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#1
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Re: Did I read <R32> Correctly? Six CIMs?
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#2
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Re: Did I read <R32> Correctly? Six CIMs?
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#3
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Re: Did I read <R32> Correctly? Six CIMs?
how much weight can a CIM pull? just a quick question..
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#5
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Re: Did I read <R32> Correctly? Six CIMs?
This is dependent on a number of factors, including but not limited to gear ratios, lever length, and the method of lift.
- Sunny G. |
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#6
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Re: Did I read <R32> Correctly? Six CIMs?
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Adding extra motors can be a benefit if you need more torque than was available with the previous CIM limit, but as Al mentioned you might not have enough power available to power that many CIMs anyway. Additional CIMs don't change how much power you can draw from the battery at one time. |
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#7
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Re: Did I read <R32> Correctly? Six CIMs?
Also on the subject of reading it correctly, are Fisher Price motors in the doghouse this year?
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#8
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Re: Did I read <R32> Correctly? Six CIMs?
I didnt read most of the posts but is there a limit of how many motors or motor controllers a bot can have this year.
Thanks team 3633 |
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#9
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Re: Did I read <R32> Correctly? Six CIMs?
Goodness, YES. Please get a copy of the rules. It explains a lot, and if your robot does NOT follow the rules, you will not be allowed to compete. Really, it is very important.
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#10
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Re: Did I read <R32> Correctly? Six CIMs?
CIM motors stall at at 133 amps, PROVIDED that the battery can provide 133 amps. We discovered in 2010, when we had two CIMs and one FP in each transmission, that under load, the battery sags way down, and cannot provide more than about 95 amps. With a fully charged, brand new battery, we could, on very rare occasions, blow the breaker, but once a battery had a few charge/discharge cycles, it wouldn't provide enough current to blow the breaker, even with all drive motors stalled. We actually got more torque by removing the FP from the drivetrain because the battery didn't sag as much.
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#11
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Re: Did I read <R32> Correctly? Six CIMs?
I have an earnest desire to estimate how much our battery's voltage will drop as a result of a given amount of demand by the motors. How does one do that?
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#12
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#13
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Re: Did I read <R32> Correctly? Six CIMs?
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The current of DC motors is approximatley linearly related to the torque applied to the shaft, and is usually assumed to of the form I=k*T. where I the the current and T is the applied torque and k is the "motor constant" (that's what we call it in hebrew, I imagine the name is similar in english). so no matter how you split the torque between any number of motors, the current draw will be roughly the same (if you neglect friction, which isn't always a wise thing to do). If i'm not mistaken, the motor's speed is also approximately linearly related to the torque, but it reduces, for example: w=3000-a*T where w is the rotational velocity. so just like Trent said, reducing the torque on each motor would increase their rotation speed. -Leav |
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#14
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Re: Did I read <R32> Correctly? Six CIMs?
Nemo,
Here are a few items for your calculations. The internal impedance of the battery is 0.011 ohms fully charged. The #6 wire is 0.0005 ohms per foot, #10 is 0.001 ohms per foot. I am trying to remember, I think Jaguars are 0.004 ohms, Victors are 0.006 ohms. So all you need to do is add up the losses and solve for Ohm's Law. Al's Rule of the Wire Foot (WF) states 100 amps in one foot of #10 or two feet of #6 equals 0.1 volt drop per foot. Starting six CIM motors would max out the current handling of the battery at 600 amps. With a typical FRC robot the wire/foot losses would be 11 (battery resistance) + 2 (four feet of #6)= 13 WF. 13 * 6 (for max battery current) would result in 7.8 volts of drop in the path to the PD. Since the power supply for the Crio drops out at 4.5 volts, this would produce a sufficient drop to reboot the Crio each time the robot started. Predictably, the 120 amp main breaker may or may not trip at this initial start but the temperature in the breaker is certain to rise at that demand. Now all things being equal, the losses in the remainder of the wiring feeding the speed controllers and the motors would prevent maximum stall currents to be reached. However, it is still likely that six CIM motors could under certain circumstance max out the current ability of the battery and main breaker. Teams did report main breaker trips last year with additional motors. My recommendation is to monitor your currents and adjust your design appropriately. |
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#15
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Re: Did I read <R32> Correctly? Six CIMs?
Al,
That's very helpful - thanks very much for posting. |
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