Nonlinear Friction Characteristics Research Articles

Actuation strategy characterizations of a vibro-impact crawling robot driven by dielectric elastomer actuators

Vibro-impact crawling robots driven by the emerging dielectric elastomer actuators (DEAs) feature the advantages of reduced system complexity and bidirectional locomotion capability. However, due to the lack of systematic dynamic models and in-depth investigations, the fundamental locomotion principles of the robots are still unclear and therefore their applications in real-world scenarios remain hindered. In this paper, a comprehensive dynamic model of this robot is developed by considering the complex interactions between the electro-mechanical coupling, impact mechanism, and multiple nonlinear friction characteristics. By incorporating extensive modeling and experimental studies, we explain the fundamental principles that lead to the bidirectional locomotion of the robot. The actuation strategies for both forward and backward locomotion are characterized in-depth, which include the actuation frequencies, relative phases, and waveforms. Three typical contact surface cases (i.e. rigid & dry, rigid & viscous, compliant & viscous) are considered in this paper, where we report the changes in the optimal actuation strategies for bidirectional locomotion with the contact surfaces and reveal the influences of key factors of friction coefficient, viscosity, and material compliance. The key findings reported in this work can build foundations for developing a highly robust and efficient DEA-driven vibro-impact crawling robot for broad applications.

Electro-Mechanical Joint Simulation Technology of 3D Printing Equipment of Outer Space

The outer space environment is complex, so the research and development of 3D printing equipment depends on the prototype test that is difficult to achieve on the ground, therefore, the performance of the equipment must be investigated by means of simulation. In this paper, the electro-mechanical joint simulation model of the outer space 3D printing equipment is established based on ADAMS and MATLAB/Simulink, which taking into account the overall micro-gravity environment, the nonlinear friction characteristics of each motion-axis, the inertia of each axis, and the overall frame vibration. Based on this model, the point-to-point control and the interpolation motion errors of X and Y axes with external disturbance is realized. At last the comparative control of motion error is realized, which provides a basis for the development of 3D printing equipment in outer space environment.

An exponentially convergent adaptive robust based space manipulator noise control method

Because most of the traditional nonlinear control methods are model-based control methods, it will dramatically increase the parameter identification burden of multi-rigid-body system dynamic model. Meanwhile, Unmodeled dynamics, parameter drift, model uncertainty, complex motor hysteresis characteristics, etc are unfortunately in the practical systems, which makes it very hard to accurately build the noise model and identify the model parameters. Among them, the accurate values of the viscous friction coefficient, elastic friction coefficient, the distance between the centroid and the rotation center of the connecting rod, the inertial moment and products of each joint, these exact values can hardly be obtained. In this paper, direct adaptive control and the robust model reference adaptive control based on Lyapunov direct method are adopted, which can make the noise error close to zero under the condition of parameter uncertainty and unknown nonlinear friction characteristics.

Development and study of a microprocessor automatic control system for a mono-switch tie type with a linear inductive electric motor and a discrete speed controller

Introduction. The article is devoted to the development of the microprocessor automatic control system for a gearless controlled electric drive of a mono-switch tie based on a linear inductor electric motor. This solution provides control the position of the switch point, to carry out the transfer process with a smooth drive of shanks to the frame rail, to protect electric motor elements from overloads. Goal. development and study of the behavior of microprocessor automatic control system for mono-switch tie type with linear inductive electric motor and discrete PID speed controller which coefficients are adjusted according to Chin-Hrons-Reswick method to meet modern traffic safety requirements and improve operational reliability factors. Methodology. On the basis of electric drive theory, a kinematic line of a mono-switch tie type with nonlinear friction characteristic is presented. Using differential equation theory and Laplace transformation, a mathematic description of a linear inductor electric motor has been made. Using the z-transform method, a difference equation for describing a discrete PID speed controller is obtained, the coefficients of which are derived using the Chin-Hrons-Reswick method. A simulation mathematical model of the electric drive mono-switch tie type as the microprocessor automatic control system with linear inductive electric motor and discrete PID speed controller and nonlinear friction characteristic was built in MATLAB. Results. Simulation modelling of a mathematical model of the microprocessor automatic control system of the electric drive mono-switch tie type with the linear inductive electric motor and discrete PID speed controller and nonlinear friction characteristic have been developed and performed. Studies of dynamics of switch point movement have shown that, a drive time of less than 0.7 s at a constant speed motor armature of 0.2 and 0.3 m/s provides to meet modern requirements for railway switch points. The application of discrete PID speed controller has shown improved dynamics of switch point. Originality. First for the electric drive of the mono-switch tie type with linear inductive electric motor a mathematical model of the discrete PID speed controller and nonlinear friction characteristic as an object of speed control of switch point movement, has been developed. Practical value. Mathematical model of a railway track switch of the mono-switch tie type with linear inductive electric motor and discrete PID speed controller has been developed to carry out the control of the position of the switch point, process with a smooth drive them to the frame rail, to protect electric motor elements from overloads.

