Section: New Results


Cable-driven parallel robots (CDPR)

Analysis of Cable-driven parallel robots

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

We are still investigating the extremely complex analysis of the kinematics [24] of CDPRs assuming either rigid [21] [20] , elastic or sagging cables.

We have also started an analysis of cable configuration of redundantly actuated CDPRs for control purposes. Indeed we have shown that for robot with rigid cables it is impossible to have, in a given pose, more than 6 cables in tension simultaneously: the set of cables under tension is called the cable configuration. However at a pose there may be different sextuplets of under tension cables that satisfy the kinematico-static equations. Each of these sextuplets exhibits different performances (e.g. maximal tension in the cables or sensitivity of the positioning to errors in the cable lengths). Hence it may be interesting for control purposes to select one of the sextuplet that is optimal with respect to a performance criteria and to enforce this configuration by letting voluntary the cables that are not in the sextuplet being slack (i.e. adjusting their lengths to be larger than the one required for the pose).

We have generalized this approach for a trajectory of a 4 cables CDPR with all cables attached to the same point of the platform. In that case only up to 3 cables may be under tension at the same time. We have designed an algorithm that determine the optimal cable configuration on the whole trajectory.

Simultaneously we have addressed part of an ambitious goal: a full simulation tool for CDPR. We assume a high level motion planning loop that calculate a motion order every Δt1 second and send this command to an inner motor control loop that execute it by sending a command to the motor every Δt2 second. Then we have a continuous time model of the motor that determine its velocity. The whole purpose is to calculate the pose of the platform together with the tensions in the cables. This simulation is extremely demanding and cannot be performed with classical software because of the changes in the cable configuration that have to be detected for determining the platform pose and cable tensions. We have succeeded for CDPR with rigid and elastic cables, furthermore introducing random errors in the cable length measurements. This tool has allowed us to show that cable tensions are very sensitive: for example a high level loop that is designed to minimize τj2, where τ are the cable tensions, exhibits large difference with the objective as soon as discrete time-control is taken into account.

Certified Calibration of a Cable-Driven Robot Using Interval Contractor Programming

Participants : Julien Alexandre Dit Sandretto, David Daney, Gilles Trombettoni.

An interval based approach is proposed to rigorously identify the model parameters of a parallel cable-driven robot. The studied manipulator follows a parallel architecture having 8 cables to control the 6 DOFs of its mobile platform. This robot is complex to model, mainly due to the cable behavior. To simplify it, some hypotheses on cable properties (no mass and no elasticity) are done. An interval approach can take into account the maximal error between this model and the real one. This allows us to work with a simplified although guaranteed interval model. In addition, a specific interval operator makes it possible to manage outliers. A complete experiment validates our method for robot parameter certified identification and leads to interesting observations [9] , [16] , [15] .

Tool for Agencement Analysis and Synthesis of CDPRs

Participants : Laurent Blanchet, Jean-Pierre Merlet [correspondant] .

In the frame of FP7 project CABLEBOT, we are developing a methodology to analyze or synthesize a Cable Driven Parallel Robots configuration i.e. either to determine the performances of a given CDPR (e.g. maximal wire tensions over a given workspace) or, being given a list of requirements, to determine what what are all possible CDPR geometries that are guaranteed to satisfy the requirements. This tool relies heavily on our analysis of the CDPRs and on interval analysis.

To illustrate this approach we have developed a software that can be used to illustrate the workings/operating procedure of interval analysis through a 3D visualization. This software sets up a scenario of a CDPR in a warehouse and computes in real time its workspace under different constraints.

Visual-servoing of a parallel cable-driven robot

Participants : Rémy Ramadour, Jean-Pierre Merlet [correspondant] , François Chaumette [correspondant] .

MARIONET-ASSIST is a parallel cable-driven robot designed to move through large rooms in order to provide services such as walking-aid, lifting people or manipulating heavy loads. In order to experiment, a full-scaled flat with a crane robot has been built. Adding one or several low-cost cameras (the cost being here a fundamental constraint), visual-servoing control is used to provide a whole new set of useful services such as grasping objects in order to bring them to the end-user (if they are too heavy, too far, high or low), or cleaning the table after lunch. Using a parallel crane robot, we are able to cover a large workspace, the vision-control allowing us to obtain the precision required by the manipulation of daily-life objects. The collaborative implementation of the vision and the kinematic control of the robot gives us a way to make best use of the advantages of both parts, while overcoming their respective drawbacks.

This project is supported by the large-scale initiative PAL.

Experimentation showed that we are able to provide a much better accuracy and repeatability using visual-servoing. However, the velocity of the process is slowed because of several encountered problems :

  • when there are changes in the distribution of tension between the wires, oscillations are occurring on the end-effector, affecting the movement of the camera in such a way that we can not rely on the measurements

  • the methods used to first detect the object are not satisfactory. Also, the actual segmentation is not robust to luminance changes, the target may thus be lost during the process.

