Recently, the development of collaborative robots that have to work in a cooperative way with humans, has become an important trend both in academia and in the industry. Thereby, safety has increasingly become an essential research aspect. Today, the main techniques to ensure safety are based on reducing 1) the stiffness of the actuators and/or links, 2) the speed of the robot when it approaches humans or obstacles, and 3) the weight of the robot. In all these techniques, the robot's end-effector position is measured by position sensors on the joints, whereby the robot's limbs have to be as rigid as possible. This method has as possible drawbacks that the limbs have to carry unnecessary load (i.e. the robot's weight), and that the position errors increase with increased payload so that the robot can only be used for low payloads. To tackle these problems, we proposed a novel error compensation method based on the use of an additional measurement arm in parallel with the main load bearing arm, whereby the two arms are only coupled between the base and the end-effector. We designed a proof of concept robotic arm and validated the feasibility of our method. This paper presents the End-Effector Position Measuring (EEPM) method and introduces the Independent Load And Measurement Arm (ILAMA) to demonstrate the EEPM concept. With this novel method, the robot can be designed by strength instead of by stiffness. As a consequence, the weight of the limbs can drastically be reduced and the payload to mass ratio can be increased to a value that is bigger than one, while preserving the high end-effector position accuracy, as shown in the experiments. These advantages make the EEPM method very promising to use in collaborative robots or in mobile robot arms. Future works will investigate the feasibility of the proposed concept for real industrial robots with 6 to 7 DOF.

Variable stiffness actuators (VSAs) are complex mechatronic devices that are developed to build passively compliant, robust, and dexterous robots. Numerous different hardware designs have been developed in the past two decades to address various demands on their functionality. This review paper gives a guide to the design process from the analysis of the desired tasks identifying the relevant attributes and their influence on the selection of different components such as motors, sensors, and springs. The influence on the performance of different principles to generate the passive compliance and the variation of the stiffness are investigated. Furthermore, the design contradictions during the engineering process are explained in order to find the best suiting solution for the given purpose. With this in mind, the topics of output power, potential energy capacity, stiffness range, efficiency, and accuracy are discussed. Finally, the dependencies of control, models, sensor setup, and sensor quality are addressed.

In bio-inspired robotic applications, multiple-degree-of-freedom actuators are often desired. The current state of the art of variable stiffness actuators consist mostly of different concepts for single-degree-of-freedom joints. An innovative Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator (MACCEPA) concept is presented to specifically accommodate implementation in two-degree-of-freedom joints. A prototype design, based on this concept, is furthermore presented. The working principle and relevant design choices are explained, a zero-dynamic model is derived for future design iterations, and the implementation in a first prototype is discussed.

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.

Walking behavior is modulated by controlling joint torques in most existing passivity-based bipeds. Controlled Passive Walking with adaptable stiffness exhibits controllable natural motions and energy efficient gaits. In this paper, we propose torque-stiffness-controlled dynamic bipedal walking, which extends the concept of Controlled Passive Walking by introducing structured control parameters and a bio-inspired control method with central pattern generators. The proposed walking paradigm is beneficial in clarifying the respective effects of the external actuation and the internal natural dynamics. We present a seven-link biped model to validate the presented walking. Effects of joint torque and joint stiffness on gait selection, walking performance and walking pattern transitions are studied in simulations. The work in this paper develops a new solution of motion control of bipedal robots with adaptable stiffness and provides insights of efficient and sophisticated walking gaits of humans.

Actuators are key components for moving and control ling a mechanism or system. However, the torque to weight ratio and the energy efficiency of the curre nt state of the art actuators is much lower than in human muscles. As a consequence, among others, manipulato rs for human robot interaction in industry or servi ce applications have a low payload to weight ratio and low energy efficiency. Therefore, we developed a n ovel actuation concept which we name Series-Parallel Ela stic Actuation (SPEA). The concept enables variable recruitment and locking of multiple springs in para llel. After a problem analysis, the Series-Parallel Elastic Actuator schematic will be introduced. Next, our tw o prototypes will be discussed. The experimental re sults endorse the practicability of the SPEA concept and the modeled trend of a lowered motor torque and inc reased energy efficiency.

Powered lower limb exoskeletons require high-performance actuator systems, capable of producing zero to high-assistive torque and at the same time yielding to human interaction torques. Such variable impedance can be achieved by the means of compliant actuators. Because of their intrinsic compliance and high force-to-weight ratio pneumatic muscles are a viable option. However, previous pneumatic muscle powered exoskeleton designs either used them as a position source or failed to meet the high-dynamic torque requirements when using them as a torque source. This paper contributes to the improvement of pneumatic muscle-based actuator systems as a torque source for exoskeleton-type robots. The use of pleated pneumatic artificial muscles in a novel actuator system design allows for a higher torque range in a larger range of motion. Performance evaluation results are given for a 1 DOF test setup and a powered knee exoskeleton. The proposed torque controller achieves the dynamic torques required for zero to full assistance at moderate walking speeds.

