INTRODUCTION TO ROBOTICS Part 3: Propulsion System Robotics and Automation Copyright © Texas Education Agency, All rights reserved. 1

Robot Systems Structural System Physical system that provides support and stability Propulsion System (motion) Drive system includes motors, wheels, and gears Control System Microcontroller, operating program, electrical power, and joystick Tool and Actuator system Arms, grippers, manipulators Sensor and Feedback system Perception, transducers 2

Propulsion System Components Also called the motion system. The most important propulsion system components are gears and motors. All examples shown are for permanent magnet type DC motors. Copyright © Texas Education Agency, All rights reserved. 3

Motors A motor converts electrical energy into mechanical energy. Video: How DC motors workHow DC motors work 4

DC Motors DC motors are 2 wire. The higher the voltage, the faster they spin. 5

6 A DC motor has several parts

Rotor 7 The Rotor (the spinning part) Connected to an axle which is also the rotor shaft

Stator 8 The Stator (stationary part) The frame Supports the permanent magnets

Bearings Commutator 9 The Commutator Switches the DC voltage polarity for continuous rotation (polarity has to switch every half rotation) The Bearings Supports the spinning rotor and keeps magnets from touching each other.

Armature Electromagnetic Field Coil Windings Field Coil Poles 10

Permanent Magnet Field Poles Brushes 11 The Brushes Gets electricity into the rotor

12 DC Motor

DC Motor Speed A DC motor is a variable speed device. Speed is controlled by the amount of DC voltage applied: As the voltage increases, the speed increases As the voltage decreases, the speed decreases 13

Motor Specs The physical load applied to the motor also affects its speed: A higher load slows it down The torque a motor provides is always equal to its load. 14

Stall Torque Stall Torque is the torque load that causes the output rotational speed of a device to become zero (stops spinning). 15

Motor Characteristic Curves All motor specifications are at 7.2 volts 16

Motor Current and Torque Torque (N-cm) Speed (RPM) Current (amps) This line shows that current and torque are directly proportional 17

Motor Speed and Torque Torque (N-cm) Speed (RPM) Current (amps) This line shows that current and speed are inversely proportional, meaning that as torque goes up, speed goes down 18

Motor Characteristic Curves No-Load Speed ω n No-Load Current 19

Motor Characteristic Curves Stall Current Stall Torque τ s 20

Servo Motors Have 3 wires Rotates 180° (+90° to -90°) 21

Servo Motors The width of the pulse on the signal in line determines the rotor location. 22

Stepper Motors Stepper motors are 4 or more wires. They are a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any feedback sensor. 23

Stepper Motors Each wire is connected to a coil. As each coil is energized individually, the rotor turns. 24

Gears Gears are used for several things: To increase the speed of rotation To increase the torque, or the rotating force applied to a load To change the direction of a torque Gears trade one for the other: If you use gears to increase speed, torque will decrease. If you use gears to increase torque, speed will decrease. 25

Gears A big gear driving a small gear increases speed. A small gear driving a big gear increases torque. 26 Driving Gear Speed Increases Here Driving Gear Torque Increases Here Speed decreases Torque decreases

Optimal Gear Ratio There is an optimal gear ratio. The optimal gear ratio puts load torque on the motor at exactly half stall torque. Load torque on the motor is due to: The weight of the robot The number of drive wheels (motors used) The diameter of the drive wheels This gear ratio will maximize robot speed and motor efficiency. 27