The Bearingless Rotary-Linear Reluctance Motor (BRLRM) – Position Measurement and Control in 6 Degrees of Freedom

Abstract:
The novel bearingless rotary-linear reluctance motor (BRLRM) enables fully active magnetic bearing as well as rotation and translation of a ferromagnetic rotor. This is realised with a very compact double stator structure and a total of only 12 phases. The main challenges are the position measurement and control in all 6 degrees of freedom. In order to avoid measuring tracks in the rotor, the position measurement is carried out by means of a capacitive sensor array, which utilises the rotor toothing. For the radial axes, a differential signal evaluation is used which is immune to changes in the rotor ground capacitance and voltage. The position control in all 6 degrees of freedom is done by using the pseudoinverse of the flux square force transfer matrix and iterative addition of those flux square vectors that do not generate any forces. In the static case, this method yields solutions that differ only slightly from the optimal solution, while massively reducing the computational and memory efforts. The currents are regulated by analogue amplifiers in order to reduce electromagnetic interference in the experimental setup. The controllability and stability of the entire system were verified for virtually arbitrary tangential positions. The measured position errors are adequate. For tangential setpoint position jumps, the system also shows adequate dynamic behaviour and position errors and is also robust to radial position changes. Due to the non-ideal properties of the current controllers and the measuring system, the different axes of motion show a moderate residual mutual influence.