2 Motors is Faster?!

[waialua359]

Originally Posted by 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).

sounds right on to me.

[Uberbots]

Originally Posted by 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.

Why would you use an automatic traction system when there isnt any traction in the first place?

[AdamHeard]

Originally Posted by Uberbots

Why would you use an automatic traction system when there isnt any traction in the first place?

if you can keep your wheels from slipping, that is a significant advantage. I've heard .06 and .05 for static, .05 and .04 for dynamic. Either way, you lose about 20% of your traction once you start slipping.

[=Martin=Taylor=]

Originally Posted by AdamHeard

if you can keep your wheels from slipping, that is a significant advantage. I've heard .06 and .05 for static, .05 and .04 for dynamic. Either way, you lose about 20% of your traction once you start slipping.

The effects are enormous. Try it out and see.

A bot moving very slowly pushes much harder than one with spinning wheels.

[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)

[Dominicano0519]

actually how does a motor on each wheel sound and maybe even an 8 wheel drive for the traction it would give you and a really big and slow gear ratio