Research on the forcefree control of cooperative robots based on dynamic parameters identification

2019 ◽  
Vol 46 (4) ◽  
pp. 499-509 ◽  
Author(s):  
Juliang Xiao ◽  
Fan Zeng ◽  
Qiulong Zhang ◽  
Haitao Liu
Keyword(s):  
Dynamic Model ◽  
Control Algorithm ◽  
Angular Displacement ◽  
Friction Model ◽  
Friction Torque ◽  
Parameters Identification ◽  
Dynamic Parameters ◽  
Cooperative Robots ◽  
Content Type ◽  
Direct Teaching

Purpose This paper aims to propose a forcefree control algorithm that is based on a dynamic model with full torque compensation is proposed to improve the compliance and flexibility of the direct teaching of cooperative robots. Design/methodology/approach Dynamic parameters identification is performed first to obtain an accurate dynamic model. The identification process is divided into two steps to reduce the complexity of trajectory simplification, and each step contains two excitation trajectories for higher identification precision. A nonlinear friction model that considers the angular displacement and angular velocity of joints is proposed as a secondary compensation for identification. A torque compensation algorithm that is based on the Hogan impedance model is proposed, and the torque obtained by an impedance equation is regarded as the command torque, which can be adjusted. The compensatory torque, including gravity torque, inertia torque, friction torque and Coriolis torque, is added to the compensation to improve the effect of forcefree control. Findings The model improves the total accuracy of the dynamic model by approximately 20% after compensation. Compared with the traditional method, the results prove that the forcefree control algorithm can effectively reduce the drag force approximately 50% for direct teaching and realize a flexible and smooth drag. Practical implications The entire algorithm is verified by the laboratory-developed six degrees-of-freedom cooperative robot, and it can be applied to other robots as well. Originality/value A full torque compensation is performed after parameters identification, and a more accurate forcefree control is guaranteed. This allows the cooperative robot to be dragged more smoothly without external sensors.

scholarly journals A new joint friction model for parameter identification and sensor-less hand guiding in industrial robots

2020 ◽  
Vol 47 (6) ◽  
pp. 847-857
Author(s):  
Guanghui Liu ◽  
Qiang Li ◽  
Lijin Fang ◽  
Bing Han ◽  
Hualiang Zhang
Keyword(s):  
Industrial Robot ◽  
Angular Displacement ◽  
Weighted Least Squares ◽  
Friction Model ◽  
Industrial Robots ◽  
Model Parameters ◽  
Dynamic Parameters ◽  
Content Type ◽  
Joint Friction ◽  
Six Dof

Purpose The purpose of this paper is to propose a new joint friction model, which can accurately model the real friction, especially in cases with sudden changes in the motion direction. The identification and sensor-less control algorithm are investigated to verify the validity of this model. Design/methodology/approach The proposed friction model is nonlinear and it considers the angular displacement and angular velocity of the joint as a secondary compensation for identification. In the present study, the authors design a pipeline – including a manually designed excitation trajectory, a weighted least squares algorithm for identifying the dynamic parameters and a hand guiding controller for the arm’s direct teaching. Findings Compared with the conventional joint friction model, the proposed method can effectively predict friction factors during the dynamic motion of the arm. Then friction parameters are quantitatively obtained and compared with the proposed friction model and the conventional friction model indirectly. It is found that the average root mean square error of predicted six joints in the proposed method decreases by more than 54%. The arm’s force control with the full torque using the estimated dynamic parameters is qualitatively studied. It is concluded that a light-weight industrial robot can be dragged smoothly by the hand guiding. Practical implications In the present study, a systematic pipeline is proposed for identifying and controlling an industrial arm. The whole procedure has been verified in a commercial six DOF industrial arm. Based on the conducted experiment, it is found that the proposed approach is more accurate in comparison with conventional methods. A hand-guiding demo also illustrates that the proposed approach can provide the industrial arm with the full torque compensation. This essential functionality is widely required in many industrial arms such as kinaesthetic teaching. Originality/value First, a new friction model is proposed. Based on this model, identifying the dynamic parameter is carried out to obtain a set of model parameters of an industrial arm. Finally, a smooth hand guiding control is demonstrated based on the proposed dynamic model.

