How many CIMs in your drivetrain?

[CryptoStorm]

I started as a FIRST student in 2004. to my knowledge we've always used 2 CIMS per side on the standard "tank drive" type setup. We used mecanum wheels once which required independent drive of 1 CIM per wheel.

Does anybody have any (scientific) input on using only 1 CIM per side? (2 total)

how about 6 CIM drive?

[pfreivald]

Originally Posted by CryptoStorm

I started as a FIRST student in 2004. to my knowledge we've always used 2 CIMS per side on the standard "tank drive" type setup. We used mecanum wheels once which required independent drive of 1 CIM per wheel.

Does anybody have any (scientific) input on using only 1 CIM per side? (2 total)

how about 6 CIM drive?

We prototyped a 6 CIM drive during the offseason using VexPro's dog shifting dual-speed gearboxes, and it works fantastically.

We also did octocanum for about five years, and could beast through just about everyone...except for 6 CIM drivetrains, despite being geared down about as preposterously low as you could get, and with great traction.

I would strongly recommend not going to one per side--you'll lose every pushing match you get into with a four-CIM drivetrain, and each of those CIMs are going to draw more current, work harder, get hotter, and drain your battery more than four or six. The only advantage is weight, and it's not a good enough tradeoff.

[MaGiC_PiKaChU]

has anyone ever done a drivetrain with more than 6 motors? (not including steering for swerve)

[Ether]

Here's a quick calculation using MCALC to compare

one CIM drawing 60 amps at 12 volts
vs
two CIMs each drawing 30 amps at 12 volts:

Motor Calculator  build MCALC_2014d 2/3/2014 1255pm


Select motor:
1)CIM     6)FP0673   d)FP2719  f)FP9012     g)FP9015    h)FP9013 j)MiniCIM
a)am-0912 b)am-0914  7)DensoL  8)DensoR     c)Denso0160 k)BAG    u)UserDefined
2)RS395   3)RS540    4)RS550   e)RS775-12   5)RS775-18  m)RS555  n)am-0915
r)am-2193 s)am-2235  t)RS390   v)RS545      p)am-2161&2194
w)VEX2177hi  z)VEX2177std  1


CIM FR801 001, AM802 001A @ 12.00 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    5310   100.0     2.7         0.0        32.4     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   343.4   2.425       0     0.0   133.0         0.0      1596.0     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   171.7   1.212    2655    50.0    67.8       337.1       477.1    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    42.8   0.302    4648    87.5    18.9       147.1        80.2    64.7

Select input:
1)oz-in  7)watts_in    3)rpm    5)amps   b)rpm&amps  8)eff%
2)Nm     6)watts_out   4)rpm%   9)volts  a)rpm&ozin  m)main menu   x)exit 5


enter amps: 60

CIM FR801 001, AM802 001A @ 12.00 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   151.0   1.066    2975    56.0    60.0       332.2      387.8    46.1


Select motor:
1)CIM     6)FP0673   d)FP2719  f)FP9012     g)FP9015    h)FP9013 j)MiniCIM
a)am-0912 b)am-0914  7)DensoL  8)DensoR     c)Denso0160 k)BAG    u)UserDefined
2)RS395   3)RS540    4)RS550   e)RS775-12   5)RS775-18  m)RS555  n)am-0915
r)am-2193 s)am-2235  t)RS390   v)RS545      p)am-2161&2194
w)VEX2177hi  z)VEX2177std  u


Enter the following motor specs in the order and units indicated,
separated by spaces or tabs:
volts   stall_ozin      free_rpm        stall_amps      free_amps
12       686.8           5310             266            5.4

UserDefined-> 12.0 686.8  5310 266.0  5.4 @ 12.00 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    5310   100.0     5.4         0.0        64.8     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   686.8   4.850       0     0.0   266.0         0.0      3192.0     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   343.4   2.425    2655    50.0   135.7       674.2       954.2    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    85.6   0.605    4648    87.5    37.9       294.3       160.4    64.7

Select input:
1)oz-in  7)watts_in    3)rpm    5)amps   b)rpm&amps  8)eff%
2)Nm     6)watts_out   4)rpm%   9)volts  a)rpm&ozin  m)main menu   x)exit 5


enter amps: 60

UserDefined-> 12.0 686.8  5310 266.0  5.4 @ 12.00 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   143.9   1.016    4197    79.0    60.0       446.6       273.4    62.0

one CIM drawing 60 amps at 12 volts:
332 watts output mechanical power
388 watts waste heat

two CIMs each drawing 30 amps at 12 volts:
447 watts output mechanical power
273 watts waste heat

[GeeTwo]

We did a 2 CIM drivetrain last year, but Recycle Rush was definitely a special case; you didn't want to accelerate quickly or drive very fast. We're starting with 4 this year, and if we change it, we'll be going to six.

