Concept Of Impedance Control Research Articles

Rehabilitation of a Human Arm Supported by a Robotic Manipulator: A Position/Force Cooperative Control

Problem statement: The assistance of person with limited ability of ar m movement is necessary for rehabilitation reasons. This aid is r equired not only to cover the human performances of the arm in motion and force but also to have a stri ctly stable dynamics. In this study, we proposed a cooperative system between a disabled arm and a robotic manipulator to reach such objectives. Desired positions and contact forces were imposed by the di sabled human whereas appropriate torques were applied by the manipulator to follow human intensio n. Approach: Various control strategies were proposed during recent years to solve position/forc e control problem. The impedance control concept was used in this study. A relationship between the dynamics of the robot and its energy was developed to derive stability conditions of the robotic syste m at the constrained motion phase using a suitable Lyapunov approach. Results: New sufficient conditions of asymptotic stability were developed. To prove the efficiency of the proposed approach, a pr ototype of a human arm coupled to cooperative constrained robotic manipulator was used. The simul ation results showed the stability and the performances of the proposed approach. Conclusion: Results showed the possibility of their use in a real context of rehabilitation of injured and disab led people.

High-Precision Impedance Control Method for Flexible Base Moving Manipulators

The general problem of impedance control is addressed for a robotic manipulator with a moving flexible base. Impedance control imposes a relation between force and displacement at the contact point with the environment. The concept of impedance control of flexible base moving manipulators (FBMMs) is rather new and is being considered. The dynamic of the manipulator is decomposed into slow and fast dynamics using the singular perturbation method. A new sliding mode impedance control (SMIC) method using an element on the end-effector is proposed for high-precision impedance control of FBMMs. The SMIC method as a robust impedance control law is derived for the slow dynamics. The asymptotic stability of the overall system is guaranteed using a combined control law comprising the impedance control law for the slow dynamics and a feedback control law for the fast dynamics. Active control on the end-effector (ACEE) is able to eliminate the force chatter at the contact. This proposed sliding mode impedance controller with active end-effector control (SMIC/ACEE) was simulated for two models. The first model is a simple FBMM composed of a 2-d.o.f. planar manipulator and a 1-d.o.f. moving base with flexibility in between. The second model is an advanced 10-d.o.f. FBMM composed of a 4-d.o.f. manipulator and a 6-d.o.f. moving base with flexibility. This controller provides the desired position/force control accurately with satisfactory damped vibration at both soft and hard contact. ACEE as the control element on the end-effector was able to eliminate chattering of position and force about their desired values. Composite SMIC/ACEE represents a new framework for high-precision impedance control of moving and/or flexible base robotic manipulators.

Impedance control of a two degree-of-freedom planar flexible link manipulator using singular perturbation theory

In this article, impedance control of a two link flexible link manipulators is addressed. The concept of impedance control of flexible link robots is rather new and is being addressed for the first time by the authors. Impedance Control provides a universal approach to the control of flexible robots, in both constrained and unconstrained maneuvers. The initial part of the paper concerns the use of Hamilton's principle to derive the mathematical equations governing the dynamics of joint angles, vibration of the flexible links and the constraining forces. The approximate elastic deformations are then derived by means of the Assumed-Mode-Method (AMM). Using the singular perturbation method, the dynamic of the manipulator is decomposed into fast and slow subsystems. The slow dynamic corresponds to the rigid manipulator and the fast dynamic is due to vibrations of flexible links. The sliding mode control (SMC) theory has been used as the means to achieve the 2nd order target impedance for the slow dynamics. A controller based on state feedback is also designed to stabilize the fast dynamics. The composite controller is constructed by using the slow and fast controllers. Simulation results for a 2-DOF robot in which only the 2nd link is flexible confirm that the controller performs remarkably well under various simulation conditions.

Impedance Control for A Class of Parallel Robots

In this paper the impedance control concept is applied to a class of six-DOF parallel robots. A geometrically consistent six-DOF impedance behaviour is enforced at the end-effector to manage the interaction with the environment. The overall control scheme is based on an inner/outer strategy with an outer impedance control loop and an inner motion control loop of inverse dynamics type. An approximate dynamic model of the parallel structure is adopted for on-line computation of the compensation terms required by the inverse dynamics control law. Simulation results are presented to show the effectiveness of the proposed strategy.

A Concept of an Intelligent Fuzzy Control for Assembly Robot

Active accommodation of robotic manipulator in assembly applications is considered. Mechanical impedance control concept has been chosen for development an intelligent controller for assembly robots, where target impedance appears as a control variable. Basic assumption for successful part mating process according to part mating theory is mechanical isotropy of support system. Consequently, target impedance should be adopted as isotropic. The main problem appearing here is complex analytical formulation of isotropic target impedance and uncertainty of parameters related to the robot and environment model. To overcome this problem an adaptive fuzzy model of isotropic target impedance is proposed. The fuzzy model is incorporated into general impedance control law form, to obtain new fuzzy-impedance control law. Proposed control law has been verified by computer simulation.

Experimental Object- Level Strategic Control With Cooperating Manipulators

This article presents the high-level control module and user interface of the Dynamic and Strategic Control of Cooperating Manipulators (DASCCOM) project at Stanford University's Aerospace Robotics Laboratory. In addition to cooperative dynamic control, DASCCOM incorporates real-time vision- based feedback, a novel strategic programming technique, and an iconic "object-only" graphical user interface. By focusing on the vertical integration problem, we are examining not only these subsystems, but also their interfaces and interactions. The control system implements a multilevel hierarchical structure interconnected via a real-time network. At the highest level, a mouse-driven graphical user interface allows an opera tor to direct the activities of the system conceptually. Strategic command is provided by an event-driven finite state machine. This methodology provides a powerful yet flexible technique for managing concurrent system interactions. The dynamic controller implements object impedance control —an extension of Neville Hogan's impedance control concept to cooperative multiple-arm manipulation of a single object. This article concentrates on user interfacing techniques and strategic programming capabilities. These modules allow the user to directly specify conceptual object relationships. Experimental results are presented, showing the system lo cating and identifying a moving object, "catching" it, and performing a simple cooperative assembly. Each of these operations is executed autonomously, with only object-level task-specification direction from a remote operator.

Impedance Control of a Pneumatic Servo System with Adaptive Control Method

One of the typical features of a pneumatic servo is a relatively high compliance due to air compressibility. This feature may be useful for constrained tasks such as deburring, polishing, and assisting humans, in which the relationship between position and force is important. If this relationship of a pneumatic servo becomes actively controllable, it can be effectively applied to these tasks. In order to control this relationship, an impedance control concept has recently been proposed. The impedance of the overall control system depends not only on the manipulator but also on the manipulated object of which the characteristics are usually unknown. Therefore, to attain the desired impedance over extensive operating conditions, an adaptive control strategy is required. This paper proposes an impedance control method of a pneumatic servo, using a position based approach, where an adaptive position control system is constructed inside the force feedback loop. The proposed method is applied to an experimental pneumatic servo system comprised of a pneumatic cylinder, electro-pneumatic proportional control valves, and a spring object. From the experiments, the following has been verified: 1) both static stiffness and dynamic impedance of the pneumatic servo system can be independently regulated by setting a desired reference model; 2) the impedance can be held constant with changes in system parameter such as object stiffness; and 3) the instability problem for the low stiffness setting can be overcome by setting high damping in the reference model. The proposed impedance control method may prove to be effective for both improving a pneumatic servo and developing its new applications.