction-gripping system is integrated to grasp objects in cluttered, narrow spaces. Figure 2. The closing sequence of a three-phalanx robotic ﬁnger. Figure 2. The closing sequence of a three-phalanx robotic finger. Thus, the initial motivation of this research is to propose a new three-phalanx robotic ﬁnger that can achieve multiple grasping modes. To fully actuate this three-phalanx robotic ﬁnger, another design goal is to install all the actuators at the base. Furthermore, the motion of the distal phalanx is expected to be decoupled from the proximal and intermediate phalanxes to reduce control complexity and increase dexterity. Finally, a suction-gripping system is integrated to grasp objects in cluttered, narrow spaces. Figure 2. The closing sequence of a three-phalanx robotic finger. Appl. Sci. 2019, 9, 5266 4 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 21 2.1. Thr 2.1. T ee-DOF hree-DLinkage-Driven OF Linkage-DriRobotic ven Robo Finger tic Finger In this paper, we propose a three-DOF linkage-driven robotic finger whose all three actuators In this paper, we propose a three-DOF linkage-driven robotic ﬁnger whose all three actuators required for full actuation can be installed at the base, and the orientation of the distal phalanx can required for full actuation can be installed at the base, and the orientation of the distal phalanx be independently controlled. The schematic diagram of the three-DOF linkage-driven robotic finger can be independently controlled. The schematic diagram of the three-DOF linkage-driven robotic is shown in Figure 3. We can see that this finger mechanism is constructed by stacking one five-bar ﬁnger is shown in Figure 3. We can see that this ﬁnger mechanism is constructed by stacking one mechanism (ABCD) over one double parallelogram (ABFE, BKHG). Figure 3a shows that three ﬁve-bar mechanism (ABCD) over one double parallelogram (ABFE, BKHG). Figure 3a shows that three independent links ( L , L , and L ) can be used to fully control this three-phalanx robotic finger. independent links (L , L , and L ) can be used to fully control this three-phalanx robotic ﬁnger. AE AE AD AD AB AB Because three joints driving those three independent links are coaxial, all three actuators required for Because three joints driving those three independent links are coaxial, all three actuators required for full actuation full actuat can ion be can b installed e instal at lethe d at base. the base. (a) (b) Figure 3. (a) Architecture of the three-DOF (degrees of freedom) linkage-driven robotic finger. (b) Figure 3. (a) Architecture of the three-DOF (degrees of freedom) linkage-driven robotic ﬁnger. (b) Using Using torsion spring and mechanical stopper to realize the underactuated finger. torsion spring and mechanical stopper to realize the underactuated ﬁnger. 2.2. Two-Finger Underactuated Gripper and Integrated Suction System 2.2. Two-Finger Underactuated Gripper and Integrated Suction System Even though the robotic ﬁnger shown in Figure 3a can be fully actuated to achieve high dexterity, Even though the robotic finger shown in Figure 3a can be fully actuated to achieve high in the current study, we focus on developing an underactuated type that can realize parallel and dexterity, in the current study, we focus on developing an underactuated type that can realize shape-adaptive grasping. Similar to the two-phalanx underactuated robotic ﬁnger shown in Figure 1, parallel and shape-adaptive grasping. Similar to the two-phalanx underactuated robotic finger one torsion spring and one mechanical stopper are installed between the proximal and intermediate shown in Figure 1, one torsion spring and one mechanical stopper are installed between the phalanxes to realize an underactuated ﬁnger, as shown in Figure 3b. The torsion spring is used to proximal and intermediate phalanxes to realize an underactuated finger, as shown in Figure 3b. The prevent free motion between the proximal and intermediate phalanxes. The proximal and intermediate torsion spring is used to prevent free motion between the proximal and intermediate phalanxes. The phalanxes are passively coupled with each other by the torsion spring and mechanical stopper. There proximal and intermediate phalanxes are passively coupled with each other by the torsion spring exist two independent motions in this ﬁnger, i.e., the open-close motion of the ﬁnger and distal-phalanx and mechanical stopper. There exist two independent motions in this finger, i.e., the open-close orientation adjustment. Two motors are required to control this underactuated ﬁnger. One motor motion of the finger and distal-phalanx orientation adjustment. Two motors are required to control rotating link L is used to control the open-close motion of the ﬁnger, and the other one rotating AD this underactuated finger. One motor rotating link L is used to control the open-close motion of AD link L is used to control the orientation of the distal phalanx. Thus, the open-close motion and AE the finger, and the other one rotating link L is used to control the orientation of the distal AE distal-phalanx orientation adjustment are decoupled from each other. The grasping sequences of the phalanx. Thus, the open-close motion and distal-phalanx orientation adjustment are decoupled from parallel and shape-adaptive grasping are shown in Figure 4a,b, respectively. each other. The grasping sequences of the parallel and shape-adaptive grasping are shown in Figure For the parallel grasping shown in Figure 4a, when independent link L is actuated and if AD 4a,b, respectively. no external contact occurs at the proximal phalanx, the proximal and intermediate phalanxes move For the parallel grasping shown in Figure 4a, when independent link L is actuated and if no AD together as a single rigid body (from phase I to phase III) because the torsion spring prevents a relative external contact occurs at the proximal phalanx, the proximal and intermediate phalanxes move free motion between them. In general, preloading of the torsion spring is required to prevent any together as a single rigid body (from phase I to phase III) because the torsion spring prevents a undesired motion due to the gravity and inertia eects during the open-close motion. relative free motion between them. In general, preloading of the torsion spring is required to prevent any undesired motion due to the gravity and inertia effects during the open-close motion. For the shape-adaptive grasping shown in Figure 4b, if no external contact occurs at the proximal phalanx, activating independent link L generates a free open-close motion (from phase AD Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 21 I to phase II), similar to that of the parallel grasping. Starting from phase II, physical contact occurs between the object to be grasped and the proximal phalanx. In this case, the proximal phalanx stops the movement, and the intermediate phalanx continues to move against the torsion spring until it makes contact with the object (from phase II to phase III). Thus, the torsion-spring stiffness should be designed as small as possible, but sufficiently big to prevent undesired motion during the free open-close motion. Furthermore, from phase III to phase IV, we can see that the orientation of the distal phalanx can be adjusted by activating independent link L to add one more contact with the AE object. Actively adjusting the orientation of the distal phalanx allows this robotic finger to perform multiple grasping tasks. Controlling orientation of the distal phalanx is a special feature that other contemporary grippers have not had. It is noted that after preforming the shape adaptive grasping and Appl. rele Sci. 2019 asing , 9, t 5266 he object, stored load in the torsion spring will force the proximal and intermedi 5 of at 21 e phalanxes to go back to their original configuration. (a) (b) Figure 4. Grasping sequence of the underactuated finger. (a) Parallel grasping. (b) Shape-adaptive Figure 4. Grasping sequence of the underactuated ﬁnger. (a) Parallel grasping. (b) Shape-adaptive grasping. grasping. For the shape-adaptive grasping shown in Figure 4b, if no external contact occurs at the proximal phalanx, The proposed robotic fin activating independent ger can be link L use generates d to desi a fr gn multi-finger ee open-close motion robotic (fr hands/ om phase grippers. In this I to phase II), AD similar to that of the parallel grasping. Starting from phase II, physical contact occurs between the work, we use the underactuated-type robotic finger to design a two-finger underactuated gripper, as shown i object tonbe Figrasped gure 5. The ki and the nema proximal tic para phalanx. meters of In th this e underactuated case, the proximal gripper phalanx are listed in stops the Ta movement, ble 1. The and the intermediate phalanx continues to move against the torsion spring until it makes contact two fingers of this underactuated gripper are independently operated. Furthermore, for each undera with the ctua object ted f(fr inger, two om phase acII tua to tors are phase III). requ Thus, ired to the cont torsion-spring rol its open-clos sti eness motishould on and t be he or designed ientation as small as possible, but suciently big to prevent undesired motion during the free open-close motion. of the distal phalanx. For each motor, a worm