1. Robot Chassis and Drivetrain Fundamentals
Andy Baker, Team 45
John Neun, Team 20
Thank you for taking the time to listen to us talk about robot transmissions.
My name is John Neun, and this is my son John V-Neun.
I have been an engineer for about 30 years now, and it is a great and rare opportunity to give a technical presentation with my son. I went to Rensselaer Polytechnic Institute, the oldest civilian engineering school in the US. John is about to graduate from Clarkson University, and I am now suffering my new identity, “John V-Neun’s Dad.”
I’ll do a somewhat formal presentation of some material, John will do a spreadsheet demo, and there should be plenty of time for discussion either during or after.
2006

2. I am not John V-Neun (sorry
I am not John V-Neun (sorry!) John Neun Senior Development Engineer Albany International Mentor on team 20, the Rocketeers

3. Andy Baker TechnoKats team leader (#45)
Sr. Mechanical Engineer: Delphi Corporation
Co-Owner: AndyMark, Inc. (
2003 Championship Woodie Flowers Award Winner

4. What is most important? Drive Base Drive Base*
* - stolen from Mr. Bill Beatty (team 71)

5. Objectives Review “Base” Design Chassis Drivetrain Structure Geometry
Material
Examples
Drivetrain
Wheels
Motors
Transmissions

6. fear

7. Chassis Design Review principles of chassis design Examine trade-offs
Material
Weight
We would like to build on the presentation on drive system fundamentals done by the now two Woody Flowers award winners, Ken Patton and Paul Capioli. Essentially, we will add a little mathematics to the process after reviewing some basic principles, and then present a tool in the form of a spreadsheet to implement these ideas. I’ll review some slides outlining the physics, and John will do an example with his spreadsheet. In case you haven’t figured this out, he is the real brains behind this operation.

8. Chassis Function Provide platform for everything
Strong
Stable
Well laid out and accessible
Light
Resist, defend against shock

9. 200! 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!

10. CG d
Keep it Low!!
spreadsheet

11. Given the will, any configuration can work

12. Geometry
Strength
Space
Accessibility

13. Example

14. Bumpers

15. 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)

16. T-slot style
Advantages: quick to build, standard parts, easy to create tension and to add fastening points
Disadvantages: heavy, expensive

17. Welded Aluminum Tube & Plate
Advantages: lightweight, strength, fits your design
Disadvantages: takes time, requires skill, non standard parts

18. Unique Drive Bases Advantages: fits your design, unique
Disadvantages: takes much time, requires skill, non standard parts

19. Chassis Materials Aluminum Extrusion Aluminum plates and bars
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

20. luck

21. Drivetrain Design Review basics Examine trade-offs
Formulas for modeling and design
Sample Calculations
We would like to build on the presentation on drive system fundamentals done by the now two Woody Flowers award winners, Ken Patton and Paul Capioli. Essentially, we will add a little mathematics to the process after reviewing some basic principles, and then present a tool in the form of a spreadsheet to implement these ideas. I’ll review some slides outlining the physics, and John will do an example with his spreadsheet. In case you haven’t figured this out, he is the real brains behind this operation.

22. Drivetrain: #1 What must the robot do?
Speed
Force
Maneuverability
Game rules and team strategy: set specs

23. 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
Paul and Ken revieweed all of thi stuff, and I will not presume to teach you about torque, power, or Newtonian laws of motion. I suspect you all understand this stuff at least as well as I do.

24. Wheels Provide contact with ground Drive Traction Steering
Support and stability

25. Wheel Friction Theory: F = kN Drive direction vs. lateral friction
Frictional force has no dependence on contact area
HOMOGENEOUS, 2 dimensional surfaces
Drive direction vs. lateral friction N F

26. Steering wheels “Car steering:” complex “Tank steering:” simple
Wheels skate

27. Tank Steering Hi CG Short wheelbase “Bouncy” wheels Solutions:
Smaller Dia. Wheels
Use wider Frame (see Chris Hibner’s white paper on
Use Omni-wheels (

28. 6 Wheel Drive Teams can purchase these treaded wheels at…

29. Crab or Swerve Steering

30. Tank Tread Drive

31. Fall Over Drive Bases

32. 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.

33. Max Motor Load TL = Torque from load
IM = Maximum current draw (motor limit)
Ts = Stall torque
IF = Motor free current
IS = Motor stall current
What about a motor determines its limits? A DC motor has two ultimate points of operation. The first is at zero rpm, a stall condition. The second is its free speed, the speed at which theoretically no load is applied to the shaft. At stall, motor load is maximum, and at free speed, motor load is minimum. Of course, there is no such thing as “no load.” There is always some power consumed to overcome windage, friction, and other parasitic effects. Thus, there is some current associated with this free speed. You can simply assume a straight line between the two. If there is a current limit imposed by a system constraint like, for example, a circuit breaker, you can calculate the torque associated with that limit with the assumed line between stall and free speed with this simple interoplative equation.

