Figure 3. Fabrication process of the gripper. a)
Electroadhesive pad fabrication. A three-layer structure, which consists
of a bottom silicone layer (i), a conductive silicone layer (ii), and a
top silicone layer (iii) is blade-casted and cured in an oven one on top
of the other. The electrode pattern (ii) is engraved using
CO2 ablation before the top silicone layer (iii) is
cast. b) i) The fabricated pad is placed in an empty mold. Then, liquid
silicone is poured into the mold to form the silicone bag and
simultaneously connect the bag and the pad in a monolithic structure.
ii) The silicone is solidified by thermal curing and the gripper is
removed from the mold. iii) The silicone bag is filled with coffee
granules (in brown).
Grasping force
characterization
The grasping performance of the gripper in GJ mode results from three
different mechanisms: physical interlocking between the gripper and
object, static friction caused by normal stresses in the contact
regions, and suction effect due to a reduced air pressure over part of
the contact surface (the latter can only occur when the gripper
generates a vacuum seal against the object).[11]Static friction is the main factor because it contributes to both
interlocking and suction modes.[5] Thus, the
characterization of the grasping force of a GJ gripper is usually made
for the static friction mechanism, which depends on the size and shape
of the manipulated object, the contact angle, surface conditions, and
applied force at the moment of grasp.[5,11]
Figure 4. Characterization of grasping performance for an
object with hemispheric shape. Each data point corresponds to the
average measurement of three grippers tested five times each (15
measurements). a) Grasping force for granular jamming and combined
(granular jamming with electroadhesion) modes as a function of object
size. Diameters of hemispheres used in these tests range from 33% to
89% of the gripper’s active area. The gripper was pushed down and
contacted the hemisphere until it reaches 5N of applied force in all
experiments. In the combined mode, the EA pad was activated by applying
2.5 kV and GJ was triggered by the air evacuation with the pressure drop
of 84 KPa. Grasping forces increase with increasing object diameter, but
the combined mode displays 15.2% superiority for largest object
diameter compared to granular jamming alone. b) The measured total
electroadhesion force of the gripper as a function of applied voltages
on a flat surface. The normal force scales from 0 to 0.2 N with the
voltage applied from 0 to 5 kV with a step size of 500V. c) The applied
force as a function of a grasped object size. In experiments b and c,
the gripper was pulled down to a certain absolute coordinate above all
hemispheres. The applied force increases with the growth of the object
size from 1.2 to 2.2 N. d) The contact angle as a function of a grasped
object size. The contact angle changes from 73° to 30°.
In this study, the static friction mechanism in the combined mode is
enhanced by EA, which increases the friction forces and normal forces of
the gripper.[1,28] Thus, to quantify the grasping
performance, we characterize the grasping force in GJ mode and in
combined mode for different manipulated object sizes (Figure
4a ). The gripper is connected to the tensile testing machine equipped
with a load cell to perform vertical displacement and applied force
measurements on the grasped object. The objects to be grasped are fixed
on a plate beneath the gripper and in line with its central axis. The
objects consist of a set of hemispheres printed in polylactic acid (PLA)
with the diameters from 12 to 32 mm that corresponds to 33 to 89% of
the gripper diameter. The shape of the hemispheres does not allow
interlocking grasp. The gripper is moved down to a preprogrammed height
by the tensile machine and presses against the object (see Experimental
Section). After that, the GJ or combined mode is activated, and the
tensile machine lifts the gripper up until it releases the object. In
the combined mode, the EA pad is activated by applying 2.5 kV and GJ is
triggered by the air evacuation with a pressure drop of 84 KPa. Although
the EA pad can operate up to 5 kV (Figure 4b), the applied voltage here
is set to 2.5 kV to prevent possible breakdowns that may happen when
squeezing the EA pad during the grasping operation. The grasping force
data for three experiments is collected using the load cell of the
tensile machine.
In the GJ mode, the grasping force varies from 5.8 (standard deviation
(SD) = 0.38 N) to 10.6 N (SD = 0.62 N) for object sizes from 33 to 89%
of the gripper diameter. Interestingly, the maximum grasping force is
achieved for the hemisphere with the size of 78% of the gripper
diameter and then starts decreasing. In the case of combined mode, the
grasping force varies from 6.1 (SD = 0.18 N) to 12.1 N (SD = 0.83 N) for
object sizes from 33 to 89% of the gripper diameter. In this case,
there is a continuous increase in grasping force with the size of the
hemisphere. For the hemisphere size of 89% of the gripper diameter, the
grasping force of the combined mode is 15.2% larger than the GJ mode’s
force.