gear is used as a non-back-drivable transmission mechani Furthermor sm t e, o ensure from phase grasping. The spe III to phase IV cif , icat we ion can s see of the actuation that the orientation system, whic of the h include the distal phalanx motor can be adjusted by activating independent link L to add one more contact with the object. Actively type, controller type, and gear ratio, are listed in Table 2. AE adjusting the orientation of the distal phalanx allows this robotic ﬁnger to perform multiple grasping tasks. Controlling orientation of the distal phalanx is a special feature that other contemporary grippers have not had. It is noted that after preforming the shape adaptive grasping and releasing the object, stored load in the torsion spring will force the proximal and intermediate phalanxes to go back to their original conﬁguration. The proposed robotic ﬁnger can be used to design multi-ﬁnger robotic hands/grippers. In this work, we use the underactuated-type robotic ﬁnger to design a two-ﬁnger underactuated gripper, as shown in Figure 5. The kinematic parameters of the underactuated gripper are listed in Table 1. The two ﬁngers of this underactuated gripper are independently operated. Furthermore, for each underactuated ﬁnger, two actuators are required to control its open-close motion and the orientation of the distal phalanx. For each motor, a worm gear is used as a non-back-drivable transmission mechanism to ensure grasping. The speciﬁcations of the actuation system, which include the motor type, controller type, and gear ratio, are listed in Table 2. Appl. Sci. 2019, 9, 5266 6 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 21 Figure 5. Two-ﬁnger underactuated gripper + suction mechanism. Figure 5. Two-finger underactuated gripper + suction mechanism. Table 1. Kinematic parameters of the underactuated gripper. Table 1. Kinematic parameters of the underactuated gripper. Length (mm) Length (mm) Angle (Deg) Length Length Angle (Deg) L = 23.0 L = 23.0 AE BF (mm) (mm) L = 62.0 L = 20.0 AD BG = 135 KBC L =23.0 L =23.0 L = 80.0 AE L = 60.0 BF AB BK = 45 FBG L = 20.0 L = 60.0 BC L =62.0 GH L =20.0 φ = 135 AD BG 135 KBC (constrained by the stopper) KBA L = 52.0 L = 20.0 CD KH L =80.0 L =60.0 φ = 45 AB BK FBG L = 80.0 L = 45.0 EF KI L =20.0 L =60.0 φ ≤ 135 (constrained BC GH 1 KBA Referring to Figure 3a. L =52.0 L =20.0 by the stopper) CD KH L =80.0 Table 2. Speciﬁcation L =45.0 of the actuation system. EF KI Referring to Figure 3a. Motor 1 Motor 2 Motor 3 Motor 4 (FAULHABER) Table 2. Specification of the actuation system. BLDC 1226S012B BLDC 1226S012B BLDC 1226S012B BLDC 1226S012B Motor type Gear head 256:1 256:1 256:1 256:1 Motor 1 Motor 2 Motor 3 Motor 4 Worm gear 20:1 20:1 20:1 20:1 (FAULHABER) BLDC BLDC BLDC BLDC Rated torque 1.97 mNm 1.97 mNm 1.97 mNm 1.97 mNm Motor type 1226S012B 1226S012B 1226S012B 1226S012B Controller type MCBL 3002S MCBL 3002S MCBL 3002S MCBL 3002S Gear head 256:1 256:1 256:1 256:1 Worm gear 20:1 20:1 20:1 20:1 The suction-grasping system consists of an air compressor, an ejector, a ﬁlter, a vacuum cylinder, Rated torque 1.97 mNm 1.97 mNm 1.97 mNm 1.97 mNm and a suction cup. The vacuum-lifting cylinder attached to the gripper palm has an 80-mm stroke. Controller type MCBL 3002S MCBL 3002S MCBL 3002S MCBL 3002S Figure 5 shows that when the two robotic ﬁngers stay at an open conﬁguration, suction grasping can be performed to grasp objects in a cluttered narrow environment. The system overview of the The suction-grasping system consists of an air compressor, an ejector, a filter, a vacuum multi-function grasping system including a six-DOF commercial robot arm, is shown in Figure 6. cylinder, and a suction cup. The vacuum-lifting cylinder attached to the gripper palm has an 80-mm stroke. Figure 5 shows that when the two robotic fingers stay at an open configuration, suction grasping can be performed to grasp objects in a cluttered narrow environment. The system overview of the multi-function grasping system including a six-DOF commercial robot arm, is shown in Figure 6. Appl. Sci. 2019, 9, 5266 7 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 21 Figure 6. System overview of the multi-function grasping system. Figure 6. System overview of the multi-function grasping system. 2.3. Comparison with One Commercialized Three-Phalanx Robotic Gripper 2.3. Comparison with One Commercialized Three-Phalanx Robotic Gripper To demonstrate the special feature of our robotic ﬁnger. A comparison is made between our To demonstrate the special feature of our robotic finger. A comparison is made between our three-DOF robotic ﬁnger and the well-known three-DOF underactuated ﬁnger used in Robotiq’s three-DOF robotic finger and the well-known three-DOF underactuated finger used in Robotiq’s three-ﬁnger adaptive gripper (licensed from [27]). three-finger adaptive gripper (licensed from [27]). The architecture of the ﬁnger used in Robotiq’s three-ﬁnger adaptive gripper is shown in Figure 7b,c. The architecture of the finger used in Robotiq’s three-finger adaptive gripper is shown in Its design principle can be explained as follows: First, a general three-DOF shape adaptive ﬁnger, as we Figure 7b,c. Its design principle can be explained as follows: First, a general three-DOF shape explained earlier, is shown in Figure 7a. This ﬁnger mechanism can be used to perform shape-adaptive adaptive finger, as we explained earlier, is shown in Figure 7a. This finger mechanism can be used grasping, with at most three contact points at the three phalanxes in sequence. However, this ﬁnger to perform shape-adaptive grasping, with at most three contact points at the three phalanxes in mechanism cannot be used to perform parallel grasping, as its distal phalanxes cannot be maintained sequence. However, this finger mechanism cannot be used to perform parallel grasping, as its distal to be parallel to each other. To achieve parallel grasping mode, Gosselin and Laliberté [27] developed phalanxes cannot be maintained to be parallel to each other. To achieve parallel grasping mode, a three-DOF underactuated ﬁnger by adding two parallelograms to the three-DOF shape-adaptive Gosselin and Laliberté [27] developed a three-DOF underactuated finger by adding two ﬁnger paralshown lelograms to the thr in Figure 7a.ee- Their DOF sha design pe-is ada shown ptive in fing Figur er shown in Fig e 7b,c. From ure Figur 7a. e Their design 7c, we can see is shown that the two in Fi parallelograms gure 7b,c. From ar F e icoupled gure 7c, w toepr can oximal see th and at th intermediate e two parallelo phalanxes grams are co of the upshape-adaptive led to proximal ﬁnger and . intermediate phalanxes of the shape-adaptive finger. By adding two mechanical stoppers and two By adding two mechanical stoppers and two springs at the bottom and top ends of the ﬁnger, parallel springs at the bottom and top ends of the finger, parallel grasping can be achieved as shown in grasping can be achieved as shown in Figure 7b,c. However, in their design, the orientation of the distal Figure 7b,c. However, in their design, the orientation of the distal phalanx is passively coupled with phalanx is passively coupled with the intermediate phalanx through mechanical elements and cannot the intermediate phalanx through mechanical elements and cannot be actively controlled. Hence, be actively controlled. Hence, except for parallel and shape-adaptive grasping, this ﬁnger design except for parallel and shape-adaptive grasping, this finger design might not be appropriate for might not be appropriate for performing other challenging grasping tasks. But it has the advantage of performing other challenging grasping tasks. But it has the advantage of achieving stable achieving stable shape-adaptive grasping by using only one actuator to control the three phalanxes in shape-adaptive grasping by using only one actuator to control the three phalanxes in sequence, as sequence, as shown in Figure 7c. shown in Figure 7c. Compared with Robotiq’s ﬁnger, our design has a simpler structure in terms of the design Compared with Robotiq’s finger, our design has a simpler structure in terms of the design complexity and number of mechanical links. The orientation of the distal phalanx of our robotic ﬁnger complexity and number of mechanical links. The orientation of the distal phalanx of our robotic can be controlled independently by activating link L , shown in Figure 3. This feature enables the AE finger can be controlled independently by activating link L , shown in Figure 3. This feature AE ﬁnger to perform multiple grasping tasks. Actively adjusting the orientation of the distal phalanx is enables the finger to perform multiple grasping tasks. Actively adjusting the orientation of the distal also important for design a multi-ﬁnger robotic hand, which is our on-going work. phalanx is also important for design a multi-finger robotic hand, which is our on-going work. Appl. Sci. 2019, 9, 5266 8 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 21 Figure 7. (a) A three-DOF shape adaptive ﬁnger. (b,c): Three-DOF underactuated ﬁnger proposed by Figure 7. (a) A three-DOF shape adaptive finger. (b,c): Three-DOF underactuated finger proposed by Gosselin and Laliberté [27]. (b) Parallel precision grasping. (c) Shape-adaptive grasping. Gosselin and Laliberté [27]. (b) Parallel precision grasping. (c) Shape-adaptive grasping. 