Robot-assisted rehabilitation devices can provide intensive and precise task-based training that differs from clinician-facilitated manual therapy. However, industrial robots are still rarely used in rehabilitation, especially in bilateral exercises. The main purpose of this research is to develop and evaluate the functionality of a bilateral upper-limb rehabilitation system based on two modern industrial robots. A ‘patient-cooperative’ control strategy is developed based on an adaptive admittance controller, which can take into account patients’ voluntary efforts. Three bilateral training protocols (passive, active, and self) are also proposed based on the system and the control strategy. Experimental results from 10 healthy subjects show that the proposed system can provide reliable bilateral exercises: the mean RMS values for the master error and the master-slave error are all less than 1.00 mm and 1.15 mm respectively, and the mean max absolute values for the master error and the master-slave error are no greater than 6.11 mm and 6.73 mm respectively. Meanwhile, the experimental results also confirm that the recalculated desired trajectory can present the voluntary efforts of subjects. These experimental findings suggest that industrial robots can be used in bilateral rehabilitation training, and also highlight the potential applications of the proposed system in further clinical practices.

Introduction

Stroke is one of the leading causes of disability [1], with many stroke survivors suffering from complications such as paralysis of the limbs on one side of the body, or difficulty speaking and understanding words [2]. About 50% of them will suffer from upper-limb disabilities after the subacute stage (6 months from stroke onset), limiting their abilities to perform Activities of Daily Living (ADLs) [3]. This affects survivors and their families physically, emotionally, financially and socially. Bilateral upper-limb training has been widely researched as a new rehabilitation intervention in clinical practices [4]–[۶]. According to the theory of neuroplasticity, neural networks in a damaged hemisphere could be reconstructed by an undamaged hemisphere through symmetrical movements [7], [8]. The possibility of voluntary muscle contractions in an affected limb could also be increased via the activation of the primary motor cortex and the supplementary motor area of an unaffected limb during symmetrical movements [9]. Compared to traditional manual therapy, robot-assisted training has been developed as an effective rehabilitation intervention which can provide precise training for a sufficiently long timeframe, regardless of physiotherapists’ experience and fatigue level [10], [11]. A similar trend has been occurring amongst bilateral upper-limb robots/systems.