Speed control of a DC motor using a feedforward computed torque control scheme

This paper presents a feedforward control scheme using an inverse dynamic model of a DC motor for speed control in the presence of changes in dynamics during motion. Conventional velocity control schemes perform well for nominal operating conditions of the motor as the gains are tuned for such operating conditions. Some of the operating conditions considered are the changes in damping torque as speed changes, the vertical load which results in frictional load when the motor is used to drive a wheeled mobile robot or automated guided vehicles, and gravity loading and centrifugal effects when the motor is used as an actuator for robot joints. The gains and the acceleration values of conventional velocity control schemes need to be retuned in order to achieve a satisfactory performance under such circumstances. Further, the amplifiers used for conventional velocity control schemes operate in voltage mode and the current output of the amplifiers is not limited. The feedforward control scheme proposed in this paper predicts the current requirement based on the changes in the dynamics of the motor and the operating environment and limits the current drawn by the motor instead of simply adjusting the voltage. The amplifier provides as much voltage needed to achieve the desired speed while the current drawn by the motor is limited to the value allowed by the inverse dynamic model. Experimental results indicate that the proposed feedforward control scheme exhibits good speed control compared to the conventional velocity control scheme.