DC Motors are widely used in robotics because

DC Motors are widely used in robotics because of their small size and high energy output. Key characteristics of DC motors include: 1. High Speed 2. Low Torque 3. Reversibility Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

DC Motor - How it Works • A DC Motor has 4 major components: – A permanent magnet that doesn’t move, called the stator. – An electromagnet (usually wound bare wire) – A frame on which the electromagnet is wound, called the armature – A set of brushes for transferring voltage to the armature Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

DC Motor - How it Works • If a loop of wire were placed within a magnetic field, and current were applied to it, a magnetic field would be induced in the loop and it would try to rotate until both magnetic fields lined up. • As the loop of wire rotates, its connection with the brushes is broken, but because of momentum, the loop will continue to rotate until, after 1800, the other sides of the loop are in contact with the brushes. • As long as current is applied to the brushes, the loop of wire will continue to rotate again and the rotor will continue rotation. Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

DC Motor - Label Permanent Magnet Armature (Rotor) Electromagnet (Wound Wire) Brushes Voltage Source Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

Power, Torque and Speed • The power of a DC Motor is proportional to the product of its torque and its speed. • As the speed of the motor decreases, the torque increases proportionally until maximum torque is achieved. At this point, the motor is stalled, meaning that the motor is not turning even though power is being supplied to it. This is known, appropriately, as the “Stall Torque. ” Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

Power, Torque and Speed • The DC Motor has two “End States”- the Unloaded Speed (maximum speed, no torque) and Stalled Speed (zero speed, maximum torque). • Current ratings of DC Motors are given at the stall torque, since this is the point of maximum current. Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

Power, Torque, and Speed • Maximum power is achieved at a point between Stall and Unloaded Speed, where the speed and torque curves intersect. • The speed of the DC Motor is directly proportional to the applied voltage. Increase the voltage for higher speeds and decrease the voltage for lower speeds. • (INSERT SPEED & TORQUE CURVES) Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

Direction of Rotation • Direction of rotation depends upon the polarity of the applied voltage. • To change the direction of a DC Motor, simply change the polarity of the applied voltage. • (INSERT VEX PICTURES WITH LEADS REVERSED) Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

Output Gears • DC Motors run at very high speed with very low torque – too low to be able to do any useful work directly. • If you were to connect a DC Motor directly to your robot, you wouldn’t have enough torque to overcome starting friction and your robot wouldn’t move. Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

Output Gears • To make a DC Motor do usable work, the speed needs to be reduced and the torque needs to be increased. Speed can be reduced by decreasing the input voltage, but this also decreases the power from the motor. • The solution to this problem is to use a train of gears. If you put a small gear on the shaft of the motor to drive a larger gear, the larger gear will rotate slower and, because it has a larger diameter, it will have a larger torque. Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

Output Gears • Even though this train of gears has a very high gear ratio (the ratio of the number of teeth on the driven gear to the number of teeth on the driving gear), it is called a Gear Reducer. – (NEED A PICTURE OF A GEAR REDUCER) Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.

Pulse Width Modulation (PWM) • Pulse Width Modulation is used when the amount of power delivered to the DC Motor is to be reduced without decreasing the input voltage. • Full power to the Motor is switched on and off rapidly, usually 60 times per second. • To decrease the power output to 50%, the motor has full voltage to it 50% of the time and zero voltage to it 50% of the time for each cycle. • (INSERT GRAPH) Vex 1. 0 © 2005 Carnegie Mellon Robotics Academy Inc.