IEEE Robotics & Automation Magazine - December 2016 - 97

are placed on an integrated conveyor belt and transported
further along the handling chain.

decided to endow the gripper with two fingers, each of
which forms a planar rotary-rotary manipulator. In this
way, all of the links were involved in enveloping grasps
of cylindrical and deformable goods, whereas, for the
grasping of boxes, the proximal links are involved to a
lesser extent. To keep the mechanical design conceptually simple, allow for simpler control schemes, and keep
the weight of the gripper low, we also opted for underactuation of the joint between the two links of each finger.
Finally, the motor drives chosen for actuating the gripper were outfitted with back-drivable gear boxes to allow
for compliant behavior via current control.
The design of the gripper is also affected by the environment in which it operates. As the containers in our
scenario can feature a lot of clutter and closely packed
goods, it is desirable for grasps to be acquired with a
minimal gripper footprint. This requirement would
favor less-stable fingertip grasps, however, instead of
firm enveloping grasps. To solve this problem, the velvet
fingers gripper is also endowed with in-hand manipulation capabilities through sliding surfaces (implemented
with controlled conveyor belts) on the inner side of the
links. Analysis of the manipulability ellipsoids of the
gripper [2] has demonstrated a clear improvement in
terms of dexterity when using active sliding surfaces.

Empticon II Robot
Unloading coffee sacks implies the handling of tightly
packed and heavy, yet deformable, goods that do not
readily allow for stable top or side grasping. Thus, we
opted for a manipulation solution that ensures the sacks
are continuously supported from below during the
unloading process. As standard industrial robots are not
well suited for this task, we designed and implemented a
special purpose robot, i.e., the Empticon II [Figure 1(a)].
The robot design was optimized to minimize the number
of joint movements under typical unloading sequences,
while maintaining sufficient freedom in the range of
reachable target-grasping poses. The implemented
machine consists of a single DoF wheeled platform, on
which a manipulator with five actuated joint axes (with
two pairs passively coupled) is mounted, resulting in a
4-DoF system. A conveyor belt runs along the whole
manipulator chain, allowing for a smooth transition of
sacks from the container onto the robot body and further
along the material flow chain into the warehouse.
Velvet Fingers Gripper
In response to the challenges of the robust grasp acquisition of heterogeneous goods, we designed a new underactuated gripping device, i.e., the velvet fingers gripper
[2], [3], [Figure 3(a)]. Our application scenario demanded a versatile device that can reliably secure grasps on
objects of varying shape, material, weight, and deformability and do so under moderate object-pose uncertainties. After analyzing the properties of typical goods in
our scenario, we opted for a grasping principle that
would allow for both embracing grasps with four contact
areas for cylindrical and deformable goods as well as
grasps involving two opposing surfaces of parallelepipedic goods like boxes. To satisfy these requirements, we

Needle Chain Gripper
A special-purpose gripping device, i.e., the needle chain
gripper [Figure 3(b)], was developed to address the coffee-sack-unloading task. The gripper was designed to
grasp and pull a coffee sack from a pile onto its frame
and further transport it to the conveyor belt of the Empticon II robot, thus facilitating a continuous material
flow. For this purpose, six chains equipped with needles
running synchronized and parallel along the transport
direction of a grasped sack are activated by a servo electrical motor. The tip of the gripper constitutes the grasping zone where the needles pierce into the sack for the

5 4

E1 2

9
J2 E3

TB1

E5
DP

TB2
7

B1
(a)

(b)

Figure 3. The design of the developed gripping devices: (a) the velvet fingers gripper design and (b) the needle chain gripper design.

December 2016

IEEE ROBOTICS & AUTOMATION MAGAZINE

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