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Robot Proof of Concept


#1

I wanted to get a small robot for me and my son to play with, I originally looked at the Fez Mini Robot kit but it was out of stock at the time. So I did a little digging. Since I already have a Several Dominos I thought I would just build one myself. I found a chasis that looked nice here http://www.seeedstudio.com/depot/2wd-arduino-compatible-mobile-platform-p-657.html?cPath=170_172 so I thought I would just get a motor controller shield that was on sale for a great price and go that way. Well in the mean time the robots came in stock and the motor control shield is out of stock. So I guess I could just get the robot, but since I started down the road to build my own I want to continue that way now.
I’m a little into RC so I thought why not just use a brushed electronic speed controller for the motors. Hobby King has a 20 amp one for $5 http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9090 since I already have a couple of these I thought why not just use them for the motor controllers. They are controlled just like a servo so I could just control them with a pwm pin. One for each motor and I’m ready to go.
The only problem is that they expect to be connected to a RC receiver which powers up and holds the servo in a neutral state. So I’m not sure what will happen while the Fez is booting. To alleviate that problem I will add a relay to the power lead of the ESCs. That way the motors can not power up until I “turn them on” with the relay.
I plan on connecting an xbee to talk back to my computer for control.
Any thoughts or glaring problems with this concept?

Thanks,
Craig


#2

I’ve never tried using a brushed ESC before, but it looks like it may be controlled the same as a brushless ESC. Check this out.

Also, most ESCs have to be “armed” to prevent them from running during bootup. The software takes care of this.

There’s also an article on my blog concerning XBee.


#3

Yes brushed ESCs work the same way but for brushed rather than brush-less motors. When I got into RC there was no such thing as brush-less motors :wink: or at least not used for RC. Based on your results it looks like it should work fine. I’m going to use a stand-alone UBEC to provide 5v to the Fez from my lipo pack. Probably this one http://www.hobbyking.com/hobbyking/store/__15212__HobbyKing_Micro_UBEC_3A_5v.html

Now to order.


#4

That ESC has a 5V BEC built-in. Why not use it? It’ll save you a month of waiting for HK shipping…


#5

It was mostly because I was going to isolate the battery power to the ESC with a relay, if I did that the computer could not run to turn the relay on and off. That idea was out of concern about the ESC state on power up. I was going to power up and initialize the fez, when all was ready I would power up the ESCs. But that might not be necessary if the fez can come online before the ESC is ready. I need to test that but it appears to work for you so that is good.
Very good point though. Maybe I’m over thinking it. I tend to do that.

Craig


#6

Most ESCs have a programming mode where you can configure various things like the startup mode. But, most are built with airplanes/copters in mind where it can be very dangerous for them to automatically spin up when they are powered on. So, I think you’ll find that it requires a signal in order to be “armed”.


#7

Yeah I wish China was closer to Utah for shipping reasons!


#8

Uh… Well, I at least wish they stocked more items in the US warehouse :wink:


#9

So I guess my next question is how stable of a 5v do they put out? I’ll need to test that. I know some are configured for 5 or 6 volts. I might want to get a 6v one and let the Fez do the fine regulation. I’m not much of a hardware guy so I have to think about the dang wires a lot. :slight_smile:


#10

That power should be just fine. The real concern would be about the it dropping too low. For an airplane/copter it’s never recommended to wire the micro to the same battery as your motors. But, for your case I don’t think it’s really going to matter much if it craps out because the voltage gets too low.


#11

I think a 2s lipo pack should not be discharged lower than about 6 volts so I was going to put a lipo voltage monitor on it. That should keep me in a safe voltage range for the UBEC to make the 5v. Gotta love Hobby King for these few dollar gadgets!
Wish they would stock a lot more in that US warehouse.