Measurement method and a circuit arrangement of the natural vibration characteristics of a rotating object

摘要

A method and system for measuring the dynamical behavior of an elongate body which has a floating electrical potential and only forms a total capacitance with its environment and which has a longitudinal axis about which the body rotates, which method includes a contact-free capacitive measurement of changes in the position of the body transverse to the longitudinal axis by means of at least two measuring probes each disposed to form with the body a respective capacitor having a capacitance value corresponding to the distance between the probe and the body, the measurement being performed by: applying to each measuring probe a respective alternating current signal having a frequency selected such that the frequency of the signal applied to each probe differs from that applied to each other probe by at least a selected frequency difference value; deriving from each probe an output signal having an amplitude representative of the capacitance value of the capacitor formed by that probe with the body; and passing the output signal from each probe through a respective signal processing circuit having a lowpass characteristic with an upper limit frequency value which is less than the selected frequency difference value, which the signal processing circuits associated with all of the probes having the same upper limit frequency value.