I am considering to build and axe robot i was wondering what mechanism to choose . I know there is pneumatic ones but could that be in corparated in a featherweight?
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I am considering to build and axe robot i was wondering what mechanism to choose . I know there is pneumatic ones but could that be in corparated in a featherweight?
Its certainly possible, and I think its been done. The main issue I can see is whether youd have enough weight left over for a nice heavy axe head.
The other common option is an electric one.
Blue (from the big nipper team) is a pneumatic axe. it uses a rack n pinion system (same as thor and THz). It currently runs off a compressor, but there were talks about it going to CO2. I have considered building an axe, but i decided to stick with the flippers.
There is a co2 axe being made by Wayne cummings. called Basher. its got a revolutionary design like no other axe. its also 16 bar which will make it the most powerfull axe around. Rc Wars have produced an electric axe and boy is it good. its debue will be at the Rc Wars Championships
Our Project One had a CO2 powered axe (and flipper). Mario had a nice system for it, I think its called an actuator. Youll have to ask him for details though.
Rotary actuator to be exact. Very easy.
HOW DOES THE ROTARY ONE WORK?
WOULD A WINDOW WIPER MOTOR AT 24V WORK ?
A rotary actuator is a kind of pneumatical cylinder , but not with a rod that has a linear motion, but with an axle that turns a certain amount of °
How does it work, You put on air on one side and the axle turns, and visa versa.
WOULD A WINDOW WIPER MOTOR AT 24V WORK ?
Thats like asking how long is a piece of string.
There are so many different ways a ww motor could be used so the answer is maybe.
Also please there is no need for caps.
Regards
Ian
To the left on your keyboard, theres a button called Caps Lock. Press it, and you can type without annoying people. Capital letters are most commonly used when screaming, something that you generally stay away from on a public forum.
But as Ian says, there are tonnes of diffrent windscreen wiper motors. Some might work, but theyre most likely too slow. Some cordless drills would work better.
Even the cheap 24v drills you can get from maplin or argos (and other places) have lots and lots or torque- arguably motor than WW motors.
As someone with no expirience with WW motors :sad: Can I ask what the output shaft usually consists of? Is it a simple keyed axle?
And yes, capitals are annoying, I also find it annoying when people use capitals on ebay or the for sale section of this forum, believe me- IT DOES NOT WORK :)
WWmotors usually have a shaft with a slight cone and then a thread on the end (either M6 or M8).
Sorry about the capitals :)
Note that the weight of the axe head is not particularly important (for power) in a pneumatic axe, as long as you have good ram speed. A lighter axe head also tends to make the robot jump less.
However, on an electric axe, the only way of getting a good level of enery into the axe is by having a very heavy head on the end of a long shaft. The motor is producing energy at a constant rate, so the longer the swing time, the more energy will be in the axe head at the end.
Ah, the master have spoken... :)
We ran a motor similar to a WW motor on 2meat hammers electric axe, and at 24v it provided just the right sort of whacking speed, about 2 hits a second.
We coupled the output shaft to a crank, and con rod to an accelerator copied from dominator. It worked fine in tests, but eventually tore half the teeth off the gearbox. I have tried replacing the gearbox with soemthing a little stronger, but have given up, and am converting it to pneumatics.
John,
However, on an electric axe, the only way of getting a good level of enery into the axe is by having a very heavy head on the end of a long shaft. The motor is producing energy at a constant rate, so the longer the swing time, the more energy will be in the axe head at the end.
The weight of the axe head doesnt have any bearing on the energy in the axe on impact. The problem part of your comment is that the motor is producing energy at a constant rate. The power (rate of energy) the motor is taking from the battery is Volts x Current - voltage is fixed but the current varies hugely. At start, it is very high (motor is stalled), and drops off as the speed of the axe increases.
