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Robotics Intensive: Day 6

Robotics Intensive: Day 6. Gui Cavalcanti 1/17/2012. Class Goals. Electric motor theory and selection Team integration meeting (30-45 minutes) Preliminary Design Review Continued work. Electric Motors. Electric Motors. Motor are electromechanical transducers

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Robotics Intensive: Day 6

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  1. Robotics Intensive:Day 6 GuiCavalcanti 1/17/2012

  2. Class Goals • Electric motor theory and selection • Team integration meeting (30-45 minutes) • Preliminary Design Review • Continued work

  3. Electric Motors

  4. Electric Motors • Motor are electromechanical transducers • Convert voltage (V) into rotational speed • Convert amperage (A) into torque

  5. Torque • Rotational force • Linear force multiplied by a radius

  6. General Motor Principles • High speed, low torque devices • Require significant gearing to produce low speeds/high torques • Useful operating rotational velocities (joints, wheel velocities, etc.) are between 50 and 1,000 rpm • Typical motor velocities are 4,000 to 20,000 rpm

  7. Gearing • Method of changing both torque and speed in equal proportion

  8. General Motor Definitions • Stall Torque: Torque generated when the motor is not moving, at a given voltage • Free Speed: Speed generated when the motor is at top speed, at a given voltage • No-Load Current: Current demanded by the motor at free speed, at a given voltage • Maximum Power: Power generated at 50% stall torque, 50% free speed. Generally only possible for short times. • Efficiency: The amount of mechanical power the motor produces, divided by the electrical power put in to the system

  9. Motor Value Relationships • Linear relationship between stall torque (0 speed, Max torque) and free speed (Max speed, 0 torque) • Mechanical Power = Speed * Torque • Maximum Mechanical Power = 50% Max Speed * 50% Max Torque • Electrical Power = Voltage * Current • Maximum Electrical Power = Nominal Voltage * Maximum Current • Maximum Efficiency around 10-20% Max Torque, 80-90% Max Speed

  10. Motor Value Relationships • To the whiteboard!

  11. Motor Selection • Application Similarity • Continuous Power Capacity • Maximum Speed Required • Maximum Torque Required • Mission Profile/Thermal Capacity

  12. Application Similarity • What is the motor you’re buying used for? • Powering a competition BattleBot? • Powering a wheelchair? • Powering a small hand drill? • Closing a van door?

  13. Continuous Power Capacity • Calculate continuous power use • Continuous Power = Force * Velocity • Continuous Power = (Rolling Resistance + Linear Resistive Force) * Speed • Make sure motor is near its maximum efficiency at the speed desired

  14. Maximum Speed Required • Assume your robot will somehow short circuit the motor to the battery • Never allow the robot to move faster than walking speed (around 3mph) if this happens

  15. Maximum Torque Required • Calculate the maximum torque the robot is likely to expect, make sure it can be provided easily • Stopping torque for vending machine • Pushing torque for vacuum cleaner

  16. Mission Profile/Thermal Capacity • How does an electric motor fail?

  17. Break for Integration Work

  18. Last Classes Tasks

  19. Conceptual Design • Conceptual design: what are the big ideas? What can we set in stone? • By the start of next class, each team must have the following done: • Mission profiles • Expected robot dimensions and weight • Gross component placement • All major components selected • Mechanical, electrical, sensors

  20. Mission Profile • What: Representative average description of everything the robot does between ‘off’ periods • Why: To lay the groundwork for generating power system requirements • How: Ask yourself questions • What will the robot do between charges? • How long will these actions take? • What could go wrong to delay the robot?

  21. Mission Profile – Vending Machine • Drive full circle around the space with no customers (30 min) • Pause, keep vending (2.5 hrs) • Drive around space, stop halfway (15 min) • Wait for 3 customers (10 min) • Keep driving (10 min) • Encounter obstacle, pause (5 min) • Go back home the long way (15 min) • Pause, keep vending (2.5 hrs)

  22. Preliminary Design Review

  23. Presentation • Present your mission profile • Discuss your robot’s size and weight, and how you estimated those values • Present nice-to-have features in order of priority • Present continuous mechanical power, limiting values and your motor selection (with matrix) • Present electrical power draw and batteries selected (with matrix) • Present thoughts on control system design • Problems and questions

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