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Publication# CPG-based prostheses control

Résumé

In this thesis we describe a strategy to control robotic knees and ankles. A dynamical system is used to generate a position trajectory to control a servo motor replacing the missing joint. The dynamical system consists in a pool of coupled oscillators modeling a central pattern generator (CPG). As a first step, anthropometric trajectories of the knee and ankle are learned by the system through the convergence of the oscillators to the specific frequencies, corresponding amplitudes and phase relations. The same system is then used to play back these trajectories. As a sensory feedback to trigger the playback we use one adaptive frequency oscillator to synchronized with the acceleration from the thigh. We use a bipedal model in a physics-based robot simulation environment to test the proposed system. Finally we present a simple hardware implementation of this system on the Agonist-Antagonist Active Knee prototype.

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Concepts associés

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Concepts associés (14)

Réseau locomoteur spinal

Le réseau locomoteur spinal, ou central pattern generator (CPG) en anglais, est un réseau de neurones localisé dans la moelle spinale responsable de la locomotion. La particularité de ce réseau est

Oscillateur harmonique

Un oscillateur harmonique est un oscillateur idéal dont l'évolution au cours du temps est décrite par une fonction sinusoïdale, dont la fréquence ne dépend que des caractéristiques du système et dont

Trajectoire

vignette|En physique la trajectoire est une ligne décrit après le déplacement d'un mobile
En mathématiques et en sciences physiques, la trajectoire est la ligne décrite par n'importe quel point d'un

Publications associées (33)

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Coupled dynamical systems are omnipresent in everyday life. In general, interactions between
individual elements composing the system are captured by complex networks. The latter
greatly impact the way coupled systems are functioning and evolving in time. An important
task in such a context, is to identify the most fragile components of a system in a fast and
efficient manner. It is also highly desirable to have bounds on the amplitude and duration
of perturbations that could potentially drive the system through a transition from one equi-
librium to another. A paradigmatic model of coupled dynamical system is that of oscillatory
networks. In these systems, a phenomenon known as synchronization where the individual
elements start to behave coherently may occur if couplings are strong enough. We propose
frameworks to assess vulnerabilities of such synchronous states to external perturbations. We
consider transient excursions for both small-signal response and larger perturbations that can
potentially drive the system out of its initial basin of attraction.
In the first part of this thesis, we investigate the robustness of complex network-coupled
oscillators. We consider transient excursions following external perturbations. For ensemble
averaged perturbations, quite remarkably we find that robustness of a network is given by
a family of network descriptors that we called generalized Kirchhoff indices and which are
defined from extensions of the resistance distance to arbitrary powers of the Laplacian matrix
of the system. These indices allow an efficient and accurate assessment of the overall vulnera-
bility of an oscillatory network and can be used to compare robustness of different networks.
Moreover, a network can be made more robust by minimizing its Kirchhoff indices. Then for
specific local perturbations, we show that local vulnerabilities are captured by generalized
resistance centralities also defined from extensions of the resistance distance. Most fragile
nodes are therefore identified as the least central according to resistance centralities. Based on
the latter, rankings of the nodes from most to least vulnerable can be established. In summary,
we find that both local vulnerabilities and global robustness are accurately evaluated with
resistance centralities and Kirchhoff indices. Moreover, the framework that we define is rather
general and may be useful to analyze other coupled dynamical systems.
In the second part, we focus on the effect of larger perturbations that eventually lead the sys-
tem to an escape from its initial basin of attraction. We consider coupled oscillators subjected
to noise with various amplitudes and correlation in time. To predict desynchronization and
transitions between synchronous states, we propose a simple heuristic criterion based on the
distance between the initial stable fixed point and the closest saddle point. Surprisingly, we
find numerically that our criterion leads to rather accurate estimates for the survival probability and first escape time. Our criterion is general and may be applied to other dynamical
systems.

Nathan Rafaël Bernier, Tobias Kippenberg, László Dániel Tóth

Isolation of a system from its environment is often desirable, from precision measurements to control of individual quantum systems; however, dissipation can also be a useful resource. Remarkably, engineered dissipation enables the preparation of quantum states of atoms, ions or superconducting qubits as well as their stabilization. This is achieved by a suitably engineered coupling to a dissipative cold reservoir formed by electromagnetic modes. Similarly, in the field of cavity electro- and optomechanics, the control over mechanical oscillators utilizes the inherently cold, dissipative nature of the electromagnetic degree of freedom. Breaking from this paradigm, recent theoretical work has considered the opposite regime in which the dissipation of the mechanical oscillator dominates and provides a cold, dissipative reservoir to an electromagnetic mode. Here we realize this reversed dissipation regime in a microwave cavity optomechanical system and realize a quasi-instantaneous, cold reservoir for microwave light. Coupling to this reservoir enables to manipulate the susceptibility of the microwave cavity, corresponding to dynamical backaction control of the microwave field. Additionally, we observe the onset of parametric instability, i.e. the stimulated emission of microwaves (masing). Equally important, the reservoir can function as a useful quantum resource. We evidence this by employing the engineered cold reservoir to implement a large gain (above 40 dB) phase preserving microwave amplifier that operates 0.87 quanta above the limit of added noise imposed by quantum mechanics. Such a dissipative cold reservoir forms the basis of microwave entanglement schemes, the study of dissipative quantum phase transitions, amplifiers with unlimited gain-bandwidth product and non-reciprocal devices, thereby extending the available toolbox of quantum-limited microwave manipulation techniques.

2017Auke Ijspeert, Ludovic Righetti

Central Pattern Generators (CPGs) are becoming a popular model for the control of locomotion of legged robots. Biological CPGs are neural networks responsible for the generation of rhythmic movements, especially locomotion. In robotics, a systematic way of designing such CPGs as artificial neural networks or systems of coupled oscillators with sensory feedback inclusion is still missing. In this contribution, we present a way of designing CPGs with coupled oscillators in which we can independently control the ascending and descending phases of the oscillations (i.e. the swing and stance phases of the limbs). Using insights from dynamical system theory, we construct generic networks of oscillators able to generate several gaits under simple parameter changes. Then we introduce a systematic way of adding sensory feedback from touch sensors in the CPG such that the controller is strongly coupled with the mechanical system it controls. Finally we control three different simulated robots (iCub, Aibo and Ghostdog) using the same controller to show the effectiveness of the approach. Our simulations prove the importance of independent control of swing and stance duration. The strong mutual coupling between the CPG and the robot allows for more robust locomotion, even under non precise parameters and non-flat environment

2008