Base Fundamentals
Beach Cities Robotics – Team 294
Andrew Keisic
June 2008
Sources
Copioli and Patton’s “Robot Drive Systems Fundamentals”
presentation
2 - Introduction / Agenda
Topics
Center of Gravity
Types of Drive Trains
Maximizing Design
Motor Performance
Gear Ratio Calculation
3 - Introduction / Agenda
Center of Gravity
A point in space where gravity acts
Why it’s important?
Determines the balance and stability of an object
Center of Gravity
Stability - what ball is the most stable? the least?
Center of Gravity
What robot is the most stable? The least?
How do you know?
What systems are inherently stable?
Center of Gravity
Putting math behind intuition
Stability Triangle
α2
h
α1
 b1 
 1  tan  
h
1
 b2 

h
 2  tan 1 
b1
b2
Center of Gravity
Limit of stability is determined by the CG location
In other words – the maximum ramp
angle of a stationary robot
α2
α1
β1
b 
1  1  tan  1 
h
1
β2
 b2 

h
 
 2   2  tan 1 
Center of Gravity
Why keep it low?
Lowering the center of gravity maximizes alpha!
Stability Triangle
α2
h
α1
b1
b2
Center of Gravity
BCR 2008 FRC initial CG estimate
Type of Bases
Type of Bases
Drive train configurations
simple
rear wheel drive
simple
front wheel drive
simple
all wheel drive
tracked drive
swerve/
crab drive
other?
simple
center drive
6 wheel
drive
There is no “right”
answer!
Types of Bases
simple
rear wheel drive
Type of Bases
simple
front wheel drive
Types of Bases
simple
all wheel drive
Type of Bases
simple
center drive
Types of Bases
6 wheel
drive
Types of Bases
tracked drive
swerve/
crab drive
other?
Maximizing Design
Designing is all about tradeoffs
Speed vs torque
Low CG vs reaching high
Weight vs features
Control vs power
Maximizing Design: Motor Performance
Maximizing Design: Motor Performance
Maximizing Design
Requirements
Before designing a system, we must know what it needs to do
The design requirements usually stem from the game
Strategy plays a big part in the requirements
Decide the requirements as a team
For competitive robots, torque is always needed
We’re going to design for maximum torque – pushing ability
Note: Slide from Copiloi & Patton presentation
Traction Fundamentals: “Normal Force”
weight
normal
force
(rear)
front
normal
force
(front)
The normal force is the force that the wheels exert on the floor, and is equal
and opposite to the force the floor exerts on the wheels. In the simplest case,
this is dependent on the weight of the robot. The normal force is divided
among the robot features in contact with the ground.
Note: Slide from Copiloi & Patton presentation
Traction Fundamentals
weight
tractive
force
torque
turning the
wheel
maximum
tractive
force
=
friction
coefficient
x
normal
force
normal
force
The friction coefficient for any given contact with the floor, multiplied by
the normal force, equals the maximum tractive force can be applied at
the contact area.
Tractive force is important! It’s what moves the robot.