Torque control based direct teaching for industrial robot considering temperature-load effects on joint friction

2019 ◽  
Vol 46 (5) ◽  
pp. 699-710 ◽  
Author(s):  
Liming Gao ◽  
Jianjun Yuan ◽  
Yingjie Qian
Keyword(s):  
Teaching Method ◽  
Industrial Robot ◽  
Viscous Friction ◽  
Joint Space ◽  
Friction Model ◽  
Friction Torque ◽  
Torque Control ◽  
Content Type ◽  
Friction Resistance ◽  
Direct Teaching

Purpose The purpose of this paper is to design a practical direct teaching method for the industrial robot with large friction resistance and gravity torque but without expensive force/torque sensor, where the gravity torque is just a function of joints position, whereas the friction is closely associated with joint velocity, temperature and load. Design/methodology/approach In the teaching method, the output torque of joint motor is controlled through current to compensate gravity torque completely and friction resistance incompletely. Three variables closely associated with friction are investigated separately by experiment and theoretical analysis, and then a comprehensive friction model which is used to calculate the required compensated friction torque is proposed. Finally, a SIASUN 7 degrees of freedom robot was used to verify the model and the method. Findings Experimental results demonstrated that the teaching method enables an operator to teach the robot in joint space by applying small force and torque on either end-effector or its body. The friction investigation suggests that the velocity and temperature have a strong nonlinear influence on viscous friction, whereas load torque significantly influences the Coulomb friction linearly and causes a slight Stribeck effect. Originality/value The main contribution includes the following: a practical joint space direct teaching method for a common industrial robot is developed, and a friction model capturing velocity, temperature and load for robot joints equipped with commercialized motors and harmonic drives is proposed.

scholarly journals Force-Free Control for Direct Teaching of a Surgical Assistant Robot End Effector with Wire-Driven Bidirectional Telescopic Mechanism

Sensors ◽  
2021 ◽  
Vol 21 (10) ◽  
pp. 3498
Author(s):  
Youqiang Zhang ◽  
Cheol-Su Jeong ◽  
Minhyo Kim ◽  
Sangrok Jin
Keyword(s):  
Control Algorithm ◽  
Position Control ◽  
Friction Model ◽  
Surgical Instruments ◽  
Control Input ◽  
End Effector ◽  
Motor Current ◽  
Direct Teaching ◽  
End Effectors ◽  
Teaching Operation

This paper shows the design and modeling of an end effector with a bidirectional telescopic mechanism to allow a surgical assistant robot to hold and handle surgical instruments. It also presents a force-free control algorithm for the direct teaching of end effectors. The bidirectional telescopic mechanism can actively transmit force both upwards and downwards by staggering the wires on both sides. In order to estimate and control torque via motor current without a force/torque sensor, the gravity model and friction model of the device are derived through repeated experiments. The LuGre model is applied to the friction model, and the static and dynamic parameters are obtained using a curve fitting function and a genetic algorithm. Direct teaching control is designed using a force-free control algorithm that compensates for the estimated torque from the motor current for gravity and friction, and then converts it into a position control input. Direct teaching operation sensitivity is verified through hand-guiding experiments.

A novel reduced order dynamic model of axial piston motors with compression flow losses and Coulomb friction losses

2019 ◽  
Vol 72 (5) ◽  
pp. 567-573
Author(s):  
Lichen Gu ◽  
Rui Xu ◽  
Nan Wang
Keyword(s):  
Dynamic Model ◽  
Coulomb Friction ◽  
Friction Torque ◽  
Content Type ◽  
Reduced Order ◽  
Flow Losses ◽  
Compression Flow ◽  
Flow Loss ◽  
Torque Loss ◽  
Axial Piston

