Gyro Effect!

[6600gt]

I have searched the site and have not found any specific thread on it.

A single spining at such high rpms will act like a very powerful gyro. It will make the turning harder and if the wheel is placed high on the robot it will help you tip over when you trying to turn while resisting the turn. This is because of the direction its spinning relative to the direction of the robot forward direction.

I think a horizontal wheel will not have as much affect and with 2 then should counteract each others forces, right?

What do you guys think?

[Chriszuma]

I had brought this up with my team, but it's not actually a problem and here is why: It is highly unlikely that you decide to use a material for the spinning wheel that is of enough mass to create a noticeable gyroscopic effect. Yes, a horizontal spinner would eliminate this issue, but they are not as good. The only problem i forsee with gyscopicness would be making sure your mounting brackets are reinforced; it won't have enough torque to tip the robot, but it will definitely be pushing against the mounting brackets.

[6600gt]

But if you make a sharp turn or another robot tilts you a little bit then the added gyro effect could tilt you over. Our team has been debating this, and then want to shoot low and funnel out ball over the robot. This keeps the spinning wheel and the motor low but I believe we will not have the accuracy that a top mounted cannon. Because it would just shoot it in the direction instead of guiding it to a direction we want.

You said a single horzontal wheel would not work but how about two, similar to how other teams are doing?

[Chriszuma]

i was actually talking about the two-horizontal-wheel setup, whether or not i actually said that. It would mostly eliminate gyroscopic forces (except for the fact that it still has to be tilted a little bit to aim high, but this is nearly negligible). Now what i'm saying is that if you use a light enough wheel, with a low rotational inertia, gyroscopic forces won't be a problem. This, however, is a problem in and of itself; you need to have a bit of momentum carried by the wheel to shoot the balls, so finding just the right amount of mass is going to be an exercise in experimentation.

[KenWittlief]

If the flywheel is mounted with the axis pointing up and down, it wont have any effect on your robot, until you try to climb the ramp.

This is a good time for teams to learn about gyroscopes. Most people think they know what a gyroscope does, but they are wrong :^)

using the front wheel of a bike as a reference, when the wheel is spinning if you turn the handlebars to the LEFT the gyroscopic effect makes the bike lean to the RIGHT

likewise if you lean the bike to the LEFT the wheel turns to the LEFT.

the first one doesnt sound right, but it is. Hang a bike from the beams in your garage and try it. The second one is the same action with the axis of forced movement rotated 90 degrees. This is what makes a bike balance on its own, if the bike starts to fall over the front wheel turns to correct the fall (turns into the fall).

what this means for your robot:

if you have a flywheel with a horizontal axis and someone tips your bot to one side, your bot will turn in one direction. And if they push your bot so that it turns, then the bot will also tip to one side. When you turn the bot it will also try to tip to one side- the faster you turn, the stronger the tipping torque!

if you have a flywheel with a vertical axis nothing will happen until you pitch the nose of the robot up or down. Then it will tip to one side or the other (left or right).

If you have two flywheels spinning in opposite directions, the gyroscopic effect is completely canceled out. Ive seen bicycles rigged up with a wheel on top of the front and back wheels, so that as you ride they spin backwards. The bike is almost impossible to ride, and if you give it a forward motion shove by itself, it falls right over.

You would need to have a pretty massive flywheel for these effects to be of much concern. One last thing, it doesnt matter where you mount the flywheel on the bot (high or low). The gyroscopic effect is the same - it applies a torque to the frame of the bot, no matter where its mounted, the same torque.

[Joe Johnson]

Let's calculate a bit:

Torque From Gyro = (d/dt) AngularMomentumVector

Assume Gyro spinning with axle in plane parallel to the ground

AngularMomementumMagnitude = Iw

Assume wheel is a spoked wheel with all mass out OD
I = MassWheel RadiusWheel^2


Assume Gyro w is constant and robot turns at W about its center.

