Section: New Results


Cable-driven parallel robots (CDPR)

Analysis of Cable-driven parallel robots

Participants : Alessandro Berti, Laurent Blanchet, Houssein Lamine, Jean-Pierre Merlet [correspondant] , Yves Papegay, Rémy Ramadour.

We have continued the analysis of suspended CDPRs for control and design purposes. For control it is essential to determine the current pose of the robot for given cable lengths (forward kinematics, FK) and to be able to calculate the cable lengths for a given pose of the platform (inverse kinematics, IK). If the cables are supposed to be non-deformable the IK problem is trivial and has a single solution but the FK is complex, admits several solutions and raises several issues. We have shown in the past that to get all FK solutions for a CDPR with m cables we have to consider not only the case where all cables are under tension but also have to solve the FK for all combinations of cables under tension with 1 to m cables. Surprisingly the FK is more difficult if the CDPR has less than 6 cables under tension. Our team, in collaboration with M. Carricato of Bologna University, is the first to have designed a solving algorithm that allow to compute in a guaranteed manner all FK solutions  [21] , [22] . The FK problem is different if it is intended to be used in a real-time context as in that case we have the extra information of the platform pose a short time before. After a small change in the cable lengths we may assume a small change in the pose platform but using Newton method with the previous pose cannot guarantee to provide the current pose. We have proposed an algorithm that is guaranteed to get the current pose and is also able to determine if the CDPR may be sufficiently close to a singularity so that multiple solutions are possible [11] . However the assumption of a small change in the platform pose may not always hold, a point that we have shown theoretically and experimentally. We have then proposed an algorithm that uses a model of the coiling process to determine if a drastic change in the pose may occur between two sampling time [11] and also allows one to better estimate the cable tensions on a trajectory. We have for example shown that sudden and important changes in these tensions may occur. Another issue arises for non-deformable cables and CDPR with more than 6 cables in a suspend configuration. In the past we have shown that there always will be at most 6 cables under tension whatever the number of cables. For a given pose there may be several possible set of cables under tension (called cable configuration), each of them having different characteristics in terms of maximal tension, sensitivity to disturbances, .... From a control viewpoint it makes sense to impose a given cable configuration at the pose by setting the lengths of slack cables to larger values than the one required for the pose. To determine the best cable configuration we have proposed several ranking index [12] .

Even more complex kinematic problems are involved if we assume that the cable are deformable (e.g. are elastic or catenary-like). The cable model is included in the kinematic equations for getting a complete model. We have been interested in the catenary-like model that involves inverse hyperbolic functions and is valid for steel cable of relatively high length. As the IK has never been addressed with such a model we have proposed a solving algorithm [10] that has shown that the IK may have multiple solutions but also may have no solution for poses that are reachable with non-deformable cables. In the same way the DK has several solutions [13] . Finally efficient cables interference detection for sagging cables and the management of modular CDPR, whose geometry may be changed according to the task at hand, have been addressed [9]

Cable-Driven Parallel Robots for additive manufacturing in architecture

Participant : Yves Papegay.

Easy to deploy and to reconfigure, dynamically efficient in large workspaces even with payloads, cable-driven parallel robots are very attractive for solving displacement and positioning problems in architectural building at scale 1 and seems to be a good alternative to crane and industrial manipulators in this area.

In a collaboration with CNAM and Ecole Nationale Supérieure d'Architecture Paris-Malaquais, we worked on additive manufacturing of building based on ultra-high performance concrete and developed a CDPR as a proof of concept to power a large scale 3D-printer.

A real size industrial robot will be developed by the XtreeE start-up company.


This is now the core of our activity and our work on CDPR is deeply connected to this field as they are an efficient solution for mobility assistance, a high priority for the elderly, helpers and medical community. We have presented our vision of assistance robotics in several occasions [24] , [25] , [23] .

Assessment of elderly frailty

Participants : Karim Bakal, Jean-Pierre Merlet.

The assessment of elderly frailty is a difficult concept because it involves the physical capacities of a person and its environment (health-care services, families, funds...). We consider the assessment of upper limb capabilities by looking at the joint torques τ of the arm and the maximal force F that can be exerted by the hand, which are related by the equation

