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Axe mechanisms


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kkeerroo
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Joined: 17 Jun 2004
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Axe mechanisms

Jeff doesn't have a build report so I'll have to dump this info in here. This post is from a thread in the FRA forum where I asked John Reid (KillerHurtz, Beta) how to calculate the gear ratio I'll need for an EV Warrior powered axe.


quote:
An EV Warrior powered by two 24V 3Ah Nicad sets would have a peak power output of around 600 Watts.

Say the axe sweep lasted 0.2 secs, then the maximum possible energy would be 120 Joules.

To get a crude approximation to the sweep time, use the formula: t=(2 * s^2 * m / P)^0.3333
where s is the sweep distance of the axe, m is the mass of the axe head and P is the power of the motor. Assuming a 300mm long shaft, then this gives 0.14 secs and 86 Joules.

If you doubled the length of the axe shaft to 600mm and doubled the axe head to 2kg, then you could get it up to 0.29 secs and 170 Joules. With this setup, you would want a gear ratio of around 20 to 1.

To get a very crude idea of the gear ratio required, use the time of the swing to get the average rotational speed of the axe. With the current setup 0.14 secs for half a revolution = 3.5 revs/sec = 210 rpm.

The no load speed of an EV on 24V is 5000 rpm, so the speed at which peak power is produced is 2500 rpm. So the gear ratio should be around 10 to 1.


So I'll have a go at using Jeffs axe setup.

300W Scooter from Oatley electronics. I'm guessing he was using 24V's worth of SLA's in the melee, or maybe drill batteries. Either way I'll forgot current draw limatations of the batteries.

300W and 2750 rpm at no load.

t=(2 * s^2 * m / P)^0.3333

s = sweep distance of the axe. If Jeff used the same sort of Hatchet I was playing with it'll have a handle of about 30cm. If it swings though 180 degrees the distance = pi * 0.3m = 0.942m

m = My hatchet weighed 1.1kg. I'll guess Jeff's is the same.

P = 300W

So
t = (2 * 0.942^2 * 1.1 / 300)^0.3333
t = 0.187 seconds

The energy in the axe = t * P = 0.187 * 300 = 56 joules

The axe is moving at ((1/2*t) * 60) rpm's = 160.4 rpm.

Since the motor will be under load it won't be doing that rated rpm. I'll guess it'll only won't to do half that. About 1375 rpm.

So the best gear ratio will be 1375 rpm / 160.4 rpm = 8.57 : 1.

The sproket set from Oatley is 80:11 or 7.27:1

Not quite ideal. The higher gear ratio means the axe could be faster and have more energy, but will mean the robot will self right easier.
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Last edited by kkeerroo on Thu Mar 02, 2006 8:13 am; edited 1 time in total

Post Thu Jun 09, 2005 1:18 pm 
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kkeerroo
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Joined: 17 Jun 2004
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Here's a good post from the RFL forum.


quote:
Ok, first I gotta back up a bit.
Motors turn current into torque (force). Force times time = energy.

Spinners get like 5 seconds worth of 'burn time'. Over that 5 seconds, the spinner builds up a lot of energy, which is imparted to the opponent in a short time. A directly driven hammer only gets to turn the motor on for like 0.1 seconds - that's just not enough time to build up as much energy (assuming similar motors/batteries)

If you change it so that the motor drives hard for 5 seconds to compress a spring, you get to build up a similar level of energy, again released in in a (hopefully) quick big hit.

Now pneumatics are a bit different... on one hand, an air cylinder at pressure is like a compressed spring - a lot of energy ready to be released quickly. On the other, you don't get to go from zero to full pressure instantly - it's all about the flow.

Like a spring, you could envision a pneumatic hammer where the cylinder is pressurized, but the hammer held by a catch (like the spring hammer). Then when you fire (release the catch), the full force is instantaneously available, without flow limitations. This would allow pretty simple pneumatics (small valves)... but to my knowledge, nobody's tried this - again, maybe the problem is the structure to hold the forces, and the tricky release mechanism.




And another:


quote:
There's an interesting discussion to be had here with respect to energy, I think. Hammering is arguably a more effective method of energy delivery, because the floor naturally forces the opponent to absorb the bulk of the blow.

The disadvantage spinners have is that they're forced to go through the entire power curve of the PMDC motor. This means that at the onset of spinup, they draw disastrously inefficient and huge levels of amps, and past 50% rpms, they are making progressively less power.

With a nautilus style gear, not unlike what BETA employs, You could maintain the output torque of the the motor-gear system so that it is matched with a certain ratio to the spring constant, in an attempt to hold the motor around full power IIRC, with 50% efficiency. The nice thing about springs is that they can be tremendously energy dense, in the same way that a spinning flywheel is, but without the spinning. Perhaps if the design was worked on, and taken to the next level with I-L style pneumatics, or spring-hammer systems coupled to infrared switches to avoid missing the opponent and losing alot of time.


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Post Thu Jun 09, 2005 1:54 pm 
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DumHed
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as long as you can aim your axe / hammer directly at the opponent's off switch (like Jeff) you don't need much power at all Smile
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Post Thu Jun 09, 2005 2:45 pm 
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kkeerroo
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Glen was asking about Fusee drives and by chance I had already done all the calculation for the profile of a fusee cam in an EV Warroir powered axe robot.

From the FRA forum:


quote:
Also I'm intersted in the snail cam on Beta. I used the time for the axe to swing as you showed me, broke down the equation for power and rearranged with respect to acceleration (power = force x velocity) so I had a = (p*t)/(m*s). I used pi for s as I wanted the answer to be repesented in angular acceleration measured in radians (pi = 180 degrees). Once I had the acceleration I multiplied it to the time to get the angular velocity at the end of the swing. I converted it to rpm and used it to find the best gear ratio. I used this as the gearing on one end of the cam. For the other end I decided to use it and the average gearing to find the other end so the average radius of the cam is equal to the average gearing. For one set of sums I ended with 13.5:1 for the average gearing, 7.7:1 at one end of the cam and 19.3:1 at the other.



For those of you not sure about what a fusee drive is here's a piccy from John Reid's web site about Beta. http://www.johnhmreid.dsl.pipex.com/beta/images/gearbox11.gif
(He even modelled the chain. Bloody Hell)

And how he built Beta:
http://www.johnhmreid.dsl.pipex.com/beta/progress/



And what is John planning on building now?
http://www.extremefightingmachines.co.uk/cgi-bin/discus/board-profile.cgi?action=pict&file=1895.jpg
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Post Thu Jun 30, 2005 8:12 am 
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Glen
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geezus that guy knows his way around a cad program,

cheers for those calculations too
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Post Thu Jun 30, 2005 7:49 pm 
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