Vision-Based Hybrid Controller to Release a 4-DOF Parallel Robot from a Type II Singularity.

José L Pulloquinga,Vicente Mata,Mónica Urízar,Marina Vallés,Jesús Ferrándiz,Rafael J Escarabajal

doi:10.3390/s21124080

José L Pulloquinga, Vicente Mata + Show 4 more

Open Access

https://doi.org/10.3390/s21124080

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Abstract

The high accuracy and dynamic performance of parallel robots (PRs) make them suitable to ensure safe operation in human–robot interaction. However, these advantages come at the expense of a reduced workspace and the possible appearance of type II singularities. The latter is due to the loss of control of the PR and requires further analysis to keep the stiffness of the PR even after a singular configuration is reached. All or a subset of the limbs could be responsible for a type II singularity, and they can be detected by using the angle between two output twist screws (OTSs). However, this angle has not been applied in control because it requires an accurate measure of the pose of the PR. This paper proposes a new hybrid controller to release a 4-DOF PR from a type II singularity based on a real time vision system. The vision system data are used to automatically readapt the configuration of the PR by moving the limbs identified by the angle between two OTSs. This controller is intended for a knee rehabilitation PR, and the results show how this release is accomplished with smooth controlled movements where the patient’s safety is not compromised.

Highlights

Parallel robots (PRs) are composed of two or more closed kinematic chains connecting a fixed and a mobile platform that defines the end-effector to be controlled [1]
The main experimental results show the effectiveness of the hybrid controller using singularity releaser module (SRM)-V1 to release the PR under study from a type II singularity
The SRM launched has a lapse of 15s, allowing it to move the PR under study to a non-singular configuration

Summary

Introduction

Parallel robots (PRs) are composed of two or more closed kinematic chains connecting a fixed and a mobile platform that defines the end-effector to be controlled [1]. As opposed to their serial counterpart, they benefit from greater accuracy, stiffness, and load capacity, making them suitable for a great variety of applications [2,3]. Human–robot interaction is one of the major applications, for instance, in the context of medical rehabilitation [4]. The former can be addressed by means of a proper mechanical design of the PR to cover the workspace as required, while the latter requires further analysis

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