Patent ID: 7554280

Claim:
A monitoring device for monitoring a position of a rotor of an electric motor, said monitoring device comprising: an input that receives a signal representing actual zero crossings of a back-electromotive force of said electric motor; detecting means for detecting said signal in first periods of time arranged around instants of time in which the zero crossings of the back-electromotive force are expected; setting means for setting second periods of time that are less than said first periods, said second periods of time being centered on the instants of time in which the zero crossings of the back-electromotive force are expected; checking means for checking whether each actual zero crossing of the back-electromotive force occurs inside a respective one of said second periods of time; modifying means for modifying by a quantity a subsequent period of electric revolution time between two consecutive expected instants of zero crossing of the back-electromotive force if one of said actual zero crossings of the back-electromotive force occurs outside the respective one of said second periods of time.