# A Soft Robotic Gripper With an Active Palm and Reconfigurable Fingers for Fully Dexterous In-Hand Manipulation

Amir Pagoli, Frédéric Chapelle, Juan Antonio Corrales Ramon, Youcef Mezouar, Yuri Lapusta

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Amir Pagoli, Frédéric Chapelle, Juan Antonio Corrales Ramon, Youcef Mezouar, Yuri Lapusta. A Soft Robotic Gripper With an Active Palm and Reconfigurable Fingers for Fully Dexterous In-Hand Manipulation. IEEE Robotics and Automation Letters, 2021, 6 (4), pp.7706. 10.1109/LRA.2021.3098803. hal-03583792

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Submitted on 16 Dec 2025

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# A Soft Robotic Gripper with an Active Palm and Reconfigurable Fingers for Fully Dexterous In-Hand Manipulation *

A. Pagoli<fnref n="1" />, F. Chapelle<fnref n="1" />, J. Corrales<fnref n="2" />, Y. Mezouar<fnref n="1" />, Y. Lapusta<fnref n="1" />

**Abstract**— This study is focused on developing a new dexterous soft robotic gripper with three fingers and an active palm capable of performing in-hand manipulation purposes. This innovative design meets all the dexterous manipulation requirements without any increase in mechanical complexity. In each finger, the bending position can be modified and controlled by moving a stiff rod inserted inside the center hole of the finger. In this way, the effective length of the manipulation can be changed. As a result, these reconfigurable fingers provide a more accessible workspace than conventional soft grippers. Besides, a large diversity of the finger's shape configurations results in more dexterity and in-hand manipulation capability. Workspace analysis is accomplished to characterize the advantages of the proposed design. The effectiveness of this soft robotic gripper is validated by different in-hand manipulation experimental tests, including rotation, regrasping, and rolling. The results suggest a promising solution to bridge the design gap between hard and soft robots for dexterous manipulation tasks. The hybrid design carries advantages of these two classes, such as reconfigurability, position, and shape control from hard robots, with large degrees of freedom (DOFs), complex deformations, and lightweight from soft robots. Like human manipulation, the palm plays a major role in stable grasping, especially for enhancing the in-hand manipulation capability. Therefore, we also investigate two types of vacuum palms (suction cup and granular particles) to guarantee a wide range of object manipulation tasks that cannot be completely performed by previously suggested soft grippers.

**Index Terms**—Soft robotics, in-hand manipulation, dexterous gripper, shape adaptation, active palm.

## I. INTRODUCTION

The soft fluidic actuator (SFA) is one of the most widely used actuation mechanisms in soft robotics. SFA has several advantages such as simple assembly, cost-effective materials, large deformation, and high generated force [1].

Manuscript received: February 7, 2021; Revised: May 31, 2021; Accepted: June 29, 2021.

This paper was recommended for publication by Editor Kyu-Jin Cho upon evaluation of the Associate Editor and Reviewers' comments. This research has received funding from the French government research program "Investissements d'Avenir" through the IDEX-ISITE initiative 16-IDEX-0001 (CAP20-25), the European Union's Horizon 2020 research and innovation program under grant agreement n° 869855 (Project SoftManBot) and the ANR agency (MANIMAT ANR-20-CE33-0005 project).

Digital Object Identifier (DOI): see top of this page.

