An apparatus includes a drum to draw in or let out a line and a motor and transmission coupled to the drum to apply a torque thereto. A tracking module tracks an actual amount of line let out from the drum. A servo control unit receives the actual amount, compares the actual amount to a desired amount of line to let out from the drum, and generates an error signal reflecting a difference between the actual amount and the desired amount. A modulation module generates, from the error signal, a control signal to control the motor, thereby bringing the actual amount into better alignment with the desired amount.