A Simple Controller for a Variable Stiffness Joint with Uncertain Dynamics and Prescribed Performance Guarantees (original) (raw)
Dynamic modelling and control of variable stiffness actuators
2010 IEEE International Conference on Robotics and Automation, 2010
After briefly summarizing the mechanical design of the two joint prototypes for the new DLR variable compliance arm, the paper exemplifies the dynamic modelling of one of the prototypes and proposes a generic variable stiffness joint model for nonlinear control design. Based on this model, the design of a simple, gain scheduled state feedback controller for active vibration damping of the mechanically very weakly damped joint is presented. Moreover, the computation of the motor reference values out of the desired stiffness and position is addressed. Finally, simulation and experimental results validate the proposed methods.
Prescribed Performance Tracking of a Variable Stiffness Actuated Robot
2015
This paper is concerned with the design of a state feedback control scheme for variable stiffness actuated (VSA) robots, which guarantees prescribed performance of the tracking errors despite the low range of mechanical stiffness. The controller does not assume knowledge of the actual system dynamics nor does it utilize approximating structures (e.g., neural networks and fuzzy systems) to acquire such knowledge, leading to a low complexity design. Simulation studies, incorporating a model validated on data from an actual variable stiffness actuator (VSA) at a multi-degrees-of-freedom robot, are performed. Comparison with a gain scheduling solution reveals the superiority of the proposed scheme with respect to performance and robustness.
Robust control of robots with variable joint stiffness
2009
Abstract The development of safe and dependable robots for physical human-robot interaction requires both the mechanical design of lightweight and compliant manipulators and the definition of motion control laws that allow compliant behavior in reaction to possible collisions, while preserving accuracy and performance during the motion in the free space. For these motivations, great attention has been posed in the design of robots manipulators with relevant and programmable joint/transmission stiffness.
Experimental comparison of nonlinear motion control methods for a variable stiffness actuator
5th IEEE RAS/EMBS International Conference on Biomedical Robotics and Biomechatronics, 2014
Variable compliant actuators play a key role in the development of efficient biomechatronic systems since energy can be stored in the compliant element thus leading to consumption reduction. In this paper, experimental results comparing passivity-based control (PBC) and feedback linearization (FL) for motion control of an actuator with variable torsional stiffness (VTS) aiming at applications like prosthetic knee joints are presented. The concept of VTS and the experimental setup are described and a mathematical model of the latter one is derived. Based on this, a control architecture consisting of an extended Kalman filter (EKF) to estimate the velocities, a friction compensation as well as the mentioned controller types is developed. Both control methods are analyzed in terms of accuracy, dynamics and their control torque. FL and PBC lead to a stable control with high performance whereas the robustness is low by reason of the model-based control design. FL is superior to the PBC in terms of accuracy and control torque, which is mainly due to the high sensitivity of PBC regarding the discrete position signals. In addition, it is shown that FL can be applied for stable operation near the second natural frequency for different stiffness values.
Design of a robot joint with variable stiffness
2008 IEEE International Conference on Robotics and Automation, 2008
A robot joint with a variable stiffness unit is presented. The variable stiffness unit (VSU) is composed of a motor, two rings that consist of arc-shaped magnets separated by spacers, and a linear guide to change the cross-sectional area of the two rings. Angular displacement between two rings causes the magnets to generate torque, which acts as a nonlinear spring. The stiffness of the joint is varied via changing the overlapping area of the magnets. The VS J exhibits nearly zero stiffness, which enables robot manipulator to be harmless to humans at a wide range of operating speed. Connected to a joint motor in series, the stiffness by the VSU and the position of the joint are controlled independently by two motors. The torque generated by the magnets is analyzed. Using dynamics of the joint, feedback linearization method is adopted to control the VSJ. In addition to feedback linearization, an integral controller is augmented in order to reduce the effect of model uncertainty and di...
