ESCs for 18V 775 Motors

Currently looking at building a feather for the champs in April, I've bought a few parts and want to start testing things. The ESC's which I've been recommended are the BotBitz 80A ones, but I'm not ready to drop £100 on two ESC's for a project I'm still trying to get my feet wet with. I was just wondering if anybody had some cheaper options that they have had success with in the past?

I've ordered some of the cheap chinese "320A brushed ESCs" from a UK supplier, which of course are nowhere near that amperage rating, but I'm going to try and upgrade the connectors and wiring in hopes that it can withstand the draw from the 775 motor. Bit of an experiment, but if it works then its an ESC for only £10.

I know the general consensus is "you get what you pay for", and i totally agree. I will no doubt buy myself a pair of BotBitz ESC's in the future, but for now, I just want to be able to get my bot driving around the garage so I can test out various aspects of its design before commiting to spending a lot on it.

Hate to say it, but you've wasted your money on those "320A" escs. If they don't burn up, you won't be able to sell them. Money down the drain and they won't do the business in the arena.

You might be able to find a set of electronize cheap second hand if you put a post in the wanted section which would do the job better than the 320A esc.

If their the ones that can handle 16v and have the then they will probably work as ive usiveed them in my 2 and you can run 2 550 drill motors at 4s on each esc but I found that you either need a gyro or a good transmitter strait line

If your over volting them then they probably wont last long and there are also 2 versions 1 will go from forward to reverse with no problem ant the other stops when u slam it into full reverse making driving a proble so I hope u get the good 1s

We'll see! Yes indeed they are the 16V versions, and I'll be be running them on a 4S lipo, so it should be fine on the voltage side of things. The reversal thing sounds like firmware problems, I wonder if I can flash them with different firmware? Hopefully I won't need to though and they'll be the right ones.

I believe I am using the same ESC as you. I had a problem with having too lower ampere, yet for me 3s 2.2 amps lipo on each work well, using 12v drill motors.

I had a look at various of these cheap ESCs with mixed results... Have a look here: http://www.fightingrobots.co.uk/thre...heap-drive-esc

General consensus was though that the 20A ones work with 550s after a lot of modding, but those "340A" ESCs are pretty naff for anything larger than a 550 even with mods..

If you're struggling for cash have a go at the smaller weight classes first.

It's not that I'm struggling for cash. I just want to explore cheaper avenues before settling with the expensive "it just works" option :) I am building a beetle at the moment too though!

I wanted a cheap option for my first featherweight, so I opted to build my own ESC.. Cost me about £25 and lasted fine for 12 fights before blowing up in testing. Got a pair of those cheap 20A chinese ESCs, and they lasted a good number of fights before blowing up, then I stuck my heavyweight ESC in there whilst waiting for my next design of ESC to be built, then my next ESC blew up on the first day of the championships, so I've stuck a 5th new one in there now, and this is all within the space of one year.. In total I've probably spent over £150 designing and building these ESCs, and they're always the main point of failure with my bot (though getting more reliable as I get better :) ), and all but my heavy ESC wouldn't be good enough to drive those 18v 775s. On the other hand, if I'd just bought a pair of TZ85s I'd likely have had basically no problems all year and only spent £60 (would reprogram them myself as that's easy enough). Picture below shows all but the latest version (which is currently in my feather), and is in the order of oldest on the left.

http://www.fightingrobots.co.uk/atta...tid=5549&stc=1

Sadly don't think there is a cheap option when it comes to ESCs, unless you're already experienced in building them (I can build my own for <£20 now). I would still encourage more people to try building their own; I've learnt so much from it and developing new controllers is one of the most interesting parts of the sport for me, but in the long run it's not necessarily cheaper.

So, Rapid, between you and my teammate, I've been convinced to try and build my own ESC's. Even if its a failure it'll be an experience. My team mate, Dave, is a bit more in touch with electronics than I am, so he'll probably be doing most of the work on these. One thing we're both wondering though is how stall currents are handled. Do your ESC's have a way to protect themselves from the high stall currents? Or are they simply beefy enough to deal with it? I was planning on using an ATMega control board to read currents across the circuit and apply a pulse to the input power when it goes too high, bringing it back down to a reasonable level. But maybe I'm over thinking this?

There are two ways that stall currents are usually handled: Most commonly you just over rate the controller enough to deal with peak currents. This works fine unless things are heavily overvolted or you're using particularly large motors. The other alternative is to use current limiting as you say, where you read off the current and ramp down the input until it comes back down below the limit. Charles Guan has many pages on the development of the Rage Bridge which is definitely worth a read; this uses current limiting so you can get an idea of how it works. For featherweight motors though I'd advise just over rating the controllers enough to handle what you need. A good page to read for that is Chuck McManis' MOSFET rating page , which gives you an idea of what FETs you need. For an 18v 775 you'll probably want a continuous rating of about 60A or near enough.

As for components, I'd advise using HIP4028 driver chips to drive the MOSFETs; I tried using other things but this is by far the easiest. Few things to watch out for though with the HIP4082; need to run them on at least 10v (I use ~15v), have a PWM frequency of >1kHz, and don't let the high side signals go above about 98% duty cycle else the charge pump will run out of juice. Probably won't make much sense atm but hopefully after doing some research you'll see what I mean :L

Some really great information both in your post and in the links, thanks a bunch! I'll take a look at the driver chip you recommended, although we were planning to use an ATMega and write our own firmware, mainly due to wanting to incorporate a few quality of life features, like a gyro to help steering. Might be a good idea to make a barebones version first though and worry about fancy features later.

The driver chip needs to be driven by a microcontroller; all it does is interface between the microcontroller and the MOSFETS. The stuff in my previous post is mostly stuff I wish I knew when I started; hopefully should save you from a lot of the mistakes I made during my ESC's development

Ahh right. Sorry I jumped to conclusions that it was a replacement for a microcontroller! Lots of reading to do for me this weekend I think :)

Rapidrory, where did you buy your HIP4028's from? Having a hard time tracking them down online.

I get them from RS: http://uk.rs-online.com/web/p/mosfet...ivers/0496818/

Awesome thanks. They seem like they are pretty straight forward to use. One thing that is confusing me though is that their BHS (pin 15) is getting connected to the main circuit power, surely this would fry the controller with such high currents. I'm sure I'll figure it out though. Thanks again!

That's a high impedance sense line, so no current is going to it (It's the reference voltage for the charge pump). It's isolated up to 80v (Hence the 80v rating) so the controller won't care what sort of currents are flowing through the H bridge as long as the voltage doesn't exceed 80v (which it won't in your application).

One thing the data sheet doesn't make very clear is that you'll need some ~10 ohm resistors between the driver chip and the MOSFET gates (gate resistors). Keeps the peak drive currents to below what the chip is rated for (1.25A). Also stops resonance in the drive lines and the like but you don't really need to worry about that; just fit some 10 ohm gate resistors and you'll be fine :L

Ah perfect, all makes sense now! Let the breadboarding begin :)