This paper discusses a shaping strategy of a rheological object whose deformation characteristic includes both elasticity and plasticity. We first introduce a seven-nodes viscoelastic model for approximating the outline and the dynamic characteristics of the object. Then, we show a shaping method of the object's outline, where the ratio between the object's length in the grasping direction and that in the perpendicular one is controlled by using a parallel jaw gripper. Based on the plastic deformation distribution for the integrated input stress, the proposed method can actively manage the final object's outline. In addition to the contribution on simplifying the gripper's degree of freedom, this method has the advantage that the handling time is drastically reduced, compared with the position based passive method. We finally show the experimental results for confirming the validity of the proposed method.

This paper discusses an active shaping method for an unknown rheological object by considering the characteristics of viscoelasticity. By utilizing a four-element model for approximating the dynamic characteristics of object's deformation, we drive the deformation decomposition into the elastic response and the plastic one. For shaping the object, we then propose a two-phase strategy for controlling the resultant deformation; in the first phase the viscoelastic parameters are estimated with avoiding the over deformation, based on the elastic response; in the second phase the desired resultant deformation is generated by actively managing the integral force, based on the plastic response. This strategy has an advance that the handling time of the robot is given by a finite time, while the desired resultant deformation is theoretically completed in the infinite time. We finally show experimental results for confirming the validity of the proposed strategy.

Viscoelastic materials are known to exhibit temporal response that changes force or displacement at the contact interface under position or force control, respectively. In this paper, we conduct experimental study using force control to explore and observe creep phenomenon in robotic grasping in order to better understand the nature of such contact interface, which has been widely used in soft robotic fingers, robotic feet, and contact surface of robotic arms. We found that the creep response under a constant external force exhibits the characteristics of exponentially increasing or decreasing temporal response. Such characteristics are similar in nature to those found in the relaxation response of viscoelastic materials when the grasping is under position control. Two different types of creep responses are found, depending on the state of grasping. Both Types I and II in creep response mirror the Types I and II in relaxation response. We also found that different loading rates under force control result in different elastic response, in addition to the temporal response. This is an interesting finding because the Fung's model postulates for an elastic response that is independent of, and can be separated from, the temporal response. The experimental results do not show such independence.

The latency model is an analytical model for describing the behavior of nonlinear viscoelastic contact interface in robotic grasping and manipulation. The latency model is based on experimental observation of viscoelastic materials which exhibit the behavior of both elastic and temporal responses when subject to external force or displacement. It is postulated that such materials display latency in response of external influence by the rearrangement of molecules, holes, and structures in order to achieve an equilibrium state corresponding to the instantaneous loading. As a result, we propose that there are temporal latent activities in progress before the material reaches the equilibrium state. In the previous study [21], the latent activity of strain re-distribution with a prescribed constant displacement was presented using both theoretical modeling and experimental results. In this paper, we build upon this latency model to study the behavior of viscoelastic materials under different loading rates with experimental results. The latency model is employed to explain the behavior of responses of hard and soft viscoelastic materials typically found in robotic contact and grasping.