Current Monitoring system questions

[PyroPhin]

hello, gots some questions on something im developing for the 2003 bot

my idea is to have several taps on the motors/compressor to record all the voltage levels of them and send that data into the RC.. then put it thru the V= i*r formula to get the current of a particular device. i figured the R of the wire isnt gonna change that much in a 2 minute compeition, so im pretty well off setting that as a constant.

then the other half is getting one of the BS2's we got laying around to talk to the Dashboard port of the OI and send the stuff across to be displayed on a LCD mounted in the comtrol system.

i know how to do all of the programming for this.. minus the communication part. every time i have tried to duplicate the Serin command used in the RC, i get a stamp error.. so i can only assume that the form they use is for IFC stuff only.

well, if you have any suggestions... please drop me a line!

~PyroPhin

[Jnadke]

I don't wish to intrude but... how exactly are you going about getting the current draw from the wire to the robot controller? Just checking.

Ugggh... I don't know where to start...

The serin command in the default program cannot be used for any ordinarly stamp chip. Everything in the robot controller is actually performed by a central processor. The stamp chip only serves as an easier way for us to tell the processor what we want it to do. The serial input/output commands allow the two chips to interact.


I have a difficult time picturing what you are trying to do... I certainly hope you're not trying to do what I am picturing...

Why are you trying to connect the bs2 to the dashboard port? If you are trying to get the data from it then you must read the Dashboard Port documentation and serin the bytes in that format.


IMO... you are biting off more than you can chew.

[PyroPhin]

the current draw thing is one of the things i am trying to figure out.. gotta find a way to get the 12V down to 5v.. was thinking voltage regulators of some sort...

and.. bah... it can be done.. and we shall do it!!

[Bill Beatty]

Wildstang team#111 measured the current at all motors and sent the data back through the dashport. They could record the whole two minute match from an amperage point of view. I believe they just used a certain length of motor lead wire as a shunt and measured across it.

[Rob Colatutto]

do you know if team 111 had an lcd display of sorts to tell them what the current draw was?

[Bill Beatty]

They monitored the current on seven or eight motors and sent the data back to the operator's station. They would record the data on a laptop or palm pilot for later analysis. I don't think they monitored the data during the match, but I imagine they could have.

Hey Raul, any chance you can have one of your electrical guys publish a white paper?

[PyroPhin]

okay.. so i know it's possible.....

you know any way of getting in touch with team 111's programmer? i gots a couple questions for him.

Thanks!
~Pyro

[ChrisH]

We had current monitoring as part of our control program. When the current exceeded a certain limit, the computer would back off the controlers until things were down to an acceptable level. Since the current value was part of the control algorithm, I assume it could be transmitted back through the dashport pretty easily.

It worked great at LA and Nationals, then caused no end of trouble at IRI. We think we finally figured out what is going on this weekend. If we're right some minor program tweaking should make it behave. We think it was just a little too sensitive to voltage drops and have a work-around designed but haven't tried it yet.

I'm suprised gwross hasn't put his $0.02 in yet, as he was much more involved than I was. I'm just a mechanical, I leave the electron pushing to others whenever I can.

ChrisH

[Matt Reiland]

If you look on the wildstang site, I think it was under the Invention award or something they showed the circuit they used to create the current sensor and send it to a color palm pilot. Very slick, also fairly complex, if they didn't have a custom circuit board it probably was a lot of breaboarding to make the circuit in the schematic.

See here:
http://www2.wildstang.com/2002/inventor/schematics.asp

Very nice job Wildstang!

[ntroup]

Thanks for the Kudos, Matt.

I was one of the Motorolans that worked on the current-sensing (err.. StangSense) for Wildstang.

We basically did what was discussed here earlier, taking a known voltage difference across a known resistance to get current. That was accomplished by using a known length of known guage of wire, to which you can find the natural resistance of, and finding the voltage difference across. That voltage difference was piped into a Maxim current-sensing op-amp (can't recall the part number offhand), which made that voltage difference (on the scale of mV) and converted that to a range from 0-5V.

That value was then piped into an HC08 uprocessor, through an A-D, and we used the HC08 to do some time-averaging of the currents of the eight inputs (4 drive motors, shooter, conveyer, etc).

All of this was then sent to the Robot Controller through a few of the analog and digital ports, and sent to the player side, where we had a color Palm processing the data, showing live current values, max current values, etc. We also had the ability to save that data during each match, so that after wards, we could look at/analyze why our current spiked at some particular moment, or what drive motors weren't spinning, etc.

Definitely a lot of work, and a pain in the rear to solder together, but it was a fun little adventure!

-Nate

[Adam Y.]

Ok I may sound a little dumb but what is the point of doing it this way? It sounds way too complicated.

[ChrisA]

http://www2.wildstang.com/2002/inven...schematic9.gif

i think that is the final schematic for it

[Matt Krass]

Ok, we have most of this figured out, I hope, one last thing, how do we build a circuit that can measure DC current? Any schematic links?

Thanks in advance,
Matt

[ntroup]

Originally posted by wysiswyg
Ok I may sound a little dumb but what is the point of doing it this way? It sounds way too complicated.

What is the point? The point was to see if we could do it... that was all the fun right there. Where else can you stare at wires until you go crosseyed, all the while getting tipsy on solder smoke!

Seriously, though, while building this past year's robot, we were concerned that we may end up drawing too much current and blowing breakers. Since this years challenge was more agressive than years past, we didn't want to end up dead in the middle of a tug-o-war. So, a few students and a few engineers spent way too much time designing the board.... and ended up getting to done about 4 hours before ship. We ended up building a second one for debug purposes after the ship date just to see if the thing even worked

-Nate

[Al Skierkiewicz]

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,