Robot Chassis and
Drivetrain Fundamentals
Andy Baker, Team 45
John Neun, Team 20
2006
I am not John
V-Neun (sorry!)
John Neun
Senior Development
Engineer
Albany International
Mentor on team 20,
the Rocketeers
Andy Baker
 TechnoKats team leader (#45)
 Sr. Mechanical Engineer: Delphi
Corporation
 Co-Owner: AndyMark, Inc.
(www.andymark.biz)
 2003 Championship Woodie Flowers
Award Winner
What is most important?
1. Drive Base
2. Drive Base
3. Drive Base*
* - stolen from Mr. Bill Beatty (team 71)
Objectives
Review “Base” Design
 Chassis
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Structure
Geometry
Material
Examples
 Drivetrain

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Wheels
Motors
Transmissions
Examples
fear
Chassis Design
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Review principles of chassis design
Examine trade-offs
Material
Weight
Chassis Function
 Provide platform for everything

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Strong
Stable
Well laid out and accessible
Light
 Resist, defend against shock
Weight
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Develop a weight budget and stick to it!
Start coarse: chassis = 60 lbs, tower = 60 lbs
Tip: parts far from the floor should be the lightest
Refine:
 ie Chassis



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Frame
Wheels
Gearbox
Controls
 Trade-off
 How many ½ inch diameter holes in .100 Al are needed
for 1 pound?
200!
CG
 Keep it Low!!
d
spread
sheet
Given the will, any
configuration can work
Geometry
 Strength
 Space
 Accessibility
Example
Bumpers
Kit Chassis
(pictures available at www.innovationfirst.org)


Advantages: lightweight, quick to build, uses standard parts
Disadvantages: may not fit your design, requires added structure (that will
most likely be put on anyway)
T-slot style
 Advantages: quick to build, standard parts, easy to create tension
and to add fastening points
 Disadvantages: heavy, expensive
Welded Aluminum Tube &
Plate
 Advantages: lightweight, strength, fits your design
 Disadvantages: takes time, requires skill, non standard parts
Unique Drive Bases
 Advantages: fits your design, unique
 Disadvantages: takes much time, requires skill, non standard parts
Chassis Materials
 Aluminum Extrusion
 1/16” – 1/8”: usable but will dent and bend
 T-slot: use 1” sized profiles or higher
 Aluminum plates and bars
 3/16” – ¼” used often
 Plastic Sheet
 Spans structures, provides bracing
 Polycarbonate (LEXAN, etc.) NOT Acrylic (Plexiglas, etc.)
 Wood
 Lightweight and easy to use
 Will splinter and fail but can be fixed
 Steel Tube and Angle
 Strong, but heavy, 1/16” wall thickness is plenty strong
luck
Drivetrain Design
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Review basics
Examine trade-offs
Formulas for modeling and design
Sample Calculations
Drivetrain: #1
 What must the robot do?
 Speed
 Force
 Maneuverability
 Game rules and team strategy: set specs
Drivetrain Foundation
Basics
 Physics




Force = mass x acceleration (pounds)
Frictional force = constant x Normal force
Torque = force x distance (foot-pounds)
Power = force x velocity (HP, watts)
= amps x volts
 Work = power x time (HP-hour)
 Efficiency = (power out)/(power in)
 Principles of DC Motors
 Principles of Gear Trains
 Reduction
 Mechanical advantage
Wheels
 Provide contact with ground

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Drive
Traction
Steering
Support and stability
Wheel Friction
 Theory: F = kN
 Frictional force has no dependence on contact
area
 HOMOGENEOUS, 2 dimensional surfaces
 Drive direction vs. lateral friction
N
F
Steering wheels
 “Car steering:” complex
 “Tank steering:” simple
 Wheels skate
Tank
Steering




Hi CG
Short wheelbase
“Bouncy” wheels
Solutions:
 Smaller Dia. Wheels
 Use wider Frame (see Chris
Hibner’s white paper on
www.chiefdelphi.com)
 Use Omni-wheels
(www.andymark.biz)
6 Wheel Drive
Teams can purchase these
treaded wheels at…
www.andymark.biz
www.innovationfirst.com
Crab or Swerve Steering
Tank Tread Drive
Fall Over Drive Bases
Motors
 Fixed population of choices
 Range of speed and torque
 Specifications readily available
 DC motors with speed controlled via PWM
 Last year’s motors:
Use these numbers, but DON’T
assume they are all true. For
instance, the Fisher-Price motor
could not be operated at 12
volts, and was later
recommended to run at 6 volts.
Max Motor Load





