Method for contactlessly determining the position of a driven rotor of an electric motor, electric motor, and pipetting system for aspirating and dispensing pipetting liquid having such an electric motor

An electric motor with contactless position determination, comprising:

• a driven moving portion having a plurality of permanent magnets which generate a moving-portion magnetic field having a plurality of periodically spaced apart maxima;
• a stator, with said driven moving portion being arranged so as to be movable with respect the stator;
• a plurality of magnetic field sensors which are arranged along a movement path of the moving portion for measuring the magnetic field present along the movement path;
• and
• a position determination unit which receives measured data from the plurality of magnetic field sensors and is configured to:
• provide a plurality of measured values from the measured data, the plurality of measured values being sampling points for a momentary magnetic field that is generated by the plurality of permanent magnets and dependent on the position of the moving portion,
• select a subset of the plurality of measured values, wherein the subset of the plurality of measured values is taken from magnetic field sensors arranged adjacent each other,
• determine a specific spectral signal component from the subset of the plurality of measured values, said specific spectral signal component having the spatial frequency corresponding to the distance between adjacent like maxima of the moving-portion magnetic field, and
• determine the position of the driven moving portion on the basis of the specific spectral signal component by means of the phase angle of the specific spectral signal component, wherein the position determination unit is configured to convert the phase angle of the specific spectral signal component into an offset of the driven moving portion with respect to a known position.