| In comparison with the conventional rolling element or journal bearings, magnetic bearings, which use magnetic force to support freely a rotor, are being widely used in rotating machinery. The magnetic bearing has many advantages such as no wear, no need of lubrication, long life and high rotation speed. Magnetic bearing is a typical mechatronical(mechanical-electrical integration) system, which includes electronic system (sensors, controller and power amplifier) and mechanical system. In the electronic system, a power amplifier which provides magnetic coil with exciting current to produce electromagnetic force is one of the main components. General commercialization of power amplifiers can not meet the application requirements, but should be designed.Power amplifiers are divided into linear amplifier and switching amplifier. Switching power amplifier is popularly used because of its virtues of low power losses, high efficiency and high frequency bandwidth. By comparing with various modulate techniques used in the switching power amplifiers at present, summed up the advantages and disadvantages of the Sample/Hold Current, Hysteresis Control, PWM and Minimum Pulse Width Modulation. This paper also describes the general design rules of switching power amplifier.The standard current controlled bi-state PWM switching power amplifier has its intrinsic deficiencies, such as significant harmonic distortion and restriction of the dynamic characteristic, which have serious negative effects on the magnetic bearing system. To overcome these drawbacks, the application of current controlled tri-state PWM technology to the switching power amplifier is proposed, which can provide better dynamic response combined with lower harmonic distortion. This paper introduces the current controlled tri-state PWM technology and the formula of the current ripple. Theoretical analysis showed that the tri-state PWM technology can reduce effectively the current ripple, improve the current response speed of the switching amplifier and then improve the performance of overall magnetic bearing system. The simulation results show the superiority of current controlled tri-state PWM technology.After introducing the composition and characteristics of the FPGA which produces PWM pulse, a new tri-state PWM power amplifier based on FPGA is designed. And the way which select to the main components of the isolation circuit, driving circuit, the current detection circuit and the half-bridge circuit in power amplifier is given. Some characteristics are simulated and tested, which include the static input-output, current ripple, dynamic step-response, frequency response of the power amplifier. Experimental results indicate that the current-controlled tri-state PWM switching power amplifier based on FPGA has good static and dynamics characteristics, which well meet the design requirements of magnetic bearings.The tri-state PWM switching power amplifier based on FPGA has simple structure, high efficiency, high frequency bandwidth and fast dynamic response in a wide load range, which can get a good control effect in application. |
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