2.1Foundingprinciples CONCEPTDESIGN CHAPTER 2

CHAPTER

2

CONCEPT DESIGN

This chapter will describe the concept design of the Soft Claw Gripper (SGC), a novel robotic end-eector entirely made in elastomer material, designed to handle delicate objects. As stated in the previous chapter, the ambitious idea of intro-ducing Soft Robotics in surgery stems from the belief that a smart design based on the intrinsically compliant properties of the material, could allow to realize a new kind of instrument equipped with a kind of low-level intelligence, sucient to ensure a safe and stable interaction with the environment, without the need of a real force-feedback.

2.1 Founding principles

The designing assessment of SCG has been carried out in collaboration with the Articial Intelligence Laboratory of the University of Zurich, birthplace of Embodied Intelligence concept. Embodied Intelligence establishes a new paradigm of connection between morphology and behavior. New design principles for sensing, actuation and locomotion components and for robot architectures must deeper understand the role of form and material properties in shaping behavior, which is not strictly programmed but robustly emerges from the interaction of the various components, or with the environment [Pfeifer and Bongard, 2007].

2.1.1 Compliance

The compliance property enables to orient the surfaces of hand to that of an object in response to contact forces, obtaining a shape matching. A good shape matching increases the contact surface between hand and object without the need

2.1. Founding principles Chapter 2. Concept Design for explicit sensing and control. It also increases the robustness to uncertainties in hand position, nger control, and the model of the environment. In addition, a hand which is passively compliant in all directions can resist contact with the envi-ronment without getting damaged. Thus the envienvi-ronment is used as a guide during grasping motion, furthermore increasing the robustness of grasping. Compliance therefore represents a major factor in improving grasp success under uncertainty, a key object in the design of robotic grippers. Moreover, the passive compliance makes the instrument safe enough to be used nearby soft and delicate structures like vessels and body organs. For the SCG fabrication, silicone material has been chosen, to exploits its intrinsic compliance property to safely achieve shape match-ing.

2.1.2 Under-actuation

Matching shape of an object without a complex control is possible implement-ing an under-actuated mechanism, i.e. a mechanical concept with less control inputs(active joints) than DOFs. This mechanism has many advantages such as lightness, low cost and low energy consumption, and allows an adaptive closure on the surface of an object. In particular, under-actuated robotic hands are the intermediate solution between robotic hands for manipulation (which have the advantages of being versatile, guarantee a stable grasp, but they are expensive, complex to control and with many actuators) and robotic grippers (the benets of which are simplied control, few actuators, but they have the drawbacks of being task specic, and often perform an unstable grasp). In an under-actuated mechanism, actuators are replaced by passive elastic elements (e.g. springs) or limit switches. These elements are small, lightweight and allow a reduction in the number of actuators. They may be considered as passive elements that increase the adaptability to the shape of the grasped object, but cannot and should not be handled by the control system. An under-actuated mechanism allows the grasping of objects in a more natural and more similar way to the movement obtained by the human hand. The geometric conguration of the nger is automatically de-termined by external constraints related with the shape of the object [Rea, 2011]. The joints' under-actuation of a multi-rigid link nger which grasps an object, al-lows the links' adaptation to the shape of the object. When the base link reaches the surface, it stops, while the other links pursue the movement until they meet also the surface of the object (Figure 2.1).

Grasping a variety of objects from at to wavy surfaces and irregularly shaped ob-jects essentially requires adaptability for safe and reliable gripping performance.

2.1. Founding principles Chapter 2. Concept Design

Figure 2.1: Sequences for grasping a regularly and irregularly shaped object: a) starting phase; b) rst phalange is in its nal conguration; c) second phalange is in its nal conguration; d) third phalange is in its nal conguration. From [Rea, 2011].

For achieving high adaptive robotic hands, many researchers have employed var-ious under-actuated mechanisms such as dierential and compliant mechanisms. Making a sucient number of contacts is the primary purpose of under-actuated mechanisms. The mechanical intelligence embedded into the design of the hand allows the automatic shape adaptation of the nger.

There are two dierent types of passive adaptation by under-actuation when a robotic hand grasps objects: the rst is under-actuation of one nger, and the second is under-actuation over more than one nger. In the rst case, the nger can adapt to a two-dimensional (2-D) curved surface such as a cylinder, whereas multi-nger under-actuation can adapt to a three-dimensional (3-D) curved sur-face such as a sphere [In et al., 2011]. Some robotic hands use only one-nger under-actuation. For multi-nger under-actuation, a robotic hand generally uses a dierential mechanism[Carrozza et al., 2004]. An innovative strategy to make one-nger under-actuation has been conceived for the SCG nipper design, to extends shape matching also to 3-D curved surfaces.

2.2. Size Constraints Chapter 2. Concept Design

2.1.3 Scalability

Several areas where robotics was introduced would nd benet from the intro-duction of tools made with the principle of soft robotics, especially those in which it is necessary to safely handle delicate objects and/or to work nearby human op-erators. Food and Space industry and Surgery are only a few examples [Pfeifer et al., 2013] as well as robotic service. The SCG design has been conceived in such a way that its dimension can be easily scaled, to nd application in all those elds where a safe interaction with fragile items, or human co-workers is needed.

