Exploiting Variable Stiffness in Explosive Movement Tasks

Abstract

Abstract-It is widely recognised that compliant actuation is advantageous to robot control once high-performance, explosive tasks, such as throwing, hitting or jumping are considered. However, the benefit of intrinsic compliance comes with high control complexity. Specifically, coordinating the motion of the system through a compliant actuator and finding a task-specific impedance profile that leads to better performance is non-trivial. Here, we utilise optimal control to devise time-varying torque and stiffness profiles for highly dynamic movements in compliantly actuated robots. The proposed methodology is applied to a ballthrowing task where we demonstrate that: (i) the method is able to tailor impedance strategies to specific task objectives and system dynamics, (ii) the ability to vary stiffness leads to better performance in this class of movements, (iii) in systems with variable physical compliance, our methodology is able to exploit the energy storage capabilities of the actuators. We illustrate these in several numerical simulations, and in hardware experiments on a device with variable physical stiffness.

Energy-efficacy comparisons and multibody dynamics analyses of legged robots with different closed-loop mechanisms

“…The conventional definition requires the summation of the absolute power consumption ignoring the negativity, called 'Type 1' [55], while the summation of the power consumption only if it is positive is called 'Type 2' [50,52]. Depending on the type of an animal, except small size insects, biological mechanisms potentially have a capability to store elastic strain energy in muscle and other tissues under ideal conditions [4,5], and robots with elastic materials were designed to maximize the capability [11,12]. The details will be discussed in Sect.…”

“…A famous hopping robot, Gregorio's ARL monopod I [24], which was a 15 kg planar one-legged robot, and an ARL monopod II [2], which was a 18 kg planar one-legged running robot with hip and leg compliance, were included, and their specific resistances were similar, ∈ [0. 2,11], to the three closed-loop mechanisms, even though the mechanism for the hopping movement was quite different from the controlled passive dynamic running strategy.…”

“…The evidence indicates that the same mechanism can commonly be introduced to closed-loop linkages. Another possible option of the energy regeneration of the energy is energy storage with passive compliance, which is obtained from spring and elastic materials [11]. The property of the energy storage is known in the artificial legs made of elastic materials, such as carbon fiber prostheses [65].…”

Exploiting variable physical damping in rapid movement tasks

“…The benefits of such actuators include high dynamic range (e.g. due to the ability to store energy in spring-like actuators) [4] and a stable and fast response (since compliance is built into the actuator mechanically, sensory feedback is not required to respond to perturbations).…”

“…For example, optimal control approaches have been shown to be highly effective in exploiting the elastic properties of variable stiffness actuators in explosive tasks such as throwing and hitting [4], [7] as well as in periodic tasks [8], [9], [10]. In addition, stochastic optimal control with model adaptation has also been exploited in order to cope with model uncertainty and perturbations [11], [12], [13].…”