Let's Make Robots!

Gumpy the off-road robot


My first full project. This is a suprisingly capable little robot built on top of a R/C crawler chassis I found at a local thrift store. It can handle hills and long grass with ease. A wireless PS2 controller and a Pololu TReX Jr. motor controller makes it very responsive. With two drive motors and one steering motor this bot can generate pretty good torque and still has room for expanding the onboard electronics with things like GPS, accelerometer, gyroscope, etc.

The power source is a 9.6V 1600 mAh NiMH R/C battery pack for the motor driver. It works okay, but a 2000+ mAh will definately give it the kick to go up stairs and tackle much rougher terrain. Future (possible) modifications will probably include stronger motors, a wireless camera, mixed autonomous/human control, and a revamped suspension system.

Since I had a hard time finding code for the TReX Jr I decided to share my code in the hopes that it will make it easier for a beginner to interface a TReX to an arduino: PS2 motor control. I'm working on a full tutorial but until I get it posted I think the code shall suffice. 

 

~UPDATE: Well, I was going to post a video but than I realized that the steering system broke. My guess is that a cheap R/C car has a hard time handling the forces of sudden turns that the motor controller is capable of providing. When I turn, the gears that turn the wheels slip, causing a very annoying grinding sound and preventing the wheels from fully turning. So a note and a word of caution, if you use cheap parts don't overexert them more than they are meant to. The easiest solution is to program some kind of safety into the code until you determine how much stress the vehicle can withstand.

I might try to replace the entire steering system with a more reliable servo instead of a toy motor, however that is gonna take a lot of carving.

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Technically the motors will draw all the current they need, but most likely its the current draw on the motors that are the problem. LiPo batteries and Stronger motors would solve the problem.

I've heard good things about LiPoly-batteries, they can supply more current, however; you are probably better of getting more powerful motors.

Thanks for clearing that up Birdmun. I thought that if you use more mAh it provides more current to the same part for the same amount of time. For motors then, wouldn't that mean that as the power draw increases, the motor uses more current (amps), so being able to provide more current helps the motor turn more effectively?

as you have stated are rated at 1600mAh. Which means, roughly, that it will supply 1600 mA or 1.6 A for an hour. Therefore, higher current draw means the batteries won't last as long and lower draw means they will last longer. Having a higher current battery only means that, everything else being equal, your robot will last longer.

Your motors are going to pull as much current as they need no matter how much you offer. Voltage, on the other hand, will give you more speed at the expense of more wear and tear on your motors.

Current is pulled or requested by the load, be it a motor, a LED, or a uC. Voltage is pushed and it won't care what the load is expecting it will get the full voltage. LEDs and uCs are particularly unhappy about such a possiblity. You can hook a deep cycle marine battery up to a microcontroller, uC, and as long as there is some manner of voltage regulation/step down. The uC will run until you turn it off or the battery is depleted.

On a similar note, you will notice many people around here asking about powering their robots with 9v batteries. While the voltage is high enough to allow blue/magic smoke to escape, the current is not high enough to allow for any significant run time of the motors as they can pull upwards of 1A and a 9v battery offers in the neighborhood of 500mAh. In other words, maybe, 1/2 A for an hour or less.

I hope someone will set me straight if I am wrong on any of this.

This is a great explanation, voltage vs. current makes a lot more sence now, thanks a lot.

How old are the batteries? Perhaps a new set of batteries may improve output.

Thanks for sharing your knowledge.

Higher current batteries w/o changing the motors should not give you anything, but, longer run time.