Purpose The purpose of this paper is to identify the energy losses factors during the hydro-mechanical conversion process at high pressure via a novel reduced order dynamic model. Design/methodology/approach A novel reduced order dynamic model of the axial piston motor was proposed, which provides an explicit insight to the compression flow losses and the Coulomb friction losses. A fully coupled dynamic model of the piston motor was obtained based on the array bond graph method. And then, a reduced order model was obtained by the composition analysis of flow and torque of the axial piston motor. After that, the energy losses estimation model was presented to predict the energy loss of the piston motor under a wide range of working conditions. The model was verified by comparing the experimental and simulation results. Findings The simulation result indicates that the flow loss caused by oil compression accounts for 59 per cent of the total flow loss, and the Coulomb friction torque accounts for 40 per cent of the total torque loss under a specific working condition. The compression flow loss and Coulomb friction torque are the major factors that lead to the aggravation of energy loss under extreme working conditions of the piston motor. Originality/value At high-pressure condition, the compression flow losses due to fluid compressibility cannot be neglected, and the hydro-mechanical losses in varies friction pairs should involve Coulomb friction losses. Flow and torque loss analytical expression in the model involve the design and control parameters of the piston equipment, which can realize the parameter optimization of the piston equipment for the purpose of energy-saving.

Parameter Identification of LuGre Friction Model for Robot Joints

2012 ◽  
Vol 479-481 ◽  
pp. 1084-1090 ◽  
Author(s):  
Ya Qing Zheng
Keyword(s):  
Theoretical Foundation ◽  
Friction Model ◽  
Parameters Identification ◽  
Dynamic Parameters ◽  
Lugre Model ◽  
Friction Behavior ◽  
Research Issues ◽  
Lugre Friction Model ◽  
Lugre Friction ◽  
Experimental Validations

The LuGre friction model well captures most of the friction behavior, but it was very difficult to identify the parameters of the LuGre model. The LuGre friction model, theory of static and dynamic parameters identification of the LuGre model as well as the algorithm based on particle swarm optimization are summarized according to the previous work. Then the programs for the static and dynamic parameters identification are made and analyzed in the environment of Matlab software in detail, and the identification results are given. The work mentioned above will lay the theoretical foundation for the future experimental validations and provide the detailed models, algorithms and programs for the corresponding research issues.

Study on the influence of radial stiffness on the nonlinear vibration of brake system

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Xiaoyu Yan ◽  
Chenglong Fan ◽  
Wei Wang ◽  
Xiaojun Liu ◽  
Bingsan Chen
Keyword(s):  
Nonlinear Vibration ◽  
Dynamic Model ◽  
Periodic Motion ◽  
Friction Model ◽  
Brake System ◽  
Lumped Mass ◽  
Content Type ◽  
Tangential Stiffness ◽  
Radial Stiffness ◽  
The Stability

Purpose A dynamic model of the brake system considering the tangential and radial motion of the pad, and the torsion and wobbling motion of the disk is established in this paper. The influence of radial stiffness on the brake system is investigated under different tribological conditions. This paper aims to prove that sufficient radial stiffness is indispensable in the design of the brake system with good tribological performance. Design/methodology/approach By using the lumped mass method, a dynamic model of the brake system is established. A Stribeck-type friction model is applied to this model to correlate the frictional velocity, pressure and friction force. The stability of pad vibration is analysed by analysis methods. A new stability evaluation parameter is proposed to study the influence of radial stiffness on stability of pad vibration in a certain friction coefficient brake pressure range. Findings The findings show that the tangential vibration of the pad transits from periodic motion to quasi-periodic motion under a low tangential stiffness. The influence of radial stiffness on motion stability is stronger under a low nominal brake radius. The stability of the brake system can be ensured when the brake radius and radial stiffness are sufficient. Originality/value The influence of tangential stiffness of pad on stability of the brake system has been researched for decades. The insufficiency of stiffness in radial direction may also generate certain levels of instabilities but has not been fully investigated by modelling approach. This paper reveals that this parameter is also strongly correlated to nonlinear vibration of the brake pad.

