Abstract—This paper presents a novel robotic palm with a

dual-layered structure designed to yield high surface conformity

and controllable rigidity for enhanced grasping performance. It

comprises a vacuum chamber for adjusting the stiffness of the

palm via particle jamming and an air chamber for actively

controlling the palm deformation. An auto-jamming control

scheme that automatically solidifies the palm by sensing the

internal pressure of the palm without any tactile sensors or

visual feedback was also proposed. Given the design and control

of the robotic palm, the performance of the dual-layered

jamming mechanism was characterized in terms of shape

adaptability and stiffness controllability. The contact surface

areas increased by 180% compared with the single-layered

robotic palm, and the adjustable stiffness was within a range of

0.20–2.53 N/mm for varying vacuum pressures. Moreover, the

palm can act as a universal gripper for small objects, yielding a

holding force of up to 13.9 N. The grasping performance of the

palm in conjunction with robot fingers was evaluated for

various objects for varying palm stiffness. The palm increases

the grasping force by 2.0–3.1 times compared with flat skin.

Multimodal grasping strategies for various objects were

demonstrated by manipulating a robot arm.

I. INTRODUCTION

Human hands stably grasp and manipulate objects with high conformability and dexterity. They provide effective strategies for grasping objects by adjusting the configuration of the fingers and shape of the palm depending on the task [1]. A baseball bat, for example, is firmly held by partially flexed fingers and opposition pressure by the thumb. The palm also flexes to conform to the cylindrical shape of the bat during the swing [2]. Such a grip pattern is taxonomically called “power grasp.” It allows holding objects firmly with a multi-fingered hand by confining the objects with a large contact area between the finger segments and palm [3].

The palm comprises the area over the metacarpals between the five phalanges (finger bones) and the carpus (wrist joint). The palm consists of 17 of the 34 muscles that articulate the fingers and thumb. They connect to the hand skeleton through a series of tendons. The palmar surface is thus unique, with characteristics for special functions. The

* This work was supported by Korea Institute of Science and Technology

under Grant 2E31062. J. Lee is with the Center for Intelligent and Interactive Robotics, Korea

Institute of Science and Technology, Seoul, 02792 and also with the

Department of Electrical and Computer Engineering, Korea University, Seoul, Korea. (e-mail: gardenlee@kisr.re.kr)

J. Kim, S. Pack, D. Hwang, and S. Yang are with the Center for Intelligent

and Interactive Robotics, Korea Institute of Science and Technology, Seoul, 02792, Korea (corresponding author to provide phone: 82-2-958-5747; fax:

82-2-958-5304; e-mail: swyang@kist.re.kr)

palmar skin is thick and not as pliable as the dorsal skin. It is strongly anchored to the bone structure by a layer of connective fibrous tissue that links the skin with the skeleton. This enables the hand to grip without the skin sliding out of position. The palmar concavity contributes to various grasp patterns by providing a postural base to the fingers [4]. These benefits have led to the development of reconfigurable and/or stiffness-tunable robotic palms inspired by the human hand [5]–[8]. Previous studies suggested that deformable robotic palms with variable stiffness can improve grasping performance [9], [10]. These robotic palms deform to adapt to various shapes of objects under low stiffness and are solidified to provide a stable base for grip by increasing the stiffness. The palm stiffness can easily be controlled by particle jamming to provide a transition between a solid-like state and a fluid-like state of the granular material used [11].

Various other approaches have been introduced to adjust the stiffness of structures. Magnetorheological and electrorheological fluids can be utilized for stiffness control, where high voltage is required for the state transition [12], [13]. Shape memory alloy and low-melting-point alloy can substantially change their stiffness according to applied temperature [14], [15]. However, the particle jamming mechanism is most suitable for application to robotic hands including palms because it is robust to such external environments as undesired electromagnetic fields or ambient temperature during control. It can be controlled without strict kinematic modeling and complex sensors [16].