A theoretical analysis of friction and vibration characteristics of wiper reversal process

This paper is concerned with the friction and vibration characteristics of a vehicle wiper-windscreen contact. The dynamics and friction models of the wiper-windscreen system are established to study the frictional vibration and noise of the wiper-windscreen system. The dynamic model of the wiper-windscreen system is established and the analytical formulas are derived. Based on the negative slope formula of friction coefficient with respect to relative velocity, the Coulomb’s friction law, and the fluid lubrication characteristics, the friction models of the system are built in dry and wet operating conditions, respectively. Meanwhile, the formulas of friction coefficient and relative velocity in time domain are presented. The changes of blade angle and friction with time in different working conditions are investigated. The Runge-Kutta method is used to analyze the nonlinear dynamic response and friction characteristics of the friction models. The simulations successfully reproduce the vibrations of the wiper blade in actual motion. The effectiveness of the dynamic model and the friction models is verified by comparing the simulation results under different working conditions. The dynamics and friction models presented in this paper can provide theoretical references for structural optimization of the wiper blade, and thereby the vibration and noise control of the wiper-windscreen system.

Discovering Stick-Slip-Resistant Servo Control Algorithm Using Genetic Programming.

The stick-slip is one of negative phenomena caused by friction in servo systems. It is a consequence of complicated nonlinear friction characteristics, especially the so-called Stribeck effect. Much research has been done on control algorithms suppressing the stick-slip, but no simple solution has been found. In this work, a new approach is proposed based on genetic programming. The genetic programming is a machine learning technique constructing symbolic representation of programs or expressions by evolutionary process. In this way, the servo control algorithm optimally suppressing the stick-slip is discovered. The GP training is conducted on a simulated servo system, as the experiments would last too long in real-time. The feedback for the control algorithm is based on the sensors of position, velocity and acceleration. Variants with full and reduced sensor sets are considered. Ideal and quantized position measurements are also analyzed. The results reveal that the genetic programming can successfully discover a control algorithm effectively suppressing the stick-slip. However, it is not an easy task and relatively large size of population and a big number of generations are required. Real measurement results in worse control quality. Acceleration feedback has no apparent impact on the algorithms performance, while velocity feedback is important.

Mathematical modelling of transients in the electric drive of the turnout of the mono-sleeper type with switched-inductor motor

Introduction. The study is devoted to the development of the functionality of a railway track switch by introducing a switched-inductor electric drive. This solution justifies simplifying the mechanical part of the switches by changing the gearbox to a ball-screw and locating the all kinematic line of the switches on the mono-sleeper type. Goal. A study of the mono-sleeper turnout type behaviour to meet modern traffic safety requirements and improve operational reliability factors. Methodology. Based on electric drive theory, a kinematic line of a mono-switch turnout type with nonlinear friction characteristic is presented. Using differential equation theory and Laplace transformation, a mathematic description of a four-phase switched-inductor motor with ball-screw in a mechanical line of a single-mass electromechanical system has been made. A simulation mathematical model of the electric drive of mono-sleeper turnout type as the control system with a switched-inductor motor and nonlinear friction characteristic was built in MATLAB. Results. Simulation modelling of a mathematical model of a mono-sleeper turnout type with a switched-inductor motor and ball-screw gear has been developed and implemented. Studies of dynamics of turnout point movement have shown that, in contrast to the motors used today, the switched-inductor motor makes it possible to simplify the mechanical part of the drive, which leads to reduced time spent on laying and maintenance of turnout points, and therefore makes the design more reliable. The application of PID controller and fuzzy speed controller has shown improved dynamics of turnout point, while the fuzzy PID controller provides better performance of the set values and turnout point movements. Originality. First developed a mathematical model of the electric drive of the mono-sleeper turnout type, taking into account nonlinear friction characteristic, as an object of speed control of turnout point movement, is developed. Practical value. The developed mathematical model of a railway track turnout of the mono-sleeper type with a switched-inductor motor and ball-screw gear enables more efficient use of a microprocessor control system, creation of promising electric motor protection means and control of a turnout point.