On my axe weapon webpage (http://homepages.which.net/~paul.hills/AxeWeapons/AxeWeapons.htmlhttp://homepages.which.net/~paul.hil...xeWeapons.html) I have a spreadsheet for performing the physics of an electric axe: http://homepages.which.net/~paul.hills/AxeWeapons/MotorDriven.xlshttp://homepages.which.net/~paul.hil...otorDriven.xls. In this spreadsheet, you can alter the moment of inertia of the axe (which is the rotational mass), and see what energy you get after a certain angle (I chose 180 degrees in which case you may have to extend the table a bit). If you alter the moment of inertia, you find that the energy after a certain angle stays pretty much constant. The slight variation is because this calculation is using an iterative sampling approach rather than the pure maths which is rather hard!
Thats the theory that I worked out anyway. If you have any practical experimental results (and I know you have a lot of experience with these things) that disagree I would be very interested - it would mean my theory has a mistake somewhere!
If I can have an intuitive stab at what Johns saying (to see if Ive got the gist):
With a pneumatic axe, the amount of energy imparted to the weapon before impact is independent of its weight (and presumably depends on the energy of the air expanding in the cylinder), no matter the speed/weight trade-off.
With an electric axe, the more weight is present for a given sweep, the longer the motor will have to impart energy to it - and therefore the more energy the axe will have on impact.
i.e. a pneumatic ram imparts a given amount of energy regardless of how long it takes to expand, whereas an electric motor imparts energy (potentially) proportional to the time its running - with provisos about efficiency, variable gearing, etc.
Did that help, or have I just muddied the waters? (Its one of those things which could make sense, but could easily *not* make sense, too).
--
Fluppet
Youve got it spot on Andrew.
Paul, the energy delivered by an electric axe is pretty much proportional to the weight of the axe head, assuming that you optimise the gearing for a particular axe weight. You need to include a gearing factor in your spreadsheet, which you can adjust to get the maximum energy output. Judging by how flat the speed line is, it would produce much more energy with a gear reduction.
The motor is steadily churning out energy (OK, maybe not at an absolutely constant rate, although with the snail cam on beta we manage to keep the motor pretty close to max power output). The longer you can keep it feeding energy into the axe, the more it will have at the end. The way to make it take longer is to increase the weight of the axe head or increase the length of the arm.
Voltage x Current of course gives you the electrical power input to the motor, not mechanical power output. As the revs rise (and the current decreases), the efficiency rises. Hence it produces over 75% of maximum power within the centre 50% of the rev range. Which is nice for those of us making electric axes, as it means you dont have to get the gearing spot-on.
Sorry, energy delivered is not purely proportional to head weight, but it is highly dependant on it.
If you have a 10,000 Watt peak output motor attached to a very light axe, so that it only took only 1 millisecond to fire, then there is no way that it can deliver more than 10 Joules of energy.
Whereas, if you connected it to a very heavy axe, so that it took 0.5 secs to fire, then it has the potential to deliver up to 5000 Joules.
Im building a Rexs Robot Challenge style FW for my IVA project at college, and I want an axe robot rather than a flipper, and I am unsure of a mechanism that uses both rams. Any clues ? I can use a motor, but Id need extra batteries for that
A motor would be easier, as you cant get much space and pressure in a fizzy drinks bottle (meaning less potential shots from the rams than Co2). Ive got to admit a Co2 axe would look better though. It would be quite easy to couple two bicycle rams together and use the same rack & pinion meathod as used on terrorhurtz, although the rack and pinion would probably require machining a new rod (the moving bit) for the ram(s).
You could use a crank lever system powered by a motor. Similar to what Mortis has.
for electric featherweight axes, e-mail me on james@jamesbaker.f9.co.uk and Ill send you a pic and info.
the design was entirly from my own knowledge of degree level engineering, and nothing else.