Soft Robotic Palm with Tunable Stiffness Using Dual-Layered

Particle Jamming Mechanism

Jeongwon Lee, Jaehee Kim. Sungwoo Park, Donghyun Hwang, Member, IEEE, and Sungwook Yang,

Member, IEEE

Surface Membrane

Intermembrane

Coffee Grounds

Upper Rim

Jamming

ChamberAir

Chamber Fig. 1. Robotic palm with a dual-layered particle jamming mechanism.

2021 IEEE/ASME International Conference onAdvanced Intelligent Mechatronics (AIM)

978-1-6654-4139-1/21/$31.00 ©2021 IEEE 1270

(a) (b) (c) (d) (e)

Fig. 2. Four states of the robotic palm according to the pressures of the vacuum and the air chambers: (a) neutral, (b) unjammed, (c) deformed, (d) jammed with

an object, and (e) jammed without the object.

The application of particle jamming to robotic hands has focused on the development of robotic grippers [11], [17]–[19]. A robotic gripper based on particle jamming (a “universal gripper”) can adapt and conform around the surface of complex target objects. Air is evacuated using vacuum suction [11]. Further development of the universal gripper incorporated an air compressor for the control of positive pressure, which allows the rapid release of an object and the return to a deformable state between gripping tasks [17]. However, these universal grippers are designed primarily to perform pick-and-place tasks [11]. Other types of soft gripper adopting the jamming principle focus on conformably gripping variously shaped objects using fingers [20], [21]. None of these grippers utilizes an inherent advantage of human palms: a stable base for gripping and manipulating objects. Thus, gripping an object with a specific configuration of fingers to perform the next task or interaction, such as tossing or manipulating an object, becomes difficult.

Recent studies showed the potential of robotic palms [9], [10], [22]–[24]. Harada et al. adopted a granular jamming mechanism as a shape-adaptive gripper for robotic assembly tasks [22]. Although a granular bag is attached to the palm of a robot hand with a multi-finger mechanism, it acts rather as a universal gripper introduced in [11]. Thus, there was no stiffness control strategy or evaluation of the palm performance. Other particle jamming palms were introduced for climbing robots [23], [24]. The particle jamming palm in [23] enables the spiny paws of a climbing robot to contact a rough surface. The hardened palm helps to share vertical load during climbing. A robot for climbing vertical shafts was also developed [24]. These works focused primarily on load sharing or the increase in friction during climbing rather improving grasping performance. Li et al. introduced a soft-actuated palm with a particle sac for jamming, of which the shape adaptability and variable stiffness help increase grasping force [9]. A stiffness-controlled robotic palm was recently developed for improving grasping performance. A single granular chamber provides a deformable surface and tunable stiffness. It is designed primarily for use on top of conventional robot hands as an extra feature [10]. However, the limited volume change of the chamber could limit its surface conformity because a thinner structure is preferable for embedding in an existing palm.

Therefore, we propose a new robotic palm incorporating a dual-layered jamming mechanism to attain high surface conformity with variable stiffness in a thin profile, as shown in Fig. 1. Auto-jamming control makes it possible to tune the palm stiffness automatically without tactile sensors and vision control. These features make the robotic palm versatile, so it can act as an advanced universal gripper and a stable base for

gripping with enlarged contact surface areas. Given the design and control of the robotic palm, the performance of the dual-layered jamming mechanism is characterized in terms of shape adaptability and stiffness controllability. The grasping force of the palm in conjunction with robot fingers is evaluated for various objects while varying the palm stiffness. Multimodal grasping strategies for different objects are demonstrated by manipulating a robot arm.

II. DESIGN AND CONTROL

The robotic palm consists of a dual-layered chamber and its controller to adjust the surface shape and stiffness of the palm by regulating inward and outward airflow of the chamber.

