Motors, and a hello

Well, this is my first post on the forums, so hello!

My friend and I have wanted to build a heavyweight since the old series of Robot Wars - we have a little experience with small, non-combat robots (the Micromouse competition), and with the new series currently airing, we thought we'd dive right into the deep end of building a heavyweight.

Motor Controller wise, we're leaning towards the Talon SRX, which I've heard a couple of good things about and seems straightforward enough to get a grasp on.

With motors - I have almost no idea where to start. Doing some research, I keep coming back to the Ampflow/Magmotor E30-400, which strikes me as a little underpowered for getting any rotating weapon up to speed in a reasonable time. Purely due to easily available materials and a fairly simple design, we're thinking of going for dual drum weapons.

(it's probably worth mentioning that I'm a cheapskate here)

So, my questions:

Weapon motors - I want to get the weapon up to ~1500ish RPM in under 3 seconds. What would be the best motor for achieving this (PMS 100F seems to be the sort of spec my mind wants), and what would be the motor that could do it from a cost standpoint?

Drive motors - I'm OK (for now, after several arguments about cost) to sacrifice speed as long as we still have some torque behind it. Again - what would be the best and what would work from a cost perspective?


For those who are interested, neither of us are engineers or machinists or CAD experts. We've got a good enough grasp to deliver 2D drawings (I learned on a drafting board, thanks to my Dad and Great Uncle), and I've got a fair few fabrication skills, although nothing coming close to perfection.

Thanks!

-Rob

Welcome.

If cost is a real concern then I would recommend getting your feet wet with a featherweight before a heavyweight. Your first machine will have issues (everyone's does) and you will make a bunch of mistakes. It's easier to swallow these mistakes when a motor costs £15 from an argos drill than £200 for a suitable heavyweight motor.

As to your questions, a drum spinner by it's nature has a small radius. This reduces the kinetic energy it can store over a larger diameter spinner at the same rpm. To compensate for this, the drum spinners spin much much faster. Pulsar spins at between 8500 and 9000rpm in the latest series. Not sure what sabretooth spins at but it's also quick. You need to increase your velocity well beyond 1500rpm to be competitive.

As to the motor you suggested, the PMS 100F is a brushless motor and so requires fancy electronic speed control. A bit more complex than a simple on off power solenoid for a brushed motor. You will be wanting to look for the neodymium magnet ampflow motors or a motenergy motor of some description. The days of using car starter motors have long since passed.

Drive motors, wheel chair motors will work but will be incredibly slow and won't be competitive. The cheapest option that is being experimented with is cheap ebay scooter motors. No idea if they are suitable so proceed at your own risk here.

I'm not going to sugar coat it, even the simplest cheapest looking heavyweights will cost upwards of £1500 to get yourself started and that doesn't include any spares.

Gary, thank you for the welcome!

I'm well aware that we will have issues - there's an idea to make a 1:4 scale version to make sure we can actually do it first, but we've set aside around £2500 for the bot, not including spares. Ideally, we don't really want to spend more than that on a MK1 robot.

It would seem I'm working with some mildy outdated information regarding drum spinners. 9000rpm+ would be an excellent goal, but motor wise, I'd assume some gearing would be involved - is there a particular one you'd recommend? Do most people run 48V or 24V?

To get to those crazy speeds you really need to be using brushless motors as Ellis did in Pulsar. He's on here from time to time or drop him a note on the pulsar facebook and he'll be only too happy to discuss how he went about it.

Voltages differ between machines and even within machines. With lipo technology being so light and powerful you can have your drive system running off a different set of batteries at a different voltage very easily.

Quote:

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Originally Posted by emisnug

I'm well aware that we will have issues - there's an idea to make a 1:4 scale version to make sure we can actually do it first, but we've set aside around £2500 for the bot, not including spares.

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Heya,

Just a tip on making a scale model, due to the lack of cross compatibility between major components, its worth building it to 13.6kg as then youll have 2 robots with which you can actively compete. Otherwise your effectively throwing away money

Quote:

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Originally Posted by emisnug

It would seem I'm working with some mildy outdated information regarding drum spinners. 9000rpm+ would be an excellent goal, but motor wise, I'd assume some gearing would be involved - is there a particular one you'd recommend? Do most people run 48V or 24V?

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You are looking a the larger Brushless motors, 3000W+, or large bushed motors.

Pulsar used 2 Align 800MX Brushless motors on 12S, and what looks like roughly a 3:1 ratio; usually done with pulley/chain, not gears.

Other machines use the Turnigy Rotomax 80CC to 150CC (They are designed as replacements for nitro engines), a LEM Motor (not sure on the size... but BIG), Lehner 3080 Brushless (Hypershock uses this motor), I believe Yeti uses 2 A28-400s, and Warhead uses 2 of Scorpions HK 7050's at 10'000W EACH!

To use these motors to their max, you need to run 48V, but you don't need to run the whole robot on that. You can have 2 separate power systems with their own Links and Power lights. It adds a little bit of complexity but nothing major. We used dual voltage setups in both our spinner FW's. Mine uses a 5S for drive and 12S for the weapon.

In terms of Mass, you want to be looking at 15Kg I'd say absolute minimum, 20-25Kg ideally. (This is entirely down to your robots design but anything less and you won't have the energy to do any damage.) Tombstone in the US has a 40KG bar? I think? Carbide's is 25KG.

