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

Conversion of sub-tropical native vegetation to introduced conifer forest: Impacts on below-ground and above-ground carbon pools

Land-use change can have a major influence on soil organic carbon (SOC) and above-ground C pools. We assessed a change from native vegetation to introduced Pinus species plantations on C pools using eight paired sites. At each site we determined the impacts on 0–50 cm below-ground (SOC, charcoal C, organic matter C, particulate organic C, humic organic C, resistant organic C) and above-ground (litter, coarse woody debris, standing trees and woody understorey plants) C pools. In an analysis across the different study sites there was no significant difference (P > 0.05) in SOC or above-ground tree C stocks between paired native vegetation and pine plantations, although significant differences did exist at specific sites. SOC (calculated based on an equivalent soil mass basis) was higher in the pine plantations at two sites, higher in the native vegetation at two sites and did not differ for the other four sites. The site to site variation in SOC across the landscape was far greater than the variation observed with a change from native vegetation to introduced Pinus plantation. Differences between sites were not explained by soil type, although tree basal area was positively correlated with 0–50 cm SOC. In fact, in the native vegetation there was a significant linear relationship between above-ground biomass and SOC that explained 88.8% of the variation in the data. Fine litter C (0–25 mm diameter) tended to be higher in the pine forest than in the adjacent native vegetation and was significantly higher in the pine forest at five of the eight paired sites. Total litter C (0–100 mm diameter) increased significantly with plantation age (R2 = 0.64). Carbon stored in understorey woody plants (2.5–10 cm DBH) was higher in the native vegetation than in the adjacent pine forest. Total site C varied greatly across the study area from 58.8 Mg ha−1 at a native heathland site to 497.8 Mg ha−1 at a native eucalypt forest site. Our findings suggest that the effects of change from native vegetation to introduced Pinus sp. forest are highly site-specific and may be positive, negative, or have no influence on various C pools, depending on local site characteristics (e.g. plantation age and type of native vegetation)

An energy efficiency index for elastic actuators during resonant motion

The energetic advantages of series and parallel elastic actuators have been characterized in the literature considering different elastic systems and different tasks. These characterizations usually determine the energy consumption of a specific system during a specific task and generalize poorly. This paper proposes an energetic characterization of elastic actuators, following an analytical approach, rather than a data-driven one. In particular, this work analyzes the energy consumption of elastic actuators during resonant motion and introduces a novel efficiency index. This index characterizes energy consumption as a function of inherent actuator parameters only, generalizing over the specific tasks. The proposed analysis is validated using simulations and experiments, demonstrating its coherence with analytical results

Comparison of drivetrain topologies : an ankle gait cycle

Electric actuation is used in a wide variety of applications and fields. One of the fields is prosthesis actuation where energy efficiency and mass are important characteristics. A lot of applications within this field (e.g. knee prosthesis, ankle prosthesis,...) require a high torque at low speed and a low torque at high speed. These motion requirements are difficult to effectively combine into a single electric motor. A possibility is to use an oversized direct--drive motor that can deliver the peak torque. However, this results in a heavy actuation system and low overall efficiency. A common practice is combining a smaller electric motor with a gear reduction system. In literature, novel redundant electric actuation systems were presented with or without locking mechanisms. In this work, multiple electrical actuation systems composed of electric motors, gear reduction systems and locking mechanisms are compared to each other. This is done using the gait cycle of a human ankle as a case study

Comparison of Drivetrain Topologies for an Ankle Gait Cycle

Powered prosthesis actuation is a field where energy efficiency and mass are important characteristics. The motion requirements of high torque at low speed and low torque at high speed are difficult to effectively combine in a single electric motor. A possibility is to use an oversized direct–drive motor that can deliver the peak torque. However, this results in a heavy actuation system and low overall efficiency. A common practice is combining a smaller electric motor with a gear reduction system. In the literature, novel redundant electric actuation systems have been presented with or without locking mechanisms to mitigate this problem. In this work, we provide a comparison of multiple electrical actuation systems composed of electric motors, gear reduction systems and locking mechanisms. This is done using the gait cycle of a human ankle as a case study. An electric motor with a double stator and a single rotor combined with a gearbox shows the most promising overall results when taking energy loss, total mass and complexity of the system into account

Minimizing torque requirements in robotic manipulation through elastic elements optimization in a physics engine

