paper: Battery Voltage in Robot Drivetrain Simulation and Modeling

[apalrd]

Thread created automatically to discuss a document in CD-Media.

Battery Voltage in Robot Drivetrain Simulation and Modeling by apalrd

[apalrd]

A few weeks ago, JVN told me I should write a paper on this. So, after promising it to the CD community, I finished collecting all of the log files and assembling the paper. I hope you all like it. Consider it my Christmas present to Chief Delphi forums.

As always, I'm open to questions and comments, and always looking for ways to improve my design methods.

[apalrd]

Aaaand I shouldn't post things at 11:30 PM.
The first graph labels are wrong. The orange graph is 2.56:1, the blue graph is 4:1. I'll update the PDF.

[AllenGregoryIV]

Andrew,

Thanks for putting this together, I was just thinking about this problem, and this is going to be very helpful. This is a very nice Christmas present indeed.

Allen

[Al Skierkiewicz]

Andrew,
Nice paper! If you look at the battery voltage graph and compare to acceleration you will get an idea on what might be taking place. As the load changes, the current drawn by the motors change, and so to the voltage drop across the internal resistance of the battery. Of course, teams should also be aware that using other motors in the robot (at the same time as driving), also modifies the voltage drops and available current. The length and wire size also have a great effect on the current available to the motors. Longer and smaller diameter wire, adds significant resistance to the individual motors. At 100 amps, #10 wire will drop 0.1 volt per foot. So if the motor is wired with two feet of wire, the drop will be 0.4 volts since you must take into account both the red and black paths. If a team uses #12 for the motor path, double that loss.

[marccenter]

Andrew,

Merry Christmas.

I was wondering if your model takes into account gathering a game piece? In a robotic arm application, the manipulator could take a trapezoidal profile to accelerate, cruise at steady speed, and then decelerate to zero speed. This is important because the robot manipulator does not desire to ram into it's final position and
damage the manipulator. This velocity profile would seem to match a FRC robot obtaining a game piece. Would this change your model characterisitics significantly and hence your ratio match?

BTW, thanks for "thinking out of the box" for your analysis. Off to breakfast.

[ParkerF]

Andrew,

Thanks so much for posting this technique. I've been working a lot this semester on getting a better understanding of drivetrain design. This has definitely furthered my knowledge and I intend to apply this as a part of my 'lessons learned'.

Again, thanks for sharing!

-Parker

[apalrd]

Originally Posted by Al Skierkiewicz

Andrew,
Nice paper! If you look at the battery voltage graph and compare to acceleration you will get an idea on what might be taking place. As the load changes, the current drawn by the motors change, and so to the voltage drop across the internal resistance of the battery. Of course, teams should also be aware that using other motors in the robot (at the same time as driving), also modifies the voltage drops and available current. The length and wire size also have a great effect on the current available to the motors. Longer and smaller diameter wire, adds significant resistance to the individual motors. At 100 amps, #10 wire will drop 0.1 volt per foot. So if the motor is wired with two feet of wire, the drop will be 0.4 volts since you must take into account both the red and black paths. If a team uses #12 for the motor path, double that loss.

I eventually measured (sorta) the resistance of a battery using the Battery Beak. It told me the exact battery I measured (plus it's half of the cable and connector) had an internal resistance of 0.021 ohms. I had already picked 0.03 because that number matched the actual data fairly well. I think the extra 0.009 ohms accounts for some of the voltage drop in electrical components, as we would see ~0.004 ohms from the #10 wire alone (by Al's numbers). The simulation uses 12v as the base battery voltage, a good moderately charged battery is usually higher also.

Al, at 400a, what are the losses of a #6 vs #4 battery cable? We always keep the length short (never adding length to the 12" COTS cable and using a ~6" wire between Main Breaker and PD), and wondered if switching to #4 on one or both ends would help.

[AllenGregoryIV]

What other ways have people optimization power flow? I remember reading something about Jim Zondag figuring out a way to optimally charge the minibot batteries before matches in 2011 anything like that for standard FRC battery?

What about resistance added by Anderson connectors going to the motors or other quick disconnects?

Any specific type of 10 AWG wire better than others in terms of power loss?

Do these actually make a noticeable difference in performance?

[Ether]

Originally Posted by AllenGregoryIV

Any specific type of 10 AWG wire better than others in terms of power loss?

Yes. Solid silver

[apalrd]

Originally Posted by AllenGregoryIV

What other ways have people optimization power flow? I remember reading something about Jim Zondag figuring out a way to optimally charge the minibot batteries before matches in 2011 anything like that for standard FRC battery?

We never did a ton of analysis of charging FRC batteries, but we did some for the minibot batteries.

Since the use time was so short, we were trying to push the charge (and surface charge) as high as possible, as quickly before a match as possible (to reduce fade of the surface charge). We ended up with a cycle using 2x RC car battery chargers. One was 115vAC powered, with our battery cart, and would charge it 'normally'. We would then put the charged battery into the minibot (where it was secured with zip ties), and leave a lone zip tie sticking out and the minibot power switch off to indicate that the battery was disconnected. Before a match, we had another 12vDC powered battery charger (running off a robot battery) which would very rapidly (40s or so) top off the battery at a very high charge current (10a or so), then we would zip tie the battery connection and go onto the field.

We tested the charge profile using a battery discharger (functionality included in one of the RC car battery chargers), to verify the length/current of the top-off charge. It was a fairly time consuming process. Eric Yahrmatter did most of it. We did all of this development mid-comp season, we didn't use a special charge process at the first two or three competitions, but we did have many batteries and chargers in parallel then.

A cycle like that wouldn't work nearly as well for normal FRC robots.

[Ether]

Originally Posted by apalrd

Since the use time was so short, we were trying to push the charge (and surface charge) as high as possible, as quickly before a match as possible (to reduce fade of the surface charge).

Did you run any minibot pole-climbing tests to compare climb times using a given battery fresh off the charger vs the same battery 10 minutes after coming off the charger?

[apalrd]

I don't remember doing any, although I wasn't working on that project directly. We did a lot of minibot pole tests, and our lead minibot mentor worked on it during the day a lot (he's good at highly iterative machining projects like that).

[BJC]

Originally Posted by Ether

Did you run any minibot pole-climbing tests to compare climb times using a given battery fresh off the charger vs the same battery 10 minutes after coming off the charger?

I worked pretty closely on this. Yes and no, we iterated the minibot to the point of ridiculousness. Minibots were certainly both run on high and low batteries throughout the course of testing but we were never specifically interested in the difference between the times of minibots other then to tell which was fastest and why. As such we don't have any of the kind of data that you're looking for. We knew that a fresh battery couldn't possibly be less charged then one that had been off the charger for a while so we always made sure that we used a battery straight off the charger. Does 10 minutes of sitting actually reduce the battery voltage any significant amount? Maybe not, but we were squeezing every bit of power out of those motors to make it up the pole in under a second so we did it anyway.

Hope that kind of answers your question.
Regards, Bryan

[Ether]

Thanks for the detailed explanation Bryan