A survey of the field of control for flexible multi-link robots is presented. This research area has drawn great attention during the last two decades, and seems to be somewhat less “attractive” now, due to the many satisfactory results already obtained, but also because of the complex nature of the remaining open problems. Thus it seems that the time has come to try to deliver a sort of “state of the art” on this subject, although an exhaustive one is out of scope here, because of the great amount of publications. Instead, we survey the most salient progresses – in our opinion – approximately during the last decade, that are representative of the essential different ideas in the field. We proceed along with the exposition of material coming from about 119 included references. We do not pretend to deeply present each of the methods quoted hereafter; however, our goal is to briefly introduce most of the existing methods and to refer the interested reader to more detailed presentations for each scheme. To begin with, a now well-established classification of the flexible arms control goals is given. It is followed by a presentation of different control strategies, indicating in each case whether the approach deals with the one-link case, which can be successfully treated via linear models, or with the multi-link case which necessitates nonlinear, more complex, models. Some possible issues for future research are given in conclusion.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0263574703005642

Control of flexible manipulators: A survey

This chapter considers the early research on flexible manipulators, and then goes on to summarize more recent work on the subject.

Past and Recent Works

Reducing the weight of robotic manipulators has been identified as one of the important factors that can reduce the production and installation cost and allow for safe physical human-robot interaction but it results to flexible robotic manipulators. Motion control of flexible manipulators is a challenging task, particularly when the flexibility is arising not only because of flexible joints but also due to flexible links. To address this problem, more advanced real-time control strategies that effectively use more accurate mathematical models are required. This thesis deals with the various aspects of modelling, design, simulation and control of robotic manipulators that have both flexible joints and flexible links. For a manipulator with long-reach elastic arms (links), the simulations show that a lumped mass-spring-damper model with enough number of elements can adequately capture the critical modes of the distributed link flexibility of a robot arm. The main contributions of this work is the use of lumped inverse dynamic models to generate the required actuator references in addition to feedforward computed torque for high performance end-effector’s trajectory tracking. To pursue this target, the dynamic differences between lumped models and elastic links are compensated both in the frequencyand time-domain. The compensation in frequency-domain relies on frequency response matching of zeros and poles between lumped models and models of the manipulator arms and joints. By using lumped model in the inverse dynamic models for feedforward control, real-time implementation will be possible in robot controllers used today. The compensation in the time-domain is based on feed-forward compensation of estimated link position errors obtained from measurements or FEM model simulations. Trajectory tracking results using different model implementations are presented and analysed and they show that the performance of the proposed modelling and control methodologies is promising.

Modelling and feed-forward control of robot arms with flexible joints and flexible links

A new method of computing optimum mass and rigidity distributions for composite flexible manipulators with tip loads is presented. The design problem is classified as minimum weight design or maximum speed design with appropriate constraints. Multiple tip load designs are considered, as well as practical issues related to link construction.

J.L. Russell

Papers

A new approach to optimum flexible link design