Visual Servoing Based on Image Motion

International audienceThe general aim of visual servoing is to control the motion of a robot so that visual features acquired by a camera become superimposed with a desired visual pattern. Visual servoing based on geometrical features such as image point coordinates is now well established. Nevertheless, this approach has the drawback that it usually needs visual marks on the observed object to retrieve geometric features. The idea developed in this paper is to use motion in the image as the input of the control scheme since it can be estimated without any a priori knowledge of the observed scene. Thus, more realistic scenes or objects can be considered. Two different methods are presented. In the first method, geometric features are retrieved by integration of motion, which allows the use of classical control laws. This method is applied to a 6 degree-of-freedom positioning task. The authors show that, in such a case, an affine model of 2-D motion is insuffi- cient to ensure convergence and that a quadratic model is needed. In the second method, the principle is to try to obtain a desired 2-D motion field in the image sequence. In usual image-based visual ser- voing, variations of visual features are linearly linked to the camera velocity. In this case, the corresponding relation is more complex, and the authors describe how it is possible to use this relation. This approach is illustrated with two tasks: positioning a camera parallel to a plane and following trajectory

Application of Motion-Based Visual Servoing to Target Tracking

International audienceIn this paper, the classical task of mobile target tracking using a pan-and-tilt camera is considered. The authors use recent results in motion-based visual servoing to deal with complex targets for which shape and texture are unknown. The first method consists of design- ing a control law directly from the estimated image motion. This leads to the computation of the pan-and-tilt acceleration necessary to reduce the tracking error. A second method, more efficient for target tracking, consists of retrieving the target position in the im- age from its estimated motion. This leads to classical image-based visual servoing. For both methods, experimental results obtained at video rate are presented and discussed

Complex object tracking by visual servoing based on 2D image motion

International audienceEfficient real-time robotic tasks using a monocular vision system were previously developed with simple objects (e.g. white points on a black background), within a visual servoing context. Due to recent developments, it is now possible to design real-time visual tasks exploiting motion information in the image, estimated by robust algorithms. This paper proposes such an approach to track complex objects, such as a pedestrian. It consists in integrating the measured 2D motion of the object to recover its 2D-position in the image. The principle of the tracking task is to control the camera pan and tilt such that the estimated center of the object appears at the center of the image. Real-time experimental results demonstrate the efficiency and the robustness of the method

Collision Avoidance With Multiple Walkers: Sequential or Simultaneous Interactions?

Collision avoidance between multiple walkers, such as pedestrians in a crowd, is based on a reciprocal coupling between the walkers with a continuous loop between perception and action. Such interpersonal coordination has previously been studied in the case of dyadic locomotor interactions. However, when walking through a crowd of people, collision avoidance is not restricted to dyadic interactions. We examined how dyadic avoidance (1 vs. 1) compared to triadic avoidance (1 vs. 2). Additionally, we examined how the dynamics of a passable gap between two walkers affected locomotor interactions. To this end, we manipulated the starting formation of two walkers that formed a potentially pass-able gap for the other walker. We analyzed the interactions in terms of the evolution over time of the Minimal Predicted Distance and the Dynamics of the Gap, which both provide information about what action is afforded (i.e., passing in front/behind and the pass-ability of the gap). Results showed that some triadic interactions invited for sequential interactions, resulting in avoidance strategies comparable with dyadic interactions. However, some formations resulted in simultaneous interactions where the dynamics of the pass-ability of the gap revealed that the coordination strategy emerged over time through the bi-directional interactions between all walkers. Future work should address which circumstances invite for simultaneous and which for sequential interactions between multiple walkers. This study contributed toward understanding how collision is avoided between multiple walkers at the level of the local interactions

Comment rejoindre un groupe quand on se déplace en fauteuil ? Etude observationnelle biomécanique des interactions piétons-usagers de fauteuil roulant