The positive or negative pressure exerted inside the chamber can cause the soft actuator to bend, extend, twist, or contract (depending on the type of surface on which the pressure is applied) [2]. These unique characteristics make them promising candidates for various applications, such as gripping [3], mobility [4], robotic manipulation [5], as well as medical applications [6], and rehabilitation or assistive robotics [7]. Recent advances in soft grippers have been reviewed by Shintake et al. [3]. They found that SFA grippers can grasp a wide variety of objects of different sizes and shapes. They can mimic the behavior of human hand manipulation with more compatibility. They can securely handle objects with different shapes and dimensions without needing exact knowledge of their properties. Human hands can either move or translate an object using the fingers and the palm. In-hand manipulation is defined as the ability to hold and reposition an object within one hand and translate or rotate it between the fingers and the palm. It is referred to as one of the fundamental tasks for investigating the dexterity of robotic hands. This kind of manipulation has several advantages over full-arm manipulation, including its better performance in the presence of obstacles or joint singularities. Different manipulation approaches have been proposed in recent years. Ma et al. [8] categorized various within-hand manipulation strategies of hard robots into six groups: grasping [9], finger gaiting [10], in-grasp manipulation [11], finger pivoting/tracking [12], rolling [13], and sliding [14]. Shadow Hand is considered as one of the most dexterous robotic hands, including five fingers with a total of 24 DOFs [15]. Andrychowicz et al. [16] developed a reinforcement learning (RL) framework for the Shadow Hand to perform in-hand manipulation strategies such as pivoting and finger gaiting. Due to complex contact interaction between the hand and the object, these kinds of rigid dexterous hands require accurate planning and control strategies (e.g., machine learning) based on models of the object and fingers. The design and manufacturing of soft robots are more straightforward and more cost-effective than a dexterous hard robot that needs a lot of joints and motors for achieving dexterity. Consequently, soft robots can be an appropriate alternative for dexterous grippers. The use of soft robots as anthropomorphic robotic hands has emerged quickly in the last decades [17-18]. For instance, Batsuren et al. [19] proposed a soft robotics gripper with three fingers. Each finger includes three independent chambers, which can be actuated pneumatically to provide distinct types of motions. This gripper can rotate a lamp at around 35°. Shih et al. [20] integrated a similar design with a flexible sensor and identified an object's shape. This soft gripper could rotate an object at around 30°. Abondance et al. [21] suggested four soft robot grippers for finger gaiting and translation of objects. The anthropomorphic and dexterous soft hand

<footnote n="1">¹Université Clermont Auvergne, CNRS, Clermont Auvergne INP, Institut Pascal, F-63000 Clermont-Ferrand, France. Emails: {amir. pagoli, frederic.chapelle, youcef.mezouar, yuri.lapusta} @sigma-clermont.fr.</footnote>

<footnote n="2">²CiTIUS (Centro Singular de Investigación en Tecnoloxías Intelixentes), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain (email: juantonio.corrales@usc.es).</footnote>
<figure><img src="image_2.png" /><figcaption>Figure 1. Schematic view of the proposed gripper.</figcaption></figure>

proposed by Zhou et al. [22] consists of three fingers and a thumb with 13 DOFs, which enables the hand to carry out different types of in-hand manipulation tasks such as sliding, rolling, and translation. They revealed that in-hand manipulation capability is empowered by increasing the common workspace area of the fingers. The purpose of this paper is to develop a new type of soft gripper with three reconfigurable fingers and an active palm. Like human manipulation, the palm plays a substantial role in grasping, especially enhancing in-hand manipulation capabilities. For instance, this active palm can not only improve the stability of an object's grasping but also provides different in-hand manipulation tasks such as fingers and palm's regrasping, full rotation, and in-grasp manipulating that cannot be completely performed by the previously suggested soft grippers [20-21]. Each finger includes three chambers and a movable stiff rod as a backbone in the center. The bending point and the effective length of the finger can simultaneously be changed by moving the stiff rod inserted inside the finger. This reconfigurable design results in enhancing the shape control of the finger, while the shape adaptability of the finger to the object is achieved by more possible grasping configurations. Furthermore, moving the bending points along with the fingers increases their workspace with a larger and more complex intersection area. The rest of the paper is organized as follows. The next section presents the conceptual and structural design for the proposed gripper. Section III discusses the workspace area of the proposed reconfigurable soft fingers. The manufacturing procedure is introduced in section IV. In section V, experimental in-hand manipulations are performed for five different tasks. Finally, a conclusion and future work are reported.

