Well I guess I better check my e-mail a little more often...
You have no doubt read the above posts so let me fill in the holes. During the design and building phase of our robot we noticed that some of the drive motors appeared to be drawing way too much current. Since we have utmost faith in our mechanical design team (not to mention their years of experience) we made the assumption that the drive train was correctly designed so the fault must lie elsewhere. As most of you know by now, FIRST was shipping a slightly different motor (the Fisher Price look alike) than past years and the power curve and efficiency was far enough off to cause most teams problems until it was addressed.
Some students and design engineers began to ask if it ws possible to accurately sense the current in individual motors and use the data for analysis or robot feedback. So began the circuit design known as "StangSense". See the link
http://www2.wildstang.com/2002/inve.../schematic9.gif.
This was really a team effort with 10-15 students and engineers all contributing to design, building and software. Essentially it is easy to measure AC current by using a current transformer but with the speed controllers at full output the pulsed motor drive goes to full DC so the output of a current transformer would fall to zero at maximum current. So the search was on for a device that would measure current at each motor and would be small enough to fit inside the enclosure. One of the engineers found a reference to Maxim for current monitors. We found one, the MAX 4172, that was designed to sense charge current in laptop batteries. The 4172 scales a voltage drop across a very small (much less than an ohm)series resistor into a current source. We scratched our heads over what to use for the small resistance, such as how do you fit high power resistors into the project box, the weight of the wire in our weight budget, etc. It was then that we made some calculations and determined that a one foot piece of #10 wire is approximately .001 ohms. That turned out to be just right for the 4172 design and the only weight we had to add was #22 wire from both ends of the 1' #10 we were using to feed the drive motors anyway. When you choose the right components, you can scale full expected current to be represented by a variable voltage between 0 and about 3.5 volts. (For our purposes this represents 0-135 amps) Motorola (did you expect us to use anything else?) makes a series of microcontrollers that have A to D converters available on some of the pins. So we fed the sense voltage to the A to D's which converted and multiplexed the signals so that they could be passed down just one input to the robot controller and then out to the Operator interface and dashboard port. Then it was a simple matter (I am not the software guy so it looks simple to me) of writing Palm software to interpret was was being received. In using a color Palm, the drive team had a choice of displays. They could see the current represented as moving bars that were normally green but would change to yellow and then to red when the current reached predetermined trip points. Or they could see a graphical top view of the robot with colors representing the current in each of the motors, changing again from green to yellor to red.
We also stored the entire two minute match as data and were able to download that to a PC from the Palm, import the data into a spread sheet to see quantified data and chart the current. Since the dashboard also has a clock of sorts we could get a pretty good idea of what was happening during a match by replaying the video tape and comparing to approximate timing of the robot clock. Although we never used it for feedback, it became very useful for risk assessment vs. strategy, and since the battery voltage is also part of the dashboard output we were able to trace dips in battery voltage to high current demand which led to some changes in software. (don't let too many high current motors turn on at the same time.)
We built a second portable unit that allowed us to diagnose other robots who were having problems with robot reset, damaged motors, burned speed controllers, etc. We offered this service to anyone who asked during regionals and nationals and helped many teams diagnose their problems. It is an immediate response for teams who had been scratching their heads over some serious issues. The data we collected during our first regional actually caused us to change the electrical layout of our robot to make the controller operation more reliable on high current draw. As you know the robot controller will reset when the supply voltage falls below eight volts, so it is imperative to prevent this from occuring.
Thanks to this board and all the teams who began discussing the problems early on. Thanks to Andy Baker and the TechnoKat team for their input on breaker trip vs. temp, thanks to Joe Johnson and all the other dedicated engineers who helped us understand some of what we were seeing, and thanks to all the teams who let us hook up to their robot and gather data. You all helped make this project a success as well.
Good luck to you all,