Research On High-Precision Speed Control Of Low-Speed Direct-Drive Permanent Magnet Servo Motor

Direct-drive permanent magnet synchronous motor(DD-PMSM)has several advantages:1)compact structure;2)high positioning accuracy;3)high transmission efficiency;4)fast dynamic response,due to the elimination of reduction gearbox.Thus,DD-PMSM is widely used in spacecraft attitude regulation,telescope observation,and other low-speed servo occasions.However,low-speed control of DD-PMSM is a troublesome problem,which has important research significance and engineering value.Based on the linear control theory,this thesis aims at improving the control accuracy of DD-PMSM at low speed(within 100r/min,especially within 10r/min),and carries out the following studies mainly from a perspective of controller design:1.Analysis of key problems in controller design.Based on the linear control theory,the prerequisites and conditions for“perfect”control are analyzed,and methods of evaluating the advantages and disadvantages of a controller are given.Then,the block diagram of DD-PMSM speed control system is established,and the components and characteristics of disturbances and noises in current loop and speed loop are analyzed.It is found that the speed loop with high disturbance-rejection ability is the key to realize high-precision low-speed operation.The above researches lay the foundation for the subsequent controller design.2.Design of High-performance current loop.Based on s-domain model and precise-discretization model of the controlled object in current loop,different controllers are designed in s-domain or z-domain.Through theoretical analysis,simulations,and experiments,the merits and demerits of each controller are demonstrated.In addition,the principle analysis and compensation strategy design are conducted for inverter nonlinearity.The above researches provide support for speed controller design.3.Comparison of speed measurement methods.Appropriate feedback speed measurement method is the premise of speed controller design.The direct differential method with a low-pass filter,phase-locked loop method,and speed observer are analyzed by theory and simulations,and the merits and demerits of each method are summarized.Finally,the appropriate methods are applied for subsequent experimental verification.4.Investigation of the speed loop disturbance-rejection control strategies without observer.According to the analysis of traditional PI controller and active damping method,the defects of one-degree-of-freedom(1-DOF)structure and advantages of two-degree-of-freedom(2-DOF)controller are pointed out.Considering the shape of the sensitivity function determines the anti-disturbance performance of feedback system,a novel controller design idea of shaping the sensitivity function directly based on a 2-DOF structure is proposed,which shows better performance.In addition,the merits and demerits are compared between this new method and the mixed weighted sensitivity H_∞method.Further,a disturbance-rejection strategy based on the precise frequency characteristics of the disturbance,the proportional-integral-resonant controller,is analyzed.Finally,simulations and experiments demonstrate the effectiveness of the above control strategies,and the applicable conditions of different controllers are concluded.5.Investigation of the speed loop disturbance-rejection control strategies with observer.The system performance with disturbance observer(DOB)or extended state observer(ESO)is analyzed in detail.According to the experiments,when using the high-order ESO or the sensitivity function direct shaping method,the motor can operate accurately at 1r/min or even0.05r/min in the sense of average speed.Besides,it is found that at ultra-low speed situation,the inverter nonlinearity compensation and high-bandwidth current loop can both improve the position/speed smoothness to some extent.