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Originally Posted by Matt Krass
I see, but I'm still not convinced it would help enough to warrant the extra power source. And a COTS computing device doesn't necessarily have a power source, especially if you use a small embedded mini-ITX board or something.
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That's very true, but it's allowed if you want it (so far as I can tell at least).
The way I look at it, I can do this right now (and on two I did)....but I probably have to ruin the speed control for competition and I see no reason for that.
There's no reason you couldn't use the main battery for everything if you really want to and don't mind the issues from the motor. In point of fact, if you separate the control supplies and the motor supplies you could just use a big capacitor on the existing battery. The control logic doesn't require much (a couple of C or D cells is enough for what I have). A capacitor could probably provide just enough 'ride-through' for a short time. Actually, if you wanted you could get a little exotic and use a LiPo battery back for that if a suitable charger was available cheap. Then it would be cell phone weight.
Actually for fun my existing test frame has a Boss Audio 3.5 Farad (you didn't read that wrong) capacitor on the frame with the voltage meter 'cap'. The whole reason it was there was to try providing a lower internal resistance to hold up the battery if the speed controls get too much and cause a brown out. Just something I was playing with and it was less than $100. (Yes now besides stick welding with the robot's battery you have a capacitor to insure you can lightly weld with your robot as well....lol)
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I am convinced, being able to log and time data offboard would be useful, actually there were a couple times we were debugging an intermittent task crash this season that would have been useful. Once the system dies, there was no way to get the information we needed. It was a lot of blind guessing, and a lot of ibuprofen.
I see, I guess I was thinking too high level (monitoring the current and logging spikes and dips and breaker pops), you just mean detecting and recording a transient over-current system. It might be worth considering, in addition to a digital signal indicating the spike, perhaps one that indicates voltage present at the node with a comparator (to determine if the breaker is currently open or closed, maybe a moot point with self resetting breakers?) and a reset line to clear the latched fault?
Matt
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Right now my logger prototype uses the CAN bus and has an RS232 level serial port as a back up (so you can put it on a cRIO serial port if you tinkered a bit, or put it on the CAN bus). I originally built it both to monitor the CAN bus and because I figured CAN was more direct than bit banging out serial on digital I/O.
Yes I agree, there are going to be times when under or over is not enough. Right now my current monitor uses a window comparator set with fixed resistors so you can actually tell if you have too much or too little (if you want...otherwise ignore one side). The same with my voltage monitor. I thought about using 'digital potentiometers' and I tried it...but I'm not sure the extra cost is all that necessary when you can get cheap good enough resistors at RadioShack. Besides the worst that happens is someone uses the wrong resistor and it trips at 60A instead of 30A...it's not the circuit protection you'll live. People just need to learn Ohm's law or use a look up table and learn how to read color code (actually right now the resistors can be fixed into a connector so you don't even need that, could just get the connector with resistors in it for that setting).
Course I used rail-rail JFET opamps so there's the whole slew rate business, but still that's much faster than a DMM and much more practical in this application than an oscilloscope. I actually previously discussed basically what I'm talking about here in these forums. I could have done this with discrete transistors but even with surface mount I'm not sure that saves me anything over the integrated package...besides then there's that whole SWARF! thing and more possible points of contact.
I think in order for this to useful you need the reset (set reset flip-flop (or one made of gates) or JK flip-flop). Otherwise basically all that the thing is good for is the first over/under limit. Hopefully this is set below the circuit protection threshold and it actually doesn't matter if it's above the battery negative...it's easy enough to clamp the high impedance input if it's put in backwards. Though back to my previous point I'd prefer that putting things in backwards requires at least as much effort as a square peg in a round hole to achieve.