TorqueGyro = AngularMomementumMagnitude X W

Plug in some reasonable numbers:
8.8lbs wheel = 4kg = MassWheel
16" wheel = .200m = RadiusWheel
2400RPM = 250 Radians/Second = w

AngulareMomementumMagnitude = 4 kg X .2m X.2m X 250 Rad/Sec = 40 kg-m^2 / sec

1 Rev Per Second = 2 X PI Radians / Sec = W

TorqueGyro = 40 X 2 X Pi = 250 N-m

WOW! 250 N-m? How much is THAT?

Well, if you have a robot that is 130lbs and 28" wide, it would take 1800in-lbs to tip it (a simple static analysis 130lbs X 14 inches).

1800in-lbs = 200 N-m

So 250N-m is considerable.

Note that there are a lot of assumptions that will probably mean you will get much much less torque from your gyro than this.

• Your gyro will probably not weigh 9 lbs AND
• Your gyro will probably not be a hoop (though maybe) AND
• Your gyro will probably not be 16" in diameter AND
• Your gyro will probably not be spinning 2400RPM AND
• Your robot will probably not turn at 1 rev/sec

But if you did, there would be some significant tipping torques generated (though the dynamics would be more complex than the static analysis I did - for example in order to get that kind for torque, your wheels would have to be on the ground providing the forces needed to turn at 1 rev/sec, but of course as soon as you begin to tip, you'd loose some of that turning force -- it is a pretty complex dynamic situation to figure out).

It is something to think about.

Joe J.

[KenWittlief]

reading Joe's post I realized something awesome

if you had a single flywheel in your robot, with the axle horizontal pointing left and right

and the wheel spun backwards from the way the front wheel on a bike would (top of wheel moving towards the back of the robot)

then if you shot across the floor at a high speed and turned suddenly, the flywheel would exhert a torque that would try to lean your robot into the center of the turn (lean the way you are turning). This would counter the centripital force that would normally make your bot tip over if you turned too fast with forward motion.

that means your bot could corner at high speeds like it was on rails! (if the flywheel had enough angular momentum)

if you tried to turn fast while going backwards, however, the force would be against you, and your bot would easily flip over.

So it you designed a launcher wheel the right way, it would have a helpfull 'side effect' on the handling of your robot (and if you spin it the wrong way, yikes!)

[Joe Johnson]

Originally Posted by KenWittlief

...

using the front wheel of a bike as a reference, when the wheel is spinning if you turn the handlebars to the LEFT the gyroscopic effect makes the bike lean to the RIGHT...

...This is what makes a bike balance on its own, if the bike starts to fall over the front wheel turns to correct the fall (turns into the fall).

...

...
If you have two flywheels spinning in opposite directions, the gyroscopic effect is completely canceled out. Ive seen bicycles rigged up with a wheel on top of the front and back wheels, so that as you ride they spin backwards. The bike is almost impossible to ride, and if you give it a forward motion shove by itself, it falls right over.

...

This idea is a common idea that is just plain wrong.

If you read my unsung FIRST Hero response (Dr. Joe Speaks) you will see that one of the non-random whacks in my life is R.E.Klien.

He has made bicycle stability one of his life's works.

Here is a zero-gyro bicycle.



I assure you it is easily ridden.

The reason a bicycle stays up is complex, but it is MOSTLY due to the forward rake angle of the steering fork and the positive caster of the front wheel (positive caster is when the point of contact of the wheel is behind the line where the steering axis intersects the ground).

In its simpliest form a bike does not fall because these two factors elegantly conspire to provide a stable steering angle that will put the bike in a circular arc where the gravity moment trying to make the bike fall is exactly cancelled by the centripital acceleration. Above a critical speed, the fork & wheel geometry provide a stable feedback mechanism that keep the bike from falling.

Go Here and Here and Here for more detailed explanations of why a bike does not fall and why gyroscopic effects an negligible compared to other factors.