TL = Torque from load
IM = Maximum current draw (motor limit)
Ts = Stall torque
IF = Motor free current
IS = Motor stall current
Calculate the Max Motor Load
Torque = Stall torque - {speed x (stall torque/free speed)}
Current Draw vs. Load Torque
1 Chiaphua Motor
120
Motor Current Draw (Amp)
stall
100
80
60
40
20
0
0
0.5
1
1.5
Load Torque (N*m)
2
2.5
Free
speed
Gearbox Design Process
First, choose “Motion” Objective: Robot Speed 13 fps, full speed within 10 feet
•Pick motor
•(load vs amps)
•Pick wheel config.
•no. of wheels
•material
•diameter
Calculate required gear
ratio from motor and
output torques
•Motor running
characteristics
Max torque per
current limit
Calculate speed
& acceleration
Running characteristics
Current limits
•Determine maximum
drive train load from
“wall push”
Iterate
Transmission Goal: Translate
Motor Motion and Power into
Robot Motivation
 Motor
 Speed (rpm)
 Torque
 Robot
 Speed (fps)
 Weight
First Step:
Pushing against a wall…
 Objective: Determine maximum load limit
(breakaway load for wheels)
 System must withstand max load




Run continuously under maximum load
Not overload motors
Not overload circuit breakers
(Not break shafts, gears, etc.)
 Suboptimum – ignore limit (risk failure)
Pushing against a wall…
 Known Factors:
 Motor Usage
 Motor Characteristics
 Wheel Friction
 Max Motor Load (at 40 amps)
 Solve For:
 Required Gear Ratio
Robot Weight
Motor specs
Frictional coef.
Gear Ratio
Speed
acceleration
Calculate the Gearbox Load
Find Required Gearbox Ratio
 Friction between wheel
and carpet acts as a
“brake”, and provides
gearbox load.
 Find torque load per
gearbox.
Frictional
 Now Solve for Required force
Gear Ratio
Gearbox Load
Gear Ratio 
Motor Max Load
Weight
no. of wheels
Check Robot Speed
 How fast will the robot go with this
required gear ratio?
Output RPM  Motor RPM * Gear Ratio * Speed Loss
Robot Velocity  Output RPM
* Wheel Circumfera nce * Unit Conversion
 Remember Units!!!
Be Careful!
Is this fast enough?
 Major Design Compromise…
 Is this speed fast enough?
 No?


