Printable View
Nick, 90 Degree postion. you mean falling from vertical, the original Xterminator style?
Not what I had planned. 180 degree.
and yes, I need this Axe to self right. without going 180 degrees it cannot self right and theres nowhere else in my design (on profile) for a schrimech.
The figures arent too good for using a gold im afraid. Youd only get about 11mph speed on the axe if the arm is 320mm long and has a tip torque of 13kg (using 18:1 ratio). It really depends on the robot shape to if it will be able to self-right at say 10kg torque at the tip.
I knew that from the start, I just didnt want to admit it.
It would be better if I used Pneumatics but I have no idea how to implement them into a feather.
The figures for an old-style 18v DeWalt drill motor are far better though (available from the US - http://www.robotcombat.comwww.robotcombat.com or http://www.teamdelta.com/products/prod5.htmwww.teamdelta.com/products/prod5.htm). If you brought the matching gearbox (maybe the kit from team delta which includes a mount) then you could get 34mph hammer speed with 14kg tip torque if you use it in low gear geared up by 1:2.
Sorry Ceri, I meant if you have an axe with a 180 degree swing. Using the same ratio as a variable ratio axe system should have at about 90 degrees gives you a fairly decent ratio for a fixed ratio system.
Im afraid Ewans right about the power, a DeWalt or EV warrior is a lot more powerful but of course you have to up-rate everything else to get the best out of it.
How much torque you need to self-right is fairly dependant on robot length, the shorter it is the easier it gets.
Going up to 36 volts from 24 increases the available energy by about 70% if that is an option.
Thanks for the advice on motors but the reason I chose the gold for the example was because I could get it easily in Britain.
36V isnt an option. I have 24V 1.2AH Drill batteries, of which I dont know is gonna be enough anyway... Unless you fancy bending the voltage rule for 48V theres not much of an opportunity...
Length, will be easy what with it being essentially half a cone. It cant be longer than it is high.
BTW. Im getting ~19Joules for the swing with just one set of 1.2Ah (Nicads?) at 24V, thats about a 17MPH hit. It is very nearly as much at 18:1. This is neglecting motor inertia and friction.
Hi guys,
With an electric axe you should be relying on the weight of the head more than the speed its traveling at, thats not to say speed isnt important !
If youre after a good motor,go to your local scrapyard and look under the bonnet of a peugeot 405 ,the radiator fan motor is as good as a EV Warrior, and cheaper.
Progress on my feather hammer is slow but sure,i hope to have something driveable for RAF Fairford.
Pic on profile
:)GORD
koooooool... looks evil
Cool. Reminds me of Beta.
Actually it was torque I put above speed as I understand the sums better...
lol
Ill consider the radiator motor. Since Bosch made the EVs, (to my recollection) any similar motor should be reasonably the same. same logic as - It it aint broke, dont fix it.
Hopefully I would like to fight your hammer bot with my Axe bot but I would warn you Im planning around weapons on a vertical axis.
that looks great gordon. we produced an axe for a customer last year. it performed really well and was powered by a diesil mondeo fan motor. Ye electric axes dont need the speed, its more about the torque of the motor and the weight of the head to create the damage. whereas on punmatics its the opposite, a nice light axe head is best. thinking of making another one some time. Wayne cummings was designing a pumatic axe and the design concept was really good, unlike an axe out there.
if you look at the formula for the energy for a hammer/axe:
Energy = (M P2 s2)1/3
you will see that the power of the motor and the length of the sweep have a bigger effect than the mass of the hammer. Doubling the mass of the head will only increase the energy by 26%. Doubling the power the motor kicks out will increase the energy by 56%.
So to crudely simplify things, if you have to choose between a heavier hammer head and more batteries, go for more batteries.
I am ignoring the 9/8 in the above formula, as it only makes four percent difference.
Then just remember to nail it to the floor ! :)
:)Gord
In a similar vein to what John said, the equations make a kind of specific prediction, that if you have already decided that maximum length of shaft length you can allow, then for maximum energy it is worth spending roughly twice as much weight on power generation as on head mass. At the same time an amount of weight roughly proportional to the amount of energy you expect to achieve must be spent on the drivetrain. Power generation would include the motor and its associated batteries and the switch which drives it, but nothing else except perhaps the first stage gearing and the wiring.
Batteries are available with relatively good granularity, ie. in theory you can add cells one at a time. Head mass and shaft length have unbeatable granularity. Motors are typically not available in such a huge variety of sizes, the equation makes no prediction on the optimal balance of mass between the motor and the batteries, but that is not hard to work out. Granularity is important, because it forces you out of the optimal region of weight expenditure. In effect it is hard to find a motor of just the right size, with just the right windings. If you are limited to packs of 24volts and you are stuck with a motor which makes hardly any power at that voltage your out of luck with that combination. The balance between the motor, the switch and the batteries is a sub-problem to be optimized, in some systems you can fail to get good performance by failing on any sub-problem. The sub-problem for the power-generation system is simply maximise power to weight.
