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[on:]

Hello!

This is my fist post in this forum so I hope I'm in the right place.

I have a cheap RC monster truck that I want to hack in a UGV. This is my first time playing with motors and I quickly got stuck with this problem: the adafruit motor shield on my arduino uno that's meant to drive the motors can only provide 600mA of current and the rear motor probably draws 1 to 2 Amps. I thought of using transistors and optocouplers but that would require two separate power sources.

This may seem trivial to you but as a noob I want to play it safe and see how others did it.

So how would you do it? What's the best solution?

Thanks allot for your time!!

[Reply #1 on:]

Too bad you took out the original receiver.  You should be able to replace the receiver chip with the Arduino and leave the motors connected to the driver transistors designed to drive them.

[Reply #2 on:]

...the rear motor probably draws 1 to 2 Amps.

Probably? Have you measured it? If not, you want to do that first. You want to measure the total running current draw, as well as the current draw when the motor is stalled; when stalled, the motor may pull quite a few amps, depending on its design.

If you don't still have the original receiver board (as mentioned, this would be the best way to go - see this mega thread for ideas: http://arduino.cc/forum/index.php/topic,86883.0.html), then you'll need another h-bridge solution.

If the motor really doesn't pull more than 3 amps stalled, then you could use an L298 in bridged mode (read the datasheet on how to do this), which will give you up to 4 amps (you need a heatsink for the chip, though - they sell cheapo driver boards on ebay out of china that have a heatsink fitted, but you may have to mod the board to use the bridged mode of the L298). If it pulls more than this, then you'll need to use something else.

Pololu (http://www.pololu.com/) may have what you need. You didn't mention the steering system - were you going to use the built-in method (usually another small motor driven by an h-bridge or an electromagnet system)? If you are, then you'll need another h-bridge for that (and you'll need to measure the current needs there as well); you might find that the current motor driver you have is powerful enough.

What would be better for the steering, though, would be to add a standard R/C servo to drive the steering; if you do this, then you might also look into getting an R/C ESC (electronic speed control) for a -brushed- motor, and use it to control the drive motor. That way, you can easily control the platform using the Servo library. Most ESCs these days, though, tend to be for brushless motors (which won't work for your motor). Also make sure you get an ESC with "reverse" mode, so you can drive the motor in both directions (some ESCs are single direction only, meant for R/C airplanes and helicopters).

Note that ESCs tend to be rated in the number of "cells" they support (NiMH or LiPO cells); figure approximately 1.2 volts per cell; so a 5S ("S" for "cell" - don't ask me why - maybe homophone reasons?) ESC is meant to control 6 volt motors; you'll find 6S (7.2 VDC), 8S (9.6 VDC) and higher out there as well. Make sure you get an ESC that is as close as possible for your motor voltage (whatever battery it was originally using will tell you). You can go a little over, but not much (not more than 25%, and even that will cause the motor to have a shorter life). You might also find them with integrated BEC (battery eliminator circuit) - some of these will be 5 volt, others will be 6 volt; if you can get a 5 volt one, you can use that to power your Arduino (bypassing the Arduino's on-board regulator, of course). Otherwise, if your car's battery is 7.2 or 9.6 volts, then just split it directly off to the barrel connector and let the Arduino's regulator do the work.

Hope that helps.

[Reply #3 on:]

Hello Friends,

I am running the AHP piston and have the head decked (10 thous I believe is what Mark had it done) we did not fly cut the piston for the BVH but then again it wasn't necessary. I know compression is lower then it should be which is why I was looking at installing the thinner head gasket. I guess I could have the head decked some more and the piston flycut to accomodate it but I dont want to keep moving in this direction if its the wrong direction. I would rather sell what I have while its in good condition and start on a new setup. The cam I have installed is the AHP race cam for the BVH. I dont have anymore specs then that.
Nick, please read the documentation. RAMPS is happy with up to 35V. However if you go over 12V you must remove the diode D1 so that the Arduino Mega does not draw power from the RAMPS board, but instead gets it from the USB. There are compelling advantages to running above 12V including less current draw, faster heating of the print bed, and the stepper motors and stepper drivers will run significantly cooler.

Thanks And Regards.
Anthony Martello 

[Reply #4 on:]

Thanks guys for your help!


Crosh, how do you measure amperages higher than 250mA? most multimeters only go up to 250mA and the most expensive I've seen went up to 1A or something like that... I've already fried a fuse once so I take my precautions now. Oh, I've read part of the thread you've linked.

I actualy haven't tooken the cars original board out yet but it's tightly fitted up side down so it would require cutting cables to investigate it.
So here's what I'm going to do: cut the connections remembering where they where were soldered, take the board out, turn it over, take a look at the IC if it's worth replacing. If yes, hack it the way Crosh sugests in his thread. If no, try to replace the brain completely with the arduino.

Thanks again for your help!