Model-based Friction Compensation Research Articles

POSITION CONTROL OF ELECTRO-HYDRAULIC SERVO SYSTEM WITH FRICTION COMPENSATION

This work presents a model-based friction compensation technique and an integral adaptive backstepping controller to improve the position control of the electrohydraulic servo system. The controller design uses a continuous approximation of the LuGre friction model and the Lyapunov theory of nonlinear systems. The friction compensation enhances system performance, and comparison tests are conducted on a hydraulic servo test bench to validate the control strategy's tracking performance under various conditions. KEYWORDS: Electrohydraulic servo systems; Position control; Integral adaptive backstepping controller; LuGre dynamic friction model; Lyapunov theory; Friction compensation.

Adaptive FIT-SMC Approach for an Anthropomorphic Manipulator With Robust Exact Differentiator and Neural Network-Based Friction Compensation

In robotic manipulators, feedback control of nonlinear systems with fast finite-time convergence is desirable. However, because of the parametric and model uncertainties, the robust control and tuning of the robotic manipulators pose many challenges related to the trajectory tracking of the robotic system. This research proposes a state-of-the-art control algorithm, which is the combination of fast integral terminal sliding mode control (FIT-SMC), robust exact differentiator (RED) observer, and feedforward neural network (FFNN) based estimator. Firstly, the dynamic model of the robotic manipulator is established for the n-degrees of freedom (DoFs) system by taking into account the dynamic LuGre friction model. Then, a FIT-SMC with friction compensation-based nonlinear control has been proposed for the robotic manipulator. In addition, a RED observer is developed to get the estimates of robotic manipulator joints’ velocities. Since the dynamic friction state of the LuGre friction model is unmeasurable, FFNN is established for training and estimating the friction torque. The Lyapunov method is presented to demonstrate the finite-time sliding mode enforcement and state convergence for a robotic manipulator. The proposed control approach has been simulated in the MATLAB/Simulink environment and compared with the system with no observer to characterize the control performance. Simulation results obtained with the proposed control strategy affirm its effectiveness for a multi-DoF robotic system with model-based friction compensation having an overshoot and a settling time less than 1.5% and 0.2950 seconds, respectively, for all the joints of the robotic manipulator.

Periodic adaptive learning control of PMSM servo system with LuGre model-based friction compensation

In this paper, based on LuGre friction model, a compensation method by combining of model reference adaptive control and periodic adaptive learning control (MRAC-PALC) is proposed to eliminate the adverse influence of nonlinear friction disturbance on tracking performance in permanent magnet synchronous motor (PMSM) servo systems under periodic task conditions. Specifically, the controller consists of a linear algorithm with PD, feedforward and velocity feedback components and a nonlinear friction compensator which is divided into two parts as follows: in the first time period, an MRAC algorithm is developed to ensure the boundedness of tracking errors; from the second time period, a PALC compensation algorithm, which learns from past information and updates the controller parameters in real time, is deployed to accurately capture the friction dynamics and guarantee the tracking performance. The stability of the proposed MRAC-PALC approach is guaranteed through Lyapunov stability theorem. Some comparative simulations and experiments are conducted to illustrate the superiority of the proposed MRAC-PALC strategy.

Control of an Anthropomorphic Manipulator using LuGre Friction Model - Design and Experimental Validation

Automation technology has been extensively recognized as an emerging field in various industrial applications. Recent breakthrough in flexible automation is primarily due to deployment of robotic arms or manipulators. Autonomy in these manipulators is essentially linked with the advancements in non-linear control systems. The objective of this research is to propose a robust control algorithm for a five degree of freedom (DOF) robotic arm to achieve superior performance and reliability in the presence of friction. A friction compensation-based non-linear control has been proposed and realized for the robotic manipulator. The dynamic model of the robot has been derived by considering the dynamic friction model. The proposed three-state model is validated for all the joints of the manipulator. The integral sliding mode control (ISMC) methodology has been designed; the trajectories of system every time begin from the sliding surface and it eliminates the reaching phase with assistance of integral term in the sliding surface manifold. The designed control law has been first simulated in Matlab/Simulink environment to characterize the control performance in terms of tracking of various trajectories. The results confirm the effectiveness of the proposed control law with model-based friction compensation. The transient parameters like settling and peak time have improvement as well have better results with friction than without considering the friction. The proposed control law is then realized on an in-house developed autonomous articulated robotic rducational platform (AUTAREP) and NI myRIO hardware interfaced with LabVIEW. Experimental results also witnessed the trajectory tracking by the robotic platform.

