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Micro Magnum, Team Magnum, NSW
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MoonSet416



Joined: 25 Sep 2016
Posts: 297
Location: Sydney


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Micro Magnum, Team Magnum, NSW

Recently I have been working on very early stages of my first combot, Micro Magnum. I have made changes to the design according to my friends' suggestions and I think it is good enough to put the pics on here and ask for your opinions on the design. Feel free to give suggestions!

The following are pics of different evolutions of the design from the earlest to the latest.

http://imgur.com/2Zd8OSb

http://imgur.com/TpSaj06

http://imgur.com/1JaIUZR

http://imgur.com/iYO0YA5

http://imgur.com/BJ89XaS

Post Sat Oct 01, 2016 2:31 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 297
Location: Sydney


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Some additional info on it:

It is a featherweight combot with a spinning drum as the main weapon, and you can think of it as a featherweight K2 (though probably not nearly as good).

Drive:

two Banebots P60 16:1 gearbox driven by two 755 motors

4inch Banebots wheels at the back, and 2inch ones at the front, connected by 15mm timing belts


Batteries:

4*Lectron Pro 3000mAh 4S 14.8V Lipo


Frame:

10mm aluminum sheets (haven't decided on 6061 or 7075 yet)

4mm aluminum sheet for bottom armor

5mm polycarbonate for top armor


Weapon Motor:

Turnigy TrackStar 1/8th Sensored Brushless Motor 2400KV
(a 42*70 sensored brushless with 2300w max power)

Post Sat Oct 01, 2016 2:41 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 297
Location: Sydney


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Btw although it seems pretty big in the pics, it's actually not.

The size of the frame is, 33cm (length), 36cm (width)

One extra pic on the layout of the internals.

http://imgur.com/6A5tWjJ

Post Sat Oct 01, 2016 2:47 pm 
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Nick
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Joined: 16 Jun 2004
Posts: 11713
Location: Sydney, NSW


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I like it, we need more vertical spinners! Here is some constructive feedback based on 12 years of building spinners:

* 10mm aluminium armour is a bit borderline nowadays, even with 7075 you will be replacing it frequently. BTW, 2024 can often do better as it is less brittle and won't crack around screw holes so easily. Its also quite a bit cheaper.

I would look at 3 to 5mm Hardox steel or similar for the sides if you can find the spare weight.

* The side panels need to be really well attached; you probably didn't render the connector parts but whatever you think looks right, double it Smile, particularly at the front corners.

* The little articulated wedges won't work. They look good in theory but in the arena, they will always ride up against bots with fixed wedges as they have no weight bearing on the leading edges. Its also possible for them to get bent under the bot and prop the wheels off the floor. Try fixed steel wedges that bolt on and make them at least as thick as the frame to take a beating from other spinners.

* The pulleys will give the wrong gearing between the back and front wheels: if the size ratio of the wheels is 2:1 then the pulleys need to have the opposite ratio to make the smaller front wheels drive the bot at the same speed - it looks wrong in the renders.

* You probably don't need such a large battery pack. From the renders and the motors you selected, I guess they are wired in parallel to give the bot a 14.8V, 12AH pack with a 120C discharge rating. Based on experience with Mr Mangle, I would recommend a capacity of 3.5 to 4 AH, which will save heaps of weight.

Using just one pack really cuts down on messy wiring and also saves money on chargers. I'd look for a 3.8 or 4AH pack with a 50 to 65C rating to get the job done.

* The weapon motor has plenty of power but with a no-load speed of 35,500 RPM, it will need a large gear reduction to get a useful speed on the drum. It might be easier to use a lower KV motor. The 1,900 KV version and a 3:1 reduction should get you in the ballpark.

* Is that a Sabertooth ESC in the renders? I haven't used one, but they have a bad reputation for reliability.
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Post Sat Oct 01, 2016 7:07 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 297
Location: Sydney


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First, a lot of thanks for your advice! It's great to listen to experienced builders like you.

*Yes, I do have some spare weight. When I did my weight estimate a few days back, this bot was actually underweight. so I guess I will change the aluminum to something like hardox or maybe Ti if I have access to it.

*Yes I actually did leave out the connections for the side armor. In my design there should be an aluminum plate right behind the drum running through the entire body from left to right. The front and back armor plates, which are also left out in the render, will also be connecting to the sides.

*I have no problem throwing the little wedges away, but now without them I face a ground clearance issue, especially against those 2WD bots with a wedge at the front touching the ground as a support. So how does articulated front armor + wedge assembly sound? (but that way I lose a connection point for my side armor)

*I think the gear ratio on the rendering is right though. To drive at the same speed, the small wheel must be turning twice as fast as the large wheel. Let's suppose we have N teeth on the pulley for the small wheel, M teeth on the pulley for the large wheel and for a certain distance of travel the small wheel turns 2X times, and thus the large wheel turns X times. On each side of the timing belt the number of teeth coming out of the small pulley must equal to the number of teeth the large pulley is taking in, otherwise the length of the belt will be stretched. We can get N*2X=M*X, which simplifies to 2N=M, so the large wheel needs a pulley that is twice the size.

*I did a calculation based on the tools provided in Ask Arron, and the spreadsheet said the drum will eat up 8AH of electricity, and so come the four batteries. However some of my friends are telling me that the numbers are wrong in the spreadsheets. I am very confused about this right now.

*I chose those batteries because the size is right for my bot, otherwise I could not have squeezed so much into it, but if it turns out that I don't need so much power after all, I will surely go for a single larger battery.

*Yeah 2300 KV is really too much, I will be switching to a lower KV one.

