Autodesk's VEX Robotics Curriculum. Unit 5: Speed, Power, Torque, and DC Motors

Transcription

1 Autodesk's VEX Robotics Curriculum Unit 5: Speed, Power, Torque, and DC Motors 1

2 Overview In Unit 5: Speed, Power, Torque, and DC Motors, you build a VEX test stand winch that enables you to learn key engineering concepts and principles so you can directly apply mathematics and science to robotics. The test stand will be used again and enhanced in Unit 6: Gears, Chains, and Sprockets. The concepts regarding speed, power, torque, and DC motors have countless real-world applications. In STEM Connections, we present one application involving the operation of a free fall amusement park ride. After completing the Think and Build phases, you see how the concepts of speed, power, torque and DC motors come into play in the real world. Unit Objectives After completing Unit 5: Speed, Power, Torque, and DC Motors, you will be able to: Demonstrate the physical concepts of speed, force, torque, power, acceleration, and characteristics of DC Motors. Create and define characteristics of a shaft using Autodesk Inventor Professional Test a VEX motor and build a simple winch. Determine and calculate the free speed and stall torque of a VEX motor. Prerequisites and Resources Related resources for Unit 5: Speed, Power, Torque, and DC Motors are: Unit 1: Introduction to VEX and Robotics Unit 2: Introduction to Autodesk Inventor Unit 3: Building a Protobot Unit 4: Microcontroller and Transmitter Overview Key Terms and Definitions The following key terms are used in Unit 5: Speed, Power, Torque, and DC Motors. 2 Term Definition Acceleration A change in speed over time. Chamfer A placed feature that bevels a part edge and is defined by its placement, size, and angle. DC motor An electric rotating machine energized by direct current and used to convert electric energy to mechanical energy. Direct current (DC) An electric current flowing in one direction only. Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

3 Term Definition Force Accelerations are caused by forces. For example, when a robot is accelerating it does so because of the force its wheels exert on the floor Keyway A slot for a key in the hub or shaft of a wheel. This permits the shaft and wheel to turn together. Power Energy that is produced by mechanical, electrical, or other means and used to operate a device. - OR - The time-rate of doing work, measured in watts or less frequently horsepower. Relief An undercut on a shaft. Typically used between the shoulder of a shaft and a threaded section. This makes it easier to cut a thread on a shaft. Shaft Often referred to as a drive shaft. A mechanical device for transferring power from the engine or motor to where it is wanted. Speed Measure of how fast an object is moving, that is, how much distance it will travel over a given time. Torque Torque is the application of force where there is rotational motion. Work The measure of a force exerted over a distance. Wrench flat Flat surfaces cut on opposite sides of a shaft. These flats are sized for standard wrench openings and allow for assembly of the shaft. Required Supplies and Software The following supplies and software are used in Unit 5: Speed, Power, Torque, and DC Motors. Supplies Software One VEX Transmitter Autodesk Inventor Professional 2010 One VEX Microcontroller One 7.2V VEX Battery One assembled VEX motor test stand from the Unit 5: Speed, Power, Torque, DC Motors > Build Phase One zip tie Notebook and pen Overview 3

4 Supplies Software Work surface One Stopwatch 36 of 1/8 braided nylon and polyester cord, or equivalent rope/string Set of masses or other weights Small storage container for loose parts Required VEX parts The following VEX parts are required in Unit 5: Speed, Power, Torque, and DC Motors > Build Phase. 4 Quantity Part Number Abbreviation 4 BEAM-1000 B1 2 BEAM-2000 B2 1 BEARING-BLOCK BB 3 BEARING-FLAT BF 1 DRIVE-WHEEL TS 1 DRIVE-WHEEL TS15 6 NUT-832-KEPS NK 2 SCREW SS4 8 SCREW S2 4 SCREW B4 6 SCREW S6 1 SHAFT-4000 SQ4 2 SHAFT-COLLAR COL 2 SPACER-THICK SB2 12 SPACER-THIN SP1 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

5 Quantity Part Number Abbreviation 15 TANK-TREAD-LINK TL 1 VEX - MOTOR MOT 1 VL-CHAN RevA C15 2 VL-CHAN RevA CW25 Academic Standards The following national academic standards are supported in Unit 5: Speed, Power, Torque, and DC Motors. Phase Academic Standard Think Science (NSES) Unifying Concepts and Processes: Form and Function Physical Science: Motion 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) Connections: Recognize and apply mathematics in contexts outside of mathematics. Measurement: Understand measurable attributes of objects and the units, systems, and processes of measurement. Overview 5