Fault-tolerant scheme for robotic manipulator-Nonlinear robust back-stepping control with friction compensation.

Emerging applications of autonomous robots requiring stability and reliability cannot afford component failure to achieve operational objectives. Hence, identification and countermeasure of a fault is of utmost importance in mechatronics community. This research proposes a Fault-tolerant control (FTC) for a robot manipulator, which is based on a hybrid control scheme that uses an observer as well as a hardware redundancy strategy to improve the performance and efficiency in the presence of actuator and sensor faults. Considering a five Degree of Freedom (DoF) robotic manipulator, a dynamic LuGre friction model is derived which forms the basis for design of control law. For actuator’s and sensor’s FTC, an adaptive back-stepping methodology is used for fault estimation and the nominal control law is used for the controller reconfiguration and observer is designed. Fault detection is accomplished by comparing the actual and observed states, pursued by fault tolerant method using redundant sensors. The results affirm the effectiveness of the proposed FTC strategy with model-based friction compensation. Improved tracking performance as well robustness in the presence of friction and fault demonstrate the efficiency of the proposed control approach.

A Study on the Automated Guided Vehicle Platform for a Logistics Robot

The need for automated material handling inside the factory has been steadily increasing, especially due to implementation of intelligent manufacturing for better productivity and product quality. Automated material handling devices include logistics robots, automated guided vehicles, industrial robots, collaborative robots, and pick-and-place devices. This study focuses on the development of a low-cost logistics robot that works effectively within a simulated smart factory environment. A nominal PID controller is implemented to guide the robot to follow the line painted on the factory floor. The tracking error information is generated by four down-facing infrared sensors and is fed into the controller. The line-following performance is significantly improved with augmentation of a model-based friction compensator. Optimization of battery power depending on the remaining charge status enhances the reliability. All hardware/software development is supported by the Arduino platform. The step-by-step movement and performance of the logistics robot is verified inside the simulated smart factory environment that includes a robot arm, three conveyors, and two processing stations.

Synchronization Control With Adaptive Friction Compensation of Treadmill-Based Testing Apparatus for Wheeled Planetary Rover

This article studies synchronization control of a novel devised treadmill-based testing apparatus for wheeled planetary rover roaming at low speed. To offer satisfactory tracking performance and to ameliorate internal conflicts amongst individual treadmills during rover-treadmill interaction, a decentralized synchronization control strategy integrating an adaptive friction compensation scheme is proposed. To overcome the nonlinear friction effect at low-velocity scenarios, the LuGre model based friction compensation scheme is constructed with a dual-observer structure that can handle parametric uncertainties without recourse to the high-resolution encoder-required parameter identification. With the proposed control design, asymptotic stability of the closed-loop system is guaranteed with the tracking error and synchronization error converging to zero in presence of parametric uncertainties. Experimental results demonstrate the effectiveness of the proposed control strategy, which significantly improves the tracking performance in comparison with the existing proportional differential (PD)-type controller and synchronization controller without friction compensation.

Master cylinder pressure reduction logic for cooperative work between electro-hydraulic brake system and anti-lock braking system based on speed servo system

As one feasible solution of brake-by-wire systems, electro-hydraulic brake system has been made available into production recently. Electro-hydraulic brake system must work cooperatively with the hydraulic control unit of anti-lock braking system. Due to the mechanical configuration involving electric motor + reduction gear, the electro-hydraulic brake system could be stiffer in contrast to a conventional vacuum booster. That is to say, higher pressure peaks and pressure oscillation could occur during an active anti-lock braking system control. Actually, however, electro-hydraulic brake system and anti-lock braking system are produced by different suppliers considering brake systems already in production. Limited signals and operations of anti-lock braking system could be provided to the supplier of electro-hydraulic brake system. In this work, a master cylinder pressure reduction logic is designed based on speed servo system for active pressure modulation of electro-hydraulic brake system under the anti-lock braking system–triggered situation. The pressure reduction logic comprises of model-based friction compensation, feedforward and double closed-loop feedback control. The pressure closed-loop is designed as the outer loop, and the motor rotation speed closed-loop is drawn into the inner loop of feedback control. The effectiveness of the proposed controller is validated by vehicle experiment in typical braking situations. The results show that the controller remains stable against parameter uncertainties in extreme condition such as low temperature and mismatch of friction model. In contrast to the previous methods, the comparison results display the improved dynamic cooperative performance of electro-hydraulic brake system and anti-lock braking system and robustness.