The case with pneumatics is rather similar, except the amount of power available for a given effort is usually very high, potentially so high that jumping has already become a more important factor thing than pure energy transfer, particularly for a feather. In this case to reduce the jumping, you can spend more weight on power generation while reducing the head-mass, you are losing potential energy transfer ability but it is a useful thing to do.
To get a further appreciation of why compromise is important the energy equations also predict that if you have chosen a shaft cross section, it is not energy optimal to have any head mass at all, but to split available mass roughly equally between shaft and power generation. Clearly there have been things left out of this analysis, It further predicts that having a constant shaft dimension is not optimal, and the shaft should taper....If you follow all these temptations you are not left with even much to hit with.
The robot as a whole is a compromise between a great many factors including how well all the pieces fit together, and how hard they are to armour amongst other things, so many that even a much more complicated analysis which bears this in mind can only give reasonable suggestions. Hints, perhaps even insight, but not complete designs.
How much output torque does a Radiator Fan motor have, and what voltage is required for it ? Hells Angel is running on 24v, 3Ah.
I wanna get the best out of my axe, and so I need to know if itll do what I need it to do.
I have to work out the gear ratio so that it can self right my robot (12kg, 59cm in length, top panel is about 39cm due to shape)
The axe length is undecided, because it has to swing over the scoop at the front (meaning it has to travel over 180 degrees, like Behemoths or Shunts)
The first version of our Axe robot Blue only had about 8kg of lift on the axe head but this was plenty to self right.
ok, and from what I hear, the radiator fan motor should be able to muster that at least, but I need to find out how much whack power I can get out of it at 24v, and at what gear ratio.
Assuming the motor is very similar to the EV warrior, see John Reids post on 10 November, 2004 - 12:18 pm earlier in this thread.
20Nm at the output stage is probably just enough to self-right your robot, and you should have much more than that available.
David , The stall torque of fan motors I spoke of, is about 1Nm per 6 volts applied.
I cant remember what the no load speeds were ,I think they maybe around 200rpm per volt.
There nominal voltage is 12v, but Ive had them running very happly at 28.8 volts in Aaargh!.
:)GORD
I would reccommend having a belt or chain in there, we found on the Rad fan motor axe mentioned by James Cooper that the shaft was a simple push-fit, so with the impacts directly onto the gear welded to the motor, it seperated the shaft from the armature. Some shock protection is needed. Also, dont weld the case of the motor, it melts the glue on the magnets and allows the magnets to move inside the motor case. I wanted a fusee drive in this robot but was out-voted, so Id recommend it here.
Lastly, watch your gear reduction, we used approx 6.6:1 but this was way to low and was basically a cost driven solution. Invest the extra few quid and do it properly. If using the mondeo deisel fan motor aim and asuming a shaft length of 330mm and an arm weight of 1.5kg, I would suggest 9:1 or even 12:1, if for no other reson than it makes self righting less of a battery smoker.
Also, the fan motor worked quite well at 12v, but was insane at 24v (until the motor was damaged by welding) so if you get the gear ratio right there should be no need to have silly voltages, and of course as the voltage goes up, the gear reduction can go up to, so 9:1 at 12v can easily be canged to.... say... 12:1 or 14:1 at 24v. all of which helps self righting.
Hope this helps.
I remember quite well the conversations between you and james when decided on what to use. You were saying 6:1 was perfect and james thought around 12-15:1. After long discusions and some talking to john ried, the outcome was 6.6:1 would be the best and most efficient for the job.
Worked well in the end, just the quality of the motor wasnt much to be desired. Might look at making another along the same line, but with a better quality motor.
hummm.......... you seem to have proved me right all along then... see, I do know what I am talking about.
Increased ratio suggestion is purely to help self righting, as the one we built was very high current draw on self righting. But, 6:1 (or 6.6:1) to be exact is theoretically the best (doing the maths) as I said in the beginning.
I would stil reccommend the fussee drive though, and to be honest for the £5 the modeo motor costs, its worth a try, just dont weld it. Welding motors usually breaks them, it isnt worth risking.
Welding things onto motors = bad idea
we used approx 6.6:1 but this was way to low and was basically a cost driven solution.
nah well. welding wasnt a good idea
Deviating a lot from the theoretical peak does hurt the energy of fixed ratio systems quite a lot. More so than with a torque shaping mechanism. With such a mechanism, you may want to let the motor rev quite a bit high, but for reasons other than limiting current.
Whats a Fusee drive ?
Like the arrangement on beta, using snail cams.
http://www.johnhmreid.dsl.pipex.com/beta/progress/leafchain.jpghttp://www.johnhmreid.dsl.pipex.com/.../leafchain.jpg
Its only one option, there are others. In a heavyweight a clutch is practically mandatory especially if you use anything resembling an ETEK to power it. You can probably get away without one in a feather with some motors if the hammer shaft is sufficiently flexible. In that case, drivetrain mass will have to suffer so the gears dont get damaged.
