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INTRODUCTION TO ROBOTICS Part 3: Propulsion System Robotics and Automation Copyright © Texas Education Agency, 2012. All rights reserved. 1.

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Presentation on theme: "INTRODUCTION TO ROBOTICS Part 3: Propulsion System Robotics and Automation Copyright © Texas Education Agency, 2012. All rights reserved. 1."— Presentation transcript:

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

2 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 Copyright © Texas Education Agency, All rights reserved. 2

3 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. We will discuss servos in another section because they are used primarily in arms, actuators, and grippers. Copyright © Texas Education Agency, All rights reserved. 3

4 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. Copyright © Texas Education Agency, All rights reserved. 4

5 More Gear Info Gears use teeth to transmit torque. Teeth must be the same size, even on different size gears. The number of teeth varies for different size gears: A smaller gear has fewer teeth A larger gear has more teeth A big gear driving a small gear increases speed. A small gear driving a big gear increases torque. Copyright © Texas Education Agency, All rights reserved. 5

6 Gear Calculations Copyright © Texas Education Agency, All rights reserved. 6

7 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. Copyright © Texas Education Agency, All rights reserved. 7

8 More Gear Info For further information, there are some great gear video tutorials available on-line. Copyright © Texas Education Agency, All rights reserved. 8

9 Motors A motor converts electrical energy into mechanical energy. The mechanical energy comes from the interaction between two magnetic fields. Magnetic fields produce physical forces: Like poles repel (N – N, S – S) Unlike poles attract These forces make a motor spin. One magnetic field is usually a permanent magnet, the other is an electromagnet. Copyright © Texas Education Agency, All rights reserved. 9

10 Types of Motors Most motors are 2 wire, but some hobby motors are 3 wire because they are modified servos. 2 wire motor may require a motor driver board to provide higher current. 3 wire motors use a servo type RC signal output and are generally low current. Copyright © Texas Education Agency, All rights reserved. 10 Photo Credit: VEX Robotics, Inc.

11 Example Motor Specs Free Speed: 100 rpm Stall Torque: 8.6 in-lbs Stall Current: 2.6A Free Current: 0.18A All motor specifications are at 7.2 volts. Often designed to connect to a specific structural system: Drive shaft connection Mounting connections Note screw connections sizes such as 6-32 or 8-32 Copyright © Texas Education Agency, All rights reserved. 11 Photo Credit: VEX Robotics, Inc.

12 DC Motor Speed A DC motor is a variable speed device. Speed is controlled by the amount of DC voltage applied: Varying the amount of DC voltage is covered under control systems The physical load applied to the motor also affects its speed: A higher load slows it down Copyright © Texas Education Agency, All rights reserved. 12

13 Motor Specs One interesting note is that as the load on a DC motor increases, it will draw more current from the power supply and make the motor rotate more slowly. DC motor speed is inversely related to motor current (but proportional to voltage). The torque a motor provides is always equal to its load. We will discuss this in more detail later. Copyright © Texas Education Agency, All rights reserved. 13

14 Motors turn and spin, so we have to start thinking about rotating motion. Many motor formulas and equations involve rotational units and concepts. Angular velocity is the primary term used in rotational motion. Greek symbols are used for the quantities. Copyright © Texas Education Agency, All rights reserved. Motor Specs 14

15 Angular Velocity Angular velocity has a symbol, ω (omega) The speed that something is rotating In America we use RPM, or rotations per minute Science uses units of radians per second There are 60 seconds per minute and 2π radians per rotation, so: Copyright © Texas Education Agency, All rights reserved. 15

16 Conversion Practice Copyright © Texas Education Agency, All rights reserved. 16

17 DC Motors A motor has several parts: The rotor (the spinning part) Connected to an axle which is also the rotor shaft The stator (stationary part) The frame Supports the permanent magnets The commutator Switches the DC voltage polarity for continuous rotation (polarity has to switch every half rotation) The brushes Gets electricity into the rotor Copyright © Texas Education Agency, All rights reserved. 17