6 Phase Academic 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 Physical Science: Motion 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) Connections: Recognize and apply mathematics in contexts outside of mathematics. Measurement: Understand measurable attributes of objects and the units, systems, and processes of measurement. Amaze Science (NSES) Unifying Concepts and Processes: Form and Function Physical Science: Motion and Forces Science and Technology: Abilities of Technological Design Technology (ITEA) 5.8: The Attributes of Design Mathematics (NCTM) Connections: Recognize and apply mathematics in contexts outside of mathematics. 6 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

7 Think Phase Overview This phase explains some of the fundamental concepts of physics which apply to VEX Robotics. It also covers the basics of DC Motor Theory. Objectives After completing this phase, you will be able to: Describe the physical concepts of: Speed Force Torque Power Acceleration List the four primary characteristics of DC Motors. Prerequisites and Resources Related phase resources are: Unit 1: Introduction to VEX and Robotics. 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 To understand the performance of a robot one must first understand the basic concepts of physics. Speed The first concept is the concept of speed. Speed is a measure of how fast an object is moving, that is, how much distance it will travel over a given time. This measure is given in units distance per time (some common ones include miles per hour or feet per second). Rotational Speed Speed can be expressed rotationally as well. This refers to how fast something is moving in a circle. It is measured in units of angular-distance per time or rotational cycles per time. Common examples include degrees per second or revolutions per minute (RPM). Acceleration A change in speed over time is known as acceleration; the higher the acceleration the faster the change in speed. If something is moving at a constant speed, it is not accelerating. Force Accelerations are caused by forces. When you press on something you are exerting a force on it. When a robot is accelerating it does so because of the force its wheels exert on the floor. Force is measured in units such as pounds and newtons. 8 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

9 Torque Force directed in a circle is called torque. Torque is a spinning force; however in the instance of a wheel, this spinning force creates a linear force at its edge. This is how we define torque, as a linear force at the edge of a circle. Torque is described by the magnitude of the force multiplied by the distance it is from the center of rotation (Force x Distance = Torque). Torque is measured commonly in units of inch-pounds and newton-meters. So if we know how much torque is applied to an axle with a wheel on it, we can find out how much force the wheel is applying on the floor. Force = Torque / Wheel Radius In this case, the wheel radius would be the distance from the center of rotation. Work Work is the measure of a force exerted over a distance. If I lift something 5 feet, it requires less work than if I lifted it 10 feet. It can also be thought of as a change in energy. Think Phase 9

10 Power Power is another concept important in robotics. Most people are more familiar with power as an electrical term, but it is part of mechanical physics as well. Power is defined as the rate that work is performed. How fast can you do your work? In robotics, it is handy to think of power as a limit. If you need to lift a 10 lb weight (exerting a 10 lb force) the amount of power you have available limits how fast (the rate) at which you can lift it. If you have lots of power available, you will be able to lift it quickly; if you don't have a lot of power, you will lift it slowly. Power can be defined as force multiplied by velocity. (How fast can you push with a known force?) Power is frequently measured in terms of watts. Power = Force [Newtons] x Velocity [Meters / Second] 1 Watt = 1 (Newton x Meter)/ Second The descriptions above only scratch the surface of these physical properties. The concepts discussed above are significantly more advanced that touched upon here. However, the basic understanding is enough to apply them. DC Motors DC (Direct Current) motors convert electrical energy into mechanical energy. Motors can have very different characteristics depending on their manufacture. When a voltage is applied to the motor, it outputs a torque inversely proportional to its speed. (That means the faster the motor is going the LESS torque it outputs, and the slower the motor is going the MORE torque it outputs.) The motor will also draw current proportional to this torque (more torque means more current draw). You can think of the motor as working to overcome a load. With NO load on the motor, it will spin very fast and draw almost no current. As the load increases on the motor it must output more torque to overcome the load, and as it increases the torque it draws more current. Eventually if enough load is placed on the motor, it will stop moving or stall." There are four main characteristics which define DC motor performance and define the relationships described above: Stall Torque (N*m): The amount of load placed on a motor which will cause it to stop moving. Free Speed (RPM): The maximum rotational speed a motor will run at when it is under no load. Stall Current (Amp): The amount of current a motor will draw when it is stalled. Free Current (Amp): The amount of current a motor will draw when it is under no load. These four characteristics change proportionally depending on how much voltage is applied to the motor. Below is a graph which shows these characteristics for a given voltage: 10 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

