A Practical Force Correction Method for Over-Constrained Cable-Driven Parallel Robots

Abstract

Cable-driven parallel robots can have a much larger workspace than other parallel robots with rigid links or conventional serial robots. This property comes at the cost of more complex workspace calculations and control schemes that are necessary to account for the elasticity and unilateral force transmission of their parallel cable links. In practice, most cable-driven parallel robots cannot achieve the full workspace that is predicted by theoretical models. This is due to calibration errors and simplified modelling assumptions in the control schemes. While most previous works on this subject have focused on creating more accurate and complex models, the goal of this work is to increase the workspace volume that cable-driven parallel robots can realize in practice by using a simple model coupled with a new force correction method that is robust to modelling errors and uncertainties. The new method applies force corrections within the nullspace of the structure matrix in order to keep the cable forces within their limits. Experiments show that the new method can significantly increase the workspace when used in addition to basic kinematic codes. In simulations this combination achieves the same workspace as complex controllers that require the precise knowledge of many additional model parameters.

Force-Sensor-Free Implementation of a Hybrid Position–Force Control for Overconstrained Cable-Driven Parallel Robots

“…• the classical approach that exploits force sensors (HC-f ), as applied in [38][39][40]; • the nullspace control strategy (NC) proposed in [4,42], which exploits load cells to maintain cable forces always positive without computing a force distribution; • a pure inverse kinematic controller (IKC), which (as the new controllers proposed in this paper) does not use force sensors, but it does not involve any correction to maintain cable tensions within predefined bounds.…”