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Unread 04-06-2018, 03:29 PM
Anxjos Anxjos is offline
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Question Number of motors used for drive train

Our team usually uses a total of 4 cim motors for drivetrain and with 2 toolbox to use as tank drive.At the competition we saw some teams that use 3 CIMs for just one side or toolbox/side. Why would they do so? What are the benefits?
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Unread 04-06-2018, 03:32 PM
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Re: Number of motors used for drive train

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Originally Posted by Anxjos View Post
Our team usually uses a total of 4 cim motors for drivetrain and with 2 toolbox to use as tank drive.At the competition we saw some teams that use 3 CIMs for just one side or toolbox/side. Why would they do so? What are the benefits?
Higher pushing power and accelleration, also if you want a 2 speed gearbox generally youll want the extra push in each mode. If you have the weight to spare its normally one of the easier ways to improve your robot quickly.

Other power drivetrain configs are 4 mecanum wheels with 2 cims each for a total of 8 in the drivetrain or an H drive with omnis, 3 on each side and 1 or 2 for the center wheel for a 7 or 8 total.

Last edited by darajaco : 04-06-2018 at 03:34 PM. Reason: typo
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Unread 04-06-2018, 04:53 PM
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Re: Number of motors used for drive train

To amplify: you can get more power for a given amount of current, and both reduce and spread the waste heat around when you distribute it among more motors. For example, assume you have a 120A budget for your drivetrain for a given maneuver.

If you have four CIMs, each one can draw 30A. Checking the Vex power curve, a CIM drawing 30A can deliver about 225W of mechanical power, so four gives you about 900W.

If you have six CIMs, each one can draw 20A. This time, each one can deliver about 157W, for a total of 942W. What's more, each CIM is now sucking up 83W of heat as opposed to 136W in the four-CIM case, meaning they can do this at least 64% longer before overheating.

The benefits go up as you have a higher current budget. The cost is that it becomes easier to brown out the RIO or trip the main breaker.
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Unread 04-06-2018, 04:54 PM
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Re: Number of motors used for drive train

Most robots use either 4 or 6 CIMs for driving. 4 CIMs weight 5lbs less than 6 CIMs, and you don't have to deal with as much voltage sag from the battery. On the other hand, 6 CIMs gives you more power, meaning you can have a faster to speed, more pushing power, and/or higher acceleration.

Until this season, I didn't think a 2 CIM drivetrain was feasible until I saw 5291 ride one to 7th place in the district and a ticket to Houston. The bottom line is that with whatever number/type of motors you choose, you need to do the calculations to make sure that you won't draw too much current and still have enough pushing power.
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Unread 04-06-2018, 05:37 PM
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Re: Number of motors used for drive train

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Originally Posted by AriMB View Post
Most robots use either 4 or 6 CIMs for driving. 4 CIMs weight 5lbs less than 6 CIMs, and you don't have to deal with as much voltage sag from the battery. On the other hand, 6 CIMs gives you more power, meaning you can have a faster to speed, more pushing power, and/or higher acceleration.

Until this season, I didn't think a 2 CIM drivetrain was feasible until I saw 5291 ride one to 7th place in the district and a ticket to Houston. The bottom line is that with whatever number/type of motors you choose, you need to do the calculations to make sure that you won't draw too much current and still have enough pushing power.
4513 has as many event wins as they have CIMs on their drivetrain this year.

That being said, I don't think I'll ever recommend anything less than 4 CIMs for a typically sized FRC bot.
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Unread 04-06-2018, 07:35 PM
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Re: Number of motors used for drive train

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Originally Posted by Cothron Theiss View Post
That being said, I don't think I'll ever recommend anything less than 4 CIMs for a typically sized FRC bot.
This year and 2015 are the two years where 4 MiniCIMs are completely acceptable. Of course the strategy, and strategic design choices, would have to shift a bit with 4 Minis.
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Unread 04-06-2018, 09:37 PM
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Re: Number of motors used for drive train

Don’t discount the 6 Mini-CIM option!
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Unread 04-06-2018, 11:28 PM
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Re: Number of motors used for drive train

Or 8 775pros with voltage limiting. Havenít had a single motor failure all season, and the extra weight and space saved is a huge plus.
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Unread 04-07-2018, 06:57 AM
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Re: Number of motors used for drive train

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Originally Posted by granjef3 View Post
Or 8 775pros with voltage limiting.


Do you limit it to a fixed value, or do you change the limit based on motor speed ?


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Unread 04-07-2018, 07:48 AM
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Re: Number of motors used for drive train

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Originally Posted by Ether View Post


Do you limit it to a fixed value, or do you change the limit based on motor speed ?


I’m afraid its quite simple. Every method more complex then a simple 80% max output cutoff has not been successful for us!

We use the talonSRX output limit function, which limits max voltage to 80% of robot voltage. Nothing else used, no motor speed adjustments or current limiting. Our gearboxes are geared for 17fps free speed, so with this limit it ends up being around 14fps. Motors end a match warm to touch.

I’ve considered setting up the limit to raise after some time spent accelerating, but with our current gear ratio you don’t get much more usable top speed on an FRC field.

