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Moment of inertia and turning performance
Smooth turning has been a challenge for our robot this year. We noticed that unlike our 6-wheel tank drivetrain, several teams have been able to create 6WD with butter-smooth turning. We recognized that our off-center CG degraded our turning abilities. However, I feel that there should be another cause that adds to the "jumpiness" of our turns. My prime suspect is the moment of inertia, both about the turning axis and the middle wheels. Suppose that you have a robot with a fairly well-centered CG. Will large or small moment of inertia about the turning axis result in smoother turning performance?
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Re: Moment of inertia and turning performance
The moment of inertia for the robot is a complex system. (Assuming from here a simple normal system) A low moment of inertia results in a lower rotational momentum and therefor a greater ability to start and stop turning. A high moment of inertia results in a higher rotational momentum and therefor a greater desire to keep turning at the same rate.
If you only want to have a consistent, smooth turn, a high moment of inertia is what you want BUT at the cost of responsiveness and control. also keep in mind that your CG is only ideally placed when centered if your robot turns exactly around the CG. Finally: the choppy turning may be because of your programing or your power transfer method. |
Re: Moment of inertia and turning performance
Usually it's a matter of traction vs. center wheel drop. If you have a bit too much traction, or not enough center wheel drop, then it's kind of hard to turn.
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The biggest factors affecting turning for a drop-center 6WD are - amount of drop of the center wheels - wheel tread material - type of flooring - wheelbase/trackwidth ratio - location of center of mass |
Re: Moment of inertia and turning performance
For wheels having similar traction, wheelbase length is the primary factor in how easily a robot turns. The two typical ways to eliminate bouncy turns on a 6-wheel drivebase are to 1) get the center of mass directly over slightly-lowered center wheels, and 2) use omniwheels at the corners to remove friction in the sideways direction.
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Re: Moment of inertia and turning performance
No lie. At last year's inspection, we weighed in a tiny bit over the weight limit. Me, wanting to tidy the robot, trimmed all the ends of the zipties we used and, miraculously, we weighed in exactly at 120 pounds!
We used a lot of zip ties that year.... |
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Lol at your story anyway |
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Re: Moment of inertia and turning performance
We usually do a 6wd with a 1/8 inch drop center and roughtop tread on all wheels. Our robots always turn smooth as silk but this years robot didn't turn well at all. I think our weight was too centered over the middle wheels. we were able to bolt a 7 pound weight to the back of the robot which completely cured it. With a 1/8 drop center those front wheels are really close to the carpet and I think you need to keep them from grabbing as much as possible for smooth turning.
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Re: Moment of inertia and turning performance
The prime suspect for a 'jumpy' 6 wheel drivetrain would be a combination of traction materials and CG in relation to the center wheels. Also, it's possible for a small center drop to be overwhelmed by the frame flexing enough to allow the corners wheels to see more weight then is ideal.
However there are instances where turning problems can be related to control setup or PID loop tuning, and not just hardware. Are you using a PID loop on your drive train? Are you using one joystick drive? Two? A gamepad? Simple mistakes in the axis mixing or PID terms can result in really wonky turning behavior. It can also be helpful, though not at all necessary, to use Jaguars on drive motors. They have slightly better performance that can help improve turning at low speeds. The victor/jaguar thing is still a touchy subject, I know, but this is one thing the jags really do have going for them. |
Re: Moment of inertia and turning performance
Our team has done a lot of personal testing and evaluation on the top teams in the league and we have found that the CoG and tortional stiffness is generally what sets their robot's drivetrain apart from most others.
CoG, contrary to popular belief we have found that setting the CoG far to one side on a 6wd is best. A 6wd performs best when it is actually a 4wd with 2 extra wheels. Placing the CoG in the middle generally results in the turning point of the robot changing in the middle of the turn as the robot accelerates/decelerates and the outer wheels alernating touching the ground. This is especially important because a 6wd does not turn about the middle wheels but rather between the middle wheels and whichever pair of outer wheels happens to be touching the ground. So by limiting rocking as much as possible performace greatly increases. Tortional stiffness is absolutely critical to a good 6wd. In 2008 we discovered that without a very stiff chassis the opposite outer wheels will touch down essentially creating a long wheelbase while turning. This will also seriously limits turning. Edit: I forgot to add, an 8wd has mathmatically much better turning characteristics than a 6wd provided the wheels are properly spaced. But don't take my word for it, if you don't believe me run some experiments. Regards, Bryan |
Re: Moment of inertia and turning performance
From watching a few of your matches on youtube, the symptoms (rocking "hops" when turning, with size of rock roughly proportional to speed of turn) seem very much like those of a drivetrain with too much resistance to turning, the result of having too much traction at the corners of a drivebase with a long wheelbase and narrow track.
