Incorporating contact area in soft finger grasp models

The ability to grasp is a significant function of many robotic hands, whether it be for fruit-picking or manufacturing tasks. Two major theories often used in conjunction to model grasping are Nguyen’s theorem and Coulomb’s friction law. Both theories model the contact between the fingers and the object in grasp as at a point. This runs true when the fingers and the object in grasp are made of hard materials. However, when the fingers or the object are soft, they deform around each other and result in a contact area between the objects rather than a contact point. This contact area means that soft-fingered grasps can apply an additional moment to balance any external moments, of which hard fingers cannot. To account for this, the soft-fingered grasp is often modelled as a point contact with a moment about the normal direction. However, in scenarios where there is no external moment both soft and hard fingers would exert a moment of 0 Nm and their grasps are modelled identically using Nguyen’s theorem and Coulomb’s law. Since soft-fingered grasping has been used extensively due to its superiority to hard-fingered grasping it does give rise to the question of whether the soft fingered grasp should be modelled identically to the hard fingered grasp in this case. This research expands the current grasp models to better showcase the differences between soft and hard finger grasps. The approach taken to achieve this was to incorporate the contact area of soft fingered grasping into the contact model so as to highlight the differences between soft and hard fingers when grasping an object. The research utilised this expansion in the contact model employing Nguyen’s theorem and Coulomb’s friction law. Nguyen’s theorem is a condition that must be met for a grasp to be in force closure, while Coulomb’s friction model limits the forces that can be applied by a finger onto an object to being within a friction cone at a point for no slipping to occur. Both Nguyen’s and Coulomb’s theories model the friction cone of the soft finger at the centre-point of the area of contact. A set-of-cones theory was proposed, where the area of contact is comprised of many friction cones corresponding to the points that make up the contact area instead of a single friction cone. An experiment was devised for a two-fingered symmetrical grasp of a cylinder, where the maximum angle of contact above the horizontal before slipping was investigated. The set-of-cones approach was made into a resultant friction cone model. The resultant cone model and the original centre-point cone model were used to predict the forces due to Coulomb’s theory at the maximum angle of contact. The predictions were compared to the data obtained from the experiment. It was found that the resultant cone contact model predicted the normal force applied at the maximum angle before slipping more accurately than the centre-point contact model for both soft finger materials being investigated in this research. When the resultant cone model was translated for use in the prediction of a force closure grasp by using Nguyen’s theorem, the range of positions where the object was grasped in force closure increased as compared to when using the centre-point contact model. If further verified, the resultant cone model would be used for soft fingers while the centre-point model would be used for hard fingers. This extends the modelling of soft finger contact so as to illustrate the differences in the stability of a grasp between hard and soft finger contact