A novel pressure control strategy of an electro-hydraulic brake system via fusion of control signals

With the development of electro-hydraulic brake system in the automotive application, pressure control is at the top of a brake system engineer’s agenda. This work focuses on the development of a pressure-loop controller for a motor-type electro-hydraulic brake system, which is composed of an electro-mechanical actuator and a hydraulic link. The pressure control issue of motor-type electro-hydraulic brake system is influenced intensely by the nonlinearities (i.e. friction) and uncertainties (e.g. temperature variation, brake pad wear, and so on) of the system and by the very demanding performance specifications (i.e. supporting cooperative work with hydraulic control unit of anti-lock brake system). The pressure control of motor-type electro-hydraulic brake system is investigated, and a novel pressure–based control strategy via fusion of control signals is proposed to improve the pressure tracking performance. The control strategy comprises online model–based friction compensation, online dither–based friction compensation, and feedback control. Four original contributions make this work distinctive from the existing relevant literature. Selecting the Coulomb+viscous friction model can maximize to reduce difficulty of parameter identification and Stribeck effects detection based on maintaining the pressure tracking accuracy. Thanks to the model-based friction compensation torque, the signal magnitude of dither-based friction compensation torque can be decreased so that the vehicle comfort can be improved. The compensation parameters of both the model-based and dither-based friction compensation can be online modified according to the operating point of system. The robustness of the fusion controller is enhanced by employing the sliding mode control algorithm with conditional integrator. The performance of the proposed control strategy is evaluated by hardware-in-the-loop-simulation and vehicle experiment in typical braking situations. The experimental results with fusion control show improved pressure tracking performance in comparison with that without fusion control.

Friction compensation and observer-based adaptive sliding mode control of electromechanical actuator

The performance of the electromechanical actuator system is usually affected by the nonlinear friction torque disturbance, model uncertainty, and unknown disturbances. In order to solve this problem, a model-based friction compensation method combined with an observer-based adaptive sliding mode controller for the speed loop of electromechanical actuator system is presented in this article. All the disturbances and model uncertainty of electromechanical actuator system are divided into two parts. One is model-based friction torque disturbance which can be identified by experiments, and the other is the residual disturbance which cannot be identified by experiments. A modified LuGre model is adopted to describe the friction torque disturbance of electromechanical actuator system. An extended state observer is designed to estimate the residual disturbance. An adaptive sliding mode controller is designed to control the system and compensate the friction torque disturbance and the residual disturbance. The stability of the electromechanical actuator system is discussed with Lyapunov stability theory and Barbalat’s lemma. Experiments are designed to validate the proposed method. The results demonstrate that the proposed control strategy not only provides better disturbance rejecting ability but also provides better steady state and dynamic performance.

Control of Pneumatic Position Servo with LuGre Model-based Friction Compensation

Control of Pneumatic Position Servo with LuGre Model-based Friction Compensation

Output feedback adaptive robust control of hydraulic actuator with friction and model uncertainty compensation

This paper studies output feedback control of hydraulic actuators with modified continuous LuGre model based friction compensation and model uncertainty compensation. An output feedback adaptive robust controller is constructed which combines the adaptive control part and the robust control part seamlessly. The adaptive part is constructed to handle the parametric uncertainties existed in the model. The residuals coming from parameter adaption and the unmodeled dynamics are taken into consideration by the robust part. Since only the position signal is available, the velocity, pressure, and internal friction states are obtained by observation or estimation. The errors coming from observation and estimation are also dealt with the robust part. Furthermore, the convergence of the closed-loop controller–observer scheme is achieved by the Lyapunov method in the presence of parametric uncertainties only. Extensive comparative experiments performed on a hydraulic actuator demonstrate the effectiveness of the proposed controller–observer scheme.

