Number of CIMs on Drivetrain

[viveksridhar]

Hey,

Sorry about the seemingly simplistic question, but would having 4 CIMs on the drivetrain (two per side) impact speed in any way over having 2? We're using the KOP belt drive system on a custom chassis. Our robot is primarily offensive, so we want it to be at maximum speed to be able to get to the feeder station and back to the scoring area as fast as possible, would putting 4 CIMs on the chassis speed it up significantly?

Also, would 5 CIMs total on the robot (4 on chassis, 1 on shooter) bring up any battery issues? I remember someone on our team a couple years ago saying something about how running 4 CIMs at once draws too much power from the battery, so it couldn't be done, but the rule this year seems to be 6, so I'm a bit confused about that.

Thanks,

[jafisher]

The number of CIMs doesn't effect the speed because the motors still turn at the same rate. With only 1 CIM on each side you'll have less torque and pushing power though.

[Walter Deitzler]

Originally Posted by jafisher

The number of CIMs doesn't effect the speed because the motors still turn at the same rate. With only 1 CIM on each side you'll have less torque and pushing power though.

Same amount of torque (in the same way there is the same amount of speed), but there is less power. What will happen is your motors will stall out quicker, due to taking a bigger load (not split between two motors), or you will trip your breaker. Also, you have slower acceleration. We have done it, but with a high torque gearbox, and not much pushing. Video here:http://www.youtube.com/watch?v=gI2htgElMKE

If you get into a pushing match, you are more likely to stall your motors or trip a breaker, thus downing you the entire match.

tl.dr: Yes it can be done, but not advisable.

[theawesome1730]

2 CIMs per side increases resistance to stalling your CIMs in a pushing match. I'm sure there are some battery issues, but those will be encountered well after 2 and a half minutes. This year we are running 6 CIMs and 4 miniCIMs with no battery issues

[Walter Deitzler]

Originally Posted by theawesome1730

2 CIMs per side increases your torque and stopping/pushing power. I'm sure there are some battery issues, but those will be encountered well after 2 and a half minutes. This year we are running 6 CIMs and 4 miniCIMs with no battery issues

Can someone explain how torque is increased? I thought it was determined by the gear ratio?

[theawesome1730]

Originally Posted by LedLover96

Can someone explain how torque is increased? I thought it was determined by the gear ratio?

Sorry for the misinformation. I edited my response to make it accurate. You are correct that gear ratio increases (or decreases) torque. 2 CIMs carry half the load of 1 CIM and therefor are harder to stall giving the impression of more torque

[Billfred]

Originally Posted by LedLover96

Can someone explain how torque is increased? I thought it was determined by the gear ratio?

It can be, yes. Adding motors by itself won't increase speed, but the torque contributed by each individual motor allows you to change the design to achieve higher speeds without blowing every breaker and motor in the system. We robbed our 2011 robot of two CIMs and then used it for driving practice. The motors were dangerously hot quickly.

This is all assuming the motors are turning at the same speed; when you start combining different motors with different speeds, things can get fuzzy. This is what's made the AndyMark AM Planetary so popular; it would take Fisher-Price or 550 or BaneBots motors and get them Close Enough To A CIM that you could more or less ignore the differences in speed.

For reference, last year we ran seven motors on our robot. Four CIMs on drive, two AM-9015s, and a BaneBots 550. Never had a battery issue. This year, it's 8. If you have the capacity (space/weight/electrical/financial) to add them, I'd almost always say do it.

[viveksridhar]

OK, We'll go with four CIMs on the drivetrain then, thanks for your help!

[Bob Steele]

As Chris mentioned above... don't confuse speed with acceleration.
A VW can have the same speed as a Ferrari....
But the Ferrari has more acceleration....

Acceleration requires more force... (Newton's 2nd Law)
More motors will usually give you the opportunity to provide more force and thus acceleration.

[Johnny]

Originally Posted by LedLover96

Can someone explain how torque is increased? I thought it was determined by the gear ratio?

Torque is increased when adding more motors in the same way that it's easier to lift a heavy object with more than one person. When you have 2 motors i.e cims on one transmission, you are doubling your torque. You are decreasing the workload on each motor. Of course there're more complicated ways of explaining it, but that's the simple way. Hope that helps

[Chris is me]

Originally Posted by jafisher

The number of CIMs doesn't effect the speed because the motors still turn at the same rate. With only 1 CIM on each side you'll have less torque and pushing power though.

It doesn't affect top speed, but it definitely affects acceleration.

A 5 CIM robot is well within the bounds of normal. I don't think we've ever built a robot with fewer motors than five.

[TheMadCADer]

Originally Posted by viveksridhar

I remember someone on our team a couple years ago saying something about how running 4 CIMs at once draws too much power from the battery, so it couldn't be done, but the rule this year seems to be 6, so I'm a bit confused about that.

I have no idea where they could have got that idea from. 4 CIMs is pretty much the 'standard' drivetrain in FIRST. This year, you will likely see teams using all 6 of their CIMs, and probably quite a few with 6 motors in their drivetrain (though most teams do 2 CIMs and another motor per side). It does drain a bit more of your battery, but not enough to make a difference in a 2 minute match.