Current Draw vs. Stall current

[Adam.garcia]

I am currently working on a new drive train for our off-season robot chassis. I am using JVN's Design Calculator, and it is saying that, at Max Continuous Load for our High Gear (15fps), the current draw per motor is 121.57 Amps. I looked at the Cim Motor specs, and it says that the stall current is 107 Amps.

Does this mean that if the robot stalls in High Gear, the robot will trip a fuse, or blow the Jaguar into my favorite blue smoke of death?

Thanks,
Adam Garcia
Team 4 Element

[Trent B]

I am seeing the spec to be 133amp at stall http://files.andymark.com/CIM-motor-curve.pdf

As for breakers
http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf

Based on that spec sheet at about 120amperes a 40 amp would trip in .5-1.1 (300% load)

Their graph is flawed I believe, if you count there are 11 graduations starting at any number x and going to 10x including x's and 10x's, conventional logarithmic scale should be 10, eg 1 2 3 4 5 6 7 8 9 10. Additionally the data seems to be a little different than what they say in text.

Maybe provide some more information so people more experienced than I can help figure out your issues.

What is your:
Wheel Diameter?
Gear Reduction?
Coef-of Friction (if the calc has it)?
# of CIM's in Drivetrain.

[Adam.garcia]

Originally Posted by Trent B

What is your:
Wheel Diameter?
Gear Reduction?
Coef-of Friction (if the calc has it)?
# of CIM's in Drivetrain.

Wheel Diameter: 4 in
Gear Reduction: Still being determined, but prob going with 4-5 fps in low and 14-15 fps in high.
Coef-of-Friction: 1.4
# of Cims in Drivetrain: 2 per gearbox for a total of 4 in total.

[Al Skierkiewicz]

Adam,
I would check your calculations as your result sounds pretty high. The net effect on the Jaguars would be that they would go into current protect mode at that high current. The stall spec on the CIM is 133 amps but generally the wiring teams use and the length of wire will result in lower currents at stall. As to the breakers, they may trip but effectively reset within milliseconds. As suggested the breakers are a temperature controlled device and so can stand loads in the 600% of rated current range for a couple of seconds. However, once at an elevated temperature, the trip point becomes much lower.
I always ask why teams want to go so fast. At 15 fps you will be covering end to end in a little over two seconds. Control becomes an issue and collisions will cause some serious damage. We recommend speeds of 10-12 fps for high gearing as a good balance of speed, maneuverability, control and the ability to avoid potentially fatal collisions. While many teams like to go fast, only a select few do it well.

[JamesTerm]

Originally Posted by Al Skierkiewicz

Adam,
I always ask why teams want to go so fast. At 15 fps you will be covering end to end in a little over two seconds. Control becomes an issue and collisions will cause some serious damage.

I can think of one reason... one that has haunted me for quite some time... we were scrimmaging against the robowranglers where they were playing defense against us. We could not get past them at all! The only way I could think that would have worked would be to go faster doing a lateral maneuver, and then curve past them. (A short burst of speed here).

[Jared Russell]

Originally Posted by JamesTerm

I can think of one reason... one that has haunted me for quite some time... we were scrimmaging against the robowranglers where they were playing defense against us. We could not get past them at all! The only way I could think that would have worked would be to go faster doing a lateral maneuver, and then curve past them. (A short burst of speed here).

There are a few ways to defeat defense...

Speed (works great against most, but who is to say your defender isn't just as fast?)

Momentum (a 150-lb loaded robot at 15fps is going to make whatever it is hitting move backwards at least a little...as long as the rules - and your robot's construction - permit)

Pushing (works great against most, but just as with speed, you will eventually encounter a bot that can push you to a standstill)

Agility (turning on a dime and/or strafing let you outmaneuver some opponents, but fast, powerful bots can still be a problem)

Deception (in a hectic match, most defenders go do something else when they see you driving away from a goal. Take advantage!)

Teamwork/Blocking (every sport has some variation of a "pick and roll" or "lead blocker". So does FRC!)

Teamwork/Blitzing ("send more men than the enemy can block")

Against a great robot and great drive team, the last two options are often your best (and only) hope.

[JamesTerm]

Good info here... Thanks... Jared

[kws4000]

[quote=Jared341;1077997]There are a few ways to defeat defense...


Pushing (works great against most, but just as with speed, you will eventually encounter a bot that can push you to a standstill)

In 2009 (remember the slick surface?) our lead rogrammer added in code to increase the allowable pushing power of our bot. We weighed 85 lbs on the chart, and we were pushing around 120 lb bots with those ungainly trailers.

//Traction control calculations
if(m_rightStick->GetRawButton(1) == 0) {
if (ncount1 == 0 && ncount2 == 0) {
ncount1 = count1;
ncount2 = count2;
} else {
speed_error1 = ncount1 - count1;
speed_error2 = ncount2 - count2;
ncount1 = count1;
ncount2 = count2;
}

//ajjy = ajjy * .95;

if (speed_error1 > 30 && speed_error2 > 30 ) {
float ajjy = ajjy * .9;
float ajx = ajx * .9;
printf("Motor Y: %f", ajjy);
printf("Motor X: %f", ajx);
}
}

It worked, and it might help with mechano wheels, but I don't think this code will really be usefull for at least another 5 years...

Ready for kickoff?

[the man]

Now ask your self how long will a 17 amp hour battery last while four motors draw 110 ish amps? How long untill your wire melts? I feel there is some thing wrong with your calculations.

[Adam.garcia]

Originally Posted by the man

Now ask your self how long will a 17 amp hour battery last while four motors draw 110 ish amps? How long untill your wire melts? I feel there is some thing wrong with your calculations.

The 121 Amps only occur when at max continuous load (ie. We are pushing against a wall, or an immovable robot). There is no feasible reason for pushing against an immovable object for more than a few seconds.

To get around ever having to go near this 121 Amps, our programmer is working on some code which will sense if the Amps rise too high while in high gear. If the robot senses high Amps, it will automatically shift to low gear in order to prevent possible breaking.

We just saw this a few days ago while looking at 399's drive code. Ingenuity to the max!

[Adam.garcia]

Originally Posted by Al Skierkiewicz

Adam,
We recommend speeds of 10-12 fps for high gearing as a good balance of speed, maneuverability, control and the ability to avoid potentially fatal collisions. While many teams like to go fast, only a select few do it well.

I have done some further calculations, and I have a potential gearbox which goes 4.17 fps in low, and 12.51 fps in high. In high gear, the current draw per motor at Max Continuous Load is 119.41 Amps. Would this be sufficient to use without having to worry about tripping the breakers?

If not, what is the max current draw you would recommend for a robot in high gear?

Thanks for all your help!

[Chris is me]

That is the max current draw, i.e. you are pushing against an immovable object. As long as you're not pushing you should be fine.

[Brandon Holley]

Where did you get your 1.4 CoF #?

-Brando

[Adam.garcia]

Originally Posted by Brandon Holley

Where did you get your 1.4 CoF #?

-Brando

According to Chris is me,

"For calculation purposes in a drivetrain, I've been told 1.4 is a safe static CoF for roughtop on carpet in the direction of motion (basically, when gearing for traction limited pushing power / acceleration)

If you need more precision than a good estimate, I would experimentally determine this for yourself."

[Chris is me]

In retrospect, something like 1.2 or 1.3 is probably more accurate, but 1.4 does give you a nice and comfortable safety margin.