Variable impedance actuators: A review <s> Antagonistic Springs with Antagonistic Motors <s> This paper is concerned with the design and control of actuators for machines and robots physically interacting with humans, implementing criteria established in our previous work [1] on optimal mechanical-control co-design for intrinsically safe, yet performant machines. In our Variable Impedance Actuation (VIA) approach, actuators control in real-time both the reference position and the mechanical impedance of the moving parts in the machine in such a way to optimize performance while intrinsically guaranteeing safety. In this paper we describe an implementation of such concepts, consisting of a novel electromechanical Variable Stiffness Actuation (VSA) motor. The design and the functioning principle of the VSA are reported, along with the analysis of its dynamic behavior. A novel scheme for feedback control of this device is presented, along with experimental results showing performance and safety of a one-link arm actuated by the VSA motor. <s> BIB001 </s> Variable impedance actuators: A review <s> Antagonistic Springs with Antagonistic Motors <s> This paper reports on the second generation of the pleated pneumatic artificial muscle (PPAM) which has been developed to extend the lifespan of its first prototype. This type of artificial muscle was developed to overcome dry friction and material deformation which is present in the widely used McKibben muscle. The essence of the PPAM is its pleated membrane structure which enables the muscle to work at low pressures and at large contractions. There is growing interest in this kind of actuation for robotics applications due to its high power to weight ratio and adaptable compliance, especially for legged locomotion and robot applications in direct contact with a human. This paper describes the design of the second-generation PPAM, for which specifically the membrane layout has been changed. In terms of this new layout the mathematical model, developed for the first prototype, has been reformulated. This paper gives an elaborate discussion on this mathematical model that represents the force generation an... <s> BIB002 </s> Variable impedance actuators: A review <s> Antagonistic Springs with Antagonistic Motors <s> This paper presents design and performance of a novel joint based actuator for a robot run by variable stiffness actuation, meant for systems physically interacting with humans. This new actuator prototype (VSA-II) is developed as an improvement over our previously developed one reported in [9], where an optimal mechanical-control co-design principle established in [7] is followed as well. While the first version was built in a way to demonstrate effectiveness of variable impedance actuation (VIA), it had limitations in torque capacities, life cycle and implementability in a real robot. VSA-II overcomes the problem of implementability with higher capacities and robustness in design for longer life. The paper discusses design and stiffness behaviour of VSA-II in theory and experiments. A comparison of stiffness characteristics between the two actuator is discussed, highlighting the advantages of the new design. A simple, but effective PD scheme is employed to independently control joint-stiffness and joint-position of a 1-link arm. Finally, results from performed impact tests of 1- link arm are reported, showing the effectiveness of stiffness variation in controlling value of a safety metric. <s> BIB003 </s> Variable impedance actuators: A review <s> Antagonistic Springs with Antagonistic Motors <s> Facing new tasks, the conventional rigid design of robotic joints has come to its limits. Operating in unknown environments current robots are prone to failure when hitting unforeseen rigid obstacles. Moreover, safety constraints are a major aspect for robots interacting with humans. In order to operate safely, existing robotic systems in this field are slow and have a lack of performance. To circumvent these limitations, a new robot joint with a variable stiffness approach (VS-Joint) is presented. It combines a compact and highly integrated design with high performance actuation. The VS- Joint features a highly dynamic stiffness adjustment along with a mechanically programmable system behavior. This allows an easy adaption to a big variety of tasks. A benefit of the joint is its intrinsic robustness against impacts and hard contacts, which permits faster trajectories and handling. Thus, it provides excellent attributes for the use in shoulder and elbow joints of an anthropomorphic robot arm. <s> BIB004 </s> Variable impedance actuators: A review <s> Antagonistic Springs with Antagonistic Motors <s> We present a powered knee prosthesis with two series-elastic actuators positioned in parallel in an agonist-antagonist arrangement. To motivate the knee's design, we developed a prosthetic knee model that comprises a variable damper and two series-elastic clutch units that span the knee joint. Using human gait data to constrain the model's joint to move biologically, we varied model parameters using an optimization scheme that minimized the sum over time of the squared difference