Browsing by Author "Doppmann, C."

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Open 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.
Open 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.
Open Access
Towards balance recovery control for lower body exoskeleton robots with variable stiffness actuators: spring-loaded flywheel model
(IEEE, 2015) Doppmann, C.; Uğurlu, Regaip Barkan; Hamaya, M.; Teramae, T.; Noda, T.; Morimoto, J.; Mechanical Engineering; UĞURLU, Regaip Barkan
This paper presents a biologically-inspired real-time balance recovery control strategy that is applied to a lower body exoskeleton with variable physical stiffness actuators at its ankle joints. For this purpose, a torsional spring-loaded flywheel model is presented to encapsulate both approximated angular momentum and variable physical stiffness, which are crucial parameters in describing the postural balance. In particular, the incorporation of physical compliance enables us to provide three main contributions: i) A mathematical formulation is developed to express the relation between the dynamic balance criterion ZMP and the physical ankle joint stiffness. Therefore, balancing control can be interpreted in terms of ankle joint stiffness regulation. ii) `Variable physical' stiffness is utilized in the bipedal robot balance control task for the first time in the literature, to the authors' knowledge. iii) The variable physical stiffness strategy is compared with the optimal constant stiffness strategy by conducting experiments on our exoskeleton robot. The results indicate that the proposed method provides a favorable balancing control performance to cope with unperceived perturbations, in terms of center of mass position regulation, ZMP error and mechanical power.
Open 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.