my new axe bot

[psycho_fling]

hi all

making a new fw for the coming years

been talking about it for a while now, and while im off for Christmas i have time to design it and pick my parts

my question to those in the know is, for the weapon if i am using a speed 900 motor on 15 volt (rpm= 8125) powering a 1.2kg axe head and handle using timing belts and pulleys, what ratio would give the ideal setup ??

more questions to come and designs posted might even start a build diary if i really crack on

cheer guys alex

[psychostorm]

hi all

my question to those in the know is, for the weapon if i am using a speed 900 motor on 15 volt (rpm= 8125) powering a 1.2kg axe head and handle using timing belts and pulleys, what ratio would give the ideal setup ??

I'm not in the know but I fancied a laugh using the australian's formulae

Firstly, you lack the handle length and power, so I guess 30cm handle and 15V*8A=120W
sweep= pi * 0.3m = 0.942m

Time =(2*Sweep^2*Mass/Power)^0.3
T=(2*0.942^2*1.2/120)^0.3
T=(2*0.89*0.01)^0.3
T=(0.89*0.02)^0.3
T=0.0178^0.3
T is roughly 0.3 seconds

Power*Time= Axe Energy
120*0.3= 36 Joules

[1/(2*Time)]*60 =Axe speed
[1/(2*0.3)]*60 =
[1/0.6]*60=
1.7*60= 100 RPM

Speed have/Speed needed = gearing
8125/100= 81.25(:1)

Not entirely impossible - a speed 900's 6mm shaft should fit straight into an RS worm gear. Feel free to laugh at my crap estimations and stuff.

[RogueTwoRobots]

I'm not entirely sure, but I think that a lot of the electric axe robots that use the 200W - 300W range of scooter motors tend to use a ratio of between 7:1 and 9:1.
I don't doubt Ceri's mathematics or calculations and I know the formula he's using is from the Australian electric axe wiki page, but a ratio of 81.25:1 sounds far too much to me for an axe.

A windscreen wiper motor runs at somewhere in the region of 40-50rpm. I used one of these for an axe back in the Rex's Challenge days (axe arm connected directly to the output shaft) and, quite frankly, it was crap. It seemed fast in tests but when surrounded by 12 kilos of robot it looked really slow. Doubling that speed would put it in at the 100rpm mark but to me that is still too slow for an electric axe.

It's a trade-off though, as with most aspects of robots; an electric axe is effectively a segment of a spinning disc so you want as much weight in the axe head as possible to increase the energy transferred in the attack in the same way as you want as much weight on the outer rim of a spinning weapon. But unlike a spinning weapon, you don't have time or space to get your axe up to full revolutionary speed, therefore you have to accelerate your axe to as high a speed as possible in as small a distance as possible. So for that you want high torque, thus an 81:1 ratio would be excellent, but once it was up to speed, it wouldn't be travelling very fast therefore wouldn't be very effective (picture Hornet 2 with a windscreen wiper motor instead of a mag). So the trick is getting a ratio that's high enough to get the axe up to speed quickly but low enough that there is some serious speed in the swing of the axe in order to cause damage.

The Speed 900s are known for producing high torque at low speeds so if you had a ratio of, say, 10:1 I imagine the motor wouldn't struggle to move the axe. Working with your figure of 8125rpm, that would mean your axe would be rotating at 812.5rpm if it was able to fully revolve. Seems a bit fast but it might be what's needed. Then again, there needs to be enough torque in the system to lift a featherweight robot (aka self-righting) so 10:1 might not produce enough overall torque.

So for all that rambling, I cannot actually say what the best ratio would be. Technobots sprockets are pretty cheap (relatively speaking) so it may be worth getting a few sizes and experimenting with a prototype setup and see what one produces the best results.

[psychostorm]

I don't doubt Ceri's mathematics or calculations

Really? I do most of the time...

but a ratio of 81.25:1 sounds far too much to me for an axe.

As it does me. The most particularly embarassing thing is everything else looks feasible but that one is stupid.

Personally, for the record, I say ditch the speed 900 and get a scooter motor - at least you'll know it'll work.

[RogueTwoRobots]

I think the Speed 900 should suffice; it gives a right good kicking when attached to a spinning mass so should be equally as effective for an axe. It's just all about getting the right ratio.
Or you could always just do what I'm doing with my currently-being-designed axe robot - throw in a Bosch 400 and hope for the best

[jonny]

i believe that another robot using a speed 900 to power an axe used a 20:1 ratio and it was pretty effective. i can't remember the name of the robot or what voltage the speed 900 was on though.

[leorcc]

Kashei uses 6 cells A123, so 19.8V nominal.
It's gear ratio is 20.25:1.

[psycho_fling]

cheers guys given me a few things to think over

@leo so is that a speed 900 powering that? either way very impressive

[RogueTwoRobots]

Nice victory in that video Leo

Out of curiosity, what is the control setup for that axe? What is used to fire it? (esc or relays)
And does it use limit switches to cut power once the axe is fired/retracted or is the motor just stopped?

[leorcc]

Yes it is a Speed 900 on the axe, and it uses an H-bridge with Mosfets made by Rick Maas (who also repairs the Robotpower ESC's in Europe). We do not use end switches but we time the pulse giving power to the motor for just 0.2 seconds either direction. I can also switch it on the transmitter to full power without limit should the pulse not be adequate, but I've not used that before.