A selective recruitment strategy for exploiting muscle-like actuator impedance properties

Two leading qualities of skeletal muscle that produce good performance in uncertain environments are damage tolerance and the ability to modulate impedance. For this reason, robotics researchers are greatly interested in discovering the key characteristics of muscles that give them these properties and replicating them in actuators for robotic devices. This paper describes a method to harness the redundancy present in muscle-like actuation systems composed of multiple motor units and shows that they have these same two qualities. By carefully choosing which motor units are recruited, the impedance viewed from the environment can be modulated while maintaining the same overall activation level. The degree to which the impedance can be controlled varies with total activation level and actuator length. Discretizing the actuation effort into multiple parts that work together, inspired by the way muscle fibers work in the human body, produces damage-tolerant behavior. This paper shows that this not only produces reasonably good resolutions without inordinate numbers of units, but gives the control system the ability to set the impedance along with the drive effort to the load

Bi-directional series-parallel elastic actuator and overlap of the actuation layers

Several robotics applications require high torque-to-weight ratio and energy efficient actuators. Progress in that direction was made by introducing compliant elements into the actuation. A large variety of actuators were developed such as series elastic actuators (SEAs), variable stiffness actuators and parallel elastic actuators (PEAs). SEAs can reduce the peak power while PEAs can reduce the torque requirement on the motor. Nonetheless, these actuators still cannot meet performances close to humans. To combine both advantages, the series parallel elastic actuator (SPEA) was developed. The principle is inspired from biological muscles. Muscles are composed of motor units, placed in parallel, which are variably recruited as the required effort increases. This biological principle is exploited in the SPEA, where springs (layers), placed in parallel, can be recruited one by one. This recruitment is performed by an intermittent mechanism. This paper presents the development of a SPEA using the MACCEPA principle with a self-closing mechanism. This actuator can deliver a bi-directional output torque, variable stiffness and reduced friction. The load on the motor can also be reduced, leading to a lower power consumption. The variable recruitment of the parallel springs can also be tuned in order to further decrease the consumption of the actuator for a given task. First, an explanation of the concept and a brief description of the prior work done will be given. Next, the design and the model of one of the layers will be presented. The working principle of the full actuator will then be given. At the end of this paper, experiments showing the electric consumption of the actuator will display the advantage of the SPEA over an equivalent stiff actuator

Potential merits for space robotics from novel concepts of actuation for soft robotics

Autonomous robots in dynamic and unstructured environments require high performance, energy efficient and reliable actuators. In this paper we give an overview of the first results of two lines of research regarding the novel actuation principle we introduced: Series-Parallel Elastic Actuation (SPEA). Firstly, we introduce the SPEA concept and present first prototypes and results. Secondly, we discuss the potential of self-healing materials in robotics, and discuss the results on the first self-healing pneumatic cell and selfhealing mechanical fuse. Both concepts have the potential to improve performance, energy efficiency and reliability

A muscle-like recruitment actuator with modular redundant actuation units for soft robotics

Human muscles contrast sharply with traditional robot actuators in that they consist of several motor units, connected in series and parallel, which can be progressively recruited. Some roboticists have explored this idea in robotic actuators, striving for improvements such as the ability to withstand partial damage, inexpensive repeatability by discrete open loop control and the potential of modular actuators. These systems, however, become rather complex or rely on less widely used actuation techniques such as piezo-actuators or SMAs to produce a compact implementation. This paper presents a novel design of a modular redundant actuation unit which can be combined in various combinations to form compliant actuators with varying characteristics. The actuation unit consists of discretely activated solenoids with an integrated compliant coupling. This paper presents the working principle and the physical implementation in detail. Failure of a single motor unit will merely lead to a loss in performance rather than failure of the actuator. Since each motor unit is discrete, neither power electronics nor control requires analog signals. Isometric experiments display the actuation characteristics and demonstrate the repeatability. The platform can be used in future work to further explore the virtues of exploiting discretization and redundancy in muscle-like control

Concept of a Series-Parallel Elastic Actuator for a Powered Transtibial Prosthesis

