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Unread 25-01-2002, 08:50
Joe Johnson's Avatar Unsung FIRST Hero
Joe Johnson Joe Johnson is offline
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An ME sticks his oar in again...

When I suggested that teams infer battery current, my real point is that there is no reason to put a so-called shut resistor in line with main battery feed because there IS a resistor in series with the battery already... ...it is called the internal resistance of the battery! In fact, that is how simple battery models work -- they simply put an ideal battery and an internal resistance value in series as the model of the battery. To first order, it is a pretty fair approximation.

The trick is that battery resistance is not completely fixed, it varies from battery to battery and (I am guessing) from a fully charged battery to a less fully charged battery. My proposal was just to get a fix on what that internal resistance was by putting another resistor across the battery leads (say a stalled motor) and then using the internal battery voltage measurement on the RC to infer the internal battery resistance. Knowing that resistance, it is not that hard to then watch battery voltage and infer what current is going through the battery.

Now, that is a lot of words and ideas, but in the end, I am not really recommending that anyone measure battery current because I think it is a red herring.

From my experience designing power hogging robots, I do not think very many teams will have trouble tripping the 60 Amp fuse. AND (more importantly) those few that do will not be likely to have the cleverness to make the current monitoring circuit work, yet alone do something intelligent with it once they HAVE a working current signal.

It is not a simple matter to go from current measurement to how soon will my breaker trip. Yes, you can easily just clip current to 60 Amps but that using sledgehammer to kill a gnat. Getting closer to the edge of tripping but NOT tripping is tricky business involving keeping track of current usage over time and estimating how hot the breaker is at this moment based on that current history. The problem is made even trickier when you start to realize that you should limit current not to what will trip the breaker in the next few seconds but before the match is over. Now you are really into the crystal ball realm.

My bottom line, finish your robot early. Give your drivers time to practice. If you see that it is possible to trip the breaker, try to engineer your way out of it by changing ratios for example. Failing that, have your drivers LEARN the edge of the cliff. With time, they will be better current monitors than anything you could code in Pbasic.

Just one man's opinion.

Joe J.
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