This paper is concerned with summarizing the categories of impedance control showing the feature and the limitations of each category. Much attention is paid to variable impedance control considering the possible control schemes, performance, the stability, the integration of constant/variable compliant elements with the host robots, etc.
For a long time, the robotics community concentrated on improving the performance of the robotic systems in free space. The advanced control strategies for position control could not be sufficient to stabilize the motion of robots in constrained spaces. In general, there are two categories of force control schemes with miscellaneous subdivisions: hybrid position-force control and impedance control. The former is well suitable for well-known interaction environment, however, it does not consider the dynamic interaction between robot end-effector and the environment. In contrast, impedance control includes regulation and stabilization of robot motion via creating a mathematical relationship between the interaction forces and the reference trajectories.
In general, a mass-spring-damper impedance filter is used for stabilization purposes. Tuning the parameters of the impedance filter is not trivial and may lead to unstable contact if an unsuitable strategy is used. The human, however, has amazing control systems with advanced biological actuators. He/she can manipulate the muscles stiffness to softly comply to the interaction forces. Accordingly, the parameters of the impedance filter should be time-varying rather than value-constant in order to meet the human behavior during interaction tasks.
Table of Contents
1. Introduction
2. Background
2.1 Dynamic modeling of robots in constrained motion
2.2 Force or torque-based impedance control
2.3 Position-based impedance control vs. force-based impedance control
4. Position-based impedance control with force tracking
4.1 Dependence of force tracking error on the knowledge of the environment parameters
4.2 Methods
5. Variable impedance control
6. Active impedance control flexible joints actuated systems with inner torque loop
6.1 Constant impedance-joint robots
6.2 Variable impedance-joint robots
7. Conclusions
Objectives and Core Topics
This paper aims to systematically summarize and evaluate various categories of impedance control strategies for robotic mechanisms. The primary focus lies on analyzing their performance, stability, and suitability for interaction in constrained environments, with a particular emphasis on variable impedance control to better replicate human-like motion capabilities.
- Comparison of force-based and position-based impedance control schemes.
- Methods for achieving effective force tracking in uncertain environments.
- Integration of variable impedance control to enhance safety and adaptability.
- Control strategies for robots with flexible joints, specifically using cascade control.
- Analysis of biomimetic approaches for human-robot interaction.
Excerpt from the Book
1. Introduction
When a robot is in contact with the environment via its end-effector, some important points should be noted:
• Position control strategies are not sufficient for precise tracking of the desired position and force references; both the position and interaction force should be controlled carefully. For example, if the task of target robot is to write something, neglecting control of interaction force may lead to either loss of contact or hard pressure on the target environment [1]. In general, for rigid or dynamic interaction environments, pure position control schemes are not recommended especially if the environment is stiff; the contact forces may reach unsafe values [2].
• Besides, the robot looses some degrees of freedom (DoFs) during the contact phase. Consequently, the generalized coordinates of the target robot could be larger than its DoFs due to its constrained motion; it constitutes closed chain mechanism with redundant coordinates [3].
• The robot may change its configuration during a transition from open chain mechanism to a closed chain. In effect, three motion phases could be produced: free motion phase, contact motion phase (impact phase), and constrained motion phase. Every phase may have its own features and control law [3].
Summary of Chapters
1. Introduction: Outlines the limitations of standard position control in constrained environments and introduces the necessity of impedance control to regulate interaction forces.
2. Background: Provides the foundational modeling of robots in constrained motion and details the two primary categories of conventional impedance control.
4. Position-based impedance control with force tracking: Investigates the challenges of achieving accurate force tracking when environment parameters are uncertain and discusses methods to mitigate tracking errors.
5. Variable impedance control: Discusses the motivation for adapting impedance parameters, inspired by human biological movement, to improve safety and performance during contact.
6. Active impedance control flexible joints actuated systems with inner torque loop: Explores the use of cascade control for robots with flexible elements, comparing constant and variable impedance joints.
7. Conclusions: Summarizes the key findings regarding the features, limitations, and future research directions for impedance control in robotic systems.
Keywords
Impedance control, Force control, Admittance control, Variable impedance control, Series elastic actuators, Variable impedance actuators, Human-robot interaction, Constrained motion, Force tracking, Cascade control, Robot stability, Compliance control, Biomimetic control, Flexible joints, Interaction forces
Frequently Asked Questions
What is the primary scope of this paper?
This paper systematically reviews various categories of impedance control techniques used to regulate robotic interaction forces, particularly in constrained environments.
What are the main control categories discussed?
The paper classifies approaches into force-based impedance control, position-based impedance control (admittance control), and various variable impedance control strategies.
What is the core challenge regarding force tracking?
A significant challenge is that conventional impedance control often fails to track desired interaction forces accurately in the presence of uncertain environment stiffness or modeling errors.
Why is variable impedance control considered superior in some cases?
It allows the robot to adaptively modify its stiffness, similar to the human arm, which enhances safety and performance during tasks with unpredictable contact.
What is the role of cascade control in this context?
Cascade control is frequently used in robots with flexible joints to integrate outer impedance loops with inner torque or velocity control loops for better performance.
What characterizes the biomimetic approach to control?
The biomimetic approach seeks to mimic human muscle characteristics—such as variable stiffness and compliance—to enable safe and dexterous human-robot physical interaction.
How is the environment stiffness handled?
The paper highlights that knowledge of environment parameters is often necessary for high-performance tracking, leading to techniques like trajectory modification and stiffness adjustment.
Why is the "tank-based" impedance control mentioned?
It is proposed as a solution to maintain passivity and stability when using variable stiffness matrices, addressing potential instability issues found in other variable impedance methods.
- Arbeit zitieren
- Dr. Hayder Al-Shuka (Autor:in), Kareem Jawad Kadhim (Autor:in), 2020, An Introduction to Impedance Control of Constrained Robotic Mechanisms, München, GRIN Verlag, https://www.hausarbeiten.de/document/958860
