Autodesk's VEX Robotics Curriculum Unit 9: Drivetrain Design 1 1
Overview In Unit 9: Drivetrain Design 1, you learn the basic principles of drivetrain design, build a basic VEX drivetrain, and test your creation using varying wheelbases. Key concepts from previous units are applied and you document and present your findings. The concept of a skid steer system has a variety of real-world applications. In STEM Connections, we present a scenario involving the design of a personal golf cart. After completing the Think Phase and Build Phase in Unit 9: Drivetrain Design 1, you will see how skid steering comes into play in the real world. Unit Objectives After completing Unit 9: Drivetrain Design 1, you will be able to: Describe how turning scrub and turning torque affect a robot s ability to turn, and how chassis geometry affects turning scrub and turning torque. Build a gearbox with bevel gears using Autodesk Inventor Professional. Build a simple VEX drivetrain. Describe the relationship between length of wheelbase and turning ability. Prerequisites Related resources for Unit 9: Drivetrain Design 1 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. Key Terms and Definitions The following key terms are used in Unit 9: Drivetrain Design 1: 2 Term Definition Center of Rotation The point that a robot turns around, frequently variable, and dependant upon associated forces, torques, and frictions. 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. Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
Term Definition Scrub Torque (Turning Scrub) In a skid steer drivetrain, the frictional force between the wheels and the ground when a wheel or wheels slide(s) across the surface in order for the robot to turn. Skid Steer Another name for tank drive, consisting of two independent sets of powered wheels (or treads), one on each side of its chassis. Turning Torque Torque created by the force of driven wheels to the ground or surface. This torque must be greater than the scrub torque in order for a robot to turn. Wheelbase Distance between the center of front and rear wheels of a robot or other vehicle. Worm A worm is a gear that resembles a screw. It is a species of helical gear, but its helix angle is usually large (somewhat close to 90 degrees) and its body is usually fairly long in the axial direction. It is these attributes that give it its screw like qualities. Worm Gear Meshes with the worm. It is an ordinary looking, disk-shaped gear, and is sometimes called the wheel or the worm wheel. Worm Gear Set Consists of a worm and worm gear. The prime feature of a worm-and-gear set is that it allows the attainment of a high gear ratio with few parts in a small space. Required Supplies and Software The following supplies and software are used in Unit 9: Drivetrain Design 1: Supplies Software VEX Classroom Lab Kit Autodesk Inventor Professional 2010 The drivetrain built in the Unit 9: Drivetrain Design 1 > Build Phase Notebook and pen Work surface Small storage container for loose parts Overview 3
VEX Parts The following VEX parts are used in Unit 9: Drivetrain Design 1 > Build Phase: 4 Quantity Part Number Abbreviations 1 BATTERY-STRAP BST 4 BEAM-0500 B0.5 5 BEAM-1000 B1 4 BEAM-2000 B2 2 BEAM-3000 B3 14 BEARING-FLAT BF 28 BEARING-RIVET BR 238 CHAIN-LINK CL 4 MEDIUM WHEEL W4 1 MICROCONTROLLER VMC 4 NUT-832-KEPS NK 1 PLUS-GUSSET G+ 1 RECEIVER RX75 4 SCREW-632-0250 SS2 25 SCREW-832-0250 S2 9 SCREW-832-0375 S3 1 SCREW-832-0750 S6 4 SHAFT-3000 SQ3 8 SHAFT COLLAR COL 1 SPACER-THICK SP2 5 SPACER-THIN SP1 8 VEX-24-TOOTH-SPROCKET CS24 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
Quantity Part Number Abbreviations 2 VEX Motor MOT 1 VL-PLATE-5-15 RevA P15 4 VL-RAIL-21-25-RevA R25 4 WASHER-STEEL WS Academic Standards The following national academic standards are supported in Unit 9: Drivetrain Design 1. 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. Communication: Communicate mathematical thinking coherently and clearly to peers, teachers, and others. Connections: Recognize and apply mathematics in contexts outside of mathematics. Overview 5
Phase Standard 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. 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) 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. Connections: Recognize and apply mathematics in contexts outside of mathematics. 6 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
Phase Standard 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: Applying the Design Process Mathematics (NCTM) 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. Communication: Communicate mathematical thinking coherently and clearly to peers, teachers, and others. Connections: Recognize and apply mathematics in contexts outside of mathematics. Overview 7
