Calculating Electrical Requirements for Direct Current Electric Actuators

Servo control systems require accurate control of motion parameters such as acceleration, velocity, and position. This requires a controller that can apply current (torque) to accelerate a motor in a given direction, as well as provide an opposing current to decelerate it. When this application of aiding and opposing torque can be carried out in both directions, it is referred to as four quadrant motor control (Figure 1).

In four quadrant electric actuation systems, energy changes its form from electrical current flow to mechanical motion and vice versa. This conversion of energy is performed by an electric motor. An electric motor can be modeled electrically as a resistor, an inductor, and a voltage source. The resistor represents the resistance of the windings and internal wiring. The inductance is created from the turns of the wire that make up the windings. The voltage source is a result of the back electromotive force (EMF) created by the rotation of the motor shaft. When an electric motor shaft rotates, it produces an opposing voltage proportional to the motor's angular velocity. When the applied voltage exceeds the back EMF voltage, motoring occurs. When the back EMF voltage is greater than the applied voltage, braking occurs and the motor generates energy. In steady state, the difference between the applied voltage and the motor's back EMF, divided by the circuit's resistance, gives the current flowing in the motor windings. A motor's current is directly proportional to its mechanical output torque.