Originally Posted by MaGiC_PiKaChU

has anyone ever done a drivetrain with more than 6 motors? (not including steering for swerve)

I've read about people considering 4 CIM + 4 min-CIM designs for Mecanum and 4-wheel Killough, but was unable to find any examples that had been built and played.

[dellagd]

Originally Posted by GeeTwo

I've read about people considering 4 CIM + 4 min-CIM designs for Mecanum and 4-wheel Killough, but was unable to find any examples that had been built and played.

Here you go

That year we ran 4 CIMs + 4 Mini-CIMs in our shifting 8in Mecanum Drivetrain. Looking back, mecanum probably wasn't the best choice that year with the heavy defense (), but we sure tried our hardest to make it work. For the most part it did, and we were able to handle many defensive situations with the great traction of the extremely wide wheels, but the cons far outweighed the benefit.

With 8 CIM-motors, we had the potential to run 320 Amps of current into our drivetrain before any of the individual 40A breakers would trip. Even operating well below that, there was incredible risk for popping the main breaker, not to mention browning out components, both of which happened in matches on multiple occasions. In high gear, any defensive situation was almost always a brown-out and radio reboot on the field. In low gear, we had maybe 10 seconds before a brown-out. With the current sensing capabilities of the new PDP, careful software could allow a team to more safely run this kind of setup, but back then, it was far more risk than the team could effectively manage or I would ever recommend.

[CryptoStorm]

We talked about using 6 cims for a all terrain off season robot. I've got a couple 36v motors I'm going to donate though.

The team is considering 2 CIM drive this year. I'm not recommending that, especially with tank treads. I think they will change their minds when they see how poorly it performs after the chassis is built. Hopefully we can drive it this weekend.

Maybe I'll be proven wrong though..

[CryptoStorm]

Originally Posted by Ether

Here's a quick calculation using MCALC to compare

one CIM drawing 60 amps at 12 volts
vs
two CIMs each drawing 30 amps at 12 volts:

Motor Calculator  build MCALC_2014d 2/3/2014 1255pm


Select motor:
1)CIM     6)FP0673   d)FP2719  f)FP9012     g)FP9015    h)FP9013 j)MiniCIM
a)am-0912 b)am-0914  7)DensoL  8)DensoR     c)Denso0160 k)BAG    u)UserDefined
2)RS395   3)RS540    4)RS550   e)RS775-12   5)RS775-18  m)RS555  n)am-0915
r)am-2193 s)am-2235  t)RS390   v)RS545      p)am-2161&2194
w)VEX2177hi  z)VEX2177std  1


CIM FR801 001, AM802 001A @ 12.00 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    5310   100.0     2.7         0.0        32.4     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   343.4   2.425       0     0.0   133.0         0.0      1596.0     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   171.7   1.212    2655    50.0    67.8       337.1       477.1    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    42.8   0.302    4648    87.5    18.9       147.1        80.2    64.7

Select input:
1)oz-in  7)watts_in    3)rpm    5)amps   b)rpm&amps  8)eff%
2)Nm     6)watts_out   4)rpm%   9)volts  a)rpm&ozin  m)main menu   x)exit 5


enter amps: 60

CIM FR801 001, AM802 001A @ 12.00 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   151.0   1.066    2975    56.0    60.0       332.2      387.8    46.1


Select motor:
1)CIM     6)FP0673   d)FP2719  f)FP9012     g)FP9015    h)FP9013 j)MiniCIM
a)am-0912 b)am-0914  7)DensoL  8)DensoR     c)Denso0160 k)BAG    u)UserDefined
2)RS395   3)RS540    4)RS550   e)RS775-12   5)RS775-18  m)RS555  n)am-0915
r)am-2193 s)am-2235  t)RS390   v)RS545      p)am-2161&2194
w)VEX2177hi  z)VEX2177std  u


Enter the following motor specs in the order and units indicated,
separated by spaces or tabs:
volts   stall_ozin      free_rpm        stall_amps      free_amps
12       686.8           5310             266            5.4

