Technical session talks from ICRA 2012
TechTalks from event: Technical session talks from ICRA 2012
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Embodied Intelligence - Complient Actuators
A Versatile Biomimetic Controller for Contact Tooling and Tactile ExplorationThis article presents a versatile controller that enables various contact tooling tasks with minimal prior knowledge of the tooled surface. The controller is derived from results of neuroscience studies that investigated the neural mechanisms utilized by humans to control and learn complex interactions with the environment. We demonstrate here the versatility of this controller in simulations of cutting, drilling and surface exploration tasks, which would normally require different control paradigms. We also present results on the exploration of an unknown surface with a 7-DOF manipulator, where the robot builds a 3D surface map of the surface profile and texture while applying constant force during motion. Our controller provides a unified control framework encompassing behaviors expected from the different specialized control paradigms like position control, force control and impedance control.
Passive Impedance Control of a Multi-DOF VSA-CubeBot ManipulatorThis work presents an example of the application of passive impedance control of a variable stiffness manipulator, which shows the actual benefits of variable stiffness in rejecting disturbances without resorting to the closure of a high level feedback loop. In the experiment a 4-DOF manipulator arm, built with the VSA-CubeBot platform, is controlled to hold a pen and draw a circle on an uneven surface. The control is designed calculating joint and stiffness trajectories with a Cartesian approach to the problem, thus designing the optimal workspace stiffness at first. Then, the joint stiffness yielding the closest workspace stiffness is searched for. Experimental results are reported, which agree with the theoretical outcomes, showing that the sub-optimal joints stiffness settings allow the arm to follow the circular trajectory on the uneven surface at best.
Optimality Principles in Stiffness Control: The VSA KickThe importance of Variable Stiffness Actuators (VSA) in safety and performance of robots has been extensively discussed in the last decade. It has also been shown recently that a VSA brings performance advantages with respect to common actuators. For instance, the solution of the optimal control problem of maximizing the speed of a VSA for impact maximization at a given position with free final time is achieved by applying a control policy that synchronizes stiffness changes with link speed and acceleration. This problem can be regarded as the formalization of the performance of a soccer playerâ€™s free kick. In this paper we revisit the impact maximization problem with imposing a new constraint: we want to maximize the velocity of the actuator link at a given position and fixed terminal time - applicable e.g. to maximize performance of a first-time kick. We first study the problem with fixed stiffness and show that under realistic modeling assumptions, there does exist an optimal linear spring for a given link inertia, final time and motor characteristics. Results are validated with experimental tests. We then study optimal control of VSA and show that varying the spring stiffness during the execution of the kick task substantially improves the final speed.
Optimal Control for Exploiting the Natural Dynamics of Variable Stiffness RobotsIn contrast to common rigid or actively compliant systems, Variable Stiffness Arms are capable of storing potential energy in their joint and convert it into kinetic energy, respectively speed. This capability is well known from humans and is a good example for the outstanding performance of biological systems. However, only since some years intrinsic compliance is considered as a key feature and not a drawback in robot design. Therefore, only very little work has been carried out for exploiting the natural dynamics of elastic arms for such explosive motion sequences. In this paper, we treat the problem of how to optimally achieve maximum link velocity at a given final time for Variable Stiffness Arms. We show that solutions to this problem lead to excitation motions, which enable the robot to move on the link side at much higher speed on the motor side. In particular, the robot uses the dynamic transfer of elastic joint energy into link side kinetic energy for further acceleration. In our work we consider the practically relevant input and state constraints, and give experimental verification of the developed methods on the new DLR Hand-Arm system.
The vsaUT-II: A Novel Rotational Variable Stiffness ActuatorIn this paper, the vsaUT-II, a novel rotational variable stiffness actuator, is presented. As the other designs in this class of actuation systems, the vsaUT-II is characterized by the property that the output stiffness can be changed independently of the output position. It consists of two internal elastic elements and two internal actuated degrees of freedom. The mechanical design of the vsaUT-II is such that the apparent output stiffness can be varied by changing the transmission ratio between the elastic elements and the output. This kinematic structure guarantees that the output stiffness can be changed without changing the potential energy stored internally in the elastic elements. This property is validated in simulations with the port-based model of the system and in experiments, through a proper control law design, on the prototype.
pVEJ: A Modular Passive Viscoelastic Joint for Assistive Wearable RobotsIn complex dynamical tasks human motor control notably exploits the possibility of regulating joints mechanical impedance, both for stability and for energetic optimization purposes. These biomechanical findings should translate in design requirements for wearable robotics joints, which are required to produce adaptable intrinsic viscoelastic behaviors. This paper describes the design of a purely mechanical, rotary, passive ViscoElastic Joint (pVEJ), functionally equivalent to a torsional spring connected in parallel to a rotary viscous damper. The device has a modular design, which allows to modify the stiffness characteristics by replacing cam profiles. Damping coefficient can be also regulated off-line, manually acting on a valve. Prototype performances are characterized using a custom-developed dynamometric test-bed. Results demonstrate the capability of the system to render both the desired stiffness and damping values, in a range of impedance and peak torque compatible to that of wearable robotics for gait assistance.