Joe J.

[KenWittlief]

ok, maybe I should not have said "impossible to ride", but if you take a normal bike and give it a shove it will go pretty far before it falls over

if you give the bike in that photo the same shove, it wont get far.

Ive seen it demonstrated on some TV/PBS/Science channel program a while back.

A human learns to balance a bike with little flywheel action (super light tires and wheels) - but on a riderless bike the gyro effect makes a real difference.

either way, the important point for the bots is that two flywheels spinning the same speed in opposite directions do cancel each other out.

edit: ok, I checked some of Joes "here" links. I think maybe we are saying the same thing different ways.

The gyroscopic effect is not what keeps a bike from falling over while its being ridden, I agree with that. The way the front fork is angled and the patch of the wheel being infront of the center line of the fork - yes, that makes the front wheel want to stay straight, and it moves the contact patch under the bike as you lean left and right

but the gryo precession does make the front wheel turn left and right as the bike leans. The author of the first 'here' link at first dismisses this, but towards the bottom of the post he asserts it is a factor when riding hands-free:

if the rider lets go of the handlebars, the front wheel is free to precess about the approximately vertical steering axis. By following the right-hand rule, we see that as the bicycle leans to the right the front wheel precesses to the right, and as it leans to the left it precesses to the left. This is precisely the correct direction, but not necessarily the right amount, to help keep the bicycle upright.

BTW, this all becomes very important when bike riders try to ride a full sized motorcycle. Because of the mass of the wheels, at high speed if you turn the handlebars to the right, the motorcycle will lean left, and go left. Many new motorcycle riders learn this the hard way, going off the road at the first high speed curve they encounter on their new motorcycle (myself included!)

The obvious way to see how much the gyro effect helps to keep a bike upright? Turn it upside down, then push it over, and see how fast it falls down. Then stand it upsidedown again, spin the back wheel up to speed, and push the bike over and see if it falls down at the same speed. (stand back when you do this! :^)

[Leo 1529]

i think that if it was heavy enough it would also raise your center of gravity.
and in all possibility it could tip over. i could see using a softball pitcher but the wheel on that thing is so heavy that it will easily tip you over.

we have mounted one to the top of last years robot and tried driving it. it is easy to turn in one direction but insanely hard to turn the other. and it has a lot of difficulty going straight.

[6600gt]

Is that with a double wheel design or single?
The two wheels in a double wheel design would cancel each other out, according to KenWittlief, and I understand it now too.

It really dosen't take that much energy to launch the poof balls at 12m/s, even the kit of part wheels can do it.

So according to everyone else that posted here:
It doesn't matter how high or low you mount the wheel for the gyro effect
2 wheels create no gyro effect(opposite spinning)
2 horzontal wheels should cancel out all the effect, right?
Bikes are pretty stable.

Did I miss anything?

So the only problem of having a cannon with a spinning wheel high up is the center of gravity?

Thanks a lot everyone.

[Biff]

Three wheels? Effects.. Anyone....

[KenWittlief]

three flywheels, mounted on the three XYZ axis, spinning so that motion in one axis will cause a flywheel to twist into another axis, which will make the second flywheel twist into the 3rd axis?

sounds like perpetual motion ?! I wonder what that would do? It seems like it would be a very ustable system, and as soon as you disturbed it the slightest bit, it would spin faster and faster on all three axis?

is that possible?

I think you just blew my mind!

[Biff]

More like rotating, to impart motion into a ball, (launcher) mounted on three sides of a triangle canted at 15 degrees from the center if the triangle was extruded. Shove ball in you get some spin around the axis of travel. Haven't tried to move test contraption yet will have to see what kinds of motion it resists.

[Alex Burman]

I dare someone to make a bicycle robot that shoots, you can use training wheels to start but thats it.

I never thought to take any of this into account, I'll have to be sure the rest of my team did also.