Abstract
In recent years, there has been a growing interest in the development of
universal soft grippers that can handle objects of varying form factors
(including flat objects), surface condition (including moistened or oily
objects), and mechanical properties (deformable and fragile). Yet, there
is no single gripper that can gently grip objects with such a wide range
of properties. In this paper, we present a soft gripper that combines
granular jamming (GJ) and electroadhesion (EA) to gently grasp and
release a large set of diverse objects. The gripper can operate in GJ
mode only, in EA mode only, or in a combination mode that simultaneously
activates GJ and EA. In GJ mode, the gripper can grasp objects with
different surface properties, lift objects 38 times its own weight using
negative pressure, and release objects by applying positive pressure,
but has difficulty in handling flat and fragile objects. In EA mode, the
gripper can manipulate flat and fragile objects but encounters
difficulties with different surface properties such as oily or
moistened. In the combination mode, the gripper can generates grasping
forces up to 35% higher than in the GJ mode for all object sizes and
certain shapes such as a cylinder.
Introduction
The softness of the human hand is a critical factor that allows us to
hold, lift, and manipulate a variety of objects and has inspired
roboticists to incorporate softness in gripper design and materials. The
compliance of soft materials enables passive adaptation of the gripper
during grasping operations allowing manipulation of a wide range of
objects without bringing additional control
complexities.[1,2] In recent years, there has been
a growing interest in the development of universal soft grippers that
can work with objects of different form factors, rigidity, surface
properties, and level of fragility.[3–6] A
possible approach to create such highly versatile grippers is to combine
different gripping technologies that complement their individual
limitations.[1,3,7–10] Yet, it is still
challenging to develop a single gripper that can grasp and release
objects of different form factor including flat objects, surface
conditions (wet, porous, oily, and powdered), and mechanical properties
(fragile and deformable).
In this paper, we present a soft gripper capable of manipulating
different objects with varying physical properties, such as shape,
surface conditions, and rigidity. The proposed gripper combines two
different technologies: granular jamming to control stiffness and
electroadhesion to control adhesion. Here we show that not only does
this combination mutually compensate for the limitations of each
individual technology, but it also makes the gripper capable of
performing multi-stage grasping tasks that consist of diverse grasping
and releasing operations on objects made of different material, surface,
and shape. The manipulation of a book is an example of multi-stage
operation that requires grasping and turning a rigid cover and flipping
through single pages.
Granular jamming (GJ) enables reversible stiffness change between soft
and rigid configurations by means of negative pressure[5,11,12]. High compliance in the soft state
allows a GJ gripper to envelope the manipulated object by pressing on
it. When negative pressure is applied, the gripper becomes stiff and
holds the encaged object.[2] Variable stiffness
can also be achieved by integrating phase-change materials that vary
mechanical properties under thermal
stimulation.[7,13] However, granular jamming
offers comparatively faster response time (~100ms),
independence from environmental temperature, higher lifting force,
easier fabrication, higher robustness, and lower
cost.[2,5,14,15] The grasping force produced by
granular jamming is sufficient to grasp objects of different
morphologies, almost independently of the surface conditions of the
object.[5,11,16] The grasping force of GJ grippers
can vary from 0.09 to 1.2 kN.[12] GJ has been
combined with soft pneumatic actuators to provide more dexterous grasp
and lift heavier objects because of the enhanced holding
forces.[10,17] However, GJ grippers cannot lift
flat objects, such as a sheet of paper. Also, the grasping performance
of delicate, fragile and easily deformable objects such as a thin layer
of cloth, an egg, or water balloons, as well as larger objects than the
active area of the granular bag can be challenging and have not been
demonstrated so far.
Electroadhesion (EA) instead is an adhesive technology that leverages
the shear force generated by electrostatic
forces.[18] Electroadhesive pads have been
combined with different actuation technologies, such as dielectric
elastomer actuation,[1] soft pneumatic
actuation,[19] layer
jamming,[20] and Fin-Ray structured
actuation[21]. The enhanced shear force makes
EA-based grippers capable of delicately grasping both flat and fragile
objects without squeezing or breaking
them.[1,22–26] While the adhesive force of EA
pads can be tuned by regulating electrical input, EA effectiveness is
highly dependent on the environmental and surface conditions of the
object being grasped.[18] In particular
electroadhesion is less effective for objects that are greasy, rough, or
wet.[2] An additional challenge of soft grippers
that rely on electroadhesion is the residual electrostatic charge that
remains for a few seconds after removing the voltage and can result in
difficult release of light objects.[27]