4-cim vs. 6-mini-cim drivetrains

Paul Copioli had some interesting points about how 6 mini-cim drivetrains can have a lot of advantages over more traditional 4 cim drivetrains. What are everyones thoughts on the pros and cons of 6 mini-cim drivetrains.

Another concern for this debate would be whether or not adding CIMS to a drive train increases pushing power, or simply lets you run a bit faster / increase acceleration. In the context of 4CIM vs 6Mini, would the 6Mini be more likely to push the 4CIM in a pushing contest if all else were equal?

As in Cory’s quote the limit is when the wheels spin. You could have a 20 cim drive but if your wheels spin your not pushing any more than any other bot that can spin it’s wheels. (assuming equivalent COF and weight)

so 179’s tacodile 2016 (off season robot) had a 6 mini cim while the original had 4 cim’s with the same reduction (bout 12-13 fps with the 4cim single speed, i don’t remember exactly so sue me) . it obviously was faster however the motors where warm after some run time (bout 3-4 min of constant running) i don’t know if this was due to the 10 wheel drive and its turning, but either way couldn’t push that well.

1 Like

It would be interesting to see the results of a controlled real world performance comparison. Two identical kit bots. One with 4 CIM & the other with 6 miniCIM. Both with mainstream single speed transmissions & same gear ratio. Test acceleration, top speed (field length runway), efficiency, etc. Maybe even have some drivers do blind A-B comparisons.

That link is either broken or somethings wrong

you do not have permission to access this page

Same message is popping up for me. What is this trying to communicate?

A Paul Copioli post about 6 mini cims

The bottom line from this post appears to be that spreading the heat among three miniCIMS produces much better endurance than two CIMs for such high-heat cases as long practice sessions, heavy defense, and finals matches.

[My observation]: As the best route for heat out of the rotor of a sealed brushed motor appears to be the output shaft, having three shafts rather than two would tend to improve the dissipation. It seems that a back shaft would be even better - you could put a heat sinking fan right on the shaft!

Another (perhaps more significant) factor that comes into play is the heat generated in the first place. MiniCIMs are designed with a bearing on the output shaft in place of a bushing in the CIM’s design. The bearing has less friction than the bushing and therefore generates less heat.

Heat Accumulated = Heat Generated - Heat Dissipated

The peak power tests performed by VexPro tell the story:


Mostly due to a disproportionate amount of electrical energy going to heat generation by friction, a higher amount of mechanical power loss over time is observed in the CIM motor as compared to the MiniCIM.

1 Like

I wish I could have watched the FUN stream live, as I wanted to clarify a few things from memory. Hopefully Paul can weigh in here.

MiniCIMs, when compared to CIMs

  • Draw less power overall due to lower motor specs
  • Have less inertial mass, therefore pulls less current during acceleration
  • Have less inertial mass, therefore respond faster to rapid changes in acceleration/deceleration. This has two effects: 1. Better maneuverability; 2. Less acute torsional stress on the gearbox shafts when the robot slams into something.

Paul’s (convincing) argument was that since a robot that plays a game effectively spends most of its time accelerating, a 6 CIM drive train tapers off towards the end of a match whereas a 6 MiniCIM drive train will not. The caveat I noticed, and what I’d like to get more data on, is that his robot had a bunch of ‘silly wheels’. Therefore the power draw during turns for his bot was reduced to a tiny fraction of skid steer traction, which may have also led to more power at the end of the match. Yet the concepts make enough sense that we’re thinking about a 4 or 6 MiniCIM drive for the upcoming season, with final speeds dependent upon the game.

But, did you look at the locked rotor tests? At 6V, using the first and last “good” point for each:

           CIM           mini-CIM
Time   Amps  Torque    Amps  Torque
  ~0s  60.25  1.288    50.86  1.028
~300s  38.07  0.807    28.09  0.719

I selected the 6V curve because the electrical heat generation should be similar to the peak power case, but with no mechanical energy converted to heat. Over three minutes, the CIM current and torque both fall about 37%. Curiously, the mini-CIM’s current falls 44% but its torque only 30%. I’m not sure what to make of that, but it isn’t due to heat differences from friction in the bearing vs bushing. Strange as it sounds, it appears as if the mini-CIM’s torque constant increases as it heats up - it becomes a more effective torque generator.

People keep saying this. Would you mind explaining how it tells to story for those of us looking to understand DC motor behavior better? For instance, to me it looks like for the time period from 0s - 140s the CIM would always be providing more power than than the MiniCIM.

I’m assuming that this test was done with enough load to get peak power from the motors, and the frequent near-stall conditions of match would change the situation a bit but I’m not sure if that is reflected in that data.

Be careful what you say here on chief, now we’re going to have teams taking heat guns to their 6 minicim drives next year in order to get a leg up.

1 Like

Heat guns? Why not just lock the wheels and give the motors 6V for a minute or so?:]

It’s important to remember that these charts are per motor. So, the comparison is really between 4 CIMs and 6 miniCIMs. The attached chart factors that in and compares the two, and I think helps illustrate the point.

Comparison.xlsx (69.9 KB)

According to the VEX motor data, the power-to-weight ratio of the CIM drops below that of the MiniCIM at just fifteen seconds at peak power (where both are at roughly 100W/lb.)

“just” 15 seconds is 11% of a match. At full power, that’s enough time to sprint the length of the field three times. Advantage CIMs.

As others have said, I’m not so concerned with “peak power over time” as much as “efficiency over time”. Our robots have been power hungry the last few years (4 CIM’s and 10-11 775pro’s). I’ll take the slightly lower “initial peak power” to have an overall more efficient robot that loses less energy via heat.

-Mike

Well, also people seem to be comparing a single miniCIM to a single CIM.

The appropriate comparison (as you know) is 4 CIMs versus 6 miniCIMs combined. Colin’s post above shows that the peak power of 6 Minis stays above 4 CIMs for the entire duration of the dyno test.

The difference is the motor count. The CIM has better efficiency than the miniCIM over essentially all of the power curve when they’re driven using the same current or torque. Even when asked to run at the full power of each, the CIM is still ahead, at 42% vs 39% according to the power curves. I am confident that running six CIMs with current limiting (perhaps 42A per motor) would be even better on all fronts but weight, as it is ~56% efficient in that range. It should both out power and out last 6 miniCIMs.

Also, I just noticed something else weird in the test data. The peak power of the mini-CIM in the up/down test is 215W at 45.3A and 39% efficiency, but in the peak power test, it starts out at 232W at 42.0A which is 46% efficient. By comparison, the CIM results are consistent: 337W at 66.9A, 42% vs 332W at 66.6A, 42%.

Also, there was a comment back a ways about the lower inertia of the miniCIM’s rotor, and how that made it more effective when accelerating the robot. When compared to the inerta of the robot, it doesn’t make much difference, especially if you have three miniCIM rotors vs two CIM rotors.