Glenn Mathijssen

Variable Recruitment of Parallel Elastic Elements: Series–Parallel Elastic Actuators (SPEA) With Dephased Mutilated Gears

The development and control of variable stiffness actuators (VSAs) led to the capability of embodying physical principles of safety and energy-efficiency compared to traditional stiff servomotors. However, the output torque range and efficiency of servomotors and VSAs are still insufficient which hinders the development of machines with performances comparable to a human. We have developed a novel compliant actuation concept, series-parallel elastic actuation (SPEA), that addresses these problems. The novelty being the variable recruitment of parallel elastic elements and adaptive load cancellation. In this paper, we propose the use of multiple dephased mutilated gears with locking ring and plate, as intermittent mechanisms, linked in parallel to the motor. As a result, the motor torque requirements can be lowered, as such the motor can be downscaled and the efficiency can be drastically increased. After an abstract description of the SPEA concept and an outline of the biological basis, we present the first unidirectional SPEA proof of concept (PoC) setup. Experiments on this PoC setup endorse the feasibility of the SPEA concept. The results match the modeled trend of a lowered motor torque and increased energy efficiency.

Lock Your Robot: A Review of Locking Devices in Robotics

Locking devices are widely used in robotics, for instance to lock springs and joints or to reconfigure robots. This review article classifies the locking devices currently described in the literature and performs a comparative study. Designers can therefore better determine which locking device best matches the needs of their application. The locking devices are divided into three main categories based on different locking principles: 1) mechanical locking, 2) friction-based locking, and 3) singularity locking. Different locking devices in each category can be passive or active. Based on an extensive literature survey, this article summarizes the findings by comparing different locking devices on a set of properties of an ideal locking device.

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 tomorrows 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.

Energy Consumption of Geared DC Motors in Dynamic Applications: Comparing Modeling Approaches

In recent years, many works have appeared, which present novel mechanical designs, control strategies, or trajectory planning algorithms for improved energy efficiency. The actuator model is an essential part of these works, since the optimization of energy consumption strongly depends of the accuracy of this model. Nevertheless, various authors follow very different approaches, often neglecting speedand load-dependent losses and inertias of components such as the motor and the gearbox. Furthermore, there is no consensus on how negative power affects power consumption. Some authors calculate energy consumption by integrating the electrical power entirely, by integrating its absolute value, or by integrating only positive power. This letter assesses how well commonly used models succeed in predicting the energy consumption of an 80 W geared DC motor performing a dynamic task, by comparing the results they produce to experimental baseline measurements.

Investigation of self-healing compliant actuators for robotics

Last 15 years, a wide range of self-healing (SH) materials has been developed and recently these materials are increasingly used in applications in multiple fields, like the automotive industry and aerospace. However, so far this material technology is not yet explored in robotics. The introduction of these materials in robotics will potentially reduce the over-dimensioning of current robotic systems, leading to lighter systems and eventually to more efficient designs. Compliant elements used in next generation soft robots, can be constructed from available SH-materials, making them able to autonomously heal cuts and perforations caused by sharp objects in unstructured environments. In addition, the use of SH-materials will have a beneficial impact on the life span of robotic components, reducing the required maintenance drastically. This paper presents the innovative concept of implementing a SH-mechanism in compliant actuators, using dynamic covalent polymer network systems based on the reversible Diels-Alder (DA) reaction. For two entirely different compliant actuators, a series elastic actuator (SEA) and a soft pneumatic actuator (SPA), an analysis is presented on the integration of the DA-polymers in the actuator designs. For both actuator types, a prototype was designed, developed and validated.

Series and Parallel Elastic Actuation: Influence of Operating Positions on Design and Control

It is well-established that properly tuned elastic elements can make robotic actuators more energy-efficient, especially in cyclic tasks. Considering a drive train topology, two important subcategories of elastic actuators are series elastic actuation (SEA) and parallel elastic actuation (PEA). There is still no definite answer to the fundamental question which topology consumes less energy in a given task. This paper approaches the problem by studying oscillatory motions of a single degree-of-freedom link in a gravitational field. The imposed motion is a sinusoid with a nonzero offset requiring a static torque that needs to be compensated by the actuation system. Simulations and experiments show that the SEA consumes less energy up to certain offset angles. At high offsets, the PEA becomes the more energy-efficient alternative, provided that its no-load angle is properly tuned. Inverse dynamics simulations show how a threshold offset angle can be determined for a given task.

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.

Toward motor-unit-recruitment actuators for soft robotics

Among the many features of muscles, their softness, (the ability to deform to accommodate uncertainty in the environment), and their ability to continue functioning despite disturbances, even partial damage, are qualities one would desire to see in robotic actuators. These properties are intimately related to the manner in which muscles work since they arise from the progressive recruitment of many motor units. This differs greatly from current robotic actuator technologies. We present an actuation platform prototype that can support experimental validation of algorithms for muscle fiber recruitment-inspired control, and where further ways to exploit discretization and redundancy in muscle-like control can be discovered. This platform, like muscles, is composed of discretely activated motor units with an integrated compliant coupling. The modular, cellular structure endows the actuator with good resilience in response to damage. It can also be repaired or modified to accommodate changing requirements in situ rather than replaced. Several performance metrics particular to muscle-like actuators are introduced and calculated for one of these units. The prototype has a blocked force of 2.51 N, a strain rate of 21.1 %, and has an input density of 5.46 ×103 motor units per square meter. It consumes 18 W of electrical power during a full isometric contraction. The actuator unit is 41.0 mm3 in size. The force during isometric contractions as it varies with activation is evaluated experimentally for two configurations of modules.

Toward Self-Healing Actuators: A Preliminary Concept

Natural organisms have a unique property not yet available in robotics, i.e., a self-healing (SH) ability. This powerful biological healing function has inspired chemists to impart similar properties to synthetic materials to create “SH materials.” Recent developments in SH polymers led us to investigate the potential of using these materials in robotics. This paper presents an innovative approach of using SH polymers, based on the reversible Diels-Alder (DA) reaction, in a compliant actuator. Using DA polymers, a sacrificial SH mechanical fuse (SH-MF) is designed, developed, and validated by placing it in a cable-driven robotic system. The fuse is designed as weakest element and will sacrificially fail if a damaging overload occurs, protecting the compliant element and other components of the system. The experimental results showed that this SH-MF could be healed at a relatively low temperature, recovering the initial mechanical properties. This first working prototype indicates the feasibility to use SH materials in robotics. “SH robotics” will lead to more sustainable and lighter systems, and eventually to more efficient designs.

Cylindrical cam mechanism for unlimited subsequent spring recruitment in Series-Parallel Elastic Actuators

Series-Parallel Elastic Actuators (SPEA) enable variable recruitment of parallel springs and variable load cancellation. In previous work, we validated a MACCEPA-based SPEA prototype with a self-closing intermittent mechanism, to reduce motor load and improve energy efficiency. However, the mechanism only allowed for 4 parallel springs and a limited equilibrium angle range, which limits the variable load cancellation and operation range. Therefore, we developed a novel cylindrical cam mechanism for unlimited subsequent spring recruitment. This paper describes and validates the working principle of the cylindrical cam mechanism. Furthermore, the latest MACCEPA-based SPEA is presented with a maximum output torque of 40Nm and variable stiffness. Additive and traditional manufacturing techniques go hand in hand to overcome the actuators complexity. The experiments endorse the working principle, demonstrate the variable stiffness, and prove the motor torque can be reduced to 5Nm while an output torque of 40Nm can be achieved.