3. Grasping Modes and Grasping Strategy 3. Grasping Modes and Grasping Strategy This multi-function gripper is developed to grasp general objects in dierent working environments. This multi-function gripper is developed to grasp general objects in different working It can be used to achieve multiple grasping modes, such as parallel grasping, non-parallel grasping, environments. It can be used to achieve multiple grasping modes, such as parallel grasping, shape-adaptive power grasping, suction, suction-and-pinch grasping, and many other grasping tasks. non-parallel grasping, shape-adaptive power grasping, suction, suction-and-pinch grasping, and In this section, we present several examples that are selected to investigate their corresponding many other grasping tasks. In this section, we present several examples that are selected to grasping strategies. Moreover, the possibility of using this new linkage-driven gripper to perform investigate their corresponding grasping strategies. Moreover, the possibility of using this new some challenging grasping tasks is presented. linkage-driven gripper to perform some challenging grasping tasks is presented. 3.1. Parallel Grasping 3.1. Parallel Grasping 3.1.1. Sequence Demonstration of Parallel Grasping 3.1.1. Sequence Demonstration of Parallel Grasping As explained earlier, two motors are required to control each underactuated ﬁnger. One motor is As explained earlier, two motors are required to control each underactuated finger. One motor used to control the open-close motion, and the other one is used to control the orientation of the distal is used to control the open-close motion, and the other one is used to control the orientation of the phalanx. When both the two ﬁngers are simultaneously controlled to perform the open-close motion distal phalanx. When both the two fingers are simultaneously controlled to perform the open-close and no external contact occurs at the proximal linkages, parallel grasping can be achieved, as shown motion and no external contact occurs at the proximal linkages, parallel grasping can be achieved, from Figure 8a–c. It is noted that to make sure that these two independent ﬁngers can be synchronized as shown from Figure 8a-c. It is noted that to make sure that these two independent fingers can be during parallel grasping, initialization is required before performing a grasping task. Initially, we synchronized during parallel grasping, initialization is required before performing a grasping task. need to move the gripper to the home position where both of the two ﬁngers are fully opened. Due to Initially, we need to move the gripper to the home position where both of the two fingers are fully the mechanical limitation, after moving to the fully opened conﬁguration, the ﬁngers cannot move opened. Due to the mechanical limitation, after moving to the fully opened configuration, the anymore, and the motor current will be increased rapidly. Then we use the current feedback from the fingers cannot move anymore, and the motor current will be increased rapidly. Then we use the motor to detect whether these two ﬁngers move to the fully opened conﬁgurations or not. Because current feedback from the motor to detect whether these two fingers move to the fully opened we can ﬁnd the absolute position of each ﬁnger at the fully opened conﬁguration from the 3D model, configurations or not. Because we can find the absolute position of each finger at the fully opened we can control the synchronization of the two ﬁngers. Video attachment demonstrates how motion configuration from the 3D model, we can control the synchronization of the two fingers. Video synchronization is achieved. attachment demonstrates how motion synchronization is achieved. Appl. Sci. 2019, 9, 5266 9 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 21 fL=+ τθ sin θ () () (6) aa AK c The power-transmission system consists of the motor gear head and worm-gear set. By (a) (b) (c) considering the power efficiency, the relationship between the actuation torque of independent link Figure Figure 8. 8. Demonstration Demonstration of p of parallel arallel g grasping. rasping. L and input motor torque can be derived as AD 3.1.2. Analysis of Parallel Grasping Force 3.1.2. Analysis of Parallel Grasping Forcττ e = rrηη (7) ammw m w The relationship between the parallel-grasping force and actuation torque can be evaluated with The relationship between the parallel-grasping force and actuation torque can be evaluated where τ is the input motor torque. r and η are the gear ratio and efficiency of the motor m m m the quasi-static modeling of the ﬁnger. As mentioned earlier, during the parallel-grasping sequence, the with the quasi-static modeling of the finger. As mentioned earlier, during the parallel-grasping gear head, respectively. r and η are the gear ratio and efficiency of the worm-gear set, w w entire ﬁnger moves as a single rigid body through using one motor to control the link L . The parallel AD sequence, the entire finger moves as a single rigid body through using one motor to control the link respectively. grasping model is shown in Figure 9a. By equating the input and output virtual power, we have L . The parallel grasping model is shown in Figure 9a. By equating the input and output virtual AD Moreover, the motor capacity can be determined according to the payload requirement using power, we have T the above-mentioned relationship between the grasping force and input motor torque. f v = ! (1) P a a Furthermore, because the efficiency coefficients ( η and η ) shown in Equation (7) are fv⋅=τω m w (1) P aa where referring to Figure 9a, represents the contact force at contact point P during parallel grasping, difficult to accurately evaluate, accurately measuring the energy efficiency of the where referring to Figure 9a, represents the contact force at contact point P during parallel grasping, represents the instantaneous velocity of the contact point P, represents the actuation torque exerted on power-transmission system becomes difficult. Thus, Equation (6) can only be used to estimate the represents the instantaneous velocity of the contact point P, represents the actuation torque exerted the link used to perform the open-close motion of the ﬁnger, represents the angular velocity of the grasping force. In order to accurately control the grasping force, we need to install a force sensor at on the link used to perform the open-close motion of the finger, represents the angular velocity of active link, i.e., and denotes the open-close angle of the active link with respect to the horizontal axis. the distal phalanx. the active link, i.e., and denotes the open-close angle of the active link with respect to the horizontal axis. During the parallel-grasping sequence, the entire finger moves as a single rigid body to perform the open-close motion. Thus, point K shown in Figure 9a will follow a circular trajectory with its rotation center located at point A. Because the orientation of the distal phalanx remains constant during the open-close motion, all its points have the same instantaneous velocity. By defining a virtual parallelogram OA −−K−P (OA = PK ) shown in Figure 9a, we know that the P P contact point P follows a circular trajectory with its rotation center located at the virtual point O .The motion of contact point P during parallel grasping is shown in Figure 9b. The velocity of contact point P is the same as the velocity of point K, which can be derived as vv==L φ (2) PK AK As we know, the whole finger moves as a single rigid body, we have φ+− 2πθ =θ (3) ac where θ is a constant value that can be found from our design. Then we have  (a) (b) φ = θ (4) Figure 9. (a) Parallel-grasping model. (b) Trajectory of contact point P. From Figure 9 Figure a and substituti 9. (a) Parallel-grasping ng Equations model. (2)( a b)n Td rajectory (4) into ofEqu contact ation point (1),P Equati . on (1) can be rewritten as 3.2. Shape-Adaptive Grasping During the parallel-grasping sequence, the entire ﬁnger moves as a single rigid body to perform   fv⋅= fv sinφφ = fL sinφ= fL θθ sin +θ =τθ () (5) the open-close motion. Thus, P pointPa K shown AKin Figure 9 AK a will followac a ciraculara trajectory with its During performing the open-close motion of the two fingers, when the grasped object makes rotation center located at point A. Because the orientation of the distal phalanx remains constant contact (external forces F and F are applied) with the proximal phalanx, the parallel-grasping Thus, the parallel-grasping force can be obtained as 1 2 mode shown in Figure 8 will transform into shape-adaptive grasping, as shown in Figure 10. Keeping closing the finger forces the angle between the proximal and intermediate phalanxes to decrease. During this process, the torsion spring is twisted by the motor. Thus, the stiffness of the torsion spring should be designed as small as possible, but sufficiently large to prevent undesired motion due to the weight and inertia. Appl. Sci. 2019, 9, 5266 10 of 21 during the open-close motion, all its points have the same instantaneous velocity. By deﬁning a virtual parallelogram O A K P (O A = PK) shown in Figure 9a, we know that the contact point P P P follows a circular trajectory with its rotation center located at the virtual