Control Strategies Of AC Servo System And Field Bus Interface Technology

AC servo system is used widely with the development of power electronic technology, rare-earth permanent magnet, microelectronic technology and control theory. While foreign enterprises monopolize high-grade servo system maket for technology and other factors in china. The research on high-grade servo system brooks no delay to ensure that the economic development will not be restricted. The control strategies on AC servo system are studied in this thesis based on the predecessors's research, and the thesis is supplied by the national 973 projects and national natural science foundation. The issues such as high speed, high precision, high response and high stability control are discussed in this thesis. In addition, the field bus interface technonogy is also analyzed to realize servo system networking. The details are shown as follows.Typical servo involves three loop feedback controls, such as current-loop, speed-loop and position-loop. The current-loop is the inner loop, which effects on the speed performance of motors. The vector control scheme based on current feedback control is extensively applied. But this scheme can only approximate decoupling control. In fact, the effect of dq-axis current coupling and the phenomenon of torque current drifting are existed. So, two methods to solve these issues are proposed. With these methods, the current control precision can be improved.The ultra-low speed control performance of permanent magnet synchronous motor is the significant factor to position-precision control. The conventional M/T method affects the stability of motor, which is characterized by detection dead-time and delay for the resolution of encoder. Two new methods to improve the ultra-low speed control performance of ac servo system are described in this thesis. One is improved M/T method, which could minimize the influence of speed detection dead time and smooth the measurement. Another is instantaneous speed detection method based on constant jerk model, which assumes the rotor jerk is constant at the interval of sampling period. The instantaneous speed is calculated by analytic geometry. This method can minimize the speed detection dead time and detection delay, which is the premise of low-speed control.The torque fluctuate would affect the stability of low-speed control, the stability of high-speed control and precision of position control. Many factors are related to this fluctuate, one of them is the permanent magnet flux fluctuate. The model reference adaptive identification method (MARS) used to identify the permanent magnet flux is proposed, and the compensation methods is also put forward. The experiment confirmed that the method is useful to the position precision.There are many advantages for the permanent magnet synchronous motor (PMSM) than AC induction motor (ACIM) such as higher power density, lower heating on the rotor and compacter structure. So, PMSM are also suitable to use as spindle motor. To realize that the torque could linear control by Iq, The conventional method is set excitation-current (Id) to zero, which would result in the max speed can not exceed the rated speed, and the torque output can not meet the demand of spindle motor. An optimal route field-weakening control strategy is proposed, which could greatly increase the max speed of rotor. At the mean time it could also keep the torque controllability at any speed.The bus interface technology could solve the issue of communication band-wide limitation in CNC system. The servo could also get the faster and more accuracy command from superior unit with bus interface technology. Ethernet, with the virtue of mature technology and fast communication, become the trend of networking CNC system. The limitation of mechanism of CSMA/CD used in CNC system is confirmed, through analysis to the synchronism and the real-time property. Two realization scheme to establish networking CNC system are proposed, one is based on conventional Ethernet chip (called soft real-time scheme), another is based on expert hardware structure (called hard real-time scheme). In addition, the principle of current-loop-rectify and position subdivision base on cubic spline fitting are proposed. The feasibility and effectiveness are confirmed by application and experiment.