Stefan S. Groothuis

The Variable Stiffness Actuator vsaUT-II: Mechanical Design, Modeling, and Identification

In this paper, the rotational variable stiffness actuator vsaUT-II is presented. This actuation system is characterized by the property that the apparent stiffness at the actuator output can be varied independently from its position. This behavior is realized by implementing a variable transmission ratio between the internal elastic elements and the actuator output, i.e., a lever arm with variable pivot point position. The pivot point is moved by a planetary gears mechanism, which acquires a straight motion from only rotations, thereby providing a low-friction transmission. The working principle details of the vsaUT-II are elaborated and the design is presented. The actuator dynamics are described by means of a lumped parameter model. The relevant parameters of the actuator are estimated and identified in the physical setup and measurements are used to validate both the design and the derived model.

The vsaUT-II: A novel rotational variable stiffness actuator

In this paper, the vsaUT-II, a novel rotational variable stiffness actuator, is presented. As the other designs in this class of actuation systems, the vsaUT-II is characterized by the property that the output stiffness can be changed independently of the output position. It consists of two internal elastic elements and two internal actuated degrees of freedom. The mechanical design of the vsaUT-II is such that the apparent output stiffness can be varied by changing the transmission ratio between the elastic elements and the output. This kinematic structure guarantees that the output stiffness can be changed without changing the potential energy stored internally in the elastic elements. This property is validated in simulations with the port-based model of the system and in experiments, through a proper control law design, on the prototype.

Lending a helping hand: toward novel assistive robotic arms

Assistive robotics is an increasingly popular research field, which has led to a large number of commercial and noncommercial systems aimed at assisting physically impaired or elderly users in the activities of daily living. In this article, we propose five criteria based on robotic arm usage scenarios and surveys with which assistive robotic arms can be classified. Different possibilities and implementations to obtain each criterion are treated, and examples of current assistive robotic arms are given. Implementations and systems are discussed and rated qualitatively, which leads to the observation that variable stiffness actuation offers great benefits for assistive robotic systems despite an increase in the overall complexity.

Compliant robotic systems on graphs

In this paper, a modular method of modeling compliant robotic systems using graph theory is treated. Graph theoretic analyses ensure a structured way of describing a system and allow a straightforward extension to more complex systems. The graph models of a series elastic actuator, a variable stiffness actuator and a multi degrees of freedom compliant system are derived. These systems are controlled using an optimal control law that is able to find the optimal stiffness setting and distribution to accomplish a certain task. A case study shows a multi degrees of freedom compliant system which is required to resonate at the output and to accomplish a back-and-forth motion. It is shown that a constant optimal stiffness is found in the resonance simulation, and a varying optimal stiffness in case of the back-and-forth task. This indicates that this methodology can assist in finding an optimal stiffness distribution of complex robotic systems for a given task.

Modeling Robotic Manipulators Powered by Variable Stiffness Actuators: A Graph-Theoretic and Port-Hamiltonian Formalism

This paper proposes a modeling method for generic compliant robotic manipulators. It is based on graph theory and the port-Hamiltonian formalism, which allows a modular approach to the interconnection of rigid bodies with compliant actuators by means of kinematic pairs. This modularity enables a simple and straight-forward adaption the model when a manipulators actuator morphology is changed. An example of a spatial three degree-of-freedom manipulator shows that this modeling method is more suitable for modeling changes in actuator placement than the traditional Euler–Lagrange method.

Compliant manipulators on graphs

This paper proposes a modeling method for generic serial-chain compliant robotic manipulators. It is based on graph theory and port-Hamiltonian systems, which allows a modular approach to the interconnection of rigid bodies with compliant actuators by means of kinematic pairs. This modularity allows a very simple and straight-forward change in a manipulators actuator morphology. An example of a two degree of freedom planar manipulator shows that this modeling method is more suitable for modeling changes in actuator placement than traditional Euler-Lagrange models.

A General Approach to Achieving Stability and Safe Behavior in Distributed Robotic Architectures

This paper proposes a unified energy-based modeling and energy-aware control paradigm for robotic systems. The paradigm is inspired by the layered and distributed control system of organisms, and uses the fundamental notion of energy in a system and the energy exchange between systems during interaction. A universal framework that models actuated and interacting robotic systems is proposed, which is used as the basis for energy-based and energy-limited control. The proposed controllers act on certain energy budgets to accomplish a desired task, and decrease performance if a budget has been depleted. These budgets ensure that a maximum amount of energy can be used, to ensure passivity and stability of the system. Experiments show the validity of the approach.

Control of a variable stiffness joint for catching a moving object

The paper presents a control method to catch a moving object with a joint actuated by means of a variable stiffness actuator. The controller is designed such that the variable stiffness joint acts as a virtual damper that absorbs the kinetic energy of the moving object. The virtual damping and the output stiffness of the variable stiffness actuator are the control variables. To obtain a critically damped system, the damping coefficient is scheduled on both the output stiffness and the inertia of the system. Experiments on the rotational variable stiffness actuator vsaUT-II validate the control method.

Single motor-variable stiffness actuator using bistable switching mechanisms for independent motion and stiffness control

This paper presents a proof of concept of a variable stiffness actuator (VSA) that uses only one (high power) input motor. In general, VSAs use two (high power) motors to be able to control both the output position and the output stiffness, which possibly results in a heavy, and bulky system. In this work, two small and light-weight clutches are used to lock either one of the degrees of freedom, allowing the other to be controlled by the input motor. These clutches are realized by friction belts that can engage to a surrounding cylinder. The clutches are operated by solenoids, and small bistable mechanisms ensure that no electrical energy is lost in keeping a degree of freedom locked or unlocked. An experiment with a prototype of the system is performed which validates the proof of concept of this Single Motor-VSA.

On the modeling, design, and control of compliant robotic manipulators

In the future of the aging society it becomes increasingly difficult to provide personal care to people who are in need of help due to physical impairments caused by various diseases or disorders. The number of caregivers is limited and the cost of individual personal care will increase. Although personal care is desired, the downside is that people in need of care become dependent on caregivers for accomplishing their activities of daily living. This dependence has a big influence on one’s quality of life and self esteem. A solution to this situation is the deployment of assistive robotic aids like multifunctional robotic arms mounted to a table or wheelchair. These systems can aid in various tasks like opening doors and picking up a drinking glass, as well as personal hygiene tasks. Because many of today’s wheelchair mounted robotic arms are based in some aspects on existing designs for industrial applications, they are not safe to be used in human care services. Therefore, there is a need for safe robotic arms that are applicable for this kind of care. This thesis describes various ways of accomplishing safety in robotic arms, and argues that using variable stiffness actuation, which has not been applied before in assistive robotic arms, is a way to accomplish that. These actuators provide the possibility to not only control the position of the actuator output, but also to influence the stiffness that can be perceived at this output. Various concepts of these actuators, using different principles, are investigated in Part I, and Part II treats the modeling of robotic arms that are driven by these actuators. Modularity was a specific goal, because of the large diversity of connections between the actuators and the arm. The result is a model based on graph theory, which consists of vertices and edges, in which the repositioning of an actuator means changing the connections of one or more edges. This thesis also presents a structured method of analyzing an arms stiffness, and treats the control of the actuator stiffnesses to optimally approximate a desired end effector stiffness.