Active Impedance Control of Bioinspired Motion Robotic Manipulators: An Overview

Hindawi Applied Bionics and Biomechanics Volume 2018, Article ID 8203054, 19 pages https://doi.org/10.1155/2018/8203054 Review Article Active Impedance Control of Bioinspired Motion Robotic Manipulators: An Overview Hayder F. N. Al-Shuka ,1 Steffen Leonhardt,2 Wen-Hong Zhu,3 Rui Song ,1 Chao Ding,1 and Yibin Li1 1 School of Control Science and Engineering, Shandong University, Jinan, China Philips Chair for Medical Information Technology (MedIT), Helmholtz Institute, RWTH Aachen University, Aachen, Germany 3 Canadian Space Agency, Longueuil, Canada 2 Correspondence should be addressed to Rui Song; Received 6 April 2018; Revised 6 June 2018; Accepted 24 June 2018; Published 18 October 2018 Academic Editor: Dongming Gan Copyright © 2018 Hayder F. N. Al-Shuka et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. There are two main categories of force control schemes: hybrid position-force control and impedance control. However, the former does not take into account the dynamic interaction between the robot’s end effector and the environment. In contrast, impedance control includes regulation and stabilization of robot motion by creating a mathematical relationship between the interaction forces and the reference trajectories. It involves an energetic pair of a flow and an effort, instead of controlling a single position or a force. A mass-spring-damper impedance filter is generally used for safe interaction purposes. Tuning the parameters of the impedance filter is important and, if an unsuitable strategy is used, this can lead to unstable contact. Humans, however, have exceptionally effective control systems with advanced biological actuators. An individual can manipulate muscle stiffness to comply with the interaction forces. Accordingly, the parameters of the impedance filter should be time varying rather than value constant in order to match human behavior during interaction tasks. Therefore, this paper presents an overview of impedance control strategies including standard and extended control schemes. Standard controllers cover impedance and admittance architectures. Extended control schemes include admittance control with force tracking, variable impedance control, and impedance control of flexible joints. The categories of impedance control and their features and limitations are well introduced. Attention is paid to variable impedance control while considering the possible control schemes, the performance, stability, and the integration of constant compliant elements with the host robot. 1. Introduction When a robot is in contact with the environment via its end effector, some important points should be noted: (i) Given a specific degree of freedom, it is not possible to independently regulate the position and the contact force. For example, if the task of the target robot is to write something, neglecting control of the 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] (ii) In addition, the robot loses some degrees of freedom (DoFs) during the contact phase. Consequently, the generalized coordinates of the target robot might be larger than its DoFs due to its constrained motion; this constitutes a closed-chain mechanism with redundant coordinates [3] (iii) The robot may change its configuration during a transition from an open-chain mechanism to a closed-chain mechanism. In effect, three motion phases can be produced: the free motion phase, the 2 Applied Bionics and Biomechanics Impedance control Force/torque-based impedance control (impedance control) Sect. 2.2 Position/velocity-based impedance control (admittance control) Sect. 2.3 Admittance control with force tracking Sect. 3 Conventional impedance/admittance control Active variable impedance control Sect. 4 Impedance control of constant impedance flexible joints Sect. 5 Extended impedance/admittance control Figure 1: A general classification of the impedance control approaches. The paper is organized according to the depicted classification. contact motion phase (impact phase), and the constrained motion phase. Each phase can have its own features and control law [3] One of the solutions to regulate and control the interaction forces is hybrid position/force control proposed by Raibert and Craig [4]. The hybrid force-position control decouples the task space into position-controlled space and force-controlled space. Then the hybrid position/force control law is designed to track the desired position and force references. However, this scheme does not take into consideration the impedance effect between the environment and the robot end effector. In effect, impedance control plays an important role in any workspace that involves human-robot interactions. The idea behind it is to control the mechanical impedance of a host robot regulating the interaction forces produced by the coupling between the robot and the environment dynamics; mechanical impedance can be defined as the ratio of the output force to the input velocity (motion). For linear systems, mechanical admittance is the inverse of mechanical impedance; it can be defined as the ratio of input velocity (motion) to the output force. In general, the robot can ideally behave as an impedance and the contact environment is an admittance; however, this could not be the case for multibody robotic systems with heavy links and actuators [5, 6]. Impedance control is inspired by the human behavior during contact with different environments. Humans have a considerable amount of adaptability to change muscle impedance (e.g., stiffness) when in contact with an unknown environment. If the environment is stiff, the robot should be soft and vice versa. Rigid robots, however, do not have this capability; in principle, they are stiff. They are well suited for precise free motion space, but problems can occur when moving in an unstructured environment. Excessive interaction forces should be avoided. This can be achieved by making the robots change their stiffness. Therefore, Hogan proposed active impedance control which is based on the biomechanics of human motion in free and constrained spaces [5, 6]. The idea behind impedance control is to design a user-defined dynamic relationship between the reference trajectory of the end effector and the interaction contact force/torque along each axis. However, a trade-off occurs between the tracking of the position and the interaction forces [7]. Hogan proposed two models of impedance control [5, 6]: torque- or force-based impedance control and position-based impedance control. Due to the related limitations of conventional impeda (...truncated)