point O .The motion of contact point P during parallel grasping is shown in Figure 9b. The velocity of contact point P is the same as the velocity of point K, which can be derived as v = v = L (2) P K AK As we know, the whole ﬁnger moves as a single rigid body, we have + 2 = (3) a c where is a constant value that can be found from our design. Then we have . . = (4) From Figure 9a and substituting Equations (2) and (4) into Equation (1), Equation (1) can be rewritten as . . . f v = f v sin = f L sin = f L sin( + ) = (5) P P AK AK a a c a a Thus, the parallel-grasping force can be obtained as f = /(L sin( + )) (6) a AK a c The power-transmission system consists of the motor gear head and worm-gear set. By considering the power eciency, the relationship between the actuation torque of independent link L and input AD motor torque can be derived as = r r (7) a m m w m w where is the input motor torque. r and are the gear ratio and eciency of the motor gear head, m m m respectively. r and are the gear ratio and eciency of the worm-gear set, respectively. w w Moreover, the motor capacity can be determined according to the payload requirement using the above-mentioned relationship between the grasping force and input motor torque. Furthermore, because the eciency coecients ( and ) shown in Equation (7) are dicult m w to accurately evaluate, accurately measuring the energy eciency of the power-transmission system becomes dicult. Thus, Equation (6) can only be used to estimate the grasping force. In order to accurately control the grasping force, we need to install a force sensor at the distal phalanx. 3.2. Shape-Adaptive Grasping During performing the open-close motion of the two ﬁngers, when the grasped object makes contact (external forces F and F are applied) with the proximal phalanx, the parallel-grasping mode 1 2 shown in Figure 8 will transform into shape-adaptive grasping, as shown in Figure 10. Keeping closing the ﬁnger forces the angle between the proximal and intermediate phalanxes to decrease. During this process, the torsion spring is twisted by the motor. Thus, the stiness of the torsion spring should be designed as small as possible, but suciently large to prevent undesired motion due to the weight and inertia. Appl. Sci. 2019, 9, 5266 11 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 21 (a) (b) (a) (b) Figure 10. Demonstration of shape-adaptive grasping. (a) State when objects start making contact Figure 10. Demonstration of shape-adaptive grasping. (a) State when objects start making contact Figure 10. Demonstration of shape-adaptive grasping. (a) State when objects start making contact with with thproximal phalanxes. (b) State when the shape-adaptive grasping is achieved. with thproximal phalanxes. (b) State when the shape-adaptive grasping is achieved. thproximal phalanxes. (b) State when the shape-adaptive grasping is achieved. 3.3. Combination of the Suction and Mechanical Grippers 3.3. Combination of the Suction and Mechanical Grippers 3.3. Combination of the Suction and Mechanical Grippers As explained earlier, conventional mechanical grippers/hands provide the advantage of achieving As explained earlier, conventional mechanical grippers/hands provide the advantage of As explained earlier, conventional mechanical grippers/hands provide the advantage of stable grasping in open space. Meanwhile, applying them in a cluttered narrow space is dicult achieving stable grasping in open space. Meanwhile, applying them in a cluttered narrow space is achieving stable grasping in open space. Meanwhile, applying them in a cluttered narrow space is because multiple physical contacts might occur. In comparison, the suction grasper oers the advantage difficult because multiple physical contacts might occur. In comparison, the suction grasper offers difficult because multiple physical contacts might occur. In comparison, the suction grasper offers of grasping objects in a cluttered narrow environment. Meanwhile, pure suction grasping is generally the advantage of grasping objects in a cluttered narrow environment. Meanwhile, pure suction the advantage of grasping objects in a cluttered narrow environment. Meanwhile, pure suction unstable because the suction cup is too soft to maintain the conﬁguration of the grasped object. Hence, grasping is generally unstable because the suction cup is too soft to maintain the configuration of the grasping is generally unstable because the suction cup is too soft to maintain the configuration of the combining the mechanical gripper/hand with the suction grasper is an ecient method to grasp general grasped object. Hence, combining the mechanical gripper/hand with the suction grasper is an grasped object. Hence, combining the mechanical gripper/hand with the suction grasper is an objects in dierent types of working environments. The grasping sequence of this multi-function efficient method to grasp general objects in different types of working environments. The grasping efficient method to grasp general objects in different types of working environments. The grasping gripper is shown in Figure 11. sequence of this multi-function gripper is shown in Figure 11. sequence of this multi-function gripper is shown in Figure 11. Figure Figure 11. 11. Grasping-sequence demonstration of Grasping-sequence demonstration of the multi-fu the multi-function nction gripper. gripper. Figure 11. Grasping-sequence demonstration of the multi-function gripper. 3.4. Contact-Based Grasping 3.4. Contact-Based Grasping 3.4. Contact-Based Grasping In addition to the above-mentioned three types of grasping modes, this gripper can be used to In addition to the above-mentioned three types of grasping modes, this gripper can be used to In addition to the above-mentioned three types of grasping modes, this gripper can be used to perform some other challenging grasping tasks. One special feature of this linkage-driven gripper is perform some other challenging grasping tasks. One special feature of this linkage-driven gripper is perform some other challenging grasping tasks. One special feature of this linkage-driven gripper is that the orientation of the distal phalanx of each ﬁnger can be actively controlled and decoupled from that the orientation of the distal phalanx of each finger can be actively controlled and decoupled that the orientation of the distal phalanx of each finger can be actively controlled and decoupled the open-close motion of the ﬁnger. Thus, during the parallel-grasping sequence, the distal phalanx from the open-close motion of the finger. Thus, during the parallel-grasping sequence, the distal from the open-close motion of the finger. Thus, during the parallel-grasping sequence, the distal can be maintained at a desired orientation even when it contacts with the external environment, as phalanx can be maintained at a desired orientation even when it contacts with the external phalanx can be maintained at a desired orientation even when it contacts with the external shown in Figure 12. This grasping mode is quite useful when we plan to grasp relatively thin objects environment, as shown in Figure 12. This grasping mode is quite useful when we plan to grasp environment, as shown in Figure 12. This grasping mode is quite useful when we plan to grasp lying on a ﬂat surface. Many grippers/hands fail to grasp objects in such a manner because when they relatively thin objects lying on a flat surface. Many grippers/hands fail to grasp objects in such a relatively thin objects lying on a flat surface. Many grippers/hands fail to grasp objects in such a make contact with the supporting base, the orientation of the distal phalanx of their robotic ﬁnger will manner because when they make contact with the supporting base, the orientation of the distal manner because when they make contact with the supporting base, the orientation of the distal change because of the coupled structure. To achieve grasping using other grippers/hands, a good phalanx of their robotic finger will change because of the coupled structure. To achieve grasping phalanx of their robotic finger will change because of the coupled structure. To achieve grasping using other grippers/hands, a good calibration algorithm and a highly accurate computer-vision using other grippers/hands, a good calibration algorithm and a highly accurate computer-vision Appl. Sci. 2019, 9, 5266 12 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 12 of 21 calibration algorithm and a highly accurate computer-vision system are generally required to grasp system are generally required to grasp such objects without making any contact with the such objects without making any contact with the environment. environment. (a) (b) Figure 12. Parallel grasping during contact with the environment. (a) State when the gripper starts Figure 12. Parallel grasping during contact with the environment. (a) State when the gripper starts making contact with the supported surface. (b) State when the contact-based parallel grasping is making contact with the supported surface. (b) State when the contact-based parallel grasping is done done without lifting up. without lifting up. 3.5. Grasping Thin Objects via Scooping 3.5. Grasping Thin Objects via Scooping Because the two ﬁngers of the underactuated gripper are independently controlled and the Because the two fingers of the underactuated gripper are independently controlled and the orientation of the distal phalanx of each ﬁnger can be actively adjusted, using our gripper to pick orientation of the distal phalanx of each finger can be actively adjusted, using our gripper to pick up thin objects lying on a ﬂat surface via scooping becomes possible [28]. We note that to ensure up thin objects lying on a flat surface via scooping becomes possible [28]. We note that to ensure that the ﬁngertip can scoop in the space between the object to be grasped and the supporting base, that the fingertip can scoop in the space between the object to be grasped and the supporting base, the ﬁngernail for insertion should be specially designed to be frictionless and suciently sharp. The the fingernail for insertion should be specially designed to be frictionless and sufficiently sharp. The following experiment demonstrates the whole grasping sequence. following experiment demonstrates the whole grasping sequence. 