The majority of the commercial transtibial prostheses are purely passive devices. They store energy in an elastic element during the beginning of a step and release it at the end. A 75 kg human, however, produces on average 26 J of energy during one stride at the ankle joint when walking at normal cadence and stores/releases 9 J of energy, contributing to energy efficient locomotion. According to Winter, a subject produces on average of 250W peak power at a maximum joint torque of 125 Nm. As a result, powering a prosthesis with traditional servomotors leads to excessive motors and gearboxes at the outer extremities of the legs. Therefore, research prototypes use series elastic actuation (SEA) concepts to reduce the power requirements of the motor. In the paper, it will be shown that SEAs are able to reduce the power of the electric motor, but not the torque. To further decrease the motor size, a novel human-centered actuator concept is developed, which is inspired by the variable recruitment of muscle fibers of a human muscle. We call this concept series-parallel elastic actuation (SPEA), and the actuator consists of multiple parallel springs, each connected to an intermittent mechanism with internal locking and a single motor. As a result, the motor torque requirements can be lowered and the efficiency drastically increased. In the paper, the novel actuation concept is explained, and a comparative study between a stiff motor, an SEA and an SPEA, which all aim at mimicking human ankle behavior, is performed

Advances in Propulsive Bionic Feet and Their Actuation Principles

In the past decades, researchers have deeply studied pathological and nonpathological gait to understand the human ankle function during walking. These efforts resulted in the development of new lower limb prosthetic devices aiming at raising the 3C-level (control, comfort, and cosmetics) of amputees. Thanks to the technological advances in engineering and mechatronics, challenges in the field of prosthetics have become an important source of interest for roboticists. Currently, most of the bionic feet are still on a research level but show promising results and a preview of tomorrow's commercial prosthetic devices. In this paper, the authors present the current state-of-the-art and the latest advances in propulsive bionic feet with its actuation principles. The context of this review study is outlined followed by a brief description of the basics in human biomechanics and criteria for new prosthetic designs. A new categorization based on the actuation principle of propulsive ankle-foot prostheses is proposed. Based on simulations, the general principles and benefits of each actuation method are explained. The corresponding latest advances in propulsive bionic feet are presented together with their main characteristics and scientific outcomes. The authors also propose to the reader a comparison analysis of the presented devices with a discussion of the general tendencies in new prosthetic feet

A Two-Degree of Freedom Variable Stiffness Actuator Based on the MACCEPA Concept

The current state-of-the-art of variable stiffness actuators consists mostly of different concepts for single-degree of freedom joints. However, in bio-inspired robotic applications, multiple degrees of freedom variable stiffness actuators are often desired. Currently, this is usually achieved by cascading single-degree of freedom actuators. The innovation presented in this work is a two-degree of freedom variable stiffness actuator using the mechanically adjustable and controllable equilibrium position actuator (MACCEPA) concept. The presented actuator is not a cascade of two single-degree of freedom actuators, but centralizes the two degrees of freedom in one single joint. Equilibrium position and stiffness of the actuator are, furthermore, independently controllable in both degrees of freedom. The design and experimental validation of the actuator are discussed in this work. The independence of adjusting the equilibrium position and stiffness of the actuator are experimentally validated. The results show that the measured characteristics of the actuator sufficiently match the theoretically calculated ones. Future work includes implementing the presented two-degree of freedom actuator in an application, like a bipedal robot or a robotic arm

Novel lockable and stackable compliant actuation unit for modular +SPEA actuators

On compliant robotic systems, modularity is recently adopted for the ease of up- and downscaling and the possibility to downgrade the costs, by moving towards the combination of standard units instead of custom designs. However, modularity on the actuator level itself lacks more thorough evaluation. We have developed a novel lockable and stackable compliant actuation unit which can be used to form modular series-parallel elastic actuators (+SPEA). This paper describes the modular +SPEA layer architecture and discusses its two-way overrunner and rubber springs in detail, while providing experimental validation on each component as well. First experiments show the layer can deliver up to 20 Nm. Finally, we present how a manipulator can be equipped with the modular +SPEA layers

Benchmarking Human Likeness of Bipedal Robot Locomotion: State of the Art and Future Trends

The difficulty of defining standard benchmarks for human likeness is a well-know problem in bipedal robotics. This chapter reviews methods and criteria for the assessment of the sensorimotor mechanisms involved in human walking and posture. We focused on the potential of the reviewed methods to be used as benchmarks for human-like locomotion of bipedal robots. For walking conditions, several criteria and methods related to dynamic similarity, passivity and dynamicity, static stability, and energy consumption have been identified. As for standing functions, we identified the most relevant features characterizing the human postural sensorimotor mechanisms, and presented the experimental protocols currently used to evaluate the human-like robotic performance. Furthermore, we discussed how the current robotic competitions such as RoboCup and DARPA Robotics Challenges can contribute to the identification of relevant benchmarks. Finally, we speculated about the importance of international consensus on the quantitative definition of human likeness, and suggested some future actions for improving collaboration and standardization within the scientific community.This research activity has been founded by the European Seventh Framework Programme FP7-ICT-2011-9, under the grant agreement no 60069 - H2R “Integrative Approach for the Emergence of Human-like Robot Locomotion”