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Unread 06-01-2013, 14:30
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Re: Did I read <R32> Correctly? Six CIMs?

Quote:
Originally Posted by aldaeron View Post
2 questions related to the number of motors allowed

1) Do you think the rules would be changed to allow a large capacitor across the inputs to the Power Distribution Board to regulate voltage levels? Look at Figure 4-7 in the rules - the capacitor would be hooked across the wires where the text "Power Distribution Board" appears

2) Do you think in the future that they will allow two 12V batteries or larger 12V batteries (automotive instead of motorcycle) to allow for more powered motors?

Cheers!

-matto-
Not likely on both cases. 1 12V 18AH battery has been plenty for all FRC applications when designed properly. You don't need more than one.
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Unread 07-01-2013, 08:39
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Re: Did I read <R32> Correctly? Six CIMs?

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Originally Posted by aldaeron View Post
1) Do you think the rules would be changed to allow a large capacitor across the inputs to the Power Distribution Board to regulate voltage levels? Look at Figure 4-7 in the rules - the capacitor would be hooked across the wires where the text "Power Distribution Board" appears
No currently made capacitor is at all near large enough (in terms of energy storage) to do this.
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Unread 06-01-2013, 14:39
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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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Unread 06-01-2013, 14:41
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Re: Did I read <R32> Correctly? Six CIMs?

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Originally Posted by AhAhPatel View Post
how much weight can a CIM pull? just a quick question..
A better question is how much torque or how much power can it generate. See here.
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Unread 06-01-2013, 14:41
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Re: Did I read <R32> Correctly? Six CIMs?

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Originally Posted by AhAhPatel View Post
how much weight can a CIM pull? just a quick question..
This is dependent on a number of factors, including but not limited to gear ratios, lever length, and the method of lift.

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Unread 06-01-2013, 18:11
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Re: Did I read <R32> Correctly? Six CIMs?

Quote:
Originally Posted by Trent B View Post
Not to mention Banebots puts a max torque spec of 85 lb-ft or 115.2 N-m of torque on their P80 gearboxes.

As for the 2 vs 3 CIM Motors, correct me if I am wrong, but based off of this http://content.vexrobotics.com/docs/...otor-specs.pdf

2 Motors at 300oz-in each will draw about 115 amps a piece. 230 amps total.
3 Motors at 200oz-in each will draw about 75 amps a piece for 225 amps total.

Because the curves are linear, total current draw shouldn't matter on number of motors. However, the motors will be rotating faster on the 3 motor setup at ~2200rpm compared to around ~700rpm because each motor has to put out less torque.
Although this specific example is not advised (75 amps still blows the breakers!) what you're saying is correct. If you drive the same load at the same speed with 2 or 3 motors, then the total power used will be roughly the same. Each motor individually will draw less power in a 3 motor configuration but you still need the same total power to move the load (power = torque * angular velocity, none of which are motor dependent). There will be a slight difference in total power due to where the motors are running in their efficiency curve but I think it would be negligible for most applications.

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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Unread 07-01-2013, 00:15
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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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Unread 12-01-2013, 11:08
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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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Unread 12-01-2013, 19:25
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Re: Did I read <R32> Correctly? Six CIMs?

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Originally Posted by Herbie_3633 View Post
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
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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Unread 06-01-2013, 13:13
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Re: Did I read <R32> Correctly? Six CIMs?

Quote:
Originally Posted by Al Skierkiewicz View Post
Please read the blue box following the motor list very carefully. Running many high current motors is a risky choice. CIM motors stall at 133 amps.
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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Unread 06-01-2013, 13:07
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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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Unread 06-01-2013, 14:46
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Quote:
Originally Posted by theprgramerdude View Post
The motors will heat up extremely quickly at those current levels, too. I wouldn't be surprised if some team that doesn't think too deeply into current levels runs a dual or even single CIM setup at 115 amps enough to effectively ruin the entire motor (demagnetization, something melting, etc.)
Yeah, someone was just asking the difference between 2 and 3. For the same torque output you will have same current consumption but 3 motors will run at higher rpm.
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Unread 07-01-2013, 01:11
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Re: Did I read <R32> Correctly? Six CIMs?

Quote:
Originally Posted by Trent B View Post
Yeah, someone was just asking the difference between 2 and 3. For the same torque output you will have same current consumption but 3 motors will run at higher rpm.
Just to slightly elaborate on this:

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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Unread 07-01-2013, 07:53
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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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Unread 07-01-2013, 07:59
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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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