During the last decade, the usage of robotic arms for passive rehabilitation of post-stroke patients has been gaining relevance, considering how it can assist medical personal to carry out their tasks properly. However, when it comes to controlling these kinds of devices, it normally has to deal with Non-Linearities resulting from the mathematical model of robotic arms, which is the case for a three -degrees of freedom (3DoF) articulated arm. Besides, while treating real patients, eventual uncertainties and constraints must be considered, as well as proper reduction of error signals.For the given case of study, a robust control strategy might deliver the best results as it considers uncertainties as well as disturbances under constraints. Therefore, the control strategy implemented for the 3DOF robotic arm was Sliding Mode Control (SMC), as it is robust and appropriate for this kind of dynamic system and dealing with non-linearities, allowing thus a wide operation range.The mathematical model of the 3DOF robotic arm was derived from the Lagrangian formulation, which is based on the energy equations used for checking stability criteria. As it is for the non-linear dynamics model as well as the non-linear controller design, the&nbsp;&nbsp; Sliding Mode Control approach was applied, and handle constraints were implemented in the Simulink environment. The whole dynamic system must be adaptable to several physical parameters, such as passive compensation works differently depending on the patient's arm's physical parameters and the amount of potential energy that is to be compensated.&nbsp; Some restrictions need to be set, considering that the range of motion of the human arm is not the same as a conventional robotic arm which can cover an entire spherical working space. The Sliding Model Control was designed using a scheduling switch and to solve the chattering problem a solution based on the limit region was used.As a part of the Sliding Mode Control design, a definition of the Sliding Surfaces “which affects the implemented switch function directly” is to be discussed. Recent studies and investigation projects have shown that traditional Sliding Surfaces based on the error signals might lead to higher chattering effects due to an undesired coupling of the input signal as well as the presence of discontinuous functions. Thus, further simplification of the control modeling might be considered as they may also increase stability.Finally, all movement restrictions need to be included in the Simulink model, and the stability criteria also need to be set by recurring to Lyapunov functions based on the mechanical energy of the system.

On the Dynamic and Control for a Three Degree-of-Freedom Robotic Arm Used for Rehabilitation Purposes in Medicine

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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