Modeling, Identification, and Control of a Discrete Variable Stiffness Actuator (DVSA)
Actuators
A branch of robotics, variable impedance actuation, along with one of its subfields variable stiffness actuation (VSA) targets the realization of complaint robotic manipulators. In this paper, we present the modeling, identification, and control of a discrete variable stiffness actuator (DVSA), which will be developed for complaint manipulators in the future. The working principle of the actuator depends on the involvement of series and parallel springs. We firstly report the conceptual design of a stiffness varying mechanism, and later the details of the dynamic model, system identification, and control techniques are presented. The dynamic parameters of the system are identified by using the logarithmic decrement algorithm, while the control schemes are based on linear quadratic control (LQR) and computed torque control (CTC), respectively. The numerical simulations are performed for the evaluation of each method, and results showed the good potentialities for the system. Future w...
Sliding Mode Control of Compliant Joints Robot
IFAC Proceedings Volumes, 2003
The lack of knowledge of robot dynamics and joint elasticity for precise tracking control can be overcome by using either robust or adaptive (Ieaming) control. The standard fonn of adaptive control does not appear to be applicable since the basic assumptions on the system dynamics and non-linear characteristics are rarely satisfied. On the other hand, variable structure control (YSC) is a powerful control method that has its main attractiveness in the invariance to disturbances and parameter variation. The paper presents a robust control for elastic joint robots (EJR) based on variable structure systems (YSS). The controller needs the measurement of position and velocity and the knowledge of the upper bounds of the uncertainties.
The Arched Flexure VSA: A compact variable stiffness actuator with large stiffness range
2015 IEEE International Conference on Robotics and Automation (ICRA), 2015
The high stiffness of conventional robots is beneficial in attaining highly accurate positioning in free space. High stiffness, however, limits a robot's ability to perform constrained manipulation. Because of the high stiffness, geometric conflict between the robot and task constraints during constrained manipulation can lead to excessive forces and task failure. Variable stiffness actuators can be used to adjust the stiffness of robot joints to allow high stiffness in unconstrained directions and low stiffness in constrained directions. Two important design criteria for variable stiffness actuation are a large range of stiffness and a compact size. A new design, the Arched Flexure VSA, uses a cantilevered beam flexure of variable cross-section and a controllable load location. It allows the joint to have continuously variable stiffness within a finite stiffness range, have zero stiffness for a
Automatica, 2013
In this paper, we consider the design of tracking controllers for flexible joint robots with unknown and possibly variable elasticity, achieving pre-set performance attributes on the link position error. The developed full state feedback controller, is realized without incorporating knowledge relative to the actual system nonlinearities. Furthermore, no approximators are employed to acquire such information. Comparative simulation results on a 2-d.o.f. flexible joint manipulator, illustrate the efficiency of the approach.
Variable stiffness actuators: The user's point of view
The International Journal of Robotics Research, 2015
Since their introduction in the early years of this century, variable stiffness actuators (VSA) witnessed a sustained growth of interest in the research community, as shown by the growing number of publications. While many consider VSA very interesting for applications, one of the factors hindering their further diffusion is the relatively new conceptual structure of this technology. When choosing a VSA for their application, educated practitioners, who are used to choosing robot actuators based on standardized procedures and uniformly presented data, would be confronted with an inhomogeneous and rather disorganized mass of information coming mostly from scientific publications. In this paper, the authors consider how the design procedures and data presentation of a generic VSA could be organized so as to minimize the engineer's effort in choosing the actuator type and size that would best fit the application needs. The reader is led through the list of the most important parameters that will determine the ultimate performance of their VSA robot, and influence both the mechanical design and the controller shape. This set of parameters extends the description of a traditional electric actuator with quantities describing the capability of the VSA to change its output stiffness. As an instrument for the enduser, the VSA datasheet is intended to be a compact, self-contained description of an actuator that summarizes all of the salient characteristics that the user must be aware of when choosing a device for their application. At the end some examples of compiled VSA datasheets are reported, as well as a few examples of actuator selection procedures.