A. Design

The dual-layered robotic palm is designed to gain increased surface conformity with tunable stiffness. To achieve this, the robotic palm consists of a surface membrane, a jamming chamber at the top, an inflatable air chamber at the bottom, an intermembrane between the chambers, and coffee grounds as granular material (Fig. 1). The jamming chamber filled with coffee grounds passively deforms to adapt to the shape of an object under contact and then varies its stiffness by evacuating the air inside the chamber via vacuum suction. For example, the jamming chamber has a fluid-like state (easily deformable) at ambient or low negative pressure because of the relatively large distance between particles. However, it can be solid-like (jammed and rigid) when particles resist small stresses without deforming irreversibly at a high negative pressure.

The air chamber at the bottom provides extra space to accommodate the object. The inflation of the bottom chamber helps the top chamber encompass the object, resulting in increased surface conformity. The overall dimensions of the palm are designed to satisfy the workspace of the robot fingers used. The surface membrane is a convex shell to ensure a sufficient cavity for the jamming layer, whereas the intermembrane is a flat shell. The coffee grounds occupy 95% of the cavity volume between the membranes. The membranes were fabricated by a soft-silicone-rubber molding process (Ecoflex 00-20, Smooth-On, Inc., USA) using 3D-printed molds. The specifications of the robotic palm are summarized in Table I.

B. Control system

The control system of the robotic palm regulates the pressure of each chamber for jamming the upper chamber while supplying air pressure to the bottom chamber. The stiffness of the palm can thus be adjusted by evacuating the air

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inside the jamming chamber with a desired level of negative pressure. The bottom chamber can be inflated or maintained by applying the desired level of positive air pressure. Simultaneously controlling both chambers provides four grasping states: a neutral state without any pressure controls, an equilibrium (ready for being deformable) state with inflation of the upper chamber, a deformed state with passive compliance from contact, and a jammed state with vacuum suction (Fig. 2).

To control the air pressure of each chamber, the system has two electrically driven diaphragm pumps (DAP-370P, MotorBank, South Korea), two three-way solenoid valves (D070C-5CG-32, SMC Corp., Japan), and two electro-pneumatic regulators—one for the air chamber (ITV0010-2N, SMC Corp., Japan) and another for the vacuum chamber (ITV0090-3BL, SMC Corp., Japan), as Fig. 3 shows. All components are digitally controlled by a microcontroller (Teensy 4.0, PJRC, USA) interfaced with a host PC via RS-232 communication. The specific pressure of the vacuum chamber is set by a custom-built user interface program and delivered to the microcontroller. The digital-to-analog converter (MCP4725, Microchip, USA) of the controller converts the digital input to an electrical signal proportional to the set pressure of the vacuum regulator. The pressure is maintained by feedback control of the regulator. The current pressure of the chamber is monitored for further control using an analog-to-digital converter (ADC122S655, Texas Instrument, USA) for analog output from the regulator.

The solenoid valve opens to supply external air to the chamber to return back to ambient pressure jamming is complete. Given positive pressure as a target, the air chamber controls the air pressure regulator in the same manner. The vacuum pressure of the upper chamber can be lowered to −35 kPa below the atmospheric pressure, while the bottom-chamber air pressure can reach 100 kPa with this control system. An air pressure of 5.4 kPa is set for the bottom chamber to inflate the surface membrane. Its bulging shape is steadily maintained by reaching equilibrium with the pressure.

C. Auto-jamming control

An auto-jamming control scheme is proposed for the dual-layered robotic palm. Given the current pressure of the air chamber, it makes it possible to automatically set the upper chamber jammed by detecting the beginning of contact and the maximum volume change caused by deformation. Thus, various objects can be gripped without tactile sensors or visual feedback.

The air pressure of the bottom chamber instantly increases as the palm deforms from contacting an object. The change is more pronounced at the bottom chamber. No significant pressure change occurs in the upper chamber because a substantial volume change occurs only at the air-filled bottom chamber. The pressure data passes through a second-order Butterworth lowpass filter with a 20-Hz cutoff frequency in real time to mitigate high-frequency noise; the pressure data from the regulators used suffers from periodic noise of 100 Hz caused by repeatedly opening and closing the valve for feedback control. Fig. 4(a) shows the algorithm flow of the auto-jamming control. The air chamber pressure is set to 5.4 kPa to reach equilibrium, while the vacuum chamber is unjammed. Once the palm is ready for grasping, the palm approaches a target object. The air pressure increases at contact. For instance, the pressure increases sharply with high-speed contact by the deformation of the chamber and gradually with low-speed contact. When the air pressure is higher than a baseline threshold, the object is in contact with the palm. Then, the peak pressure is detected by running an online peak detection algorithm with a 20-ms search window. The auto-jamming is completed by jamming the upper chamber.

