Autodesk's VEX Robotics Curriculum. Unit 10: Drivetrain Design 2

Transcription

1 Autodesk's VEX Robotics Curriculum Unit 10: Drivetrain Design 2 1

2 Overview In Unit 10: Drivetrain Design 2, you design your own drivetrain, building on knowledge and skills from previous units. You also calculate, document, and communicate both theoretical and measured speeds for your drivetrain. The concept of a drive train has a variety of real-world applications. In STEM Connections, you are presented with a scenario involving the design of a pitching machine for baseball and softball. After completing the Think Phase and Build Phase in Unit 10: Drivetrain Design 2, you will see how a drivetrain comes into play in the real world. Unit Objectives After completing Unit 10: Drivetrain Design 2, you will be able to: Determine how fast a wheel is rolling based on its rotational speed, and calculate the load on a motor based on wheel traction. Build a gearbox with bevel gears using Autodesk Inventor Professional software. Apply your VEX expertise gained from prior units to design and build your own drivetrain. Calculate theoretical speed of a given drivetrain and explain the differences between the theoretical and measured speeds of a drivetrain. Prerequisites and Resources Related resources for Unit 10: Drivetrain Design 2, are: 2 Unit 1: Introduction to VEX and Robotics Unit 2: Introduction to Autodesk Inventor Unit 4: Microcontroller and Transmitter Overview Unit 5: Speed, Power, Torque, and DC Motors Unit 6: Gears, Chains, and Sprockets Unit 7: Advanced Gears Unit 8: Friction and Traction Unit 9: Drivetrain Design 1 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

3 Key Terms and Definitions The following key terms are used in this phase. Term Definition Bevel Gear Bevel gears are gears that have the axes of the two shafts intersect and the toothbearing faces of the gears themselves are conically shaped. Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. The pitch surface of bevel gears is a cone. Diametral Pitch The number of teeth of a gear per inch of its pitch diameter. A toothed gear must have an integral number of teeth. The circular pitch, therefore, equals the pitch circumference divided by the number of teeth. The diametral pitch is, by definition, the number of teeth divided by the pitch diameter. Functional Design Designers use functional design to analyze the function of their products and the design problems they are trying to solve, rather than spending time on the modeling operations necessary to create 3D representations. Gearbox The simplest form of a transmission. Uses multiple gears to alter motor speed to achieve desired output. Geartrain (Powertrain) Represents the part of the Drivetrain which transmits power from the Motor to the ground. Motor Load Gearing Determining drivetrain gearing based on maximum motor load. Transmission Mechanism by which power is transmitted from a power source to the wheels of a robot or other vehicle. Wheel Speed How fast the robot wheel is spinning. Used to calculate how fast the robot moves across the floor. Overview 3

4 Required Supplies and Software The following supplies and software are used in Unit 10: Drivetrain Design 2: Supplies Software VEX Classroom Lab Kit Autodesk Inventor Professional 2011 Have an assembled drivetrain from Unit 10 Build Phase Notebook and pen Work surface Small storage container for loose parts 4 x25 of open floor space Masking tape Measuring tape One stopwatch One calculator 4 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

5 Academic Standards The following national academic standards are supported in Unit 10: Drivetrain Design 2: Phase Standard Think Science (NSES) Unifying Concepts and Processes: Form and Function; Change, Constancy, and Measurement Physical Science: Motions and Forces Science and Technology: Abilities of Technological Design Technology (ITEA) 5.8: The Attributes of Design Mathematics (NCTM) Algebra: Analyze change in various contexts. Measurement: Understand measurable attributes of objects and the units, systems, and processes of measurement. Measurement: Apply appropriate techniques, tools, and formulas to determine measurements. Communication: Communicate mathematical thinking coherently and clearly to peers, teachers, and others. Connections: Recognize and apply mathematics in contexts outside of mathematics. Create Science (NSES) Unifying Concepts and Processes: Form and Function Physical Science: Motions and Forces Science and Technology: Abilities of Technological Design Technology (ITEA) 5.8: The Attributes of Design 5.9: Engineering Design 6.12: Use and Maintain Technological Products and Systems Mathematics (NCTM) Numbers and Operations: Understand numbers, ways of representing numbers, relationships among numbers, and number systems. Algebra Standard: Understand patterns, relations, and functions. Geometry Standard: Use visualization, spatial reasoning, and geometric modeling to solve problems. Measurement Standard: Understand measurable attributes of objects and the units, systems, and processes of measurement. Overview 5