11 Based on the above relationships, you can see how the concept of power comes into play. With a given loading, the motor can spin only at a certain speed. Since the relationships shown above are linear, it is a simple matter of plotting the torque-speed and torque-current graphs by experimentally determining (2) points on each graph. Designing with DC Motors Like all motors, the VEX Continuous Rotation Motors have certain limitations. They have a limited amount of power to output. If the motor draws more than 1 amp of current, it trips its internal thermal limiter; designers need to utilize the motor so it will not be placed under loads greater than this. Once you determine what load the motor will be operating under (how heavy an arm is it lifting?) and you ensure that it is under the limit, you can calculate how fast the arm will move at this loading. The key thing to remember is that motors have a limited amount of power they can output. There is a balance between speed and torque; you can't have both at the same time. Think Phase 11

12 Create Phase Overview In this phase, you learn about creating shaft components in the context of an assembly, using predefined shape elements to quickly create the geometry. When you use the Shaft Generator to create and edit a shaft design, predefined shapes help ensure that the model conforms to industry standards. Editing your shaft design is easier and quicker than editing shafts that are created using sketched and placed feature-based modeling techniques because of the ease-of-use and consistent modeling techniques offered with the Shaft Generator. The following image shows a common use of shafts in a design. In the motor assembly, shafts are required to transmit power from the electric motor to the gear assembly. Objectives After completing this phase, you will be able to: Define the characteristics of a shaft created with the Shaft Generator. Describe the options available for creating shafts using the Shaft Generator. Create shafts using the Shaft Generator. 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 5: Speed, Power, Torque, and DC Motors

13 Technical Overview The following Autodesk Inventor tools are used in this phase. Icon Name Description Shaft Generator Use to design the shape of a shaft, add and calculate loads and supports, and other calculation parameters. Perform a strength check and generate the shaft in the Autodesk Inventor model. The shaft is assembled from single sections (cylinder, cone, and polygon) including features (chamfers, fillets, neck-downs, and so on). Thread Use to create threads on existing model features. 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. Constrain Assembly constraints determine how components in the assembly fit together. As you apply constraints, you remove degrees of freedom, restricting the ways components can move. Required Supplies and Software The following software is used in this phase. Software Autodesk Inventor Professional 2010 About the Shaft Generator You use the Shaft Generator to create and edit shaft designs by combining individual shaft sections in the assembly environment. The shaft sections provided with this tool include the most common geometry found in shafts. You can base your shaft designs on data you input, part geometry in the assembly, and load calculations provided in the Shaft Component Generator dialog box. The following illustration shows a shaft design for an electrical motor. A variety of input data is used to design the shaft. In addition to supporting the loads, it needs to fit with the threaded hole on the existing motor. Create Phase 13

14 Definition of the Shaft Generator The Shaft Generator provides a series of tools that enable you to design and edit a shaft section by section. As you design the shaft, you can add multiple features to fully define each section. You can also perform calculations and create output graphs based on the loads applied to the shaft. The following illustration shows the Calculation tab of the Shaft Component Generator dialog box. 14 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

15 Example Shaft from the Shaft Generator The following illustration shows a shaft design that was created using the Shaft Generator. Multiple features were added to the shaft sections to perform the tasks of rotating and locating the various parts that will be connected to the shaft. The features created in the Shaft Generator on this design include: Cylindrical diameter change Keyway Relief Chamfer Wrench flat Shaft Component Generator Dialog Box The Shaft Component Generator dialog box offers multiple tabs for designing and analyzing a shaft. You use the Design tab to create your shaft design, and the Calculation and Graphs tabs to analyze the shaft while you design. In addition to the dialog box, the graphics area displays a 3D shaft preview based on the current settings in the dialog box. You can also quickly modify each section of your shaft using the grips provided on the 3D preview. The following illustration shows the Shaft Component Generator dialog box and the interactive 3D shaft preview. The four default sections shown are provided when you first use the tool, enabling you to immediately begin your shaft design. Create Phase 15

