| In order to realize precision feed motion in an extensive stroke range,the macro-micro composite drive is one of the current research hotspots.The scheme realizes macro motion by servo motor and micro motion by intelligent materials such as piezoelectric ceramics and flexible hinges.This combination has better positioning accuracy and dynamic response performance.Still,the shortcomings of nonlinearity,hysteresis,and creep of intelligent materials lead to their final feeding accuracy failing to meet the actual demand well.Based on the principle of dual-drive differential synthesis,our group proposes a novel dual-drive microfeeding system(DDMS):one servo motor is arranged on the screw and one on the nut,and the two motors drive the screw and the nut to rotate in the same direction and at almost equal rates.The micro-feed motion in the final table is obtained through the macro-motion differential synthesis of the two motors.Since the two motors run outside the pre-slip zone,the interference of strong non-linear friction in the pre-slip zone is avoided,and the low-speed crawling phenomenon of conventional ball screws is overcome.In this paper,a series of research works are carried out on the system working mode,dynamics model,friction model,motion planning,controller design,and motion control software development for the DDMS,which mainly include:(1)The structural composition of the DDMS is introduced,the four operating modes of the system and their characteristics are analyzed,and the definition of the table displacement coordinate system is given as the basis for subsequent research.(2)The unique ball friction coupling mechanism of the DDMS is analyzed through the force analysis of the contact surface of the screw-balls-nut.The dynamics model of the differential mode of the DDMS is established,and by comparing the dynamics model of the single drive,the influence of the table-commuted rotational inertia on the system is pointed out,and the influence of rigidity on the table feeding accuracy is analyzed.Based on the kinetic model analysis,considering the friction coupling,it is proposed to add the centrifugal friction term to the Stribeck friction model as the equivalent friction model under the single reference rate and the model of the dynamic change of friction parameters under different reference rates is established.To obtain the parameters in the proposed model,a genetic algorithm is used to realize the parameter identification of the non-linear friction model,and the required objective function of the algorithm is derived;the recursive least squares method is used to recognize the rotational inertia online,and the needed difference equation for the identification is derived.(3)The motion decomposition strategy is proposed for the unique differential and combined modes of operation of the DDMS.The trajectory planning is carried out using the five-segment sinusoidal curve acceleration and deceleration.The effectiveness of the proposed motion decomposition strategy and trajectory planning is verified by simulation.The error compensation model of the mismatch between the inertia of the screw and nut axes is established to realize the full closed-loop compensation of the servo system;in order to recognize the high-precision feed motion at the table,a discrete sliding mode controller with friction feedforward and based on the variable speed convergence law is designed.(4)The numerical control system is developed,a software framework is constructed based on the functional requirements analysis,and a set of DDMS control software is developed by integrating the logic,strategies,and algorithms required for the system.Based on the developed software,the friction parameters are identified by specific experimental sequences,and the results show that the proposed friction model can better reflect the friction characteristics of the system than the traditional Stribeck friction model.Based on the kinetic model,a separation of rotational inertia identification method is proposed,and the rotational inertia of the screw axis,nut axis,and table are obtained experimentally.The effects of friction force coupling,geometric error compensation coefficient,and servo motor torque ring on table feeding accuracy are analyzed through experiments,and the necessity and correctness of friction force coupling analysis and error compensation principle are verified.Finally,the effectiveness of the proposed sliding mode controller is proved through experiments,and the high-precision feeding motion of the table in differential mode is realized. |
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