6 Phase Standard Build Science (NSES) Unifying Concepts and Processes: Form and Function; Change, Constancy, and Measurement Physical Science: Motions and Forces Science and Technology: Abilities of Technological Design Technology (ITEA) 5.8: The Attributes of Design 5.9: Engineering Design 6.11: Apply the Design Process Mathematics (NCTM) Numbers and Operations: Compute fluently and make reasonable estimates Algebra: Analyze change in various contexts. Geometry: Use vizualization, spatial reasoning, and geometric modeling to solve problems. Measurement: Understand measurable attributes of objects and the units, systems, and processes of measurement. Measurement: Apply appropriate techniques, tools, and formulas to determine measurements. Connections: Recognize and apply mathematics in contexts outside of mathematics. Amaze Science (NSES) Unifying Concepts and Processes: Form and Function; Change, Constancy, and Measurement Physical Science: Motions and Forces Science and Technology: Abilities of Technological Design Technology (ITEA) 5.8: The Attributes of Design 5.9: Engineering Design 6.11: Apply the Design Process Mathematics (NCTM) Numbers and Operations: Compute fluently and make reasonable estimates. Algebra: Analyze change in various contexts. Geometry: Use vizualization, spatial reasoning, and geometric modeling to solve problems. Measurement: Understand measurable attributes of objects and the units, systems, and processes of measurement. Measurement: Apply appropriate techniques, tools, and formulas to determine measurements. Communication: Communicate mathematical thinking coherently and clearly to peers, teachers, and others. Connections: Recognize and apply mathematics in contexts outside of mathematics. 6 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

7 Think Phase Overview This phase describes how to calculate the gearing for a robot drivetrain. It focuses on two main methods: calculating based on output speed, and calculating based on motor load. Phase Objectives After completing this phase, you will be able to: Determine how fast a wheel is rolling based on its rotational speed. Calculate the load on a motor based on wheel traction. Prerequisites and Resources Related phase resources are: Unit 5: Speed, Power, Torque, and DC Motors Unit 6: Gears, Chains, and Sprockets Unit 8: Friction and Traction Unit 9: Drivetrain Design 1 A basic understanding of unit analysis Required Supplies and Software The following supplies are used in this phase: Supplies Notebook and pen Work surface Think Phase 7

8 Research and Activity Unit 9: Drivetrain Design 1 discussed the factors that affect what makes a drivetrain turn. This unit describes the geartrain or powertrain of the drivetrain. The geartrain represents the part of the drivetrain that transmits power from the motor to the ground. Wheel Speed The first concept to figure out and understand is how fast the robot moves across the floor based on how fast the wheel is spinning. For each time the wheel makes a full revolution, it rolls forward a distance equal to its circumference. So if you calculate the circumference of the wheel, you know exactly how far the robot goes per revolution. The circumference of a wheel is equal to its diameter multiplied by pi (about 3.14). Once you know the circumference of the wheel, you can calculate how fast it is rolling based on its rotational speed. In the example below, you assume the wheel diameter is 4 inches, and the wheel is spinning at 50 rpm. Calculate how fast the wheel is rolling in inches per second. First, you calculate the circumference: After you calculate the circumference, you can determine the ground speed based on the wheel rpm. Most of the following is just unit analysis and cancellation: Now you know the way to calculate ground speed from wheel rpm. If you know the VEX Motor has an output of approximately 100 rpm, and you know what reduction you want our wheel to spin at, you can calculate the approximate gear reduction needed. Say you have a 5 inch diameter wheel, you want the robot to travel at about 4 feet per second, and you know the VEX motor spins at about 100 rpm. The first step is to calculate the diameter of the wheel, and also convert your target speed to inches per second. 8 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

