2 Motors is Faster?!

[dtengineering]

Originally Posted by Hachiban VIII

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.

Notice the coefficients of friction published in the manual. The static coefficient (where the wheel is NOT skidding) is .6 the coefficient of dyanamic friction (where the wheel IS skidding) is .5

If a four motor drive and a two motor drive are both given the same input signal the four motor drive will generate twice as much torque. This will make the wheels skid (spin freely, do a burnout, whatever you want to call it) sooner than with a two motor drive. At this point you have shifted from static to dyanamic friction.

As soon as that happens, your friction, and thus your accelleration, drops by almost 20%.

It is quite possible for four motors to be equally as fast as two on this surface, but since the determining factor is traction, teams will have to build their robots to avoid spinning their wheels in order to achieve maximum accelleration and pushing power.

Jason

[Eugene Fang]

Originally Posted by dtengineering

Notice the coefficients of friction published in the manual. The static coefficient (where the wheel is NOT skidding) is .6 the coefficient of dyanamic friction (where the wheel IS skidding) is .5

Actually, I think they were 0.05 and 0.06. They're really easy to slide....

[Paul Copioli]

This was posted in another thread, but here are the equations:

General force equation (assuming constant mass and somewhat constant accel which is good enough for now):

F= mass * accel

The wheel to floor interaction is governed by this equation:

F = mass * g * COF (Coefficient of friction)

Set the equations equal to each other and solve for acceleration:

accel = g * COF

So your maximum acceleration is entirely based on the planet you are located and your coefficient of friction. Since the GDC couldn't control where we play the game, they simulated this by mandating a certain COF for all.

Assuming the COF is actually 0.05, then our maximum accel is 32.2 f/s^2 * 0.05, or 1.61 feet / sec^2

Anyone with more acceleration than that will slip. This is not about 1 motor or 2, but about how we manage acceleration.

For those of you that think this doesn't make sense, just think of this:

While it is true you can get more pushing force from a higher weight, it is also true that you get less acceleration due to the increased mass. This trade off is the center of drive base design for this year's game. If you can control acceleration, then you will control the game play.

[DarkFlame145]

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 give out too much torque, which when there is low traction just spins the wheels. If you have ever watched a drag race where a car didn't heat up the tires well, they go, but also just spin the tires down the track. The trick for this year is finding the right amount of torque i think.

[Daniel_LaFleur]

Originally Posted by DarkFlame145

4 motors give out too much torque, which when there is low traction just spins the wheels. If you have ever watched a drag race where a car didn't heat up the tires well, they go, but also just spin the tires down the track. The trick for this year is finding the right amount of torque i think.

Not quite true. You can gear the output of 4 CIMS so that your wheels won't slip (Read Pauls post above). That being said, I don't believe you'd gain much by doing so.


@ Paul Copioli,

Technically I believe that the wheel to floor interaction is governed by
F = CoF * downward force (which usually is gravity * mass, but can have other forces interacting with it as well)

[paulcd2000]

Because of the extremely low friction (mu=.06), the max acceleration a robot can experience is well below what the two motors can provide, so the max acceleration of a two/four motor drive are the same. HOWEVER, i think the 4 motor drive could have a much higher high speed. However, i don't think you could readily attain it on the field because the field is actually pretty short.

[Daniel_LaFleur]

Originally Posted by paulcd2000

However, i don't think you could readily attain it on the field because the field is actually pretty short.

Nor do I think you want to. Remember everyone that you'll need to stop too, and that also requires traction

[paulcd2000]

Originally Posted by Daniel_LaFleur

Nor do I think you want to. Remember everyone that you'll need to stop too, and that also requires traction

that is a very good point... while refs will be less ready to issue ramming penalties, deliberately ramming into the end wall at high speed is probably still discouraged.
My team considered using raisable and lowerable wheels as brakes, angled perpendicular to the direction of motion.

