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Author Topic: Autonomous Robot, motor driver  (Read 3508 times)
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With a 298 in there he's going to lose 2-3 volts at the motors straight off the top....

Btw, have you tried to code the acceleration in, as I suggested and as you elaborated in code earlier?

And tried it on a floor with no carpet? As the wheel sinks in the pile, as it moves off it has to flatten the pile in front and that takes energy.
« Last Edit: February 22, 2013, 11:24:11 pm by JimboZA » Logged

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With a 298 in there he's going to lose 2-3 volts at the motors straight off the top....

Btw, have you tried to code the acceleration in, as I suggested and as you elaborated in code earlier?

And tried it on a floor with no carpet? As the wheel sinks in the pile, as it moves off it has to flatten the pile in front and that takes energy.

Yes I did it didn't work, the problem is not accelerate or the code. The motor driver kept shutting off from over heating, I guess the motors were drawing more than 2.5 amps, and the motor driver couldn't handle it.
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Yes I did it didn't work, the problem is not accelerate or the code. The motor driver kept shutting off from over heating, I guess the motors were drawing more than 2.5 amps, and the motor driver couldn't handle it.

First off, what you need to do is quit guessing, and start measuring. Let's go over the specs of your motors and the driver, shall we?

First off, the motors - based on the link you gave, here are the pertinent specs:

Motor Ratings (at 7.2 VDC):

    No-load Current: 0.27 A @ 310 RPM
    Stall Current: 4.8 A
    Max Torque: 4.6 lb-inch (5.3 kg-cm)

So - with no load, these motors will only pull 270 mA - but once stalled, they pull 4.8 amps. Furthermore, they only provide 4.6 inch-lbs of torque.

We know your motors are pulling a lot of current, and that they can pull way more than your motor driver can handle (more on that later), so let's look at the torque. What "4.6 inch-lbs" means is that the motor can move (or lift, if you will) 4.6 lbs if a lever were lifting that mass one (1) inch away from the center of the shaft. If the lever were 2 inches long, you could move 2.3 lbs, if 3 inches long - 1.53 lbs - and so on.

So, what this means is that the motor can move 4.6 lbs if you have a wheel 2 inches in diameter. How big are the wheels that come with the motor? Well, according to the specs, the wheels are 4.875 inches in diameter; so that gives you a lever of 2.44 inches - giving you the ability to move 1.89 lbs (4.6 / 2.44). See how that works?

Now of course, that would be a "dead-lift" weight - rolling on the ground (carpet, floor, whatever) that number may well be enough to move the weight of the motors, batteries, platform, etc without approaching the limits of the motor, but the more weight you have (and friction with the ground, etc) - the closer to that number (and the greater the current draw) will occur.

There are ways to figure out mathematically how much torque would be required to move the mass sideways for a given mass (and things change when going uphill, of course), but you can do it empirically using a fishing scale or similar - simply hook one end onto your platform, and pull it, measuring the amount of lbs needed to pull it (ideally with free-rolling wheels and the same weight as with the motors and real wheels installed). You should attempt to do this.

Now - with the motor driver - it's specs indicate that it "...can supply up to almost 3 A continuous current per channel...and it can tolerate peak currents up to 5 A per channel for a few seconds". Ok - well, we don't know what kind of current the motors your using are pulling. Maybe you should measure that. You can do it with a digital multimeter put into current measuring mode and inserted in series with the motor; hook it directly to the battery, run it on your surface, and measure the current draw.

I'd be willing to bet that your motor is exceeding (or coming close) to it's 2.5 amp rating:

http://www.pololu.com/catalog/product/1213/specs

Now note something about the above specs - see the number "4" next to that amperage rating? Look down at the bottom - note what it says for note "4":

"4 Can be improved by addition of heat sink or forced air flow."

Hmm - sound familiar? You might want to try adding a heatsink and a fan to those ICs - glue it on with some Artic Silver or similar heatsink glue, then add a small fan to move some air and get the heat out. Note, though, that if you find that those motors are pulling more than 2.5 amps, the driver is probably going into thermal shutdown mode, which is would explain what you're seeing.

So do your measurements, and figure out where the problem lies. You may need to purchase a more capable motor driver.
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I just cant grasp how a fan over my motor driver will help my robot drive? will a fan really cool off the warming of current that much?
and I'm just trying to learn and ask here dont fight me lol

Yes, it can really make that kind of difference and so can heatsinks.  Cooling is not optional and starts with simply moving the air.

I have a linear 25A power supply that I've had for many years now.  It ran so hot just idling that you'd get second degree burns if you touched the output transistor cases.  I put a tiny computer fan inside and now the whole thing stays at almost room temperature.  A huge difference.
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I built a RepRap 3D printer using Pololu stepper drivers with heat-sinks.  Even with the heat-sink, the motors would draw enough current to overheat the driver chips, causing them to go into fail-safe mode... they stopped operating until they'd cooled down.  While this was only a second or two at a time, this caused the motors to randomly skip steps, which is a real problem with 3D printing.  The heat-sinks, on top of chips barely 1/4" square, got nearly hot enough to burn my finger.

A tiny CPU fan over the controller board solved that problem.  So yes...  a fan can quite literally make your robot's motors run more smoothly.
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Now - with the motor driver - it's specs indicate that it "...can supply up to almost 3 A continuous current per channel...and it can tolerate peak currents up to 5 A per channel for a few seconds". Ok - well, we don't know what kind of current the motors your using are pulling. Maybe you should measure that. You can do it with a digital multimeter put into current measuring mode and inserted in series with the motor; hook it directly to the battery, run it on your surface, and measure the current draw.

I'd be willing to bet that your motor is exceeding (or coming close) to it's 2.5 amp .
So do your measurements, and figure out where the problem lies. You may need to purchase a more capable motor driver.
[/quote]

So, I did some calculations.. Long story short.. The motors are drawing 3.7 amps when at stall mode..
And basically the motor driver is not giving enough for the motors to get started.. When I gave the robot a small push, it drove fine.. So, in conclusion I'm getting a motor driver that would allow such continuous current to be achieved ..
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