Improvement of Force Transmission Performance Considering Nonlinear Friction in Bilateral Control Systems

This paper presents the results of an experimental evaluation of four-channel bilateral control systems. These systems are based on robust acceleration control, and take nonlinear friction compensation into consideration. Although force estimation by a reaction force observer is one of the key technologies used to achieve precise bilateral control without force sensors, extracting accurate force data using this method proves difficult owing to the requirement of removing disturbances such as friction, gravity, interference force, etc. It is well known that nonlinear friction exhibits a complicated behavior at the micro-displacement region and/or the reversal point of velocity direction. In particular, this characteristic is most obvious in the operability in macro/micro bilateral control with scale conversion between master and slave. This paper presents an improvement to the quality of force estimation by using nonlinear friction modeling and model-based friction compensation in the reaction force observer. The accuracy of force control performance in the four-channel bilateral control system with a reaction force observer is experimentally evaluated by using linear actuator systems with different friction dynamics.

A novel approach for mitigating the effects of pre-rolling/pre-sliding friction on the settling time of rolling bearing nanopositioning stages using high frequency vibration

Nanopositioning stages are used for ultra-precise positioning in many different applications. Among the bearing options available for long-range nanopositioning stages, rolling bearings are the most cost effective. However, rolling bearing nanopositioning (RB-NP) stages are well known to take a long time to reach their target positions in point-to-point positioning tasks due to pre-rolling/pre-sliding friction. This paper presents vibration assisted nanopositioning (VAN) – a novel approach for reducing the settling time of RB-NP stages using high frequency vibration (aka dither). VAN synergistically combines the electromechanical design and control of RB-NP stages to mitigate pre-rolling/pre-sliding friction using dither, without jeopardizing positioning precision. It is shown in simulations to exhibit superior positioning performance and robustness compared to model-based friction compensation methods. Experiments carried out on a prototype stage incorporating the VAN approach show a 52% reduction of mean settling time, based on 50 trials, when compared to a case without VAN. Moreover, the heat and potential wear induced by dither are shown to be practically insignificant.

Control of vibrations for a parallel manipulator with flexible links — concepts and experimental results

A comprehensive control approach is presented to reduce the vibrations of a parallel manipulator with a kinematic loop and two flexible links whereof the longer one can show significant oscillations. The control objectives are end-effector trajectory tracking and active vibration control. The system is modeled as a flexible multibody system and exact feedforward control based on the full dynamic flexible multibody system is applied to improve the end-effector trajectory tracking performance. Furthermore, the effect of different position control concepts for the two linear drives, such as gain scheduling for the utilized cascade control and a model based friction compensation, on the movers themselves as well as on the end-effector are discussed, which can be conflicting. Experimental results are presented illustrating the achievable accuracy of the end-effector tracking for different trajectories while showing significant error reductions for a feedforward control based on an elastic model in contrast to a rigid one. Finally, a model based curvature controller is utilized which actively controls the occurring oscillations of the parallel manipulator. Here, a proportional controller as well as a linear-quadratic regulator are applied and the impact of an additional curvature control on the end-effector tracking performance is investigated.

Integral Sliding Mode Approach to Robust Control Systems Against Friction Force

Owing to the interesting challenges in the design of control systems that involve friction force, this paper proposes a new approach to the design of robust control system against friction. The proposal is motivated by the criticism aroused against the model-based friction compensation approaches such as the need for detailed friction model, accurate observer-based friction estimation, and issues of design complexity. The proposal utilizes the inherent robustness and relative design simplicity of the integral sliding mode control to attain closed-loop system robustness against friction effects if the maximum bound of friction is known. The use of ISMC provides closed-loop system robustness without the need for detailed friction modelling or accurate observer-based friction estimation and therefore simplifies closed-loop system design complexity significantly. The friction force in a tutorial example is considered to demonstrate the robustness of the proposed algorithm.

