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

1

Autodesk's VEX® Robotics Curriculum

Unit 10: Drivetrain Design 2

2 ■ Autodesk's VEX Robotics Unit 10: Drivetrain Design 2

Overview

In Unit 10: Drivetrain Design 2, you design your own drivetrain, building on knowledge and skills fromprevious units. You will also calculate, document, and communicate both theoretical and measuredspeeds for your drivetrain.

The concept of a drive train has a variety of real-world applications. In STEM Connections, we presenta scenario involving the design of a pitching machine for baseball and softball. After completing theThink Phase and Build Phase in Unit 10: Drivetrain Design 2, you will see how a drive train comes intoplay 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 using bevel gears using Autodesk Inventor Professional.■ 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:

■ 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.

Overview ■ 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 tooth-bearing faces of the gears themselves are conically shaped. Bevel gears are mostoften mounted on shafts that are 90 degrees apart, but can be designed to work atother angles as well. The pitch surface of bevel gears is a cone.

DiametralPitch

The number of teeth of a gear per inch of its pitch diameter. A toothed gear musthave an integral number of teeth. The circular pitch, therefore, equals the pitchcircumference divided by the number of teeth. The diametral pitch is, by definition,the number of teeth divided by the pitch diameter.

FunctionalDesign

Designers use functional design to analyze the function of their products andthe design problems they are trying to solve, rather than spending time on themodeling operations necessary to create 3D representations.

Gearbox The simplest form of a transmission. Uses multiple gears to alter motor speed toachieve desired output.

Geartrain(Powertrain)

Represents the part of the Drivetrain which transmits power from the Motor to theground.

Motor LoadGearing

Determining drivetrain gearing based on maximum motor load.

Transmission Mechanism by which power is transmitted from a power source to the wheels of arobot or other vehicle.

Wheel Speed How fast the robot wheel is spinning. Used to calculate how fast the robot movesacross the floor.


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 2010

Have an assembled drivetrain from Unit 10Build 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

Overview ■ 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.


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,


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,


measurements.■ Communication: Communicate mathematical thinking coherently and clearly to

peers, teachers, and others.■ Connections: Recognize and apply mathematics in contexts outside of mathematics.

Think Phase ■ 7

Think Phase

Overview

This phase describes how to calculate the gearing for a robot drivetrain. It focuses on two mainmethods: 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.


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.


The following supplies are used in this phase.

Supplies

Notebook and pen

Work surface


Research and Activity

In Unit 9: Drivetrain Design 1, we discussed the factors that affect what makes a Drivetrain turn. Thisunit describes the Geartrain or Powertrain of the Drivetrain. The Geartrain represents the part of theDrivetrain that transmits power from the Motor to the ground.

Wheel Speed

The first concept to understand is figuring out how fast the robot moves across the floor based onhow fast the wheel is spinning. For each time the wheel makes a full revolution, it rolls forward adistance equal to its circumference. So if we calculate the circumference of the wheel, we knowexactly how far the robot goes per revolution.

The circumference of a wheel is equal to its diameter multiplied by Pi (about 3.14).

Once we know the circumference of the wheel, we can calculate how fast it is rolling based on itsrotational speed. In the example below, we assume the wheel diameter is 4 inches, and the wheelis spinning at 50 RPM. Let's calculate how fast the wheel is rolling in inches per second. First, wecalculate the circumference:

After we calculate the circumference, we can determine the ground speed based on the wheel RPM.Most of the following is just unit analysis and cancellation:

Now we know the way to calculate ground speed from wheel RPM. If we know the VEX Motor has anoutput of approximately 100 RPM, and we know what reduction we want our wheel to spin at, we cancalculate the approximate gear reduction needed.

Let’s say we have a 5 inch diameter wheel, we want the robot to travel at about 4 feet per second,and we know the VEX motor spins at about 100 RPM. The first step is to calculate the diameter of thewheel, and also convert our target speed to inches per second.

Think Phase ■ 9

So now we know that we need to move 48 inches in one second, and that each revolution is 15.7inches; knowing this we can calculate how many revolutions per second the wheel needs to turn toachieve 4 feet per second.

Now we know that we need the wheel to spin at 183.42 revolutions per minute, and we know the VEXmotor spins at 100 RPM, we can calculate the Gear Ratio needed to achieve our top speed.

To achieve a speed of 4 ft/sec with a 5” wheel and a VEX motor; we need a gear reduction of .545:1.We now need to figure out what gears to use to achieve this gear ratio, or try to get as close aspossible if we do not have the correct gears available. The VEX Robotics Design System has severalGearing Options available. The following chart shows all the gear reduction pairs possible using VEXSpur Gears and VEX Chains and Sprockets.