Having an astronomical respect for my knowledge, my friend then asked everyman and his dog for advice, including John Reid, and the solution he came up with using Johns figures and many computer programs was.... exacly the same as my solution.... exactly. So Id say that sticking to basic pricipals is by far the easiest way to build an effectice axe and if you do not have the maths background e-mail me and Ill send you info about mine
cheers
james
Ive had a look at Pauls equations and Ive fed in all the stats for my feather axe robot. I had to do this on paper as I dont have Excel. I ended with an answer that I was expecting, but when I then tried to modify the inputs for the gear reduction which my robot has the answer ended up at less then a 3rd of the energy. Anybody care to help explain this?
The robots weapon is a 1kg hatchet driven from an E.V. Warrior at 24V with a 4:1 reduction through chains and sprockets. The energy calculated before reduction was 1.4kj and after reduction was 0.4kj. A photo of the half completed robot can be found here http://www.robowars.org/forum/album_pic.php?pic_id=346http://www.robowars.org/forum/album_pic.php?pic_id=346
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.
Youll be glad to hear that you are only losing around 10% in energy by running at 4 to 1 rather than 10 to 1. You will however be drawing a lot more current from the batteries.
Ive made a fether with an axe. Its just a thwack bot, using the stinger-like system. I still have to get it a bit more reliable though :)
See my profile for a picture.
It uses 2 drill motors for each wheel. I still have to replace the actual axe, but its more of a show-bot anyway.
a better way to express the sweep time formula would be:
t = (2 * s^2 * m / P)^(1/3)
x^(1/3) is the cube root of x
s^2 = s squared
The analytic formula for energy/distance for a mass m moving under constant power P is
Energy at distance s=(9/8 M P2 s2)1/3
so...dividing by P
Time at distance s=(9/8 M P-1 s2)1/3
This is a only slightly quicker swing than in Johns formula, since this is for perfect gearing throughout the stroke, and there are
several complicating factors, you can expect the real swing to be a bit slower and achieve slightly lower energy.
Great, thats lots of help. Shame Im too close to a big comp to redesign it for a more efficiant gear ratio.
One thing is John said I need a 10:1 reduction using 2500rpm from the motor, but an EV at 24V is rated at 4480 at no load. Is there something else Im missing (wouldnt surprise me).
Also Im intersted in the snail cam on Beta. Im not quite sure how to calculate the shape of the cam. This is just a guess but I had a go. 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, but this doesnt feel right) 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.
Am I wasting my lunch breaks here?
The no load speed of an EV on 24V is 5000 rpm, so the speed at which peak power is produced is 2500 rpm.
What you might not have realized(?) is that you only get peak power for the system at around half the no-load speed, when you are very close to the no-load speed the motor is taking hardly any current and generating hardly any power so you want to avoid going near it. Its exactly the same in terms of power output if your very close to stall except your taking tons of current, and its generating tons of heat....which is bad.
Thats why were determined to stay somewhere around 2500rpm, maybe just over to reduce the amount of current flowing through the system. EV warriors are often timed so they run quicker in one direction than another, or some such. This is a bad thing if you intend to run it both ways, and I intend to ignore it too. (Theres also some contention on the no-load speed of the EV, or rather what it would be without friction, this might be what you are at issue with, 4480rpm versus 5000rpm?)
The actual energy gain using snail cams over the best fixed value gear ratio is in (the simplest)theory fairly small, it has a slightly bigger affect on swing time. However it tortures the motor and batteries with the very high currents over too much of the stroke if you go for peak energy in a fixed ratio system.
If you dont saturate the motors armature and the cells hold up for long enough under this abuse, you can get within about 7% of the max amount of energy a changing gear ratio system could provide. Those two things you cant really be sure of, so realistically your going to probably lose more than 10%. But the machines endurance should be a lot better if you use cams to make it more efficient.
I think John worked out an overall fixed ratio of about ~9:1 is about right(we dont know the length of the shaft so were guessing 0.3m from centre of pivot to centre of hammer head). That would mean a good cam would probably start of at about 20:1 and end at 6:1. When you build it you may come up against a problem fitting such a big gear change in the space available, then you just cut back until everything fits.