Think Phase Overview This phase describes the factors that affect the turning of a skid steer robot. It also shows how the physical concepts of friction and torque are applied to robot turning. Phase Objectives After completing this phase, you will be able to: Describe how turning scrub and turning torque affect a robot's ability to turn. Describe how chassis geometry affects turning scrub and turning torque. Explain some of the factors that affect center of rotation. Prerequisites Related phase resources are: Unit 5: Speed, Power, Torque, and DC Motors. Unit 8: Friction and Traction. Required Supplies and Software The following supplies are used in this phase. Supplies Notebook and pen Work surface Research and Activity One of the most common types of drivetrain is known as a skid steer drivetrain, which may also be referred to as a tank drive. A skid steer drivetrain consists of two independent sets of powered wheels (or treads), one on each side of its chassis. By running the sides of the drivetrain at different speeds, it is possible to steer the robot in arcs. This drivetrain is also capable of a zero-radius turn (it will spin in place) if the sides are run at the same speed in opposite directions. Much of this discussion focuses on skid steer drivetrains. One of the major attributes that defines a drivetrain s performance is how well it turns. There are two main properties that affect drivetrain turning: turning torque and turning scrub. 8 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
The diagram above shows a simple round 4-wheeled robot (notice these wheels are in an odd configuration). In the robot, the wheels on the left and right of the robot are drive wheels, while the wheels on the front and back of the robot are wheels only for stability. When the right wheel moves forward and the left wheel moves in reverse, the robot spins left (as shown above). This occurs because the left and right drive wheels are each providing drive force on the ground. This force provides a torque about the center of the robot. Each wheel adds to this torque, causing the robot to turn. This is called the turning torque, and is labeled (T1). This torque is the force applied by each wheel, multiplied by the distance the wheels from the center of rotation. For the robot to turn, the front and rear wheels need to slide sideways across the floor. There is a frictional force between the wheels and the ground. Think Phase 9
As seen above, this force of friction causes a torque which opposes the turn; this is known as scrub torque. Scrub torque is the result of the frictional force of the wheels multiplied by their distance from the center of rotation. For a robot to turn, the turning torque needs to exceed the scrub torque. For the purposes of simplification, the above robot design makes it easy to see the difference between scrub torque and turning torque because of the configuration of its wheels; most robot configurations are not this straight forward. A typical 4WD robot configuration is more like the one shown in the following illustration: 10 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
In this case, all four wheels contribute to the turning torque, and all four wheels contribute to the scrub torque. Each wheel applies some force that contributes to turning, and each wheel needs to slide sideways and contributes some friction to scrub. Turning torque and turning scrub are both torques about the robot s center of rotation. As you know from Unit 5: Speed, Power, Torque and DC Motors, a torque is defined by a force at some distance from a center of rotation. The following diagrams illustrate these torques and their components. Think Phase 11
As seen above, the turning torque (labeled TT) is caused by the turning force of the wheel (labeled FT) at a distance from the center of rotation (labeled X). The turning scrub (labeled TS) is caused by the scrub force caused by wheel friction (labeled FS) at a distance from the center of rotation (labeled Y). Now that you understand the components of the torques that affect robot turning, you can begin to understand how to alter them to make a robot turn more effectively. The turning scrub needs to be less than the turning torque for the robot to be effective. How do you reduce the turning scrub? You can do this by reducing FS or by reducing Y. Reducing the distance Y is dependent on the chassis design of the robot (more on this later). You can also reduce FS, which is caused by the force of friction between the wheel and the ground; you can reduce this friction by either reducing the normal force (robot weight) resting on the wheel, or reducing the coefficient of friction of the wheel itself. Similarly, you can increase the turning torque. You do this by increasing FT or by increasing X. Increasing X is dependent on the robot chassis design (more on this later). To increase FT, you need to increase the force of friction between the wheel and the ground; you can increase this friction by either increasing the normal force (robot weight) resting on the wheel, or increasing the coefficient of friction of the wheel itself. Notice that to make the robot turn by reducing turning scrub, you need to decrease wheel friction. But to increase turning torque, you need to increase wheel friction; this means you may not be affecting the overall turning ability of the robot. Often, the most effective way to affect robot turning is to modify the chassis configuration. 12 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
