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 Bought DeWalt 18V's, where do I go from here?

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dalevt



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Registration date : 2010-02-17

PostSubject: Bought DeWalt 18V's, where do I go from here?   Wed Feb 17, 2010 2:24 am

I am a mechanical engineering student, and I am assigned to build a battlebot. I am not an electrical engineering student, for the obvious reason that I don't have a clue about electricity. affraid confused

I purchased two new 18V DeWalt drills, and 2 new 18V XRP DeWalt batteries. Now What?

Someone please help me, dont be afraid to insult my intelegence with basics. I don't have a clue where to go from here.

I was told I need ESC's, a reciever, and a transmitter. Is that all I need to make this work, and are there compatability issues with these seperate parts?

I am planning on running the 2 motors, direct drive to the wheels, in a skid-steer like setup for steering, and I need a feature on the transmitter that can turn my weapon on and off.

Thank you, any information will be much appreciated!!!!

-Dale



Additional info: Motors around 250 Amp stall current, bot will be less than 30 lbs
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Dan Curhan
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PostSubject: Re: Bought DeWalt 18V's, where do I go from here?   Wed Feb 17, 2010 4:18 pm

I drew a simple diagram of how you would wire up a bot with three motors



you have a lot of options as far as ESCs are concerned. Make sure you differentiate between brushed and brushless motors. Brushless motors have three leads coming out of them, and they need a special brushless ESC. If you've got a motor with a 250A stall current, you're going to need a serious ESC. IFI speed controllers work well for brushed motors that draw large amounts of current. As far as brushless ESCs that can handle huge currents like that are concerned, I don't have much experience, so I'm not sure what direction to point you in. Castle makes some good ones (Castle's website). For your drive system, you can opt for a pair of small ESCs or a dual ESC such as a scorpion or sabertooth. The dual ESCs will drive both of your motors with one unit, taking separate signals from the transmitter, and they will give you more options as far as mixing goes (ie, to drive the bot with the right stick and control weapon throttle with the left stick).

After the ESCs, you need a transmitter/receiver setup. You can either opt for a radio setup that uses crystals, or a digital 2.4 GHz spread-spectrum Spektrum system. I prefer Spektrum, but you'll find advocates for both. Spektrum is just easier because each receiver is bound to a transmitter, and you don't really get any interference problems. To choose a receiver/transmitter setup, you have to think a little bit about the functionality you want your bot to have. A simple three-motor bot has three basic channels of functionality - one for each drive and one for the weapon (usually AIL/ELE and THR respectively). If you use a scorpion or sabertooth for drive, you can use an "invert" channel, where you flip a switch when your bot is upside down and you can still drive as normal (instead of having to drive backwards and mirrored). I suggest a 5 or 6 channel receiver/transmitter combination.

The receiver send PWM (pulse width modulation) signals to the ESCs, which then interpret them and map them to an output voltage for the motors. The PWM signal is sent via a 3-conductor cable which connects to the receiver with 3-pin pwm connectors. On the receiver you'll see a symbol that looks kind of like a square capital omega, then a + and a -. The omega-thing represents "signal" and + and - are just positive and negative. The signal cable is either white or orange (depending on the color of the cable - some are black-red-white, and some are brown-red-orange), and the ground is always black or brown. MAKE SURE you plug them in correctly!

Robotmarketplace has pretty much everything you will need to get started. If you're looking for some more advanced options, there are other places to get things that RMP doesn't carry, but you're probably not going to be going beyond the level of RMP for your first bot. Just take a look through the things they carry, read the specifications and descriptions so you can learn about what things are capable of, what typical specifications things have, and such so you get a feel for the kind of things you need.

Last year, I wrote up a wiring guide for a 15-30 lb bot. It's a 4 page pdf, so I would recommend reading over that for a basic idea of things. It's not very detailed, and largely based off of the bot I built that year, but it still has some good information. For the details (and everything you would ever want to know about building a battlebot), go check out the Riobotz tutorial.

If you have any more questions, let me know.

And... just curious (and so I can assign you a color) you're MechE at Virginia Tech?

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dalevt



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PostSubject: Re: Bought DeWalt 18V's, where do I go from here?   Thu Feb 18, 2010 10:04 pm

Thank you very much.

How will I wire it up with my 3 batteries? (I have 3 complete battery/ drill combos) (please draw another pic)

Also, does my ESC need to be able to handle the 250A at stall current, or is there another way around this? (250A ESC are VERY SEPENDY)

I am from the South Dakota State University


Thanks again
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Dan Curhan
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PostSubject: Re: Bought DeWalt 18V's, where do I go from here?   Thu Feb 18, 2010 10:19 pm

I guessed the vt in dalevt was virginia tech lol. my bad...

do you want your three batteries in series or parallel? If you go in series, you'll be running stuff at 54V. There's not a lot of easily available ESCs and such that can handle that kind of voltage. If you put them in parallel, you get 18V but 3 times the capacity of a single battery. Basically, connect every positive lead together and every negative lead together. You're going to need to figure out which contact on the drill batteries is positive, and which is negative, then either solder leads to the contacts, or take apart the drills and somehow use the battery socket that comes in the drills...

The only way to really know what your ESC needs to be able to handle is to know what kind of current your motor draws. This 250A max current draw is for the dewalt drill motors? And you're using them for a drive system? In that case, the chances of them drawing anything close to 250A are really, really small, especially if your bot is under 30 lbs.

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rdubard
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PostSubject: Re: Bought DeWalt 18V's, where do I go from here?   Fri Feb 19, 2010 6:39 am

Great advice, Dan. And Welcome, davevt!

Provided you are using the DeWalts for drive (as Dan has speculated), I would recommend you use only one of your battery packs at a time, or at most two. (When you are at competition keep the other/s on the charger--always, always, always start a match with freshly charged batteries in your bot!)

The drive alone will never draw much current, and by having the battery as your 'weak point' in the system, you will not require the expensive high current speed controllers.

The IFI Victor series is quite good, but there are some details that make them a bit frustrating to use.

First, they are a bit expensive, but this is compensated by the fact that they are pretty tough.

Second, for most receivers, you will need to buy two 'signal boosters' to get the Victors to work--a simple PWM wire will not do. This isn't so expensive, but is another thing that can go wrong. Simpler tends to be better when it comes to wiring. (But not always)

Third, they are somewhat large, mostly because of the fan. Lots of teams remove the fans when using these for drive in small (15 lbs) bots, and while this is explicitly not recommended by the manufacturer, it seems to work OK. I do not know about a 30 lber, and I wouldn't risk it with a 60.

There are a kabillion other brands of speed controllers out there, and it's worth your time (and will likely save you money) to look around. Two interesting places to look are.Hobby City/Hobby King, and Tower Hobbies

I would definitely recommend the Spektrum transmitters 5 channel or 6 channel 'mode 2' radio systems if for no other reason than that they are essentially interference-proof. The main reason, though is that they will allow you more options/flexibility/programming as you build more robots!!! which we all know you will
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