Spinner Energy

Elsewhere on the FRA site, you may have seen that Im working with James Baker on some guidelines for Spinning Weapons. We have a neat way of assessing the Kinetic Energy in your designs which shows how fast we would let you run. We only need to make a crude assessment of Style (mass distribution) + measured figs for actual Mass & Dia.

Now, at present the FRA are NOT sanctioning use of spinners except under controlled conditions in Class 2 arenas. James and I want to set sensible rules that will allow more spinners, albiet with a limit on permissible rpm.

We suggest that you start designing them with alternative drive systems - one geared down for live events, and another geared up for serious combat. You never know, the reduced spin-up time with a low ratio might even be an advantage ! Please note, that we do not anticipate permitting electronic speed limiters (but would allow reduced voltage).

We need to collect as much data as possible to decide where we should set the level for different arenas. For example we believe the most aggressive spinner is up at around 38KJ, with the most well know in the region of 16KJ, and Feathers peaking at 5-6KJ.

We have some experience of 6KJ in Jonnos arena, and want to collect as much additional info as poss to help establish challanging, but manageable poower limits.

Please post details here, or if you dont want to go public, then send them direct to me or James.

We would like to see Basic Dimensions of the spinning element plus its Mass and anticipated RPM. If you have a calculation for KE - then please post that as well.

Thanks,
Kevin S

Vortex - 3Kg
4500rpm
Diameter 200mm - almost all mass at the edge of rim - it only has 2 spokes on the disk.

No idea what energy that translates into, but hope the figures are of some use.

Ed
http://www.stormrobot.comhttp://www.stormrobot.com

Strip - Feather weight - 3.5Kg disc
3500rpm
Diameter 500mm - mass equally distributed apart from two 0.25kg teeth on outer edge.

Jonno
http://www.roamingrobots.co.ukwww.roamingrobots.co.uk
http://www.ukinnovations.co.ukwww.ukinnovations.co.uk

Big Daddy- Featherweight- (revised disc from my profile pic) 2kg Bar at 3000rpm, 500mm dia, vertical
with 0.8kg mallets at the end of each side.

Think I can be of a little help here;
Vortex generates 3KJ of kinetic energy
Strip produces 8KJ of kinetic energy (@3500rpm)

Rough figures based on info above

Looking at Strip and votex I think those figures sound wrong but what to I know. I dont know how to work it out either, but I do know that 90% of the mass of vortexs disc is on the rim and strips is evenly distrbuted. Going off the size of the discs i would say if the speeds are true they are very even in power. I wouldnt say that its 2.5 times the power of vortex.

Prove me wrong.

Does look wrong doesnt it? But it isnt (based on the figures above (assuming Jonnos can actually do 3,500rpm)

It has to be remembered that KE is proportional to the SQUARE of velocity. (KE=1/2MV^2)

Unlike Momentum which is directly proportional to velocity (P=MV)

Since Kinetic Energy is proportional to the Square of the Velocity, this means that a Mass traveling at 2 metres per second has 4 times the Kinetic Energy of the Same Mass travelling at 1 Metre per second, Even though it only has double the momentum.

Perhaps measurements should be compared in terms of momentum (Kgm/s) and not Kinetic Energy (KJ)?

Anyone please feel free to correct me on any of this, I am by no means skilled in this field.

Roger.

Roger,
Yes you are right in princable but we are dealing with rotating mass not mass traveling in a straight line!
There are questions involving moments of inertia to contend with as well as angular velocity.
I also confess to be a bit rusty but as I remember it the kinetic energy of a rotating mass is somthing like..


Kinetic energy = ½ moment of inertia X angular velocity squared

or E = 1/2Iw^2


I hope this helps, although as I said its been a long time since I really had a use for maths like that so dont take it as gospel!

Geoff,
Team Scorpion.

Sorry, yes KE=1/2MV^2 is for mass in a straight line - didnt mean for anyone to use it as it will not calculate KE of rotating mass - but does clearly show how KE differs from Momentum which is how most people think.

Been giving this a little more thought, The calculation of angular velocity is relatively simple. But the problem I can see in trying to calculate KE in a rotating weapon is going to be determining the moment of inertia. It will totally depend on the construction of the weapon . How mass is distributed through the disc will have a serious effect on the value. I think (and I may be wrong) but the only way to arrive at an accurate value will be to calculate it for every component of the disc and add it all together to arrive at a total value. Which is fine if you could weigh all the component parts separately, this would also mean having to calculate their velocity which of course is relitive to their radius from the center but how you calculate that once the disk is built, I have no idea!
Maybe some one who understands this better than I may have a solution. I would love to know what the answer is!

This is just an idea off the top of my head: You could produce a maximum moment of inertia, based on the assumption that all the weight is on an infinitely thin band at the circumference of the disc. It wouldnt be perfect but it would be better than no measurement at all.