4. Experimental Evaluation and Discussion 4. Experimental Evaluation and Discussion To validate the design concept of this multi-function gripper, an aluminum prototype is To validate the design concept of this multi-function gripper, an aluminum prototype is manufactured to test its performance in the real world. A commercially available six-DOF robot arm manufactured to test its performance in the real world. A commercially available six-DOF robot arm is integrated into the experimental system. Different types of grasping modes, including parallel is integrated into the experimental system. Dierent types of grasping modes, including parallel grasping, shape-adaptive grasping, combination of the suction and mechanical grippers, grasping, shape-adaptive grasping, combination of the suction and mechanical grippers, contact-based contact-based grasping, and grasping thin objects lying on a flat surface via scooping, are grasping, and grasping thin objects lying on a ﬂat surface via scooping, are experimentally veriﬁed. experimentally verified. 4.1. Independent Motion Demonstration 4.1. Independent Motion Demonstration As mentioned earlier, this linkage-driven gripper has two ﬁngers that can be controlled independently As ment . Furthermor ioned earlier, this li e, each ﬁnger nkage-driven gripper has two independenthas two fi motions, i.e., ngers th the open-close at can be contro motion of lled independently. Furthermore, each finger has two independent motions, i.e., the open-close motion the ﬁnger and distal-phalanx orientation adjustment. Figure 13 shows a demonstration of the four independent of the finger motions. and distal-ph From Figur alanxe orient 13a,b,ation the right adjustment. F ﬁnger performs igure 13 the shows open-close a demonstration motion. Fr of the om four independent motions. From Figure 13a–b, the right finger performs the open-close motion. Figure 13b,c, the distal-phalanx orientation of the right ﬁnger is adjusted. From Figure 13c,d, the left ﬁnger From performs Figure 13b the–c, t open-close he distal-p motion. halanx o From rient Figur ation o e 13 f t d,e, he right the distal-phalanx finger is adjust orientation ed. From Fig of u the re 13c t left o d, the left finger performs the open-close motion. From Figure 13d–e, the distal-phalanx orientation ﬁnger is adjusted. Finally, from Figure 13e,f, the two ﬁngers are closed simultaneously. of the left finger is adjusted. Finally, from Figure 13e to f, the two fingers are closed simultaneously. Appl. Sci. 2019, 9, x FOR PEER REVIEW 13 of 21 Appl. Sci. 2019, 9, 5266 13 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 13 of 21 Figure 13. Demonstration of the independent motions of this linkage-driven gripper. Figure 13. Demonstration of the independent motions of this linkage-driven gripper. Figure 13. Demonstration of the independent motions of this linkage-driven gripper. 4.2. Suction Experiments 4.2. Suction Experiments As mentioned earlier, suction grasping offers the advantage of grasping objects in a cluttered 4.2. Suction Experiments narrow environment. Figure 14 shows some examples of successful suction grasping, which are As mentioned earlier, suction grasping oers the advantage of grasping objects in a cluttered As mentioned earlier, suction grasping offers the advantage of grasping objects in a cluttered difficult to achieve with the mechanical gripper. However, using the suction gripper to grasp narrow environment. Figure 14 shows some examples of successful suction grasping, which are narrow environment. Figure 14 shows some examples of successful suction grasping, which are object dis made cult to achieve of fabric m with the ateria mechanical ls or obje gripper cts wit . h However multiple ho , using les the is di suction fficugripper lt. Figure to grasp 15 sho objects ws this difficult to achieve with the mechanical gripper. However, using the suction gripper to grasp made of fabric materials or objects with multiple holes is dicult. Figure 15 shows this condition. condition. objects made of fabric materials or objects with multiple holes is difficult. Figure 15 shows this condition. Figure Figure 14. 14. Using Using a s a suction uction gri gripper pper to gras to grasp p objec objects ts iin n a cluttered narrow en a cluttered narrow envir vironment. (a) onment. (aGrasp an ) Grasp an Figure 14. Using a suction gripper to grasp objects in a cluttered narrow environment. (a) Grasp an object from the mesh pen/pencil cup holder. (b) Grasp a fruit from the wine glass. (c) Grasp an object object from the mesh pen/pencil cup holder. (b) Grasp a fruit from the wine glass. (c) Grasp an object object from the mesh pen/pencil cup holder. (b) Grasp a fruit from the wine glass. (c) Grasp an object surrounded by other objects from the paper box. surrounded by other objects from the paper box. surrounded by other objects from the paper box. Appl. Sci. 2019, 9, x FOR PEER REVIEW 14 of 21 Appl. Sci. 2019, 9, 5266 14 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 14 of 21 (a) (b) (a) (b) Figure 15. The suction gripper fails to grasp certain objects. (a) Bear toy. (b) Mesh pen/pencil cup Figure 15. The suction gripper fails to grasp certain objects. (a) Bear toy. (b) Mesh pen/pencil cup Figure holder. 15. The suction gripper fails to grasp certain objects. (a) Bear toy. (b) Mesh pen/pencil cup holder. holder. 4.3. Linkage-Driven Gripper Experiments 4.3. Linkage-Driven Gripper Experiments 4.3. Linkage-Driven Gripper Experiments Even though mechanical grippers have diculty in grasping objects in a cluttered narrow space, Even though mechanical grippers have difficulty in grasping objects in a cluttered narrow using them to stably grasp objects in open space is simple. For the objects shown in Figure 15, using Even though mechanical grippers have difficulty in grasping objects in a cluttered narrow space, using them to stably grasp objects in open space is simple. For the objects shown in Figure 15, the suction gripper to grasp them is dicult. In comparison, using mechanical grippers to grasp such space, using them to stably grasp objects in open space is simple. For the objects shown in Figure 15, using the suction gripper to grasp them is difficult. In comparison, using mechanical grippers to objects is simple, as shown in Figure 16. using the suction gripper to grasp them is difficult. In comparison, using mechanical grippers to grasp such objects is simple, as shown in Figure 16. grasp such objects is simple, as shown in Figure 16. (a) (b) (a) (b) Figure 16. Two-finger underactuated gripper succeeding in grasping certain objects that are difficult Figure 16. Two-ﬁnger underactuated gripper succeeding in grasping certain objects that are dicult Figure 16. Two-finger underactuated gripper succeeding in grasping certain objects that are difficult for the for the suct suction gripper ion gripper to grasp. ( to grasp. (a) Bear a) Bear toy. (toy b) Mesh . (b) Mesh pen/pencil pen/pencil cup holder cup holder. . for the suction gripper to grasp. (a) Bear toy. (b) Mesh pen/pencil cup holder. Moreover, Figure 17 shows how the pinch motion of the distal phalanx of the robotic ﬁnger is Moreover, Figure 17 shows how the pinch motion of the distal phalanx of the robotic finger is usedMoreover, to grasp di Figure erent17 show objects in s how the pinch motion of dierent scenarios. The the distal p object shown halanx o in Figur f the e 17robotic fin a lies above ger the is used to grasp different objects in different scenarios. The object shown in Figure 17a lies above the used to gr supporting asp different obj base, i.e., thereeexists cts in differe a sparenspace t scenar between ios. The object the object shown in Fi and the base. gure Actively 17a lies contr above t olle hd e supporting base, i.e., there exists a spare space between the object and the base. Actively controlled supporting pinch motion base allows , i.e., there the ﬁnger exists toa scoop spare sp theace be objecttween the object up from the bottom and the b side. a Figur se. Acti e 17 vel b shows y control using led pinch motion allows the finger to scoop the object up from the bottom side. Figure 17b shows using pi the nch moti closed on al distal lophalanx ws the finger to scoop to grasp a cup the object having a up lifting from the bott ear. This kind om side. Figure 17b of grasping mode