Research on the Motor Friction Model of Electro-Hydraulic Servo System

2013 ◽  
Vol 579-580 ◽  
pp. 594-597 ◽  
Author(s):  
Xiao Jing Wang ◽  
Wen Xuan Han ◽  
Xiao Cui ◽  
Shuo Ze Li ◽  
Cheng Zhi Du
Keyword(s):  
Simulation Analysis ◽  
Angular Displacement ◽  
Servo System ◽  
Friction Model ◽  
Friction Torque ◽  
Servo Motor ◽  
Lugre Friction Model ◽  
Hydraulic Servo ◽  
Friction Models ◽  
Hydraulic Servo System

In order to improve the performance of the electro-hydraulic servo system, considering the special structure of servo motor, nonlinear model of the servo motor was established in time domain. Several friction models commonly used in project were introduced and applied to the simulation analysis of servo motor with friction torque. The simulation curves show the relationship between driving torque, friction torque, angular velocity and angular displacement. The result confirms the influence of the different friction torque model on the performance of motor, and the LuGre friction model is identified which is fit for the friction torque compensation. This study lays the foundation for the friction torque compensation of electro-hydraulic servo motor.

Dynamic Parameters and Friction Model Identification of an Industrial Hybrid Robot

2019 ◽  
Author(s):  
Binbin Zhang ◽  
Liping Wang ◽  
Jun Wu
Keyword(s):  
Dynamic Model ◽  
Control Method ◽  
Driving Forces ◽  
Dynamic Control ◽  
Model Identification ◽  
Friction Model ◽  
Dynamic Parameters ◽  
Typical Trajectory ◽  
Lagrangian Dynamic ◽  
Moving Speed

Abstract To obtain higher performance for an industrial hybrid robot, the dynamic control method is utilized to control the robot. For dynamic control, the control performance is directly affected by the accuracy of the dynamic model. This paper investigated a method to establish and identify an accurate dynamic model. First, based on the Lagrangian dynamic equation and the Stribeck friction model, the unidentified dynamic model of the five-DOF hybrid robot is established. Second, identification experiments are carried out. Each of the driving joints performs frequent reciprocating motions individually. In the meantime, the moving speed is gradually increased to obtain driving torques of the respective joint at different moving speeds. Then the dynamic parameters with lower coupling are identified by using the standard deviation index and the least squares methods until all parameters are gradually determined. Finally, the hybrid robot moves a typical trajectory, while the currents of each joint are collected to obtain the driving forces. The actual driving forces, the identified dynamic model, and the unidentified dynamic model are compared. The results show that the identified method could significantly improve the accuracy of the dynamic model. The method proposed in this paper is general and can be applied to the other robots without adding any sensors.

scholarly journals Nonlinear friction dynamic modeling and performance analysis of flexible parallel robot

2020 ◽  
Vol 17 (6) ◽  
pp. 172988142097251
Author(s):  
Lei Zhao ◽  
Xin-hua Zhao ◽  
Bin Li ◽  
Yu-wei Yang ◽  
Liang Liu
Keyword(s):  
Dynamic Model ◽  
Dynamic Modeling ◽  
High Speed ◽  
Parallel Robot ◽  
Friction Model ◽  
Parameters Identification ◽  
Common Method ◽  
Nonlinear Friction ◽  
Low Speed ◽  
The Stability

The article presents a friction dynamic modeling method for a flexible parallel robot considering the characteristics of nonlinear friction. An approximate friction model which is proposed by Kostic et al. is applied to establish the dynamic model with Lagrange method. Parameters identification is completed by least square method, and the tracking accuracy and the stability of the robot are systematically analyzed before and after dynamic compensation at different speeds. Its position error of the robot after compensation is only 0.98 mm at low speed. The accuracy is improved 10 times than that before compensation. In addition, the peak velocity errors are 3.97 mm·s−1 and 1.49 mm·s−1 at high and low speed, respectively, which are reduced 2.5 times than that before compensation. The experimental data also indicate that velocity tracking curve has no obvious peak error compared with the common method based on Coulomb and viscous friction model. The curve is much smoother with proposed model, and the motion stability of robot at high speed has been greatly improved. The proposed method just needs the robot to collect some positions before path tracing, and the parameters identification of dynamic model can be completed quickly. The compensation effect is much more better than common method. So the proposed method can be extended to complete the dynamic identification for complex robot with more joints. It is helpful to further improve the stability and the accuracy at high speed.

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