Citation: Wang, Z.; Wang, Y.; Cai, Z.;
Zhao, J.; Liu, N.; Zhao, Y. Unified
Accurate Attitude Control for
Dual-Tiltrotor UAV with Cyclic Pitch
Using Actuator Dynamics
Compensated LADRC. Sensors 2022,
22, 1559. https://doi.org/10.3390/
s22041559
Academic Editors:
George Nikolakopoulos,
Pablo Rodríguez-Gonzálvez,
Diego González-Aguilera and
Arturo de la Escalera Hueso
Received: 7 January 2022
Accepted: 28 January 2022
Published: 17 February 2022
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Article
Unified Accurate Attitude Control for Dual-Tiltrotor UAV with
Cyclic Pitch Using Actuator Dynamics Compensated LADRC
Zexin Wang
1
, Yingxun Wang
2
, Zhihao Cai
1,
*, Jiang Zhao
1,
*, Ningjun Liu
2
and Yanqi Zhao
1
1
School of Automation Science and Electrical Engineering, Beihang University, Beijing 100191, China;
wangzx@buaa.edu.cn (Z.W.); zhaoyanqi@buaa.edu.cn (Y.Z.)
2
Institute of Unmanned System, Beihang University, Beijing 100191, China; wangyx@buaa.edu.cn (Y.W.);
liuningjun@buaa.edu.cn (N.L.)
* Correspondence: czh@buaa.edu.cn (Z.C.); jzhao@buaa.edu.cn (J.Z.)
Abstract:
This paper proposes a unified attitude controller based on the modified linear active
disturbance rejection control (LADRC) for a dual-tiltrotor unmanned aerial vehicle (UAV) with
cyclic pitch to achieve accurate attitude control despite its nonlinear and time-varying characteristics
during flight mode transitions. The proposed control algorithm has higher robustness against model
mismatch compared with the model-based control algorithms. The modified LADRC utilizes the state
feedbacks from the onboard sensors like IMU and Pitot tube instead of the mathematical model of the
plane. It has less dependency on the accurate dynamics model of the dual-tiltrotor UAV, which can
hardly be built. In contrast to the original LADRC, an actuator model is integrated into the modified
LADRC to compensate for the non-negligible slow rotor flapping dynamics and servo dynamics.
This modification eliminates the oscillation of the original LADRC when applied on the plant with
slow-response actuators, such as propeller and rotors of the helicopter. In this way, the stability and
performance of the controller are improved. The controller replaces the gain-scheduling or the control
logic switching by a unified controller structure, which simplifies the design approach of the controller
for different flight modes. The effectiveness of the modified LADRC and the performance of the
unified attitude controller are demonstrated in both simulation and flight tests using a dual-tiltrotor
UAV. The attitude control error is less than
±
4
◦
during the conversion flight. The control rising time
in different flight modes is all about 0.5 s, despite the variations in the airspeed and tilt angle. The
flight results show that the controller guarantees high control accuracy and uniform control quality
in different flight modes.
Keywords:
dual-tiltrotor; linear active disturbance rejection control (LADRC); UAV; unified controller
1. Introduction
A tiltrotor unmanned aerial vehicle (UAV) utilizes tiltable rotors to change the direc-
tion of the thrust for high-speed cruise and vertical takeoff and landing. It is an important
category of vertical takeoff and landing (VTOL) UAVs that has gained fast-growing pop-
ularity in both academia and industry [
1
–
6
]. According to the number of rotors, tiltrotor
UAVs can be divided into two categories, dual-tiltrotor UAVs and multi-tiltrotor UAVs [
7
].
Apart from the different number of rotors, the main difference between the dual-tiltrotor
UAVs and multi-tiltrotor UAVs is that the former uses two helicopter rotors with collective
and cyclic pitch to provide control forces and moments, such as V-247 and Eagle eye [
6
].
Multi-tiltrotor UAVs, on the other hand, have multiple rotors mounted in front of and
behind the center of gravity to provide control forces and moments, just like traditional
multi-rotor UAVs. In the cruise period, their rotors that do not provide thrust become
dead weight and reduce the efficiency of cruise flight [
7
]. Owing to the high efficiency in
hover and cruise flight, the dual-tiltrotor UAV is a promising configuration for the high
performance VTOL UAV [4,6].
Sensors 2022, 22, 1559. https://doi.org/10.3390/s22041559 https://www.mdpi.com/journal/sensors
