A Test Bench for Evalulating Exoskeletons for Upper Limb Rehabilitation


Rehabilitation exoskeletons have proven to be useful in assisting clinicians in therapy and assisting users in daily tasks. While the potential of wearable robotics technology is undeniable, quantifying its value is difficult. As a result, performance in wearable robotics is becoming a pressing concern, and the scientific community requires reliable and repeatable testing methodologies to evaluate and compare the available exoskeletal systems. Various types of exoskeleton robots have already been developed and tested for upper limb rehabilitation. The problem is that evaluations are not standardized, particularly in pediatric rehabilitation.
This paper aimed to propose a methodology for the quantitative evaluation of upper limb exoskeletons that, like a test bench, would serve for replicable testing. This was accomplished by determining the range of motion (ROM) and joint torques using both kinematic modeling and experimental measurements (using sensors integrated into Dynamixel actuators, where ROM and joint torques were estimated from actuator feedback, respectively, in position and in load through the IDE Arduino).
The proposed test bench can provide an accurate range of motion (ROM) and joint torques during the pronation–supination task. The range of motion obtained with the 3D or physical prototype was approximately 156.26 ± 4.71° during the pronation–supination task, while it was approximately 146.84 ± 14.32° for the multibody model. The results show that the average range of experimental
torques (0.28 ± 0.06 N.m) was overestimated by 40% and just 3.4%, respectively, when compared to the average range of simulated torques (0.2 ± 0.05 N.m) and to the highest range of simulated torques (0.29 N.m). For the experimental measurements, test–retest reliability was excellent (α = 0.96-0.98) within sessions and excellent or good (α = 0.93 and α = 0.81-0.86) between sessions.
Finally, the suggested approach provides a range of motion close to the normal range of motion necessary during PS tasks. These results are important because they validate the measurements’ accuracy and underline the proposed methodology’s relevance. This study also confirms fluctuations in torque in human joints during motion and emphasizes the importance of considering these variations for precise quantification of joint torques by using the maximum value of estimated torques (rather than the average value).
To conclude, the proposed assessment procedure could become a reference standard for evaluating exoskeletons for the upper limb. This study also addresses a methodological aspect on the accurate assessment of joint torques that can serve in applications such as the sizing of actuators in exoskeletons or the non-invasive evaluation of muscle forces in the human body. In perspective, the concept will be expanded to additional joints, such as the elbow and wrist, to have a more complex assessment tool. Furthermore, future research will address the user’s safety by quantifying the kinematic coupling between the user and the device.
Keywords: upper limb rehabilitation, multibody modeling, exoskeleton evaluation, movement simulation, rehabilitation devices, motor sizing, wearable robotics.


Academic paper featuring our DYNAMIXEL AX-18A all-in-one smart actuators

All credit goes to Clautilde Nguiadem, Maxime Raison, and Sofiane Achiche of Polytechnique Montreal