A surgical instrument (10), comprising a housing (110)； an elongated shaft (140) extending distally from the housing (110) and defining a first longitudinal axis；a firing rod (220) disposed at least partially within the elongated shaft (140)； a drive mechanism (200,210) disposed at least partially within the housing, the drive mechanism (200,210) being in mechanical cooperation with the firing rod (220)； a motion sensor (251) that senses changes in an electric field between the firing rod (220) and the elongated shaft (140)； a signal generator that generates a pulse signal including a plurality of pulses, wherein widths of the plurality of pulses vary with the changes in the electric field between the firing rod (220) and the elongated shaft (140)； a measurement unit (470) that measures the widths of the plurality of pulses； and a data processor (474) that determines a parameter of motion of the firing rod (220) based on the measured widths of the plurality of pulses.
