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

Abstract

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.

Time-Optimal Control of Variable-Stiffness-Actuated Systems

“…Notice that the constraint on target performance (6d) in OCP (6) is here substituted by (9b), which requires performance maximization. OCP (9) is more complex than (6), because the term φ * in the constraint (9b) is in turn defined as the outcome of the maximization described in (7)- (8). As a consequence, one cannot numerically solve it as a single optimization problem (for instance using the ACADO Toolkit) as described in Section III-B: a reformulation of problem (9) is therefore needed.…”

“…The advantages of this type of robots compared to the traditional rigid ones are discussed in [3]. While researchers continue to work on ingenious mechanical designs for variable impedance actuation [4]- [8], the control of these systems started to attract more attention [9]- [12]. Task planning and control of these robots are daunting problems due to the highlyconstrained and nonlinear nature of these systems.…”

A clutch mechanism for switching between position and stiffness control of a variable stiffness actuator

“…The same concept has been implemented in the vsaUT-II, only through rotational elements [15], Fig. 4(b).…”

“…Such points works as the pivot for the lever arm (white bar). Details on the mathematical formulation of the vsaUT-II stiffness model are out of the scope of the present paper: we did not vary its mechanics, but added the switching stage, without affecting the shape of the A and B transformers in (5), for which readers can refer to [15].…”

A New and Versatile Adjustable Rigidity Actuator with Add-on Locking Mechanism (ARES-XL)

“…However, the stiffness regulation of these devices is achieved by acting directly over the springs; thus, the compliance adaptation requires a large motor or gearbox to overcome the spring forces, or the adjustment time is too slow for online adjustments during gait. On the other hand, many of the designs from the Italian Institute of Technology (IIT), such as the AWAS [15], AWAS-II [16], CompAct [17] and VSAUT-II [18] to name a few, achieve compliance adaptation by changing the transmission between the load and the spring. The main feature of this group is that the size of the actuator responsible for the stiffness regulation can significantly be reduced because it only has to overcome friction, and a percentage of the spring actions in the presence of external loads at the joint.…”