Current Sensors

[JesseK]

I think my favorite engineering question of all comes up -- is the data that's being interpreted here used in the correct context or is the whole question negligible? I think that when you connect a 10AWG wire to a 12AWG wire that comes of the CIM, the 1 ampere improvement becomes diminished, if not altogether negligible.

For fun, I have some evidence where 1885 used the proper gauge wire but due to bad connections and/or soldering, a resistance was created that was high enough to burn through the plastic casing that housed the connection. This was discovered in Atlanta '07 coming off of one of our CIM motors. After it was replaced, we saw much improved performance in our speed -- go figure . Would have been interesting to have a current sensor giving us feedback data from this.

http://s3.photobucket.com/albums/y95...t=WireBurn.jpg

/edit - note - the blue plastic housing got hot enough to melt and deform a bit, allowing it to sag to the side it's skewed on. The connectors appear to be difference sizes, however this is not the case. A terrible lesson to learn had this gone unnoticed for a few more matches.

[Richard Wallace]

Originally Posted by JesseK

I think my favorite engineering question of all comes up -- is the data that's being interpreted here used in the correct context or is the whole question negligible? I think that when you connect a 10AWG wire to a 12AWG wire that comes of the CIM, the 1 ampere improvement becomes diminished, if not altogether negligible.

For fun, I have some evidence where 1885 used the proper gauge wire but due to bad connections and/or soldering, a resistance was created that was high enough to burn through the plastic casing that housed the connection. This was discovered in Atlanta '07 coming off of one of our CIM motors. After it was replaced, we saw much improved performance in our speed -- go figure . Would have been interesting to have a current sensor giving us feedback data from this.

http://s3.photobucket.com/albums/y95...t=WireBurn.jpg

/edit - note - the blue plastic housing got hot enough to melt and deform a bit, allowing it to sag to the side it's skewed on. The connectors appear to be difference sizes, however this is not the case. A terrible lesson to learn had this gone unnoticed for a few more matches.

Many crimp contacts intended for use with 10-12 AWG wire are insulated with yellow plastic. The blue-plastic insulated contacts are usually intended for use with 14-16 AWG wire. Forcing 12 (or 10!) AWG wire into a blue-plastic insulated contact usually results in some missing strands, a poor crimp, and a potential hot spot. That appears to be what happened in the case shown by your picture.

[JesseK]

Originally Posted by Richard

Many crimp contacts intended for use with 10-12 AWG wire are insulated with yellow plastic. The blue-plastic insulated contacts are usually intended for use with 14-16 AWG wire. Forcing 12 (or 10!) AWG wire into a blue-plastic insulated contact usually results in some missing strands, a poor crimp, and a potential hot spot. That appears to be what happened in the case shown by your picture.

Definitely good to know, thanks!

[Al Skierkiewicz]

OK,
There is a lot to answer here. (You guys need to give me some time to respond, I leave work about 2 most days and Monday night is robot team night)
Phil, the analysis showed that even the addition of one foot of wire in the design of the layout results in two feet of additional loss. The split of the power wiring to high current and low current loads not only affects the series resistance but also the voltage drop that the other loads see down stream. Placing a critical device down stream from the motor wiring causes all of the currents to add together. So although the #6 is only half the series of resistance of the #10, in some designs in handles a lot of current. Think of it this way, one foot of #10 at 100 amps drops 0.1 volt (100 amps x .001 ohms). The same current flowing in a #6 drops half of that or .05 volts/ft. However, most robot designs have four Chalupas on the floor driving the robot so with just the motors the drop now becomes 0.2 volts/ft. If your team uses the full length (24" each side of the Anderson connector) of the #6 wire supplied then you are dropping 1.6 volts in the wire feeding your robot. (4 ft. x 2 wires x 400 amps x .0005 ohms) Add to that the .011 ohms of internal resistance in the battery and you suddenly get down to the critical 8 volt cutout of the RC. A simple method of looking at things is a term I use called the "wire foot". This term simplifies calculations if one remembers that 100 amps in one foot of #10 is 0.1 volt drop. With that in mind, the battery has 11 wire feet of loss, the #6 is 0.5 wire feet of loss/ft, etc. and it easy to see that even standard loads will quickly degrade the available voltage for motors, compressor and RC.
Due to the analysis we now minimize #6 wire runs, split the load as close to the main breaker as possible and separate and minimze the length of wire that feeds the RC. The RC then always becomes the first load on the fuse panel so that the other load currents don't draw the supply down. In recent years, the RC backup battery was added, but at the time we did the analysis, a drop below 8 volts shut down the RC for several critical seconds. If you run the calculations on just the battery alone, 11WF x 400 amps= 4.4 volts. And with the other motors running the current on a fully charged battery is even higher. Remember, (and this is critical) every motor that is not moving is in stall. So everytime one of your motors starts, it is drawing stall current. If you use tank design for your robot (without omni wheels), the drive motors are at or near stall in every turn.
So you bring up the #12 on the Chalupa, but if you reduce that length, the drop in the motor wire is reduced. Don't forget the other losses to consider, a Victor has some "ON" resistance and amounts to about 6 WF, a breaker may have 1 WF of loss, bad crimps can amount to 3-4 WF, loose screw terminals as much as 10 WF, etc. It is obvious then if care isn't taken in the layout and design, proper crimps, tight hardware, and correct load balance that your are lucky if the motors turn at all. Another way to look at the issue is to examine a motor curve. Add up your losses, perhaps as much as 30 WF, and then slide the motor curve down from 12 volts to 9 volts and see where your designs start to fall apart. Add that to every one of the six drive motors and your hoped for 10 ft./sec drive speed may have fallen to 8 or even six. That difference can mean beating the competition to a ball or climbing the ramp at the end of the match.