[team2039] Re: Drive Train going over Bump
- From: "Mary Johnson" <MJohnson@xxxxxxxxxxxxxxx>
- To: "'team2039@xxxxxxxxxxxxx'" <team2039@xxxxxxxxxxxxx>
- Date: Mon, 09 Jan 2012 13:50:43 -0600
Here is a question from the non-engineer... is there any chance of the
four wheel narrow orientation drive getting stuck with one set of wheels
on one side of bump and the second set on other side? Or is this what
omni wheels are supposed to correct?
>>> On 1/9/2012 at 1:38 PM, in message
<3DF86F824940F1478BBB61678F47B79F01F0A439@xxxxxxxxxxxxxxxxxxxxx>,
Adam Czerwonka <Adam.Czerwonka@xxxxxxxxxxxx> wrote:
Hello Team,
Based on the great discussions I heard about drive trains this weekend,
I decided to look at how different drive trains would perform while
going over the bump. I put together scale drawings of the drive trains
(with a few assumptions made).
For the 4 and 6 wheel designs, I assumed we have 8” wheels, for the
tank tread, I assumed an angled tread pattern on both sides with 4”
rollers (probably doesn’t impact the results if we change the roller
size and keep the overall tread pattern).
Here are my observations. I invite everyone to include their comments,
questions, or new ideas so we can repeat this experiment with other
designs.
A brief discussion of the physics:
You’ll notice that I have some of the designs reaching very high angles
before they tip over and drive down the other side of the bump. This
was determined based on principles of physics, the robot will only tip
(toward the front of the robot) when the center of gravity passes over
the pivot point, or when forces from the wheels or treads cause the
tipping force. This means that this performance is impacted by the
location of the center of gravity (CG) of the robot. The center of
gravity is the average location of the weight of the robot. The point
at which the weight of the robot is balanced in every direction. I
placed the center of gravity in the middle of the frame, but above the
frame since there will be a bunch of hardware sticking above the drive
frame of the robot.
For just about all of these drive train designs, it would be ideal to
have the center of gravity very low to the ground, and near the front of
the robot too for others. I assumed a somewhat central location not
knowing the robot design yet. Much of the upper part of the robot will
be designed based on functional constraints, so we may have limited
flexibility in where the center of gravity is located. We can probably
improve upon the center of gravity that is shown in the images, but the
question of how much will not be answered for quite a while. One thing
is for certain, the Center of Gravity cannot be located at the edge of
the robot, since there is no weight beyond this point to balance out the
robot weight. My advice would be as we look at robot designs, we can
assume that we might be able to improve these worst case numbers by a
conservative (that means small) amount, but it might be challenging to
move this location by much more than 6” to 12”. For scale, the robot
length in the narrow orientation as drawn is 34”.
I looked at the following 4 designs: 6 wheel narrow orientation, 4
wheel narrow orientation, 4 wheel wide orientation, tank treads
Ranked from most likely to tip over to least likely to tip over we
have:
4 wheel wide orientation *almost guaranteed to tip over
Tank Treads* high probability of tipping unless the center of gravity
is very low and very close to the front of the robot
6 wheel narrow drive *medium probability of tipping – might tip if we
get hit by another robot
4 wheel narrow drive* low probability of tipping
Ranked from biggest landing shock on the robot to smallest landing
shock on the robot we have:
Tank treads *unless the cg (center of gravity) is very close to the
front, this design practically has to jump over the bump, large impact
on landing (16 inch drop at front of bot)
6 wheel drive *medium impact (8” drop at front of bot)
4 wheel wide *low impact 4” drop at front of robot (high probability of
tipping over when coming down)
4 wheel narrow * low impact 4” drop at front of robot
Hopefully this will get people thinking of other configurations, and
what the impact on speed, maneuverability, pushing power, and bump
performance will mean for the robot design.
Happy Designing!
-Adam
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