between the model's joint torque and biological knee values. We then used these optimized values to specify the mechanical and control design of the prosthesis for level-ground walking. We hypothesized that a variable-impedance control design could produce humanlike knee mechanics during steady-state level-ground walking. As a preliminary evaluation of this hypothesis, we compared the prosthetic knee mechanics of an amputee walking at a self-selected gait speed with those of a weight- and height-matched nonamputee. We found qualitative agreement between prosthetic and human knee mechanics. Because the knee's motors never perform positive work on the knee joint throughout the level-ground gait cycle, the knee's electrical power requirement is modest in walking (8 W), decreasing the size of the onboard battery required to power the prosthesis. <s> BIB005 </s> Variable impedance actuators: A review <s> Antagonistic Springs with Antagonistic Motors <s> The variable stiffness actuation concept is considered to provide a human-friendly robot technology. This paper examines a joint concept called the bidirectional antagonistic joint which is a extension of antagonistic joints. A new operating mode called the helping mode is introduced, which increases the joint load range. Although the joint can not be pretensioned in the helping mode, it is shown that a stiffness variation is possible, assuming a suitable torque-stiffness characteristic of the elastic elements. A methodology to design such characteristics is presented along with several example cases interpreted in a torque-stiffness plot. Furthermore, a stiffness adaptation control scheme which ensures mechanism safety is described. Finally, the design methodology and the control are evaluated on an implementation of a bidirectional antagonistic joint. <s> BIB006 </s> Variable impedance actuators: A review <s> Antagonistic Springs with Antagonistic Motors <s> We propose a prototype of a Variable Stiffness Actuator (VSA) conceived with low cost as its first goal. This approach was scarcely covered in past literature. Many recent works introduced a large number of actuators with adjustable stiffness, optimized for a wide set of applications. They cover a broad range of design possibilities, but their availability is still limited to small quantities. This work presents the design and implementation of a modular servo-VSA multi-unit system, called VSA-CubeBot. It offers a customizable platform for the realization and test of variable stiffness robotic structures with many degrees of freedom. We present solutions relative to the variable stiffness mechanism, embedded electronics, mechanical and electrical interconnections. Characteristics, both theoretic and experimental, of the single actuator are reported and, finally, five units are interconnected to form a single arm, to give an example of the many possible applications of this modular VSA actuation unit. <s> BIB007
Agonist/antagonist actuator pairs are common in nature  and robotics. Rotation of the actuators in the same sense generates a net joint torque, while counter-rotation sets different levels of effective joint stiffness. Although agonistic-antagonistic actuation for variable stiffness is well known, it is useful to recall the three different possible embodiments (see Fig. 3 ). These will be referred to as "uni-directional", "cross-coupled", and "bi-directional" antagonistic arrangements. When tension-only tendons are considered for the uni-directional case BIB002 , the maximum joint torque can not be more than that of each single motor, and no net torque is available when stiffness is at maximum. To overcome these limitations, a third compliant element (possibly different from the two antagonists) may be introduced to cross-couple the two prime movers (see Fig. 3 -b). Cross-coupling allows setting of preload forces to tune it to nominal working conditions, using (a fraction of) each motor's torque in both directions. The VSA-I prototype is an example of a such a cross-coupled device BIB001 . One further step introduces a fourth spring to connect each actuator to the link via two compliant elements (not necessarily symmetric) in push-pull configuration (see Fig. 3 -c). This configuration is used in the VSA-II prototype BIB003 , the VSA-cube that is a hobby servomotor style of VIA actuator BIB007 and in the VSJ BIB004 , used to actuate the wrist and forearm of the DLR hand/arm BIB006 . The big advantage here is that the sum of the two motor torques are available at the joint side if you drop the requirement to track a desired stiffness. The passive joint range of a setup of antagonistic springs is that both the angular and passive joint ranges are limited to the maximum extension of the springs. An interesting approach to reduce the energy consumption of an active knee prosthesis was introduced in BIB005 , where during the walking cycle of the knee, not always both springs were engaged due to a clutch mechanism. Most of the VIAs are powered by electromagnetic motors, but also other means are possible like the hydraulic pistons used in an antagonistic setup for the NEUROexos elbow exoskeleton  or pneumatics BIB002 [37].