9 So now you know that you need to move 48 inches in one second, and that each revolution is 15.7 inches; knowing this you can calculate how many revolutions per second the wheel needs to turn to achieve 4 feet per second. Now you know that you need the wheel to spin at revolutions per minute, and you know the VEX motor spins at 100 RPM, you can calculate the Gear Ratio needed to achieve your top speed. To achieve a speed of 4 ft/sec with a 5 wheel and a VEX motor; you need a gear reduction of.545:1. You now need to figure out what gears to use to achieve this gear ratio, or try to get as close as possible if you do not have the correct gears available. The VEX Robotics Design System has several Gearing Options available. The following chart shows all the gear reduction pairs possible using VEX Spur Gears and VEX Chains and Sprockets. Using the above table, you can see the ratios closest to.545. From here it is a question of choosing which option best suits the design. In this case there is no combination that yields exactly the ratio you want, but there are several that are close. You can see how one would follow this same process to calculate the gearing in other situations as well. Think Phase 9

10 Motor Load Gearing The second concept to understand is calculating the maximum load applied to the motor by the drivetrain. This occurs during the wall push situation; that is, when the robot is up against an immovable object and is running full throttle into it. In this situation the wheels should slip on the floor, but the friction between the wheels and the floor will act as a brake on the motor. The first step is determining how many wheels are acting as a brake on the gearbox you are looking at. Only wheels directly linked through gearing or chain apply load to the gearbox. In this example, consider a 4WD robot that has two wheels linked to the same gearbox on each side of the robot. In this case, half the robot weight will be resting on the wheels. This weight is the normal force, which determines the force of friction applied by these two wheels on the gearbox. This torque is applied through the gearbox onto the motor or motors. If a gearbox has multiple motors, then the torque will be divided evenly between them. It is important to design the gearing so that the load applied on each motor is not higher than the motor limit. VEX Motors cannot draw more than 1 amp of current for an extend period of time. Use the principles learned in Unit 5 and Unit 6 to ensure this limit is not exceeded. The two methods listed above are critical for drivetrain design. A good designer will not only gear a robot so that it moves at the desired speed but will also ensure that there is not excessive loading on the motors. 10 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

11 More Information on Transmissions Using different combinations of gear ratios and the principle of mechanical advantage, transmissions provide a torque increase and speed reduction from the high speed and low torque supplied by the driving motor or engine. Transmissions are used in practically every industry that uses machinery. They are sometimes used in stationary applications to move or lift heavy loads, but are more easily recognized and appreciated in vehicles to direct power to make them move. They not only transmit power out of the engine, but more simplistic gearboxes redirect the power out to the wheels. In addition to geared transmissions, heavy construction and agricultural equipment sometimes utilize hydrostatic and electric drives. Hydrostatic drives utilize fluids pumped through hoses to drive components instead of heavy spinning driveshafts and mechanical connections. Electric drives use direct-driven systems in which the motors are actually mounted at the location where the power is needed. Electricity is sent from the power source to the motors through wires, eliminating the need for mechanical power transfer. Gearboxes are the simplest form of a transmission. Transmissions do just what their name implies: they transmit torque and speed. Gearboxes are different from the multispeed or shiftable transmissions most people think of when they hear the term, because their gear ratio is fixed when it is assembled. A gearbox is incapable of shifting, which means that its gear ratio cannot be changed while it is operating. Gearboxes are found in a wide variety of different applications. They were first used in windmills, grain mills, and steam engines. They sometimes supported a 90-degree change in the direction of rotation and occasionally had multiple output shafts to run several different machines at the same time. They were particularly desirable in operations where a large speed reduction and torque increase were needed in lifting loads and pumping liquids. You could say that gearboxes were perfected down on the farm. Low gear ratios provide lots of torque with little speed, while higher gear ratios provide speed with little torque. Think Phase 11

12 Create Phase Overview In this phase, you learn about creating a gearbox with bevel gears. You use the Bevel Gears Generator option of Design Accelerator. The completed exercise Objectives After completing this lesson, you will be able to: Describe options for bevel gear generation. Build a gearbox using bevel gears. Prerequisites and Resources Before starting this phase, you must have: A working knowledge of the Windows operating system. Completed Unit 1: Introduction to VEX and Robotics > Getting Started with Autodesk Inventor. Completed Unit 2: Introduction to Autodesk Inventor > Quick Start for Autodesk Inventor. 12 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