The example above shows a drivetrain configuration which is long and narrow. This configuration will likely have poor turning characteristics because of its low turning torque and high turning scrub. In this example (above), the drivetrain configuration is short and wide. This configuration will likely have good turning characteristics because of its high turning torque and low turning scrub. All the examples so far have been greatly simplified to help illustrate the major underlining concepts. There is another important consideration that changes the dynamics of these systems, the location of the center of rotation. In all the examples presented so far, the center of rotation has been fixed in the exact center of the robot; this is not always the case. The center of rotation often varies based on the differences between the wheels. This is primarily based on the friction between the individual wheels and the floor. (As you know, this friction is based on the weight resting on the wheels, and the coefficient of friction of the wheels.) To help understand this, imagine a 4WD robot similar to the ones above, except the rear wheels are Think Phase 13
made of slippery ice. In this imaginary robot, the rear wheels have almost no traction and contribute almost nothing to the turning torque and the turning scrub. It is easy to imagine how the center of rotation would move up to the very front of the robot, directly in between the two front wheels. Another thing to consider is a wheel with high traction going forward and in reverse, but with low traction going from side to side. In this case, the wheel helps the turning torque, but contributes very little to the turning scrub. The traction of the different wheels and the location of the weight of the robot affects the location of the center of rotation, and this in turn affects the turning torque and turning scrub of the robot. To recap, to make a robot turn better, you primarily adjust two things: the chassis geometry (wide or narrow, long or short) and the difference in traction between the various wheels (primarily the difference between the front wheels and the back wheels). 14 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
Create Phase Overview In this phase, you learn about creating a gearbox with a worm gear set. You use the Worm Gears Generator option of Design Accelerator. The completed exercise Objectives After completing this phase, you will be able to: Describe options for worm gear generation. Build a gearbox using a worm gear set. Prerequisites 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 15
Technical Overview The following Autodesk Inventor tools are used in this phase. Icon Name Description Return Use Return to quit in-place editing and quickly return to the desired environment. The destination depends on which modeling environment you are working in. Worm Gear Calculates dimensions, force proportions, and loading of worm gearing with common or spiral teeth. The gear has cylindrical worm and globoidal worm gears. It contains the geometric calculation of center distance or the calculation based on the center distance and the calculation of gearing ratio, which enables the design of a gear correction. Create 2D Sketch Consists of the sketch plane, a coordinate system, 2D curves, and the dimensions and constraints applied to the curves. Circle Creates a circle from a center point and radius, or tangent to three lines. 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. 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). Mirror Features can be mirrored about any work plane or planar face. You can mirror solid features, work features, surface features, or the entire solid. A mirror of the entire solid allows mirroring of complex features such as shells or swept surfaces included in the solid. Part, sheet metal, surface, and assembly features can be mirrored to create and maintain complex symmetrical features, which can also reduce the amount of time required to create a model. Features can also be mirrored when a model body is in an open or surface state. Insert ifeature 16 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. Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
Required Supplies and Software 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. 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. Create Phase 17
Worm Gear Options The following options are available for creating worm 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. Use to input data specific to the worm. Use to input data specific to the worm gear. Display a page containing all input data and calculations. 18 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