Some ways to reduce this issue: - Reduce traction at corners by: * Making corner tires less grippy (slick wheels, or something less extreme) * Making corner tires move sideways easily (90 degree omnis) * Reducing weight on corners (increasing "rocker" and/or stiffening frame so that sag doesn't eliminate "rocker") * Reducing weight on corner wheels by centering weight over center wheels - Shorten wheelbase and/or widen track to get a footprint more advantageous for turning I wouldn't recommend a rocker greater than 1/8"... If putting the center wheels 1/8" lower isn't sufficient, look at your frame sag. Then consider the lateral grippiness of your corner tires and where your CoG is. Note that you don't want the robot to turn without any resistance, as it'd be nearly uncontrollable; however, you don't want so much resistance that you're hopping at even fairly low-speed turns... Some teams prefer just using omnis at the corners, while others prefer using rocker, or a more advantageous wheelbase/track ratio. Personally, I prefer a combination of the rocker, CoG, and wheelbase/track ratio to provide a drive base that goes straight naturally but still turns well and won't be spun around by a defender. |
Re: Moment of inertia and turning performance
Building on what Bryan said...
CoG is a very overlooked thing in 6WD/8WD design. Its important to really consider where the approximate CoG of your robot will be as its being designed. This becomes especially true as you begin to build taller structures on top of your drive system. Weight higher up will tend to amplify the issues you are seeing with turning performance. -Brando |
Re: Moment of inertia and turning performance
Torsional stiffness and a low CoG are absolutely critical to smooth turning in a dropped-center, skid steer drivetrain.
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A 6WD with omnis at all four corners is like a Segway with training wheels. *well, for all practical purposes |
Re: Moment of inertia and turning performance
Our robot was symmetrical this year. Well, not exactly symmetrical, but so much so that our driver has trouble telling one end of the robot from the other. We are using 6wd off a CIM and a FP a side. All 6 wheels are 6in performance wheels with blue nitrite tread, and the center wheels are dropped about an eighth of an inch.
We turn very easily, at least much more easily than most 6wd robots with all traction wheels. However, if you watched our robot play, you would say that it turned very jerkily. Why? The driver and the programming. |
Re: Moment of inertia and turning performance
This is merely an explanation for why a LOW c.g. matters.
Consider a c.g. that's off-center to the right and is 18" off the ground (really high for FRC). When moving forward, that c.g. creates a moving inertia. During a sweeping left-hand turn while moving, that inertia becomes a centrifugal force that attempts to lift the left side of the robot. The higher the c.g., the more leverage the force has to lift the left side. This centrifugal force is easily noticable on robots that do high-speed 0-radius turns in place. It's like an off-balanced top. This lifting causes a major traction loss, even if it only lifts a few tenths of an inch. Realize that with even a few degrees of lateral lift, a wheel that makes flat contact with the field (such as the KOP wheels) will lose nearly all of its traction. Rounder wheels, like the AM pneumatic wheels, will only lose some of their traction. This traction loss is why high c.g. robots cannot do sweeping turns like the more agile robots. A good picture of this concept is on page 41 of this slide deck: http://www.chiefdelphi.com/media/papers/2597 |
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In the "real world" (i.e. with cars, airplanes, segways, etc.) you're definitely right - you basically always want no mechanical resistance in the drive mechanism. As a result, you have a drive mechanism that is capable of being quite responsive. However, if that mechanism is controlled carelessly, it could result in a vehicle that responds easily to accidental maneuvers. These potential side-effects are mitigated by control mechanisms with resistance or "feedback" (such as the yoke or stick on a plane), control motions that aren't inherently twitchy (two hands on a reasonably sized steering wheel), software damping of otherwise twitchy motions (a Wii uses some of this, I conjecture), and/or by training the manipulator to use the responsive mechanism advantageously (fighter pilot). These are all better ways of getting good controlability out of an otherwise difficult-to-control system; however, I made several unspoken assumptions when I thought what I wrote. I figured teams would be working with control mechanisms that aren't perfect (the current joysticks at least have a little more resistance than the old ones...), that they'd be using control motions that aren't always easy to make steady (unsupported arms on two different joysticks), that most teams wouldn't be using software to damp out twitchiness (perhaps some teams do), and that teams wouldn't be requiring a level of responsiveness that could only be acquired with a resistance-free mechanism. From our own team's experience, our drivers had a hard time getting accustomed to our very low-resistance drive trains... but we ended up with robots that could drive fairly well by compensating. We didn't want all the responsiveness we had, so we used software to map the distance from the neutral axes exponentially, so that small motions would be less significant. I consider this raising the tolerances, rather than "damping out twitchiness..." which I presume would be feasible but significantly more difficult, and perhaps not even preferable. We also used software so that when the driver hit a button they could limit the robot to half (or quarter) power for fine motions, such as placing a tube on a peg last year or lining up for a shot in 2010. Then we got the drivers to practice a lot. So, in the FRC robot world, my experience has been that a hyper-sensitive robot is not preferable and that a good way to limit the sensitivity is to leave a little resistance in the system (i.e. put grippy tires at the corners rather than omnis); however, some teams may think otherwise. Perhaps your team disagrees... I could imagine some teams preferring hyper-sensitive robots that are driven well solely by the skill and practice of their drivers... I apologize for my non-absolute statement that could easily be misinterpreted. |
