An investigation of stiffness modulation limits in a pneumatically actuated parallel robot with actuation redundancy

Actuation redundancy in parallel mechanisms has been predominantly investigated assuming electromechanical actuators. Pneumatic actuation offers new alternatives to achieving stiffness modulation in parallel robots. This paper investigates the limits of stiffness modulation using two alternatives: modulation of joint-level stiffness using antagonistic pneumatic actuators (passive stiffness modulation) and stiffness modulation using antagonistic actuation at the structure level (active stiffness modulation). A surrogate translational parallel robot architecture, which is a variant of the Delta robot with one redundant kinematic chain, is used in this investigation. A simplified model of a pneumatic double-acting piston actuator is used to establish upper and lower bounds for attainable joint-level stiffness. A kinematic model of the parallel robot is presented along with its passive and active stiffness models. A simulation study is carried out assuming attainable joint-level stiffness based on the simplified pneumatic actuator model. The simulations show that, due to the achievable low pneumatic actuator stiffness, the active stiffness contribution can be as high as 65% of the passive stiffness within the robot workspace. This preliminary investigation suggests that pneumatic actuators are uniquely suited for robot interaction tasks (e.g. assembly or rehabilitation) where stiffness control and modulation can offer increased safety and task-specific end-effector stiffness.