Spinner Energy

[tim_jones]

Providing any figure arrived at is achievable by a good engineer, it should not be a problem. There should be no issue with a system that rewards good, efficient and safe engineering.

My thought was more along the lines of the inevitable % loss that everyone suffers in their drivetrain. I have no idea how to quantify this, but am very aware that in our case the calculation of theoretical figures is usually tragically inaccurate ;(

Losses due to our standard of engineering are our own problem and no-one elses. Subject to common sense safety concerns of course.

Regards

Tim Jones

[jamesb]

Hi Tim,
I have been working with Kev on this, and at our last discussion at the FRA meeting, we discussed a method of determining the max rpm of the system. It was suggested that, as it would be dangerous to get close to a spinning disc to measure it, the output shaft attaching to the disc should be tachod for the rpm. not only will this eliminate the need to work out gear ratios etc, it has the side effect to factoring in losses and inefficiencies as we are getting output rpm not input power.

The things kev and I are investigating are to allow spinners to run, and although its unlikely the final solution will be 100% acurate for every spinner, it means they can come and play, so if we are talking about 5 or 10% inaccuracy, thats better than staying at home. As the live circuit gains experience in running spinner, Im sure further developments can be made to get closer to exact figures.

Hope this helps.
James

[richard_morris]

OK - I am going to have my say now.
Firstly - There has been a lot of rubbish written above - I wish some people would check their physics books before describing facts.

As we have a spinner, we want to use it and use it in a safe way that is reasonably competitive. Therefore I endorse the concept of trying to arrive at a workable ruling - but have seen little to encourage me yet. Let me explain.

1)Simple estimations of MOI dont work in many cases. Forget mass and guessing where most of it is - just use a rotating pendulum and measure the thing directly. I have had several communications with Paul Hill about his site - some of his equations are wrong (still) but he insists in trying to derive them from first principles. OK thats fine for him, but if he gets it wrong.....

2)Measuring rotational speed - this is easy - yes you can use a tacho - but I for one will not stand next to 13B at full disk speed - it needs to be remote - either a laser or on a very long stick with remote read out. This we have done before. Question - since it takes several minutes to achieve maximum speed (which you never do in the arena) just when will you take the reading? After 1 sec, 10 sec, 30 sec, 60 sec? All will give different readings.

3)Blade design - This is a fun one. Imagine an infinitely smooth drum as a weapon. OK you might not want to put your hand on it at high speed, but it is hardly a threat to an arena. Compare that with a triangular section hardened steel point for maximum crack inducement - as we have on 13B - a little more dangerous perhaps even if the disk has the same energy.

4)What is acceptable? I saw Kevins fancy curve plots on Sat - but how on earth has that any relation to Jonnos arena? Or anyone elses?

The crucial question in my mind is :
How much energy will this disk impart to this arena wall in a likely (or a worst case) impact, and can the arena withstand this impact without any ejection of material outside the arena?

Disk speed, MOI, blade shape, blade material, blade stiffness, arena shape, arena stiffness and arena material, potential angles of attack etc ALL NEED TO BE TAKEN INTO ACCOUNT.

Some empirical data can be obtained by testing. I strongly suggest we do this so that we all can learn real numbers and make sensible decisions.

Rant over!

Richard
13 Black

[robotwars]

as i said, im willing to make the arena available for testing purposes before an event.

Jonno

[jamesb]

Richard, as an experienced roboteer it is clear you know what you are talking about, but it is almost impossible to implement a system based on your rant. Kev and I are not inexperienced know nothings, and have spoken about everything you mentioned, but it is impractical to factor in everything or we would be there most of the morning deciding if a spinner can run.

This is not an attack on you, as personally Ive a great deal of respect toward you, but we must make assumptions or we wont be able to run anything, as I said earlier, the system can evolve once in place.

In responce to your points,

1) we MUST simplify the design and obtain an approx MOI, or we will be there all day.

2) By measureing the shaft speed, we iliminate most of the problem you mentions, as we are in effect measuring the unloaded speed plus friction.

3) we are not bringing tooth design in as this is a point of contention with many people sure their differing solutions ore more efficient. It also depends on what materials we are hitting and the weight of them. In practice, best assume the teeth are 100% efficient and impart the full energy without loss.

4)we dont know.... until testing is done, we have no way of knowing what energy levels apply to what arenas, I have personally run a 5kJ spinner in my arena, but the opinion on that is divied, and I think that is going to be a major problem we will face as diffenet groups argue what they can run.

Until a basic system is in place, we cant really obtain the amount of data wed like, you can only do so much controller testing, the real figures come out in real fights.

If you have any suggestions I would be very keen to hear them, as I this is not an easy task, for me anyway.

Cheers
James

[big_nipper]

Beening serious for a moment you boffins with spinners will know if this will work. Its just an idea but as it seems that a method for limiting spinners is undecided I thought Id mention it. If you attached a G-Force meter on or near to the blade of the disc and span it very slowly (a known rpm) and hit a solid object that stopped the disc could you calculate the energy of the impact? If so you could then recalculate this for the top speed of the disc. A simple Accelerometer (Spelling) such as those used in Car Airbag systems could be used and are relativly cheap.

[jamesb]

possible I suppose, but youre not using mine

This is the next thing we need to look at I think. We know what energy is in a disc, thats the easy bit, its how it imparts that energy to the other person that causes the headaches.

Well get there, but it wont be a simple solution I dont think.

[james...venom]

also an important fact that i feel hasnt been brought up is the amount of traction the robot has. If you have 6 wheel drive, 200mm wide sticky wheels you will put more energy into the a hit as a 2 wheel drive robot with 25mm wide hard wheels . if both robots had exactly the same disc etc etc.

[jamesb]

true, we saw this with Tantrum, with just 2 hard wheels he did spin away from the hits, which a 6 wheeler would be less likely to do.

There are other things that effect the potency of hits too, but I wont go into the one I mean or everyone will build one!

[stinger]

WRT Richards comments (an a few others) - we have to start by assuming worst case :-
1. You can impart Total KE into the oposition
2. Disc can achieve Max Possible rpm from your motor, voltage and gearing.

Of course very few will get anything this good - but there are some damn fine engineers out there who will (are) getting close.

Now - this pendulum thing. I rather like that.

If we get people to remove their disc - measure the shaft speed, and swing the disc to work out its inertia - then I think we could do a very similar set of curves showing the allowables.

I was going to work up those mass*dia^2 lines - but recognise that relies on the selection process to decide which chart to apply.

If we built a pendulum rig and callibrate it - then well only need ONE chart. Can anybody share the formulae with me (wasnt it posted here a while back ?)

Would really like to take this concept further - but finding the time is tough :sad:
Kev.