Article
Estimation and Control of Cooperative Aerial Manipulators for
a Payload with an Arbitrary Center-of-Mass
Hyeonbeom Lee
1
and Uikyum Kim
2,
*
Citation: Lee, H.; Kim, U. Estimation
and Control of Cooperative Aerial
Manipulators for a Payload with an
Arbitrary Center-of-Mass. Sensors
2021, 21, 6452. https://doi.org/
10.3390/s21196452
Academic Editor: Andrey V. Savkin
Received: 9 August 2021
Accepted: 23 September 2021
Published: 27 September 2021
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4.0/).
1
School of Electronic and Electrical Engineering, Kyungpook National University, Daegu 41566, Korea;
hbeomlee@knu.ac.kr
2
Department of Mechanical Engineering, Ajou University, Suwon 16499, Korea
* Correspondence: ukim@ajou.ac.kr; Tel.: +82-31-219-2341
Abstract:
This paper presents an integrated framework that integrates the kinematic and dynamic
parameter estimation of an irregular object with non-uniform mass distribution for cooperative
aerial manipulators. Unlike existing approaches, including impedance-based control which requires
expensive force/torque sensors or the first-order-momentum-based estimator which is weak to noise,
this paper suggests a method without such sensor and strong to noise by exploiting the decentralized
dynamics and sliding-mode-momentum observer. First, the kinematic estimator estimates the relative
distances of multiple aerial manipulators by using translational and angular velocities between aerial
robots. By exploiting the distance estimation, the desired trajectories for each aerial manipulator
are set. Second, the dynamic parameter estimation is performed for the mass of the common object
and the vector between the end-effector frame and the center of mass of the object. Finally, the
proposed framework is validated with simulations using aerial manipulators combined with two
degrees-of-freedom robotic arms using a noisy measurement. Throughout the simulation, we can
decrease the mass estimation error by 60% compared to the existing first-order momentum-based
method. In addition, a comparison study shows that the proposed method satisfactorily estimates an
arbitrary center-of-mass of an unknown payload in noisy environments.
Keywords:
parameter estimation; arbitrary center-of-mass; cooperative aerial manipulation; dynamic
uncertainty
1. Introduction
Multirotor UAVs (unmanned aerial vehicles), which are highly maneuverable and can
be made small, are gaining popularity as a major air transportation platform [
1
]. Among
them, cooperative UAVs are widely exploited to handle a heavy or large payload [
2
] beyond
the limits of a robot’s transportation capabilities. Recently, researchers have developed
cooperative mobile manipulators [
2
–
9
] by exploiting grasping capability. However, due to
several issues including the complexity associated with multiple aerial robots, they have
focused on solving a control and coordination problem. To make the problem simple, they
have assumed that a relative distance from the payload frame can be easily calculated
because the regular payload has a uniform mass distribution [
2
,
4
–
9
]. For these reasons,
cooperative aerial manipulations to handle a payload with the non-uniform mass distri-
bution (i.e., the geometry of the payload and center of mass (COM) of the payload are
different) are quite complicated.
1.1. Contribution
This paper is interested in resolving the aforementioned problem of handling a pay-
load with an arbitrary center-of-mass for cooperative aerial manipulators. The contribu-
tions of this paper can be summarized as follows. First, we propose a kinematic estimation
algorithm for unknown kinematics of the payload and the trajectory generation algorithm
using the estimated kinematic parameter. The existing method in [
10
] estimates the relative
Sensors 2021, 21, 6452. https://doi.org/10.3390/s21196452 https://www.mdpi.com/journal/sensors
