Predictor-Based Time Delay Control of A Hex-Jet Unmanned Aerial Vehicle

TL;DR

Addressing the large actuator delay in turbojet-powered UAVs, the paper develops Hex-Jet, a platform that combines uniaxial thrust vectoring and differential thrust to simplify attitude control. It adopts predictor-based time-delay control built on a frequency-domain model, comparing a Smith predictor and a $d$-step ahead state predictor; the Smith predictor shows superior disturbance rejection and robustness to modeling errors, especially under delays on the order of $h=25$ samples ($\approx0.1$ s). The authors validate the approach with a quadrotor-based experimental study and flight tests on a scaled Hex-Jet, achieving robust roll control with reduced overshoot and faster settling compared to a baseline. The work demonstrates a practical route to reliable jet-powered UAV attitude control and informs design choices for future full-scale demonstrations.

Abstract

Turbojet-powered VTOL UAVs have garnered increased attention in heavy-load transport and emergency services, due to their superior power density and thrust-to-weight ratio compared to existing electronic propulsion systems. The main challenge with jet-powered UAVs lies in the complexity of thrust vectoring mechanical systems, which aim to mitigate the slow dynamics of the turbojet. In this letter, we introduce a novel turbojet-powered UAV platform named Hex-Jet. Our concept integrates thrust vectoring and differential thrust for comprehensive attitude control. This approach notably simplifies the thrust vectoring mechanism. We utilize a predictor-based time delay control method based on the frequency domain model in our Hex-Jet controller design to mitigate the delay in roll attitude control caused by turbojet dynamics. Our comparative studies provide valuable insights for the UAV community, and flight tests on the scaled prototype demonstrate the successful implementation and verification of the proposed predictor-based time delay control technique.

Figures (11)

• Figure 1: (a). The scaled Hex-Jet and its coordinate system: blue arrows indicate the body frame and black arrows stand for the inertial frame. (b). 3D model of the Hex-Jet's parallelogram uniaxial thrust vectoring mechanism. (c). Schematic view of the scaled Hex-Jet. The numbers in the circle indicate the index of the turbojet. $L_x$ and $L_y$ stands for the distance between the turbojet and the center of gravity of the vehicle. (d). Hex-Jet differential thrust for pitch rotation. The blue and red arrows represent the magnitude and direction of thrust on the left and right sides, respectively. (e). Hex-Jet thrust vectoring for roll rotation. (f). Hex-Jet thrust vectoring for yaw rotation.
• Figure 2: The attitude controller diagram.
• Figure 3: System identification results, line with asterisk marker present frequency response from sweep data, solid line represent the fitted result from frequency response.
• Figure 4: Smith Predictor structure.
• Figure 5: State Predictor with Observer structure.