IEEE Robotics & Automation Magazine - December 2018 - 68

been chosen following the principles described in the technical report attached in the multimedia material.
The GM: LBR-iiwa
We used the KUKA 7-DoF LBR-iiwa 14 R820 as the GM
(Figure 1), allowing a 14-kg payload and 820-mm reach (see
Table 1 for more data), which satisfies the task requirements
for the GM in terms of payload and workspace (see the
attached technical report). The embedded joint torque sensors allow the retrieval of the external wrench without the
need to integrate a force/torque sensor at the EE. Despite the
significant payload, its maximum admissible joint torque for
the last joint is 40 N, and its rated torque of 23.3 Nm corresponds to holding a 1.42-kg, 3-m-long bar horizontally, which
may not reflect the typical bar weight for several of the considered applications.
The software architecture of the LBR-iiwa is dictated by the
proprietary solution. The control cabinet contains two computers, a Windows machine for the high-level interface and a

3
4

Vx-Work machine for real-time execution of the applications.
In our setup, we use a KUKA library, the Fast Robot Interface
(FRI), which employs the User Datagram Protocol to achieve
real-time control from a remote computer. The application on
the control cabinet runs a server (simultaneously running the
low-level safety features) to which the client sends commands
at a high frequency (up to 1 kHz) and from which the client
receives system information. In this modality, the remote computer has access to various quantities to close the control loop
and sends joint commands to a low-level loop that runs on the
control cabinet. This allows a real-time interaction between
the operator and the manipulator. Any other manipulator
could be used as long as the hard real-time remote control
is supported.
During our implementation, only the joint controllers
(position or impedance) were available in the FRI library.
Hence, we built our control architecture on top of the joint
position controller by implementing both an admittance filter
and an inverse kinematics layer. For compliance, the admittance filter is preferred over impedance control, despite being
more tedious to tune, because of the availability of the lowlevel position controller [16]. This means that the new, compliant reference trajectory for the EE pose is decoded into the
joint trajectory based on closed-loop inverse kinematics [17].
As for the gripper, we integrated a Schunk PGN-plus-E
80-1 gripper with custom-designed claws on the flange of the
LBR-iiwa. The choice of this gripper was made considering
the significant torques that could be involved at the EE during
our experiments (as indicated in the technical report).

The AM: OTHex
The OTHex is an AM specifically developed for this project
and tailored to perform physical interaction tasks with the
environment, i.e., meeting the requirements presented in the
Figure 1. The experimental setup with an LBR-iiwa l mounted
technical report. Its main features and peculiarities are preon a FlewFellow m . The smartPad n is carried by the
sented here (more details can be found in [15]), and its key
FlexFellow, and the gripper k grasps the bar via customspecifications are listed in Table 2.
made claws j.
The first feature is the tilted propeller actuation. In classical multirotor
design, all the propellers are collinear,
Table 1. The characteristics of the GM
which leads to underactuated system
used in the experiments (KUKA LBR-iiwa 14).
dynamics, i.e., position and orientation
cannot be controlled independently.
Description
Value
The OTHex design enforces noncolReach
820 mm
linear propeller orientation, as illustrat3
Volume
1.8 m
ed in Figure 2(a). This design feature
Weight
29.9 kg
allows the system to have multidirecPosition repeatability
±0.15 mm
tional thrust ability, i.e., the total thrust
can be exerted in a polytope attached
Maximum rated payload
14 kg
to the body frame instead of just along
Maximum rated torque at EE
23.3 Nm
a single upward direction, as in the
Maximum joint angle
±170, ±120, ±170, ±120, ±170, ±120, and ±175°
underactuated case. This means that
Maximum joint angular velocity 85, 85, 100, 75, 130, 135, and 135°s-1
the robot can exert lateral forces withMaximum joint torque
320, 320, 176, 176, 110, 40, and 40 Nm
out needing to reorient itself; thus, it is
able to track a decoupled reference traFRI control frequency
500 Hz
jectory in position and orientation
1

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IEEE Robotics & Automation Magazine - December 2018

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