Variable admittance time-delay control of an upper limb rehabilitation robot based on human stiffness estimation

•A variable admittance time-delay cooperative controller is developed for upper limb rehabilitation training.•An iterative optimization algorithm is proposed to estimate human arm stiffness and adjust interaction compliance.•The training trajectory is determined based on minimum jerk cost principle to ensure smoothness and continuity.•The effectiveness is validated via trajectory tracking experiments with variable admittance regulation.