Research On The Expansion Method Of Current Loop Bandwidth For Ac Servo System

Permanent magnet synchronous motors(PMSM)have gradually replaced asynchronous motors and widely used in the AC servo system because of its high efficiency,low noises,low maintenance cost and good performance in dynamic response.Currently,the AC servo systems are widely used in the electric vehicle,robot control,numerical control machine and many other fields in the industrial field.With the development of science and technology,the requirements for the performance of AC servo systems have become much higher.As a result,improving the dynamic performance has become one of the research hotspots.The current loop is the innermost loop in the AC servo system,and the bandwidth of the speed loop and position loop are all related to the current loop.Therefore,the current loop bandwidth strategy is proposed to improve the dynamic response of the AC servo system in this thesis.The mathematical model of the PMSM is established in the synchronous rotating coordinate axises.Then the digital delay in the current loop is equivalent t o the firstorder inertia.The transfer function of the current loop in the AC servo system is built and the relationship between the delay time and the bandwidth of the current loop is calculated from the transfer function above.In this thesis,a current sampling and PWM duty cycle update method is adopted and implemented in the Field Programmable Gate Array(FPGA).This method can reduce the time delay and expand the bandwidth of the current loop in the AC servo system.The dead time effect is analysed theoretically and the dead time compensation method is proposed in this thesis.It is hard to determine the current direction when the current is in the zero-crossing region because of the zero-current clamping effect in the inverter.A dead time compensation method without the use of the current direction is proposed in this thesis.The low frequency harmonics in the current can be eliminated effectively by using this online compensation method.Simulation and experiment results prove that the dead time compensation method can reduce the current ripple and improve the performance of the AC servo system effectively.The simulation model is built in the MATLAB/Simulink Cosimulation platform.And the PMSM experimental platform is based on the simulation model.The dynamic performance of the PMSM control system based on the FPGA is verified through the simulation model.The open loop experiment of current loop is to test the function of the SVPWM module.And the close loop experiment of current loop is to test the bandwidth.The proposed algorithm in this thesis is verified according to the result of the experiment and simulation.