Model-based Friction Compensation

Friction is present in all mechanical systems and causes a wide range of problems for control. The development of model-based strategies accounting for Friction in the designed control has been a vast area of Research since these last decades. A promising Friction model which received a lot of attention these last years is the so-called LuGre model, which originates in the collaboration between the two control Research groups of Lund (Sweden) and Grenoble (France). This model is quite simple and is capable of capturing a wide range of well known friction phenomena, such as the Stribeck effect or the frictionnal lag. In particular this model is used in [Robertsson et al., 2004], where a general method for friction compensation for nonlinear systems is presented. The compensation strategy is simple: it just consists in adding to the control signal a friction estimate, computed using a LuGre model based observer. This thesis deals with the application of the theory of this article on a real experiment: the stabilization of the Furuta pendulum in the upright position. First, attention is paid so that the initial hypothesis of this article be satisfied. These hypotheses consist in finding a stabilizing control for the system when Friction is neglected, and an associated Lyapunov function verifying some properties. Then, Friction is included by following the procedure presented in the article. The friction estimate is computed according to the discretized LuGre form, presented in fFreidovich et al., 2006g, and the main result of the article is verified both in Simulation and on the real process, the simulations being carried out with Matlab-Simulink and the real experiments by using a dSPACE device. From a practical point of view, the implemented compensation scheme works perfectly in Simulation: the limit cycles originating from an uncompensated friction are totally annihilated, while for real experiments this oscillating behaviour is still remaining, but happens to be significantly reduced. From a theoretical point of view, the results of [Robertsson et al., 2004] are fully verified in Simulation, while for real experiments the presence of remaining limit cycles prevents a perfect verification of the theory

Identification and model-based compensation of Striebeck friction

The paper deals with the measurement, identification and compensation of low velocity friction in positioning systems. The introduced algorithms are based on a linearized friction model, which can easily be introduced in tracking control algorithms. The developed friction measurement and compensation methods can be implemented in simple industrial controller architectures, such as microcontrollers. Experimental measurements are provided to show the performances of the proposed control algorithm

Research on parameter identification of nonlinear friction on cantilever beam

Free section friction on cantilever beam is the object in this paper. Practical experiment and virtual simulation are combined to gain understanding of the whole mechanical system. The classical Tustin model cannot provide perfect description of the actual friction process. Friction compensation model 1 is established through introducing time-varying compensation into the classical Tustin model based on the classical friction model and the theory of Fourier transform. A modified genetic algorithm is proposed by introducing self-adaptive strategy. The parameter identification based on the time-varying friction compensation model is performed by using the modified genetic algorithm. Friction compensation model 2 is established by introducing the improved time-varying compensation strategies which are more in line with the friction process. The numerical results demonstrate the high iterative search capability and computation efficiency of friction compensation model 2

Optimal control design for robust fuzzy friction compensation in a robot joint

This paper presents a methodology for the compensation of nonlinear friction in a robot joint structure based on a fuzzy local modeling technique. To enhance the tracking performance of the robot joint, a dynamic model is derived from the local physical properties of friction. The model is the basis of a precompensator taking into account the dynamics of the overall corrected system by means of a minor loop. The proposed structure does not claim to faithfully reproduce complex phenomena driven by friction. However, the linearity of the local models simplifies the design and implementation of the observer, and its estimation capabilities are improved by the nonlinear integral gain. The controller can then be robustly synthesized using linear matrix inequalities to cancel the effects of inexact friction compensation. Experimental tests conducted on a robot joint with a high level of friction demonstrate the effectiveness of the proposed fuzzy observer-based control strategy for tracking system trajectories when operating in zero-velocity regions and during motion reversals

STUDY OF FRICTION COMPENSATION MODEL FOR MOBILE ROBOT’S JOINTS

Frictional forces inside the joints of mobile robots hurt robot operation's stability and positioning accuracy. Therefore, establishing a suitable friction force compensation model has been a hot research topic in robotics. To explore the robot joint friction compensation model, three friction compensation models: linear, nonlinear, and neural network models, are developed in this paper. Based on the deep learning algorithm for three models at low speed, high speed, acceleration, and uniform speed training test, respectively results have been obtained. The test results show that the best friction compensation effect comes from combining neural network models in acceleration and a consistent speed state way. The friction compensation model trained this way yielded superior results to the other combinations tested. Finally, using the method, a friction compensation model trained by adding a neural network to the feedforward control torque was tested on a four-wheeled mobile robot platform. The test results show that the relative error of the torque caused by the friction of each joint is reduced by 15%-75% in 8 groups of tests, which indicates that our friction compensation method has a positive effect on improving the accuracy of the joint torque

Friction Compensation by the use of Friction Observer

The deteriorating effects of friction on the control of mechanical systems is a major problem in a multitude of applications such as high-performance robotics and pointing systems. This thesis aims at improving control by using a nonlinear friction observer to detect friction in the system and use this information to modify the control input. After some initial attempts with the LuGre friction model, the observer was decided to be based on the Dahl model of friction. Simulations and experiments were made using the unstable Furuta pendulum and the efficiency of the observer aproach was compared to other standard methods for friction compensation. Although being able to serve its purpose very well in the simulated environment the observer friction compensation could not outperform the other compensation methods in the experiments, only being capable of slightly improving the control of the pendulum. The cause of this lack of success is still unclear

Research on parameter identification of nonlinear friction on cantilever beam

Free section friction on cantilever beam is the object in this paper. Practical experiment and virtual simulation are combined to gain understanding of the whole mechanical system. The classical Tustin model cannot provide perfect description of the actual friction process. Friction compensation model 1 is established through introducing time-varying compensation into the classical Tustin model based on the classical friction model and the theory of Fourier transform. A modified genetic algorithm is proposed by introducing self-adaptive strategy. The parameter identification based on the time-varying friction compensation model is performed by using the modified genetic algorithm. Friction compensation model 2 is established by introducing the improved time-varying compensation strategies which are more in line with the friction process. The numerical results demonstrate the high iterative search capability and computation efficiency of friction compensation model 2

An Angular Position-Based Two-Stage Friction Modeling and Compensation Method for RV Transmission System

In RV transmission system (RVTS), friction is closely related to rotational speed and angular position. However, classical friction models do not consider the influence of angular position on friction, resulting in limited accuracy in describing the RVTS frictional behavior. For this reason, this paper proposes an angular position-based two-stage friction model for RVTS, and achieves a more accurate representation of friction of RVTS. The proposed model consists of two parts, namely pre-sliding model and sliding model, which are divided by the maximum elastic deformation recovery angle of RVTS obtained from loading-unloading tests. The pre-sliding friction behavior is regarded as a spring model, whose stiffness is determined by the angular position and the acceleration when the velocity crosses zero, while the sliding friction model is established by the angular-segmented Stribeck function, and the friction parameters of the adjacent segment are linearly smoothed. A feedforward compensation based on the proposed model was performed on the RVTS, and its control performance was compared with that using the classical Stribeck model. The comparison results show that when using the proposed friction model, the low-speed-motion smoothness of the RVTS can be improved by 14.2%, and the maximum zero-crossing speed error can be reduced by 37.5%, which verifies the validity of the proposed friction model, as well as the compensation method

Friction and Friction Compensation in the Furuta Pendulum

Inverted pendulums are very well suited to investigate friction phenomena and friction compensation because the effects of friction are so clearly noticeable. This paper analyses the effect of friction on the Furuta pendulum. It is shown that friction in the arm drive may cause limit cycles. The limit cycles are well predicted by common friction models. It is also shown that the amplitudes of the limit cycles can be reduced by friction compens ation. Compensators based on the Coulomb friction model and the LuGre model are discussed. Experiments performed show that reduction of the effects of friction can indeed be accomplished