Quote:
Originally Posted by Joe Johnson
Aspect ratio is all.
Wider is better. Longer is less good.
...snip... Dr. Joe J.
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Gulp, I can't believe I am going to do this, but I somewhat disagree with Dr. Joe.....
While aspect ratio will dictate whether or not you can turn, the overall length and weight distribution on the wheels touching dictates how much power it takes to turn at a given turn rate. What is turn rate you ask? It is angle of the turn divided by the time to execute the turn.
For instance, lets compare two scenarios of wide and long for those "back in the old days" of 28x38 dimensions.
Long bot was a 6x6 drop, and on 8" wheels, the point of contacts with the carpet were at least 4 inches from each edge and then of course the center. total length touching was 38-4-4 or 30 inches. If you dropped the center, they you were riding on an effective wheelbase of 15" while the width was typically 1" inside each for frame, and the crown of the tire was inboard another inch each side (2" wide pneumatic tires) for a effective width of 24". This robot turns well. Assum that the CG is right in the middle so all four touching wheels have the same contact. In this scenario, the lateral forces are the same as the longitudinal and for a COF of 1.3, the resultants work out to about 0.91. The longitudinal forces when pivoting act about a 12 inch radius (24/2), where as the laterals act a a 7.5" radius (15/2). To turn 90 degrees in 1 second, the tires end up traveling around a 14" arc, and 90 degrees of that arch would be 22 inches of travel. 22 inches of scrub in 1 second on a 120 lbs robot is about 2400 in*lbs/second or about 268 watts (also 268 J).
For the old wide bot, the wheel base was 28-4-4 or 20 inches, and the width was 38-2-2= 34 inches. This robot will turn well, but to turn the same 90 degrees in 1 second it will have the tires traveling about a 19.7" radius for a total travel of 31 inches which for a 120 lbs robot is 3700 inch*lbs/sec or 420 Watts of power (also since 1 second 420 J)
So to perform the same turning action, the wide bot actually required about 56% more energy to execute the action.
While it will turn just fine, this will cause your motors to get hot. Especially if chasing down balls.
*I did this math back in 2010 after watching 1918 have hot motor problems with their awesome wide bot that year.