We’ve tried the current limiting feature as well, but as each side operates independently you get fun behavior if one side limits before the other (violent sudden turns)
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Unread 04-07-2018, 08:06 AM
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Re: Number of motors used for drive train

Drive train power really depends game and what you want to do in that game. I would state that the gold standard for any FRC team is the 6-wheel (centered dropped) with two cims per side. That should be what you measure all other drive trains from. Min/maxing with 8 775's or 6 cims can be a loosing proposition for many teams because it requires some current control that might be beyond those teams. Don't get caught up with the new hotness because it, well...new. The drive train should be something you forget about during your season because it should always work. Use that time and energy in perfecting your game manipulators and game specific programming.
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Unread 04-07-2018, 08:10 AM
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Re: Number of motors used for drive train

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Originally Posted by MrT8 View Post
Donít discount the 6 Mini-CIM option!
It's been a slow process for us, we use alot of caution. Our standard for the longest time was Victor 884/888 Speed controllers on the drive motors with PWM signaling. Mainly because of CAN-BUS failure, we wanted to still be able to drive. We hardly EVER noticed a brown out with 6-CIM motors powered by Victors. This year, we finally went all CAN-BUS and using TalonSRX speed controllers. In my opinion, right out of the box, the TalonSRX are not setup the same way or something as tried and true Victor 884/888, so they tend to pull more current/voltage and brown out the RIO with a same setup gear box with 6-CIM motors. You have to get your software kids involved to make a 6-CIM and TalonSRX "work".

So I agree, 6-miniCIM motors with TalonSRX seems to be pretty nice setup with nothing laboring in the software side. We are slow adopters into the 775 PRO or Red Line motor world, we've been watching other teams and seeing what the results are. It seems on the field, we can push a robot with a 775 PRO drive train pretty easy, which is not desirable in my book depending on the game. Most of these 775 PRO drive trains I've seen so far are single speed setups, I haven't seen any two speed ones yet to know or seen on the field or tried to push with our robot let's say around. This summer we will probably built a mule robot with a 775 PRO drive train and see what we get. But for TalonSRX and 6-miniCIM +1 for us. That's the "Paul Copioli" setup bty. That's always the best tact, built what you want to try on the field in the off season and learn from the experience for sure. 775 Pro / Red Line teams, share some defense stories, we would love to learn about it.
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Unread 04-07-2018, 08:28 AM
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Re: Number of motors used for drive train

Quote:
Originally Posted by Chris_Elston View Post
It's been a slow process for us, we use alot of caution. Our standard for the longest time was Victor 884/888 Speed controllers on the drive motors with PWM signaling. Mainly because of CAN-BUS failure, we wanted to still be able to drive. We hardly EVER noticed a brown out with 6-CIM motors powered by Victors. This year, we finally went all CAN-BUS and using TalonSRX speed controllers. In my opinion, right out of the box, the TalonSRX are not setup the same way or something as tried and true Victor 884/888, so they tend to pull more current/voltage and brown out the RIO with a same setup gear box with 6-CIM motors. You have to get your software kids involved to make a 6-CIM and TalonSRX "work".

So I agree, 6-miniCIM motors with TalonSRX seems to be pretty nice setup with nothing laboring in the software side. We are slow adopters into the 775 PRO or Red Line motor world, we've been watching other teams and seeing what the results are. It seems on the field, we can push a robot with a 775 PRO drive train pretty easy, which is not desirable in my book depending on the game. Most of these 775 PRO drive trains I've seen so far are single speed setups, I haven't seen any two speed ones yet to know or seen on the field or tried to push with our robot let's say around. This summer we will probably built a mule robot with a 775 PRO drive train and see what we get. But for TalonSRX and 6-miniCIM +1 for us. That's the "Paul Copioli" setup bty. That's always the best tact, built what you want to try on the field in the off season and learn from the experience for sure. 775 Pro / Red Line teams, share some defense stories, we would love to learn about it.
We have been thrilled with our 6 mini drive. It has been fast and perfectly reliable. I don't think we will change.
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Unread 04-07-2018, 09:18 AM
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Re: Number of motors used for drive train

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Originally Posted by Chris_Elston View Post
TalonSRX are not setup the same way or something as tried and true Victor 884/888, so they tend to pull more current/voltage and brown out the RIO with a same setup gear box with 6-CIM motors.
@anyone: what could possibly be the cause of the above?

Perhaps because the SRX has lower resistance than the 884/888?


Quote:
we've been watching other teams and seeing what the results are. It seems on the field, we can push a robot with a 775 PRO drive train pretty easy
Perhaps because voltage-limiting the 775Pro substantially limits their torque and power.


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Unread 04-07-2018, 05:08 PM
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Re: Number of motors used for drive train

Cyber Blue is doing a presentation at both CHP forums on our motor testing from last fall. Testing included CIM, mini-CIM and 775ís in different combinations.

We will also include our decision making process for our 2018 drive train and our results with that decision.

For info - based on our testing, and the VEX motor test data, we selected 6 mini-CIM.
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