Using the above table we can see the ratios closest to .545. From here it is a question of choosingwhich option best suits the design. In this case there is no combination which yields exactly the ratiowe want, but there are several which are close.

You can see how one would follow this same process to calculate the gearing in other situations aswell.


Motor Load Gearing

The second concept to understand is calculating the maximum load applied to the motor by thedrivetrain. This occurs during the “wall push” situation; that is, when the robot is up against animmovable object and is running full throttle into it. In this situation the wheels should slip on thefloor, 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 we are looking at.Only wheels directly linked through gearing or chain apply load to the gearbox.

In this example, let's consider the example of a 4WD robot where each side of the robot has 2 wheelslinked to the same gearbox. In this case 1/2 the robot weight will be resting on the wheels. Thisweight is the Normal Force which determines the Force of Friction applied by these two wheels on thegearbox.

This torque is applied through the gearbox onto the motor or motors. If a gearbox has multiplemotors, then the torque will be divided evenly between them.

It is important to design the gearing such that the load applied on each motor is not higher than themotor limit. VEX Motors cannot draw more than 1 Amp of current for an extend period of time. Usethe 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 arobot such that it moves at the desired speed but will also ensure that there is not excessive loadingon the motors.

Think Phase ■ 11

More Information on Transmissions

Using different combinations of gear ratios and the principle of mechanical advantage, transmissionsprovide a torque increase and speed reduction from the high speed and low torque supplied by thedriving motor or engine.

Transmissions are used in practically every industry that uses machinery. They are sometimes used instationary applications to move or lift heavy loads, but are more easily recognized and appreciatedin vehicles to direct power to make them move. They not only transmit power out of the engine, butmore 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 spinningdriveshafts and mechanical connections. Electric drives use direct-driven systems in which the motorsare actually mounted at the location where the power is needed. Electricity is sent from the powersource 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 multi-speed or shiftabletransmissions most people think of when they hear the term, because their gear ratio is fixed whenit is assembled. A gearbox is incapable of shifting, which means that its gear ratio cannot be changedwhile it is operating.

Gearboxes are found in a wide variety of different applications. They were first used in windmills, grainmills, and steam engines. They sometimes supported a 90-degree change in the direction of rotationand occasionally had multiple output shafts to run several different machines at the same time. Theywere particularly desirable in operations where a large speed reduction and torque increase wereneeded in lifting loads and pumping liquids. You could say that gearboxes were perfected down on thefarm.

Low gear ratios provide lots of torque with little speed, while higher gear ratios provide speed withlittle torque.


Create Phase

Overview

In this phase, you learn about creating a gearbox with bevel gears. You use the Bevel Gears Generatoroption 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.


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.

Create Phase ■ 13

Technical Overview

The following Autodesk® Inventor® tools are used in this phase.

Icon

Name

Description

Create 2DSketch

A sketch consists of the sketch plane, a coordinate system, 2D curves, andthe dimensions and constraints applied to the curves.

Bevel GearsGenerator

Calculates dimensions and strength check of bevel gearing with straightand helical teeth. It contains geometric calculations for designing differenttypes of correction distributions, including a correction with compensationof slips.

Center PointCircle

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 andangle from an edge, or a different distance from the edge for each face.

CircularPattern

Part, surface, and assembly features can be arranged in a pattern torepresent hole patterns or textures, slots, notches, or other symmetricalarrangements.

Extrude Creates a feature by adding depth to a sketched profile. Feature shape iscontrolled by profile shape, extrusion extent, and taper angle. Unless theextruded feature is a base feature, its relationship to an existing featureis defined by selecting a Boolean operation (join, cut, or intersect withexisting feature).

GeneralDimension

Adds dimensions to a sketch. Dimensions control the size of a part. Theycan be expressed as numeric constants, as variables in an equation, or inparameter files.

InsertiFeature

An iFeature is one or more features that can be saved and reused in otherdesigns. You can create an iFeature from any sketched feature that youdetermine to be useful for other designs. Features dependent on thesketched feature are included in the iFeature. After you create an iFeatureand store it in a catalog, you can place it in a part by dragging it fromWindows Explorer and dropping it in the part file or by using the InsertiFeature tool.

Rib Ribs and webs are often used in molds and castings. In plastic parts, theyare commonly used to create rigidity and to prevent warping.



The following software is used in this phase.

Software

Autodesk Inventor Professional 2010

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. Theinformation varies depending on the method.

