We’ve found that the soft rubber on 4 inch wheels provide much better grip on the platform and facilitates easier climbing. But the 4 inch wheels have many disadvantages mainly that it is too large and wide. The 2.75 inch ones have a similar problem of being a little too wide. It’s also not very easy to get a good gear ratio out of a efficient transmission with these. So is there a way I can make a drive on 3.25 inch omnis go up the platform as easily as one using other omnis?
It’d be best if we didin’t add traction wheels of any sort. We have thought of this so please don’t suggest it.
You could try using 2.75 omnis, which would have the same grip as 4" but much smaller. A good ratio on those would be 360 (600 3:5, about the speed of a 247 rpm tank drive), or 333 (200 3:5, about the same speed as a 230 rpm 4").
We’d consider this as a last resort.
does your robot have it’s full weight yet? or is it just the chassis?
because if not, once your robot weighs more that added weight might have a significant impact on the amount of traction you can gain climbing the platform.
@Xenon27 it’s 6.5 kilos
with 2 mogos it’d be around 9
interesting, we’ve seen robots using 3.25" wheels climb the platform with ease already, see this clip from 62a’s amogo and dogo reveal.
This robot uses 4 normal 3.25" omni wheels, and while I think it probably weighs more than yours, it don’t think it weighs that much more.
It’s not that we can’t climb at all, but it does not climb as nicely as our sister team’s 4 inch wheel robot. They have an entirely different design from ours that doesn’t require small wheels.
My team previously had a bot that had 3.25 and was supposed to carry and climb with 4 mogos, but we seemed to lose more traction the more goals it carried. When we tried to climb with all 4 goals, the bot wasn’t able to climb at all. The wheels were spinning at full or close to full speed but they would quickly slip on the platform.
You can have a 6 wheel 3.25" omni drive. As long as all wheels are powered, you would increase traction by 1/3 of the original configuration (assuming no wheel slippage).
that’s a very surprising result, I wonder why additional weight would cause a decrease in traction?
I’m no physics expert, but some googling tells me that the formula for the tractive force applied by a wheel on a surface is:
F = μt W
where μt is the coefficient of friction between the wheel and the surface (which in the scenario of using only 3.25" omni wheels, should remain constant), and W is the weight (or vertical force) applied on the wheel.
So physics tells us that as weight increases, the tractive force should also increase.
however, there might be other factors at play here such as the pivoting center of mass causing the robot to want to tilt backwards, taking weight off of the front wheels and onto the rear wheels, which would probably reduce the traction of the wheels on the platform.
this is true when all 6 wheels are contacting the platform, but if the issue is with climbing the platform, the middle wheels would be lifted momentarily into the air and not contribute anything to the traction of the drive. If you can make to the point where all the wheels of the robot are contacting the platform though, this should help.
This is not the case for the platform. When on an incline, the traction is a function of the angle the robot is at. So, really, it’d be
F = μ • Fn where
Fn is the normal force on the robot from the platform. In this case, the normal force is equal to
cos(Θ) • W where
W is the weight of the robot and Θ is the angle of the robot relative to the ground. Basically, the steeper the incline, the less traction
This simplification assumes the robot is in continuous contact with the platform and friction coefficent is constant.
Like you said, I assume the reason this happens is because the bot is trying to go uphill rather than a flat surface so it also needs to overcome its own weight with the wheels’ friction rather than having the weight be purely increasing effective friction.
We did have a 6 wheel drive with all wheels being powered, but even when the bot had all 6 wheels fully contacting the platform, it still wasn’t enough to consistently drive up or even stay on the platform. Poor weight distribution also could’ve played a role as we had 3 out of the 4 goals held at/behind the rear wheels which could’ve caused the front wheels to have less traction.
Yeah depending on where that weight is it could actually hurt more than it helps
This is an added effect of the incline. So not only does it decrease the robot’s traction, but the robot also has to increase its gravitational potential energy some amount. Thinking of it in terms of energy is an elegant way of visualizing what’s going on, otherwise, you need to do fairly complex free body diagrams to account for all the other variables. The work needed to do this is significantly more than just translating across a level surface.
If you think about it, the formulas for normal force, reactive friction force in the drivetrain, friction converted to traction force, or force required to move the robot forward uphill - they all look like this: F = W * some_coefficient, where the coefficient depends on various things, like drivetrain properties, slope incline, etc…
The important point is that, if system behaves linear, and you write an equation to compare, let say, force required to push robot uphill against the force of max traction you can get, given the robot weight, then Ws on the left and right sides will cancel out.
This means that it shouldn’t matter how much weight you add to the robot: increase in the force required to drive it uphill will be matched by the increased wheel traction.
So, if you have a counterintuitive case of wheels slipping after you add weight to the robot, it means that something is non-linear.
For example, adding more weight to the back of the robot will actually decrease the normal force on the front wheels and you will end up with front motors being underused and the power of the back wheel motors may not be enough to move the robot.
Do you have front and back wheels of the drivetrain linked to redistribute the power for such use cases?
of course 20 characters
All sorts of stuff accumulates on the omni rollers’ surface and their little axles which makes them less grippy and harder to turn.
You’ve got to make sure they are clean - wash them in the warm soapy water and air dry well.
Also, 3.25" rubber is not as soft as for 4" and hardens over time - some people boil their omnis after a few years to get it softer again.
clean the wheels
you could do some tests before/after to measure the difference
would make for a good notebook entry
How about doing locked omnis? Like Blank’s bot in turning point!