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
Decrease Gearbox Load
Increase Gearbox Power
Live with the low speed…
Design two speeds!
 Low speed/high force
 High speed/low force
 Risk failure
 Design is all about tradeoffs
Secondary Analysis
Plotting Acceleration
 Calculate Motor Current Draw and Robot
Velocity over time (during robot acceleration).
 Time to top speed
 Important to show how drivetrain will perform (or
NOT perform!)
 If a robot takes 50 feet to accelerate to top speed, it
probably isn’t practical!
 Performance on flat floor is VASTLY different on a
ramp (2003 example)
Plotting Acceleration
 Voltage to resting motor
 Start at stall condition (speed = 0)
 Stall torque  initial acceleration
 Robot accelerates
 Motor leaves stall condition
 Force decreases as speed increases.
Instantaneous Motor Torque
Stall Torque
Motor Torque  - (
) * Motor RPM  Stall Torque
Free Speed
 When Motor RPM = 0,
Output Torque = Stall Torque
 When Motor RPM = free speed
Output Torque = 0 (in theory)
 (.81)
Gearbox (reduction) basics
 Chain, belt
 Gear Ratio = N2/N1
N2
N1
 Spur gears
 Gear Ratio = N2/N1
N1
N2
Gearbox Torque Output
Robot Accelerating Force
Gearbox Torque  Motor Torque * Gear Ratio * Efficiency
Gearbox Torque
Accelerati on Force  2 * (
)
Wheel Radius
Instantaneous
Acceleration and Velocity
Accelerati on Force - Friction Resistance
Accelerati on 
Robot Mass
 Instantaneous Acceleration (dependant on
robot velocity, as seen in previous equations).
 The instantaneous velocity can be numerically
calculated as follows:
V2  V1  1 * (t)
(thanks, Isaac)
Velocity vs. Time
 The numerical results can be plotted, as
shown below (speed vs. time):
Robot Velocity vs. Time
8
Robot Velocity (ft/s)
7
6
5
4
3
2
1
0
0
0.5
1
1.5
2
2.5
Tim e (s)
3
3.5
4
4.5
5
Current Draw Modeling
 The current drawn by a motor can be
modeled vs. time too.
 Current is linearly proportional to torque
output (torque load) of the motor.
Stall Current - Free Current
Current Draw 
* Torque Load Free Current
Stall Torque
Current Draw vs. Time
 The numerical results can be plotted, as
shown below:
Gearbox Current Draw vs. Time
250
Current Draw (Amp)
200
150
100
50
0
0
1
2
3
Time (s)
4
5
It’s just a
little volts
& amps
What does this provide?
 Based on these plots, one can see how
the drivetrain will perform.
 Does current draw drop below “danger”
levels in a short time?
 How long does it take robot to accelerate
to top speed?
Are things okay? NO?!?
 How can performance be increased?
 Increase Drivetrain Power
 Use Stronger Motors
 Use Multiple Motors
 Increase Gear Ratio (Reduce top speed)
 Is this acceptable?
Adding Power – Multiple
Motors
 Combining Motors Together – Not Voodoo!
 2 Motors combine to become 1 “super-motor”
 Match motors at free speed
 Matching does not have to be exact
 Sum all characteristics
 Motor Load is distributed proportional to a ratio of free
speed.
 2 of the same motor is easy!
 4 Chiaphua Motors
Multiple Speed Drivetrains
 Allows for multi-speed setup using max
motor power:
 1 “pushing” speed & 1 “cruising” speed
 1 “cruising” speed & 1 “very fast” speed
 Shift-on-the-fly allows for accelerating
through multiple gears to achieve high
speeds.
 Shifting optimizes motor power for
application at hand.
 www.andymark.biz sells 2-speed
transmissions for FIRST applications.
Take necessary
precautions
The big picture…
 These calculations are used to design a
competition drivetrain.
 Rather than do them by hand, most
designers use some kind of tool.
 Excel Spreadsheet
 Matlab Script
 Etc…
And then…
 This is a starting point
 Iterate to optimize results
 Test
 Use your imagination




Infinite speeds
Multiple motors
Many gears
This isn’t the “end all” method.
Gearbox Design Process
Set “Motion” Objective: Robot Speed 13 fps, full speed within 10 feet
•Pick motor
•(load vs amps)
•Pick wheel config.
•no. of wheels
•material
•diameter
Calculate required gear
ratio from motor and
output torques
•Motor running
characteristics
Max torque per
current limit
Calculate speed
& acceleration
Running characteristics
Current limits
•Determine maximum
drive train load from
“wall push”
Iterate
Automation
 Spreadsheet to do drivetrain design at
 www.team229.org
Calculation Example
Peak
Power
(W)
Free
Speed
(RPM)
Stall Torque
(N*m)
Stall Current
(Amp)
Free
Current
(Amp)
321
5500
2.22
107
2.3
407
24000
.647
148
1.5
FP w/Gearbox
407
193
80
148
1.5
124:1
Globe Motor
(With Gearbox)
50
100
19
21
.82
117:1
Van Door Motor
69
75
35
40
1.1
22
92
9.2
24.8
3
18.5
85
8.33
21
3
Motor Name
Atwood Chiaphua
Motor
Fisher Price Johnson (2005)
(No Gearbox)
Nippon Window
Motor (2002)
Jideco Window
Motor (2005)
Gearbox
Ratio
Remember:
It’s no big deal!
Thanks!
“Robot System Drive
Fundamentals”
Ken Patton
Paul Copioli
Questions?