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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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now class repeat after me..
"You will *not* save money making your own speed controller !"
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Hoenstly.. the parts you need are 3-5 as expensive in small quantities than they are in bulk (100 controllers worth) price that is available to the commercial manufacturers.
There is also the high likelyhood of an assembly error causing half the speed controller to go up in smoke and cost you the parts over again, and quite likely the fight.
If your budget is tight, look for a cheaper commercial speed controller, (Scorprions, Electronize's), or hunt around and find what you need second hand. Most ESC's are regularly traded on the for sale sections of the various forums.
I'm not being difficult, I'm a very do-it-yourself kinda guy do.. the difference is.. I've *done it*. I designed and built the IBC prototypes and I can tell you they worked out far more expensive each than just buying a production unit would be now.
You will find that applies to all ESC's. They do not use parts that are available from your local Dick Smith or Jaycar at a good price.
For example, the IRF1405 FET has just been started carried in stock by jaycar (so I read) for `$5 per FET. (An IBC uses 16 of them). Jason buys them in 1000 packs for around $2 each, so your $50 behind the commcerial unit cost already, and youve just started.
Sorry. There are plenty of plans on the net for simple and cheapo (Transistor bases) speed controllers tht can handle a maximum of 5-10 amps.. beyond that, powerful speed controllers need big $$ and time to make work.
Seriously, look at the Robot Power Scroprion, the English Electronize and shop 2nd hand, thats as cheap as it gets. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Fri Nov 10, 2006 8:17 am |
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Fish_in_a_Barrel
Joined: 30 Sep 2006
Posts: 673
Location: Perth, Western Australia
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I had a look at this design a few weeks ago, and one interesting part was the power dissapation information.
http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html
My $5 60V 100A FETs can only put out 71A max, while putting out 300 watts of heat. And to get rid of all that heat for a 3 phase H-bridge will cost me around $300 in fans & heatsinks.
For your situation, presuming your drive motors (2) are stalled (10A each) and your using a H-bridge driver (4 FETs per motor, 2 in use at any one time), you need to find a heatsink that will dissapate 9* degrees/Watt, not too hard, as long as there is some air movement. (If your bot is fully sealed then it will have thermal capacity and there will be a time limit before the air inside the bot gets too hot to cool the FETs.)
*I think this is right but it's getting late.
If for some reason you go all out on you motors and find some with a stall current of 50 amps then you need to get a 0.35 degree/Watt heatsink, which will total ~$80 from jaycar. (This would require fans venting outside the bot.)
Since it is low power, perhaps you could save some heat by doing PWM with one or two FETs and forward and reverse with relays.
Back to your first post: I haven't built it, but i haven't noticed any technical errors in his site, I looks like it should work fine. Good luck if you decide to build it.
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Sat Nov 11, 2006 1:23 am |
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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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Chucks website is full of good information, you wont go wrong following the principles of his design.
Thigs that'll get you in real life ESC design are the practicalities caused by things like PCB track Layout, EMI caused spikes, power supply filtering and hidden parasitic capcitance, and inducatance altering the H-bridge timing and causing shoot through.
Little things like having the FET gate drive tracks too long can trip you up (long tracks hanve solf inductance which is bad when you are trying to shove amps of gate charge current through them in microseconds)
Enjoyment and Learning is the only reason to consider building your own speed controller in *small* quantities. You will learn a lot about power electronics and if learning is enjoyable and more important to you than winning, go for it.
Just be prepared to be let down by your ESC on occasion while you iron out the wrinkles in the *impleementation* (not the design).
Thats why I said its cheaper to buy commerical. If you take into account a couple of likely smoke-bangs to repair that happen under combat stress, and the damage that will likely happen to your bot from being immobilisied mid-fight, then home made ESC's start to look very expensive against a proven reliable unit.
Especially seeing as how most people let time run away from them and are usually committing the sin of trying to finish their bot in the days before the competition. Having an unproven and untested component in there is almost guaranteeing a first round out, which doesnt please event promoters (like me) either. We want to see you put up a good fight throughout the event, not go out with a whimper first round - which is why I reccomend proven components to first time competitiors.
If you *want* to learn all about ESC's and how they work, and the primary motivation is not just to save a few $$ on a one-off unit (you wont). then by all means build your own.
Its a very rewarding process and you do learn a lot of stuff you probably never suspected (I had 10+ years of electronics experience and I found that *power* electronics encompasses a lot more subtle effects the become major ones under the influence of 50+amps that you just never see in <1amp circuits).
Just make sure you test your home brew ESC *thoroughly* before you jump into the arena with it. and try to build it in such a package shape that if it does all go haywire close to the event, you can drop in one of the commerical ESC;s to replace it and still go a fighting.
you should also join up to the OSMC (Open Source Motor Controller) yahoo group and have a read of the archives and follow the discussions for a bit. Thats where most of the friendly ESC design guru's hang out.
http://tech.groups.yahoo.com/group/osmc/ _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Sat Nov 11, 2006 8:41 am |
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