In the GJ mode, the grasping force depends not only on the size of the
object but also on the applied force with which a gripper touches the
object. This applied force determines a contact angle between the
gripper surface and the object. The contact angle and size of the object
define the contact surface i.e., amount of granules in the contact zone
between the gripper and the object. To characterize the relationship
between the applied force and the contact angle, we use the same setup
with the tensile machine, hemispheres, and an additional camera placed
in front of the gripper to capture the contact angle. The gripper is
lowered to a preprogrammed height by the tensile machine. After reaching
the hemisphere, the applied force data is collected from the load cell.
The applied force increases from 1.2 (SD = 0.03 N) to 2.2 (SD = 0.61 N)
N for the object size from 33 to 89% of the gripper diameter (Figure
4c). The applied force grows with an increase of the object size because
larger object size results in the larger contact surface area across
which more granules can cover the target. We noticed that an increase in
the applied force correlates with a decrease in the contact angle. The
lower contact angle shows that a larger area of the hemisphere is in the
contact with the gripper causing more granules to flow towards the
surface of the hemisphere, press onto it resulting in a higher applied
force. The contact angle decreases from 73° (SD = 9.6°) to 30° (SD =
4.9°) for the object size from 33 to 89% of the gripper diameter
(Figure 4d).
The EA mode allows grasping of flat objects, which is not possible in GJ
mode, by means of the normal force generated between the gripper and the
object when voltage is applied. The amount of normal force generated by
the interdigitated electrode structure depends on the design of
electrodes, dielectric constants of contact materials, thickness of the
dielectric layer, and effective area of contact. It also scales with the
square of the applied voltage.[1,29] Other
physical parameters, such as surface conditions and roughness can also
influence the normal force.[18] Thus, we performed
experimental characterization of the EA generated force (Figure 4b). The
tensile machine lowers the gripper onto the flat surface and a voltage
is applied from 0 to 5 kV with a step increment of 0.5 kV. The measured
normal force scales with the voltage as reported in the
literature.[1,28,30] The inherent stiction between
the outer dielectric layer and the flat surface causes high standard
deviation of normal force measurements at zero voltage. Normal force
reaches 0.2 N (SD = 0.02 N) at 5kV. Normal force can also be enhanced
either by decreasing the outer dielectric layer or increasing the
density of generated fringe fields by decreasing the width of the
electrodes and the gaps between electrodes.
The grasping force of conventional GJ grippers varies up to an order of
magnitude depending on the shape of object.[5]Thus, we characterized the grasping force generated by the proposed
gripper in GJ mode for different object geometries, including flat and
deformable objects (Figure 5a ). For these experiments, we used
seven different 3D shapes with the cross-sectional diameter of 80-90%
of the gripper’s diameter, which corresponds to maximum grasping force
(Figure 4a) consisting of a sphere, cube, pyramid, disks with 20 and 50
mm in a diameter, cylinder, and parallelepiped. All shapes were 3D
printed with the highest layer thickness of 0.4 mm (instead of a usual
0.06 mm). The objects were fixed at the bottom of the tensile machine,
while the gripper grasped and pulled the object up and measures the
holding force. We tested three grippers three times for each shape. We
observed the highest grasping forces of 6.1, 10.4, and 8.9 N in both
modes for a sphere, cube, and disk of 20 mm. The combined mode increases
the adhesion for these objects by 5.6%, 3.7%, and 0.5%. In comparison
to spheres, these shapes allow granules to flow more easily around the
target object and envelope it completely. Yet, when the manipulated
object design is less suitable for GJ mode, the influence of EA on the
resulted grasping force is higher. We observed the highest differences
(35%, 16%, and 15%) in grasping forces between granular jamming mode
(0.83, 1.33, 1.35 N) and combined mode (1.12, 1.54, 1.55 N) for a
cylinder, pyramid, and parallelepiped respectively. In the case of
deformable water balloon, we observed grasping forces of 1.8 and 2 N for
the GJ and combined mode indicating the grasping enhancement of 11%.
