Research On Low Speed Feed Characteristics Of Dual Direct Drive Servo System

Accurately achieving micro-displacement feed of tools or workpieces is one of the key technical bottlenecks that restrict precision and ultra-precision machining.When the conventional ball screw pair is running at a low speed,the nonlinearity of the friction causes a low-speed nonlinear creep creep phenomenon,and it is difficult to achieve precise micro-displacement feed control.In precision machining,in order to realize large-stroke,high-precision feed motion,macro-micro composite drive is currently used,that is,large-stroke feed motion is realized by macro drive,and high precision is achieved by micro-drive in a small stroke range.But this drive has defects such as low stiffness,nonlinearity,hysteresis,creep.Therefore,based on the principle of motion synthesis,a double-directed micro-feed servo system is proposed by using the hollow shaft direct drive motor and the nut-driven ball screw pair.In this paper,the design and optimization of the hollow direct drive motor,the stiffness configuration,dynamic characteristics,nonlinear friction,low-speed feed characteristics at low speeds of the double-drive servo system are studied.The main contents of this article include:(1)The direct drive hollow shaft motor is preliminarily designed based on the equivalent magnetic circuit method.According to the design requirements of the double-drive servo system for the hollow shaft motor,the basic parameters of the hollow shaft motor are calculated based on the motor magnetic circuit method.According to the basic parameters of the magnetic circuit design,the finite element model of hollow shaft motor is established in maxwell,and the no-load characteristics and load characteristics of hollow shaft motor are simulated.(2)The multi-objective optimization of the direct-drive hollow shaft motor is carried out by genetic algorithm,which improved the performance of the motor.For the preliminary design.the direct drive motor has large cogging torque,low efficiency,large torque fluctuation under rated load,and does not meet the design requirements.The design variables and the range of values that affect the performance of the direct drive motor are determined.The minimum torque,the highest efficiency,and the minimum amount of magnetic steel are the targets.A multi-objective optimization model is established.The optimized Latin hypercube sampling is used.The sampling result is calculated by the finite element method.The Kring approximation model is established and the direct drive is performed by NSGA ? multi-objective genetic algorithm.The motor is optimized to reduce torque ripple and improve the performance of the direct drive motor.(3)The dynamic stiffness of the joint of the double direct drive servo system is optimized to improve the dynamic characteristics of the feed system.For this purpose,an accurate finite element model of the double direct drive servo system including the modeling of the bolt joint,the screw bearings,the linear guides and the nut driven ball screw for the DDFS is established.The correctness of the FEM for the double direct drive servo system is proved by experimental measurements.The weak models and the weak joints of the double direct drive servo system are obtained based on the computations of modal flexibility and energy distribution.After that all weak joints stiffness values are used as the design variables and the minimum weak modal flexibility index is regarded as the objectives in the multi-objective optimization model.To implement the multi-objective optimization model,a collaborative optimization method based on combing orthogonal experiment and the grey relational analysis is used.The optimal combination of joint stiffness is obtained.(4)Based on the LuGre friction model,the critical crawling velocity characteristics of the double direct drive servo system at low speed are studied.The dynamic model and full-closed control scheme of the dual direct drive servo system are established.The critical crawling speed of the double-drive servo system using the differential double drive and the traditional single screw drive is studied.Through simulation and experiment.Comparing the speed response of the table under the two driving modes,it is concluded that the low-speed feed performance of the dual-drive differential system is significantly better than that of the conventional single-drive system.Finally,the study found the effect of the axial stiffness of the ball screw on the critical creep speed of the table.(5)The speed fluctuation characteristics of the double direct drive servo system are studied.The influence of motor torque harmonics on the motor speed for the double direct drive servo system is analyzed by the error transfer function,which indicates that the effect of torque harmonics on the motor speed decreases with increasing speed.According to the error transfer function analysis and the modal analysis of the double direct drive servo system,the reasonable speed operating ranges of a differential double drive motor are determined to obtain low smoothness velocity of the table for the double direct drive servo system.Compared with classical drive feed system,the double direct drive servo system reduces the influence of the motor torque harmonics on the speed fluctuation and displacement fluctuation of the table at low-speed operation and avoids the system resonance caused by motor torque harmonics at certain speed points.