UserDefined-> 12.0 686.8  5310 266.0  5.4 @ 12.00 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    5310   100.0     5.4         0.0        64.8     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   686.8   4.850       0     0.0   266.0         0.0      3192.0     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   343.4   2.425    2655    50.0   135.7       674.2       954.2    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    85.6   0.605    4648    87.5    37.9       294.3       160.4    64.7

Select input:
1)oz-in  7)watts_in    3)rpm    5)amps   b)rpm&amps  8)eff%
2)Nm     6)watts_out   4)rpm%   9)volts  a)rpm&ozin  m)main menu   x)exit 5


enter amps: 60

UserDefined-> 12.0 686.8  5310 266.0  5.4 @ 12.00 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   143.9   1.016    4197    79.0    60.0       446.6       273.4    62.0

one CIM drawing 60 amps at 12 volts:
332 watts output mechanical power
388 watts waste heat

two CIMs each drawing 30 amps at 12 volts:
447 watts output mechanical power
273 watts waste heat

Wouldnt a 1/4 hp motor and a 1/2 hp motor consume the same amount of power if the continuous load only required 1/8 hp. Amperage being based on load.

I know that 2 cims will be capable of more torque, I'm just wondering how noticeable it will be from the perspective of the driver.

[Ether]

Originally Posted by CryptoStorm

Wouldnt a 1/4 hp motor and a 1/2 hp motor consume the same amount of power

Take a careful look at what I posted:

one CIM drawing 60 amps at 12 volts:
332 watts output mechanical power
388 watts waste heat

two CIMs each drawing 30 amps at 12 volts:
447 watts output mechanical power
273 watts waste heat

332+388 = 720 total watts consumed (one CIM)

447+273 = 720 total watts consumed (two CIMs)

They both consume the same power @ 12 volts @ 60 amps, but you get more output mechanical power and less waste heat with two CIMs.

So you don't need as many amps with the 2 CIMs to get the same output power as 1 CIM.

[Sperkowsky]

We had 2 added 2 more today. Only problem is we were only able to get one screw in each Cim. We will try more probably making a tool to get in there but, we sadly have run a drive train an entire season using 1 screw per Cim and can verify it works.

[CryptoStorm]

You're specifying X amount CIMs at 60 amps. It is manufacturing a theoretical load that will require 60 amps.

The load is different.

What happens if you specify a load?

[D_Price]

We are going to be running either a 6 CIM drive or a 4 CIM drive depending on our lift mechanism. At this point in the build we are using the 6. 3 on each side.

[ratdude747]

General rule of thumb for motors (at least what I was taught in college): A motor is most efficient when run exactly at the rated power output. More or less will result in a drop in efficiency. Obviously the latter is a better scenario than the latter, but in terms of % efficiency, all else being equal, going overkill isn't an improvement.

That said, it's far easier for a 2 cim drive to get pushed past the rated output, and given that batteries are usually recharged between matches, the time when efficiency (for realistic practical FRC purposes) counts is under full load (read: shoving match), so that's why a 2 cim drive will overheat and (probably) draw more amps in such a scenario.

The reason 6 cim drives lead to brownouts is not a motor issue per se; it's simply that such a drivetrain has more capability than a single battery can provide (at a voltage that the control electronics/VRMs can handle). So, realistically, in a FRC legal robot, you'll never see the full potential of a 6 cim drivetrain. You may see more than a 4 cim, sure, but you'll probably never get to full load simply because the battery can only give so much power (internal resistance is the limiting factor assuming good battery wiring). Hence the need for brownout prevention for such drivetrains.

[Jared Russell]

Originally Posted by ratdude747

A motor is most efficient when run exactly at the rated power output.

Maximum power and maximum efficiency occur at two separate points on the motor curve for brushed DC motors.

Maximum power occurs when P(out)=radial velocity * torque is maximized, which is at half free speed and half stall torque.

Maximum efficiency occurs when P(out) / P(in), or (radial velocity * torque) / (current * voltage) is maximized. This always occurs somewhere between the maximum power point and free speed (it varies from motor to motor depending on free current).

[Ether]

Originally Posted by ratdude747

General rule of thumb for motors (at least what I was taught in college): A motor is most efficient when run exactly at the rated power output.

The max power (rated power) of the DC brushed motors used in FRC occurs at roughly half the free speed.

The max efficiency occurs at a higher speed.

EDIT: Jared beat me to it.