Make sure your LiPo's or A123's are able to deliver the current that the motors demand, and take in to consideration burst currents as on impact the current draw can double for fractions of a second.

PS: Apologies for the info dump. Its all necessary though.
PPS: People will disagree with my choice of weight or motors and have their own way of doing things. Find what works for you/is in your budget/you have experience with.
PPPS: Have fun!

Welcome to the group Emisnug.

The Ebay/Aliexpress 700-1000W scootermotors ain't bad actualy. Need some ruggedizing and better wiring, but are up to the task to get a heavy moving with a usable speed and acceleration. Budgetfriendly and easy to obtain.

The 1000W versions are 36-48V. (in effect, 700-800W 24V versions marked for 36 or even 48V)

On prototypes and scaled versions. Some parts don't scale well.
And with the popular classes it's 150 gram ants, 1.3kg beetles , 13.6 kg Featherweigths and 100kg heavies.
I would follow Garfies advies there.

Weaponpower. Typhoon_Driver Gary mentioned that de days of carstartermotors are over. Yep, that's true.
Series wound motors are very very powerhungery and realy ineffecient, meaning you'll fry the motor and drain big batteries quickley.

Thank you for all of your responses - extraordinarily useful!

RE: The scale model - you're very right, some parts do not scale well. Something I've earmarked for later.

Weapon Motors: running at 48V shouldn't be too far out of the realm of possibility. Eventorizion - I've looked at Turnigy motors before, but could never find specs in regard to torque or an RPM figure that wasn't +/-150rpm/V. But! It's good to hear that they are suitable for some uses. The planned weight of our drum (according to some very basic math) should fall around 27-28Kg, not including chainwheels etc.

Drive Motors - I've managed to burn out Ebay motors before (admittedly, entirely due to my very bad gearing), and I do have a Framco 240W in the garage that I've never touched.

One point about design I keep coming back to in my notes is this:

Is it better to focus on a running, driving bot and have the weapon as almost an afterthought? Or is it better to design around the weapon and its potential damage potential?

Thanks again

The 'proper' way to do it is to design around the weapon and have the rest of the robot optimised as a weapon delivery system (unless your main weapon is ramming, obv). Depending what components you have access to when, your skills, and the exact design though, that can vary a bit (certainly for a first build on this scale I'd shift slightly more towards making sure it all moves!), but weapon first is the accepted design methodology.

I'd also leave a 240w anything in the garage, modern heavyweights will probably be needing double that *at least* in drive power!

24V 240W = 36V 540W.

To compare, the "old gold standard" of heavyweight drivemotors are the Bosch GPA 750. Those are rated at 750W, but can push up to 4Kw @36V (and get very hot, melting themselfs if that load is asked for more than a few seconds) This was 16 years ago.

Storm 2 was during the "old" series the undisputed king of drivepower. The LEM's capable to push out 6Kw+

Team RCC's heavy, Bullfrog, handles 3kw for each side.

In short, 240W is good for a selfrighter. And that can be done with a modern 150 gram £15 brushless motor.

The thing I'd worry about (and, please correct me if I'm wrong) is the stall torque on smaller brushless motors - I can't seem to find specs on motors, and when doing a rough calculation, I get in the region of 100Nm for stall torque, which seems good, although I'm not too good at math, so I'm not sure how wrong I am.

Brushless motors are weird. The reason websites won't give you a stall torque or a torque curve is because... they don't have one (for our uses anyway). The ESC you use with the motor has such a major impact on how they perform and things like the prop/wheel/tracks/impeller all give the motor a different profile. Where brushed motors deliver all their torque at 0rpm, a brushless is almost the other way around (kind of).

Stalling a brushless motor isn't the big deal that it is with a brushed motor. Most good brushless ESC's can detect the stall, or jam and will cut out. And even if it doesn't, since you are not burning out brushess and creating sparks, I've heard of massive 1500W outrunners stalled for over a minute with no ill effects.

The main issue with brushless is the higher speeds (usually). Where a bosch 750 or Mag motor outputs around 3000-4000rpm at the shaft, a brushless motor will be double or even triple that. The Rotomax 50CC's in Pulsar do around 8000rpm and the motors I am experimenting with do 20'000rpm (This higher gear ratio makes position sensing for the ESC easier and allows the motor to start spinning which gives the ESC the feedback it needs to fire the phases at the right time)

We are still tuning the ESC's we use to our motor setup and the system gets better and better, giving faster acceleration, more torque, lower start speeds and better efficiency. In a weapon setup its not so critical but it can make the difference between a 10 second spin up time and a 5 second spin up.

PS: Brushless is still new in the HW category so if you take a leap like we have then there are very few people who even have the slightest clew what is going on. But if it works the benefits seem to be... substantial!

Ah, see using brushless seems like a good option to me - the ESC side of things looks like it will be a steep learning curve - if it's alright with you, I'll reach out when we're about to wire up to ask hundreds of ESC tuning questions! One of my friends uses Rotomax 100cc ones in his helicopter (does all the crazy stunts in it), I'll ask him as well.

I wonder if some of the smaller brushless motors could work as drive motors?

If you are planning to put a machine together with the intention of getting it on the tv then I wouldn't expect a large leadtime on the build. Two months heads up was given last time with a month of actual confirmation. With that little time and the fact it's your first build stick with brushed motors. There's a good reason that 95% of machines use them.