Elastic Structure Preserving (ESP) Control for Compliantly Actuated Robots

Abstract

Physical compliance can be considered one of the key technical properties a robot should exhibit to increase its mechanical robustness. In addition, the accompanying temporal energy storing capabilities enable explosive and energy efficient cyclic motions. But these advantages come at a price, as compliance introduces unwanted intrinsic oscillatory dynamics, underactuation, and reduces the natural frequency of the plant. These aspects make control of the link configuration variables a challenging task. This work presents two novel control methods for implementing link-side motion tracking capabilities and injecting a desired damping characteristic to suppress link vibrations along the reference trajectory for compliantly actuated robots with nonlinear elastic characteristics. We prove their uniform global asymptotic stability by invoking a theorem by Matrosov. Both approaches, namely ESP and ESP+, have in common that they preserve the link-side inertial properties and the elastic structure of the original plant dynamics, hence the name Elastic Structure Preserving control. Apart from that, ESP control focuses on preserving the inertial properties of motor dynamics. While ESP+ control aims at minimizing the dynamic shaping on the motor side. The performance of the feedback control laws have been evaluated on the variable stiffness robot arm DLR Hand Arm System, where the stiffness in each of its joints is highly nonlinear. To the best of our knowledge, this is the first experimentally validated tracking controller for compliantly actuated, multi-joint robots with nonlinear elastic elements.

Passivity Controller Based on Load-Side Damping Assignment for High Stiffness Controlled Series Elastic Actuators

“…In order to render the damping directly at the load port, both the dynamic model and passivity have to be taken into account in the controller design. Recently, the elastic structure preserving (ESP) control has been developed to passively add the desired force on the load side [28]- [30]. The ESP preserves the intrinsic inertia and spring properties of the system while adding the desired force on the load side.…”

Flexible Robots

“…A first example is the gravity cancelation method of De Luca and Flacco (2010), which makes a robot having joints with (constant or variable) flexibility that moves under gravity feedback equivalent to the same robot without gravity. Such an elastic structure preserving concept has been expanded by introducing a number of similar control schemes, e.g., for link damping injection or trajectory tracking (Keppler et al 2018). Recent results along this line have been obtained also for a class of soft continuous robot with distributed flexibility.…”

Control Architecture for Human-Like Motion With Applications to Articulated Soft Robots

“…In Zhakatayev et al ( 2017 ) an optimization framework to minimize time performance is proposed. In Keppler et al ( 2018 ) the Authors propose a controller to achieve motion tracking while preserving the elastic structure of the system and reducing the link oscillations. On the other hand, model-free algorithms are promising, but usually require long-lasting learning procedures and face generality issues (Angelini et al, 2018 ; Hofer et al, 2019 ).…”