*Yes it is a Sabertooth. I was putting it in there cuz it's the worse case scenario in terms of internal space. I guess I will use another motor driver for my bot anyway, so do you have some preferred ones?

Post Sat Oct 01, 2016 8:13 pm 
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Nick
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Joined: 16 Jun 2004
Posts: 11713
Location: Sydney, NSW


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Its really hard to predict how much power a spinning weapon will use; there are just so many variables. If you can accurately work out the kinetic energy of the drum (this calculator is my favourite: http://www.teamcosmos.com/ke/ke.shtml ), then you can work thorough some torturous maths to get a 'text book' battery capacity, but that doesn't allow for bearing friction, air resistance, belt slip, ESC efficiency and many more factors. The usual way that builders work out the battery capacity is to find the most similar existing bot and look at it's battery. That probably means we are all way off the ideal capacity but short of building the weapon and testing it, whatcha gonna do?

I have the same problems with battery sizes - most of them are designed to stuff into long plane fuselages, its really annoying. Perhaps two 2AH packs will do the job and still fit. Even if you don't really need a high C rating on paper, it really helps to have over rated packs as they stay cool and can be recharged as soon as they come out of the bot. Batteries that run cool usually have a longer life and will have less voltage sag under load, meaning a slightly faster weapon spin-up. They also recharge at higher currents, meaning you can be ready in time for back-to-back fights, which often happen at the end of competitions.

You probably already found Ti Joe http://www.titaniumjoe.com/ . They have a huge range of offcuts and the shipping is fairly reasonable. They are very helpful and the only thing to be aware of is that their stock list is always out of date - I always ask for the price of the best piece and two alternatives to save time with emails. If you can wait a couple of months for my new workshop to be finished, I can TIG weld Ti parts for the cost of the consumables.

I mentioned the pulley ratio on the wheels only because the front pulley looked smaller than the rear pulley in the renders I looked at. You are obviously right on top of the issue so its all good.

The front wedge issue is always a compromise. Bots with two wheel drive at the back and fixed front wedges usually get under 4 wheel drive bots - its the main reason that UK flippers are so hard to beat IMHO. On the other hand 4WD bots drive much better and usually have more grip and 'pushing power' IF they happen to get under their opponent. I haven't tried it, but perhaps you can have interchangeable forks. One set would be shorter and not touch the ground while a longer set would touch the ground and slightly lift the front wheels. The longer set would be reserved for battles against floor-hugging wedges. My bots all have rear two wheel drive but even then, different hight and shape wedges have been a major advantage.

I figured you left out quite a few bits in the renders - the side-to-side bulkhead is exactly what you need.

Choosing a weapon motor is another compromise area. In-runners are usually tougher than out-runners (depending on which crappy Chinese factory they came out of!), while out-runners usually have more torque for better spin-up times and their lower KV ratings make gearing easier. Using a large sensored in-runner is rare and in theory it will give you better spin-up times if the motor has enough torque. Perhaps allow enough space for a shorter & wider out-runner if the in-runner has issues.

For drive ESCs, you could try two hacked TZ85a controllers, they should fit if you stack them vertically. The Ragebridge 2 is a great 2 channel ESC and while it is much longer than a Sabertooth or the TZ85, it is very low profile and might fit if you mounted it vertically across the chassis. Flipping a single battery to fit across the width of the bot may also help.
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Post Sat Oct 01, 2016 10:05 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 297
Location: Sydney


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A lot of thanks, I guess I will try to figure things out as this project progresses.

I really learned a lot from you, and I can't thank you enough for your help.

Post Sun Oct 02, 2016 12:39 am 
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Philip
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Joined: 18 Jun 2004
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Location: Queensland near Brisbane


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A wider wheel base might reduce gyro dancing. Can you put the wheels on the outside and the belts on the inside? By the way, I have never built a spinner.
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Post Sun Oct 02, 2016 8:36 am 
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Nick
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Location: Sydney, NSW


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A slightly wider wheelbase doesn't do much to reduce gyro dancing but it does allow the bot to keep turning while it is tilted; definitely worth the change.
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Post Sun Oct 02, 2016 8:45 am 
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MoonSet416



Joined: 25 Sep 2016
Posts: 297
Location: Sydney


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Oh yeah that will surely help. Thanks.

Post Sun Oct 02, 2016 11:47 am 
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MoonSet416



Joined: 25 Sep 2016
Posts: 297
Location: Sydney


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Design update:

http://imgur.com/uZQC1rN

http://imgur.com/gtb27Zj

Post Sun Oct 02, 2016 1:29 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 297
Location: Sydney


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Haven't changed to hardox side armor plates because the weight limit was tighter than I thought it was...

Going to calculate the needed capacity of batteries...

Post Sun Oct 02, 2016 1:32 pm 
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Valen
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Joined: 07 Jul 2004
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Location: Sydney


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be careful with that outside, when somebody bashes that side panel if you are using it as a wheel bearing it'll bend the drive train.
If not you could potentially press the armour onto the wheel.
angling your armour makes a massive difference btw
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Post Sun Oct 02, 2016 9:06 pm 
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maddox



Joined: 21 Dec 2006
Posts: 748
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3.2mm (1/8inch) Hardox 450 is lighter than 10mm 7075 aluminium.

For the same weight as 10mm aluminium, you could look at 6mm titanium.

Post Sun Oct 02, 2016 9:11 pm 
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miles&Jules
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Joined: 19 May 2010
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Location: ipswich QLD


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Yep its a mega learning curve building fighting robots- love your designs/ renderings 😎
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Post Sun Oct 02, 2016 9:55 pm 
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