Transcript Document
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
Structure
Geometry
Material
Examples
Drivetrain
Wheels
Motors
Transmissions
Examples
fear
Chassis Design
Review principles of chassis design
Examine trade-offs
Material
Weight
Chassis Function
Provide platform for everything
Strong
Stable
Well laid out and accessible
Light
Resist, defend against shock
Weight
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
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
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
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 MotorRPM* Gear Ratio* Speed Loss
Robot Velocity Output RPM
* WheelCircumferance* Unit Conversion
Remember Units!!!
Be Careful!
Is this fast enough?
Major Design Compromise…
Is this speed fast enough?
No?
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 T orque
MotorT orque - (
) * MotorRPM Stall T orque
FreeSpeed
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 T orque MotorT orque* 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
* T orqueLoad Free Current
Stall T orque
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?