Observer-based Friction Compensation Research Articles

Observer-based friction compensation in heavy-duty parallel robot control

Observer-based friction compensation in heavy-duty parallel robot control

Passive Friction Compensation Using a Nonlinear Disturbance Observer for Flexible Joint Robots with Joint Torque Measurements

The friction and ripple effects from motor and drive cause a major problem for the robot position accuracy, especially for robots with high gear ratio and for high-speed applications. In this paper we introduce a simple, effective, and practical method to compensate for joint friction of flexible joint robots with joint torque sensing, which is based on a nonlinear disturbance observer. This friction observer can increase the performance of the controlled robot system both in terms of the position accuracy and the dynamic behavior. The friction observer needs no friction model and its output corresponds to the low-pass filtered friction torque. Due to the link torque feedback the friction observer can compensate for both friction moment and external moment effects acting on the link. So it can be used not only for position control but also for interaction control, e.g., torque control or impedance control which have low control bandwidth and therefore are sensitive to ripple effects from motor and drive. In addition, its parameter design and parameter optimization are independent of the controller design so that it can be used for friction compensation in conjunction with different controllers designed for flexible joint robots. Furthermore, a passivity analysis is done for this observer-based friction compensation in consideration of Coulomb, viscose and Stribeck friction effects, which is independent of the regulation controller. In combining this friction observer with the state feedback controller \cite{Albu-Schaeffer2}, global asymptotic stability of the controlled system can be shown by using Lyapunov based convergence analysis. Experimental results with robots of the German Aerospace Center (DLR) validate the practical efficiency of the approach.

PASSIVITY BASED ON ENERGY TANK FOR CARTESIAN IMPEDANCE CONTROL OF DLR SPACE ROBOTS WITH FLOATING BASE AND ELASTIC JOINTS

This paper presents a control structure for orbital servicing mission of CEASAR robotic arm developed by German Aerospace Center (DLR). In order to reduce mass the CEASAR arm is equipped with Harmonic-Drives with high ratio which unfortunately lead to high joint elasticity and high motor friction and have to be considered in controller design for successful manipulator in-orbit operations. Therefore, in this control structure, for high tracking control a cascaded position controller based on state feedback control structure with observer-based friction compensation and for safe interaction control with the environment a Cartesian impedance controller is used. The proposed control methods are very efficient and practicable. Furthermore, they are very robust with dynamic parameter uncertainties, coupling dynamics, and can simultaneously provide good results in term of the position accuracy and dynamic behavior. Simulation results validate practical efficiency of the controllers.

Identification and compensation of friction for a novel two-axis differential micro-feed system

Abstract Non-linear friction in a conventional drive feed system (CDFS) feeding at low speed is one of the main factors that lead to the complexity of the feed drive. The CDFS will inevitably enter or approach a non-linear creeping work area at extremely low speed. A novel two-axis differential micro-feed system (TDMS) is developed in this paper to overcome the accuracy limitation of CDFS. A dynamic model of TDMS is first established. Then, a novel all-component friction parameter identification method (ACFPIM) using a genetic algorithm (GA) to identify the friction parameters of a TDMS is introduced. The friction parameters of the ball screw and linear motion guides are identified independently using the method, assuring the accurate modelling of friction force at all components. A proportional-derivate feed drive position controller with an observer-based friction compensator is implemented to achieve an accurate trajectory tracking performance. Finally, comparative experiments demonstrate the effectiveness of the TDMS in inhibiting the disadvantageous influence of non-linear friction and the validity of the proposed identification method for TDMS.

Modeling, identification and analysis of a novel two-axis differential micro-feed system

With the development of precision and ultra-precision machining technology, the demand of drive feed system increases. Non-linear friction in a conventional drive feed system (CDFS) feeding at low speed is one of the main factors that lead to the complexity of the feed drive. The CDFS will inevitably enter or approach a non-linear creeping area at extremely low speed. A novel two-axis differential micro-feed system (TDMS) is developed in this paper to overcome the accuracy limitation of CDFS. A dynamic model of TDMS is first established. Then, a distributed component friction parameter identification method using a genetic algorithm (GA) to identify the friction parameters of a TDMS is introduced. A proportional-derivate feed drive position controller with an observer-based friction compensator is implemented to achieve an accurate trajectory tracking performance. Finally, comparative experiments demonstrate the effectiveness of the TDMS in inhibiting the disadvantageous influence of non-linear friction and the validity of the proposed identification method for TDMS.

Distributed Component Friction Model for Precision Control of a Feed Drive System

Nonlinear friction is a dominant factor of the complexity growth of feed drive dynamic behavior. In particular, a feed drive system equipped with rolling contact components such as ball screws and linear motion (LM) guides undergoes impalpable friction behavior. Therefore, proper modeling and compensation of friction in feed drive components play a crucial role in improving the precision of a feed drive system. In this study, to overcome the accuracy limitation of the simplified feed drive model, a distributed component friction model (DCFM) of a feed drive system is developed. Friction has been modeled as a single element of entire feed drive system in the conventional model. However, the proposed DCFM is composed of models of individual feed drive components such as ball screw and LM guide including their friction behaviors. To take advantage of the DCFM by considering both presliding and sliding friction behaviors of each component, LuGre model is applied. For experimental verification of the accuracy of the proposed DCFM, a specially designed feed drive testbed is constructed. Using the testbed, the friction forces of ball screws and LM guides are measured independently. Using the DCFM, a friction observer is built for real-time monitoring and compensation of the friction force. A proportional-derivative feed drive position controller with an observer-based friction compensator is implemented and the experimental results are presented.

空気圧平面パラレルマニピュレータの圧力・力センサレス位置・剛性制御(機械力学,計測,自動制御)

This paper presents a method for controlling the position and stiffness of a pneumatic planar parallel manipulator in which the pressure and force sensors are eliminated by replacing them by observers for the purpose of cost reduction. Generally, since pneumatic manipulators are characterized by high nonlinearities such as air compressibility and friction arising in their joints and sliding parts, conventional PID controllers are insufficient for precise control. Using advanced controllers aiming at higher performance requires various sensors, but their dependence on a lot of costly sensors makes pneumatic manipulators less competitive with other power source manipulators in terms of cost. Based on our previous papers, a position controller for the pneumatic manipulator is first discussed in this paper, which introduces observer-based friction compensation for enhancing motion accuracy and pressure observers as substitutes for pressure sensors. The designed controller is further extended in order to control simultaneously the position and stiffness of the manipulator. In this controller, friction observers estimated the contact force between the manipulator's end-effector and a target object, and from these results a force sensor was removed to save costs. Several experiments were carried out to verify the performance of the proposed low-cost controller.

Friction Compensation of DC Motors for Precise Motion Control Using Disturbance Observer

Friction compensation is often neglected in DC servo control systems where precision is not a main concern. However, friction compensation is necessary to achieve precision of a motion control system. In this research work frictional components have been estimated using constant angular velocity motion test and by the use of the disturbance observer. Disturbance observer based friction compensator has been Modeled and developed model was implemented in hardware setup and evaluated using simulation and practical results.