2. Robotics Intensive: Day 6 Gui Cavalcanti 1/17/2012

3. Class Goals 1.Electric motor theory and selection 2.Team integration meeting (30-45 minutes) 3.Preliminary Design Review 4.Continued work

4. Electric Motors

5. Motor are electromechanical transducers – Convert voltage (V) into rotational speed – Convert amperage (A) into torque

6. Torque Rotational force – Linear force multiplied by a radius

7. 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

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

9. 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

10. 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

11. Motor Value Relationships To the whiteboard!

12. Motor Selection 1.Application Similarity 2.Continuous Power Capacity 3.Maximum Speed Required 4.Maximum Torque Required 5.Mission Profile/Thermal Capacity

13. 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?

14. 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

15. 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

16. 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

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

18. Break for Integration Work

19. Last Classes Tasks

20. 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

21. 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?

22. Mission Profile – Vending Machine 1.Drive full circle around the space with no customers (30 min) 2.Pause, keep vending (2.5 hrs) 3.Drive around space, stop halfway (15 min) 4.Wait for 3 customers (10 min) 5.Keep driving (10 min) 6.Encounter obstacle, pause (5 min) 7.Go back home the long way (15 min) 8.Pause, keep vending (2.5 hrs)

23. Preliminary Design Review

24. 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