18 Permanent Magnet DC Motor Copyright © Texas Education Agency, All rights reserved. 18

19 Stator Rotor Copyright © Texas Education Agency, All rights reserved. 19

20 Armature Bearings Commutator Copyright © Texas Education Agency, All rights reserved. 20

21 Armature Electromagnetic Field Coil Windings Field Coil Poles Copyright © Texas Education Agency, All rights reserved. 21

22 Permanent Magnet Field Poles Brushes The positive and negative DC voltage on these wires connects to the brushes giving power to the armature field. Copyright © Texas Education Agency, All rights reserved. 22

23 Copyright © Texas Education Agency, All rights reserved. 23

24 Brief Motor Description An increase in motor load requires the motor to draw more current from the power supply. An increase in load slows the motor down, and the decrease in speed decreases something called CEMF, allowing the current to increase, creating higher motor torque which balances the increase in load. Current is directly related to motor torque because torque is produced through the interaction of 2 magnetic fields. Magnetic field strength increases with current. Copyright © Texas Education Agency, All rights reserved. 24

25 Generator Action In order to understand how a motor works, you must understand how a generator works. A generator converts mechanical energy into electrical energy. The mechanical energy comes from an external source called a prime mover. The electrical energy is created from a conductor moving relative to a magnetic field: Relative motion. Copyright © Texas Education Agency, All rights reserved. 25

26 Motor Action in a Generator Current in a conductor creates a magnetic field. The magnetic field created by the induced current always opposes the original field. The interaction of the 2 fields creates a force that opposes the applied mechanical force. This is the load on a generator. More current drawn creates a larger mechanical force which opposes the applied mechanical force from the prime mover. Copyright © Texas Education Agency, All rights reserved. 26

27 Generator Action in a Motor A motor also has induction due to conductors moving in a magnetic field. The induced voltage always opposes the applied voltage from the power supply. The induced voltage in a motor is called CEMF. A larger external load slows the motor down, it produces less CEMF and draws more current from the power supply. Copyright © Texas Education Agency, All rights reserved. 27

28 Motors and Generators Motors convert electrical energy into mechanical energy. Generators convert mechanical energy into electrical energy. All motors are generators and all generators are motors. The load on a generator is the physical force created by the interaction of 2 magnetic fields. The electrical load on a motor is the current which is controlled by the induced voltage due to conductors moving in a magnetic field. Copyright © Texas Education Agency, All rights reserved. 28

29 Motor Characteristic Curves All motor specifications are at 7.2 volts Copyright © Texas Education Agency, All rights reserved. 29

30 Motor Current and Torque Torque (N-cm) Speed (RPM) Current (amps) This line shows that current and torque are directly proportional Copyright © Texas Education Agency, All rights reserved. 30

31 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 Copyright © Texas Education Agency, All rights reserved. 31

32 Motor Characteristic Curves No-Load Speed ω n No-Load Current Copyright © Texas Education Agency, All rights reserved. 32

33 Motor Characteristic Curves Stall Current Stall Torque τ s Copyright © Texas Education Agency, All rights reserved. 33

34 Motor Characteristic Formulas Copyright © Texas Education Agency, All rights reserved. 34

35 The stall torque, τ s, represents the point on the graph at which the torque is a maximum, which is when the shaft is not rotating. No rotation means no CEMF which allows maximum current. The no load speed, ω n, is the maximum output speed of the motor (when no load torque is applied to the output shaft meaning the motor is freely spinning). Copyright © Texas Education Agency, All rights reserved. 35

36 Torque, Current, and Speed Torque is proportional to current. Current is proportional to supplied voltage. The relationship between speed and current is more complex. Speed is inversely proportional to current. Torque generated equals the load applied. As load increases, the motor slows down, current increases, torque generated rises to meet the higher load. Copyright © Texas Education Agency, All rights reserved. 36

