Nonlinear Oscillations for Cyclic Movements in Human and Robotic Arms

Abstract

The elastic energy storages in biologically inspired Variable Impedance Actuators (VIA) offer the capability to execute cyclic and/or explosive multi degree of freedom (DoF) motions efficiently. This paper studies the generation of cyclic motions for strongly nonlinear, underactuated multi DoF serial robotic arms. By experimental observations of human motor control, a simple and robust control law is deduced. This controller achieves intrinsic oscillatory motions by switching the motor position triggered by a joint torque threshold. Using the derived controller, the oscillatory behavior of human and robotic arms is analyzed in simulations and experiments. It is found that the existence of easily excitable oscillation modes strongly depends on the damping properties of the plant. If the intrinsic damping properties are such that oscillations excited in the undesired modes decay faster than in the desired mode, then multi-DoF oscillations are easily excitable. Simulations and experiments reveal that serially structured, elastic multi-body systems such as VIA or human arms with approximately equal joint damping, fulfill these requirements.

Soft Robotics

“…In our recent works [54], [55], [56], [57], [58], [58], novel concepts to exploit these natural dynamics have been derived. Especially in case of cyclic motion tasks, the capability to buffer and release elastic energy may reduce the size and weight of required actuators and save a substantial amount of energy.…”

“…For the maximally efficient controller parameters ≤ 2ϵ, this controller generates a globally asymptotically stable limit cycle without any model knowledge, only based upon measurements of the spring deflection [58].…”

Design and control of compliantly actuated bipedal running robots: Concepts to exploit natural system dynamics

“…The locomotion control concept is based on biomechanics insights, special kinematics design, and our recently proposed limit cycle control method as introduced in [15], [17], [18], [20]. The controller explained next is an overall result of the system design, the task-oriented coordinate transformation, and our recently introduced model-free control concepts.…”

“…In our recent work [15], [16], [17], [18], [19], [20], we developed concepts to excite the intrinsic oscillatory dynamics of compliantly actuated mechanical systems [21], [22] for cyclic tasks. The control approaches apply to systems in which balancing is of minor importance such as multi-legged hopping robots and thereby the focus is on exploiting the natural dynamics of the plant.…”

Damping control of variable damping compliant actuators

“…Here, the linear modeling of the system was feasible since continuous change in velocity was not taken into account. However, as is the case for robots with compliant joints, which can have highly dynamic motions [23], it is essential to consider continuous variations in velocity as well as changes in friction parameters arising from wearing and heating. Fig.…”