Self-sensing algorithms for active magnetic bearings
Niemann, Andries Christiaan
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Active magnetic bearings (AMBs) have become a key technology in industrial applications with a continued drive for cost reduction and an increase in reliability. AMBs require position feedback to suspend the rotor. Conventional contactless position sensors are used to measure the rotor's position. The major disadvantages of conventional position sensors are their cost and that the sensors are viewed as a weak point in an AMB system. A self-sensing sensor is a type of sensor which is cost effective, reduces sensor wire-length and increases reliability, thus ideal for the industry. This type of sensor relies on the current and voltage signals of the AMB's to obtain the rotor position. Due to the rapid and advanced development of digital electronics, it has become more powerful and cheaper, thus self-sensing in mass production will be cost effective. Different self-sensing approaches were developed in the past and can be divided into two main categories: state estimation and amplitude modulation approaches. In this research the focus will be on the amplitude modulation approach. Amplitude modulation makes use of two signals, namely the modulation signal and the carrier signal. In a self-sensing AMB system the carrier can be a high frequency component injected into the system or the switching ripple of the switch mode power amplifier can be used. The modulation signal is the change in rotor position which results in changing inductances. The actuator material introduces nonlinear effects on the estimated position. Due to these nonlinear effects, it is rather difficult to obtain the rotor position. The first industrial application of a self-sensing turbomolecular pump system was implemented in 2005 by S2M. The aim of this thesis is to evaluate existing self-sensing schemes, devise improvements and investigate possible new schemes. Four different demodulation methods and two new self-sensing schemes are evaluated. An AMB transient simulation model which includes saturation, hysteresis, eddy currents and cross-coupling is used to evaluate the schemes in simulation. The self-sensing schemes are implemented in hardware and evaluated on a 7 A rms 500 N AMB. A comparative study was done on the different self-sensing schemes. From the comparative study it was determined that the gain- and phase effects have a direct effect on the sensitivity of the system. It was also proved that self-sensing can be implemented on a coupled AMB with a sensitivity of 10.3 dB.
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