International audienceL’étude et la modélisation des interactions entre piétons et usagers de fauteuil roulant électrique (FRE) est un enjeu majeur pour le développement d’assistance à la conduite de tels fauteuils, notamment pour suivre l’intention de déplacement de l’usager tout en évitant une collision avec un piéton à proximité. Pour mieux comprendre les intentions et les distances de proximité piéton-FRE lors de déplacement en groupe, nous avons demandé à un usager de FRE de rejoindre un groupe de deux piétons, marchant côte à côte. Les marcheurs se déplaçaient d’un point de départ à un point d’arrivée situé en face à 20m. Le participant en FRE arrivait perpendiculairement à la trajectoire des marcheurs. Ils devaient alors tous les trois rejoindre ensemble le point d’arrivée. 9 participants marchant (8 hommes, 1 femme, 27ans) et 3 usagers de FRE (3 hommes, 43 ans) ont participé à cette étude. 2 des usagers de FRE étaient ergothérapeutes, 1 avaient une paralysie cérébrale. Trois sessions incluant 1 usager de FRE et 3 marcheurs ont été réalisées. Chaque paire de marcheurs a réalisé 10 essais, pour un total de 20 essais par marcheur et 30 essais pour l’usager en FRE. L’analyse en cours porte sur les adaptations de mouvement de la personne en FRE et des piétons ainsi que leurs positions relatives. Il s’agit alors d’étudier le comportement d’interaction qui permet aux marcheurs et à l’usager de FRE de former un groupe, les distances de proximité ainsi que de potentielles asymétries marcheur-marcheur et marcheur usager de FRE

Body movement strategies to initiate the crossing of a street in front of traditional and self-driving cars in young and older adults

International audienceBACKGROUND AND AIM: The safety of elderlies is a key societal issue, especially when considering that 48% of pedestrian fatalities involve people aged 65 or more (Sécurité Routière 2017-France). Aging affects street crossing behavior, with a decrease of walking speed or more risky decisions because elderly people have difficulties to estimate the approaching speed of vehicles, especially in complex situations. In young adults, recent work focused on body movement performed to initiate the crossing, showing a top down sequence of advancement along the antero-posterior axis: the head initiates the crossing movement, followed by the shoulders, elbows, wrists, hips, knees and ankles. Identifying such motion invariants can be particularly useful in the context of self-driving vehicles which aim at predicting the intent of crossing. In this study, we aim at investigating body movement strategies performed before crossing in older adults in complex mixed traffic. METHODS: 30 young adults (YA, 21-39yo) and 30 older adults (OA, 68-81yo) were asked to cross (or not) a virtual two-way street by walking in a simulator. Participants performed a total of 120 trials where we manipulated: the type of vehicles (Conventional and/or Self driving car, the latest always stopping to let the pedestrian cross the street), their speed (30 or 50km/h), their position on the lane (far/near lane), as well as the temporal gap available to cross the street (1,2,3,4 or 5s). After computing temporal body segment motion and orientations, we analyzed the delays in initiating the crossing movement for the head, shoulders and hips with respect to the feet. We also performed hierarchical clustering to identify specific groups of behavior. RESULTS: Preliminary results show a top-down sequence of forward body motion, starting from the head to the feet, whatever the traffic condition and the group. In OA, the head initiates the motion sooner than YA wrt their feet. Moreover, while the horizontal angle profile of the head, shoulders and hips does not allow to identify invariants due to the large variety of behaviors before crossing, the trunk tilt angle profile appears to be a relevant marker for predicting the intent to cross the street. CONCLUSIONS: While aging was shown to affect street crossing decisions, our results highlight consistent behavior between YA and OA regarding trunk tilt profile when initiating the crossing. In line with previous work on YA, we also show a top down sequence of advancement of body segments. Future work is needed to use our results to predict the intent of crossing on a new database. Beside the choice to cross the street, future work is also needed to understand body segment motion and walking speed profile while crossing

Which biomechanical models are currently used in standing posture analysis?

