The QuadSoft: Design, Construction, and Experimental Validation of a Soft and Actuated Quadrotor

Abstract

This paper presents QuadSoft, a novel fully actuated quadrotor equipped with continuous-curvature, tendon-driven soft robotic arms. The design combines a semi-rigid central frame with flexible arms, enabling controlled structural reconfiguration during flight without altering the propeller layout. Unlike existing soft aerial platforms that rely on discrete bending joints, QuadSoft utilizes a continuum deformation approach to modulate arm curvature, actively adjusting its thrust vector and aerodynamic characteristics. We characterize the geometric mapping between servomotor input and the resulting constant curvature, validating it experimentally. Outdoor flight tests demonstrate stable take-off, hover, directional maneuvers, and landing, confirming that controlled arm bending can generate horizontal displacement while preserving altitude. Measurements of pitch, roll, and curvature angles show that the platform follows intended actuation patterns with minimal attitude deviations. These results demonstrate that QuadSoft preserves the baseline stability of rigid quadrotors while enabling morphology-driven maneuverability, all under the standard PX4 autopilot without retuning. Beyond a proof of concept, this work establishes a distinctive outdoor validation of a tendon-driven continuum morphing quadrotor, opening a new research avenue toward adaptive aerial systems that combine the safety and versatility of soft robotics with the performance of conventional UAVs.

Figures (7)

• Figure 3: QuadSoft prototype during outdoor flight. The four arms are tendon-driven and flexible, actuated by servomotors to enable in-flight morphological adaptation.
• Figure 4: Schematic of the QuadSoft's flexible arm, showing the geometric variables used to calculate the bending angle $\beta$ as a function of the servo motor's rotation angle $\alpha$. The lengths $L_a$, $L_b$, $L_c$, and $L_3$ represent segments of the arm and its base, while $\beta$ describes the arm's curvature due to flexibility.
• Figure 5: Design and actuation of the QuadSoft. (a) Complete QuadSoft platform. (b) Soft arm with semi-rigid carbon fiber insert and air chambers for vibration damping. (c) In the graph, the geometric mapping between the servo input $\alpha$ and the arm curvature $\beta$ is shown in blue, while the cubic interpolation obtained using Eq.\ref{['eq6']} is shown as a red dashed line. (d) Photographs of the QuadSoft with arms at different bending angles. (e) Dual-cable tendon mechanism: one cable induces positive curvature, the other negative. This design ensures precise and reliable arm reconfiguration while balancing flexibility and structural stability.
• Figure 6: Electrical system diagram of the drone with a PX4 controller. It shows power distribution from a LiPo battery, motor control via ESCs, and servo connections through a distributor and UBEC. A capacitor reduces electrical noise, and the servos adjust tendon angles for morphing. The RC receiver enables remote control.
• Figure 7: Hover stability experiment with flexible arms. (a) 3D trajectory during take-off, hover, and landing (blue: actual, red dashed: setpoint); Background colors denote the flight modes (green: manual, yellow: hold), and the transition between modes is indicated in red. (b) Position (left) and velocity (right) tracking along $x,y,z$, showing smooth setpoint following across mode transitions.