The auto-jamming control contributes to reducing the operation time because general jamming grippers take longer to visually check contact states than the jamming itself. For example, the completion time with auto-jamming control was measured to be 55±8 ms on average for 20 trials. It was 236±30 ms for manual jamming control.

III. EXPERIMENTS AND ANALYSIS

The performances of the robotic palm were characterized in terms of shape adaptability and stiffness controllability. Whether grasping performance improves with the robotic palm in conjunction with three robotic fingers was also investigated. The multimodal grasping capability was explored for various objects.

A. Characterization of robotic palm

1) Shape adaptability The shape adaptability of the robotic palm is a significant

feature that helps increase the object’s contact area in grasping, providing a stable base for a power grasp and a higher grasping force. The shape conformity of the dual-layered robotic palm was investigated for a cube with a 20-mm edge length and a hemisphere with a 40-mm diameter. A 3D scanner (Go!SCAN 50, Creaform, Canada) was used to obtain 3D models of the palm deformed to the shapes of test objects and to analyze the cross-sections of the 3D scans. The results of the proposed palm were compared with those of a

TABLE I SPECIFICATIONS OF ROBOTIC PALM

Quantity Value

overall dimension Φ 85 mm × 30 mm

weight 220 g

cavity of jamming chamber 119,489 mm3

volume of coffee grounds 113,515 mm3

size of grain 1.0 – 1.2 mm

thickness of membrane 1.5 mm

Host PC MCU

Air pump

Solenoid valve

Compress regulator

Air pump

Solenoid valve

Vacuum regulatorDAC

ADC

Data signal

Vacuum flow

Compress flow

Fig. 3. Flow diagram of the control system for the robotic palm

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(b)(a) Fig. 5. Three-dimensional scans of the robotic palm and resulting cross sections with testing objects: (a) sphere and (b) cube. The gray arrows are the

friction cones of the single and dual palms. The red arrows are additional

friction cones for the dual palm.

(a) Auto-jamming Start

Set each pressure to unjammed state

Wait for an object

Is the current pressure in

unjammed state?

The object is in contact with the surface membrane

Is the current pressure

up to threshold?

Is a peak pressure detected?

Set a desired level of pressure

Auto-jamming End

N

N

N

Y

Y

Y

ReadyStep

DeformedStep

JammedStep

(b)

Fig. 4. (a) Algorithm flow of auto-jamming control, (b) pressure curve during

auto-jamming control—the blue region for time is ready, the red is deformation by contact, and the yellow is activating jamming after detection

of the peak pressure (indicated as a red circle). The solid line is for the

pressure of the air chamber upon object contact. The dashed and dotted lines are for the pressure of the jamming chamber in auto and manual control,

respectively.

single-layered palm comprising only a jamming chamber. The jamming volume of the single-layered palm was the same as the entire volume of jamming and air space in the dual-layered palm to maintain an identical form factor.

The overall shape of the dual palm was significantly deformed to fit its shape to the objects, while less conformity was observed for the single chamber, as shown in Fig. 5. For the hemisphere, the difference between the dual-layered and single-layered palms becomes considerable when following the hemisphere’s boundary from the bottommost contact toward the top. The contact areas were calculated as 576.9 and 1375.2 mm2 for the dual-layered and single-layered palms, respectively, and the entire area of the hemisphere was 2512 mm2. The single-layered palm leads the separation of contact between the target object and palm. In contrast, the dual-layered palm conformably adapts to the shape of the hemisphere because the air chamber has extra space for the object and also lifts the jamming chamber up along the surface of the object to encompass the object.