37 Motor Formulas From Ohms Law: Where: V S = Supply voltage (from power supply or control circuit) I = Motor Current (Amps) R = Terminal Resistance (Ohms) V e = Back EMF (Volts) (also called counter emf, or CEMF) The back EMF generated by the motor is directly proportional to the angular velocity of the motor. V e = k e · ω Copyright © Texas Education Agency, All rights reserved. 37

38 Formula Application We will only use these simple equations in our calculations. There are actually many other factors that influence motor operation. Fortunately, most of those factors are constant for a given motor and can be accounted for in a single constant of proportionality. See the next slide for an example of some of the factors we will NOT be taking into account Copyright © Texas Education Agency, All rights reserved. 38

39 Copyright © Texas Education Agency, All rights reserved. 39

40 Additional Formulas Copyright © Texas Education Agency, All rights reserved. 40

41 Typical Questions Assume you have a particular motor with specs for that motor. What is the constant of proportionality, k e, for that motor? Given a load, what is the motor speed? What current does the motor draw? How does motor speed vary with applied voltage? Copyright © Texas Education Agency, All rights reserved. 41

42 Example 1 Use the specs for the motor given on slide 8 Free Speed: 100 rpm Stall Torque: 8.6 in-lbs Stall Current: 2.6A Free Current: 0.18A All motor specifications are at 7.2 volts Calculate k e, speed, and current for this motor given motor load equals 3 in-lbs Copyright © Texas Education Agency, All rights reserved. 42

43 Example 1 Solve for R, V e = V S – IR = 7.2 V – (.18 A · 2.77 Ω) = = 6.7 V Copyright © Texas Education Agency, All rights reserved. 43

44 Example 1 continued Calculate motor speed from load To calculate current, you need V e = 65.1 RPM V e = k e · ω = · 65.1 = 4.36 V Copyright © Texas Education Agency, All rights reserved. 44

45 Efficiency Motor efficiency is power out divided by power in. Power out is mechanical energy, P = τ · ω Power in is electrical energy, P = V · I Copyright © Texas Education Agency, All rights reserved. 45

46 Robot Linear Speed Assume the motor is coupled directly to a wheel. The formula is slightly different when using American units vs. metric units. American Units: Ѵ = ω · C Metric Units: Ѵ = ω · r Units for ω in RPM, C is circumference of the wheel = 2 π r Units for ω in radians per second, r is radius of the wheel Copyright © Texas Education Agency, All rights reserved. 46

47 Calculating Linear Speed Copyright © Texas Education Agency, All rights reserved. 47

48 Optimal Robot Speed Copyright © Texas Education Agency, All rights reserved. 48

49 Robot Weight The example value of 3 in-lb load on the motor is due to the weight of the robot and the radius of the wheel used. The weight of the robot can be calculated: Assuming 2 drive wheels, the actual example robot weight equals 4.4 lb Copyright © Texas Education Agency, All rights reserved. 49

50 Additional Examples The following slides show motor specifications for the actual motors used in the BEST robotic contest. Use these specifications for example problems using real world examples. Graphs are included for visual clarity, but students can be expected to create these graphs themselves from information given. Copyright © Texas Education Agency, All rights reserved. 50

51 Motor 2 specs Free Speed: 43 RPM Stall Torque: 24 in-lbs Stall Current: 3.34 amps Free Current: 0.32 amps All motor specifications are at 7.2 volts Copyright © Texas Education Agency, All rights reserved. 51

52 Example Motor 2 Torque (N-cm) Speed (RPM) Current (amps) Copyright © Texas Education Agency, All rights reserved. 52

53 Motor 3 specs Free Speed: 90 RPM Stall Torque: 8.9 in-lbs Stall Current: 2.39 amps Free Current: 0.21 amps All motor specifications are at 7.2 volts Copyright © Texas Education Agency, All rights reserved. 53

54 Example Motor 3 Torque (N-cm) Speed (RPM) Current (amps) Copyright © Texas Education Agency, All rights reserved. 54


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