The dc motor is a commonly used electrical actuator. It converts direct current (dc) electrical energy into mechanical energy. The principle of operation is that when a conductor carrying current is placed in a magnetic field, a force is generated.
A classic problem in robotics is the case of a robotic hand gripping and turning a doorknob to open a door. The mechanism is schematically. Suppose that the actuator of the robotic hand is an armature-controlled dc motor. Note that the field circuit provides a constant magnetic field to the motor, and is not important in the present problem. The armature (motor rotor winding) circuit has a back emf. ub, a leakage inductance La, and a resistance Ra. The input signal to the robotic hand is the armature voltage ua (t) as shown.