A modally adaptive control for multi-contact cyclic motions in compliantly actuated robotic systems

Abstract

Compliant actuators in robotic systems improve robustness against rigid impacts and increase the performance and efficiency of periodic motions such as hitting, jumping and running. However, in the case of rigid impacts, as they can occur during hitting or running, the system behavior is changed compared to free motions which turns the control into a challenging task. We introduce a controller that excites periodic motions along the direction of an intrinsic mechanical oscillation mode. The controller requires no model knowledge and adapts to a modal excitation by means of measurement of the states. We experimentally show that the controller is able to stabilize a hitting motion on the variable stiffness robot DLR Hand Arm System. Further, we demonstrate by simulation that the approach applies for legged robotic systems with compliantly actuated joints. The controlled system can approach different modes of motion such as jumping, hopping and running, and thereby, it is able to handle the repeated occurrence of robot-ground contacts.

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

“…It can be interpreted as a state machine that changes finite states triggered by events. These events are only functions of the continuous system states and therefore this concept further supports the idea of exploiting the natural dynamics of the plant [17]. In the following, we present the control framework exemplary for a specific running controller.…”

“…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.…”

“…It combines a task-oriented system design and a "minimally invasive" control approach. A main requirement of the controller design is robustness against parameter uncertainties and measured signal noise, which can be achieved by (adaptive) methods exciting the intrinsic plant dynamics [17], but which in turn require the plant to fit to the dynamics of the task.…”

Energy-based and biomimetic robotics

“…Our approach is to use the motor to create a virtual spring with negative hysteresis, similar to [82]: we create the hysteresis not by switching the equilibrium position of the spring, but by changing the virtual spring's stiffness, as illustrated in Figure 3.6. The energy injected is the area enclosed by the curve and is thus directly dependent on the amount of hysteresis.…”