Transcript Session 6

Robotics Intensive:
Day 6
Gui Cavalcanti
1/17/2012
Class Goals
1. Electric motor theory and selection
2. Team integration meeting (30-45 minutes)
3. Preliminary Design Review
4. Continued work
Electric Motors
Electric Motors
• Motor are electromechanical transducers
– Convert voltage (V) into rotational speed
– Convert amperage (A) into torque
Torque
• Rotational force
– Linear force multiplied by a radius
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
Gearing
• Method of changing both torque and
speed in equal proportion
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
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, 8090% Max Speed
Motor Value Relationships
• To the whiteboard!
Motor Selection
1. Application Similarity
2. Continuous Power Capacity
3. Maximum Speed Required
4. Maximum Torque Required
5. Mission Profile/Thermal Capacity
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?
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
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
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
Mission Profile/Thermal Capacity
• How does an electric motor fail?
Break for Integration Work
Last Classes Tasks
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
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?
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)
Preliminary Design Review
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