A Comprehensive Review of Tethered Hexacopter Sprayers: Dynamics, Control Strategies, and Integration for Steep-Terrain Agriculture

Abstract

The implementation of Unmanned Aerial Vehicles (UAVs) in agriculture has transitioned from monitoring to functional intervention. However, battery endurance and instability under dynamic payloads remain critical bottlenecks for industrial-scale adoption. This paper presents a systematic review of the evolution of agricultural UAVs, focusing on the emergence of tethered hexacopter systems (TKT-6) as a definitive solution for long-endurance missions in complex topographies, such as mangosteen orchards. We analyze 67 high-impact studies (2021–2025) covering high-voltage power transmission, slosh effect mitigation, and robust control architectures. The discussion emphasizes the transition from conventional PID to adaptive-robust controls, specifically Sliding Mode Control (SMC) and Model Reference Adaptive Control (MRAC). Synthesis of the reviewed literature highlights that the synergy between Disturbance Observers and SMC can reduce position errors to below 0.2 m despite tether tension. This review consolidates recent advances into a unified framework for TKT-6 readiness in precision agriculture.