Single board computer based control of an active magnetic bearing
The McTronX Research group at the North-West University is currently conducting research in the use of active magnetic bearings (AMBs) for a flywheel energy storage system (FESS). Together with this, the aim of this project is to take the level of control of such AMB systems to an industrial level. Instead of using a rapid prototype dSPACE® controller, a single board computer (SBC) is proposed. Issues to be addressed, includes: SBC overview, control system specifications, skills development for SBC, implementation and evaluation. All the digital and analogue input/output signal requirements for the FESS are determined prior to specifying an SBC. Six different SBCs were compared and in the end the SBC6713ell from Innovative Integration (II) was chosen and sourced. The SBC6713ell complies with all the requirements specified by the client. Two interface boards were used to integrate the SBC with the FESS. The first board contained all the protection circuitry to protect the controller from faults that could occur on the sensor and actuator side of the FESS and is used to connect the dSPACE® to the FESS without the SBC. After the hardware was integrated, the software/firmware integration started. On the SBC, the PD control was implemented for the AMBs as well as the voltage over frequency control for the PMSM. A graphical user interface (GUI) was further developed on a host computer to monitor the FESS. Four tests were done on the integration of the SBC with the FESS. Firstly the performance of the controller with regard to the control algorithms was tested. The stability and sensitivity analyses of the system followed and ended with the PMSM start-up test. The control algorithm execution time was longer than expected and adjustments to the sampling time had to be made. Stability tests showed a decrease in bearing stiffness and damping, which was due to low pass filters on the analogue to digital converter board. The sensitivity of the system also increased due to the effect of the filters on the system. The inconsistency in bearing damping and stiffness, obtained from the stability tests was verified by adding the filters to the simulation. These filters caused an attenuation of less than 1 dB, but resulted in a phase shift of -36.3° in the control loop. Industrial control was realised using an SBC, but further work is still necessary. The areas identified for future work is: non linear control algorithms, low noise digital power amplifiers, speed sensor and the PMSM drive.
- Engineering