While I (Chris) suspect many budding robot builders have actually spent the Christmas period working on their robots – something evidenced by the hive of activity on the Fighting Robots Association forums – we’ve been using it as time to reflect where we are, get things ordered in ready for our next build session and crucially resolve some lingering issues with the weapon systems. I’ll leave it to Rob to explain more about that in a blog post shortly.
However, I returned to Southampton today having been away over Christmas to find a delivery – and an important one at that.
Some time back, we ordered the speed controllers – one of these (for the weapons system) was dead on arrival. We’ve since returned that one to the seller (and from our conversations over on the FRA Forum, seems like we aren’t the only ones to have “crispy” Turnigy speed controllers), and asked around the community about other options.
The “go-to” standard for high current, high voltage electronic speed controllers is pretty much a choice between MGM Controllers, and…. MGM Controllers. So, throwing the robot building handbook away once again, we’ve ignored the advice of the community (and yes, we’ve already done that several times) and settled on trying out one of HiFei’s Fulcrum Pro controllers.
As it stands, it’s been with us sufficiently little time we’ve not even put the connectors on the wires yet, but we hope to have it connected up and running next time we’re in the workshop. We have had a quick look inside it though, and there’s essentially six 120A FETs per channel meaning that their design of circuit should be good for 720 Amps!!!!! In reality, that won’t be the case, and their rating of 300A is probably about right. Since we’re below this, we should be OK.
Now, the educational bit: Before picking a speed controller, you must first pick your motors. Going the other way around is a recipe for speed controllers going up in smoke, costing you lots of money.
Once you have your motors picked – find out how much current they draw “at stall” – i.e. motor not moving at all. This gives you a number of Amps above which your speed controller must be rated. Our weapons motor draws around 225A when at stall, so we need a speed controller that’s got more “oomph” than that. By and large you want to allow as much headroom as you can above this without going completely overboard. If you think you’re going to need more powerful motors later, you probably need to look at speed controllers with a 50% margin at least. If not (as in our case), 10-20% is usually enough.
So, we’re looking at 250A or higher in our case…. and as we discovered, there’s not many options. We’ll let you know how we get on with the HiFei controllers as they make stuff that’s suitable for many robots.