Recently I was bit with with the robotics bug. After a lot of Google searching and advice from kids on my high school’s robotics team I decided that it sounded like a reasonable project for me to start. I’m still in the planning stages and I still don’t know if will actually be able to t make this project work. Essentially I just want to make robot that I can experiment with. I’m on a very limited budget which complicate things even more. At the current moment I want to use and Arduino Mega 2560 as the controller. I want to use two
RS550 Motors - 12V from banebots along with two
P60 Gearboxes: Stock, Standard Shaft, RS-540/550 Mount, 16:1 also from banebots. From what little knowledge I have I understand that I’m going to need motor controllers for each of the motors. What I don’t know is exactly what I’m looking for. I know that they will require a different power source then the arduino. So what do I use for motor controllers and what do I use to power them? I’m probably in way over my head, but I seriously can’t stop thinking about this project and all the possibilities it has. Any advice is greatly appreciated.
Can I suggest that instead of spending upwards of $65 (that seems to be a minimum for motor controllers, you build an H-Bridge. They’re a simple, intuitive, and fairly cheap (like $5-10) to make. I warn you- you do need some electronics / soldering skills and basic knowledge of circuits and common components.
Here is one decent tutorial: http://letsmakerobots.com/node/2074
And here is the chip: http://www.pololu.com/catalog/product/24
And last, here is another method of dual direction motor speed control- the darlington transistor. I very much suggest this method. It’s a better place to start. I am making a bunch of these for an animatronic I’m finishing up this summer. http://bildr.org/2011/03/high-power-control-with-arduino-and-tip120/
The problem with these methods is simply the amount of current that the controller needs to handle. Most of the pages you’ll find on the Internet describing these circuits are targeting small ~3-5W “hobby motors,” whereas something like the RS-550 will pull hundreds of Watts at stall (the specs cite a 85A stall current) (hopefully you’ll never get there if you just want a robot "that you can experiment with) and even unloaded, they’ll pull 30W.
It is possible to build your own controller that can handle these kinds of loads (the Open Source Motor Controller project comes to mind), but as squirrel says, you may end up spending as much on parts if you’re only building a couple, and this depends on much you trust your electronics skills (nasty things happen when that much current goes places it shouldn’t). It would be a fantastic learning experience, but I would suggest tracking down an electronics mentor from your friendly neighborhood robotics team to help you with the project.
If you’re more interested in the programming side of things, it’s probably easier to buy a pre-built motor controller if you’re looking to use motors that powerful. Something you may ask yourself is what the objective of your project is, and if you actually need that powerful of a motor. Honestly, you’ll be pretty hard pressed to find a better price than $60 for a motor controller that can handle 45A. If you’re concerned about dropping $60 on a motor controller, keep in mind that you’re spending that much for each of the gearboxes as well.
Unfortunately, building robots can be an expensive hobby…
Speaking from experience, the more difficult exercise than assembling your control system is designing and fabricating a chassis to hold the parts. Your robotics team may be able to help you with this, but, again, it depends on how much you’re interested in the design and construction versus the programming. Again, evaluate what your objectives are for the project. Something to look into may be some of the kits available that specifically target Arduinos as control systems.
I’m looking at the banebots website and after reading what you said about the current I was wondering if the BB-12-45 ESC ( http://banebots.com/pc/ELECTRONICS/BB-1245 ) will be able to provide enough current for the RS550 Motor - 12V?
I decided that I may as well go all out on this project. I am also interested in all the aspects of this project including the design, construction, and programing.
Nice catch, I had misread it as 45A continuous. The BB-12-45 could something like the RS-395 and probably the RS-540 if you were careful, but the RS-550 is going to be a bit much for it in most cases.
As explanation: the peak efficiency, no load, and stall currents they list on the page will give you a basic idea of the current requirements for the motor, and describe three notable operating conditions for the motor, but to get a better sense for specific applications, you can make use of the motor curve that FIRST posted for the RS-550. Figure out how much torque your motor will need to output in the worst case (if your robot is going up hills, pushing things, etc) - maybe look at some of the resources posted here about gearing choice to make sure you have the right gear reduction for your application and make sure to account for efficiency losses in the gearbox - find this torque on the X-axis, and then find the corresponding point on the current curve. You’ll also want to add some safety margin, and then this will give you a good idea of your needed continuous current; note that your motor will draw more than this at some points, but this is where the peak current rating for the motor controller comes in, as long as you have a safety margin so that you don’t jump into that region too often. If you work through this, I expect you’ll get a higher number than 12A for your continuous current. When working with these kinds of currents, you may want to consider designing in some sort of fuse or circuit breaker as well, to prevent horribly catastrophic failure.
Just a quick look around gave me this controller from robot Robot Marketplace that support 40A continuous / 60A peak for about the same price that might be an option, without looking too much into the details.
*I think I may have glossed over some details in the interest of brevity, always feel free to ask if you have any questions.
If you’re going to get into robot pushing matches, then you need a more powerful motor controller. But for just driving around, the 12 amp controller should be fine…as long as you don’t have a long skid steer robot. Using two drive wheels, and caster(s) to keep it balanced, should make it so you don’t work the motors and speed controllers very hard.
Those little motors will smoke if you try to work them too hard. Having a speed controller that will cut off like the BB ones will, could be a good thing.