## II. OPERATING PRINCIPLES AND DESIGN

Fig. 1 shows the model of the proposed soft gripper. The three symmetric fingers are radially located around a central axis passing through the palm of the gripper. Each finger is made of silicone elastomer and composed of three pneumatic chambers, which can be inflated independently (plus one central hole for reconfigurability). A wide range of

workspace is achieved by activating combinations of two or three chambers. We previously demonstrated that the bending angle depends on several elements: the cross-section shape of the chamber, the finger diameter, as well as the applied pressure, and the distance between chambers and fingers [23]. Then, a sensitivity analysis was utilized to investigate the positive or negative effects of these parameters. Consequently, an optimization algorithm was deployed to find the best geometry for a maximum bending angle. In the present study, these optimized parameters are considered for designing the dimensions of the fingers and chambers. A semi-circular cross-section design is considered to reduce the ballooning effect of the chambers (in Elsayed et al. [24] and our previous work [23]). The other innovative part of our design is related to the addition of the stiff rod, which plays a principal role in grasping and in-hand manipulation tasks. As shown in Fig. 1, the location of the bending point is modified by sliding the stiff rod. This process is carried out through the hollow center of the finger. This mechanism not only helps to control the shape of the soft finger effectively but also provides stable grasping of a wide range of objects with irregular shapes. The comparison of grasping between a conventional soft gripper and the proposed gripper is shown in Fig. 2a. The curvature control of a conventional soft finger with one section is difficult. Therefore, the gripper made of this finger cannot adapt itself to the shape of the object. Consequently, proper contact and stable grasping remain incomplete. On the contrary, the movable joint reshapes the soft finger's bending curvature according to the object's contour, and thus more contact points are provided between the gripper and object. This adaptability guarantees a stable and reliable grasping. Besides, this design creates various possible configurations to access each point. It increases finger dexterity to encounter objects under unrecognized conditions or uncertainties. Human's hands can grasp an object by using the fingers and translate it from finger to palm or vice versa. The palm significantly contributes to

<figure><img src="image_3.png" /><figcaption>Figure 2. a) Performance comparison of the proposed finger and a conventional SFA, b) In-hand manipulation ability of the proposed soft gripper with active palm.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3. a) Kinematic modeling of the soft finger with a movable rod, b, c, d): 2D Workspace analysis and common reachable area of two fingers of 10 cm length: b) Conventional SFA compared with c) Proposed movable joint design, d) Common workspace area of the proposed soft gripper in 3D space.</figcaption></figure>

supporting objects by providing an adequate and stable grasp. An active palm is attached to the center of the fingers for improving the capability of the proposed gripper. The distance between the palm and finger tipping point can be set based on the size of objects. Fig. 2b shows how the inclusion of a palm in our design improves in-grasp manipulation. An active palm enables us to complete the in-hand manipulation task without taking advantage of the ground or gravity to fix or support the object when the palm is above the object, which is more like human in-hand manipulation. Besides the palm provide a reliable grasping approach of the wider range of object's weights. At the same time, applying air pressure to the lateral chamber in each finger leads to the rotation of an object around the principal axis of the gripper, while the palm is not active and acts as a fixture to keep the position of an object. After first-round rotation, the suction palm is activated to hold the object position, and then fingers will be inactivated and repositioned. Thereafter, it is actuated again to make a second rotation after inactivating the palm. This design provides stable and full rotation of the object (i.e., 360 degrees), using the necessary number of repetitions, with only three fingers and a palm. Other types of in-hand manipulation like rolling would also be possible by using the palm of the gripper. Three stepper motors with through-type lead screws are responsible for changing the bending position in each finger. This mechanism reduces the number of connection parts between the motor and finger. Also, it increases the position accuracy for the bending point with fast reactivity. Small solenoid valves control two types of airstreams: the positive one in the chambers (to deform the finger) and the negative one in the suction cup. Furthermore, the position of the palm is changed by another stepper motor connected to the suction cup in the center of the gripper's base. This mechanism changes the position of the palm based on the size of the object and the effective length of the finger. This functionality is especially useful during manipulations.