13 Technical Overview The following Autodesk Inventor tools are used in this phase: Icon Name Description Create 2D Sketch A sketch consists of the sketch plane, a coordinate system, 2D curves, and the dimensions and constraints applied to the curves. Bevel Gears Generator Calculates dimensions and strength check of bevel gearing with straight and helical teeth. It contains geometric calculations for designing different types of correction distributions, including a correction with compensation of slips. Circle Creates a circle from a center point and radius, or tangent to three lines. Chamfer Chamfers bevel part edges in both the part and assembly environments. Chamfers may be equal distance from the edge, a specified distance and angle from an edge, or a different distance from the edge for each face. Circular Pattern Part, surface, and assembly features can be arranged in a pattern to represent hole patterns or textures, slots, notches, or other symmetrical arrangements. Extrude Creates a feature by adding depth to a sketched profile. Feature shape is controlled by profile shape, extrusion extent, and taper angle. Unless the extruded feature is a base feature, its relationship to an existing feature is defined by selecting a Boolean operation (join, cut, or intersect with existing feature). Dimension Adds dimensions to a sketch. Dimensions control the size of a part. They can be expressed as numeric constants, as variables in an equation, or in parameter files. Insert ifeature An ifeature is one or more features that can be saved and reused in other designs. You can create an ifeature from any sketched feature that you determine to be useful for other designs. Features dependent on the sketched feature are included in the ifeature. After you create an ifeature and store it in a catalog, you can place it in a part by dragging it from Windows Explorer and dropping it in the part file or by using the Insert ifeature tool. Rib Ribs and webs are often used in molds and castings. In plastic parts, they are commonly used to create rigidity and to prevent warping. Create Phase 13

14 Required Supplies and Software The following software is used in this phase: Software Autodesk Inventor Professional 2011 Gear Generator Options The Gears Component Generator dialog box is displayed after you click the tool to generate gears. Within this dialog box, you enter the method and values required to calculate the gear set. The information varies depending on the method. When you create or edit gear sets, you use the Gears Component Generator dialog box. With the Design Accelerator, you can design spur, bevel, and worm gear sets efficiently. To design and position your gear sets in your assemblies, you need to know what options are available in the dialog box and where they are located. 14 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

15 Bevel Gear Options The following options are available for creating bevel gear sets. Enter data to design the gear set. Input power and speed requirements and review calculation results. Calculations are based on power and speed inputs, and information from the Design tab. Specify information that applies to the entire gear set. Input data specific to the first gear. Input data specific to the second gear. Display a page containing all input data and calculations. Create Phase 15

16 Exercise: Build a Gearbox Using Bevel Gears In this exercise, you build a gearbox using bevel gears. You use Design Accelerator to design and calculate the gear geometry Make IFI_Unit10.ipj the active project. Open Bevel_Gear.iam. 3. In the browser, select BEARING-FLAT:1. Press and hold SHIFT. Select SHAFT-2000:2. All the parts in between are also selected. 4. Right-click any of the highlighted parts. Click Visibility to turn off the visibility of the parts. Bevel gears are used to transmit motion at a 90 degree angle. The completed exercise Open the File A robot design team started designing the gear box assembly. You inform the design team that using the Bevel Gears Component Generator is the recommended workflow. This workflow illustrates how functional design provides a quick solution to a complex design problem. The design team posted the partially complete gearbox so that you can finish the design. The following design criteria is already determined: The face width is 0.25 inches. The diametral pitch is 24 ul/in. The number of teeth for both gears is Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

17 Create the Bevel Gears 6. Click Calculate. The current design fails Click the Calculation tab. Under Loads: For Power (P), enter For Speed (n), enter 100. In this section of the exercise, you create two bevel gears. 1. On the Design tab, Power Transmission panel, click the arrow next to Spur Gear. Click Bevel Gear. 2. Under Common: For Facewidth, enter For Diametral Pitch, select ul/in from the list. 3. Under Gear1, for Number of Teeth, enter Under Gear2, for Number of Teeth, enter If the Summary window is not open, click the chevron. 9. Click Calculate. The current design is compliant. 10. Click the Design tab. 11. Under Gear 1, click Cylindrical Face. Select the outside face of the cylinder. Create Phase 17