Exercise: Build a Gearbox Using a Worm Gear Set In this exercise, you build a gearbox using a worm and a worm gear. You use Design Accelerator to design and calculate the gear geometry. 1. 2. Make IFI_Unit9.ipj the active project. Open Worm_Gear.iam. 3. In the browser, select BEARING-FLAT:1. Press and hold SHIFT. Select SPACER-THIN:4. 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. In the browser, right-click VL-WORM-BRK RevA:1. Click Edit. Worm gear sets are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater. The completed exercise Open the File A robot design team started designing the gear box assembly. You inform the design team that using the Worm Gears Component Generator is the recommended workflow. This workflow illustrates how functional design provides a quick solution to a complex design problem. 5. The design team posted the partially complete gearbox so that you can finish the design. The following design criteria is already determined: The facewidth is 0.25 inches. The number of teeth for the worm gear is 24. The worm is 0.75 inches long. Create Phase 19
6. 7. In the browser, select Extrusion1 and Extrusion2. Right-click one of the extrusions. Click Unsuppress Features. 2. Under Worm, for Worm Length, enter 0.75. 3. Under Worm Gear: For Number of Teeth, enter 24. For Facewidth, enter 0.25. 4. If the Summary window is not open, click the chevron. 5. Under Worm, click Cylindrical Face. Select the outside face of the lower cylinder. Drag End of Part below Split1. The front face of the bracket is removed, making it easier to view the internal configuration. 8. On the Quick Access toolbar, click Return. 9. Click Accept. The error message refers to the parts that are constrained to the front face of the bracket. Create the Worm Gear Set In this section of the exercise, you create a worm gear set. 1. On the Design tab, Power Transmission panel, click the arrow beside Spur Gear. Click Worm Gear. 20 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
6. Under Worm Gear, click Cylindrical Face. Select the outside face of the upper cylinder. 7. In the Gearing Choice dialog box, select the first row with a z value of 24. Click OK. 12. Click OK twice. 13. Drag the worm gear. The worm rotates at a much faster rate. 14. If required, click Cancel to close the dialog box that is displayed. Modify the Worm Gear In this section of the exercise, you reduce the weight of the gear. 8. 9. Click the Calculation tab. Under Loads: For the Worm Power (P), enter 0.003. For Speed (n), enter 300. 1. 2. 3. In the browser, expand Worm Gears:1. Rightclick Worm Gear1. Click Open. On the ViewCube, click Home. On the Sketch panel, click Create 2D Sketch. 4. Select the top face of the gear. 5. On the Draw panel, click Circle. 10. Click Calculate. The current design is compliant. 11. Click the Design tab. Create Phase 21
6. Create a circle on the sketch. Make sure it is centered on the face of the gear. 7. On the Constrain panel, click Dimension. 11. Select the bottom face of the extrusion. 12. Create a circle and add a 0.315 dimension. 8. 9. Add a 0.74 dimension to the circle. Press E to start the Extrude tool. Select inside the circle. Under Operation, select Cut. For Distance, enter 0.094. Click OK. 13. Press E to start the Extrude tool. Select inside the new sketch. For Distance, enter 0.157. Click OK. 14. On the Pattern panel, click Mirror. 10. On the Sketch panel, click Create 2D Sketch. 15. In the browser, select the two extrusions. 16. In the Mirror dialog box, click Mirror Plane. 22 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
17. In the browser, expand the Origin folder. Click XY Plane. 5. Select the top face of the extrusion. Note that a check mark is added to Profile Plane1 in the dialog box. 18. Click OK. 19. Rotate the part and review the mirrored extrusions. 20. On the ViewCube, click Home. 6. Click Finish. 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. Change the Material 2. 3. Click Browse. The default location for ifeatures is displayed. Click Workspace to navigate to your working folder. 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. 4. Expand Material. Select ABS Plastic. Select SquareHole.ide. Click Open. Create Phase 23
3. For color, select Green (Flat) from the list. Modify the Worm In this section of the exercise, you add the hole and change the material of the worm. Click Save. Click Done. In the browser, right-click Worm Gear. 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 Worm Gear window. Return to the assembly. Note that the gear is updated in the assembly. 1. 2. In the browser, right-click Worm:1. Click Open. Using the same workflow as the worm gear, add the ifeature to the worm. 3. Using the same workflow as the worm gear, change the material to ABS plastic with Green (Flat) color. 4. 5. 6. 13. Save the file. 4. 24 Note: When you become more familiar with styles and materials you will add materials to a shared material library. This makes it easier to reuse materials. Save the file. Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