16 16 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

17 Shaft Component Generator Dialog Box Options The following options are available for designing, locating, calculating, and saving shaft designs. Design tab: Design a shaft. Calculation tab: Set material, loads, and supports to calculate the shaft. Graphs tab: Display a diagram of shaft loads. Locate the shaft in the assembly. Define the axis, start plane, and orientation. Add cylinder, cone, or polygon sections to your shaft. Expand all, collapse all, or set options for the section tree display. Define each section of your shaft design. Specify shape and size of the section. Add features to each section. Drag to reorder a section. Specify the size of the base feature. Expand the Design tab and display information regarding each shaft section in a text format. Expand the Design tab to display calculation messages. Toggle the display of the Shaft Component Generator dialog box to display the Templates library. Create Phase 17

18 Creating Shafts The Shaft Component Generator tool creates a parametric shaft part using defined and consistent modeling methodologies. Because the shaft component is a parametric model, you can edit the model and add additional sketched or placed features, depending on your design needs. In the following illustration, a shaft is created using the Shaft Component Generator. Currently, a chamfer is being added to the first segment of the shaft. Sections Area You use the Sections area of the Shaft Component Generator dialog box to control the shape and size of each shaft segment and to add bores to either end of the shaft. The Sections drop-down list has three options to define the working location: Sections, Bore on the Left, and Bore on the Right. The default Sections option enables you to add multiple sections to the shaft. You can add cylinders, cones, and polygons. Each time that a section is added, a new row is added to the section tree for further refinement. The Bore on the Left and Bore on the Right options enable you to add internal shaft features. You can add cylindrical or conical bores to the shaft. Each time that a bore is added to the shaft, a new row is added to the section tree for further refinement. 18 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

19 The following options are available for designing shafts in the Sections area of the Shaft Component Generator. Section Tree The Section tree area of the Shaft Component Generator dialog box is where the majority of the shaft design is accomplished. Each Section tree row is broken down into four groups. After the shape and size of the shaft section have been specified, you can add features to the section based on options available from these four groups. The options that are available vary depending on the shape selected, and other options already applied. The following options are available in the Section tree of the Shaft Component Generator dialog box. Add features to the left edge of the shaft section. The features that are available depend on the type of section. Change the current section type. Add features to the right edge of the shaft section. The features that are available depend on the type of section. Add features to the shaft section. The features that are available depend on the type of section. Create Phase 19

20 Shaft Features The following features are available for shaft designs. 20 Shaft Feature Feature Description Cylinder Add cylindrical shaft sections. Cylinders are a base feature for shaft design. Additional features such as keyways, retaining rings, through holes, grooves, reliefs, and wrenches can be added to shaft sections. The length and diameter of cylinders can be changed using grips. Polygon Add a section to a shaft with three to 50 flat sides. Polygons are a base feature for shaft designs. Through holes can be added to polygon sections. Use grips to change the length and the section diameter, or to rotate the section. Cone Add a cone shape to a shaft. The cone is a base feature for shaft design. Fillets and chamfers can be added to cone sections. If the cone section is in between two other shaft sections, both cone diameters are initially controlled by the adjacent sections. To change either diameter of the cone section independently, you must first unlock the variables controlling their size using the Edit option. You can adjust the length and both diameters with grips. Cylindrical Bore Add cylindrical internal bores to the shaft. Internal bores can be added to either end of the shaft and can be used in conjunction with conical bores. You can adjust the length and diameter with grips. Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

21 Shaft Feature Feature Description Conical Bore Add conical bores to the shaft. Conical bores can be added to either end of the shaft and can be used in conjunction with cylindrical bores. The length and both diameters can be adjusted with grips. Chamfer Add an angle break at the end of shaft section, or provide a transition between two shaft sections. Chamfers are constructed using one of the following three methods: Distance, Distance and Angle, and Two Distances. Fillet Add a rounded shape to the outside edge of a shaft section, or provide a transition between different shaft sections. Lock Nut Groove Add a groove, chamfer, and optional thread to the end of a shaft. The Locknut Groove dialog box enables you to select from a list of standard locknut grooves, or you can define custom settings. Relief Define a relief in a shaft section. A relief is typically used to provide an undercut for clearance between two shaft sections. You select a relief from a series of standard shapes. The Relief dialog box enables you to define custom values for the relief selected. Create Phase 21