The increasing number of robots and the rising cost of electricity have spurred research into energy-reducing concepts in robotics. One such concept, elastic actuation, introduces compliant elements such as springs into the robot structure. This article presents a comparative analysis between two types of elastic actuation, namely, monoarticular parallel elastic actuation and biarticular parallel elastic actuation, and demonstrates an end-to-end pipeline for their optimization. Starting from the real-world system identification of an RRR robotic arm, we calibrate a simulation model in a general-purpose physics engine and employ in silico evolutionary optimization to co-optimize spring configurations and trajectories for a pick-and-place task. Finally, we successfully transfer the in silico optimized elastic elements and trajectory to the real-world prototype. Our results substantiate the ability of elastic actuation to reduce the actuators’ torque requirements heavily. In contrast to previous work, we highlight the superior performance of the biarticular variant over the monoarticular configuration. Furthermore, we show that a combination of both proves most effective. This work provides valuable insights into the torque-reducing use of elastic actuation and demonstrates an actuator-invariant in silico optimization methodology capable of bridging the sim2real gap

Minimizing torque requirements in robotic manipulation through elastic elements optimization in a physics engine

The increasing number of robots and the rising cost of electricity have spurred research into energy-reducing concepts in robotics. One such concept, elastic actuation, introduces compliant elements such as springs into the robot structure. This article presents a comparative analysis between two types of elastic actuation, namely, monoarticular parallel elastic actuation and biarticular parallel elastic actuation, and demonstrates an end-to-end pipeline for their optimization. Starting from the real-world system identification of an RRR robotic arm, we calibrate a simulation model in a general-purpose physics engine and employ in silico evolutionary optimization to co-optimize spring configurations and trajectories for a pick-and-place task. Finally, we successfully transfer the in silico optimized elastic elements and trajectory to the real-world prototype. Our results substantiate the ability of elastic actuation to reduce the actuators’ torque requirements heavily. In contrast to previous work, we highlight the superior performance of the biarticular variant over the monoarticular configuration. Furthermore, we show that a combination of both proves most effective. This work provides valuable insights into the torque-reducing use of elastic actuation and demonstrates an actuator-invariant in silico optimization methodology capable of bridging the sim2real gap

Introduction of a redundant actuator using planetary gear trains for human centred robotics

Matching motor efficiency and performance with the load demands can significantly improve the overall efficiency of a driveline. Inspired by the automotive sector -with the high interest of hybrid and electric cars currently-, the authors have studied how state of the art technologies can be used in the relatively new field of collaborative and Human centred robotics. Multiple transmission systems have been considered, among others redundant actuators (both static and kinematic) and continuously variable transmissions. Based on these findings and the experience of the research group on customised planetary gear trains for Human Limb Assistance and Replication, an extensive review of existing redundant actuators is presented in combination with an alternative transmission system which does not need any auxiliary gear transmissions and hence can be lighter and more compact than state of the art drivelines for Human centred robotics. A calculation was performed -including the efficiency model presented by Müller- which shows the high potential of this type of dual-motor actuator

Introduction of a redundant actuator using planetary gear trains for human centred robotics

Matching motor efficiency and performance with the load demands can significantly improve the overall efficiency of a driveline. Inspired by the automotive sector -with the high interest of hybrid and electric cars currently-, the authors have studied how state of the art technologies can be used in the relatively new field of collaborative and Human centred robotics. Multiple transmission systems have been considered, among others redundant actuators (both static and kinematic) and continuously variable transmissions. Based on these findings and the experience of the research group on customised planetary gear trains for Human Limb Assistance and Replication, an extensive review of existing redundant actuators is presented in combination with an alternative transmission system which does not need any auxiliary gear transmissions and hence can be lighter and more compact than state of the art drivelines for Human centred robotics. A calculation was performed -including the efficiency model presented by Müller- which shows the high potential of this type of dual-motor actuator

Factors influencing actuator’s backdrivability in human-centered robotics

Human-Centered Robotics aims to use robotic devices to improve our life. In Europe alone, around 650.000 people live with limb amputations, 40 Mio. have jobs with high of lumbar injuries, and 40 Mio. are 80+ years. Worldwide, active prostheses, exoskeletons, and service robots could help improve these people’s lives. However, their adoption is unfortunately strongly hindered by technological limitations in the actuators powering these devices. Backdrivability characterizes an actuator’s ability to be driven from the load side, and it is a crucial property to enabling capable human-centered robotic devices. In this paper, we describe the underlying factors that determine actuator backdrivability in robotics and investigate suitable scaling laws to understand how these factors are conditioned by the motor and gearbox selection and the specific operational cycle of a robotic device. This analysis unveils the complexity and challenges faced to accurately model and predict this complex phenomenon, contradicting an extended hypothesis in the robotics community that sees low-ratio transmissions as the best strategy to build backdrivable, lightweight actuators