The only other way that you could measure a disks stored energy would be to measure the work required to accelerate the disk to a fixed velocity over a fixed time. Havent thought this through yet, but once a formula is thought out and equipment set up, measuring a new disk would simply be like balancing a wheel.

Not saying its easier, but should be more accurate, and require only the initial effort as opposed to having to take each disk from scratch, and as you say calculating each change of shape/part as you would have to. This may be easier for one or two, but over time still being laborious.

Dont have all the answers.
Roger

MOI of a disc rotating about its centre point = 1/2 x Mr^2
MOI of a bar rotating about one end = 1/3 Ml^2
where
M = Total Mass
r = radius
l = length

If the mass is not rotating about its centre of gravity then you have to use the parallel axis theorem to calculate the additional MOI. This is
(Total MOI) = (MOI about centre of gravity) + (Area of object) x (distance of axle from centre of gravity).

so for vortexs disc if modeled as a large disc (200mm) with a smaller disc cut out of the middle (140mm?) and a bar put across (width 30mm) then total x-sectional area = 0.068 m^2
mass per unit area = 43.9 kg/m^2

Total MOI = 0.03 kgm^2

Total KE = 3.3 KJ

Similarly for strip:

MOI = 0.095 (seems a bit large)

KE = 6.4 KJ

Ive just done these in a rush and am pretty sure one of them is wrong .If Anyone wants me to explain my methods or any thing then just post your e-mail or e-mail me and i will send you the full calculations

Joe Townsend

Also other ways to calculate moment of inertia are
1: to take the disk off, place it on a rotational pendulum and calculate the difference in period.
2: place the disc and axle on two bars at a slight decline. By timing the disc rolling measured distance at a measured angle you can calculate the MOI. If anyone wants a full explanation of these method please feel free to e-mail me and I will dig out my 2nd year lab books.

Joe Townsend

I found this,
http://homepages.which.net/~paul.hills/Spinningdisks/Disks.htmlhttp://homepages.which.net/~paul.hil...sks/Disks.html
I think that covers it all!

Many thanks to you Joseph for reinforcing the difference in KE between Vortex and strip (though not quite as extreme as I got - But probably much more acurately) I feel vindicated.

Geoff - Fantastic find - Everything needed, and by your component method.

Ed - Hope you are not too dissapointed in the KE produced by Vortex - Vortex speaks for itself. (compliment).

Roger.

Im impressed with the erudition of the above posts, but the purpose of this exercise must be to define a fairly simple limit for the KE of a spinner that is a bit cleverer than Mentorns 1500rpm rule. The limit needs to be crude enough to be checkable. That rules out any consideration of weight distribution. Here is how I would do it for heavyweights:-

Spinner weight(kg) * (Radius(m) * Speed(RPM)) squared 5,000,000

Then we get into the more interesting argument about whether 5 million is too much!

Jeremy

while finishing my final degree project.... which was to design a new robot arena, i went through all of the sums.

Ill have to pop into uni and pick up the report, but as far as i can remember it is very difficult to work out the ke,

You have to know what the time taken for the disc at full speed to decelerate to nothing in Milliseconds is )


Then you put that into a formula with the other values and hey presto, you have a value.

They do the same for crash tests and if you look in the books, they use experimental method to determine the time.

I will look it out.

jonno

That website says it perfectly, good find geoff. I think it may start to get misleading though if people start to talk (boast) about their spinners in terms of KJ of energy, to me it means very little as strip has far more KJ than vortex but if is far less destructive (from what ive seen). Does anyone know a good way to measure a discs effectiveness numerically?

Guys,
If you thought that section was cool...Just check out the rest of that site!! :)

http://homepages.which.net/~paul.hills/http://homepages.which.net/~paul.hills/

Try this it does all the work for you...:)
http://www.teamcosmos.com/ke/ke.shtmlhttp://www.teamcosmos.com/ke/ke.shtml

Why would I be dissapointed ?

Vortex can go on delivering a 3Kj hit time... and time... and time... and time again :)

In terms of total Kj delivered into opponent during a fight I would fancy Vortexs figures over anyone else :)

Ed
http://www.stormrobot.comhttp://www.stormrobot.com

At least for the time being....

In terms of disc power is KE due to disc size/weight all there is? If you have a light disc with a very very powerful motor has anyone worked out the effect of the motor. The energy stored in the motor itself must add to the overall Enery in a hit. I might be wrong but the electrial power within the motor should have an effect.

Id guess that the only energy the motor can deliver is KE stored in the spinning rotor. Since that is close to the center of the disc its contribution will be very small.

Eds motor, say it was a 2kw magmotor, is producing 2Kj of energy, and as his disk is twice that, to ignore the effects of the motor in this equation is silly.