22 22 Shaft Feature Feature Description Keyway Groove Add a slot to a shaft section. A keyway groove is used to lock rotation between the shaft and added components. The keyway sizes are generated from industry standards. The Keyway dialog box enables you to create a custom keyway and to insert parts from the Content Center. Retaining Ring Add a square groove around the diameter of a cylindrical shaft section. The default sizes are based on industry standards. The Retaining Ring Groove dialog box enables you to enter custom values and insert parts from the Content Center. Wrench Opening Add a wrench opening to a cylindrical shaft section. This opening makes assembly of the shaft much simpler. The opening should match a standard wrench size. Through Hole Add a through hole to a cylinder or polygon shaft section, perpendicular to the centerline of the shaft. This hole can be used for inserting a pin. Groove Add a radius groove to a cylindrical shaft section. This groove can be used to locate an o-ring. Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

23 Exercise: Design a Shaft Design a Shaft In this exercise, you use Shaft Generator to design a shaft for a motor. 4. Click Resets Calculation Data. 5. The completed exercise 6. Place the Shaft in the Assembly Click OK. A shaft preview is placed in the assembly. The default dimensions are too large. You will modify the design in the following steps. On the ViewCube, click Home to view the motor and the shaft preview. Using the Shaft Generator, you create a shaft with two sections. On the first section, you add a keyway and a flat for a wrench. You size the second section for a thread and add a relief Make IFI_Unit5.ipj the active project. Open motor.iam Note: The shaft may not be placed in your assembly as shown. The final shaft design will still be the same. Under Sections, select Cone 100/ 65 x 100. On the Design tab, Power Transmission panel, click Shaft. Create Phase 23

24 4. For L, enter Click Yes. 10. Repeat this workflow to delete Cylinder 55 x Click OK. From the Second Edge Features list, select No Feature. Design the First Section on the Shaft 7. From the Section Features list, select Add Keyway Groove. The default dimensions are acceptable. 8. From the Section Features list, select Add Wrench. Click Feature Properties. 8. Click Delete Section and Features. In this section of the exercise, you add a keyway and a flat for a wrench Select Cylinder 50 x 100. Click Section Properties. For D, enter Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

25 10. Under Position, select Measure from Second Edge. 6. Zoom into the modified shaft design preview. 7. From the First Edge Features list, select No Feature. From the Second Edge Features list, select Chamfer. Click the check mark. From the Section Features list, select Add Relief - D (SI Units). 11. For x, enter For L, enter For W, enter Click OK. Design the Second Section on the Shaft In this section of the exercise, you size the second section for a thread and add a relief Select Cylinder 80 x 100. Click Section Properties. For D, enter 8. For L, enter Click Feature Properties. 11. For x, enter 0.4. Click OK. 12. Click OK twice to create the shaft. 5. Click OK. Create Phase 25

26 13. Click to place the shaft in the assembly Complete the Assembly You now add a thread to the shaft and constrain the shaft to the motor assembly. 1. In the browser, expand Shaft:1. Right-click Shaft:1. Click Edit. Click the Specification tab. Confirm that the thread designation is M8x1.25. Note: When you place a thread, Autodesk Inventor uses the diameter of the selected feature to determine the thread size. Click OK. On the Quick Access toolbar, click Return. On the ViewCube, click Home. On the Assemble tab, Position panel, click Constrain. 9. Under Type, click Insert. 10. Select the outside edge of the shaft. 11. Select the edge of the threaded hole. 2. On the Modify panel, click Thread. 3. Select the second section of the shaft. 26 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

27 12. Click OK. 13. Save the file. 14. Close the file. Create Phase 27

28 Build Phase Overview In this phase, students assemble a test stand that can be used to determine the specifications of a VEX motor. The completed test stand is shown in the following image. Phase Objectives After completing this phase, you will be able to: Test the specifications of a VEX motor. Build a simple winch. 28 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

29 Prerequisites and Resources Before starting this phase, you must have: Completed Unit 5: Speed, Power, Torque, and DC Motors > Think Phase. Disassembled Protobot used in Unit 3: Building a Protobot. Related phase resources are: Unit 1: Introduction to Vex and Robotics. Unit 3: Building a Protobot. Required Supplies and Software The following supplies are used in this phase: Supplies One disassembled Protobot used in Unit 3: Building a Protobot Notebook and pen Work surface Small storage container for loose parts Optional: Autodesk Inventor Professional 2010 Required VEX parts The following VEX parts are required in this phase: Quantity Part Number Abbreviation 4 BEAM-1000 B1 2 BEAM-2000 B2 1 BEARING-BLOCK BB 3 BEARING-FLAT BF 1 DRIVE-WHEEL TS 1 DRIVE-WHEEL TS15 6 NUT-832-KEPS NK Build Phase 29

