Research

2016-present: Research work in LISV, University of Versailles/Univ. Paris Saclay (with Prof. Eric Monacelli)

In the LISV lab, I am participating on the research project about a learning assessment method based on the identication of reference behaviours. It was applied for wheelchair skills assessmentand training, which is a simple, risk-free, but unusual platform. Eight experienced electric-powered wheelchair users, six novice ones and one subject with disabilities who drives a wheelchair everyday were asked to carry out several typical driving tasks for few trials. Seven classic performance indicators based on the joystick control were used to describe the users' driving behaviours. We assumed that a driving skill evaluation could be performed by using reference behaviours which could be extracted from experienced users. Therefore, in order to identify the reference behaviours, the Fuzzy C-Means (FCM) classi.cation method was performed on the data of experienced subjects, which could classify the behaviours of the experienced subjects into several types of reference behaviour. We also used the Wilcoxon test to determine whether there was a signi.cant di erence between any two classes obtained by using the FCM method. Next the evaluation was performed on novice subjects by comparing their behaviours with respect to the referenceones. The results showed that, for all the experienced subjects, there was no signi.cant di erence on their driving behaviour from one trial to another, even for the subject with disabilities. That means their driving behaviour was stable and we found the driving behaviours of them could be classi.ed into two types of reference driving behaviour. On the other side, the novice users had switching behaviours during the learning phase, but after several trials, each novice user's behaviour converged to one of the identi.ed reference behaviours, that means the novice users could obtain their stable performance after learning.

In our future work, we plan to conduct an assessment of a population of subjects with disabilities by cooperating with rehabilitation centres. The participants will be selected according to both their driving experiences (beginners and daily users) and their types of disability. They will use the same control interface and be evaluated on the same course.

Keywords: Learning assessment, non-parametric analysis, Fuzzy C-means, Electric-Powered Wheelchair, driving skill evaluation

2013-2015: Research work of Postdoc at INRIA Sophia Antipolis (with Jean-Piere Merlet, DR INRIA, Fellow of IEEE)

1. Walking analysis of young-elderly people by using an intelligent walker ANG. (Robotics and Autonomous Systems Journal-2014) This paper proposes a new method to analyze human walking by using a 3-wheels rollator walker instrumented with encoders and a 3D accelerome-ter/gyrometer. In order to develop walking quality index and monitor the health state of elderly people at home, the walking of 23 young adults and 25 elderly people (> 69 years) with the help of the walker, are compared. The results show that many general walking indicators such as walking speed, stride length do not present obvious difference between the two groups, but that new indicators obtained by using the walker measurements and not available otherwise are very discriminating, e.g., the lateral motion of elderly people is larger, their walking accuracy is lower, but their effort distributed on the handles are more symmetrical. We also show that this walker may have other purposes such as updating collaborative maps with sideway slopes and location of lowered kerbs.

2. A new application of smart walker for quantitative analysis of human walking. (ACVR'14 held in conjunction with European Conference on Computer Vision (ECCV)) This paper presents a new nonintrusive device for everyday gait analysis and health monitoring. The system is a standard rollator equipped with encoders and inertial sensors. The assisted walking of 25 healthy elderly and 23 young adults are compared to develop walking quality index. The subjects were asked to walk on a straight trajectory and an L-shaped trajectory respectively. The walking trajectory, which is missing in other gait analysis methods, is calculated based on the encoder data. The obtained trajectory and steps are compared with the results of a motion capture system. The gait analysis results show that new index obtained by using the walker measurements, and not available otherwise, are very discriminating, e.g., the elderly have larger lateral motion and maneuver area, smaller angular velocity during turning, their walking accuracy is lower and turning ability is weaker although they have almost the same walking velocity as the young people.

2008-2011: Research work of PhD (with Christine Chevallereau, DR CNRS, LS2N, Ecole Centrale de Nantes)

1. Stable walking control of a 3D biped robot with foot rotation (Robotica Journal-2013)

In order to obtain a more human-like walking and less energy consumption, a it foot rotation phase is considered in the single support phase of a 3D biped robot, in which the stance heel lifts from the ground and the stance foot rotates about the toe. Since there is no actuation at the toe, a walking phase of the robot is composed of a fully actuated phase and an under-actuated phase. The objective of this paper is to present an asymptotically stable walking controller that integrates these two phases. To get around the under-actuation issue, a strict monotonic parameter of the robot is used to describe the reference trajectory instead of using the time parameter. The overall control law consists of a zero moment point (ZMP) controller, a swing ankle rotation controller and a partial joint angles controller. The ZMP controller guarantees that the ZMP follows the desired ZMP. The swing ankle rotation controller assures a flat-foot impact at the end of the swinging phase. Each of these controllers creates two constraints on joint accelerations. In order to determine all the desired joint accelerations from the control law, a partial joint angles controller is implemented. A word “partial” emphasizes the fact that not all the joint angles can be controlled. The outputs controlled by a partial joint angles controller are defined as a linear combination of all the joint angles. The most important question addressed in this paper is how this linear combination can be defined in order to ensure walking stability. The stability of the walking gait under closed-loop control is evaluated with the linearization of the restricted Poincaré map of the hybrid zero dynamics. Finally, simulation results validate the effectiveness of the control law even in presence of initial errors and modelling errors.

2. Walking and steering control for a 3D biped robot considering ground contact and stability (Robotics and Autonomous Systems Journal-2012) This paper presents a stable walking control method for a 3D bipedal robot with 14 joint actuators. The overall control law consists of a ZMP (zero moment point) controller, a swing ankle rotation controller and a partial joint angles controller. The ZMP controller guarantees that the stance foot remains in flat contact with the ground. The swing ankle rotation controller ensures a flat foot impact at the end of the swinging phase. Each of these controllers creates 2 constraints on joint accelerations. As a consequence, the partial joint angles controller is implemented to track only 10 independent outputs. These outputs are defined as a linear combination of the 14 joint angles. The most important question addressed in this paper is how this linear combination can be defined in order to ensure walking stability. The stability of the walking gait under closed loop control is evaluated with the linearization of the restricted Poincare map of the hybrid zero dynamics. As a result, the robot can achieve an asymptotically stable and periodic walking along a straight line. Finally, another feedback controller is supplemented to adjust the walking direction of the robot and some examples of the robot steered to walk along different paths with mild curvature are given.

3. Stability analysis and time-varying walking control for an under-actuated planar biped robot. (Robotics and Autonomous Systems Journal-2011) This paper presents two walking controllers for a planar biped robot with unactuated point feet. The control is based on the tracking of reference motions expressed as a function of time. First, the reference motions are adapted at each step in order to create a hybrid zero dynamic (HZD) system. Next, the stability of the walking gait under closed-loop control is evaluated with the linearization of the restricted Poincaré map of the HZD. When the controlled outputs are selected to be the actuated coordinates, most periodic walking gaits for this robot are unstable, that is, the eigenvalues of the linearized Poincaré map (ELPM) is larger than one. Therefore, two control strategies are explored to produce stable walking. The first strategy uses an event-based feedback controller to modify the ELPM and the second one is based on the choice of controlled outputs. The stability analysis show that, for the same robot and for the same reference trajectory, the stability of the walking (or ELPM) can be modified by some pertinent choices of controlled outputs. Moreover, by studying some walking characteristics of many stable cases, a necessary condition for stable walking is proposed. It is that the height of swing foot is nearly zero at the desired moment of impact. Based on this condition, the duration of the step is almost constant in presence of initial error, so a method for choosing controlled outputs for the second controller is given. By using this method, two stable domains for the controlled outputs selection are obtained.