This issue is discussed in the in-hand manipulation and dexterity validation section. The proposed gripper has a lower design, modeling, and fabrication complexities than the other dexterous rigid hands like Shadow Hand (24 joints) [15], Pisa/II (19 joints) [25], DLR Hand II (20 joints) [26], or even some dexterous soft hand like BCL-13 (13 chambers) [22] and BCL-26 (26 chambers) [17]. Moving the rod inside the finger changes the bending point and the effective length of the finger results in different finger shape configurations with more dexterity. While in the previously cited approaches, the dexterity and in-hand manipulation capability are improved by increasing the number of joints and links.

### III. WORKSPACE ANALYSIS OF THE PROPOSED GRIPPER

This section is focused on the workspace analysis and evaluating the mechanical design, manipulation range, and dexterity of the fingers. The finger kinematic model is configured as a movable joint and a soft link. By moving the stiff rod inside the finger, the position and the length of each part will consecutively be changed. The bending point divides the link into two separate parts: the rigid link (first part) and the soft link (second part). By applying pressure to the inside of a chamber which is located head-on the object, the finger starts to bend in the XY plane from the bending point. Actuating the other side chamber rotates the tipping point of the finger in a 3D space. The transformation matrix of the tipping point for each finger can be calculated by these steps (Fig. 3a):

1) Rotation D point to D' by $\phi$ angle about $x_2$ axis

2) Rotation by $-\theta$ angle about $z_2$ axis

3) Translation by R along $x_1$ axis

4) Translation by H along $y_0$ axis

The final transformation matrix can be defined as in (1)
$$A = (R_{\phi}^{X_1})(R_{-\theta}^{Z_1})(H_{R}^{X_1})(H_{H}^{Y_0}) \quad (1)$$

The final transformation matrixes for each finger after simplification can be written as (2)

$$A = \begin{bmatrix} \cos(\theta) & \sin(\theta) & 0 & R\cos(\theta) + H\cos(\theta) \\ -\cos(\phi)\sin(\theta) & \cos(\phi)\cos(\theta) & -\sin(\phi) & \cos(\phi)*(H*\cos(\theta) - R*\sin(\theta)) \\ -\sin(\phi)\sin(\theta) & \cos(\theta)\sin(\phi) & \cos(\phi) & \sin(\phi)*(H*\cos(\theta) - R*\sin(\theta)) \\ 0 & 0 & 0 & 1 \end{bmatrix} \quad (2)$$

Monte Carlo numerical algorithm is employed to calculate the resulting workspace for every point touched by a fingertip. The outcomes are depicted in Fig. 3b, c, d. It is assumed that the length of the finger is 10 cm, and the bending point (the center of the joint) can move along with the link axis from 2 to 8 cm. The first link is fixed and the second one can be bent up to 90 degrees in the XY plane. By actuating the side chambers, the finger can rotate from -120 to 120 degrees. It is presumed that the projected distance between two fingers in the XY plane is 10 cm. In Fig. 3b, a comparison is made between the conventional soft finger with one section and the proposed finger for evaluating the accessible area in a 2D space. It shows that if the tipping point workspace of the previous traditional soft pneumatic actuator with one section is assumed to be a constant arc [27], the capability of changing the bending point enhances not only the reaching areas of the tipping point but also increases the common workspace between the fingers. The results of the references [22] and [28] revealed that the accessible area of each finger and the common workspace area are taken into account as two critical metric factors for

measuring dexterity and in-hand manipulation capability. The larger reachable area with more shared intersection workspace results in more in-hand manipulation capability. The intersection area for two fingers is highlighted in Fig. 3c by red color, and it is around 6.125 cm². For three fingers in a 3D space (Fig. 3d), the total intersection volume is about 11.22 cm³.