These characteristics were most evident with the cube, which had sharp corners yielding extremely high curvatures at the edges—see Fig. 5(b). Passive adaptation in the single-layered palm resulted in the limited contact area at the bottom of the cube: 400.0 mm2. However, the cube was well-captured inside the dual-layered palm while providing a

719.2-mm2 contact area (the entire surface area was 1200 mm2). The interlocking of the object was possible because friction cones created on the object’s sides could resist external wrenches. Thus, the robotic palm can be used as a universal gripper for small objects.

2) Stiffness controllability The stiffness controllability of the robotic palm was

evaluated for varying negative pressures of the vacuum chamber while applying constant positive pressure to the air chamber.

The palm stiffness was obtained by measuring a force–displacement curve using a force/torque (F/T) sensor (Nano-17, ATI Industrial Automation Inc., USA) and a laser displacement sensor (LK-150, Keyence Corp., Japan). The force and corresponding displacement were simultaneously measured while advancing a hemisphere ball with a 30-mm diameter attached to the F/T sensor by a z-axis motorized stage.

Fig. 6(a) shows the averaged force–displacement curves with error bars for five repeated measurements. The curve slope rapidly increases as the vacuum state of the upper chamber increases (lower negative pressure). Each box plot in Fig. 6(b) shows the range of stiffness coefficients for the applied vacuum pressure, where a set of linear stiffness coefficients (local slopes) is extracted from each nonlinear curve in Fig. 6(a) over the displacement from 0 to 20 mm. Thus, the palm stiffness was adjustable within a range of 0.20–2.53 N/mm for varying vacuum pressure. The ascending force suddenly dropped in the experiment with the lowest vacuum pressure. This occurred when the restraint of granular materials slightly loosened.

B. Grasping performance

The effect of the robotic palm on the grasping performances was investigated in terms of grip strength, slip resistance, and versatile grasping. The slip resistance and versatile grasping performances were tested by assembling three robotic fingers along the palm’s circumference. The custom-built finger had four degrees of freedom (DOFs) in actuation (three for flexion/extension, one for abduction/adduction). The payload was 0.75 kg at the distal.

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(a)

(b)

Fig. 6. Evaluation of variable stiffness.: (a) force-displacement curve with

error bars for varying vacuum pressures when the testing object pushes against the palm and (b) resulting stiffness with error bars for the decrement

of the vacuum pressure.

Fig. 7. Evaluation of gripper strength for single-layered versus dual-layered

palms.

1) Gripper strength: holding force The grip strength of the palm as a universal gripper was

evaluated for hemispherical objects of various sizes. The holding force created by jamming the upper layer to grip the objects was measured using a push–pull gauge. All surface membranes were powered with coffee grains to minimize the effect of static friction. The tests were repeated for five trials. The results were compared with those of the single-layered jamming palm. The dual-layered palm had significantly higher holding forces than the single-layered palm for all object sizes (Fig. 7). The maximum holding force of the dual-layered palm was 13.9 N with a 40-mm-diameter hemisphere. With sufficient loading force, the object became more than half enveloped by the palm, which had a 22-mm depth for the coffee grounds and air to be filled. This led to the geometric interlocking of the object and increased holding strength, because interlocking was only allowed at a contact angle >180°. The contact angle is defined as the degree to which an object is enveloped by the jamming palm [11]. However, such geometric interlocking was not attainable for an object with a diameter greater than 44 mm because the object was not fully half enveloped and the contact angle was less than 180°. For object sizes >44 mm, the effect of only vacuum suction becomes dominant in holding strength. Thus, the holding force dropped as the object size exceeded 40 mm. The contact angle affecting vacuum suction decreased from 180° for the 40-mm to 149° for the 60-mm objects. For the single-layered palm, the holding force gradually rose with the increment of the object sizes. Therefore, the maximum force of the single-layered palm was 1.1 N with a 60-mm hemisphere.

