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Unread 04-08-2018, 09:17 AM
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Re: Number of motors used for drive train

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Originally Posted by granjef3 View Post
Iím 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.

Iíve considered setting up the limit to raise after some time spent accelerating, but with our current gear ratio you donít get much more usable top speed on an FRC field.

Weíve 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)
We've used a speed-based limiting technique on CIM motors in 2016 & 2018 with great success at pushing them hard without brownout. There are some caveots we found this year, which will hopefully be better defined by an offseason project...
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Unread 04-23-2018, 07:44 PM
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Re: Number of motors used for drive train

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Originally Posted by GeeTwo View Post
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.
6 CIMs weigh about 16.8 pounds, and deliver 942W of output power.

However, a dozen BAG motors weigh only 8.5 pounds, and deliver 966W. The implications are clear. The era of BAG motor drivetrains begins today. (Alternatively, 22 BaneBots RS550 motors can deliver up to 961W and weigh only 10.6 pounds.)

If anybody is curious, I've found the "ideal" number of motors for a 120A current budget, for all motors that have curves up on the Vex website.

CIM: 6 (942.2 W)
MiniCIM: 7 (826.1 W)
775Pro: 12 (1084.7 W)
BAG: 12 (965.7 W)
AM9015: 7 (595.8 W)
AM NeverRest: 64 (857.3 W)
AM 775: 22 (687.5 W)
BB 775: 6 (891.9 W)
BB 550: 22 (961.0 W)

On a slightly more practical note, you could probably come up with a decent approximation of a function relating gear reduction to weight. Then using this spreadsheet you could provide a current budget, along with output speed, and output power to return the lightest weight option for your needs.

On a less practical note, somebody please design a drivetrain using 64 AndyMark NeverRest motors. Their speed should be around 4595 RPM for that power draw.
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Unread 04-24-2018, 07:55 AM
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Re: Number of motors used for drive train

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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 decided to run an 8x 775pro drivetrain this year. Based on our testing last off-season we were confident that with current and voltage control there wouldn't be any issues. We've ended up running the drive all season with no limiting on anything in teleop and have never had motors or the main breaker get hot in a match. We do rarely play defense but only positional defense which the added acceleration and speed are very helpful for. In autonomous mode we limit speed, acceleration, and jerk to prevent the robot from breaking traction during the paths as much as possible.

2451 ran a shifting 8x 775pro drive last year and 6x shifting this year with great success, I recommend talking to them.
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Unread 04-24-2018, 08:13 AM
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Re: Number of motors used for drive train

It is worth noting that simply looking at the power curves is likely misleading; the interaction of increased motor power with the limitations of the battery and/or a SRX-imposed current-limit are nontrivial and important.

For example, when using Talon SRX current-limiting (which, as noted in threads earlier this year, is an input current limit, not an output current limit), increasing motor power by a factor of n (i.e., by using n times the original number of motors, or equivalent) only increases effective stall torque by a factor of sqrt(n).

It would be nice to do a rigorous investigation of the performance of drives with differing amounts of motor power, and see how well it matches theory.
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Unread 04-24-2018, 08:23 AM
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Re: Number of motors used for drive train

Quote:
Originally Posted by Chris_Elston View Post
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".
How much more current/voltage? Do you have data logs showing the difference? I'm curious to know how much of a difference the motor controller makes. Where do you see the difference? Is it in the spikes, the continuous draws, or always? Is it ~5A per motor difference? ~10A per motor difference?
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Unread 04-24-2018, 08:44 AM
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Re: Number of motors used for drive train

GENERALLY SPEAKING: more motors equates to improved ultimate drive performance and increased efficiency.

There are loads of threads and posts on this topic, the search function is your friend.
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Unread 04-24-2018, 09:16 AM
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Re: Number of motors used for drive train

Engineering is all about tradeoffs.

Quote:
Originally Posted by GeeTwo View Post
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.
There's another issue here as well.
There is more weight as you have 2 more motors, 2 more motor controllers, and more wiring. This will reduce the efficiencies gained above a bit.


Quote:
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.
There are a couple of misconceptions here;
1> more speed --- is false. Top speed is dependent on gear ratio and wheel size, not output power of the motor.
2> more pushing power --- also false, as pushing power is dependent on output torque (not motor power) and traction.
3> Higher acceleration --- True, provided the gear ratio is the same and you do not exceed the tractive forces.

2 CIM drivetrains can out push ... or out accelerate a 6 CIM drivetrain if designed correctly. But there will always be tradeoffs.
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Unread 04-24-2018, 10:28 AM
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Re: Number of motors used for drive train

Quote:
Originally Posted by Chris_Elston View Post
775 Pro / Red Line teams, share some defense stories, we would love to learn about it.
We use 4 775Pros on each side of our drive and our experience has been that we can push just about anyone around. The best example I have to show is our last quals match at NEDCMP here: https://youtu.be/Q6xtpsEeM6g

We have only been using voltage limiting up until this match. Based on the data we gathered during this match, we added current limiting (30 peak and continuous). We came very close to browning out the Rio at the end of this match, which is why the current limits were added. We're considering raising the voltage limits now and just using current limiting, but need to do some experimentation with that.

Logfiles for that match are attached for anyone interested.
Attached Files
File Type: zip 2018_04_14 10_56_54 Sat.zip (71.8 KB, 3 views)
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Unread 04-24-2018, 10:55 AM
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Re: Number of motors used for drive train

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Originally Posted by Chris_Elston View Post
775 Pro / Red Line teams, share some defense stories, we would love to learn about it.
I know from experience we haven't had any issues with pushing people with our 775 Pro drivetrain. We have 8 of them geared 27.9:1 (roughly 15 fps iirc w/ 6" wheels, loaded speed). We tend to avoid pushing matches to prevent the risk of destroying them but we have current limiting installed with Talon SRXs that help improve the safety. Our motors are somewhat warm when they get off the field but barely noticeable unless it's playoffs. We have been able to push quite a few smaller bots sideways this year with no problem. Let me see if I can find some match videos later and edit this to include them of us pushing.
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Unread 04-24-2018, 11:26 AM
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Re: Number of motors used for drive train

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Originally Posted by Daniel_LaFleur View Post
There are a couple of misconceptions here;
1> more speed --- is false. Top speed is dependent on gear ratio and wheel size, not output power of the motor.
2> more pushing power --- also false, as pushing power is dependent on output torque (not motor power) and traction.
3> Higher acceleration --- True, provided the gear ratio is the same and you do not exceed the tractive forces.

2 CIM drivetrains can out push ... or out accelerate a 6 CIM drivetrain if designed correctly. But there will always be tradeoffs.
My point here was that 6 CIMs gives you more power, which translates to more speed*torque, where higher torques can give you better acceleration and more pushing force. Obviously you can gear your 2 CIM drivetrain to be faster than a 6 CIM one, but good luck pushing anything without browning out. You can gear it to be able to push a truck too, but you might not be able to cross the field within the 2.5 minute match. The whole point of the extra CIMs is to be able to increase speed or torque without having to decrease the other (obviously there will be trade offs in other ways). To say those claims are false overlooks the basic advantages of adding power to the drivetrain.
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