## IV. MANUFACTURING AND FABRICATION PROCESS

One of the promising advantages of a soft robot in comparison with a hard robot is its low manufacturing cost. Soft pneumatic actuators are usually made of silicone rubber to support large elongation. Besides, silicone is not expensive, and its manufacturing process is fast. In this study, EcoFlex was chosen with shore hardness 00-30. It is selected due to its low viscosity, which provides the ability to bend the finger up to 90 degrees by using small pumps and applying a pressure of less than 0.1 bar. The recent advances in 3D printing technology significantly facilitate the fabrication process for soft components by producing more complex parts with higher precision. In this study, the printer's resolution is set at around 20 microns, and thus the printing process of all the molds lasts for about 14 hours. For each silicone, two liquid parts must be mixed with the same ratio followed by 2-3 minutes of vacuum degassing to eliminate any entrapped air bubbles. It is performed by placing the molds in a vacuum chamber. The standard cure time of Eco-Flex 00-30 is around four hours at ambient temperature. This time can be reduced to less than an hour by putting it in an oven at 100 °C.

<figure><img src="image_5.png" /><figcaption>Figure 4. a) Electro-pneumatic control system of one soft finger, b) and c) Time-lapse of the soft finger with a movable joint workspace test in b) XY plane and c) YZ plane.</figcaption></figure>
V. IN-HAND MANIPULATION AND DEXTERITY VALIDATION

To validate the performance of the proposed movable joint, a workspace test was performed using camera recording positions. The pressure of each chamber is supplied by one micropump, and it is controlled by one solenoid valve and a piezoresistive pressure sensor. The microcontroller (Arduino Mega 2560) is used to control two different elements: (I) the switching of the micropumps and solenoid valves and (II) the handling of the electric motors following the feedback signals of the pressure sensors (Fig. 4a). The test bench has been developed for one finger to characterize the performance of the movable joint. Fig. 4b illustrates the time-lapse for different configurations of the finger in three positions of the bending points. It shows that a wide range of areas can be reached by a continuous movement of the bending point along with the finger. While the main chamber is responsible for bending in the XY plane, activating the side chambers leads to the rotation of the tipping point of the finger from -120 to 120 degrees in the YZ plane (Fig. 4c). The dimensions and ranges of variation for device parameters are tabulated in Table I. The time-lapse of the one finger in the XY plane is conducted to show different possible shape configurations with a larger reachable area by changing the bending point.

<table><caption>TABLE I. DEVICE PARAMETERS: DIMENSION AND RANGES</caption><tr><td>Finger length (mm):</td><td>L<sub>1</sub>=L<sub>2</sub>=L<sub>3</sub>=130</td></tr><tr><td>Finger diameter (mm):</td><td>30</td></tr><tr><td>Rod diameter (mm):</td><td>8</td></tr><tr><td>Chamber length, diameter (mm):</td><td>125, 9.4</td></tr><tr><td>Palm translation range (mm):</td><td>40 &lt; X<sub>p</sub> &lt; 250</td></tr><tr><td>Distance between fingers and palm (mm):</td><td>30 &lt; H &lt; 95</td></tr><tr><td>Bending point range (mm):</td><td>40 &lt; R &lt; 125</td></tr></table>

<figure><img src="image_6.png" /><figcaption>Figure 5. Schematic view of the proposed set-up assembly: a) The three soft fingers with variable lengths, b) The stiffened rod assemblies with their through-type stepper motors, c) Two easily interchangeable palms (suction cup and granular one).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 6. Different types of grasping with the proposed gripper: a) Large cube by the suction cup, b) Suction cup and soft fingers for stable grasping, c) Granular palm for a non-flat object like a pencil, d) Shape adaptability of the fingers with the object (see Fig. 2a).</figcaption></figure>

