Switching control for tracking of a hybrid position-force trajectory (1503.00603v1)

Abstract: This work proposes a control law for a manipulator with the aim of realizing desired time-varying motion-force profiles in the presence of a stiff environment. In many cases, the interaction with the environment affects only one degree of freedom of the end-effector of the manipulator. Therefore, the focus is on this contact degree of freedom, and a switching position-force controller is proposed to perform the hybrid position-force tracking task. Sufficient conditions are presented to guarantee input-to-state stability of the switching closed-loop system with respect to perturbations related to the time-varying desired motion-force profile. The switching occurs when the manipulator makes or breaks contact with the environment. The analysis shows that to guarantee closed-loop stability while tracking arbitrary time-varying motion-force profiles, the controller should implement a considerable (and often unrealistic) amount of damping, resulting in inferior tracking performance. Therefore, we propose to redesign the manipulator with a compliant wrist. Guidelines are provided for the design of the compliant wrist while employing the designed switching control strategy, such that stable tracking of a motion-force reference trajectory can be achieved and bouncing of the manipulator while making contact with the stiff environment can be avoided. Finally, numerical simulations are presented to illustrate the effectiveness of the approach.

• The paper introduces a novel switching control strategy for hybrid position-force tracking that ensures stability between free motion and contact phases.
• The methodology integrates active control damping with a compliant wrist design to manage high impact forces during stiff environmental interactions.
• Numerical simulations validate the approach, showing reduced peak forces and improved tracking performance through optimized gain tuning.

Switching Control for Tracking of a Hybrid Position-Force Trajectory

Introduction

The paper "Switching control for tracking of a hybrid position-force trajectory" explores an approach to control robotic manipulators interacting with stiff environments, focusing specifically on scenarios where the interaction affects only one degree of freedom. The proposed method involves a switching control strategy to perform a hybrid position-force tracking task. The work addresses the challenge of maintaining stability during transitions between free motion and contact with a stiff environment—a critical issue in applications such as teleoperation and automated assembly.

Control Strategy

The control strategy proposed is a hybrid position-force controller that switches based on the contact state. The key features are:

• Switching Mechanism: The controller switches modes when the manipulator makes or breaks contact with the environment. The motion control mode is active during free motion, while a force control mode takes over in contact scenarios.
• Stability Analysis: The stability of the closed-loop system is examined. The paper provides sufficient conditions to ensure input-to-state stability (ISS) with respect to disturbances arising from time-varying desired profiles.
• Propagation Damping: The analysis indicates that a significant amount of damping is usually required to maintain stability—particularly to handle abrupt changes in dynamics upon contact or detachment.

System Model and Controller Design

The manipulator-environment interaction is modeled with a dynamic system that uses the Kelvin-Voigt viscoelastic model to represent environmental contact:

• Manipulator Dynamics: The model consists of equations that describe both free motion and contact periods. The manipulator dynamics in Cartesian space integrate forces experienced due to environmental stiffness and damping properties.
• Hybrid Controller: The proposed controller effectively handles free motion to adhere to position trajectories, and switches to a force controller leveraging desired force profiles during contact. Parameters such as proportional and derivative gains are rigorously tuned.

Numerical Simulations and Results

Simulations illustrate the robustness of the approach, evidencing:

• Impact Force Management: During contact, excessive peak forces are mitigated, and force tracking performance improves when the manipulator is designed with a compliant wrist.
• Parameter Sensitivity: A high damping in the controller is necessary to prevent bouncing and ensure stable tracking when interacting with stiff environments.

Compliant Manipulator Concept

To counteract the impractical requirements for high controller damping:

• Compliant Wrist Design: The authors propose incorporating a compliant wrist to passively absorb impact energies, thus decreasing reliance on active damping from the controller.
• Model Reduction: The impact and contact phases are represented through a reduced order system, leading to improved design flexibility and ensuring stability through the physical system's inherent compliance.

Conclusion

This paper advances the field of hybrid position-force control by proposing a novel switching control strategy that simplifies the implementation of stable contact dynamics in manipulators. By introducing system compliance, they circumvent traditional limitations imposed by stiff environments. The implications of this work extend to more natural and robust control systems in robotics, primarily in applications requiring delicate interaction with solid surfaces, potentially inspiring future research into adaptive compliant designs and advanced control strategies under varying dynamic conditions.