Nonlinear Adaptive Robust Precision Pointing Control of Tank Servo Systems

This paper focuses on the high performance pointing control of tank servo systems with parametric uncertainties and uncertain nonlinearities including nonlinear friction, backlash and structural flexibility. A comprehensive dynamic nonlinear mathematical model of the two-DOF tank servo system is established. Specifically, to accurately describe the nonlinear friction characteristics in actual systems, a continuous friction model is employed. Moreover, a hybrid nonlinear model combining structural flexibility and transmission backlash is constructed to characterize the nonlinear characteristics of the backlash and flexible coupling between the input and output shafts of the drive end for the tank servo system. By using the backstepping method, a nonlinear adaptive robust controller is presented. In the controller, the adaptive law is compounded to dispose of parametric uncertainties and a well-designed continuous nonlinear robust control law is developed for the purpose of coping with unmodeled disturbances. The closed-loop system stability analysis indicates that the presented controller achieves an asymptotic tracking performance with parametric uncertainties and ensures the robustness against unmodeled disturbances theoretically. The effectiveness of the proposed control strategy is verified by a large number of comparative simulation results.

Neural network based adaptive rise control of tank gun systems

With the development of the all-electric fourth-generation tank, higher requirements have been put forward for the accuracy and response speed of tank gun control and the traditional tank gun system modeling methods and control strategies failed to meet the requirements. For this reason, a neural network based adaptive robust integral of the error sign (ARISE) control for tank gun system is proposed considering two-axis dynamic coupling and nonlinear friction characteristics. Firstly, a two-axis coupling nonlinear dynamic model of the tank gun system is established and a continuous static friction model is used to characterize the nonlinear friction characteristics of the system. Then, an adaptive law is designed based on the desired position instruction to realize the online update of unknown system parameters. In addition, the neural network is used to compensate other unmodeled dynamic errors and the influence of neural network approximation error is suppressed by the nonlinear integral robust control term. The stability analysis results show that the proposed neural network based adaptive robust integral of the sign of the error control strategy can obtain excellent asymptotic tracking performance, and the simulation results verify its effectiveness.

Partial and Full Braking Algorithm According to Time-to-Collision for Both Safety and Ride Comfort in an Autonomous Vehicle

In this study we developed an autonomous braking algorithm to satisfy both safety and ride comfort of a vehicle. The proposed algorithm is composed of two-step braking strategies depending on the value of time-to-collision. The first step is a partial braking strategy to provide not only deceleration but also good ride comfort in a normal braking situation, and the second step is a full braking strategy to avoid forward collisions in an emergency braking situation. Further, the partial braking is divided into a recovery and a release zones. The former is to apply braking pressures for the safe deceleration, whereas the latter is to release the braking pressure smoothly for good ride comfort. To determine the partial braking pressures, the nonlinear characteristics of the tire friction is considered and the linear momentum of a preceding vehicle is calculated based on the extrapolation of its motion. Computer simulations using CarSim were executed combined with MATLAB/Simulink to implement the driving situations, and finally we obtained successful performances satisfying ride comfort as well as safety of the vehicle.

Research on thrust vector control of nonlinear solid rocket motor nozzle based on active disturbance rejection technology

In this paper, based on the auto disturbance rejection control technology, the electromechanical servo system is used as the control actuator, and the thrust vector control of the solid rocket motor nozzle with typical nonlinear friction characteristics is studied and analyzed. In this paper, the realization of the classical PID algorithm and the lack of dynamic performance are analyzed, and then the compensation algorithm based on the auto disturbance rejection control technology is added. The algorithm compensates for the phase lag of the system due to the nonlinear friction characteristics. As a result, the frequency characteristics of the system have been significantly improved.