Figure 5. Characterization of grasping forces for different
geometries and surface conditions. a) The grasping force for granular
jamming (GJ) and combined mode for different object geometries that have
the same volume: sphere, cube, pyramid, disks with diameters of 20 and
50 mm, cylinder, parallelepiped, and deformable balloon with liquid. The
flat disk with a diameter of 50 mm has a diameter larger than the
diameter of a gripper and cannot be grasped using GJ mode. The highest
grasping forces of 6.1, 10.4, and 8.9 N in both modes were observed for
a sphere, cube, and disk of 20 mm. The combined mode increases the
adhesion for these objects by 5.6%, 3.7%, and 0.5%. These shapes
allow granules to flow more easily around the target object and grasp it
from all sides. The highest differences of 35%, 16%, and 15% in
grasping forces between GJ mode 0.83, 1.33, 1.35 N and combined mode
1.12, 1.54, 1.55 N are observed for a cylinder, pyramid, and
parallelepiped respectively. The combined mode shows worse performance
only in the case of a disk of 50 mm (flat object). The activation of GJ
in a combined mode makes the bottom surface involved in grasping rigid,
thus, less compliant. b) Grasping forces of a flat object as a function
of four grasping modes. Electroadhesion mode (EA) shows the highest
grasping force of 0.8 N compared to combined, granular jamming, and
nature stickiness forces of 0.15, 0.27, and 0.45 N resulting in the
grasping enhancement of 430%, 194%, and 78% respectively. c) The
grasping force for a combined mode for different surface conditions: dry
(grey), oily (yellow), porous (pink), moistened (red), and powdered
(brown). The hemisphere with the dimeter of 28 mm, which corresponds to
78% of the gripper diameter was used. The same setup and testing
protocol as for Figure 4a was used. The grasping of objects with dry,
moistened, and oily surfaces resulted in four to five times higher
grasping force compared to porous and powdered surfaces due to suction
effect formation.
We also characterized the grasping force for each of the objects when
GJ, EA, and the combined mode are turned off, and the object is lifted
only due to the inherent stickiness of silicone (Figure S1, Supporting
Information). Interestingly, for a flat object larger than the gripper
size, we observed that two highest forces of 0.42 and 0.80 N are
achieved in the soft state of the gripper using only the stickiness of
silicone and EA mode, respectively (Figure 5b). In these modes, the
gripper is in the soft state and is more compliant than GJ mode,
resulting in a better contact between the gripper and the object that
leads to higher grasping performance. The modes that includes GJ, thus,
stiff state of the gripper, result in the maximal grasping force of 0.27
N, which is three times lower than EA. The activation of GJ makes the
gripper rigid and the external contact surface of the gripper becomes
rough (Figure S2, Supporting Information). The higher surface roughness
in a rigid state makes the contact area between the device and the
manipulated object lower than in a soft state.[18]
Contrary to EA-based grippers, GJ-based grippers can grasp objects that
are powdered, moistened, and porous.[11] In this
study, we chose different gripper design and material composition from
already existing devices.[31] Thus, we performed
characterization of the grasping force at different surface conditions
in the combined mode (Figure 5c) as this mode shows the highest gasping
force among all modes (Figure 4a). We used the hemisphere with a dimeter
of 28 mm, which corresponds to 78% of the gripper diameter, following
the same setup and testing protocol as for Figure 4a. We measure the
grasping force for the gripper in combined mode under different surface
conditions of the object: dry, oily, porous, moistened, and powdered.
Porous and powdered surfaces resulted in the lowest grasping forces (1.1
N and 1.9 N, respectively). Oily, moistened, and dry objects instead
yielded substantially higher forces (14.8 N, 12.9 N, 12.6 N,
respectively). The porous and powdered surfaces prevent the formation of
sealed interface between the gripper and object surfaces, eliminating
the suction effect. In the case of oily, moistened, and dry objects, the
silicone surface of the gripper fastens tightly around the object
causing a suction effect, which magnifies the grasping force. These
results are in line with the performance of a conventional GJ gripper
measured with similar surface conditions.[11]These results indicate that the outer surface of the gripper, which is
three times thicker than conventional GJ grippers due to the presence of
the EA electrodes, does not limit grasping capabilities for different
surface conditions.
Grasping and manipulation of fragile, deformable, and flat
objects.
GJ is an effective method for grasping solid objects with different
shapes.[5,11,12] However, the grasping of
deformable, and flat objects can be challenging and is never discussed
in the literature for GJ-based grippers. Here we show that the combined
use of GJ and EA enables successful pick and place of such objects
(Figure 6 and Video S1, Supporting Information). In a first
example, we pick up two fragile quail eggs with diameters of 27 mm and a
weight of 17 grams each. The gripper in the soft state approaches the
eggs until the granules in the gripper flow around both eggs