18 12. Under Gear 1, click Plane. Select the front face of the cylinder. Modify the Faces of the Gear In this section of the exercise, you modify the faces of the gear In the browser, expand Bevel Gears:1. Rightclick Bevel Gear1:1. Click Open. On the ViewCube, click Home. On the Sketch panel, click Create 2D Sketch. 4. Select the top face of the gear. 5. Press EE Under Operations, select Cut. For Distance, enter Click the More tab. For Taper, enter -45. Click OK. 13. Repeat this workflow for Gear Click OK twice. 15. Drag one of the bevel gears. The second gear rotates in the opposite direction. 18 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

19 6. Rotate the gear to view the bottom face as shown. Create the Shaft Support In this section of the exercise, you create the shaft support. 1. On the Sketch panel, click Create 2D Sketch. 7. On the Sketch panel, click Create 2D Sketch Select the bottom face of the gear. Press EE For Distance, enter Click the More tab. For Taper, enter -45. Click OK Select the top face of the gear. On the Draw panel, click Circle. 4. Create a circle as shown. Make sure the center of the circle is coincident with the center of the projected circle. 5. In the Value Input box, enter Press ENTER. Press EE Select inside the circle. For Distance, enter Click Flip Direction. Click OK. Rotate the gear to view the extruded feature On the ViewCube, click Home. Create Phase 19

20 8. 9. On the ViewCube, click Home. In the browser, expand the last extrusion in the list. Right-click the sketch. Click Share Sketch. Insert an ifeature In this section of the exercise, you insert an ifeature for the square hole in the gear. An ifeature is one or more features that can be saved and reused in other designs. This ifeature was extracted from another gear. 1. On the Manage tab, Insert panel, click Insert ifeature. 10. Press EE Select inside the circle. For Distance, enter Click OK Click Browse. The default location for ifeatures is displayed. Click Workspace to navigate to your working folder. Select SquareHole.ide. Click Open. Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

21 5. Select the front face of the extrusion. In the dialog box, a check mark is added to Profile Plane1. 6. Click Finish In the browser, expand the Origin folder. Rightclick XZ Plane. Click Visibility to turn on the visibility of the plane. On the Model tab, Sketch panel, click Create 2D Sketch. 6. Select the edge of the work plane Turn off the visibility of the work plane. Right-click in the graphics window. Click Slice Graphics. 9. On the ViewCube, click Bottom. Reduce the Weight of the Gear In this section of the exercise, you reduce the weight of the gear using a revolve cut workflow In the browser, expand the last extrusion. Turn off the visibility of the sketch. On the ViewCube, click the edge as shown. 3. On the ViewCube, click the corner. Create Phase 21

22 10. If required, rotate the view to appear as shown. 13. Select the two edges. 14. On the Draw panel, click Line. Note: Depending on how you have rotated the model previously, the ViewCube may appear differently. Make sure your model is oriented as shown. 11. Zoom into the top half of the gear. 12. On the Draw panel, click Project Geometry Sketch a profile as shown. Make sure that the endpoints 1 and 2 are coincident to the endpoints of the projected geometry: line 3 is vertical, and line 4 is horizontal. Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

23 16. On the Constrain panel, click Dimension. 17. Add the two dimensions to the sketch. Create a Rib Support In this section of the exercise, you create a rib support. Removing the material to reduce weight has weakened the shaft support. Adding rib supports resolves the problem, and does not add significant weight. 1. On the Sketch panel, click Create 2D Sketch In the browser, under the Origin folder, click XZ Plane. Right-click in the graphics window. Click Slice Graphics. On the ViewCube, click Bottom. Zoom into the top of the gear. 6. On the Draw panel, click Project Geometry. 7. Select the two edges as shown. 8. On the Draw panel, click Line On the ViewCube, click the top right corner. 19. Press R to start the Revolve tool. Select the axis. Under Operation, select Cut. Click OK. Create Phase 23