Time domain coupled simulation of machine tool dynamics and cutting forces considering the influences of nonlinear friction characteristics and process damping

A higher machining ability is always required for NC machine tools to achieve higher productivity. The self-oscillated vibration called “chatter” is a well-known and significant problem that increases the metal removal rate. The generation process of the chatter vibration can be described as a relationship between cutting force and machine tool dynamics. The characteristics of machine tool feed drives are influenced by the nonlinear friction characteristics of the linear guides. Hence, the nonlinear friction characteristics are expected to affect the machining ability of machines. The influence of the contact between the cutting edge and the workpiece (i.e., process damping) on to the machining ability has also been investigated. This study tries to clarify the influence of the nonlinear friction characteristics of linear guides and ball screws and process damping onto milling operations. A vertical-type machining center is modeled by a multi-body dynamics model with nonlinear friction models. The influence of process damping onto the machine tool dynamics is modeled as stiffness and damping between the tool and the workpiece based on the evaluated frequency response during the milling operation. A time domain-coupled simulation approach between the machine tool behavior and the cutting forces is performed by using the machine tool dynamics model. The simulation results confirm that the nonlinear frictions influence the cutting forces with an effect to suppress the chatter vibration. Furthermore, the influence of process damping can be evaluated by the proposed measurement method and estimated by a time domain simulation.

외골격 로봇의 마찰보상제어를 위한 구동 모듈의 정적 및 동적 마찰 특성 분석

Actuators for exoskeleton robots comprise various types such as electric, hydraulic, and pneumatic and it is necessary to apply the correct actuator according to the purpose. Most exoskeleton robots mainly use electric actuators, and some special-purpose robots, such as for heavy-load transport requiring large force, use hydraulic actuators. In this paper, friction of the actuation module consisting of a harmonic drive and a brushless DC motor is measured through experiments. And the friction characteristics of the actuation module are analyzed. The harmonic drive transmission system has various advantages, but it also has hysteresis and nonlinear friction characteristics. The friction compensation control of the actuation module enables precise control of the exoskeleton robot, and improves the robot’s performance. Appropriate friction model selection and design affects friction compensation performance. In this study, static and dynamic friction models are designed and analyzed based on the friction data of the actuation module.

Improvement of Reverse Motion of an NC Moving Table Based on Vector Control Method by Friction Force Compensation

Lost motion is a phenomenon that often occurs during the motion of a moving table, which is used for machine tools to ensure their precise positioning. Lost motion occurs when the direction of the table’s motion reverses as a result of nonlinear friction characteristics between the feed drive mechanisms such as the ball screw and linear guide. Lost motion directly influences the machining accuracy of a machine tool, because the accuracy of machining depends on the relative motion between the tool and the workpiece. A number of studies have dealt with suppressing the occurrence of lost motion using model-based control. However, nonlinear friction has not been addressed to the same extent, as it is difficult to determine the motion characteristics of and therefore develop a model for the nonlinear friction. Thus, to address these problems, we propose a compensation method for revers motion based on vector control, which is used to control the torque and velocity of the alternating current (AC) servomotor in the moving table. In this study, the current applied to the AC servomotor for a vector with force components in the rotational direction (torque component) and in the direction perpendicular to the axis of rotation (field component) was measured to clarify and establish the relationship between the motion and the control current. The compensation current was then derived as a functional value based on the results of the measured torque at the occurrence of lost motion. Further, tests were carried out using the proposed method, which directly applies the drive current of the AC servomotor by using a field-programmable gate array controller to improve the reverse motion of the table. The results reveal that the motion characteristics of a numerical control (NC) table can be determined by measuring the drive current of the AC servomotor. In addition, it is verified that the proposed method can compensate for the torque command smoothly at the time of velocity reversal, resulting in suppression of the lost motion and reduction of reverse motion of the moving table.

A cable-pulley system modeling based position compensation control for a laparoscope surgical robot

Due to nonlinear friction and backlash characteristics of cable-driven mechanism (CDM) in laparoscope surgical robot end-effector, it is difficult to predict force and control position precisely during the surgical procedure. Current studies of transmission characteristics of CDM mainly focus on the tendon-sheath system (TSS) with the classical Capstan equation which neglects the effect of bending rigidity of the cable. However, analysis for transmission characteristics of a cable-pulley system (CPS) is rare. This paper presents a tension and displacement transmission model of a CPS in the end-effector of laparoscope surgical robot. The model considers the bending rigidity of the cable and spatial location of the pulleys. Experiments validate the proposed model. Based on the model, a position compensation algorithm of feedforward control is presented to reduce the tracking position errors. The experimental validations show substantial improvements on performance of position tracking errors for the use of the proposed algorithm.

