Accuracy of Robotic Elastic Object Manipulation as a Function of Material Properties

Abstract

We deal with the problem of thin string (1D) or plate (2D) elastic material folding and its modeling. The examples could be metallic wire, metal, kevlar or rubber sheet, fabric, or as in our case, garment. The simplest scenario attempts to fold rectangular sheet in the middle. The quality of the fold is measured by relative displacement of the sheet edges. We use this scenario to analyse the effect of the inaccurate estimation of the material properties on the fold quality. The same method can be used for accurate placing of the elastic sheet in applications, e.g. the industrial production assembly.

In our previous work, we designed a model simulating the behavior of homogeneous rectangular garment during a relatively slow folding by a dual-arm robot. The physics based model consists of a set of differential equations derived from the static forces equilibrium. Each folding phase is specified by a set of boundary conditions. The simulation of the garment behavior is computed by solving the boundary value problem. We have shown that the model depends on a single material parameter, which is a weight to stiffness ratio. For a known weight to stiffness ratio, the model is solved numerically to obtain the folding trajectory executed by the robotic arms later.

The weight to stiffness ratio can be estimated in the course of folding or manually in advance. The goal of this contribution is to analyse the effect of the ratio inaccurate estimation on the resulting fold. The analysis is performed by simulation and in a real robotic garment folding using the CloPeMa dual-arm robotic testbed. In addition, we consider a situation, in which the weight to stiffness ratio cannot be measured exactly but the range of the ratio values is known. We demonstrate that the fixed value of the ratio produces acceptable fold quality for a reasonable range of the ratio values. We show that only four weight to stiffness ratio values can be used to fold all typical fabrics varying from a soft (e.g. sateen) to a stiff (e.g. denim) material with the reasonable accuracy. Experiments show that for a given range of the weight to stiffness ratio one has to choose the value on the pliable end of the range to achieve acceptable results.