... and I'm just a programmer. I'm not that up on the electrical, but since I did the programming that interacted with the current sensor, I'll tell you what I know:

We didn't have a display of the current draw, and we didn't attempt to record any values. Instead, we used electronics to sense the current being drawn by our drive wheels, and programmatically limit the power being requested of the drive motors.

Some of our electrical/electronics types came up with a circuit for our electronics box which output a voltage which varied from 0 to about 3.5 volts according to the amount of current being drawn by our three drive motors. (It's ALL black box magic to me.) Ideally, the output voltage range would have been 0 to 5 volts, but we didn't have time to get it tweaked to perfection.

These voltages get read as analog inputs to the robot controller. Each time through the control loop, if any one of the values is above a certain threshold, the program scales back the power being demanded of all the drive motors. The scaling factor starts at 100%, and then every time through the control loop the current is above the threshold, the factor is cut back by 5 percentage points (but not below 10%.) Once the current gets back below the threshold, each time through the control loop the scaling factor is increased by 1 percentage point until it gets back up to 100%.

Thats the problem I am having. It should be possible to stop the abnormal current draw in the drivetrain. Of course I need to read the chapter on drivetrains and current draw in my books to be sure. I really should read the First Bill of Materials before I post because all of the information you need is there but the answer you get ends up being a coefficent of friction between the playing surface and the wheels. Of course with my specialty being with mechanics it makes perfect sense that I'd see this problem with a differnt light.

Dead link
 

That link doesn't work. In fact, there's no "www2.wildstang.com". Trying www2.wildstang.org" instead with the rest intact gave a 404 error.

What's the correct URL?

- Keith

Keith,
We are trying to get the site back up. We had to change ISP's and the former ISP refused to give up "wildstang,com" so we moved to "wildstang.org". There have been problems though and I know Mike Soukup was working like crazy to get everything moved and updated. It's a slow process and we just completed our fall classroom time with a mini-competition Monday night. (the mini-comp uses Lego RCX and Robolab to give students a leg up on design, problem solving and software.) Sorry for the problems.
I tried to include the schematic as an attachment here. I had to scale it down to upload and it was unreadable.

Al's right, we switched ISPs and the crooks at our old one won't give us our domain name, so we switched to http://www.wildstang.org. I uploaded the old site to the new server last night so the diagrams should be there. Plese look and let me know if anything isn't working or you can't find what you're looking for.

[edit]
The link you're looking for is http://www2.wildstang.org/ws/invento...schematic9.gif
[/edit]
Mike

WHoa i though it was illegal to have custom circuits provide input!!!!!!!! SWEET!!!! OMG ITS NOT?awesome!!!

Outstanding info
 

Al That is worthy of being a white paper .Can you you cut n paste it as one .Great info. Jim :cool:

Re: Current Monitoring system questions
 

You will notice that alot of teams measured across calibrated lengths of wire, or even placed very small value resistors in series.......we just measured the voltage drop across the circuit breaker and multiplied by different coeficients for 20A and 30A breakers.

Simple...and works great.

We used it in the stamp code to detect stalled drive motors...where we modulated them to keep the breakers happy.

We also used it to detect another motor stall, and used it as a limit switch. (end of travel instead of a nasty microswitch that always gives p the ghost that the most inopportune time)

I can pass along schematics if you are interested.

-Quentin

Re: Re: Current Monitoring system questions
 

Oh yea.. I thought we've ALWAYS been able to talk to the RC. Wildstang's innovation was to made a VERY NICE package for massaging and sending realtime data back to the dashboard. :)

Custom circuits just can't run DEVICES directly. No motor controller replacements, nor driving the motor controllers with the custom circuits. The RC is the ONLY device that's allowed to run the MOTION outputs.

This makes sense. This rule is to GUARANTEE the robot WILL stop when told to by the arena controller. If the custom circuit could directly affect any output device, FIRST couldn't guarantee shutdown, and you'd immediately have a safety hazard if it DIDN'T shut down. Since the inspectors can't check your circuits, it's simpler to rule that ONLY the RC can command "real" things.

Heck, if you think about it, if the custom circuit COULDN'T talk to the RC, what WOULD be the point of it beyond "fancy telemetry"??? The only real effect it's allowed is to tap the RC on the shoulder and say "Ahem... Pardon me... Here's something YOU should know about so YOU can do something about it..." :D

Oh, please DO!

