| Active magnetic bearing(AMB)has the advantages of no mechanical friction,no lubrication,low loss,high efficiency and long service life.It can be widely used in highspeed motor,centrifuge,vacuum pump and other fields.As a key component,switching power amplifier has an important impact on the performance of AMB.In this paper,the design of switching power amplifier for AMB is studied.1)The factors affecting the performance of switching power amplifier are analyzed.For the three control modes of sample and hold,hysteresis comparison and pulse width modulation(PWM),their dynamic and harmonic characteristics are analyzed by Matlab/ Simulink simulation.The sample and hold control mode has good tracking performance at high frequency,the hysteresis comparison control mode has better performance when the loop width is small,and the pulse width modulation control mode has the smallest current harmonic.Then,according to the steady-state current waveform and the change rate of coil current,the calculation formula of current ripple of two-level PWM switching power amplifier is deduced,based on this the possible way to reduce the current ripple is obtained.2)Aiming at the control problem of power amplifier,a PI control strategy based on slope matching is proposed.Firstly,the mathematical model of two-level PWM switching power amplifier is established,and the expression of system transfer function is obtained.On this basis,according to the principle of slope matching and the analysis results of cut-off frequency,a PI control strategy of power amplifier is designed.In this new strategy,the PI parameters for the second-order power amplification system with zero are set by the principle of slope matching,so that the power amplification system can obtain better dynamic performance,compared with traditional PID,the overshoot is reduced by 5%.3)Further considering the delay of power amplifier system,an internal model PID control strategy of switching power amplifier is proposed.The three parameters of traditional PID control method are simplified as one parameter in the internal PID control strategy,which simplifies the parameter tuning process and makes the system have better performance,compared with traditional PID,the overshoot is reduced by 25%and the adjustment time is reduced by 0.2ms.Finally,the hardware system of two-level PWM switching power amplifier is designed,and the experimental platform is developed.The effectiveness of the control strategy is verified by experiments.The linearity,tracking performance and current ripple of the designed switching power amplifier are tested.The experimental results are compared with that of the theory.The results show that the linearity is small when the given signal amplitude is relatively poor,but the overall performance is good.The tracking performance is good effect within the cut-off frequency range,and the value of current ripple can meet the requirements of the power amplifier system.The designed PI control method has smaller overshoot and better stability than the traditional PID method,but the design method is slightly complex.The internal model PID control method simplifies the parameter tuning problem,further reduces the overshoot,and has good robustness and dynamic performance. |
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