Control system design of a linear motor feed drive system using virtual friction

Abstract This paper proposes a friction compensator and a design method for control systems to improve the response characteristics of linear motor feed drive systems. The proposed friction compensator cancels the real nonlinear friction of feed drive systems by using the nonlinear friction model proposed in this study and introduces virtual linear friction to facilitate the control system design. The proposed design method enables the determination of servo gains and friction compensator parameters that lead to desirable tracking performance and disturbance rejection without many trial-and-error tuning processes. In addition, the proposed method facilitates the design of the velocity feedforward compensator by using the inverse transfer function of the velocity control loop to correct the position tracking errors for various position commands. The effectiveness of the proposed method with the friction compensator and the velocity feedforward compensator was verified in simulations and experiments using a one-axis feed drive system consisting of a rod-type linear motor and linear roller guides. The results confirmed that the proposed method enables desirable overshoot-free responses and corrects motion trajectory errors due to nonlinear friction characteristics, and the proposed velocity feedforward compensator can correct tracking errors in both constant velocity motion and circular motion.

Identification and Compensation of Nonlinear Friction Characteristics and Precision Control for a Linear Motor Stage

The main goal of this investigation is to improve the tracking accuracy of the stage of a linear motor. A DC brushless linear motor is used to actuate a gantry stage to perform printing. To compensate for the tracking error of the gantry stage that is associated with nonlinear friction, the dynamics of the nonlinear static friction are formulated using the Hsieh-Pan model. Particle swarm optimization (PSO), genetic algorithm, and real-coded genetic algorithm-based optimization problems are investigated to evaluate the parameters of the nonlinear friction model. The use of PSO-based optimization to tune the parameters of a disturbance-observer-based variable structure controller is also discussed to improve the tracking response. To check the consistency of the proposed controller, it is implemented in real time and an improved positional accuracy better than 0.1 μm is readily achieved.

Modeling and Robust Discrete-Time Sliding-Mode Control Design for a Fluid Power Electrohydraulic Actuator (EHA) System

This paper studies the design of a robust discrete-time sliding-mode control (DT-SMC) for a high precision electrohydraulic actuator (EHA) system. Nonlinear friction in the hydraulic actuator can greatly influence the performance and accuracy of the hydraulic actuators, and it is difficult to accurately model the nonlinear friction characteristics. In this paper, it is proposed to characterize the frictions as an uncertainty in the system matrices. Indeed, the effects of variations of the nonlinear friction coefficients are considered as norm-bounded uncertainties that span a bounded region to cover a wide range of the real actuator friction. For such a discrete-time dynamic model, for the EHA system with system uncertainty matrices and a nonlinear term, a sufficient condition for existence of stable sliding surfaces is proposed by using the linear matrix inequality approach. Based on this existence condition, a DT-SMC is developed such that the reaching motion satisfies the discrete-time sliding mode reaching condition for uncertain systems. Simulation and experimental studies on the EHA system illustrate the effectiveness and applicability of the proposed method.

グリース潤滑膜を利用した直動転がり案内の振動減衰性向上

This paper describes a linear roller guide system designed for improving the vibration damping by using grease lubrication film. This linear roller guide has a very narrow gap between the carriage and top surface of the guide rail, as compared to conventional types for gaining moderate amount of friction force of the thin film of grease that provides the feed drive system with higher damping. In this study, the friction characteristics of the linear roller guide were measured and mathematically modeled in order to investigate the friction characteristics of the thin film of lubricants and the influence of friction on the dynamic behavior of the feed drive system. In the measurement of friction force, two kinds of grease with different consistencies and viscosities of base oil are filled into the narrow gap. Step responses in microscopic displacement region were measured and simulated in order to evaluate of vibration damping of the feed direction. As the results, it was confirmed in both experiment and simulation results that the grease flow characteristic affects the nonlinear friction characteristics and the step response of the feed drive system.

Design Method for High-gain Control of Linear Motor Feed Drive Systems Using Virtual Linear Friction

This paper describes a method for stabilizing feed drive systems by introducing virtual friction characteristics having higher vibration damping with a friction compensator. The validity of the proposed friction compensator is verified by experiments using a one-axis feed drive system consisting of a rod type linear motor and linear roller guides. In addition, this paper proposes a control design method to determine the servo gains and the virtual friction parameters that lead to overshoot-free responses. It is confirmed that the step response curves under the servo gains and the virtual friction parameters determined by the proposed method have no overshoots and amplitude dependence due to the nonlinear friction characteristics.