2) Slip resistance: pull force The slip resistance of the robotic palm by inherent surface

friction properties was evaluated as a kinetic measure of a robotic hand's ability to resist slip. A test object was pulled by a fishing line connected to a single-axis motorized stage until any of the three fingers lost its grip—see Fig. 8(a). To mitigate

any undesirable effect from inconsistent loading, the fishing line was kept parallel to the ground by adjusting the height of the sensor using a z-axis manual stage according to the heights of the objects. First, a pull test was conducted for a 100-mm-long cylindrical object (a glue stick) with a diameter of 30 mm for five trials as shown in Fig. 8(a), resulting in the force–displacement curves. Fig. 8(b) shows representative force–displacement curves for the proposed robotic palm with various vacuum pressures. To exclude the friction effect of the silicone material, a rigid flat palm with a silicone skin made of the identical Ecoflex was also tested. For the flat silicone skin, the peak force reached 20.3 N at first. It then fluctuated for the rest of the displacement, possibly because of instability in grasping. However, the pull force of the unjammed palm gradually decreased along the increment of displacement after reaching the peak force, 15.3 N, because of the lateral deformation of the palm along the axis of pulling, resulting in consistent surface friction. The highest pull force (50.5 N) was attained with the fully jammed palm (−29.9 kPa) and also the medium level of peak force (32.9 N) with the partially jammed palm (−7.8 kPa). The pull force instantly dropped as one of the three fingers lost grip. Then, it gradually increased with both fully and the partially jammed palms, even under loss of grip. Additional resistance force may be augmented by the resilience of the compliant palm with surface friction. The partially jammed palm may have the potential to deform its shape adaptively under a certain level of stiffness for providing stable contact surfaces. However, a fully jammed palm in a solid-like state is unlikely to conform to the change of object contact.

The slip resistance enhanced by the use of the robotic palm was investigated using different types of object—cylindrical (a glue stick), spherical (ornament ball), and arbitrarily shaped (figure model) objects—see Fig. 8(c). As expected from the previous experiment, the proposed robotic palm had 2.2, 2.0, and 3.1 times higher pull force than those of the flat skin for the cylindrical, spherical, and arbitrarily shaped objects, respectively.

3) Multimodal grasping The robotic palm allows multimodal grasping with a

variety of different grasping strategies. The palm provides a stable base for a power grasp, providing high surface conformity to any arbitrary shape of an object in contact and also high slip resistance. Different objects were selected for the demonstration, as shown in Fig. 9(a), considering cases where the human palm contributes substantially to improving grasp performance or aiding in manipulation of an object. For example, the robotic palm helps the robot hand robustly manipulate a spray gun. One of the three fingers could freely

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F/T

sensor

Robot

fingerObject

Robotic

palm

Laser

disp.

sensor

(a)

Motorized

stage

(b)

(c)

Fig. 8. (a) Experimental setup to evaluate slip resistance, (b)

force-displacement curves for varying vacuum pressures of the robotic palm

and the flat skin when pulling the cylindrical object using a fishing line

connected to the motorized stage, and (c) maximum pull force for various

objects.

act for pulling and releasing the lever of the spray gun, while the object could be held firmly, even with the other two fingers and palm utilized as a stable base.

The dual-layered palm can act as a universal gripper so that secure grasping of small objects becomes possible, which is accomplished typically by a precision grasp. When an object is gripped by the palm first, secure grasping is accomplished by planning the poses of the fingers to shroud the object secularly. Fig. 9(b) shows a variety of secure grasping patterns for small objects.

The new grasping strategy was validated using a pick-and-place task for a small object by integrating the robot hand with a commercially available 6-DOF robot arm (VS-050, DESSO, Japan)—see Fig. 9(c). The robotic palm was set to the equilibrium state. Then, the robot arm approached a target object (e.g., a pen) and contacted the object. At the moment of contact, the auto-jamming control automatically grasped the pen without any tactile sensors or visual feedback. The robot arm then moved away from the floor to secure space for the fingers to operate. Then, the fingers were folded to secure the object in the robot hand. Next, the robot arm located its end effector (the hand) above a target position. Finally, the fingers were unfolded, and the vacuum pressure was released to detach the object from the robot.