5. Close the Worm Gear window. Return to the assembly. Note that the worm is updated in the assembly. Finish the Gear Design In this section of the exercise, you turn on the visibility of all the gear assembly parts. 1. 4. In the browser, select BEARING-FLAT:1. Press and hold SHIFT. Select SPACER-THIN:4. All the parts in between are also selected. Right-click any of the highlighted parts. Click Visibility to turn on the visibility of the parts. In the browser, right-click VL-WORM-BRK RevA:1. Click Edit. Drag End of Part above Split1. 5. On the Quick Access toolbar, click Return. 6. 7. Save the file. Close the file. 2. 3. Create Phase 25
Build Phase Overview In this phase, you build a simple and modifiable drivetrain. Phase Objectives After completing this phase, you will be able to: Build a simple VEX drivetrain. Modify and optimize a drivetrain to your liking based on the principles of drivetrain design learned in the Unit 9: Drivetrain Design 1 > Think Phase. 26 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
Prerequisites and Resources Before starting this phase, you must have: Completed Unit 9: Drivetrain Design 1 > Think Phase. Related phase resources are: 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. Required Supplies and Software The following supplies are used in this phase: Supplies Notebook and pen Work surface Small storage container for loose parts Optional: Autodesk Inventor Professional 2010 VEX Parts The following VEX parts are used in this phase: Quantity Part Number Abbreviations 1 ANTENNA HOLDER AH 1 ANTENNA TUBE AT 1 BATTERY, 7.2 VOLT RECHARGEABLE BP 1 BATTERY-STRAP BST 4 BEAM-0500 B0.5 5 BEAM-1000 B1 Build Phase 27
28 Quantity Part Number Abbreviations 4 BEAM-2000 B2 2 BEAM-3000 B3 14 BEARING-FLAT BF 28 BEARING-RIVET BR 236 CHAIN-LINK CL 4 MEDIUM WHEEL W4 1 MICROCONTROLLER VMC 4 NUT-832-KEPS NK 1 PLUS-GUSSET G+ 1 RECEIVER RX75 4 SCREW-632-0250 SS2 24 SCREW-832-0250 S2 9 SCREW-832-0375 S3 1 SCREW-832-0750 S6 4 SHAFT-3000 SQ3 8 SHAFT COLLAR COL 1 SPACER-THICK SP2 5 SPACER-THIN SP1 4 VEX-24-TOOTH-SPROCKET CS24 2 VEX Motor MOT 1 VL-PLATE-5-15 RevA P15 4 VL-RAIL-21-25-RevA R25 4 WASHER-STEEL WS Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
Activity Build a VEX Drivetrain In this activity, you build a simple drivetrain that can be modified to test the difference in turning in drivetrains of varying length. As you work on building this project, have some of your team members focus on expanding their expertise using Autodesk Inventor. Later in the curriculum, you will be challenged to come up with your own creative solutions for robot design. You will save time and maximize your ability to create winning solutions if your team understands how to leverage the power of digital prototypes using Inventor. Note: Team members can download a free version of Autodesk Inventor Professional to use at home, so you can come to class prepared to build and test your best ideas! To do this, simply join the Autodesk Student Engineering and Design Community at www.autodesk.com/edcommunity. 1. The completed model is as shown: Using Bearing Rivets [BR], attach four Bearing Flats [BF] to a 25 Hole Chassis Rail [R25]. Build Phase 29
2. 30 To complete the next step: Using Bearing Rivets [BR], attach three Bearing Flats [BF] to a 25 Hole Chassis Rail [R25]. Bolt a Motor [MOT] to the Chassis Rail. Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
3. The completed model is as shown: Using Bearing Rivets [BR], attach four Bearing Flats [BF] to a 25 Hole Chassis Rail [R25]. Build Phase 31
4. 32 To complete the next step: Using Bearing Rivets, attach three Bearing Flats to a 25 Hole Chassis Rail. Bolt a Motor to the Chassis Rail. Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
5. To complete the next step: Bolt a Receiver [RX75] to the 5x15 Plate [P15]. Bolt a Battery Strap [BST] to the 5x15 Plate. Bolt four 2 Beams [B2] to the 5x15 Plate. Build Phase 33
6. 34 The completed model is as shown: Bolt four 1 Beams [B1] to a VEX Microcontroller [VMC]. Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
7. To complete the next step: Bolt two 3 Beams [B3] to the Plus Gusset [G+]. Bolt a 1 Beam to the Plus Gusset. Build Phase 35
8. 36 The completed model is as shown: Bolt the assemblies from Steps 2, 4, 5, and 6 together. Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
9. Bolt the assemblies from Steps 7 and 8 together, adding a Thick Spacer [SP2] and a Thin Spacer [SP1] between them. The completed model is as shown: Build Phase 37
10. Bolt the assemblies from Steps 1, 3, and 9 together with four Beams [B0.5] and four Steel Washers [WS] in between them. The completed model is as shown: 38 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
11. To complete the next step: Insert a 3 Shaft [SQ3] into both the Motor Clutch and one of the three open Bearing Flats at the Rear of the robot, while sliding a Collar [COL] on the Shaft between the outside Bearing Flat and the Inner Chassis Rail. Slide a Thin Spacer [SP1], a 4 Wheel [W4], a 24 Tooth Chain Sprocket [CS24], and a Collar onto the Shaft. Tighten the Collars in place. Build Phase 39
12. Repeat Step 11 on the opposite side of the robot. The completed model is as shown: 40 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