James thats not entirely true in that were talking about potential energy in flywheels, not the fact that his 2kw motor is producing 2kj/s of energy of energy, which obviously affects how fast the disc spins up, but not the amount of stored energy. In terms of what Mark and John where talking about, the MOI of a mag rotor would have little bearing on the final outcome. So ignoring the effects of the motor in that equation is entirely justifiable :-p

Having said that, with talk of LEMs and such, I would have thought the MOI of a LEM rotor, especially in terms of featherweight weapons, is reasonably substantial, and so should be taken into consideration.

With reference to Marks question, taking it to the extreme of, for example, an antweight spinning blade attached to a GPA 750, then obviously the MOI of the rotor would have a big effect, but more importantly its about the maximum potential K.E your flywheel can store- a smaller, lighter disc, will not have such a large potential. This means that it does not take your very big powerful motor much time to fill it up (a yuck way to put it, but you get my point) with energy. So, choosing some fairly arbitary values, your disc may have a maximum potential of 1kJ, and your motor may be 4kW- or 4000 Joules/sec, so obviously the motor will fill (yuck again) the disc with energy rather quickly. So, as the light disc is rapidly dissipating its energy into its opponent that its just hit, the motor is pumping loads of energy back in, thus keeping the disc going and going- its the relationship/ratio between the power of the motor and the energetic potential of the disc. I suppose this could bring us to the same point that the flipper folks have got to- trying to find the sweet combination or golden ratio between larer bore (so more powerful) rams which consequently take longer to fill up, and less powerful rams which actuate more quickly. In our case, its a question of having a bigger/heavier/faster disc that takes longer to gain its max potential K.E, or more powerful motors/bigger batteries that get a disc with less potential up to speed more quickly, and keep it there.

To add more controversy over how good a measure KE is of a disks potential damage / safety issue etc.

The disk on Revolution 3 is a 12kg 8inch disk @2,500rpm and is very capable (done it) of flipping a 100kg robot clean over - easily.

If you calcualte its KE, it is not much more than that of Strip (about 7,500 KJ) - But in terms of damage potential / safety concerns, it is far greater (or so I would like to think).

Perhaps a simple table would work ie Disk Weight crossed with Disc Diameter and find Max permitted speed - This would still allow people more scope to optimise designs within the restraints.

Roger, what makes you think Revolutions disc is more dangerous than Strips? (Dont take that question the wrong way!) To my mind, flipping a 100kg robot should take a certain amount of energy; if Strips disc has that then it should be able to do it too. The only differences I can think of would be the length of the collision, which would be the most difficult variable of all to pin down.

The little bit on the theoretically distance you will throw your opponent when you hit them with you spinning disc, whilst highly theoretical, and ignoring a million other factors, is nonetheless quite interesting!

The maths says my feather should throw another feather 60m. Ill beleive that when I see it!

In our case, its a question of having a bigger/heavier/faster disc that takes longer to gain its max potential K.E, or more powerful motors/bigger batteries that get a disc with less potential up to speed more quickly, and keep it there.

or in our case, shove half the weight in the disc and spin it up slowly (compared to other discs) and rely on driving skill.

this is a crazy random idea i had......

wot if a machine were somehow created that could take a blow from the most powerful disc but measure how much energy (or give a rough estimate) was delivered. Sorry its crazy and there are a hundred reasons against it but i thought id share it! :)

WOW - thanks for all the interest - especially the websites, and even more to the souls at the start who published their statistics. It only seems like a moment ago that I launched this - now 2 archives already :) :sad:

The formula derive from that bloke Newton and his Laws of Motion. They are Fixed Laws resulting in Kinetic Energy which is measured in Joules (Kilo Joules fit better in our situation). The skill is in how to make a simple assessment of all possible discs and work out the max permissible rpm for anything that might turn up at an event.

Also, each arena will need to have an Energy Rating, which will need careful monitoring by the FRA.

I believe it is possible to create a range of charts which represent all potential styles of disc (these represent different Mass Distributions) and with a few diagrams, its feasable to select the most appropriate chart by visual assesment of the subject.

Ive done the prelims which show Mass x Dia^2 on the vertical axis (log basis - sorry) and Speed on the Horriz. These each have 5 curves drawn that show 5, 10, 15, 20 & 25 KJ.

Visually assess to choose the best chart, Weigh, Measure and Multiply to find your M.D^2 value, plot this Horrizontally and compare your max permissible speed with the agreed allowable for that event.

The BIG problem is deciding where the limit should be set for different arenas. Clearly experience is going to play a major role and organisers (FRA) will need to be extremely cautious (coz if we get it wrong and hurt somebody - weve made ourselves liable for prosecution!)

