High performance capacitive position sensing device for compact active magnetic bearing spindles

High precision rotation of compact spindles that are contact-free suspended by active magnetic bearings, require high performance position sensors placed in suitable locations around the spindle. Industrial available position sensing devices either use large sensing probes, or are too expensive and are often already too application specific to be adapted. Therefore, an alternative position sensing device based on the electrostatic sensing principle measuring mutual capacitance is studied in this paper. For getting insight into the general tendency of mutual capacitance, an overview is elaborated by finite element modeling, mathematical derivations and experiments. It shows that there are three different types of spindle surface materials, each influencing the mutual capacitance in a different way. In addition, an optimized sensor pattern has been manufactured using a standard, low cost printed circuit board technology. First experimental results demonstrate a position measurement with a 40 [nm] resolution and a signal-to-noise ratio of 74 [dB].