TITLE:
Design and Composite Disturbance-Rejection Control of an Underactuated End Effector for Apple Harvesting
AUTHORS:
Jianan Gao
KEYWORDS:
Apple Harvesting Robot, End Effector, Feedforward Compensation, Disturbance Rejection
JOURNAL NAME:
Agricultural Sciences,
Vol.17 No.9,
September
18,
2026
ABSTRACT: To improve the harvesting adaptability and motion-control performance of apple-harvesting end effectors in complex canopy environments, an underactuated enveloping-and-cutting end effector integrating fruit capture, stem cutting, and fruit collection was developed. The device employs a cylindrical fruit passage and a pair of symmetrically arranged swinging cutter heads actuated by traction wires and torsion springs, enabling the enveloping harvesting of apples of different sizes while reducing large-amplitude twisting and pulling motions. Considering the displacement-dependent elastic load generated by the torsion springs, a dynamic model of the electric drive system was established with the motor shaft as the reference. On this basis, a composite control strategy combining load-observer-based feedforward compensation with second-order active disturbance rejection control (ADRC) was developed. The Luenberger load-torque observer estimates the equivalent torsion-spring load online and generates the corresponding feedforward compensation term, while the extended state observer (ESO) estimates residual disturbances arising from model uncertainties and external disturbances for real-time compensation. MATLAB/Simulink simulations were conducted to compare closed-loop position control, elastic-load feedforward compensation, and the proposed composite control strategy and to evaluate its performance under time-varying external load disturbances. The results showed that the proposed strategy reached the 20 mm target position with a peak displacement of approximately 20.2 - 20.3 mm and an overshoot ratio of only 1.0% - 1.5%. The steady-state position error was maintained within approximately 0.1 - 0.2 mm, representing a reduction of about 85% compared with conventional closed-loop position control. Under imposed time-varying load torque disturbances, the maximum position deviation was approximately 0.4 mm, and the tracking error recovered to within ±0.05 mm in approximately 0.7 s. These results indicate that the proposed composite control strategy can effectively improve positioning accuracy, transient response, and disturbance-rejection performance under time-varying elastic loads and abrupt external disturbances.