Performance assessment of fractional order PID control for a 2DOF flexible joint robotic manipulator under various operating conditions

Abstract

Flexible joint robot manipulators (FJM) exhibit high nonlinear dynamics and coupling effects, which make their control a challenging task. This paper proposes a fractional order PID (FOPID) cascade control strategy for a 2DOF FJM. The controller performance is compared with that of a sliding mode controller (SMC) under identical operating conditions. Both controllers’ parameters are optimally tuned using the particle swarm optimization (PSO) algorithm to ensure a fair performance evaluation. The proposed control consists of an inner loop that regulates motor dynamics and an outer loop that ensures accurate link position tracking. Additionally, a gravity compensator is integrated to improve control efficiency by reducing the nonlinear load on the controller. Both controllers are evaluated under nominal conditions, external disturbances, and variable payloads. Numerical results show a performance trade-off: under nominal conditions, FOPID achieved superior tracking performance (ITAE = 0.1165 for link 1) compared to SMC (ITAE = 0.2423), while SMC provided a smooth, zero-overshoot response and consumed much less actuator energy (ISCE = 23.35 vs. FOPID's 154.78). Furthermore, when subjected to a severe 1.0 N.m external disturbance, FOPID showed superb robustness by maintaining an ITAE of 0.178, whereas SMC suffered severe performance degradation. These findings illustrate the trade-offs between energy efficiency and robust precision in FJM control.