An integrated controller for an active magnetic bearing system
Van Rensburg, Jacques Jansen
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An active magnetic bearing (AMB) involves the suspension of a rotor by means of electromagnetic actuators. An AMB system comprises actuators, position sensors, a controller and power amplifiers. Conventional position sensors for rotor position sensing are expensive. The McTronX Research Group at the North-West University is currently conducting research on self-sensing, a method of extracting rotor position from the current and voltage actuation signals of the AMB. It was deemed necessary to develop a complete platform that will enable the implementation of self-sensing algorithms. The platform incorporates analogue circuitry, embedded devices and switching power amplifiers. The system will thus integrate all the electronics required for AMB suspension, and is thus dubbed: an Integrated Controller. The development of the integrated controller presented unique design challenges not yet faced by the McTronX group. To increase the chances of success and minimize some of the risks, the integrated controller was developed in collaboration with an industry partner. The collaboration required a systems engineering approach to the project. A detail type B specification was drawn up to aid in the project management process. The specification includes important detail such as the functional architectures, functional capabilities, performance specifications and physical constraints of the integrated controller. Some self-sensing algorithms can be implemented by means of analogue circuitry, and some can only be implemented in the digital domain. These digital algorithms are computationally intensive and require powerful processors. Digital signal processors (DSPs) and field programmable gate arrays (FPGAs) both exhibit unique architectures ideal for digital signal processing. Both these devices were implemented in the integrated controller, with the DSP as the main processor and the FPGA the co-processor. Although the integrated controller can operate as a stand-alone system, it has several communication interfaces (RS485, USB and RS232). This allows the system to work in conjunction with other integrated controllers, to be controlled by a master controller or to communicate to a personal computer (PC). Testing the integrated controller involved the closed loop current control of an AMB power amplifier, facilitating all critical functional aspects. The integrated controller's performance meets the required specifications and is expected to successfully facilitate future self-sensing investigations.
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