shows is similar using the closed distal phalanx to grasp a cup having a lifting ear. This kind of grasping mode is similar tto he closed d caging [29 is ,30 tal p ]. Figur halanx t e 17 oc gra shows sp a c how up hav to grasp ing a objects lifting ear with . This a cone kind o shape. f grasp Figur ing m e 17 od de shows is simone ilar to caging [29,30]. Figure 17c shows how to grasp objects with a cone shape. Figure 17d shows one to cagin examplegof [29, grasping 30]. Figure though 17c shows how making contacts to grasp at both obje phalanxes cts with a co andne sh palm. ape. Figure 17d shows one example of grasping though making contacts at both phalanxes and palm. example of grasping though making contacts at both phalanxes and palm. 4.4. Combination of the Suction and Mechanical Grippers The suction gripper cannot stably grasp heavy objects that lie in a narrow space. In this case, the combination of suction and linkage-driven grippers will be an ecient method for achieving stable grasping. Figure 18a shows an example of grasping a 1.35 kg dumbbell from a narrow space. Figure 18b shows how the multi-function gripper is used to grasp an object with a cone shape and that with an irregular shape. Appl. Sci. 2019, 9, x FOR PEER REVIEW 15 of 21 Appl. Sci. 2019, 9, 5266 15 of 21 (a) (b) (c) (d) Figure 17. Examples of grasping by using the pinch motion of distal phalanx. (a) Grasping a roll of Figure 17. Examples of grasping by using the pinch motion of distal phalanx. (a) Grasping a roll of toilet paper lying above a supporting base. (b) Grasping a cup through caging. (c) Grasping a cup toilet paper lying above a supporting base. (b) Grasping a cup through caging. (c) Grasping a cup with with an irregular shape. (d) Grasping a baseball. an irregular shape. (d) Grasping a baseball. 4.4. Combination of the Suction and Mechanical Grippers The suction gripper cannot stably grasp heavy objects that lie in a narrow space. In this case, the combination of suction and linkage-driven grippers will be an efficient method for achieving stable grasping. Figure 18a shows an example of grasping a 1.35 kg dumbbell from a narrow space. Figure 18b shows how the multi-function gripper is used to grasp an object with a cone shape and that with an irregular shape. Appl. Sci. 2019, 9, x FOR PEER REVIEW 16 of 21 Appl. Sci. 2019, 9, 5266 16 of 21 Appl. Sci. 2019, 9, x FOR PEER REVIEW 16 of 21 (a) (b) Figure 18. Combination of the suction and mechanical grippers. (a) Grasping a heavy object from a (a) (b) narrow space. (b) Grasping objects with certain shapes that are difficult to directly grasp by the Figure 18. Combination of the suction and mechanical grippers. (a) Grasping a heavy object from a Figure 18. Combination of the suction and mechanical grippers. (a) Grasping a heavy object from mechanical gripper. narrow space. (b) Grasping objects with certain shapes that are difficult to directly grasp by the a narrow space. (b) Grasping objects with certain shapes that are dicult to directly grasp by the mechanical gripper. mechanical gripper. 4.5. Environment Contact-Based Grasping 4.5. Environment Contact-Based Grasping Figure 19 shows contact-based grasping. We note that during physical contact with the 4.5. Environment Contact-Based Grasping supporting base, the torsion spring is twisted. Thus, after the contact-based grasping is finished and Figure 19 shows contact-based grasping. We note that during physical contact with the supporting Figure 19 shows contact-based grasping. We note that during physical contact with the the six-DOF robot arm is lifting the gripper up, the torsion spring will be released. Because the base, the torsion spring is twisted. Thus, after the contact-based grasping is ﬁnished and the six-DOF supporting base, the torsion spring is twisted. Thus, after the contact-based grasping is finished and proximal and intermediate phalanx are passively coupled by the torsion spring, the distance robot arm is lifting the gripper up, the torsion spring will be released. Because the proximal and the six-DOF robot arm is lifting the gripper up, the torsion spring will be released. Because the between two robotic fingers might increase if we don’t control the gripper actively. Hence, to intermediate phalanx are passively coupled by the torsion spring, the distance between two robotic proximal and intermediate phalanx are passively coupled by the torsion spring, the distance prevent the objects from falling, we need to actively control the closing speed of the gripper to ﬁngers might increase if we don’t control the gripper actively. Hence, to prevent the objects from between two robotic fingers might increase if we don’t control the gripper actively. Hence, to ensure that the closing speed is faster than the finger-opening speed caused by the lifting of the falling, we need to actively control the closing speed of the gripper to ensure that the closing speed is prevent the objects from falling, we need to actively control the closing speed of the gripper to six-DOF robot arm. faster than the ﬁnger-opening speed caused by the lifting of the six-DOF robot arm. ensure that the closing speed is faster than the finger-opening speed caused by the lifting of the six-DOF robot arm. Figure 19. Example of environment contact-based grasping. Figure 19. Example of environment contact-based grasping. 4.6. Grasping Thin Objects via Scooping Figure 19. Example of environment contact-based grasping. 4.6. Grasping Thin Objects via Scooping Figure 20 shows an example of grasping thin objects lying on a ﬂat surface by scooping. We note that the ﬁngernail of the left robotic ﬁnger used for scooping needs to be suciently sharp and Figure 20 shows an example of grasping thin objects lying on a flat surface by scooping. We 4.6. Grasping Thin Objects via Scooping frictionless to scoop at the bottom side of the object. Moreover, the object to be grasped should not be note that the fingernail of the left robotic finger used for scooping needs to be sufficiently sharp and Figure 20 shows an example of grasping thin objects lying on a flat surface by scooping. We too rigid, and an appropriate control algorithm is required for a stable and robust scooping task. This frictionless to scoop at the bottom side of the object. Moreover, the object to be grasped should not note that the fingernail of the left robotic finger used for scooping needs to be sufficiently sharp and frictionless to scoop at the bottom side of the object. Moreover, the object to be grasped should not Appl. Sci. 2019, 9, x FOR PEER REVIEW 17 of 21 Appl. Sci. 2019, 9, 5266 17 of 21 be too rigid, and an appropriate control algorithm is required for a stable and robust scooping task. example This examp only le only provide provides a simple s a simple de demonstration monstrat ofio the n of the possib possibility of ility of performing a performing a scooping scooping task task using our using our proposed gripper. proposed gripper. Figure 20. Example of scooping and picking up thin objects lying on a ﬂat surface. Appl. Sci. 2019, 9, x FOR PEER REVIEW 18 of 21 Figure 20. Example of scooping and picking up thin objects lying on a flat surface. 4.7. Discussion coincide and friction forces generated are big enough, the parallel grasping generally can be 4.7. Discussion Two-ﬁnger grippers have been extensively used in industrial application and automated assembly achieved stably. The friction force depends on the nature of two surfaces in contact. For example, Two-finger grippers have been extensively used in industrial application and automated because 60 to 70% of human’s grasping of objects of cylindrical, parallelepiped, and pyramidal shapes the sliding contact and rolling contact would have different friction forces. And the friction assembly because 60 to 70% of human’s grasping of objects of cylindrical, parallelepiped, and is performed with only two ﬁngers, and two-ﬁnger grippers are generally cheaper and easier to use as coefficient between the contact surface and the object can be increased by adding a friction pad at pyramidal shapes is performed with only two fingers, and two-finger grippers are generally compared with multi-ﬁnger grippers/hands [31,32]. Our prototype gripper was manufactured using the fingertip. Furthermore, because the two-finger parallel grasping has only two contact cheaper and easier to use as compared with multi-finger grippers/hands [31,32]. Our prototype aluminum alloy. Because ball bearings are used at all joints, friction in the joints can be considered to areas/points, grasping points would also influence the stability of the two-finger parallel grasping. gripper was manufactured using aluminum alloy. Because ball bearings are used at all joints, be negligible. For example, it might be unstable to grasp a relatively long object by selecting the corresponding friction in the joints can be considered to be negligible. Dierent phases of two-ﬁnger grasping with only two contact points are shown in Figure 21. The grasp contact points shown in Figure 23a. This is because the gravity center of the object is far from Different phases of two-finger grasping with only two contact points are shown in Figure 21. two ﬁngers will make contact with the object in sequence. From Figure 21b,c, we can see that the object the grasping zone. The gravity force will generate some rotation torque, which is a leading cause of The two fingers will make contact with the object in sequence. From Figure 21b,c, we can see that is dragged by the ﬁnger during continuous closure movement. Figure 21d demonstrates the phase the unstable two-finger parallel grasping. Experiments of grasping the same object at different the object is dragged by the finger during continuous closure movement. Figure 21d demonstrates where a static grasp is achieved. In order to model these phases, the planar grasp model [33] for a contact points were performed as shown in Figure 23b. We note that the grasping motor was the phase where a static grasp is achieved. In order to model these phases, the planar grasp model two-ﬁnger gripper is shown in Figure 22. The static equilibrium condition of a planar grasp can be actuated by using almost the same current in these two experiments. The left panel shown in Figure [33] for a two-finger gripper is shown in Figure 22. The static equilibrium condition of a planar expressed in the directions of the contact and squeezing line as follows 23b demonstrates the success of a stable grasping by selecting the grasping points/area near the grasp can be expressed in the directions of the contact and squeezing line as follows gravity center. The right panel shown in Figure 24b demonstrates one case of unstable grasping. F cos F cos + F sin F sin + m g cos sin + a = 0 (8) 1 1 2 2 1 1 1 2 2 2 obj w w y As far as the objects FF cosψψ−+ with irregu cos μ lF ar sisha nψ p− es μaF re concerned, sinψ+m igf the two conta cosψ sinφ+act points a= 0 re not well () (8) 11 2 2 11 1 2 2 2 obj w w y selected, the static force equilibrium might not be achieved. One example is shown in Figure 24, F sin F sin + F cos + F cos + m (g cos cos + a ) = 0 (9) 1 1 2 2 1 1 1 2 2 2 obj w w z where the two contact points A and B are relatively far from each other. During applying the −−FF sinψψ sin +μF cosψ+μF cosψ+m g cosψ cosφ+a = 0 () (9) 11 2 2 11 1 2 2 2 obj w w z contact forces, the configuration of the object might be changed (Initial configuration of the object is r F (sin cos ) r F (sin cos ) N r m g+a sin = 0 (10) A 1 1 1 1 B 2 2 2 2 G obj y w indicated by yellow color; the new configuration is indicated by transparent green color) because rF() sinψμ−− cosψ r F(sinψ−μ cosψ)−N−r m g+a sinφ= 0 () (10) A1 1 1 1 B 2 2 2 2 G obj y w where F and F are the grasping forces at contact points A and B. and are the friction coecients 1 2 1 2 the two contact forces do not coincide, a winding moment will be produced on the object. After at contact points A and B. r and r represent the distances of contact points A and B. r represents the A B G continually applying the grasping force, it is not sure whether the static equilibrium between two where F and F are the grasping forces at contact points A and B. μ and μ are the friction 1 2 1 2 distance of the gravity center. W = m g represents the weight vector of the objects. It is orientated obj fingers can be achieved or not. In this case, a multi-contact grasping generally is required for coefficients at contact points A and B. r and r represent the distances of contact points A and B. A B with an angle with respect to the squeezing line and an angle with respect to the perpendicular w w achieving stable grasping. For certain objects to be grasped, the grasping mode and grasp contact r represents the distance of the gravity center. Wm = g represents the weight vector of the G obj axis to the y–z plane. a and a represents the acceleration components of the gravity center point. N y z points should be carefully selected. It is also possible to increase the grasping stability by adding an objects. It is orientated with an angle φ with respect to the squeezing line and an angle ψ with is an external torque acting on the object and the inertia eect due to the manipulator movement is w w appropriate compliant structure to the fingertip because the compliance increases the ability of a respect to the perpendicular axis to the y–z plane. a and a represents the acceleration also included. This grasp model describes all situations of a two-ﬁnger grasping as pointed out by y z gripper to conform to the shape of the object being grasped, and also increases the area of the Ceccarelli [34]. components of the gravity center point. N is an external torque acting on the object and the inertia contact patches, increasing the grasp wrench space [37]. effect due to the manipulator movement is also included. This grasp model describes all situations of a two-finger grasping as pointed out by Ceccarelli [34]. From Figures 21 and 22, it can be found that the static equilibrium might be difficult to achieve by using only two grasping forces, especially in the case that the two fingertips are not parallel to each other. From Equations (8) to (10), we can see that with only two grasping forces (two unknowns), it might be difficult to ensure that all those three equations hold. As mentioned earlier, both parallel grasping (two fingertips are parallel to each other) and non-parallel grasping (through adjusting the orientation of the distal phalanx) can be realized by using our two-finger gripper. As far as the parallel grasping is concerned, the parallel grasping force depends on friction forces at the fingertip [35,36]. The parallel grasping can be achieved by using the outer or interior Figure 21. Dierent phases of grasping objects. (a) Initial impact. (b) Second impact. (c) Applying Figure 21. Different phases of grasping objects. (a) Initial impact. (b) Second impact. (c) Applying surface parallelepiped of the fingertip. For cylindrical objects or those with parallel surfaces to both grasping force. (d) Static equilibrium. grasping force. (d) Static equilibrium. fingers, the parallel or opposed grasping force can be produced. If the directions of these forces Figure 22. Planar grasp model of a two-finger gripper. Appl. Sci. 2019, 9, x FOR PEER REVIEW 18 of 21 coincide and friction forces generated are big enough, the parallel grasping generally can be achieved stably. The friction force depends on the nature of two surfaces in contact. For example, the sliding contact and rolling contact would have different friction forces. And the friction coefficient between the contact surface and the object can be increased by adding a friction pad at the fingertip. Furthermore, because the two-finger parallel grasping has only two contact areas/points, grasping points would also influence the stability of the two-finger parallel grasping. For example, it might be unstable to grasp a relatively long object by selecting the corresponding grasp contact points shown in Figure 23a. This is because the gravity center of the object is far from the grasping zone. The gravity force will generate some rotation torque, which is a leading cause of the unstable two-finger parallel grasping. Experiments of grasping the same object at different contact points were performed as shown in Figure 23b. We note that the grasping motor was actuated by using almost the same current in these two experiments. The left panel shown in Figure 23b demonstrates the success of a stable grasping by selecting the grasping points/area near the gravity center. The right panel shown in Figure 24b demonstrates one case of unstable grasping. As far as the objects with irregular shapes are concerned, if the two contact points are not well selected, the static force equilibrium might not be achieved. One example is shown in Figure 24, where the two contact points A and B are relatively far from each other. During applying the contact forces, the configuration of the object might be changed (Initial configuration of the object is indicated by yellow color; the new configuration is indicated by transparent green color) because the two contact forces do not coincide, a winding moment will be produced on the object. After continually applying the grasping force, it is not sure whether the static equilibrium between two fingers can be achieved or not. In this case, a multi-contact grasping generally is required for achieving stable grasping. For certain objects to be grasped, the grasping mode and grasp contact points should be carefully selected. It is also possible to increase the grasping stability by adding an appropriate compliant structure to the fingertip because the compliance increases the ability of a gripper to conform to the shape of the object being grasped, and also increases the area of the contact patches, increasing the grasp wrench space [37]. Appl. Sci. 2019, 9, 5266 18 of 21 Figure 21. Different phases of grasping objects. (a) Initial impact. (b) Second impact. (c) Applying grasping force. (d) Static equilibrium. Figure 22. Planar grasp model of a two-ﬁnger gripper. Figure 22. Planar grasp model of a two-finger gripper. From Figures 21 and 22, it can be found that the static equilibrium might be dicult to achieve by using only two grasping forces, especially in the case that the two ﬁngertips are not parallel to each other. From Equations (8) to (10), we can see that with only two grasping forces (two unknowns), it might be dicult to ensure that all those three equations hold. As mentioned earlier, both parallel grasping (two ﬁngertips are parallel to each other) and non-parallel grasping (through adjusting the orientation