2.2 Size Constraints

(a)

(b)

Figure 2.2: Soft Claw Gripper in closed conguration: (a) isometric view and (b)top view.

2.3. Actuation mechanism Chapter 2. Concept Design As a case of study, this work is devoted to analyze the possible application in the medical eld, as a tool for manipulation of soft tissues in MIS. MIS operations are performed placing a trocar through the abdomen. The trocar functions as a portal for the subsequent placement of other instruments. Hence, to meet the constraint imposed by the trocar inner diameter, the total diameter of the system and the length of the nipper should not exceed respectively 18 mm and 20 mm. It is possible to insert the system in closed conguration, with an hindrance of 17 mm (Figure 2.2). The total length of each nger is 20 mm. Each nger has a cross section of 6 × 2 mm2 _{at the top and 6 × 6 mm}2 _{at the bottom. The ngers}

are mounted with a 39.3◦ _{angle respect to the vertical.}

2.3 Actuation mechanism

The two main types of continuous manipulator present in literature are tendon-[Hannan and Walker, 2001; Immega and Antonelli, 1995; Laschi et al., 2012] or pneumatically-driven [Tsukagoshi et al., 2001; Ilievski et al., 2011; Deimel and Brock, 2013]. In the OCTOPUS Project [Cianchetti et al., 2011], soft robotics technologies and the embodied intelligence principle have been combined to de-velop a robotic tentacle equipped with the dexterity of the biological homologous. In the octopus arm, active bending requires selectively contracting the longitudi-nal muscle bers along one side of the arm, creating an asymmetrical longitudilongitudi-nal compressional force that shortens one side of the arm and thus causes bending [Kier and Stella, 2007]. In the OCTOPUS arm robot the arrangement of sheathed wires along the tentacle's length is assimilable to a muscle longitudinal ber and the fastening of a cable produces the selective contraction of the side subjected to the action of the same cable. In the SGC the same bio-inspired strategy has been adopted, but a dierent tendon route inside the nger has been thought to extend nipper shape matching also to 3-D curved surfaces (Figure 2.4).

2.4 Design description

The SCG is composed by three nger-like nippers in elastomeric material, since they were demonstrated to be sucient in grasping task [Laliberte et al., 2002]. The frame of SCG consists of an hexagonal 3D-printed lodging platform that contains a palm, also made of silicone, and the ngers, positioned sloped with respect to the palm plane, so that the o-position is the open one, and the pulling of the cables brings to the close position. Releasing the cables, the natural elasticity

2.4. Design description Chapter 2. Concept Design

Figure 2.3: Nipper actuator arrangement: tendon route(red) inside the nger. of the material brings back the ngers to the open position. The frame presents three holders tighter to the base and wider to the top to prevent the slipping of silicone during an anti-gravitational grasping. This assembly is robust, easy to manufacture and quick to adapt during rapid prototyping. Initial studies have focused on the choice of the optimal prole for the nger, and on the arrangement of cables inside it. Both indeed decreed what will be the behavior of the physical body during its actuation in the interaction with the environment. To meet a prole that forces the structure to bend itself in only one direction and to increase the stability during the grasping, a truncated pyramid prole was chosen (Figure 2.5). Each nger of the SGC has innite DOFs actuated by one cable and shows an under-actuated behavior obtained combining the passive deformation property of the silicone and an innovative strategy of embedding sheathed cable inside. One nylon cable, previously inserted in a silicone sheath, is placed inside the nger with the central part xed by two points at the tip and the rest running in two parts longitudinally down to its base, and coming out of the nger body. The eccentric plane containing the cable is parallel to the nger ventral surface and just below it (Figure 2.4). Thanks to the silicone sheath, the wire can slide with low friction inside the nger body, obtaining a mechanism intrinsically under-actuated, due to the naturally adaptability of the soft material. The under-actuated mechanism confers robustness to the gripper in term of functionality and allows the nippers to adapt to the shape of the object in contact, 3-D surfaced included. Indeed, if a complex-shaped object is grasped, sliding along the sheath path the nipper can passively adapt its surface to that of the object, while the cables tips are xed to the actuation source(for example a servomotor). Then, two behaviors can be obtained in one time, a curling motion on the sagittal mid-plane due to the pulling of the cable and a 3-D shape matching motion due to the relative slide between nipper body and cable.

2.4. Design description Chapter 2. Concept Design A bio-compatible silicone rubber has been chosen as the material for the

nger-Figure 2.4: Sections containing the cable(red) obtained from longitudinal cutting planes.

like nippers. Basic capability implemented is grasping, accomplished by the full winding of the ngers around the object grasped, exploiting the unlimited DOFs of the material during the bending mechanism. Rubber is subject to wear and fatigue, and "remembers" episode of excessive stretch (Mullins eect [Mullins, 1969], for an analysis of mechanical fatigue in silicones, see [Meier et al., 2005]). The cable breaking load specied by the manufacturer is 3.5 Kg, equivalent to 34.335 N. To keep the device usable for a long time, the maximal stretch has to be kept as low as possible. in order to avoid that the wire.

2.4. Design description Chapter 2. Concept Design

(a) Front view. (b) Top view.

(c) Isometric view.

Figure 2.5: CAD drawings of the Soft Claw Gripper in open conguration: (a) front view, (b)top view and (c) isometric view