An Adaptive Grasping Force Tracking Strategy for Nonlinear and Time-Varying Object Behaviors

TL;DR

This work tackles adaptive grasping force tracking for objects with unknown, nonlinear, and time-varying properties by introducing generalized stiffness $k(t, \mu(t), F)$ and an online LSTM-based stiffness estimator to dynamically tune a PI controller. The estimator augments a physics-based baseline with neural correction, achieving high accuracy with short probing times (mean validation loss $0.144$ vs $142.4$ for the baseline). The force-tracking controller adjusts PI gains by $1/\hat{k}(t)$, and stability requires $\eta(t)=\hat{k}/k$ to lie in $(0.634,2.366)$, enabling rapid convergence when $\hat{k}$ is close to the true stiffness. Experimental results over 14 objects show a mean asymptotic error of about $0.16\,\text{N}$ and probing time of $3.34\,\text{s}$, substantially outperforming prior methods and demonstrating robust generalization to elastic, viscoelastic, plastic, and variable-stiffness materials.

Abstract

Accurate grasp force control is one of the key skills for ensuring successful and damage-free robotic grasping of objects. Although existing methods have conducted in-depth research on slip detection and grasping force planning, they often overlook the issue of adaptive tracking of the actual force to the target force when handling objects with different material properties. The optimal parameters of a force tracking controller are significantly influenced by the object's stiffness, and many adaptive force tracking algorithms rely on stiffness estimation. However, real-world objects often exhibit viscous, plastic, or other more complex nonlinear time-varying behaviors, and existing studies provide insufficient support for these materials in terms of stiffness definition and estimation. To address this, this paper introduces the concept of generalized stiffness, extending the definition of stiffness to nonlinear time-varying grasp system models, and proposes an online generalized stiffness estimator based on Long Short-Term Memory (LSTM) networks. Based on generalized stiffness, this paper proposes an adaptive parameter adjustment strategy using a PI controller as an example, enabling dynamic force tracking for objects with varying characteristics. Experimental results demonstrate that the proposed method achieves high precision and short probing time, while showing better adaptability to non-ideal objects compared to existing methods. The method effectively solves the problem of grasp force tracking in unknown, nonlinear, and time-varying grasp systems, demonstrating the generalization capability of our neural network and enhancing the robotic grasping ability in unstructured environments.

Figures (16)

• Figure 1: Grasping force tracking system.
• Figure 2: State change of the object.
• Figure 3: Neural network workflow. $F$ is the grasping force, $x$ is the finger displacement, $\hat{k}_E$ is the stiffness estimated by LSM, and $\hat{k}$ is the generalized stiffness estimated by our method.
• Figure 4: Random data sampling process. (a) Generated grasping force $F(t)$. (b) Generated surface $x(t, F)$. (c) Simulated deformation $x(t)$.
• Figure 5: (a) Training and validation loss. (b) Stiffness estimation comparison.