As a EE, I'm a big fan of custom circuits, and appreciate seeing the neat things everyone has come up with! :)

- Keith

Re: Re: Re: Current Monitoring system questions
 

Done...I just uploaded it a few seconds ago...and I suppose, after the sysop reviews it, they will make it visible.

I hope you find it interesting. If you (or anyone) has any questions, please feel free to email me.

Enjoy,
-Quentin

Re: Re: Current Monitoring system questions
 

We had thought about doing this, but our experience from previous years was that any breaker that had gone through reset cycles changed resistance. We were afraid that that these changes would make results unreliable since our original plan had been to use the data for feedback to control systems.
As to the custom circuit, I agree with the other posts that a custom circuit gives us the opportunity to teach electrical to students in addition to mechanical. I also think that teams should be able to demonstrate during inspection, the ability to remove the custom circuit and still run. Even without this restriction, teams should make that their first priority. If it fails, remove it and still compete.

current measurement?
 

bigqueue, you talked about measureing the current over the breakers to check the current going to the motors, how exactly does that work? I am trying to find a way to do exactly that, measure the current drawn by my 2 main drive motors. How/Where do you physically connect up wires to test this, and do I need a custom circuitry board to interpret this voltage measured? Thanks for the help.

Re: current measurement?
 

I have written a short whitepaper on this. You can see it at:
http://www.chiefdelphi.com/forums/pa...le&paperid=155

You basically use an Op-AMP that has a common mode voltage at and/or above the power rail. Then you build a difference amp, and measure across the breaker.
(actually, the side at the top of the breaker has to be done through a separate breaker because of the 1st rules of connection....but the current through that breaker is so low powering the OP-AMP that the drop is essentially zero)

Look at that paper and tell me what you think. Then, ask me any questions and I will answer them. (I can also get you a full schematic and board layout if you want)

-Quentin

confusion!
 

Hey bigqueue, thanks for the help, but being a novice at electronics, I can understand BASIC schematics, but complicated ones overwhelm me. The 2nd schematic, I guess of the custom circuit itself, is a little intimidating. Could you explain what's goin on in plain english, and exactly what parts (ie. resistors, capacitors, input types) i would need to build this circuit. Once again, thanks for your help.

Re: confusion!
 

I'm not sure I understand the question. (sorry) Let me try to take a crack at giving more detail.

1) The circuit pictured is a difference amplifier made with a single OP-AMP. The voltage gain is given by the ratio of R1 and R1 (and R7 and R9 which are the same values 12.1K and 100K respectively) I will not have enough space here to explain exactly how an OP-AMP works, but surfice it to say that this circuit gives you a voltage output that is 8.26 times the difference of the voltage difference between MS1+ and MS1-.

2) Given that this voltage is measured across the circuit breaker, and that we measured the 30A breaker to have a .006 ohm resistance, the output of this circuit when it measures across the 30A breaker is 49.4 millivolts per amphere flowing through the breaker. (it actually measures the voltage across the breaker, and assumes that ohms law actually works) I=V/R.

3) The zener diode at the output is used to clamp the output which is capable ot going up to 12v. (since the circuit is powered off 12v) I added this top protect the A/D inputs......(I'm not sure it is needed, especially given the series resistor and potential clamp diode in the A/D input...but I thought it was better to be safe than sorry)

4) The resistor and capacitor at the output (R51 and C10) are used to filter the output. Don't forget that the motor is controlled via a PWM, so the voltage to the motor is pulsed on and off...andthis will show up in the motor current....and this RC filters all that switching out into a steady DC voltage which is fed to the analog to digital converters of the robot interface.

5) Given that the input of the A/D is 0-5v, and that it is a
n 8 bit converter (256 counts), each count is 19.5 millivolts of input....or about .4Amps per count. (for a total of 101Amps full scale)

Sorry...but as I read this, it looks basically like what I wrote in my paper.
I'm not sure this is any help.

Please give me some more specific questions and I will try to answer them.

Please note: picking the OP-AMP for this applicaiton is important. Its input characteristics must be specific to allow it to operate in this environment. (high common mode inout voltage...that is, both inputs must be able to sense at or above the power supply voltage of the OP-AMP.)

Ask me more and I will reply.

-Quentin
:ahh: