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

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

Stick Slip Friction Models Control Design Approach For Friction Compensation In Machine Tools Drive System

In machining process, positioning accuracy of the drives system is always the key element in producing good products with great precision and minimal or zero defects. Positioning accuracy of an electrical drive system is measured by two types of errors; tracking and contour errors. Reducing tracking error will reduce positioning error and thus increase motion accuracy. Meanwhile, reducing contour error will improve quality of machined surface that leads to improvement in overall precision. Position accuracy and precision are subjected to input disturbance acting on the drive system. A special form of error produced as a result of friction is quadrant glitches. Quadrant glitches are spikes occurred at each quadrant angle in a circular motion due to the effect of highly non-linear friction force acting on the feed drive mechanism influenced by pre-sliding friction characteristics at low velocity. At pre-sliding, friction is pre-dominantly a function of displacement that behaves as hysteresis function with non-local memory. This thesis aims at enhancing knowledge and contributes towards compensating quadrant glitches in circular motion for a ball screw driven XY milling positioning table by means of control design approach using enhanced friction force models. The objectives are to model friction behaviour, design and validate the friction compensation performance at low tracking velocity. Two models of friction forces were introduced; the Sigmoid-Like-Curve-Slip (SLCS) model and the Pseudo-Like-Curve-Slip (PLCS) model. Compensation via friction model based method was implemented in this thesis with different position controllers; namely, Proportional Integral Derivative Controller (PID), Cascade Proportional/Proportional-Integral (P/PI) Controller and Sliding Mode Controller (SMC). The effectiveness of the two proposed friction models were validated against the Generalized Maxwell Slip (GMS) friction model – a model known for effective friction compensation in pre-sliding regime. The numerical analyses and experimental validation performed showed improved performance with reduced contour errors. The SLCS model managed to produce a 99% reduction in the magnitude of the quadrant glitches when combined with cascade P/PI position controller at tracking velocity of 2 mm/s. For similar position controller, the PLCS model was able to produce a maximum quadrant glitches reduction of 70%. In comparison, the GMS model was only able to produce a maximum reduction of 40%. Also, both SLCS and PLCS models demonstrate better friction compensation performance when applied with cascade P/PI position controller compared to SMC. Whereas, PID controller has limited ability to sufficiently compensate quadrant glitches even with feedforward of friction models. In conclusion, this thesis has successfully presented significant improvement in accuracy of drives system made with implementation of the two new improved friction models combined with a cascade P/PI position controller. The new models are able to accurately describe friction behaviour in pre-sliding regime by providing smooth transition between pre-sliding and sliding regimes. However, further researches are desired in enhancing the capability of the friction compensation performance in terms of adaptive ability and robustness. Also, further analyses are necessary in the design of SMC robust controller for friction compensation

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

A State-Space Model of a Large, Lightly Damped Space Structure Using the Eigensystem Realization Algorithm

Large, lightweight space structures of the future will require state-of-the-art vibration suppression systems. To design such a system, it is necessary to have a mathematical model that adequately describes the motion of the system. Coulomb damping compensation was utilized to remove a known nonlinearity from the frequency response functions obtained from the Passive and Active Control of Space Structures (PACOSS) Dynamic Test Article (DTA). Via the Eigensystem Realization algorithm (ERA), the frequency response functions were then used to generate a mathematical representation of the structure\u27s dynamics. Two models were obtained. The first model, without friction compensation, acted as a baseline. The second was obtained using friction compensation. Comparing the models, it was determined that active friction compensation was worthwhile and resulted in a more accurate mathematical description of the system

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

Effects of friction in the system of vibration-isolation platform with gyroscopic stabilizer

In the previous works the mathematical models of vibration-isolation platform with gyroscopic stabilizer did not count with a friction in the joints of precession and stabilizer frames and especially the friction in the correction system drive. The works about gyroscopic systems for indication and measuring purposes presumed and required very low friction which could be neglected. However the friction occurs in the mentioned cases and could not be neglected in our gyroscopic stabilizer system. In the paper will be introduced, discussed and evaluated the effects of friction on the system behavior. The evaluation of impacts of the friction on the system behavior is necessary for adaptation of correction and compensation controllers to reduce the negative effects of the friction as well as for possible choice of more suitable correction system drive. Some effects will be compared with the experimental results

Friction compensation in TP model form - Aeroelastic wing as an example system

The aim of this paper is to fit the friction compensation problem in the field of modern polytopic and Linear Matrix Inequality (LMI) based control design methodologies. The paper proves that the exact Tensor Product (TP) type polytopic representations of most commonly utilized friction models such as Coulomb, Stribeck and LuGre exist. The paper also determines and evaluates these TP models via a TP model transformation. The conceptual use of the TP model of the friction is demonstrated via a complex control design problem of a 2D aeroelastic wing section. The paper shows how the friction model and the model of the aeroelastic wing section can be merged and transformed to a TP type polytopic model - by TP model transformation - whereupon LMI based control performance optimization can immediately be executed to yield an observer based output feedback control solution to given specifications. The example is evaluated via numerical simulations. © 2015, Budapest Tech Polytechnical Institution. All rights reserved

Framework for Static and Dynamic Friction Identification for Industrial Manipulators

Even if friction modeling and compensation is a very important issue for manipulators, quite simple models are often adopted in the industrial world to avoid too heavy solutions from the computational point of view, and because of the difficulty of finding and identifying a model applicable in any motion condition. This article proposes a general framework for friction identification for industrial manipulators with the goal of solving the previous problems through: first, a complete procedure managing all the steps from data acquisition and model identification up to the generation of the code for the implementation into the robot software architecture, second, the possibility of adopting static or dynamic models of different complexity, and third, the development of some modifications in the dynamic friction model so to achieve a reliable friction torque estimation at any velocity and acceleration regime, avoiding unfeasible peaks and overestimation. The results of experimental tests carried out for different manipulators prove the validity and generality of the proposed friction model and identification procedure

Modelling and control of position and velocity drives subject to friction

Performance degradation in most mechanical systems with friction which are not easily eliminated through design mechanisms can be greatly reduced within acceptable limits through the process of friction compensation. Generally friction compensators are used to improve system performance in terms of error reduction, transient response, thereby countering the effects of friction. Model-based techniques for friction compensation require an accurate model of the system friction. This is very important for high precision mechanical systems where excellent positioning and motion tracking, especially in the low velocities, is critical. This thesis proposes a new integrated friction model structure capable of modelling known friction dynamics. The new friction model incorporates a pre-sliding friction function with non-local hysteretic features. Analysis of the model shows the model to possess dissipative, boundedness, passivity and uniqueness properties. Results of sensitivity and robustness analysis indicate the new friction model is robust to parameter variations. A friction characterisation test-bed was designed and constructed for the purposes of friction identification, compensation and control. A set of experiments were designed and implemented on the test rig/bed to demonstrate friction dynamics. The input- output results of the experiments were used for parameter estimation of the proposed new friction model and some other relevant friction model structures. The performance of the new friction model for position and velocity control was studied using the experimental friction test-bed and simulations. The result of such analysis underscores the advantage of integrating a friction observer in the system control loop. The new friction model provided better position and velocity control of the experimental friction test-rig when compared with other well known models of friction