2 Motors is Faster?!

[=Martin=Taylor=]

Okay, so I think we all saw the demo where Dean and Dave race a 2 motor robot against a 4 motor robot and they reach the finish at the same time.

Having taken my fair share of physics I do not for the life of me understand how this is possible.

More force should equal greater acceleration, for the same mass. Correct?
They will reach the same speed, but the 4 motor bot will accelerate faster.

So did they reach the end at the same time simply because the 2-motor bot's lightness made up for its lack of power?


After several transmission/motor swaps in '08 our team pretty much convinced ourselves that 4 motors was ALWAYS the way to go.

[Thermal]

If torque is greater than traction/FoF, then the wheels will skid reducing your ability to apply your forward force to the ground.

2 motors have less torque than a 4 motor setup, so the wheels skidded less

[GUI]

They accelerated at the same rate because acceleration is a function of mass and force. If both bots were maximum weight, the mass and the force of friction provided by the wheels would be the same. Since the motors provided more force than the friction of the wheels, either robot could only apply, at most, the frictional force of the wheels. This is different than in previous years where the frictional force of the wheels was greater than the output possible with the motors, a situation where the motor output was the limiting factor (and more motors = more force, assuming identical motors). The difference is that this year the limiting factor is friction, while previously it has been motor power.

[=Martin=Taylor=]

I see... That makes sense...

Two FP's for drive and four CIMS driving a beastly manipulator...

Originally Posted by Hachiban VIII

I see... That makes sense...

Two FP's for drive and four CIMS driving a beastly manipulator...

I predict this ending in a LOT of magic smoke. At least use the AM planetary so you can switch back to CIMs when you smoke your drive train...

[gburlison]

Originally Posted by Hachiban VIII

Okay, so I think we all saw the demo where Dean and Dave race a 2 motor robot against a 4 motor robot and they reach the finish at the same time.

Having taken my fair share of physics I do not for the life of me understand how this is possible.

More force should equal greater acceleration, for the same mass. Correct?
They will reach the same speed, but the 4 motor bot will accelerate faster.

So did they reach the end at the same time simply because the 2-motor bot's lightness made up for its lack of power?


After several transmission/motor swaps in '08 our team pretty much convinced ourselves that 4 motors was ALWAYS the way to go.

4 motors can create more force, but the low friction wheels will spin before that additional force can be used to accelerate the robot, The acceleration is limited by the wheels, not the motors. I think that the 2 motor robot accelerated faster because of the gear ratios used.

[GUI]

Originally Posted by CraigHickman

I predict this ending in a LOT of magic smoke. At least use the AM planetary so you can switch back to CIMs when you smoke your drive train...

Depending how it is set up, I think this could work fine. The best idea would be a software traction control system, so the FPs are only ever doing as much work as is required. If they aren't spinning too fast, and only doing as much work as the wheels allow, I don't think they would have too much trouble. Then again, it's nice to keep the cg low, so having the heavy motors in the bottom would probably be preferable. Note that I've never used the Fisher Prices, so I'm just guessing. If someone who knows a bit more than me could comment, that would be helpful.

[Akash Rastogi]

High speed and low torque is the way to go. Try using some encoders in a special way as to create your own ABS. Might just work out better than you plan. Try it out.

[GUI]

I think it wouldn't be too difficult to figure out if the robot is slipping. If you have encoders and an accelerometer you could compare the measured acceleration to the expected acceleraion at the measured output speed. If the measured acceleration is significantly less than what is expected, you would simply reduce the power to the motors. Does this sound like a good system (I haven't messed with any of the sensors in the kit, but this is what I assume would be the simplest solution).

[=Martin=Taylor=]

Originally Posted by GUI

Depending how it is set up, I think this could work fine. The best idea would be a software traction control system, so the FPs are only ever doing as much work as is required. If they aren't spinning too fast, and only doing as much work as the wheels allow, I don't think they would have too much trouble. Then again, it's nice to keep the cg low, so having the heavy motors in the bottom would probably be preferable. Note that I've never used the Fisher Prices, so I'm just guessing. If someone who knows a bit more than me could comment, that would be helpful.

I punched the numbers into the JVN calc and with a reduction of 48:1 (big reduction) I got a good speed of around 8 fps with a current draw of 13 Amps per Motor. Perhaps the smallest amp draw I've ever seen in a drive.

Going from CoFs of 1.4 to 0.1 makes a huge difference

[GUI]

48:1 is a bit of reduction, but if you use the AM planetary adapters then you could use the Toughbox and get a ratio close to that, though I suspect you would want to get gears for the toughbox for less reduction, maybe the 8:1 set, to eliminate the need for a funky sprocket ratio.

[artdutra04]

I see no reason to use four CIM motors in a drive train this year.

There isn't really a way in which a team would be torque-limited this year (unless you use the Mabuchi motor); you will be entirely traction-limited. Using four CIMs on the drive is just wasting hundreds of Watts of power.

[Katie_UPS]

Another thing answering your question;
In this game traction is everything. And you want the most traction.
So with the two wheel bases, they both are relitively the same weight (I know weight is added for extra motors and wheels, but I'm talking on a very general level).
So with four wheels, if the weight was 100, each wheel carried 25 pounds (ok, so I know this is a little off, and I don't have enough physics background to properly explain this, but once I'm done, this should make sense), but on the two wheel, each wheel carried 50 pounds, which created more friction which gave it more... power (if thats the right word....)


EDIT: No one really explained that to you, and thats why the two-wheeled went faster (or as my mentors explained)...

[Akash Rastogi]

Originally Posted by Katie_UPS

Another thing answering your question;
In this game traction is everything. And you want the most traction.
So with the two wheel bases, they both are relitively the same weight (I know weight is added for extra motors and wheels, but I'm talking on a very general level).
So with four wheels, if the weight was 100, each wheel carried 25 pounds (ok, so I know this is a little off, and I don't have enough physics background to properly explain this, but once I'm done, this should make sense), but on the two wheel, each wheel carried 50 pounds, which created more friction which gave it more... power (if thats the right word....)

Not necessarily, that's with the assumption that your team feels traction is most important. Even if Dean or Woodie said it doesn't make it true either. There's no defining basis that says that teams want most traction possible in this situation.

[Katie_UPS]

Originally Posted by Akash Rastogi

Not necessarily, that's with the assumption that your team feels traction is most important. Even if Dean or Woodie said it doesn't make it true either. There's no defining basis that says that teams want most traction possible in this situation.

Ok, well, in my mind, on the ride from kick-off to shop, that was the most important thing to me, because I couldn't think of anything else to worry about without knowing the rules/restrictions. And so thats where that comes from.

I should have specified that that was an "In My Opinion" thing.