of the distal phalanx) can be realized by using our two-ﬁnger gripper. As far as the parallel grasping is concerned, the parallel grasping force depends on friction forces at the ﬁngertip [35,36]. The parallel grasping can be achieved by using the outer or interior surface parallelepiped of the ﬁngertip. For cylindrical objects or those with parallel surfaces to both ﬁngers, the parallel or opposed grasping force can be produced. If the directions of these forces coincide and friction forces generated are big enough, the parallel grasping generally can be achieved stably. The friction force depends on the nature of two surfaces in contact. For example, the sliding contact and rolling contact would have dierent friction forces. And the friction coecient between the contact surface and the object can be increased by adding a friction pad at the ﬁngertip. Furthermore, because the two-ﬁnger parallel grasping has only two contact areas/points, grasping points would also inﬂuence the stability of the two-ﬁnger parallel grasping. For example, it might be unstable to grasp a relatively long object by selecting the corresponding grasp contact points shown in Figure 23a. This is because the gravity center of the object is far from the grasping zone. The gravity force will generate some rotation torque, which is a leading cause of the unstable two-ﬁnger parallel grasping. Experiments of grasping the same object at dierent contact points were performed as shown in Figure 23b. We note that the grasping motor was actuated by using almost the same current in these two experiments. The left panel shown in Figure 23b demonstrates the success of a stable grasping by selecting the grasping points/area near the gravity center. The right panel shown in Figure 24b demonstrates one case of unstable grasping. As far as the objects with irregular shapes are concerned, if the two contact points are not well selected, the static force equilibrium might not be achieved. One example is shown in Figure 24, where the two contact points A and B are relatively far from each other. During applying the contact forces, the conﬁguration of the object might be changed (Initial conﬁguration of the object is indicated by yellow color; the new conﬁguration is indicated by transparent green color) because the two contact forces do not coincide, a winding moment will be produced on the object. After continually applying the grasping force, it is not sure whether the static equilibrium between two ﬁngers can be achieved or not. In this case, a multi-contact grasping generally is required for achieving stable grasping. For certain objects to be grasped, the grasping mode and grasp contact points should be carefully selected. It is also possible to increase the grasping stability by adding an appropriate compliant structure to the ﬁngertip because the compliance increases the ability of a gripper to conform to the shape of the Appl. Sci. 2019, 9, 5266 19 of 21 object being grasped, and also increases the area of the contact patches, increasing the grasp wrench Appl. Sci. 2019, 9, x FOR PEER REVIEW 19 of 21 space [37]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 19 of 21 (a) (b) (a) (b) Figure 23. Instable parallel grasping of the two-finger underactuated gripper. (a) Demonstration of Figure 23. Instable parallel grasping of the two-finger underactuated gripper. (a) Demonstration of Figure 23. Instable parallel grasping of the two-ﬁnger underactuated gripper. (a) Demonstration of the the instable grasping due to unsuitable contact points. (b) Experimental verification. the instable grasping due to unsuitable contact points. (b) Experimental verification. instable grasping due to unsuitable contact points. (b) Experimental veriﬁcation. Figure 24. Instable parallel grasping of irregular objects. Figure 24. Instable parallel grasping of irregular objects. Figure 24. Instable parallel grasping of irregular objects. 5. Conclusions 5. Conclusions 5. Conclusions TTo gr o grasp aspgeneral general object objects s in in di diff erent erent wo working rking envi envir ron onments, ments, a mult a multi-function i-function gragrasping sping systsystem em is To grasp general objects in different working environments, a multi-function grasping system is isdeveloped developed in in th this is st study udy. .A new A new tw two-ﬁnger o-finger under underactuated actuated gripper is propos gripper is proposed ed an and d int integrated egrated toto a a developed in this study. A new two-finger underactuated gripper is proposed and integrated to a suction-grasping system. The performance of this multi-function gripper is evaluated through both suction-grasping system. The performance of this multi-function gripper is evaluated through both suction-grasping system. The performance of this multi-function gripper is evaluated through both simulations and real-world experiments. We verify that this multi-function gripper can be used to simulations and real-world experiments. We verify that this multi-function gripper can be used to simulations and real-world experiments. We verify that this multi-function gripper can be used to perform many types of grasping tasks. perform many types of grasping tasks. perform many types of grasping tasks. Two-finger grasping with only two contact points might not be stable in some grasping Two-ﬁnger grasping with only two contact points might not be stable in some grasping scenarios. Two-finger grasping with only two contact points might not be stable in some grasping scenarios. To achieve a stable grasping, the grasping modes and contact points should be carefully To achieve a stable grasping, the grasping modes and contact points should be carefully selected by scenarios. To achieve a stable grasping, the grasping modes and contact points should be carefully selected by taking the characteristics of the object, such as the size and shape, the weight and selected by taking the characteristics of the object, such as the size and shape, the weight and location of the gravity center, and the friction knowledge of the surface, into consideration. location of the gravity center, and the friction knowledge of the surface, into consideration. For scooping and picking up thin objects lying on flat surfaces, our future work will focus on For scooping and picking up thin objects lying on flat surfaces, our future work will focus on design special fingernails and appropriate motion/force control algorithms to increase the success design special fingernails and appropriate motion/force control algorithms to increase the success rate of grasping. Currently, we are using current feedback from the motor to detect the contact and rate of grasping. Currently, we are using current feedback from the motor to detect the contact and Appl. Sci. 2019, 9, 5266 20 of 21 taking the characteristics of the object, such as the size and shape, the weight and location of the gravity center, and the friction knowledge of the surface, into consideration. For scooping and picking up thin objects lying on ﬂat surfaces, our future work will focus on design special ﬁngernails and appropriate motion/force control algorithms to increase the success rate of grasping. Currently, we are using current feedback from the motor to detect the contact and grasping state. In the future, tactile sensors will be added to the gripper to detect the contact and grasping forces. Moreover, we will use the new three-DOF robotic ﬁnger to develop a multi-ﬁnger robotic hand. Author Contributions: Conceptualization: L.K., J.-T.S., and B.-J.Y.; visualization: L.K. and J.-T.S.; software: L.K.; writing—original draft preparation: L.K.; writing—review and editing: L.K.; supervision: B.-J.Y.; experimentation: L.K., S.-H.K., J.-T.S., and W.-J.K. Funding: This research was funded by the Technology Innovation Program (or Industrial Strategic Technology Development Program) (Grant Number 20001856, Development of Robotic Work Control Technology Capable of Grasping and Manipulating Various Objects in Everyday Life Environment Based on Multimodal Recognition and Using Tools) funded by the Ministry of Trade, Industry and Energy (MOTIE, Sejong City, Korea), and performed by the ICT-based Medical Robotic Systems Team of Hanyang University, Department of Electronic Systems Engineering was supported by the BK21 Plus Program funded by the National Research Foundation of Korea (NRF). The APC was funded by Grant Number 20001856. 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On Alternative Uses of Structural Compliance for the Development of Adaptive Robot Grippers and Hands. Front. Neurorobot. 2019, 13. [CrossRef] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Design and Implementation of a Multi-Function Gripper for Grasping General Objects

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Publisher: Multidisciplinary Digital Publishing Institute
ISSN: 2076-3417
DOI: 10.3390/app9245266
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Design and Implementation of a Multi-Function Gripper for Grasping General Objects

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Design and Implementation of a Multi-Function Gripper for Grasping General Objects

Design and Implementation of a Multi-Function Gripper for Grasping General Objects

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