Rolling
Mar 14, 2016
Rolling
What is mobility? Understanding wheeled and traced
mobility How do I control the motors How do I steer How do I know how far I have gone
Mobility: Directed movement of and agent Characteristics (land)
Max height of traversable step, Max width of traversable ditch, Max speed on flat floor, Max traversable slpe, Type of terrain traversable
Cement grave, grass, loose sand etc
Characteristic
Wheeled Tracked Winner
Assuming 20 mph
Complexity •Tire Pressure•Alignment
•Tension•Alignment•Breaking•Weight
Wheeled
Hard Ground• road
Power Required on Hard Ground
3HP/ton 7HP/Ton Wheeled
Power Required on Soft Ground
15HP/ton 10HP/ton Tracked
Soft Ground10 inch layer plastics soild
Reduction in speed on soft or uneven ground if power constant
75-85% 50-75% Tracked
There are several areas of concern Simplicity vs. complexity Reliability Theoretical vs. practical concerns Can I get the parts? Is there a backup plan?
We have already seen the advantages of differentially steered wheels
Using a steering wheel allows one drive motor for
Toy servos often come with a circuit board which can be modified to steer “left”, “centre” and “right”.
Attach servo to “tiller” arm of steering wheel.
Vehicle Weight Movement causes redistribution
Yaw and Slip Yaw caused by turning wheel
side ways, slip caused by loss of friction with surface
Torque Caused by Power to axle
Forward/Reverse Caused by turning of axle due to
torque and momentum of vehicle
Yaw of steered wheels affects grip of wheels on surface
Slip Caused by friction loss between steered wheels and
surface Vehicle will not make a “true”
turn Can be reduced
(sometimes) by adding weight over steering components
Turning radius is quite wide needing lots of real estate
“Actual” turn footprint depends on weight distribution and slip
Slip will be different on all contacting wheels
“Actual” foot print can be reduced by bringing wheels closer together
Uptake hauls track up
Dispersed Weight done by virtue of
track pressure Slip
Caused by friction loss
Top Good Traction and Stability Relatively poor turning
radius Bottom
Reverse characteristics of top arrangement