# Complementary Cooperation of Dual Power Circuits to Drive Active Magnetic Bearings
## 1 Introduction  
Active magnetic bearings (AMBs) are able to levitate the interested objects in a fashion of no friction and no mechanical wear. Therefore, they are increasingly applied to the industries for which high speed and high precision are both required, such as high-speed compressors, turbo-machineries, high-precision machine tools, and flywheel energy storage systems. One of the most important components of a superior AMB unit is its drive circuit. Most of the drive circuits applied to AMBs are of analog type [1, 2]. Analog drive circuits possess the merits of high linearity and low electromagnetic interference (EMI). However, the energy efficiency of analog drive circuits is inherently poor because a great percentage of the energy power is converted into heat at the corresponding driver integrated circuits (ICs). By contrast, the power loss is relatively limited for the digital drive circuits. As a consequence, a large number of studies on digital drive circuits for AMBs were proposed during the past decades [3, 4, 5, 6, 7, 8, 9]. The main shortcoming of digital drive circuits to AMBs is: severe current ripple induced. The most popular technique for current ripple reduction is the so called three-level pulse width modulation (PWM) scheme [8, 9]. Nevertheless, the current ripple problem by digital drive circuits is still more serious than that by analog drive circuits.  
Due to analog drive circuits and digital drive circuits having their own irreplaceable advantages, many types of switch-linear hybrid (SLH) power amplifiers were proposed recently [10, 11, 12, 13, 14]. The SLH amplifier is mainly composed by a set of linear power amplifier and a set of two-level switching power supply. However, the switching frequency of the power supply has to be much higher than the bandwidth of the linear power amplifier. As well known, high switching frequency results in seriousswitching loss [11]. Therefore, a multilevel voltage supply, namely a multilevel converter, is proposed to replace the two-level switching power supply to improve the overall energy efficiency [11; 12; 13; 14]. Unfortunately, \"multilevel\" implies it requires a large number of electronic components. The reliability of the overall drive circuit would be accordingly reduced. Besides, the more complex the circuit, the larger the impact of time delay which certainly downgrades the quality of rotor dynamic control. To overcome the aforementioned shortcomings, a compact-design hybrid power amplifier for AMB units, named as dual cooperative drive circuit (DC\({}^{2}\)), is proposed by this paper. The proposed DC\({}^{2}\) is able to conduct under dual operation modes: digital driving mode (DDM) and analog driving mode (ADM). Taking advantage of the complementary characteristics of interchange between DDM and ADM, the proposed DC\({}^{2}\) manifests its superiorities on structure compactness, high energy efficiency, fast response, low tracking error and fairly mild current ripples. This work is absolutely aimed at drive circuit design. Regarding AMB position control on high-speed rotors, please refer to the previous works of the authors [15; 16; 17].