I didnt realise about clutches for HWs. Ill be sure to note that when I get back to building it.
I know that the Fan motor produces about 600W and 4800rpm at 24v, but Im trying to find the torque of the motor (I can probably work out the torque from the power and rpm, but I cant remember which way around it goes right now), so that I can work out what speed and with how many Kj my axe will hit with
I reckon the stall torque would be about 4.8 Nm if max power is 600 watts and no load speed is 4800rpm
But then Ive been wrong before.
:)GORD
i come up with the same as gordon, about 4.8Nm.
Ok, cool. Is trying to figure out how fast the axe comes down and with how much force.
If Energy = (MxP^2xs^2)1/3 and Sweep time = (2xS^2xM/P)^(1/3), I require s (sweep distance) to work it out. This is measured in Radians, right ? My axe has to travel about 200 degrees.
Say I use this radiator fan motor (No Load RPM is 4800rpm)to power my axe through a 12:1 ratio on 24v (belt or chain, I havent deciced, but I think belts would lose to much energy), the axe arm is 330mm long, and the arm weighs in at 1kg and the head weighs in at 700g.
Motor Torque: 4.8Nm
What will I end up with in terms of energy of the blow and speed of the blow ?
well 200 degrees is 3.49Radians
Im in college atm so i dont hasve time to wo4k out the whole equation
Same here funnily enough :P
For calculation purposes, all angles are measured in radians and all distances in meters. Lets also call the effective head mass 1kg and forget about the shaft.
If we assume peak power is 600watts at 2400rpm (Im really not enormously sure it is)
For fixed ratio drive you chalk up at least an effecitve 10% of the available power as lost over a varable system as it wont be running at peak power all the time, you should decrease this further to reflect additonal losses expected in the drivetrain.
So say power is 540 watts at 251 radians per second
Energy = (1.0 * 5402* (0.33*3.49)2 )-3 = 73 Joules
For the variable ratio system the ideal raito at angle x=radians_per_second*(1/3 * shaft_length2 M P-1 x-1)1/3
For a fixed ratio system a ratio equal to that of the ratio at the halfway point in the ideal variable ratio system works well.
ideal ratio at 1.75 radians=251*(1/3 * 0.332 * 1.0 * 540-1 *1.75-1)1/3 = 8.5:1
So at 12:1 that energy figure would be a little lower, the sweep angle makes a difference.
You can get an idea of the sweep time by doing 73/540 but in reality it will take rather longer.
I havent checked these figures, so you might want to.
73 joules is not a lot :S
Gord- Are you using the same fan motor in your hammer that you used in Arggh?
James and Grant...dont know what You mean by cost effective...I bought you one of the fan motors you destroyed :) so you owe me lol
At 73 Joules a conventional electric feather will already be beginning to jump. For many opponents it is a damaging level of energy. Feather hammers are handicapped in two ways compared to their heavier counterparts. For a given weight they cannot make as high a energy to weight as a heavy without generating more power to weight, as there will be less time in which to generate it. This *is* somewhat possible for motors and gearboxes which can increase in power performance as they get smaller, but is more of a challenge for available batteries which put more sharp limits on performance.
With pneumatics so much power can be made available, unless you are stuck with low performance components it is not in my mind even an issue, pneumatics scale down remarkably well. Given the need and motivation it is possible to construct pneumatic parts in a heavy capable of reaching hundreds of kilowatts of power. The second disadvantage is that gravity is a less significant effect the smaller you are, so jumping becomes a problem even with a reduced energy. It is easy for feather to lift other feathers, they dont have to move things as far so the stresses are less.
There are a couple of ways to generate more energy, the simplest is often to increase stroke or shaft length. Neither are totally problem free. It isnt worth increasing the head mass much so you need to concentrate on power. Secondly more volts, it works best to really punish the motors and the batteries. If you havent thought of overvolting your probably just not trying.
If you use a toothed belt or chain drive, plausibly you might be able to handle shock energy elastically in the tensioners, and avoid a clutch - if you are careful. The tensioners have to be sprung low friction rollers for a timing belt or sprockets for chain.
The effect on the overall energy shouldnt be too great. If you can cut your own sprockets it might be possible for you to make a torque shaping linkage. However with only one ratio per position, you cant get much of a retract jolt, the same effect is there for elliptic gears except you wont have the shock absorption either. This kind of torque shaping mechanism can be used to reduce jumping to a degree. Shaping mechanisms work efficiently with sprung energy storage, though too much use of these mechanisms can cause trouble in self-righting (and possibly retract). The former might be less of an issue in a feather.
Nick.
Dave - Yes I am using the same motors as Aaargh! but at a higher voltage. Ive designed it to take upto 32 C cells,whether i use that many will just be down to testing.
:)GORD
To tension the drive chain for the axe, cant a use a piece of self lubricatin plastic like George fransis does for Chaos 2s drive chain ? saves the weights on spring loaded ones