13. Bolt the Antenna Holder [AH] to the inside Chassis Rail, adjacent to the receiver. The completed model is as shown: Build Phase 41
14. To complete the final step: Attach Chains [CL] to the Drive Wheels. Plug in the Receiver into Rx 1 on the Microcontroller. Plug in the right and left motors into Motor ports 2 and 3 respectively on the Microcontroller. Attach a Battery using the Battery Strap and plug it in. The completed model is as shown: 42 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
15. Your drivetrain is now complete and ready for a test drive! Note: Battery and electrical connections not shown. Build Phase 43
Amaze Phase Overview In this phase, you experiment with varying lengths of drivetrain wheelbase. Phase Objectives After completing this phase, you will be able to: Explain the relationship between length of wheelbase and turning ability. Determine the optimal length of wheelbase for a given robot. Prerequisites Before starting this phase, you must have: Completed Unit 9: Drivetrain Design 1 > Think Phase. Completed Unit 9: Drivetrain Design 1 > Build Phase. Have the assembled drivetrain from Unit 9: Drivetrain Design 1 > Build Phase. Related phase resources are: 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. Required Supplies and Software The following supplies are used in this phase: Supplies The drivetrain built in the Unit 9: Drivetrain Design 1 > Build Phase Notebook and pen 44 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
Evaluation Challenge Instructions To change the length of the wheelbase, complete the following steps: 1. 2. 3. 4. 5. 6. Work on one side of the drivetrain, left or right, at a time. Remove the chain on the drivetrain. Loosen the collars on the wheel shaft not directly driven by the motor. Remove the outer collar, sprocket, wheel, and spacer from the shaft. Remove the shaft and second collar. Insert the Shaft [SQ3] into the open Bearing Flat corresponding to the desired wheelbase, while sliding a Collar [COL] on the Shaft between the outside Bearing Flat and the Inner Chassis Rail. See the following figure. The completed model is as shown: Amaze Phase 45
7. Slide a Thin Spacer [SP1], a 4 Wheel [W4], a 24 Tooth Chain Sprocket [CS24], and a Collar onto the Shaft. Refer to the previous figure. 8. Tighten the Collars in place. Refer to the previous figure. 9. Replace the chain. You will need to shorten or lengthen the chain, depending on the length of the new wheelbase. 10. Repeat these steps for the opposite side of the drivetrain. 11. For each of length of wheelbase, test drive the robot. Take note of how the drivetrain turns. Engineering Notebook In your notebook, write an analysis that compares the turning ability of the different lengths of wheelbase. Comment on the ease and smoothness of turning, and which style you prefer. Answer the following questions: What are the advantages of a drivetrain that turns easily? What are the disadvantages of a drivetrain that turns too easily? What are the advantages of a shorter wheelbase? What are the disadvantages of a shorter wheelbase? What are the advantages of a longer wheelbase? What are the disadvantages of a longer wheelbase? Aside from varying the wheelbase, how can you improve the turning ability of a drivetrain? Presentation Present your findings from this challenge and discuss one improvement you can make to the drivetrain. 46 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1
STEM Connections Background A golf cart company is considering a new design for a personal golf cart that would employ a skid steer drivetrain, like the one discussed in this unit, for greater maneuverability. Science The two main ways to adjust the turning ability of a skid steer vehicle are adjusting the chassis geometry and changing the traction of the wheels. How can you adjust the traction of the wheels? What materials do you envision using for wheels on a skid steer golf cart? Technology How might you incorporate the concept of a skid steer drivetrain into automobile design? Can you do this without abandoning the concept of traditional steering? What are the possible advantages of this car in everyday driving situations? STEM Connections 47
Engineering What types of movement does this new golf cart gain with a skid steer drive configuration? What types of movement does it lose? Will people be able to drive it? How do you keep the scrub torque associated with a skid steer vehicle from damaging the grass on a golf course? Math One idea for a skid steer golf cart uses two 18-inch diameter wheels on each side for a total of four wheels. If a 2-inch shift mounted to the motor and connected to the wheels/tires rotates at 120 rpms (revolutions per minute), how long will it take for the cart to move forward a straight line distance of 100 feet? With the same configuration, if you slowed the motor speed down to 20 rpms, how long would it take to make a 180-degree turn using the skid steering system (the right and left wheel sets spin in opposite directions)? 48 Autodesk's VEX Robotics Unit 9: Drivetrain Design 1