Your figures arent that far off these, so I doubt your lunchtime was completely spent in vain. Thats for absolute peak power, generally you want to keep just on the lower current side of peak power, you only lose about ten percent power but your power wont drop off abnormally quickly as you keep pounding your opponent.
Your approximate expression for acceleration is ok. If you chose m to be shaft_length*shaft_length*head_mass when s is in radians. I assume you did that, because your numbers do work. Using the average gear ratio as a pivot to work out the high gear ratio end is rather clever, it might not be totally rigorous but it worked. Thankfully a lot of the calculations involving the gear ratios seem to be fairly forgiving anyway.
The actual designing of the cams to have the right gear ratio is made more complicated because of the angle of the chain is changing and so forth, luckily you dont have to get it very accurate if your only interested in energy.
Back to that constant power equation...its more accurate when your using cams as then the power is much more constant.
Energy = (9/8 M P2 s2)1/3
so speed= (2 M-1 Energy)1/2
[at which point a horrible thought occurs]
Speed= (9/8)1/6 (2/M)1/2 M1/6 P2/6 s2/6
after almost losing the will to live....
Speed = (3 M-1 P s)1/3
260 is about half the motor no-load speed in radians per second...its when the motor is around peak power. You could go a bit higher so things are a bit more efficient with only a small reduction in power eg. 350 radians per second. You have to change the power in the formula accordingly.
ideal ratio at distance s=~260*shaft_length/linear_head_speed
so ideal ratio at distance s=~260*shaft_length * (3 M-1 P s)-1/3
If x is the angle in radians of the hammer, where x=0 is the start condition of the shaft you get this
ideal ratio at angle x=260*(1/3 * shaft_length2 M P-1 x-1)1/3
Plugging in shaft_length=0.3m (pivot centre to hammerhead centre) M=1.0Kg P=750Watts (optimistic...no friction)
gives an ideal ratio at x=3.14 (end of stroke) of a shade over 6:1
Obviously as you get closer and closer to the start of the swing the ideal gear ratio becomes infinite. Thankfully its not necessary to actually go this far, and big enough is....enough.
At a tenth that angle x=0.314, the ideal ratio is 101/3 times bigger...or about 13:1 so by the time your at 18:1 or more you are only left hurting a small amount of beginning of the stroke.
whats the general opinion on spring powered axes? is it better to just go with an all electric one in the first instance or what? (id like to keep away from pneumatics big time).
ive started on a little axe robot now () and i was thinking of using this huge beast of a windscreen wiper motor (http://robowars.org/forum/album_pic.php?pic_id=574http://robowars.org/forum/album_pic.php?pic_id=574) to wind back the axe and compress the springs in my design, but would it be easier and work just as well to use a couple of linkages and power the axe straight from the gearbox?
i doubt the motor itself is lacking in power. its like twice the size of a normal windscreen wiper motor but im still not sure if the plastic gear inside the gearbox would strip the teeth off even with a linkage or two on it.
thanks in advance
Glen, talk to me on msn, I have a very very simple design for you that should work well :)
Glen, Im currently building an hydraulic axe.
Also possible for an feather, or a simular system powered by a linac.
Mail me for more info.
Hate to revive an old thread but could anyone help me figure out a gear ratio for an motor powered axe?
(motor powered purely due to my lack of experience with pneumatics, you understand.)
The Axe Im planning has a roughly 700G head, 0.3M handle and will probably be driven by a 24V Gold motor.
Ive tried the formula but its making as much sense to me as an in-flight magazine produced by air belgium...
Anyone fancy lending me a hand?
Do you want it to have enough torque to allow self-righting?
Hmm, maybe about 13:1.
Its also roughly the number you get if you calculate the ideal gear ratio at the 90 degree position for a motor developing about 120watts at 2700RPM.