τ = 𝐉 𝐓 F (2)

where 𝐉 is a matrix which depends only upon the configuration of the arm. These equations constitute an underconstrained linear system. In biomechanics the torque τ is measured together with the configuration of the arm and the force F is evaluated by using the method of Chiacchio, that involves the pseudo-inverse of 𝐉𝐓 to calculate F. But there are several uncertainties that are neglected when using this method: the measurement errors on τ and on the configuration of the arm together with uncertainties on the physical parameters of the arm (such as the length of the bones). The method of Chiacchio provides one of the possible solutions of equation (2 ) and not necessary the one corresponding to the force at the hand. We use another approach based on interval analysis. We assume that all uncertainties may be bounded (τ is an interval vector τm, 𝐉𝐓 is an interval matrix) so that equation (2 ) become an interval linear system. Interval analysis then allows one to determine an approximation as accurate as wanted of the set Fs of all forces F that satisfy the equation and therefore this set includes the real force at the hand. Now assume that with the same arm configuration we measure the force at the hand, here again with some bounded uncertainties (i.e. F is an interval vector Fm). Here again we may use interval analysis applied on equation  (2 ) in order to determine an interval vector τv for the τ that is guaranteed to include the real τ. Furthermore τ must be included in the intersection τi of τv and τm while F must be included in the intersection Fi of Fm and Fs. If τi is strictly included in τm, then we may compute a better approximation of Fs. Reciprocally if Fi is strictly included in Fm we will get a better τv. If one of these situation occurs we repeat the process until no significant improvement of Fs or τv is obtained. In a second step we consider that the uncertainties that lead to uncertainties in the matrix 𝐉𝐓 are constrained as we have to satisfy τv=𝐉𝐓Fs. Here again we use interval analysis to determine if this constraint does not allow to reduce the size of the interval on the physical parameters in which case we may obtain a new 𝐉𝐓 that is included in the initial one. In turn this may allow to obtain better τv and Fs. The process stops when no improvement has been obtained for Fs, τv and the physical parameters.

To test this approach the right upper limb joint torque of 10 males and the force capacity at the right hand was measured by a dynamometer (Biodex III, Biodex Medical Systems) and respectively by a 6-axis load sensor during an experiment performed at HandiBio laboratory. The configuration of the upper limb was measured with a motion capture system (Qualisys, Sweden). The approach is currently being evaluated.

Walking analysis and Rehabilitation

Participants : Claire Maillard, Ting Wang, Jean-Pierre Merlet [correspondant] .

The walkers of the ANG family allow one to determine accurately the trajectory of the walker and therefore to analyze the walking of the user. But these walkers may also be used to assess a rehabilitation process or the progress of an end-user involved in rehabilitation. For that purpose after having identified needs and requirements [17] we developed a new walker ANG-med that used infra-red distance sensors to measure the position of the subject during a rehabilitation exercise. Furthermore the software of this walker has been designed to support a message-passing scheme based on the HOP language of the INDES project team so that the walker may exchange information and control order with an external computer, together with allowing the download of new rehabilitation exercise through the robotics RAPP-store  [26] . New exercises are designed as a set of such messages, that may include the calculation of exercise assessment indicators. ANG-med supports various modes: stand-alone (no external connection), passive mode (the walker only report indicator and status using a wifi connection) or full external control (an external computer fully control the walker except for emergency and real-time procedures).

ANG-med has been tested for one month in Centre Héliomarin de Vallauris and is now deployed in the rehabilitation center of MATIA in Spain, as part of the RAPP project. A start-up plan was proposed in November 2014 to transfer the walking analysis technology of HEPHAISTOS with the ANG walker in a company called Euthenia .

Design and evaluation of assistive devices, ethics

Participants : Marc Beninati, Bernard Senach [correspondant] , Jean-Pierre Merlet.

Providing appropriate support, services and information to the elderly, to their caregivers and to the medical profession, through a fleet of communicating devices must rely on a structured processes. A generic design and evaluation framework is being elaborated and will be validated through field experiments [20] , [19] , [18] .

Assistance robotics raises many ethical questions. We started reflection about conducting experiments with frail and old people. A listing of questions to be addressed at each step of an experiment has been written (internal document). We have also hired a joint PhD student with University Bologna about the legal aspects of assistance robotics and we have initiated, together with Nathalie <Nevejans from University of Douai, a meeting with the OPECST at the French National Assembly to discuss legal and ethical aspects of robotics.

Smart Environment for Human Behaviour Recognition

Participants : Aurélien Massein, Yves Papegay, Odile Pourtallier.

Both economic motivations due to demographic evolution and willingness of people to live independently at home when aging, facing physical impairment or recovering from injuries has raised the need for activity monitoring at home, in rehabilitation center or in retirement home. Monitoring systems provide information that can range from a broad measure of the daily activity to a precise analysis of the ability of a person performing a task (cooking, dressing, ...) and its evolution.

The broad range of needs and contexts, together with the large variety of available sensors implies the necessity to carefully think the design of the monitoring system. An appropriate system should be inexpensive and forgettable for the monitored person, should respect privacy but collect necessary data, and should easily adapt to stick to new needs. We aim to provide an assisting tool for designing appropriate monitoring systems.

As part of a PhD work, optimal motion planning of a mobile robot with range sensors to locate targets in a room has been studied. Work in progress also include algorithms to deploy infra-red barriers in a large area with several interest places, to be able to locate people. An experimental set-up is in use in the lab and data analysis methods are developed to infer people behaviors.