As mentioned previously, the in-hand manipulation technique is generally categorized into three main classes: translation, rotation of the object, and relocation of the fingers. In this section, different tasks are defined based on these three categories to demonstrate the in-hand manipulation capability for the proposed gripper. Fig. 5 shows the prototype setup for the proposed gripper to conduct the experimental tests. Three stepper motors are connected directly to the fingers for controlling the bending position (Fig. 5a), while the fourth motor is located in the center of the gripper, and it is responsible for moving the palm (Fig. 5b). In this study, two palm designs (i.e., suction cup and granular matter) are suggested to maintain the grasping of a wide variety of objects' shapes. These two designs are similar and work with the same actuation mechanism and control units. As shown in Fig. 5c, they can easily be inverted. The connection part is utilized to convert these palms very easily and quickly.

**Task 1- Stable grasping:** Like the human hand, the palm significantly contributes to supporting objects manipulated by hand. In the proposed gripper, an active palm has been designed to improve manipulation performance. The position of the palm can be changed along with the principal axis of the palm to ensure that there is proper contact between the fingers and an object in terms of the size and shape of objects. Two different types of palms are considered: (I) simple suction cup and (II) membrane palm with granular particles (Fig. 5c). A suction cup is an appropriate choice for a flat and deformable object (Fig. 6a, b), whereas the granular palm is useful for an object with an edge or non-smooth surface like a pencil (Fig. 6c). The other aspect of this task is grasping and holding the object by the palm and fingers with adjusted bending points for reliable grasping strategies (Fig. 6d).
<figure><img src="image_8.png" /><figcaption>Figure 7. In-hand manipulation task 2: full rotation of Rubik's Cube around its axis including rotation.</figcaption></figure>

**Task 2- Full rotation of the object by regrasping between the palm and the fingers:** This task is principally concerned with the issue that the object should be able to translate from palm-to-finger and finger-to-palm and change the contact points. This capability is validated using the 3×3×3 Rubik's Cube with a dimension of 5.6 cm to make a full rotation around its principal axis. In the proposed gripper, the active palm helps to make a full rotation of objects. In this regard, the first step is dealt with activating the palm to hold the object. Then, the main chamber of each finger is activated, and the fingers are bent to contact the cube and subsequently, hold it. Afterward, the palm is inactivated, and the fingers hold the cube. The palm acts as a fixture to guarantee the proper rotation of the cube (Table II). As illustrated in Fig. 7, the right or left side chambers in each finger are actuated based on the direction of rotation. The fingers are released, and the palm is activated at the same time. Thus, the current position of the cube is preserved. Thereafter, the finger bends again and grasps the cube at the

<figure><img src="image_9.png" /><figcaption>TABLE III. TASK 3: FINGER GAITING PROCEDURE</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 8. In-hand manipulation task 3: rotate one section of rubric cube to the desired angle by controlling the bending shape of the finger and active palm.</figcaption></figure>

new contact points. The cube is now ready for the second rotation. This process continues until the desired angle is reached.

**Task 3- Finger gaiting:** A cube similar to the one used in the previous task is employed to show the capability of the proposed gripper in terms of a full rotation of the last section of the cube around its axis. This process is a kind of finger gaiting task by replacing the grasping fingers with the active palm. As observed previously, the active palm enables not only the proposed gripper to grasp an object with irregular shapes or considerable weight but also to hold the object and simultaneously allows the fingers to change their contact points with the object. The palm is activated during the experimental test to hold the position (Table III). Then, the main chamber of each finger is actuated to grab the object. In the next step, the left side chambers of each finger are.

<figure><img src="image_11.png" /><figcaption>Figure 9. In-hand manipulation task 4: pouring colored water from a glass.</figcaption></figure>
simultaneously actuated to produce a clockwise rotation of the last section of the cube (Fig. 8). The ultimate angle for each rotation is around 45 degrees. The fingers are released and activated again for the second rotation. This process sticks out to reach the desired angle

**Task 4- In-grasp manipulation:** This task is involved in handlining the object through the fingers and applying small changes in its position and orientation while fingertip contact is preserved (i.e., no contact break while there are small rolling motions around the contact point). The palm and the fingers with the capability of changing the bending point enable the gripper to manipulate the objects with different shapes (Table IV). As illustrated in Fig. 9, the three fingers are actuated to grasp a glass with colored water. In this task, the palm is not active and only acts as a support to enable stable grasping. An increase in pressure inside the rear finger results in pouring some water from a cup.

