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Re: Moment of inertia and turning performance
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Even though the Segway is disabled there is something electrically that is working on the motors, akin to 'brake' mode. When you turn it off completely, it acts as you would expect. |
Re: Moment of inertia and turning performance
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However, we noted that some teams are able to create very smooth 6WD drivetrains that are similar in configuration to ours (254's Slipstream, for example). We will analyze some high-speed footage to get a better understanding of why we are suffering from this problem. |
Re: Moment of inertia and turning performance
Robots have been doing this for more than the 17 years I have been here and drop wheel drive trains have not been around that long. When you have a lot of torque and friction something has to give somewhere.
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Re: Moment of inertia and turning performance
Do a CD search for "Trapezoidal Speed Profile". Also do a CD search for 254's 2011 code, as that was posted as well. They've refined their code to optimize a couple of things. There may also be a difference in turning response with brake versus coast mode on the Jags/Vics.
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Re: Moment of inertia and turning performance
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Re: Moment of inertia and turning performance
I have not read through all the threads, but many 6 WD drives drop the center wheels to rock back and forth. This is typically in the robot 101 build stuff.
Instead we raise the front wheels and shift the CG towards the back. This allows the robot to turn on a smaller profile. There is no rocking motion except for fast stops. The height that you raise the front wheels will depend on the wheels that you use. Good luck for those continuing to St. Louis. |
Re: Moment of inertia and turning performance
Here is an update from the team regarding the "rocking while turning" problem:
"In our effort to determine why our robot 'dances' when it turns quickly, our first test was to find the horizontal center of mass/gravity (c.g.) To our surprise, we found the c.g. to be very close to the center of the middle wheel. It is about 1" forward of the center wheel, as determined by balancing the robot on blocks. ![]() Instead, we believe the problem is in the distribution of the mass. The mass has large densities like below: (S)--- | | | | | ----(B) where (S) is the shooter, and (B) is the battery. While the mass distribution is more complicated, the point is that as we rotate about the center vertical axis, the shooter wants to tilt the robot forward, and the battery wants to pull the back outward. _-\ (S) \ \ \ _(B) \- In short, we need to pay more attention to the mass distribution for dynamic balance. While we are well balanced statically, our 2012 mass distribution results in poor dynamic balance." Hopefully this can provide some new insight into this issue. There are so many complications that make creating a stable and smooth drivetrain harder than it seems :) |
Re: Moment of inertia and turning performance
Try adding about 10 lbs to the front, center, then rear. High and low. Take notes about how thw cg movement effects driving performance. These 6 trials will tell you a lot and will make a good experiment.
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Re: Moment of inertia and turning performance
Another problem, that I noticed this year, was the field. I think many fields were not completely level due to maybe a budget decrease. I know for a fact we were "bouncing", with an 1/8 in drop, and very stiff chassis(we overbuilt the drivetrain, using 3 x 1 tubing instead of 2 x 1). Furthermore, I even saw teams that are known for their drive bouncing(ie. 1538 Holy Cows).
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Re: Moment of inertia and turning performance
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I noticed many teams this year with the same condition. |
Re: Moment of inertia and turning performance
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Re: Moment of inertia and turning performance
Chinmay,
Try some smaller wheels and drop the bumper as low as you can get it. Geared correctly with the smaller wheels you will find the robot accelerates quickly. We design in the placement of the wheels close to the perimeter of the frame. The frame needs to be very stiff if you want the robot to run fast and turn gracefully. Try to keep the weight of the robot as low as possible and between the front and rear axles. Here are some pictures of our 6 wheel sheetmetal chassis for 2012: https://picasaweb.google.com/1177698...12BuildSeason# One of 971 summer projects is to invite local teams to a question and answer to our design process, materials, fabrication methods. Shoot us a email if you are interested. Roy |
Re: Moment of inertia and turning performance
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Re: Moment of inertia and turning performance
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Our wheel base was probably only about 14" due to our use of 12 inch wheels on a wide robot, but we were not in danger of tipping over. In one match we were up at nearly a 45 degree angle on a tilted bridge and we still recovered and balanced the bridge. The only time we tipped was when we drove off the side of a bridge in Boston. |
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