When you create or edit gear sets, you use the Gears Component Generator dialog box. With theDesign Accelerator, you can design spur, bevel, and worm gear sets efficiently. To design and positionyour gear sets in your assemblies, you need to know what options are available in the dialog box andwhere they are located.

Create Phase ■ 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 onpower 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.


Exercise: Build a Gearbox Using Bevel Gears In this exercise, you build a gearbox using bevel gears.You use Design Accelerator to design and calculatethe gear geometry. Bevel gears are used to transmit motion at a 90degree angle.

The completed exercise

Open the File A robot design team started designing the gearbox assembly. You inform the design team thatusing the Bevel Gears Component Generator is therecommended workflow. This workflow illustrateshow functional design provides a quick solution to acomplex design problem. The design team posted the partially completegearbox so that you can finish the design. Thefollowing design criteria is already determined:■ The facewidth is 0.25 inches.■ The diametral pitch is 24 ul/in.■ The number of teeth for both gears is 24.

1. Make IFI_Unit10.ipj the active project. 2.

Open Bevel_Gear.iam.

3.

In the browser, select BEARING-FLAT:1. Pressand hold SHIFT. Select SHAFT-2000:2. All theparts in between are also selected.

4.

Right-click any of the highlighted parts. ClickVisibility to turn off the visibility of the parts.

Create Phase ■ 17

Create the Bevel Gears In this section of the exercise, you create two bevelgears. 1.

On the Design tab, Power Transmission panel,click the arrow beside Spur Gear. Click BevelGear.

2.

Under Common:■ For Facewidth, enter 0.25.■ For Diametral Pitch, select 24.000 ul/in

from the list.

3.

Under Gear1, for Number of Teeth, enter 24.

4.

Under Gear2, for Number of Teeth, enter 24.

5.

If the Summary window is not open, click thechevron.

6.

Click Calculate. The current design fails.

7. Click the Calculation tab. 8.

Under Loads:■ For Power (P), enter 0.01.■ For Speed (n), enter 100.

9. Click Calculate. The current design is compliant. 10. Click the Design tab. 11.

Under Gear 1, click Cylindrical Face. Select theoutside face of the cylinder.


12.

Under Gear 1, click Plane. Select the front faceof the cylinder.

13.

Repeat this workflow for Gear 2.

14.

Click OK twice.

15. Drag one of the bevel gears. The second gear

rotates in the opposite direction.

Modify the Faces of the Gear In this section of the exercise, you modify the faces ofthe gear. 1. In the browser, expand Bevel Gears:1. Right-

click Bevel Gear1:1. Click Open. 2. On the ViewCube, click Home. 3.

On the Sketch panel, click Create 2D Sketch.

4.

Select the top face of the gear.

5.

Press E to start the Extrude tool.■ Select the top face of the gear.■ Under Operations, select Cut.■ For Distance, enter 0.014.■ Click the More tab.■ For Taper, enter -45.■ Click OK.

Create Phase ■ 19

6.

Rotate the gear to view the bottom face asshown.

7.


8. Select the bottom face of the gear. 9.

Press E to start the Extrude tool.■ Select the bottom face of the gear.■ For Distance, enter 0.044.■ Click the More tab.■ For Taper, enter -45.■ Click OK.

10. On the ViewCube, click Home.

Create the Shaft Support In this section of the exercise, you create the shaftsupport. 1.


2. Select the top face of the gear. 3.

On the Draw panel, click Circle.

4.

Create a circle as shown. Make sure the centerof the circle is coincident with the center of theprojected circle.

5.

On the Constrain panel, click Dimension.

6. Add a 0.286 dimension to the circle. 7. Press E to start the Extrude tool.

■ Select inside the circle.■ For Distance, enter 0.375.■ Click Flip Direction.■ Click OK.

8.

Rotate the gear to view the extruded feature.

9. On the ViewCube, click Home.


10.

In the browser, expand the last extrusion in thelist. Right-click the sketch. Click Share Sketch.

11.

Press E to start the Extrude tool.■ Select inside the circle.■ For Distance, enter 0.047.■ Click OK.

Insert an iFeature In this section of the exercise, you insert an iFeaturefor the square hole in the gear. An iFeature is one or more features that can besaved and reused in other designs. This iFeature wasextracted from another gear. 1.

On the Manage tab, Insert panel, click InsertiFeature.

2. Click Browse. The default location for iFeatures

is displayed. 3.

Click Workspace to navigate to your workingfolder.

4. Select SquareHole.ide. Click Open.

5.

Select the front face of the extrusion. In thedialog box, a check mark is added to ProfilePlane1.

6.

Click Finish.

