Quote:
Originally Posted by newton418
Could you further explain what the Rockwell blocks can actually handle?
As for the resistance in the circuitry, we kept the wire lengths to a minimum, and will definitely be more conscious of it in the future; however, I do not understand how this would fully explain our problem. At 40 amps, a small CIM outputs ~.8 N*m (please correct me if my calculations are wrong), and IF we could run them significantly beyond this, say, near stall current, then even if you take the ~2.2 N*m stall current then the wire resistance would have to be somewhere around 76 WF. Though I am not the most familiar with our electronics system, I am confident it does not have this much resistance. Now, I did make several assumptions when calculating the torque needed to turn in 2nd gear, so in actuality .8 N*m of torque might not have been enough; but, assuming that the difference is negligible, how do you explain the problems we faced?
Also, I have just talked to our electronics/code mentor and he suggested that we bypass the Rockwell block to see if it is (at least in part) the problem. He also suggested we monitor the voltage on both sides of the Rockwell block using a USB DAQ, so when we get those results I will make sure to follow up on this.
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OK,
The Rockwell blocks are simply a piece of copper with a wire clamp at each end embedded in a plastic housing. The rated current is the safe limit when the block has continuous current (measured in days) passing through it when mounted in a closed electrical enclosure. (i.e. no cooling air flowing around it) The safe limit is based on the temperature rise of the block that would cause damage to the wire and block. Since we are only using this block for a few minutes at a time, the heat rise is so low that several hundred amps of current flow will not significantly raise the temperature. However, a high resistance connection (loose, small wire size, improper crimp) will cause significant temperature rise as current flows. I would expect you to find the voltage drop across the Rockwell block to be in the millivolt range.
I did not do an actual calculation but your numbers look like they are in the ballpark. Remember that the actual performance of the motor is skewed down the curves by the series resistance of your system. The motor is tested with a power supply that has no output resistance whereas the battery is .011 ohms. The resistance limits/reduces the available current to the motor and so for a given design point, your speed and torque are reduced. Don't forget to include in your calculations the efficiency of the transmission you are using. As you add up mechanical losses and try to push the motor to gain more speed, the current begins to skyrocket. I think that if you back into the operating point by measuring the wheel speed of your robot in a turn you might be surprised to find that you have moved far to the right on the curve where current and torque is very high, but efficiency has started to drop off. (max power is at 2655RPM but current is at almost 70 amps) Without seeing your robot design and watching it perform it is hard to tell exactly where the problem might be. So a few ideas...check the wheels by hand with the power off and no brake on the Victors. Do both sides move freely and with the same force? Are drive motors on both sides at about the same temperature after a match? Is there a similar amount of wire in each side of your robot? i.e. does one side have a longer length of wire to the block and fuse panel than the other? Do you bring the black wires back to the Rockwell block (as required) or do you instead attach them to the small fuse panel? A picture could tell us a lot.