30 Quantity Part Number Abbreviation 2 SCREW SS4 8 SCREW S2 4 SCREW B4 6 SCREW S6 1 SHAFT-4000 SQ4 2 SHAFT-COLLAR COL 2 SPACER-THICK SB2 12 SPACER-THIN SP1 15 TANK-TREAD-LINK TL 1 VEX - MOTOR MOT 1 VL-CHAN RevA C15 2 VL-CHAN RevA CW25 Activity VEX Motor Test Stand In this activity, you assemble a test stand that will be used to test the specifications of a VEX motor. This stand will be reused in later units for further activities. 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 30 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

31 1. Wrap fifteen Tank Tread Links [TL] around a Tank Tread Sprocket [TS] and fasten together. Build Phase 31

32 2. 32 To complete the next step: Bolt two 2" long beams [B2] to the 1x2x1x15 C-Channel [C15]. Bolt a Motor [MOT] and a Bearing Flat [BF] to the 1x2x1x15 C-Channel. Insert the 4" Shaft [SQ4] through the bearing into the motor. Slide two Collars [COL] onto the shaft and leave them loose. Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

33 3. To complete the next step: Bolt a Bearing Flat [BF] to the 1x5x1x25 C-Channel [CW25]. Bolt 1x5x1x25 C-Channel to the assembly completed in the previous step. Adjust the two Collars up against each Bearing Flat. Build Phase 33

34 4. 34 To complete the next step: Attach four 1 long beams [B1] to the corners of the 1x5x1x25 C-Channel. Attach the rope guide [BB] to the 1x5x1x25 C-Channel. Slide the assembly from Step 1 onto the 4" Shaft. Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

35 5. To complete the next step: Bolt a Bearing Flat to the second piece of 1x5x1x25 C-Channel. Bolt the two 1x5x1x25 C-Channel pieces together. Build Phase 35

36 6. 36 Your test stand is now complete and ready for use! Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

37 Amaze Phase Overview In this phase, students use their VEX motor test stand to determine the specifications of a VEX motor. Phase Objectives After completing this phase, you will be able to: Determine the free speed of a motor. Calculate the stall torque of a motor. Describe the typical free speed and stall torque of a VEX motor. Prerequisites and Resources Before starting this phase, you must have: Completed Unit 5: Speed, Power, Torque, and DC Motors > Think Phase. Completed Unit 5: Speed, Power, Torque, DC Motors > Build Phase. Have one assembled VEX motor test stand from the Unit 5: Speed, Power, Torque, and DC Motors > Build Phase. Related phase resources are: Unit 1: Introduction to VEX and Robotics. Unit 4: Microcontroller and Transmitter Overview. Required Supplies and Software The following supplies are used in this phase: Supplies One VEX Microcontroller One assembled VEX motor test stand from the Unit 5: Speed, Power, Torque, and DC Motors > Build Phase One VEX Transmitter One 7.2V VEX Battery Notebook and pen Amaze Phase 37

38 Supplies Work surface One Stopwatch 36 of 1/8 braided nylon and polyester cord, or equivalent rope/string Set of masses or other weights One zip tie Evaluation Free Speed and Stall Torque Challenge Using the VEX motor test stand, determine the free speed and stall torque of a VEX motor. Determine Free Speed 1. Attach a zip tie to the 4 shaft between the tank tread pulley and the motor. The zip tie serves as a visual reference when counting motor revolutions. 2. Plug the motor into port 6 of your Microcontroller. 3. Plug the Transmitter into port Rx1 of the Microcontroller. 4. Plug the 7.2V battery into the Microcontroller. 5. Turn both your Transmitter and Microcontroller on. 6. Using the yellow buttons on the back of the Transmitter, you should be able to control the rotation of the pulley. 7. Perform five tests, where you run the test stand for one minute. 8. During each test, count the number of times the zip tie passes a reference point. 9. For each test, record this number in a chart in your engineering notebook. 10. Calculate the average of your five tests. This number is your experimental determination of the free speed of a VEX Motor in RPM. 38 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

