battery voltage vs current

[techhelpbb]

Originally Posted by pribusin

Can you explain what you mean by teams needing to limit power input to the drive train? I'm not familiar enough yet with the new PDB capabilities I guess. Thanks.

I will take a crack at this:

Let's say you have 6 CIMs on VexPro gear boxes on your drive train.
http://www.vexrobotics.com/vexpro/ge...llshifter.html

Now let's say you have some issues in your drive train.
Slight shaft misalignment.
Bearings loaded improperly.

Plus let's make that robot reach the FRC legal weight limit.

Add on some end effectors maybe with a few more CIM or miniCIM.

At this point the battery voltage will start to drop quite a bit during operation because the combined load is high enough.

If the battery voltage gets low enough the control system will brown out and the robot will loose field connectivity.

In the past a potential solution was to reduce the maximum output from the speed control for a given input from the operator controls via the software. This would reduce the total load on the battery even if the operators would otherwise cause issues with their request. However this is a static degrading of the system performance. It may be difficult to predict where the trade off between performance and system failure is especially since you could not previously drive around with the test equipment on the competition field and sometimes things change.

The new PDB will allow a team to monitor their power usage more continuously so they can tune the power usage dynamically. Of course they must exploit that information in their software for the full effect. With this sort of information teams can move further into the performance spectrum and further from the safe zone. That increases the loads on the electrical system in the form of higher surge currents.

However it appears that there is some sort of inherent protection at work in the underlying software anyway because I noticed Team 11 during RampRiot kept really beating on their battery until it was down to 6V and they only lost communications for at most 2 seconds each time. So far as I could see, when their lead programmer asked for my help getting the RoboRIO on the field, the loaded Java software was very similar to previous years.

It is entirely possible to not overload breakers and still create so much system load that the battery can't keep the entire control system fully powered on. Such that in previous years teams would field robots that were often near the limits of the battery and then fail to grasp the issues it would create. Then again power measurement was not a tool teams were previously provided by FIRST (except for the Jaguar) and FIRST really doesn't teach electronics. With the new tools the expectations will shift and I think for the better.