<caption>TABLE IV. TASK 4: IN-GRASP MANIPULATION PROCEDURE</caption>

1- The bending point for three fingers is set to R= 90 mm

2-Two fingers are responsible for grasping the cup
($\theta_1 = \theta_2 = 55^{\circ}, \theta_3 = 30^{\circ}$)

3- Palm is inactive but acts as a fixture to keep the position of the cup (Xp=100 mm)

4- By pressurizing another finger, the water begins to pour from the cup $\theta_3 = 95^{\circ}$

**Task 5- Rolling:** The proposed movable bending and stiff rod design enable the gripper to perform different types of movements. Driving the stiff rod towards the end of two of three fingers increases the stiffness of fingers and can be used as a rigid surface for rolling movement. The other finger with the movable joint is responsible for rolling a pencil with 8 mm in diameter and 20 mm in height. As shown in Fig. 10, the marker circle paper is attached to the pencil to facilitate the rotation tracking process. The pencil can reach a maximum rotation angle of around 160 degrees. These experimental results realize the in-hand manipulation capability of the proposed three soft fingers with the movable bending point. The active palm improves the grasping ability and the handling of various shapes of objects. The Supplementary Video provides a full sequence of these tasks (Table V).

<caption>TABLE V. TASK 5: ROLLING PROCEDURE</caption>

1- Stiffness of two fingers is increased by moving the stiff rod inside each finger R₁= R₂=125 mm, R₃=80 mm

2- These two fingers act as a support and help the rolling

3- By actuating the third finger, the pencil starts to bend up to around 160°

VI. CONCLUSION

In this paper, an innovative soft gripper with three fingers and a palm has been described. Each finger consists of three inflatable chambers and a movable stiff rod, which controls the position of the bending, and subsequently the shape of the finger. The stiff rod can longitudinally slide along with the principal axis of the finger using an electric stepper motor. The effective variable length of the fingers

<figure><img src="image_12.png" /><figcaption>Figure 10. In-hand manipulation task 5: rolling the pencil.</figcaption></figure>

with the bending capability at different points increases the hand's dexterity considering a large diversity of configurations. The workspace analysis is performed to highlight the advantages of this reconfigurability. This approach extends the available workspace of the fingertips in comparison with the SFAs. Besides, this design increases the dexterous grasping capability of the soft gripper, especially for in-hand applications. Two types of easily interchangeable palms (suction cup and granular particles) are added to improve the in-hand manipulation tasks such as full rotation by regrasping a cube between the palm and the fingers, finger gaiting, and finally pouring some water from a cup. Five popular in-hand manipulation tasks [8] are designed to investigate the performance of the movable bending points and active palm. The experimental results reveal that the proposed soft gripper can successfully carry out these in-hand manipulation functionalities. The proposed design represents a promising solution for a dexterous anthropomorphic gripper, which can potentially interact with humans. In this study, an optimized semi-circular geometry has been employed for chambers to reduce the ballooning effect. This balloon is produced, especially when the bending angle is large (around 90 degrees). Adding fiber reinforcement [27], [29] or sleeve [30] could reduce the ballooning effect but at the price of increasing the required bending pressure and the local stiffness. Also, it requires more powerful equipment (bigger pumps and solenoid valves) with drivers to be connected to the control board. Ultimately, it makes the gripper design more complex and expensive. Furthermore, the balloon does not affect the in-hand manipulation performances or decrease the capability of the proposed grippers which is the main contribution of this paper.

In future work, the real-time control of this gripper will be addressed by using SOFA software and tactile sensors will be added to increase the grasping precision.
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