Design and control of a parallel electromagnetic variable stiffness manipulator for robotic compliant grinding

Environmental adaptability is a key challenge in robotic operation, while the limited compliance of parallel manipulators hinders their application in variable-stiffness environments. This paper proposes a novel three-degree-of-freedom parallel electromagnetic variable stiffness manipulator (PEVSM) that actively adapts to the environment through self-stiffness modulation. PEVSM integrates an unconstrained variable stiffness limb driven by an electromagnetic spring and three compliant limbs actuated by Lorentz motors, enabling active and continuous stiffness modulation. Based on the established kinematic and stiffness models, a hybrid force-position-stiffness control framework is developed, integrating enhanced fractional-order adaptive impedance control and a stiffness controller based on deep deterministic policy gradient with multi-source feedback to achieve precise and compliant force regulation. For applying robotic grinding to low-stiffness workpieces, a force–deformation model and a force compensation strategy are introduced to mitigate deformation effects and improve material removal accuracy. The robotic grinding platform with PEVSM is constructed, demonstrating its advantages on improve force control and material removal accuracy in compliant grinding.