Robotics Intensive:Day 6

Gui Cavalcanti1/17/2012

Class Goals1. 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,

80-90% Max Speed

Motor Value Relationships

• To the whiteboard!

Motor Selection

1. Application Similarity2. Continuous Power Capacity3. Maximum Speed Required4. Maximum Torque Required5. 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 Machine1. 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