Browsing by Subject "Friction observer"
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Item Open Access A novel observer design for friction estimation(Taylor & Francis, 2023-05-10) Odabaş, Caner; Morgül, ÖmerIn this paper, we propose a novel adaptive friction estimator for simple mechanical systems with Coulombfriction. We assume that the friction constant is unknown and we propose an adaptive observer for itsestimation. We show that under certain conditions the observer structure is asymptotically stable. The pro-posed observer dynamics has two tuning parameters and essentially behaves as a second-order switchingsystem. These parameters could be utilised to tune some time domain performance measures such assettling time, per cent overshoot. etc., under certain conditions. Lastly, simulations reveal that when theactual friction is not confined to Coulomb friction only, the proposed design improves the position trackingperformance, especially when the position controller has low bandwidth.Item Open Access Adaptive friction compensations for mechanical systems with measurement delay(SAGE Publications, 2021) Odabaş, Caner; Morgül, ÖmerApplication performance of mechanical positioning systems might not coincide with the theory, mainly due to nonlinearities or imperfections of system models. Although it is sometimes possible to ignore these mismatches, systems generally suffer from performance degradation or even instability eventually. Especially, friction force and time delay are two major factors of these undesired effects. Hence, in this paper, Smith predictor-based controllers and an adaptive Coulomb friction observer are designed to enhance position tracking performance of a mechanical system including time delay. In fact, implemented hierarchical control scheme provides two-degree of freedom to control both velocity and position separately. The proposed observer structure is mainly motivated by the Friedland-Park observer but could be considered as an extension of it which characterizes a general class of nonlinear functions for friction estimation. To assure its functionality with delayed measurements, different velocity predictor schemes are designed and their performances are compared. As a guideline for observer design, some conditions for exponential stability and robustness analysis are presented. Simulation results demonstrate that the proposed control system enhances the tracking performance even when the actual friction is a compound of various static and dynamic terms.Item Open Access Adaptive observer designs for friction estimation in position control of simple mechanical systems with time delay(Bilkent University, 2021-09) Odabaş, CanerFriction force/torque is a well known natural effect that can cause performance degradation or even instability in mechanical systems, although it sometimes can be disregarded in closed loop feedback design phase. Hence, friction modeling and cancellation methods can be vital to achieve desired robustness and performance criteria in position control problems. Basically, the topic of friction cancellation is divided into two main categories named model based and non-model based methods. Friction modeling is a broad area of research and there are lots of different modeling approaches in various complexities. Among these approaches, Coulomb Model is one the simplest yet fundamental models. Nevertheless, in some cases, being a classical static model, it is inadequate to exhibit the dominant friction components occurring at different motion stages such as break-away force, stick-slip motion, pre-sliding behavior or friction lag. Generally, dynamical models, i.e. LuGre Model, are more advanced as a result, they are better to describe such friction effects. Unfortunately, for these cases, the number of friction parameters are increased. In fact, there is a trade-off between model complexity and parameter identification. A desired system response may not be achieved when model parameters do not coincide with the existing friction coefficients. In this manner, precise identification of each parameter can be challenging when there are many of them. Besides, some of these parameters might be time varying due to environment, temperature, material properties, position, etc. Therefore, non-model based adaptive schemes are prevalent in the literature since these methods do not require any parameter identification. In this study, we focus on adaptive observer based friction compensation techniques and provide some stability conditions. First, we consider simple second order mechanical systems with or without time delay under Coulomb friction. To estimate the Coulomb friction, we first consider Friedland-Park observer. Then, some necessary conditions are stated to extend the estimation function in the observer structure to a larger class of functions. Especially measurement delay can be significant since observers estimate friction based on the velocity measurements. Therefore, it is proposed to employ a velocity predictor either based on numerical differential equation solvers or inverse Pade approximant when the existing time delay is large. What is more, a new observer design that considers friction and velocity error dynamics together is proposed as a novel contribution. Extensive MATLAB simulations are conducted to investigate the performances of proposed observers in a closed loop position control system with and without delay. To this end, Smith predictor and ITAE index-based designs are considered to utilize a position controller. In some of these simulations, LuGre model is preferred to mimic the actual friction instead of Coulomb friction in order to observe the effects of dynamic parameters. Moreover, some experiments are performed on DC motor platform driven by Arduino Uno microcontroller. Under the light of acquired results, observer based friction compensation improves the system performance even existing friction cannot be confined to Coulomb coefficient, especially when the implemented controller has low bandwidth. Also, in terms of practicability, it is an advantage that these observer structures do not require any parameter identification.