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...

Feedforward control of Variable Stiffness Joints robots for vibrations suppression

2017 IEEE International Conference on Robotics and Automation (ICRA), 2017

This paper presents a new feedforward controller based on a continuous-time finite impulse response filter, designed to minimize the vibrations that usually affect robot manipulators with elastic joints. In particular, Variable Stiffness Joints (VSJ) robots are considered, since they are usually characterized by a very low level of damping which makes the problem of the oscillations quite important. The proposed approach allows to simplify the overall control structure of VSJ robots, which is based on a decentralized control of each servomotor, imposing the desired position and the desired stiffness at each joint, and on a novel feedforward control, filtering the reference signals. After analyzing some of the filter properties and the method for the parameters choice, experimental results on a VSJ robot demonstrate the importance of the proposed filtering action for minimizing vibrations and oscillations.

Output-Based Control of Robots with Variable Stiffness Actuation

2011

The output-based control of a redundant robotic manipulator with relevant and adjustable joint stiffness is addressed. The proposed controller is configured as a cascade system that allows the decoupling of the actuators dynamics from the arm dynamics and the consequent reduction of the order of the manipulator dynamic model. Moreover, the proposed controller does not require the knowledge of the whole robot state: only the positions of the actuators and of the joints are necessary.

Design and control of variable stiffness actuation systems

2010

Page 1. Design and Control of Variable Stiffness Actuation Systems Gianluca Palli, Claudio Melchiorri, Giovanni Berselli and Gabriele Vassura DEIS - DIEM - Universit`a di Bologna LAR - Laboratory of Automation and Robotics Viale Risorgimento 2, 40136, Bologna May 3, 2010 Gianluca Palli (Universit`a di Bologna) VIA Workshop @ ICRA 2010 May 3, 2010 1 / 20 Page 2.

A vibration control strategy using variable stiffness joints

2018

Adaptive joints are structural joints made of materials with enhanced transduction properties that can vary their stiffness via solid-state actuation (e.g. thermal, mechanical). In this work, stiffness tuning is used to switch the joint between a ‘locked’ (e.g. a moment connection) and a ‘released’ (e.g. pin) state. Previous work has looked into the feasibility of using variable stiffness joints during shape and force control in order to reduce actuation work. This paper focuses on control of the structure dynamic response to loading. The natural frequency of the structure is tuned to escape dangerous resonance conditions in two ways: 1) a geometric reconfiguration via large shape changes or 2) via the change of stiffness of the joints. Two case studies are considered: 1) an active frame integrated with four actuators fitted on tubular elements which are connected by a shape memory polymer joint 2) a planar truss structure. Experimental tests on the active frame have shown that by v...

Impedance-Controlled Variable Stiffness Actuator for Lower Limb Robot Applications

IEEE Transactions on Automation Science and Engineering, 2019

We present a novel application of the variable stiffness actuator (VSA)-based assistance/rehabilitation robotfeatured impedance control using a cascaded position torque control loop. The robot follows the adaptive impedance control paradigm, thereby achieving an adaptive assistance level according to human joint torque. The feedforward human joint torque command is used to cooperatively adjust the impedance controller and the stiffness trajectory of the VSA (this functional architecture is referred to as the cooperative control framework). In this way, the task performance during movement training can be improved regarding: 1) safety-for example, when the subject intends to contribute considerable effort, low-gain impedance control is activated with a low stiffness actuator to further decrease output impedance and 2) tracking performance-for example, for the subject with less effort, high-gain impedance control is used while pursuing high stiffness to enhance the torque bandwidth. Regarding the safety aspect, we demonstrate that the torque controller designed at low stiffness can be sensitive to the disturbance for low output impedance while maintaining tracking performance. A precondition for this is to treat the input disturbance separately. This is guaranteed by our previously proposed torque control of the VSA using the linear quadratic Gaussian technique. This approach is also employed here, but with additional discussion on the observer design to serve the proposed cooperative control approach. Here, the effectiveness of the proposed control system is experimentally verified using a VSA prototype and a one-degree-of-freedom lower limb exoskeleton worn by a human test person. Note to Practitioners-Control of "physical human-robot interaction" can be achieved by the mechanical parts of the variable stiffness actuator (VSA). However, the mechanical construction for stiffness variation may limit the capacity to achieve low output stiffness and fast stiffness variation in speed. These limitations may become more evident in the assistance/rehabilitation robot applications. To overcome these limitations, the impedance control scheme can be employed to achieve a programmable impedance range and impedance variation speed. This control scheme has been widely applied on the fixed-compliance joint but lacks a way to be implemented on the VSA joint because of its existing capacity to control the impedance with the mechanical construction. This article presents a novel application Manuscript