Re: Moment of inertia and turning performance
I think your expanded explanation makes a lot of sense. |
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I think it would be better to say that a Segway has very little wheel scrub to worry about. |
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With regards to method of using a 6WD as a short-base 4WD with 2 additional wheels, this is something that we've considered last year, but I believe we chose against it because it might've made aiming on one side of the robot, either the scoring grid or minibot tower, difficult (this was before we chose to use "alignment legs" at the bottom of the robot). In addition, if our robot's turning axis is biased to one side, other robots may have a large moment arm to turn our robot. |
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As to not wanting to be turned sideways, it seems to be mostly a non-issue compaired to the performance gains, you don't notice the powerhouse teams that use 6-8wds year after year getting pushed around very much. (and when you think about having the CoG on top of the middle wheels in the 6wd wouldn't having a changing point of rotation actually make it harder to line up to something?) Interesting thread Bryan |
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Some of my own observations: Welded tube frames are always rock solid in my experience. 1662 uses 1"x1.5", 1/8" wall tubing welded frame. No flex whatsoever. This is consistent with many WCD around California. 1678 uses 8020 extrusion frames, and they flex like mad. Maybe it was just the gussets we used, but I have a feeling that the 8020 beams are less able to resist torsional forces. (If I remembered everything I learned in mechanics of materials 4 years ago I might be able to figure it out... :rolleyes:) Last year we had to create a pyramid strut system to our arm apex in order to remove the torsion experience while turning. How is the kitbot frame's torsional stiffness? I don't have much experience with it. Awesome thread, all this talk about frame flex has got me thinking! -Mike |
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1114's Kitbot on Steroids improves this using the robust base-plate. Super structure design can make a frame stiff. Welding can make a frame stiffer. Gussets can make a frame stiffer. Additional rivets at attachment locations. Gluing. In general, Triangulation beats Boxification (I made the last one up). There are lots of ways that can make a frame stiffer, but many do not without some attention to detail. If you are doing a sheet metal chassis, making a "model" out of posterboard can be very helpful. The posterboard is much less stiff than metal, but the general weak spots should be the same locations. If you like doing FEA, you can apply moments in opposite directionon the center of the outer rails. Moments on the order of 180 to 240 in*lbs is a good place to start. Look at the displacements relative towhere the wheels would be. Displacements on the order of 1/16th of an inch or more are important. If you think about the "pile" of the carpet. A light touch to fully supported (60-75 lbs) is on the order of 1/8". If someone was interested in doing some testing, you could anchor 3 corners of a frame onto something rigid (say very stiff table or multiple pieces of plywood). You could then apply weight to the 3rd corner and measure displacement . Trying different gussets, bracket, welding... and repeating the experiement would make for a very nice paper and possibly a cool sciencefair project. |
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Why? Usually a frame stays square "just fine" until an impact. Then things can get bent out of shape. |
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X X O O X X This reaction effective turns somwhere between a long and a short wheelbase, and makes turinging more difficult. In order to counteract this, teams will often add more "drop" to the center wheel. As the CG Height increases, this increases rock which can cause the center of rotation to oscillate between front of center wheels and behind of center wheels. The stiffness of the wheel/tires, carpet then act like a spring. The softer the spring, the more displacement, which in turn will equate to more energy (E=1/2KX^2 where as F=KX decrease K, and X increases proportionally in which energy increase due to the X^2 term). This is why buck bobble is more prevalent in pneumatic tire chassis. |
Re: Moment of inertia and turning performance
Haochuan,
The "jumpiness" that teams describe is primarily the torque of your drive train overcoming friction with the floor. As a wheel breaks loose and the RPM jumps up, the result is a mini-wheelie when the tires again grab the carpet. In most cases with high friction tires (four or more wheels) this is the case when turning. |
Re: Moment of inertia and turning performance
The extreme case is "help my robot will not turn" or once you go mechanum you never go back :)
In the real world: watch the skid steers on a construction site & how they chew up the ground. |
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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. |
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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. |
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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
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 :) |
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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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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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I noticed many teams this year with the same condition. |
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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 |
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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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