24 9. Draw a line from 1 to 2. Make sure that point 1 is coincident and point 2 is at the intersection of the projected lines. If the intersection icon is not displayed for point 2, zoom in closer. 10. Press ESC to exit the Line tool. 11. Right-click the vertical projected edge (1) below the new line. Click Delete. 14. On the Create panel, click Rib. 15. To create the rib: Select the sketch line. Under Shape, click Direction. Move the cursor below the profile. The preview arrow must point down. Click when the arrow is pointing down. For Thickness, enter Click OK. 16. On the Pattern panel, click Circular Pattern. 17. To create the pattern: In the graphics window, select the rib feature. In the Circular Pattern dialog box, click Rotation Axis. Select the axis. Under Placement, for Occurrence Count, enter 4. Click OK. 12. Rotate the part as shown. 13. On the Quick Access toolbar, click Return. 24 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

25 Change the Material 13. Turn on the visibility of all the parts. Note that the gear is updated in the assembly. In this section of the exercise, you change the material to ABS plastic. 1. On the Manage tab, Styles and Standards panel, click Styles Editor. 2. Expand Material. Select ABS Plastic. Note: The second gear, Bevel Gear2, is not updated. It is a separate file. In this gearbox, both gears are identical, so you can replace Bevel Gear2, with Bevel Gear1. For the objective of the exercise, this is not required. 14. Save the file. 3. For color, select Green (Flat) from the list Click Save. Click Done. In the browser, right-click Bevel Gear1. Click iproperties. 7. Click the Physical tab. 8. For Material, select ABS Plastic from the list. 9. Click Apply. Note the properties of the gear, such as Mass, Area, and Volume. 10. Click Close. 11. Click Save. 12. Close the Bevel Gear1 window. Return to the assembly. Create Phase 25

26 Build Phase Overview In this phase, you design and build a drivetrain of your choice. Phase Objectives After completing this phase, you will be able to: Apply your VEX expertise gained from prior units to design and build your own drivetrain. Build a VEX robot of your own design. Prerequisites Before starting this phase, you must have: Completed Unit 10: Drivetrain Design 2 > Think Phase. Unit 1: Introduction to VEX and Robotics Unit 4: Microcontroller and Transmitter Overview Unit 5: Speed, Power, Torque, and DC Motors Unit 6: Gears, Chains, and Sprockets Unit 7: Advanced Gears Unit 8: Friction and Traction Unit 9: Drivetrain Design 1 Required Supplies and Software The following supplies are used in this phase: Supplies VEX Classroom Lab Kit Notebook and pen Work surface Small storage container for loose parts Optional: Autodesk Inventor Professional Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

27 Activity Design a Drivetrain In this activity, you design and build your own drivetrain that will be used in the challenge of the upcoming Amaze Phase. In the Amaze Phase, you will be required to calculate the theoretical speed of your drivetrain. This is something to keep in mind while designing. 1. In your engineering notebook, briefly describe the drivetrain you want to build. Talk about features such as size, number of wheels, type of wheels, and gearing. 2. If you are having trouble coming up with a design, look back to some of the drivetrain designs you saw in previous units. Do not be afraid to look to these designs for inspiration. For instance, you could take one of these designs and make your own improvements to it! Build Phase 27

28 3. Remember, as creative as your design may be, you are limited to the VEX parts you have in your VEX Classroom Kit. 4. Look back to your previous Think Phases. The lessons learned on topics such as gearing, torque, friction, and traction will all come in handy when designing your drivetrain. 5. Once you have settled on a design, sketch it out in your notebook. Work as professionals in the engineering and design fields by leveraging the power of Autodesk Inventor software to explore potential solutions through the creation and testing of digital prototypes. Note: Come to class prepared to build and test your best ideas! Team members can download a free version of Autodesk Inventor Professional software to use at home by joining the Autodesk Education Community today at Now that you have completed a basic design, it is time to get building! 7. For tips on best practices for the construction of VEX robots, refer to the previous Think and Build Phases, as well as your Inventor s Guide. 8. Once your robot is complete, take it for a test drive, and get ready for the next phase! Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

