A Study Of Vibration Suppression For A Lightweight Manipulator And Its Modular Joints

The lightweight manipulator has the advantages of lightweight,high load-toweight ratio,low power consumption,and good safety,which is the main development direction of the next generation of collaborative robots.In order to realize the interaction with the external environment while ensuring high positioning accuracy,the lightweight manipulator also needs to ensure the safety of human-robot collaboration.As the core drive-control unit of the lightweight manipulator,the modular joint usually adopts the harmonic driver with lightweight,zero clearance,and high reliability as its transmission part to ensure the motion accuracy of the manipulator.However,the harmonic driver has lower stiffness compared to the joint’s other structural components and the manipulator’s link,which introduces a flexible mode to the joint system.The system resonance frequency brought by the flexible mode will induce mechanical resonance during high acceleration and deceleration motion.Furthermore,the inherent transmission error of the harmonic driver will also cause periodic vibration at its output.In addition,the inevitable dynamic modeling errors,dynamic parameter identification errors,nonlinear friction and damping,unknown external disturbances,and the joint’s load inertia and coupling torque vary with the manipulator’s pose changing,which will affect the precision and stability of motion control.In view of the above problems,the main works and contributions of this study can be summarized as follows:1)First,the modular joint of the lightweight manipulator is studied.According to the structural characteristics of the joint,it is divided into the motor-side,load-side,and transmission part.The motor-side and load-side are equivalent to the inertia-damping system.The transmission part is equivalent to an inertia-spring-damping system.Then,a joint dynamic model with a normalized transmission ratio is established,which is convenient for analyzing and processing the data measured by the motor-side and loadside encoder.After that,the joint torque sensor is re-calibrated to reduce the installation error and factory error to improve the measurement accuracy.Moreover,the joint current-torque constant is identified and corrected to improve the joint’s input torque calculation accuracy.Furthermore,the modular joint’s dynamic parameters are identified through the frequency domain response method with variable frequency and amplitude to reduce the nonlinear influences,improving the identification accuracy during the low-frequency stage.2)Based on the friction model named Stribeck,the least-squares correction is performed on the friction model in the starting and low-speed stages through identification experiments to improve the friction model’s accuracy.After that,the friction is compensated to improve the response speed and the control precision.In addition,the power spectral density analysis is carried out on the load velocity vibration caused by the harmonic driver’s transmission error.Then the double frequency relationship between the motion velocity and the fluctuation frequency is obtained.Finally,a speed-acceleration controller is designed to effectively suppress the periodic vibration caused by the harmonic driver’s transmission error.3)The rigid-body velocity method is proposed.And the Rigid-Body-Solver based on the joint’s dynamic model is designed.Combined with the Rigid-Body-Solver,a Frequency-Selector method is designed.According to the motor velocity or link velocity as its feedback velocity,it can be divided into the motor-side controller and load-side controller,respectively.Finally,it is verified by simulations and experiments that the Frequency-Selector method with two different feedback ways can suppress the mechanical resonance and external disturbance effectively.4)In order to suppress the mechanical resonance and the periodic vibration simultaneously,the motor-side Tunable-Damper method and the load-side TunableDamper method are also designed according to its feedback velocity.Through simulations and experiments,it is verified that the Tunable-Damper method with two feedback ways can simultaneously suppress the mechanical resonance,periodic vibration,and external disturbance.5)In order to improve the robustness to model uncertainty,external disturbance,and load inertia changing,an equivalent rigid body state observer method is designed.The theoretical analysis and the comparisons with other controllers in the frequency domain prove that the equivalent rigid body state observer method has better robustness than others.Experiments have verified its suppression effects on mechanical resonance and periodic vibration,as well as its robustness to external disturbance and load inertia changing.6)By analyzing the structural characteristics and coupling characteristics of the 7-DOF lightweight manipulator independently developed by our team,in order to reduce the dimension and complexity of the manipulator’s controller,the 7-DOF manipulator is equivalent to a two-link manipulator.Then a complete dynamic model of the twolink manipulator,including the coupled model,is built.Next,a dynamic model decoupler and a coupling torque observer are designed to decouple the two-link manipulator’s coupling torque,according to whether the manipulator contains the joint’s torque sensors.7)So as to apply the modular joint’s vibration suppression algorithms to the manipulator,based on the coupling torque decoupled,the distributed control structure is adopted for the two-link manipulator.Its two joints are controlled as two independent systems.Then the two joints are controlled using the equivalent rigid-body state observer method based on the dynamic model decoupler and the coupling torque observer,respectively.Finally,the coupling torque decoupling effects and the vibration suppression effects of the above two methods on the manipulator are verified by simulations and experiments.