Thomas Lens

Investigation of safety in human-robot-interaction for a series elastic, tendon-driven robot arm

This paper presents the design of the lightweight BioRob manipulator with spring-loaded tendon-driven actuation developed for safe physical human-robot interaction. The safety of the manipulator is analyzed by an analytical worst-case estimation of impact and clamping forces in the absence of collision detection. As intrinsic joint compliance can pose a threat by storing energy, a safety evaluation method is proposed taking the potential energy stored in the elastic actuation into account. The evaluation shows that the robot arm design constrains the worst case clamping forces to only 25 N, while being able to handle loads up to 2 kg, and inherits extremely low impact properties, such as an effective mass of less than 0.4 kg in non near-singular configurations, enabling safe operation even in case of high velocities. The results are validated in simulation and experiments.

Design and dynamics model of a lightweight series elastic tendon-driven robot arm

This paper presents the design of a lightweight robot arm intended for safe physical human-robot interaction. The robot arm design combines tendon actuation with elasticity in the tendons to achieve a significant reduction in mass and passive compliant behavior. The use of elastic tendons in all joints in order to gain maximum safety and performance properties, however, results in a significant increase of the model complexity with oscillatory behavior and kinematic coupling of the joint equilibrium positions. Therefore, special effort needs to be made to model the robot arm dynamics, which is an essential basis for model-based algorithms utilizing and fully exploiting the particular properties of the robot arm. This paper therefore derives the full dynamics model of the robot arm with focus on the nonlinear elastic tendon actuators, the kinematic tendon coupling, and modeling complexity reduction by reflecting all model parameters to the joint space. The resulting model is validated by comparing the identified simulation model with experimental data of an application-related pick-and-place trajectory and a trajectory with undamped oscillating motions of the robot arm.

Detailed dynamics modeling of BioBiped's monoarticular and biarticular tendon-driven actuation system

Bio-inspired, musculoskeletal design of bipedal robots offers great potential towards more human-like robot performance but imposes major challenges on their design and control, as it is challenging to analyze the contribution of each active and passive series elastic tendon to the overall joint, leg and robot dynamics. In this paper, detailed mathematical models of the tendon-driven, series elastically actuated mono- and biarticular structures of the BioBiped1 robot are presented. These enable a systematic analysis of the design space and characteristic curves as well as to derive guidelines for the design of improved prototypes. The derived models are applied to investigate the effects of the active and passive, mono- and biarticular structures on different performance criteria of 1D hopping motions by means of a detailed multi-body system dynamics simulation.

Dynamic modeling of the 4 DoF BioRob series elastic robot arm for simulation and control

This paper presents the modeling of the light-weight BioRob robot arm with series elastic actuation for simulation and controller design. We describe the kinematic coupling introduced by the cable actuation and the robot arm dynamics including the elastic actuator and motor and gear model. We show how the inverse dynamics model derived from these equations can be used as a basis for a position tracking controller that is able to sufficiently damp the oscillations caused by the high, nonlinear joint elasticity. We presents results from simulation and briefly describe the implementation for a real world application.

On the influence of elastic actuation and monoarticular structures in biologically inspired bipedal robots

Implementing the intrinsically compliant and energy-efficient leg behavior found in humans for humanoid robots is a challenging task. Control complexity and energy requirements are two major obstacles for the design of legged robots. Past projects revealed that the control complexity can be drastically reduced by designing mechanically intelligent systems with self-stabilization structures. Breaking through the latter obstacle can be achieved by the development and use of compliant actuators. Mechanical elasticity and its online adaptation in legged systems are generally accepted as the technologies to achieve human-like mobility. However, elastic actuation does not necessarily result in energy-efficient systems. We show that mechanical elasticity, although being worthwhile, can have negative effects on the performance of drives. We present a methodology that introduces both elasticity and energy-efficiency to a bipedal model. To this end, we report on the influence of monoarticular structures and demonstrate that these structures have the potential to both take us a step further toward the goal of realizing human-like locomotion and reduce the energy consumption.

Dynamic modeling of elastic tendon actuators with tendon slackening

This paper presents a new, detailed dynamics model of a novel type of actuators based on tendons with integrated springs that allows for offline adjustment of the stiffness characteristics. Like other cable or belt actuators, the elastic tendon actuator allows to radically reduce the link inertia by placing the motors near the robot base. But by additionally integrating springs in the tendons, the motor and the joint are elastically decoupled, which increases the lifespan of the tendons and the safety of the actuator. A detailed mathematical model of the actuator is derived taking tendon slackening effects into consideration. The result is a degressive stiffness curve that depends on the pretension force of the integrated tendon springs. The derived model is validated against static and dynamic experimental measurement data of a robot arm equipped with elastic tendon actuators.

Compliant robot actuation by feedforward controlled emulated spring stiffness

Existing legged robots lack energy-efficiency, performance and adaptivity when confronted with situations that animals cope with on a routine basis. Bridging the gap between artificial and natural systems requires not only better sensorimotor and learning capabilities but also a corresponding motion apparatus and intelligent actuators. Current actuators with online adaptable compliance pose high requirements on software control algorithms and sensor systems. We present a novel feedforward trajectory shaping technique that allows for a virtual stiffness change of a deployed series elastic actuator with low energy requirements. The performance limits of the approach are assessed by comparing to an active and a passive compliant methodology in simulation. For this purpose we use a 2-degrees-of-freedom arm with and without periodic load representing a 2- segmented leg with and without ground contact. The simulation results indicate that the approach is well suited for the use in legged robots.

The musculoskeletal system of the human arm — More than the sum of its parts

Biological systems show outstanding performance in the control of highly redundant and nonlinear systems. The complexity of these systems has raised questions about sufficient strategies of planning and controlling movements. Although many aspects of the musculoskeletal system, like nonlinear muscle properties, redundant actuation, and mechanically coupled joints, seem to make things more complicate from a purely technical point of view, this complexity has positive influence on the control. In this paper we show first aspects on how the nonlinear characteristics of the musculoskeletal system of the human arm may influence and even support the control of movements.