Reduce the Weight of the Gear In this section of the exercise, you reduce the weightof the gear using a revolve cut workflow. 1. In the browser, expand the last extrusion. Turn

off the visibility of the sketch. 2.

On the ViewCube, click the edge as shown.

Create Phase ■ 21

3.

On the ViewCube, click the corner as shown.

4. In the browser, expand the Origin folder. Right-

click XZ Plane. Click Visibility to turn on thevisibility of the plane.

5.

On the Model tab, Sketch panel, click Create 2DSketch.

6.

Select the edge of the work plane.

7. Turn off the visibility of the work plane. 8.

Right-click in the graphics window. Click SliceGraphics.

9. On the ViewCube, click Bottom. 10.

If required, rotate the view.

Note: Depending on how you have rotated themodel previously, the ViewCube may appeardifferently. Make sure your model is oriented asshown.

11. Zoom into the top half of the gear. 12.

On the Draw panel, click Project Geometry.

13.

Select the two edges as shown.

14.

On the Draw panel, click Line.


15.

Sketch a profile as shown. Make sure thatthe endpoints 1 and 2 are coincident to theendpoints of the projected geometry: line 3 isvertical, and line 4 is horizontal.

16.

On the Constrain panel, click Dimension.

17.

Add the two dimensions to the sketch asshown.

18.

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 a Rib Support In this section of the exercise, you create a ribsupport. Removing the material to reduce weighthas weakened the shaft support. Adding rib supportsresolves the problem, and does not add significantweight. 1.


2. In the browser, under the Origin folder, click XZ

Plane. 3. Right-click in the graphics window. Click Slice

Graphics. 4. On the ViewCube, click Bottom. 5.

Zoom into the top of the gear.

Create Phase ■ 23

6.

On the Draw panel, click Project Geometry.

7.

Select the two edges as shown.

8.

On the Draw panel, click Line.

9.

Draw a line from 1 to 2. Make sure that point 1is coincident and point 2 is at the intersectionof the projected lines. If the intersection icon isnot displayed for point 2, zoom in closer.

10. Press ESC to exit the Line tool.

11.

Right-click the vertical projected edge (1) belowthe new line. Click Delete.

12.

Rotate the part as shown.

13.

On the Quick Access toolbar, click Return.

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 whenthe arrow is pointing down.

■ For Thickness, enter 0.05.■ 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.

Change the Material In this section of the exercise, you change thematerial to ABS plastic. 1.

On the Manage tab, Styles and Standardspanel, click Styles Editor.

2.

Expand Material. Select ABS Plastic.

3.

For color, select Green (Flat) from the list.

4. Click Save. 5. Click Done. 6. 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

13.

Turn on the visibility of all the parts. Note thatthe gear is updated in the assembly.

Note: The second gear, Bevel Gear2, is notupdated. It is a separate file. In this gearbox,both gears are identical, so you can replaceBevel Gear2, with Bevel Gear1. For theobjective of the exercise, this is not required.

14. Save the file.


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


■ 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.


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 2010

Build Phase ■ 27

Activity

Design a Drivetrain

In this activity, you design and build your own drivetrain that will be used in the challenge of theupcoming Amaze Phase. In the Amaze Phase, you will be required to calculate the theoretical speed ofyour 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. Forinstance, you could take one of these designs and make your own improvements to it!


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 powerof Autodesk Inventor to explore potential solutions through the creation and testing of digitalprototypes. Note: Come to class prepared to build and test your best ideas! Team members can downloada free version of Autodesk Inventor Professional to use at home by joining the AutodeskStudent Engineering and Design Community today at www.autodesk.com/edcommunity.

6. 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!

Amaze Phase ■ 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


■ 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.


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


Supplies

One stopwatch

One calculator

Evaluation

Challenge Instructions

1. 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 thetape lines.)

2. Place your robot approximately 5’ behind one of the tape lines. 3. Turn your robot and receiver on. 4.

Drive your robot at full speed towards the two tape lines. Start timing when the front of therobot passes the first tape line, and stop timing when the robot passes the second tape line. Seethe following figure.

5. Record the time in your engineering notebook.

Amaze Phase ■ 31

6. Repeat this test 10 times. 7. Calculate the average time of your trial. Record these calculations in your notebook. 8. 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 Unit6: Gears, Chains, and Sprockets > Think Phase, calculate the theoretical speed of your drivetrain in feetper 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 thereduction ratio of the gearing. Be sure to convert the free speed of the wheel from revolutions perminute to revolutions per second.

Now that you know how many times your wheel turns in one second, by multiplying this value by thecircumference 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.


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 itmakes contact with the surface of the spinning rubber wheel it is hurled forward. The speed of thepitch 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)

Documents

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