DEWBOT IX Design Team Page
Student Lead: tbd
Student Sub-Lead: Dhananjay (DJ)
Student Team Members: Jack, Kira, Tobi
Here are the motor specifications and performance curves for the 2013 motors.
A first look at climbing physics
Here is a first look at geometry, power and normal force requirements for climbing the pyramid's corner pole in the 2013 game.
Drive Train model adapted for pyramid corner climbing
The Minibot drive-train model was adapted to work with climbing the pyramid corner. For use as a design tool.
Lesson plans created in order to teach engineering analysis and synthesis on a level compatible with highschoolers.
3-Wheel Swerve Drive
Email from Clem McKown to the team - 18-June-2013:
- While putting together an updated drive-train lesson and thinking about why a 3-wheel drive robot was such a bad idea, I had an epiphany: Under the 2013 robot perimeter rules, a 3-wheel drive robot is really not such a bad idea at all. In fact, a 3-wheel drive pivot robot is perhaps a very good idea. I'll explain.
- The issue is stability. No-one want their robot to fall over. A robot will fall over if the gravitational projection of the robot's center of mass, adjusted for robot acceleration or incline, falls outside of the line described by the robot's contact points with the field (which we will call the wheel contact points). Remember, the gravitational constant is just acceleration, so we're adding (vector) apples and apples. Also, remember turning and stopping are also acceleration. Pretty simple, in principle.
- Under old robot perimeter rules (<=2006 through 2012), the robot perimeter was limited to 28" x 38". Under these rules, the most stable robot you can make has at least 4 wheels as close to the corners of this rectangle as practical. A 3-wheel drive robot under these rules seriously compromises stability. This is the same reason for there being no 3-wheel automobiles in the US (safety - there is a Buckminster Fuller story here).
- But the new (2013) perimeter rules allow up to 112" of total robot perimeter. The shape of this perimeter is not specified. When we designed our chassis, we saw this rule as allowing us flexibility in varying length versus width of a fundamentally rectangular robot (which we ended up making square). DEWBOT's 2013 perimeter was 111" (27-3/4" square). Our wheel-base ended up 21" (nominally wide) x 22.25" (nominally long). A 112" square chassis would have sides of 28"
- But if a side is eliminated, the other sides can become longer. An equilateral triangle with a 112" perimeter would have sides of 37-1/3" long. Pivots could be installed in the three corners to bring them as close to the perimeter as practical, increasing the wheel-to-wheel distance vis-à-vis DEWBOT IX. Under these conditions, I would expect a 3-wheeel drive base with stability similar to this year's 4-wheel base (which was excellent).
- But with three pivots, not 4. This would save significant weight (the 2013 pivots were 7.9 lb each, plus talons,...) which could be deployed elsewhere.
- Unlike tank or mecanum, I think pivot drive should work as well with 3-wheels as with 4. But, this poses a new control challenge for us (sorry, programmers).
- And a pointier robot (60° versus 90°) could break through defenders more easily.
- I'm excited, but I'm easily amused. Let me know what you think.
- Best regards,
- Under old rules (<= 2012), the largest circle which could be inscribed within the chassis perimeter was 28" diameter. If your contact points were at the perimeter corners, you could not tip unless the projected CoM fell outside this circle. Call this the minimum circle of stability.
- Under old rules, if I simply go to a 3-wheel isosceles triangle, the largest circle which may be inscribed shrinks to 19.524" (~70%).
- In reality, the wheel contact points are inside the perimeter, not at the corners. So this is all back of the napkin work. Our pivot drive wheelbases under 28" x 38" rules ranged from 20.27-21.5" in the 28" dimension and from 28-30.75" in the 38" dimension. This spread becomes wider when you consider earlier tank drive robots. A "selling point" of WCD is that it maximizes the minimum circle of stability (while improving agility).
- Now, in 2013, rules are changed to limit perimeter to 112". With a square robot with contact points at corners, the minimum circle of stability is still 28"; with a triangle bot with 112" perimeter it drops to 21.55" (77%). But I think we can get the contact points closer to the perimeter corners with a triangular chassis (and ambidextrous pivot modules) than with a rectangular chassis. Needs proof.