2 Motors is Faster?!
 

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? :confused:


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

Re: 2 Motors is Faster?!
 

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

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

I see... That makes sense...

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

Re: 2 Motors is Faster?!
 

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...

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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).

Re: 2 Motors is Faster?!
 

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 :)

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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)...

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is 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

Re: 2 Motors is Faster?!
 

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

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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)

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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

Re: 2 Motors is Faster?!
 

:p 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.

Re: 2 Motors is Faster?!
 

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

Re: 2 Motors is Faster?!
 

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

Re: 2 Motors is Faster?!
 

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?

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

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

Re: 2 Motors is Faster?!
 

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

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

So if you made omni wheels out of these kit wheels that would be "normal" (since the use would be used as designed)?

**note: I'm not suggesting anyone make omni wheels this year**

Re: 2 Motors is Faster?!
 

Normal: yes, but it would violate other parts of that same rule (must-be-rover-wheels parts). at least, that's the way i see it.

Re: 2 Motors is Faster?!
 

Because I'm too lazy to read through 4 pages of things :p I'm going to just go ahead and post what a mentor and I though up of as the reason for the two robots. We figured that since the fricitional force is the normal force times the coefficient of friction, then a heavier machine would generate more traction. But according to Newton's First Law, an object in motion wants to stay in motion, and an object at rest wants to stay at rest. Therefore, for the heavier machine, the inertia of it is far larger than that of the lighter machine. We both felt that the four motor machine was heavier, but could not produce significantly more traction to counteract it's inertia. Therefore the 2 motor machine accelerated faster only do to its smaller inertia.

Re: 2 Motors is Faster?!
 

While I'll agree that deliberately ramming the end wall at high speed is hardly something to encourage (although there are bumpers to help absorb the impact) high speed ramming is quite legal this year. As is pinning a robot to the boards, sitting in the middle of the playing field and spinning around with your trailer behind you like a big hammer and all sorts of fun and crazy stuff. As the head ref said at kickoff "we expect there will be very few penalties this year". That's because there are very few rules!

You may wish to go back and read the posts, where you will find several excellent explanations of the physics at play. You will find that your suggested explanation is somewhat incomplete. In the future, should you wish to join a discussion, please be respectful of those who have already posted by taking the time to at least glance at what they have written.

Jason

Re: 2 Motors is Faster?!
 

It seemed to me like the four motor drive wasn't *that* far behind. The difference in the times looked more or less neglegible. I'd wager that it could have gone either way, depending on the reaction times of the drivers, the charge of the batteries, and a multitude of other factors.

I think the point was that every drivetrain is going to be able to easily overcome the available traction, so torque is largely irrelevent this year.

Re: 2 Motors is Faster?!
 

Torque is extremely relevant, just not necessarily desired. You may find that lower torque drive trains work better, or there may be little difference in performance, but the important thing to remember is that available traction is the most limiting factor, not available torque.

Re: 2 Motors is Faster?!
 

Paul is right, being able to control acceleration is the key to this game, at least as far as mobility is concerned.

The demo at the kickoff where they drag raced the 2 robots is a great example of this. From the audio, you can hear that these robots were spinning their wheels under full thottle for about 7 seconds. Estimating the distance they both travelled in this time puts their approximate acceleration at ~.8ft/s^2. This is only about half of the theoretical maximum acceleration rate of 1.61. Why? because rapidly spinning the wheels (burning out) greatly reduces the coefficient of friction. It looks like it drops by ~50% or so in this case.

Thus, if you can figure out a good way to optimize your acceleration rate (AKA Traction Control), you will be able to easily out run anyone else who has not done so. Of course, you can only run until you get to a field boundary, but like with all games, getting there first is always a big advantage.

Re: 2 Motors is Faster?!
 

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

Re: 2 Motors is Faster?!
 

No. The axis of rotation of some of the rollers will not be parallel to the ground.

I would tend to agree with the interpretation that "sideways" wheels are allowed, because the rules seem to clearly spell out their definition of "normal" in the parenthesis following the word.

Re: 2 Motors is Faster?!
 

Be very careful regarding how you interpret items in parenthesis within the rulebook. As was demonstrated by <R16> last year, statements within parenthesis are not part of the official rule.

I'm not saying "sideways" wheels are or are not allowed, only that statements in the rules within parenthesis should not be overly applied.

Re: 2 Motors is Faster?!
 

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

Re: 2 Motors is Faster?!
 

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.

Re: 2 Motors is Faster?!
 

sounds right on to me. ;)

Re: 2 Motors is Faster?!
 

The effects are enormous. Try it out and see.

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