IV. DISCUSSION AND FUTURE WORK

A robotic palm design is proposed with a dual-layered structure that can yield high surface conformity and controllable rigidity. These features enable the robot hand to attain increased friction force with enlarged contact areas because of the deformable palm. The increased friction force contributes substantially to the minimal grasping effort while yielding greater resistances to slippage in an energy efficient manner. The auto-jamming control allows automatic solidifying of the palm by detecting the change of the internal pressure of the air chamber without tactile or vision sensors. It thus provides the greatest surface conformity within the short operation time. The proposed auto-jamming control principle could be applicable to the control of soft grippers using particle jamming. The overall system was designed to be easily embedded in any robot hand while keeping the palm height as low as possible and miniaturizing the control hardware, such as palm-sized electronics with miniature motor pumps.

The comparative study shows that the proposed palm has higher shape conformity and greater slip resistance than the latest robotic palm with single-layered particle jamming [10]. The proposed palm could increase the grasping force by 2.0–3.1 times compared with the flat skin. It was limited to 1.6 times with the single-layered palm. This can be explained by three aspects of particle jamming: static friction, interlocking, and vacuum suction [11]. Given the identical height in the two palm models’ design, the dual-layered palm has larger contact surface areas that contribute to increasing static friction because the robotic palm was made of silicone. For certain small objects, interlocking can also be possible with the dual-layered palm by pinching the edge of an object and restraining its movement. Vacuum suction also pinches the object’s edge and seals any gaps between the object and contact surface.

Regarded as n fingers, the robotic palm offers new grasping strategies with the versatile grasping modes demonstrated. An alternative grasping strategy for small objects was also demonstrated in conjunction with robot arm manipulation. A small/slender object could be easily picked up and secured by the fingers without any complex control or sensors needed for precision grip with fingered grippers. The proposed palm is more robust to unexpected disturbances than gripping the object using only fingers. Although the deformed palm in the jammed state is less stiff than any solid-surface palm, the palm also works for manipulation tasks requiring a high level of force to manipulate an object or tolerate external disturbance. A large amount of normal force to the palm applied by robot fingers causes an object to be pressed against the palm’s bottom and/or edge. In this scenario, like the bones and skin of human palms, the robotic palm offers stable support with the palm’s hard parts and also with high shape conformity providing additional contact surface areas.

The proposed palm can easily be miniaturized as desired because its form factor is determined primarily by the diameters of the chambers and tube fittings used for air supply and vacuum. Although the current height of the palm was limited because of the use of one-touch tube fittings, it could

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(b)

(c)

(ⅱ)(ⅰ) (ⅲ) (ⅵ)(ⅴ)(ⅳ) (ⅶ)

(a)

Fig. 9. Demonstration of multimodal grasping with the robot fingers: (a) power grasp of a tennis racket, spray gun, tennis ball, and door handle, (b) alternative

grasp for small objects—a figure model, flash memory, glue stick, and pen, and (c) the pick-and-place scenario of a pen with the robot arm.

be further miniaturized by directly affixing air tubing onto each chamber without tube fittings. Regarding the optimal height of each chamber, a larger jamming-chamber height and smaller air-chamber height exhibited better shape adaptability with high stiffness. However, an extremely large jamming chamber would make the overall system too heavy. Furthermore, a shallow air chamber would result in limited space for an object. Therefore, it should be further investigated regarding the robot hand size and required stiffness. The weight of the current robotic palm (220 g) can be decreased by half by changing the base materials from aluminum to plastic because the hard aluminum parts occupy more than 80% of the total weight. Future plans include performing such manipulation tasks as opening a door, drilling, and hammering while grasping the corresponding objects. A unified control system with grasp planning algorithms needs to be developed to manipulate objects seamlessly.

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