Adaptive Control of Hydraulic Actuators With LuGre Model-Based Friction Compensation

This paper concerns high-accuracy tracking control for hydraulic actuators with nonlinear friction compensation. Typically, LuGre model-based friction compensation has been widely employed in sundry industrial servomechanisms. However, due to the piecewise continuous property, it is difficult to be integrated with backstepping design, which needs the time derivation of the employed friction model. Hence, nonlinear model-based hydraulic control rarely sets foot in friction compensation with nondifferentiable friction models, such as LuGre model, Stribeck effects, although they can give excellent friction description and prediction. In this paper, a novel continuously differentiable nonlinear friction model is first derived by modifying the traditional piecewise continuous LuGre model, then an adaptive backstepping controller is proposed for precise tracking control of hydraulic systems to handle parametric uncertainties along with nonlinear friction compensation. In the formulated nonlinear hydraulic system model, friction parameters, servovalve null shift, and orifice-type internal leakage are all uniformly considered in the proposed controller. The controller theoretically guarantees asymptotic tracking performance in the presence of parametric uncertainties, and the robustness against unconsidered dynamics, as well as external disturbances, is also ensured via Lyapunov analysis. The effectiveness of the proposed controller is demonstrated via comparative experimental results.

A parsimonious friction model for efficient identification and compensation of hysteresis with nonlocal memory

A novel dynamic friction model, which allows to capture friction hysteresis with non-local memory, is presented in this paper. The model is conceived in order to find a trade-off between accuracy of the model prediction and difficulty of implementation in motion control systems with model-based friction compensation. The hysteresis function introduced into the model accounts for non-local memory, i.e., the property for which the friction output depends not only on the initial conditions but also on past extremum values of the input or the output. In comparison with other models incorporating a hysteresis function with non-local memory, the proposed model is demonstrated to reduce the number of parameters necessary to reproduce the hysteresis loops observed experimentally. Moreover, parameter identification can benefit from the availability of a closed form of the model solution.

Adaptive robust motion trajectory tracking control of pneumatic cylinders with LuGre model-based friction compensation

Friction compensation is particularly important for motion trajectory tracking control of pneumatic cylinders at low speed movement. However, most of the existing model-based friction compensation schemes use simple classical models, which are not enough to address applications with high-accuracy position requirements. Furthermore, the friction force in the cylinder is time-varying, and there exist rather severe unmodelled dynamics and unknown disturbances in the pneumatic system. To deal with these problems effectively, an adaptive robust controller with LuGre model-based dynamic friction compensation is constructed. The proposed controller employs on-line recursive least squares estimation (RLSE) to reduce the extent of parametric uncertainties, and utilizes the sliding mode control method to attenuate the effects of parameter estimation errors, unmodelled dynamics and disturbances. In addition, in order to realize LuGre model-based friction compensation, the modified dual-observer structure for estimating immeasurable friction internal state is developed. Therefore, a prescribed motion tracking transient performance and final tracking accuracy can be guaranteed. Since the system model uncertainties are unmatched, the recursive backstepping design technology is applied. In order to solve the conflicts between the sliding mode control design and the adaptive control design, the projection mapping is used to condition the RLSE algorithm so that the parameter estimates are kept within a known bounded convex set. Finally, the proposed controller is tested for tracking sinusoidal trajectories and smooth square trajectory under different loads and sudden disturbance. The testing results demonstrate that the achievable performance of the proposed controller is excellent and is much better than most other studies in literature. Especially when a 0.5 Hz sinusoidal trajectory is tracked, the maximum tracking error is 0.96 mm and the average tracking error is 0.45 mm. This paper constructs an adaptive robust controller which can compensate the friction force in the cylinder.

Application in prestiction friction compensation for angular velocity loop of inertially stabilized platforms

To overcome the influence of the nonlinear friction on the gimbaled servo-system of an inertial stabilized platforms (ISPs) with DC motor direct-drive, the methods of modeling and compensation of the nonlinear friction are proposed. Firstly, the inapplicability of LuGre model when trying to interpret the backward angular displacement in the prestiction regime is observed experimentally and the reason is deduced theoretically. Then, based on the dynamic model of direct-drive ISPs, a modified LuGre model is proposed to describe the characteristic of the friction in the prestiction regime. Furthermore, the state switch condition of the three friction regimes including presliding, gross sliding and prestiction is presented. Finally, a composite compensation controller including a nonlinear friction observer and a feedforward compensator based on the novel LuGre model is designed to restrain the nonlinear friction and to improve the control precision. Experimental results indicate that compared with those of the conventional proportion–integration–differentiation (PID) control method and the PID plus LuGre model-based friction compensation method, the dwell-time has decreased from 0.2s to almost 0s, the position error decreased to 86.7% and the peak-to-peak value of position error decreased to 80% after the novel compensation controller is added. It concludes that the composite compensation controller can greatly improve the control precision of the dynamic sealed ISPs.