In order to overcome the first problem, we are working on an algorithm able to determinate the best sequence of configurations (distribution of tension) : we can avoid singularities and provide a more stable trajectory. The second problem has yet to be solved : we are at the moment looking into several methods, using for example k-nearest neighbors algorithms with different color spaces, gradient-based information and morphological preprocessing.

Finally, we experimented our device with others technologies developed within the context of PAL, in a full-scaled apartment located in Nancy (Loria-Inria).

Assistance robotics

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 [22] , [23] , [19] .

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...). To evaluate the physical abilities, biomechanics tests can be underwent on the upper limb, lower limb or the whole body. In particularly, the motricity of the upper limb can be measured in terms of range of motion, velocity, acceleration or forces.

To analyze the velocity of the loads in the upper limb, a polytope interpretation is used. Currently the force polytope at the hand is calculated from the torques τ measured at each joint (shoulder, elbow and wrist) by a dynamometer (Biodex III, Biodex Medical Systems). But because of the redundancy of the upper limb (7 degrees of freedom), the dynamic equation (τ=𝐉TF) is difficult to solve. To find the minimal and maximal forces F that can be exerted at the hand from the measured torques, we may use the jacobian pseudo-inverse with the method of Chiacchio but this method is not well suited to manage the large uncertainties in the measurements. In the a reverse approach, the force at the hand will be measured by a 6-axis load sensor and the minimal and maximal joint torques will be computed by using interval analysis and compared with the measurements of the Biodex.

Moreover this analysis of the force capacities in the upper limb need to be connected to the daily activities or usual motion test monitored by the medical services. Therefore, a review of tests and questionnaires regularly used to measure the physical capacities has been performed. This review gather the type of mark, the exercises and the used sensors that can be employed in future experimentation. Also, this review will be discussed with medical staff to highlight relevant activities.

Walking analysis

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

In the period 2009-2013 we have conducted in collaboration with Nice hospital a large experiment involving 54 subjects (30 elderly and 24 young adults) for determining walking pattern of elderly people using our instrumented walker ANG-light . We have started the processing of this large amount of data we some interesting results [25] :

  • a classical walking test is the 10 meter walking test: the subject is asked to perform a 10m straight line trajectory and the result is the total time. Such test may have large consequences as it is used to determine the autonomy level and the resulting financial aid. Our test has surprisingly shown that when using a walker elderly people are usually faster that young adults

  • on the other hand the maximal deviation with respect to the desired trajectory is much smaller for young adults than for elderly one. Furthermore few elderly have the same deviation and it may be considered as a signature of the walking pattern that is worth measuring

Our objective is now to analyze the maneuvers (half-turn and round-about) and to compare/complement the data with the one obtained with a Kinect. A long term objective is also to implement a model of a human walking with a walker and to use this model for an inverse calculation: measuring walking patterns indicators with the walker and calculating these indicators when not using the walker.

Experimental calibration of a high-accuracy space telescope

Participants : Thibault Gayral, David Daney, Jean-Pierre Merlet.

A collaborative work began in October 2010 with Thales Alenia Space on the calibration of the mechanical structure of a space telescope. Its architecture is based on a parallel manipulator (type active wrist 6-PUS) used to correct the relative position of two mirrors. The aim is to reach a micrometer accuracy in order to obtain a suitable quality of the images provided by the telescope. Thus, a complete model of the space telescope needs to be developed and validated through calibration. Since high velocity is not required in such an application, the dynamic effects can be neglected and only geometric and/or static calibration has to be considered.

For the geometric models, measurements for calibration were performed in a clean room under controlled pressure, temperature and humidity conditions to minimize the influence of the non-geometric errors. Thus, two possible static inaccuracy sources were identified and modeled: one from the deformation of the mobile platform and the other resulting from the behavior of the flexure joints. Three incremental models of the flexure joints were developed and compared: a spherical joint model, a model issued from the beam theory and a stiffness model. Results of calibration using an accurate measurement system of photogrammetry showed that the flexure joints can be modeled by perfect spherical joints due to the small workspace of the telescope. Concerning the mobile platform deformation, two models were developed. With those models, a positioning accuracy of some micrometers was finally reached after calibration with only position and orientation measurements of the mobile platform.

Then, opto-mechanical models were developed considering experimental measurements by imaging on the prototype of the space telescope. The optical defects were analyzed considering Zernike polynomials. The aim of optical calibration was to minimize the coefficients of the Zernike polynomials in order to improve the optical properties of the space telescope. Results of calibration were studied in order to perform a proper choice of the opto-mechanical models. Finally, the optical quality was improved after calibration. This validates the fact that the telescope can be calibrated directly in space, after its deployment, with only the provided information. A second campaign of measurements by imaging was programmed to finely adjust the opto-mechanical model parameters.