High-performance actuators are required for numerous novel applications such as human-robot assistive devices. The torque-to-weight ratio and energy efficiency of current actuation technology is often too low, which limits the performance of novel robots. Therefore, we developed a Series-Parallel Elastic Actuator (SPEA) which enables variable recruitment of parallel springs and variable load cancellation. Finding suitable intermittent mechanisms for the SPEA is however still challenging. This paper reports on the innovative design of an intermittent self-closing mechanism for a MACCEPA-based SPEA that can deliver bi-directional output torque and variable stiffness, while minimizing friction levels. Experiments on a one-layer intermittent self-closing mechanism are conducted to validate the working principle and the proposed model. A demonstrator of the MACCEPA-based SPEA with intermittent self-closing mechanism is presented and the experiments validate the modeled output torque and lowered motor torque for different stiffness settings.

Variable Impedance Actuators (VIA) have received increasing attention in recent years as many novel applications involving interactions with an unknown and dynamic environment including humans require actuators with dynamics that are not well-achieved by classical stiff actuators. This paper presents an overview of the different VIAs developed and proposes a classification based on the principles through which the variable stiffness and damping are achieved. The main classes are active impedance by control, inherent compliance and damping actuators, inertial actuators, and combinations of them, which are then further divided into subclasses. This classification allows for designers of new devices to orientate and take inspiration and users of VIA’s to be guided in the design and implementation process for their targeted application.

Controlled passive walking is an approach that extends the passive walking by adapting the compliance of the joints. Natural motions can be chosen in order to obtain a controllable and energy-efficient walking motion. In this paper, actuators with online adaptable compliance are used based on the concept of controlled passive walking, to obtain adjustable step length and velocity during dynamic bipedal walking. We designed and constructed a bipedal walking robot Veronica which is actuated by the MACCEPA actuators, in which the compliance and equilibrium position can be controlled independently. In addition, a 2-D seven-link bipedal model for simulated walking of Veronica is built to analyze the relation between joint compliance and walking characteristics. Experimental results show that effective walking transitions between different walking speeds and step lengths are realized in both simulations and physical robot experiments.

Most of today's robots have rigid structures and actuators requiring complex software control algorithms and sophisticated sensor systems in order to behave in a compliant and safe way adapted to contact with unknown environments and humans. By studying and constructing variable impedance actuators and their control, we contribute to the development of actuation units which can match the intrinsic safety, motion performance and energy efficiency of biological systems and in particular the human. As such, this may lead to a new generation of robots that can co-exist and co-operate with people and get closer to the human manipulation and locomotion performance than is possible with current robots.

This work describes hardware architecture developed for ALTACRO Automated Locomotion Training using an Actuated Compliant Robotic Orthosis. The ALTACRO project focuses on identified challenges in the state of the art of locomotion rehabilitation. The research goals are aimed at improving robot-aided locomotion training by means of active ankle assistance and a better human-robot interaction. The project involves the design, control and testing of a powered exoskeleton prototype that will assist the step rehabilitation exercises of Spinal Cord Injured (SCI) patients. Keywords— Hardware Architeture, Real Time, SbRIO, Low Level Controller (LLC), FlexRayTM

Traditional stiff actuators have a high reflected inertia, which do not meet the safety requirements, are bad to absorb shocks and cannot store and release energy which are for different applications required. Initiated by the introduction of the Series Elastic Actuator (SEA) by Gill A. Pratt in 1995, a wide range of Variable Impedance Actuators (VIA) are developed, for which inspiration was found in the elastic properties of biological muscle. These compliant VIA’s benefit from the inherent series elasticity since the series elastic element can store and release energy for energy efficiency and decouple the inertia from one link to another. As a result, VIA’s significantly extended the possibilities of traditional stiff actuation. However, current actuators do not yet reach the torque and power requirements for many novel applications like exoskeletons, humanoids, agile locomotion, prostheses and manipulators. This abstract reports on the study of a novel compliant actuator: the Series-Parallel Elastic Actuator (SPEA). The difference with previous studies on parallel elastic elements in joint actuation is that the parallel springs in the SPEA are actively controlled and not passively used. Main inspiration source is again the biological muscle that consists of a large set of parallel and series muscle fibers. The difference is that not all parallel springs in the SPEA need to have their own force converter (mostly DC motor), but are tensioned by an intermittent mechanism actuated by a single motor. This abstract discusses the design and testing of a first Proof Of Concept prototype.

The MACCEPA (Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator) is an electric actuator of which the compliance and equilibrium position are fully independently controllable and both are set by two dedicated servomotor. In this paper an improvement of the actuator is proposed where the torque-angle curve and consequently the stiffness-angle curve can be modified by choosing an appropriate shape of a profile disk, which replaces the lever arm of the original design. The actuator has a large joint angle, torque and stiffness range and these properties can be made beneficial for safe human robot interaction and the construction of energy efficient walking, hopping and running robots. The benefit of the ability to store and release energy is shown by the 1DOF hopping robot Chobino1D. The achieved hopping height is much higher compared to a configuration in which the same motor is used without a series elastic element. The stiffness of the actuator increases with deflection, more closely resembling the properties shown by elastic tissue in humans.