Browsing by Author "Forni, P."

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    ArticlePublicationOpen Access
    Stable control of force, position, and stiffness for robot joints powered via pneumatic muscles
    (IEEE, 2019-12) Uğurlu, Regaip Barkan; Forni, P.; Doppmann, C.; Sarıyıldız, E.; Morimoto, J.; Mechanical Engineering; UĞURLU, Regaip Barkan
    This paper proposes a novel controller framework for antagonistically driven pneumatic artificial muscle (PAM) actuators. The proposed controller can be stably configured in both torque-stiffness control and position-stiffness control modes. Three contributions are sequentially presented in constructing the framework: 1) A PAM force feedback controller with guaranteed stability is synthesized in a way so as to contend with nonlinear PAM characteristics; 2) a mathematical tool is developed to compute reference PAM forces, for a given set of desired joint torque and joint stiffness inputs; and 3) on top of the torque controller, a position control scheme is implemented and its stability analysis is given in the sense of Lyapunov. In order to test the controller framework, an extensive set of experiments are conducted using an actuator that is constructed using two antagonistically coupled PAMs. As a result, the actuator exhibits satisfactory tracking performances concerning both torque-stiffness control and position-stiffness control modes.
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    Conference ObjectPublicationOpen Access
    Torque and variable stiffness control for antagonistically driven pneumatic muscle actuators via a stable force feedback controller
    (IEEE, 2015) Uğurlu, Regaip Barkan; Forni, P.; Doppmann, C.; Morimoto, J.; Mechanical Engineering; UĞURLU, Regaip Barkan
    This paper describes a novel controller that is capable of simultaneously controlling torque and variable stiffness in real-time, for actuators with antagonistically driven pneumatic artificial muscles (PAMs). To this end, two contributions are presented: i) A stable force feedback controller that can cope with inherent PAM nonlinearities is synthesized using the dissipativity theory, for each PAM unit. ii) On top of this force feedback controller, a mathematical formulation is developed to compute reference force inputs that correspond to desired joint torque and joint stiffness inputs, concerning both agonist and antagonist PAMs. This strategy enables us to introduce real-time sensory feedback; torque and stiffness control is addressed by means of PAM force feedback control with guaranteed stability. To validate the proposed control scheme, a series of experiments were conducted on an experimental setup. As the result, the controller exhibited favorable torque and stiffness tracking in real-time, demonstrating that it could meet the performance criteria to power exoskeleton systems.
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    ArticlePublicationOpen Access
    Variable ankle stiffness improves balance control: experiments on a bipedal exoskeleton
    (IEEE, 2016-02) Uğurlu, Regaip Barkan; Doppmann, C.; Hamaya, M.; Forni, P.; Teramae, T.; Noda, T.; Morimoto, J.; Mechanical Engineering; UĞURLU, Regaip Barkan
    This paper proposes a real-time balance control technique that can be implemented to bipedal robots (exoskeletons, humanoids) whose ankle joints are powered via variable physical stiffness actuators. To achieve active balancing, an abstracted biped model, torsional spring-loaded flywheel, is utilized to capture approximated angular momentum and physical stiffness, which are of importance in postural balancing. In particular, this model enables us to describe the mathematical relation between zero moment point (ZMP) and physical stiffness. The exploitation of variable physical stiffness leads to the following contributions: 1) Variable physical stiffness property is embodied in a legged robot control task, for the first time in the literature to the authors' knowledge. 2) Through experimental studies conducted with our bipedal exoskeleton, the advantages of variable physical stiffness strategy are demonstrated with respect to the optimal constant stiffness strategy. The results indicate that the variable stiffness strategy provides more favorable results in terms of external disturbance dissipation, mechanical power reduction, and ZMP/center of mass position regulation.