Dominic Lakatos

Nonlinear Oscillations for Cyclic Movements in Human and Robotic Arms

The elastic energy storage in biologically inspired variable impedance actuators (VIA) offer the capability of executing 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 multibody systems such as VIA or human arms with approximately equal joint damping, fulfill these requirements.

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

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.

Modal limit cycle control for variable stiffness actuated robots

This paper presents a control approach to stabilize limit cycle motions along a mechanical mode of variable stiffness actuated (VSA) robots. Thereby, first a PD controller with gravity and Coriolis/centrifugal compensation shapes a desired dynamics, which is decoupled in terms of modal coordinates. Then an asymptotically stable limit cycle is generated on the link side dynamics for a selected mode. Finally, the modal control approach first introduced for rigid robots is extended to the VSA case. This is done by a joint torque controller, which decouples the torque dynamics from the link side dynamics. Stability and convergence are proven for the dynamics resulting from each feedback control. Furthermore, the energy efficiency of the proposed approach is verified by simulation and experiments on the VSA robotic arm DLR Hand Arm System.

Nonlinear oscillations for cyclic movements in variable impedance actuated robotic arms

Biologically inspired Variable Impedance Actuators (VIA) offer the capability to execute cyclic and/or explosive multi degree of freedom (DoF) motions efficiently by storing elastic energy. This paper studies the preconditions which allow to induce robust cyclic motions for strongly nonlinear, underactuated multi DoF robotic arms. By experimental observations of human motor control, a simple 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 periodic behavior of the robotic arm is analyzed in simulations. It is found that a modal analysis of the linearized system at the equilibrium point allows to qualitatively predict the periodic behavior of this type of strongly nonlinear systems. The central statement of this paper is that cyclic motions can be induced easily in VIA systems, if the eigenfrequencies and modal damping values of the linearized system are well separated. Validation is given by simulation and experiments, where a human controls a simulated robotic arm, and the developed regulator controls a robotic arm in simulation and experiments.

The Grasp Perturbator: Calibrating human grasp stiffness during a graded force task

In this paper we present a novel and simple handheld device for measuring in vivo human grasp impedance. The measurement method is based on a static identification method and intrinsic impedance is identified inbetween 25 ms. Using this device it is possbile to develop continuous grasp impedance measurement methods as it is an active research topic in physiology as well as in robotics, especially since nowadays (bio-inspired) robotics can be impedance-controlled. Potential applications of human impedance estimation range from impedance-controlled telesurgery to limb prosthetics and rehabilitation robotics. We validate the device through a physiological experiment in which the device is used to show a linear relationship between finger stiffness and grip force.

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

Biped running can be conceptually reduced to a set of simple and quasi-independent tasks such as weight bearing, upper-body balancing, and energy injection through ankle push-off. We show in this paper that by appropriately designing multi-articular elastic actuators for biped robots in a manner inspired by human biomechanics, these tasks can be favorably expressed in a set of coordinates, in which the system is elastically decoupled. In these coordinates, the robot can be easily controlled by a set of simple and independent control laws. By exploiting the natural dynamics of the specially designed robot, the proposed controller requires only minimal model knowledge (mainly in terms of kinematic and static parameters) and is therefore robust to model uncertainties. It requires only state measurements and no measurement or model based computation of higher order state derivatives. Moreover, since the system is operated at a frequency dictated by the natural resonance, the running gait is energy efficient and resembles to a large extent natural human motion. Simulations validate the concept and demonstrate the independence of the approach from the knowledge of dynamics parameters.

Switching Based Limit Cycle Control for Compliantly Actuated Second-Order Systems

This paper derives a stability statement for a novel, switching based limit cycle control. The stability proof is based on multiple Lyapunov functions and a new interpretation of contraction analysis. By showing that the dissipated energy on the cycle increases with increasing velocity, while the injected energy is constant, the emergence of an attractive limit cycle is shown. The approach applies for general, nonlinear, and compliantly actuated second-order systems, with positive definite plant parameters and non-aperiodic solutions. An analysis of the controller parameters reveals, that for the majority of parameters, global attractiveness of the limit cycle can be guaranteed.

Jumping Control for Compliantly Actuated Multilegged Robots

A feedback control to generate jumping motions for compliantly actuated multilegged robots is proposed. The method allows to specify the direction of the jumping motion. This is achieved by a constraint that defines a one-dimensional submanifold and a bang-bang control which generates a limit cycle on this submanifold. The approach is based on classical impedance control with the difference that the stiffness on the submanifold and the force to preserve a predefined nominal body configuration result from the intrinsic mechanical springs in the joints. Furthermore, we propose two controller implementations: the first implementation does not require to detect the contact state, while the second implementation requires contact state detection, but accounts in addition for Coulomb friction constraints. The controller is validated in simulation with a compliantly actuated quadruped.

Conditioning vs. excitation time for estimating impedance parameters of the human arm

The human arms capability to alter its impedance has motivated multiple developments of robotic manipulators and control methods. It provides advantages during manipulation such as robustness against external disturbances and task adaptability. However, how the impedance of the arm is set depends on the manipulation situation; a general procedure is lacking. This paper aims to fill this gap by providing a method to estimate the impedance parameters of the human arm, while taking the numerical stability of the approach into account. A dynamic arm model and an identification method is presented. Confidential criteria to determine the accuracy of the estimated parameters are given. Finally, the procedure is validated in an experiment with a human subject and the results are discussed.

Dynamic Trajectory Generation for Serial Elastic Actuated Robots

Abstract Robotic systems can benefit from the introduction of properly chosen joint elasticity. Besides their robustness against rigid impact, the energy saving capabilities may increase the system dynamics. In this paper, a method applicable for robots with serial elastic joints is presented, which embodies a desired oscillatory behavior into the hardware and thereby leads to improved performance. This is achieved by shaping the flexible joint robot as a linear one-mode system and embodying the natural frequency of the real intrinsic behavior. An algorithm is presented for shaping the one-mode property and exciting the system via a negative definite damping term in a decoupled coordinate space. The output of the approach is a dynamic trajectory resulting in a coordinated link motion and synchronized transfer of kinetic and potential energy. Furthermore, the dynamic trajectory is commanded to the real robot via a motor PD controller, where asymptotic stability for both subsystems—i.e. the trajectory generator and the controlled robot—is proven. The method is validated on a two-link serial elastic actuated robot. Both, simulation and experiment confirm the eigenmode embodiment, energy efficiency by velocity enlargement between motor and link side motion, and synchronized joint motion.