29 Amaze Phase Overview In this phase, you measure the speed of the drivetrain designed and built in the previous Unit 10: Drivetrain Design 2 > Build Phase. You then calculate the theoretical speed. Phase Objectives After completing this phase, you will be able to: Calculate theoretical speed of a given drivetrain. Explain the differences between the theoretical and measured speeds of a drivetrain. Prerequisites Before starting this phase, you must have: Completed Unit 10: Drivetrain Design 2 > Think Phase. Completed Unit 10: Drivetrain Design 2 > Build Phase. Have an assembled drivetrain from Unit 10: Drivetrain Design 2 > Build Phase. Unit 1: Introduction to VEX and Robotics Unit 4: Microcontroller and Transmitter Overview Unit 5: Speed, Power, Torque, and DC Motors Unit 6: Gears, Chains, and Sprockets Unit 7: Advanced Gears Unit 8: Friction and Traction Unit 9: Drivetrain Design 1 Required Supplies and Software The following supplies are used in this phase: Supplies The drivetrain built in the Unit 10: Drivetrain Design 2 > Build Phase Notebook and pen 4 x25 of open floor space Masking tape Measuring tape Amaze Phase 29

30 Supplies One stopwatch One calculator Evaluation Challenge Instructions Place two strips of masking tape on the floor, 20 apart from each other. (If you do not have 20 of open space, use whatever space you have, and carefully measure the distance between the tape lines.) Place your robot approximately 5 behind one of the tape lines. Turn your robot and receiver on. Drive your robot at full speed towards the two tape lines. Start timing when the front of the robot passes the first tape line, and stop timing when the robot passes the second tape line. See the following figure. Record the time in your engineering notebook. Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

31 Repeat this test ten times. Calculate the average time of your trial. Record these calculations in your notebook. Using the average time, calculate the average speed of your robot. Remember, the equation for speed is: Speed = Distance / Time. Record these calculations in your notebook. Engineering Notebook Using the equations from the Unit 10: Drivetrain 2 > Think Phase, and lessons on gearing from the Unit 6: Gears, Chains, and Sprockets > Think Phase, calculate the theoretical speed of your drivetrain in feet per second. Use 100 rpm as the free speed of a VEX motor. Be sure to show all your calculations. Remember, the free speed of a wheel is equal to the free speed of the motor, multiplied by the reduction ratio of the gearing. Be sure to convert the free speed of the wheel from revolutions per minute to revolutions per second. Now that you know how many times your wheel turns in one second, by multiplying this value by the circumference of the wheel, you can calculate how far your robot moves in one second. Compare the values of your measured and theoretical speeds. Why are they different? List at least five factors that may have caused the difference. Why do you think you were asked to start the timing only after the robot had driven for a distance? Presentation Present your findings on the differences between measured and theoretical speeds to the class. Amaze Phase 31

32 STEM Connections Background A pitching machine works by using a spinning wheel to shoot a ball towards the plate at a given speed. The wheel s direction of rotation is towards the batter. A ball is dropped though a tube and when it makes contact with the surface of the spinning rubber wheel it is hurled forward. The speed of the pitch is controlled by changing the RPMs of the motor. Science What material should the spinning wheel be made of to grip and fling a baseball most effectively? Would you change this material based on the type of ball you are pitching? Technology You have to make a pitching machine with a motor that runs at only one speed. What other parts of the machine can you adjust to change the velocity of the pitch? How can you calculate the speed of the pitch before you put the ball in the machine? Engineering What scientific forces are involved in the pitching motion of the machine? Consider the motion of the wheel and the interaction of the wheel with the ball. Math To pitch a baseball at 30 miles per hour, what do the RPMs of a 24-inch diameter wheel on a pitching machine need to be? If a baseball player wants to hit balls that are pitched at 45 miles per hour? What is the RPM setting of the 24-inch wheel? What do the RPMs of the same wheel need to be in order to pitch balls traveling at a speed of 75 miles per hour? (Reminder: 5,280 feet = 1 mile) 32 Autodesk's VEX Robotics Unit 10: Drivetrain Design 2