39 Determine Stall Torque Remove the zip tie that was attached in the Free Speed section. Wrap the rope around the pulley, and pass the end of it through the rope guide. See Figure Stall torque is the amount of torque required to prevent the motor from spinning. Torque is defined as: Torque = Force x Radius Where force is the force of gravity downwards measured in pounds (lbs.), radius is the radius of the rotating object measured in inches (in.), and torque is the torque on the motor measured in inch pounds (in-lbs.). 4. To determine the stall torque, you need to determine what force or weight will prevent the motor from spinning. To do so, you load the test stand with a weight, and keep increasing it until the motor can no longer spin. 5. Set up the test between two tables, with the string hanging between them. 6. Attach an initial weight of approximately 3 lbs. to the string. 7. Using the Transmitter, rotate the pulley and lift the weight. 8. Increase the weight attached to the string by approximately 4 oz. and try and rotate the pulley. 9. Continue increasing the weight until the motor can no longer rotate. 10. Record this amount of weight in your engineering notebook. 11. Measure the radius of the pulley. 12. Now using the force in pounds and the radius of the pulley in inches, you can calculate the stall torque of the motor in inch-pounds. Amaze Phase 39

40 Engineering Notebook The actual free speed and stall torque of a VEX motor are approximately 100 rpm and 6.5 in-lbs. Do your results match these? Explain what factors could cause a variance between the actual results, and the results you obtained experimentally. How could these tests be improved to ensure a greater degree of accuracy? Presentation Prepare your findings from this phase, and present them to the class. Specifically talk about your recommendations to improve the accuracy of tests. If your results varied a great deal from the actual data, explain why you think this happened. 40 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors

41 STEM Connections Background Amusement park rides, such as roller coasters and drop rides, are great examples of how the principles of physics apply to the world around us. The following STEM Connection relates to a free fall amusement park ride where passengers are loaded into a car, pulled vertically to the top of a tall structural tower, and then dropped with the forces of gravity hurling them back to the ground. Science Some amusement park rides pull riders slowly to the top of a large tower, and then send them plummeting in a free fall to the bottom again. This can be achieved with a large motor that rotates a shaft with a pulley and cable attached. Why is this big drop so thrilling? Why don't you feel the same rush flying across the country in an airplane, even though the plane moves much faster than the ride? Does your body feel speed? Does it feel acceleration? As you picture an amusement park ride in which you are pulled to the top and then do a feel fall, explain how the following physics concepts come into play: power, speed, acceleration, energy, force, and work. STEM Connections 41

42 Technology One of the biggest concerns of an amusement park ride designer is safety. Based on your understanding of speed, acceleration, power, and forces, what safety features would you want to make sure are incorporated into the ride? The materials used in the construction of a free fall amusement park ride play a major part in maintaining safety. What materials and construction methods do you think would be optimal for the design of this type of ride? Engineering Is it possible to get a lighter load (fewer people) to the top faster than a maximum load? If you move a lighter load of people to the top at a faster speed, what happens with motor torque? When attempting to lift a load to the top of a drop ride, when might you reach stall torque? Why would an amusement park designer want to avoid a situation involving stall torque? What is happening to the electrical current being drawn by the motor as it draws the load from the bottom to the top of the ride? What might happen to the motor if you were to reach stall torque? Can you think of some type of warning system that can prevent the possibility of reaching stall torque? Can you think of a safety backup if the motor were to fail while pulling the load to the top? Math Given equation: Force = Mass * Acceleration. For example, a force of 12 N (that is, Newtons) could accelerate a 3 kg mass at 4 m/s2, because 12=3*4. Or, it could accelerate a 6 kg mass at 2 m/s2, because 12=6*2. When you apply the same force to double the mass, you get half the acceleration. Suppose you were building a drop ride, and you want it to lift 16 people at a time. If we plan for an average mass of 95 kg per person, how much force would the ride's motor need to apply in order to exactly counteract acceleration due to gravity (9.8 m/s2)? How much force to accelerate the riders upward at 3 m/s2? (Note: your answer to the second question should be greater than your answer to the first. Think about why this might be, and discuss with a teammate.) Final question: If 16 football players (average mass: 140 kg) decide to try the drop ride, how fast can the ride accelerate them upwards with a force of N? Remember to account for gravity. 42 Autodesk's VEX Robotics Unit 5: Speed, Power, Torque, and DC Motors