34. Calculate the Max Motor Load
Torque = Stall torque - {speed x (stall torque/free speed)}
stall
Free speed
Here is a simple illustration of these concepts. These lines are not really perfectly straight, but for our purposes, the assumption is adequate.

35. 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
Motor running
characteristics
Max torque per
current limit
Determine maximum
drive train load from
“wall push”
The first thing to do is to choose the “motion” objective. This really means the you need to understand how fast and how strong you want your robot to go. Next, you need to choose both ends of the drive train. That is, the motor or these days motors, and the “ground effector.” Somewhere between the wheels and the motor, there’s usually a gearbox or transmission. This process characterizes each of those, and then develops a means to reconcile and connect the two. I’ll review these steps.
Calculate required gear
ratio from motor and
output torques
Calculate speed
& acceleration
Running characteristics
Current limits
Iterate

36. Transmission Goal: Translate Motor Motion and Power into Robot Motivation
Speed (rpm)
Torque
Robot
Speed (fps)
Weight
Therefore, a gearbox or transmission is a device to translate motor motion into wheel and ultimately robot motion. With some basic parameters or assumptions about each of those, you can design a good transmission.

37. 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)
How do you characterize this stuff? First, determine the limit of the motor. This sets the bounds of the system.

38. Pushing against a wall…
Known Factors:
Motor Usage
Motor Characteristics
Wheel Friction
Max Motor Load (at 40 amps)
Solve For:
Required Gear Ratio
“Pushing against a wall” implies determination of the ultimate stall load of the motor, and the maximum load that the robot can impart on the motor. The first is a function of the motor. The second is mostly a function of the weight of the robot and wheel geometry and traction material.
Robot Weight
Motor specs
Frictional coef.
Speed
acceleration
Gear Ratio

39. 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.
Now Solve for Required Gear Ratio
Weight
no. of wheels
From the wheel side, we need to know what the “break away” frictional load is, and then using the wheel circumference, converting that to a torque. The gear ratio is simply the ratio of the axle load and motor load.
Frictional force

40. Check Robot Speed
How fast will the robot go with this required gear ratio?
Remember Units!!!
Now the other half of the equation. We have bounded the gear ratios with load. They also determine speed. Is the robot fast enough?

41. Be Careful!

42. 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
Is the machine fast enough? If not, then you can make the wheels slip earlier, put on more power to lessen the reduction, live with it, or design two gear boxes in one.

43. 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)
Now, how long does it take for the machine to get to speed?

44. 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.
The machine acceleration rate also determines how long the robot will spend at that “infinite” torque. Too long, and breakers start to trip. Remember those lines relating speed, torque, and load?

45. Instantaneous Motor Torque
When Motor RPM = 0,
Output Torque = Stall Torque
When Motor RPM = free speed
Output Torque = 0 (in theory)
(.81)
To calculate instantaneous torque, you just interplate on that assumed straight line between stall and free speed.

46. Gearbox (reduction) basics
Chain, belt
Gear Ratio = N2/N1
Spur gears N2 N1 N2 N1

47. Gearbox Torque Output Robot Accelerating Force
Output torque, converted to a force across the wheels, determines acceleration.

48. Instantaneous Acceleration and Velocity
Instantaneous Acceleration (dependant on robot velocity, as seen in previous equations).
The instantaneous velocity can be numerically calculated as follows:
We all learned this stuff in the first week or two of Physics I. F=ma, and V = at.
(thanks, Isaac)

49. Velocity vs. Time
The numerical results can be plotted, as shown below (speed vs. time):
It is easy to “spreadsheet” these relations, and plot the results. Speed comes to some steady state in some finite time.

50. 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.
We then move back to the other assumed line between current limits, and figure out if we are ok.

51. Current Draw vs. Time
The numerical results can be plotted, as shown below:
We come to some equilibrium current draw at speed. Power is of course voltage times current.

52. It’s just a little volts & amps

53. 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?
Does it work?

54. 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?
How to iterate.

55. 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

56. 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.
sells 2-speed transmissions for FIRST applications.

57. Take necessary precautions

58. 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…

59. And then… This is a starting point Use your imagination
Iterate to optimize results
Test
Use your imagination
Infinite speeds
Multiple motors
Many gears
This isn’t the “end all” method.

60. 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
Motor running
characteristics
Max torque per
current limit
Determine maximum
drive train load from
“wall push”
Calculate required gear
ratio from motor and
output torques
Calculate speed
& acceleration
Running characteristics
Current limits
Iterate

61. Automation
Spreadsheet to do drivetrain design at

62. Calculation Example

63. Remember: It’s no big deal!

64. Thanks!
“Robot System Drive Fundamentals”
Ken Patton
Paul Copioli

65. Questions?