A surgical instrument, comprising: a shaft; an end effector extending distally from said shaft; a firing system, comprising: an electric motor; a first jaw (10410, 10420); a second jaw (10410, 10420) movable relative to said first jaw; and a tissue cutting knife movable from a proximal position toward a distal position by said electric motor during a firing stroke to cut patient tissue captured between said first jaw and said second jaw; and a control system, comprising: a control circuit comprising a pulse width modulation circuit in communication with said electric motor, wherein said pulse width modulation circuit is configured to apply voltage pulses to said electric motor, wherein the duration of said voltage pulses controls the speed of said tissue cutting knife, characterised wherein: said control circuit is configured to evaluate the duty cycle of said electric motor by summing the total pulse duration of said voltage pulses, summing the total gap duration between said voltage pulses, and dividing said total pulse duration by the sum of said total pulse duration and said total gap duration to calculate said duty cycle; and the control system further comprises a speed sensor configured to detect the speed of said tissue cutting knife, wherein said speed sensor is in communication with said control circuit, and wherein said control circuit is configured to decrease the speed of said tissue cutting knife when said duty cycle is above a predetermined threshold or increase the speed of said tissue cutting knife when said duty cycle is below a predetermined threshold. a shaft; an end effector extending distally from said shaft; a firing system, comprising: an electric motor; a first jaw (10410, 10420); a second jaw (10410, 10420) movable relative to said first jaw; and a tissue cutting knife movable from a proximal position toward a distal position by said electric motor during a firing stroke to cut patient tissue captured between said first jaw and said second jaw; and an electric motor; a first jaw (10410, 10420); a second jaw (10410, 10420) movable relative to said first jaw; and a tissue cutting knife movable from a proximal position toward a distal position by said electric motor during a firing stroke to cut patient tissue captured between said first jaw and said second jaw; and a control system, comprising: a control circuit comprising a pulse width modulation circuit in communication with said electric motor, wherein said pulse width modulation circuit is configured to apply voltage pulses to said electric motor, wherein the duration of said voltage pulses controls the speed of said tissue cutting knife, characterised wherein: said control circuit is configured to evaluate the duty cycle of said electric motor by summing the total pulse duration of said voltage pulses, summing the total gap duration between said voltage pulses, and dividing said total pulse duration by the sum of said total pulse duration and said total gap duration to calculate said duty cycle; and the control system further comprises a speed sensor configured to detect the speed of said tissue cutting knife, wherein said speed sensor is in communication with said control circuit, and wherein said control circuit is configured to decrease the speed of said tissue cutting knife when said duty cycle is above a predetermined threshold or increase the speed of said tissue cutting knife when said duty cycle is below a predetermined threshold. said control circuit is configured to evaluate the duty cycle of said electric motor by summing the total pulse duration of said voltage pulses, summing the total gap duration between said voltage pulses, and dividing said total pulse duration by the sum of said total pulse duration and said total gap duration to calculate said duty cycle; and the control system further comprises a speed sensor configured to detect the speed of said tissue cutting knife, wherein said speed sensor is in communication with said control circuit, and wherein said control circuit is configured to decrease the speed of said tissue cutting knife when said duty cycle is above a predetermined threshold or increase the speed of said tissue cutting knife when said duty cycle is below a predetermined threshold.