All I can say is that My recomendation on Sat is that we start with an energy limit of 5KJ (which may be cut lower) - and roboteers should exploit this by building systems with alternative drive systems that have different potential speeds.

We will need proof of your potential speed and are likely to request the disc is removed (for assessment) and the shaft run up and checked with a tacho.

More after the weekend,
Cheers,
Kev.

To digress slightly - has anyone got any ideas for measuring the speed of a disc? I know we all like to talk big numbers (I do myself), but how do we get a little more science, anda little less bragging rights?

eddy, do you actually know whay a Kj is?
a Kj is watts per second.

If a disc has 4 Kj potential energy, and a 4000 watt spinning it, if it takes 1/4 second to stop it, you have applied 4x4 kj kinetic, and 4/4 motor power, ie 16Kw and 1kw = 17kw of impact energy.

ignoring the motor gives 16kw, thats 7% from the motor.

look at Eds.
3Kj in the disc, 4500watts off the motor.

1/4 second impact, and.....
12kw kinetic and 1100watts motor.
thats 10% of the total energy.

see, its not about the weight or MOI of the internals of the motor.
Hope this clarifies things.

I agree entirely with that, but from my experience the length of an impact is a lot shorter than that, perhaps a 10th of a second at most. Of course I appreciate there are situations where, say, a large tooth spinning disc burries itself satisfyingly into a sheet of poly, and so will have a longer deceleration, but in most cases impacts last a very very short time- sufficient that the amount of energy produced by the motor that is directly transfered into the opponent is negligable in comparison to the energy the flywheel is transferring. It would be interesting to get a figure on what most people would regard as a typical impact time, but I would be fairly certain that it would fall short of 1/4s. Of course a shorter impact time is desirable, as its largely the rate at which the energy is transfered into an opponent that counts. Over the course of a days sailing, your average yaught sail must have had kilowatts and kilowatts of energy transfered into it from the wind, but nout happens in the way of damage. Focus all that energy at once (imagine some uber gust of wind) and you suddenly find youre missing your sail, mast, wrigging etc.

Im sure tooth design must play a huge factor in this- do people generally prefer the sharpish armour piercing teeth, or a bludgeoning type of tooth- Im sure that should perhaps be taken into account when deciding what is safe to run and in which arena- I would have thought that a thickish steel arena sidewall would be happier taking a bludgeoning, as opposed to a piercing, toothed spinner- thoughts gentlemen?

Well the RR arena, has 6mm steel plate as the kick board all the way round, and has proven strong enough for impacts, and spikes, and i would feel comfortable that it would take a general impage from a spinner.

However....

It is the worst case scenario that worries me about allowing spinners to run under controlled restraints.

ie flipper vs disc.... where horizontal becomes vertical etc... or its flipped over a wall.

thoughts?

Strip was very much untested and only had one real fight at inspire against Aaargh, the two discs met, vertical vs horizontal, and Aaargh enged up being thrown to the other side of the arena.

The 2 teeth are not piercing teeth as such because the tilted horizontal disc is designed to hit / throw opponents rather than pierce.

Jonno

I Still think the motor plays a large part. Its not just about the stored energy. Take a stupid example where you have a 500mm disc with a tip speed of 100mph and only weights 1 gram. If you have a big enough motor (with no MOI) to maintain this speed and produces a torque of 500Nm then if Ive done my sums correcty you will have a point impact force of 100kG @ 100mph.

Dont all shout about it its probably wrong and completely ignores the stored energy in the disc as it does not weigh anything.

Jonno, would it be possible to put a thin (see-through) net or mesh in front of the polycarbonate to foul up any spinners which get above the kick board? I seem to remember chainsaw operators can get clothing with a fiberous layer designed to clog chainsaws, which is where I got the idea. Perhaps a widely-spaced net of really strong but flexible material like shark fishing line, for example?

Steve, would a strobe light be an effective way to check spinner speed? Its the only non-invasive way I can think of to check it. You could also attach a magnet to the spinner and use a coil or hall sensor to check it, but that might unbalance the flywheel.

The spinner I have right now, relies quite alot on the actual torque and inner mass of the motor, at 3000rpm, it has lots of torque in itself, and will keep hitting, hit after hit. Yet the actual mass of the weapon is only about 2kg.
I think the featherweight spinners should be looked at more carefully, as they are evolving more and more. Strip could well have the power of many heavyweight spinners...

Our tool sponsor - Teng Tools are supplying us with an laser tachometer - simply stick a white spot at the object, spin it up, and point the tachometer at it.

Were planning to bring it along to some events with us this year to give people the opportunity to measure just what their spinner does.

I think people will be surprised just how much energy (and rpm) is lost due to wind resistance alone - espcially bar spinners !

Ed
http://www.stormrobot.comhttp://www.stormrobot.com