[Daniel_LaFleur]

Originally Posted by paulcd2000

that is a very good point... while refs will be less ready to issue ramming penalties, deliberately ramming into the end wall at high speed is probably still discouraged.
My team considered using raisable and lowerable wheels as brakes, angled perpendicular to the direction of motion.

Not alloew according to <R06>

<R06>
ROBOTs must use ROVER WHEELS (as supplied in the 2009 Kit Of Parts and/or their equivalent as provided by the supplying vendor) to provide traction between the ROBOT and the ARENA. Any number of ROVER WHEELS may be used. The ROVER WHEELS must be used in a “normal” orientation (i.e. with the tread of the wheel in contact with the ground, with the axis of rotation parallel to the ground and penetrating the wheel hub). No other forms of traction devices (wheels, tracks, legs, or other devices intended to provide traction) are permitted. The surface tread of the ROVER WHEELS may not be modified except through normal wear-and-tear. Specifically, the addition of cleats, studs, carved treads, alterations to the wheel profile, high-traction surface treatments, adhesive coatings, abrasive materials, and/or other attachments are prohibited. The intent of this rule is that the ROVER WHEELS be used in as close to their “out of the box” condition as possible, to provide the intended low-friction dynamic performance during the game

[paulcd2000]

that.... is a good point. i imagine our rules guru would have caught that, but thank you.

[Madison]

Originally Posted by Daniel_LaFleur

Not alloew according to <R06>

<R06>
ROBOTs must use ROVER WHEELS (as supplied in the 2009 Kit Of Parts and/or their equivalent as provided by the supplying vendor) to provide traction between the ROBOT and the ARENA. Any number of ROVER WHEELS may be used. The ROVER WHEELS must be used in a “normal” orientation (i.e. with the tread of the wheel in contact with the ground, with the axis of rotation parallel to the ground and penetrating the wheel hub). No other forms of traction devices (wheels, tracks, legs, or other devices intended to provide traction) are permitted. The surface tread of the ROVER WHEELS may not be modified except through normal wear-and-tear. Specifically, the addition of cleats, studs, carved treads, alterations to the wheel profile, high-traction surface treatments, adhesive coatings, abrasive materials, and/or other attachments are prohibited. The intent of this rule is that the ROVER WHEELS be used in as close to their “out of the box” condition as possible, to provide the intended low-friction dynamic performance during the game

A wheel perpendicular to the direction of travel can meet all of the criteria of this rule. The tread of the wheel is contacting the ground and the wheel's axis of rotation is parallel to the ground.

If having wheels skid sideways violated this rule, wouldn't all teams violate it at some point during a match as they're pushed around by their opponents?

[paulcd2000]

Originally Posted by Madison

A wheel perpendicular to the direction of travel can meet all of the criteria of this rule. The tread of the wheel is contacting the ground and the wheel's axis of rotation is parallel to the ground.

If having wheels skid sideways violated this rule, wouldn't all teams violate it at some point during a match as their pushed around by their opponents?

This may be something that will get updated in future rule sets. I agree that as stated, it doesn't forbid it, but i would be careful about designing a robot that way.

[Daniel_LaFleur]

Originally Posted by Madison

A wheel perpendicular to the direction of travel can meet all of the criteria of this rule. The tread of the wheel is contacting the ground and the wheel's axis of rotation is parallel to the ground.

If having wheels skid sideways violated this rule, wouldn't all teams violate it at some point during a match as their pushed around by their opponents?

In his application, the wheels are purposely not in their "normal" orientation.

[paulcd2000]

Originally Posted by Daniel_LaFleur

In his application, the wheels are purposely not in their "normal" orientation.

"normal" as specified only refers to vertical orientation, not which way it's facing.

[Ziaholic]

I do not believe that they meant "normal" when they say "normal".

I beleive that they are using the word "normal" to mean perpendicular to the floor. As long as the rotational axis is parallel to the floor, then the wheel is being used in it's "normal" position.