International audienceIn 1995, David Winter concluded that postural analysis of upright stance was often restricted to studying the trajectory of the center of pressure (CoP). However, postural control means regulation of the center of mass (CoM) with respect to CoP. As CoM is only accessible by using a biomechanical model of the human body, the present article proposes to determine which models are actually used in postural analysis, twenty years after Winter's observation. To do so, a selection of 252 representative articles dealing with upright posture and published during the four last years has been checked. It appears that the CoP model largely remains the most common one (accounting for nearly two thirds of the selection). Other models, CoP/CoM and segmental models (with one, two or more segments) are much less used. The choice of the model does not appear to be guided by the population studied. Conversely, while some confusion remains between postural control and the associated concepts of stability or strategy, this choice is better justified for real methodological concerns when dealing with such high-level parameters. Finally, the computation of the CoM continues to be a limitation in achieving a more complete postural analysis. This unfortunately implies that the model is chosen for technological reasons in many cases (choice being a euphemism here). Some effort still has to be made so that bioengineering developments allow us to go beyond this limi

Visual Servoing Based on Image Motion

International audienceThe general aim of visual servoing is to control the motion of a robot so that visual features acquired by a camera become superimposed with a desired visual pattern. Visual servoing based on geometrical features such as image point coordinates is now well established. Nevertheless, this approach has the drawback that it usually needs visual marks on the observed object to retrieve geometric features. The idea developed in this paper is to use motion in the image as the input of the control scheme since it can be estimated without any a priori knowledge of the observed scene. Thus, more realistic scenes or objects can be considered. Two different methods are presented. In the first method, geometric features are retrieved by integration of motion, which allows the use of classical control laws. This method is applied to a 6 degree-of-freedom positioning task. The authors show that, in such a case, an affine model of 2-D motion is insuffi- cient to ensure convergence and that a quadratic model is needed. In the second method, the principle is to try to obtain a desired 2-D motion field in the image sequence. In usual image-based visual ser- voing, variations of visual features are linearly linked to the camera velocity. In this case, the corresponding relation is more complex, and the authors describe how it is possible to use this relation. This approach is illustrated with two tasks: positioning a camera parallel to a plane and following trajectory

Application of Motion-Based Visual Servoing to Target Tracking

International audienceIn this paper, the classical task of mobile target tracking using a pan-and-tilt camera is considered. The authors use recent results in motion-based visual servoing to deal with complex targets for which shape and texture are unknown. The first method consists of design- ing a control law directly from the estimated image motion. This leads to the computation of the pan-and-tilt acceleration necessary to reduce the tracking error. A second method, more efficient for target tracking, consists of retrieving the target position in the im- age from its estimated motion. This leads to classical image-based visual servoing. For both methods, experimental results obtained at video rate are presented and discussed

Additional energetic cost due to belt speed variations when walking on a treadmill.

International audienceBACKGROUND: Treadmill is commonly used in routine evaluation of walking capacities, especially when dealing with energetic cost, as it allows to measure this motion on a long enough period. However, even when using powerful treadmills, it is impossible to avoid belt speed variations. The objective of this paper was to demonstrate that this instantaneous treadmill speed has to be taken into account in mechanical work calculation during treadmill walking. METHODS: To do so, 11 healthy subjects participated in the study. They walked on treadmill at four different speeds: very slow (0.3 ms(-1)), slow (0.8 ms(-1)), normal (1.4 ms(-1)) and fast (1.9 ms(-1)). To compute P(Vi), the instantaneous mechanical work relatively to the instantaneous treadmill speed, we applied the work-energy theorem in a non-inertial reference frame induced by the treadmill speed variations. This value was then compared to P(V¯), i.e. the instantaneous mechanical work computed with the usual approximation of a constant treadmill speed. FINDINGS: The speed variations had an average value of 17% for the very slow speed and around 4.5% for the three other and more usual speeds. These variations have a strong influence on instantaneous mechanical work. Indeed, P(V¯) is nearly null when not considering speed variations whereas P(Vi) goes up to 113 mW kg(-1) at higher speeds. INTERPRETATION: To conclude, our work showed that the instantaneous treadmill speed has to be taken into account in mechanical work computation. This method may provide supplementary information in clinical assessments and analyses of gait. Indeed, when using the same methodology on formerly acquired data in a group of 13 hemiplegic patients, we found that if P(V¯) is not null in this impaired subjects group, P(Vi) is still 30% higher