FRC Blogged-Kit of Parts Drive System Option

[s_forbes]

Maybe AM made it easier to attach super structures because teams typically need to attach super structures? Nothing can be implied with these kit frame details other than the robots will most likely be allowed to have wheels.

We loved the kit frame last year, it's great to see some improvements are being implemented. I'm hoping the transmissions are easier to get into than they were on the previous iteration, that's what gave us the biggest setbacks.

[Siri]

Originally Posted by s_forbes

Maybe AM made it easier to attach super structures because teams typically need to attach super structures?

but...but...reality without wild suppositions is so much less convivial!

This is silly. Obviously, the point is that all robots will have to be both long- and wide- simultaneously. In fact, they'll have to be entirely symmetrical, because we're landing on the field like this* [/sarcastic wild supposition]


*Mars Pathfinder 40g "bounce" landing, if you don't want to click.

[Racer26]

Originally Posted by Nemo

It could be 6" wheels with 12.76:1 gearing or maybe 4" wheels with the 8.46:1 gearing option.

If it's 6" wheels, the gearbox options give the options of switching to 13 ft/s or 16 ft/s, at least.

It's too bad Toughboxes aren't setup with a 50:12 initial stage instead of 50:14. Then we'd also have the option of switching the first stage to 50:11 or 48:14 for a bunch more options.

If its 6" wheels, with the 10.71:1 ratio configuration, JVN's calculator drops out 10.51fps as the max speed.

[Ether]

Originally Posted by Racer26

If its 6" wheels, with the 10.71:1 ratio configuration, JVN's calculator drops out 10.51fps as the max speed

How do you know what "Speed Loss Constant" to use?

[Racer26]

I don't. I just assumed the 81% isn't wildly out to lunch. I'm also guessing that HQ/AM in saying "10.5fps" did some kind of calculation using commonly available tools -- likely JVN's calculator.

[Ether]

Does anyone know where the 81% number came from?

[Madison]

Originally Posted by Ether

Does anyone know where the 81% number came from?

Two stages of gearing, each 90% efficient -- .90 * .90 = .81

[Ether]

Originally Posted by Madison

Two stages of gearing, each 90% efficient -- .90 * .90 = .81

Are you sure that's not just a coincidence?

The "Speed Loss Constant" in cell I5 is independent of the gearing in cells C12 thru D15, or the Drivetrain Efficiency in cell J5.

.

[Nemo]

Originally Posted by Racer26

If its 6" wheels, with the 10.71:1 ratio configuration, JVN's calculator drops out 10.51fps as the max speed.

Looking at the blog again, I see that they're calling that an "estimated final drive speed," so I shouldn't have been looking at maximum theoretical speeds.

As an extra piece of information, last year they said this about the 2013 kit drive:

The 2013 gearbox is a ToughBox Mini with 10.71:1 reduction. The estimated final drive speed is about 10 feet per second.

That drive had a 42:39 timing belt reduction, so the ratio was 11.53:1. If they used the 0.81 factor last year, they'd get around 9.75 ft/s, which they might have rounded up to 10 ft/s. Based on that I'd agree that they're very likely doing 6" wheels with 10.71:1 gearboxes. If not, they're estimating the speed differently this year.

[Ether]

Originally Posted by Ether

This constant is not computed within the spreadsheet from user-entered info about physical loads and motive forces in the drivetrain: you get the same 8.83 answer from the spreadsheet if you change the number of motors, stall torque of motors, the mass of the robot, or gearbox/drivetrain friction

Also worth noting: in the real world, the % of "speed loss" is not constant when changing gear ratios either:

Team 1678 did some testing earlier this year on a bot with a 2-speed gearbox. The "Speed Loss Constant" was 92% in low gear and 72% in high gear.

[T^2]

Originally Posted by Ether

Also worth noting: in the real world, the % of "speed loss" is not constant when changing gear ratios either:

Team 1678 did some testing earlier this year on a bot with a 2-speed gearbox. The "Speed Loss Constant" was 92% in low gear and 72% in high gear.

Ether is correct on most accounts. Keep in mind that JVN's "speed loss constant" comes after a "drivetrain efficiency" (i.e. losses from gearing), therefore the 81% number cannot be attributed to gearbox losses. The testing we did earlier this year found the overall speed loss from the theoretical free speed of the motors. The numbers we found make intuitive sense, given that motor resistance, friction, rolling resistance, and air drag all tend to increase as speed increases.

My conclusion now is the same as it was then: test your drivetrain instead of blowing smoke from your behind about the theoretical speed you think you'll get.

[Ether]

Originally Posted by T^2

Keep in mind that JVN's "speed loss constant" comes after a "drivetrain efficiency" (i.e. losses from gearing), therefore the 81% number cannot be attributed to gearbox losses.

The Speed Loss Constant does not come "after" the drivetrain efficiency (at least not in the 2013 version). It is completely independent. See below:

The testing we did earlier this year found the overall speed loss from the theoretical free speed of the motors.

^^This is exactly what the "Speed Loss Constant" in the JVN spreadsheet is.

My conclusion now is the same as it was then: test your drivetrain instead of blowing smoke from your behind about the theoretical speed you think you'll get.

I love test data too.

But I still think it may be possible to develop a physics-based model that will reasonably predict overall drivetrain performance -- including not only top speed but also time to reach a desired speed, time to reach a desired distance, and accel, speed, distance, motor amps, motor volts, and Coulomb consumption versus time -- based on a priori estimates of a limited number of parameters .

[BBray_T1296]

Originally Posted by FrankJ

Wild speculations about what this means about the game.

First big assumption: You can build a competitive robot using the KOP chassis as designed.
Second implied assumption 6 in wheels based on stated speed.
Third implied assumption direct drive places the gear box center line on the wheel.

So with a 3" high nano box & 6" wheels, you have 1.5" ground clearance at the center of the robot. 8" wheels give you 2.5" of ground clearance in the center of the robot.

Leads to Big assumption based on this is no big obstacles the robot has to go over.

Wheels & stated speed also implies no water game. Again. Sorry guys.

What if the water is only 1" deep?

[Whippet]

Originally Posted by BBray_T1296

What if the water is only 1" deep?

Then that drastically reduces the coefficient of friction between the wheels and the playing surface, reducing the robot's ability to accelerate to the maximum drive speed, and possibly making it impossible to accelerate to 10 ft/s within the length of the playing field. However, they never said that the robot would be able to achieve that speed on the field. That was just the estimated speed.

[ErvinI]

I'm hoping that the dimensions are increased this year. The dimensions last year severely limited what teams could design (although it meant less clutter on the field). The fact that they somehow fit 6 wheels in a wide configuration makes me hopeful the dimensions are increased this year.

Has anyone tried modifying last year's wide 4-wheel KOP drive base in a 6 wheel base, keeping the same dimensions? This could answer the question of dimensions; if it's unfeasible or useless, then chances are that the dimensions could have been increased for next year.