4-cim vs. 6-mini-cim drivetrains

2010 was definitely the exception. Play soccer like real soccer players in order to score goals.
And to the comment by Cory on pushing power independent of no. of motors. We quickly figured that out that same year, running just 1 CIM/1 FP-AM planetary gearbox) on each side of our drive.
Cory wont remember, but I asked him during build season about our holonomic drive motors stalling. He suggested an 8 wheel drive instead during week 4 of build season. Until this day, its our best season…ever.

If that’s your concern, limit it to 80A and you’ll get better torque than a miniCIM at 89, with less current draw. Personally, I’m thinking of a current limit in the 50-55A range, but planning to experiment. I used 42A in the case above to maximize the difference between the two motors when run with the miniCIM’s profile in the max power test.

If that bushing were such a problem, it should show up in the free run conditions. The CIM dissipates 32.4W running free at 5330 rpm, and the mini-CIM dissipates 35.8W at 5840. If we assume the friction is viscous (proportional to speed), they’re practically twins. Unless you’re putting significant lateral forces on the shaft for long periods of time, I’m dismissing this as a marketing rather than engineering consideration. So far, all but one of our CIM/mini-CIM motors have had spur pinions driving spur gears on well-constrained axles. That one exception directly drove a belt; that case would likely cause me a bit of concern today.

The peak power data shows what happens in a specific, somewhat pathological use case - you’re constantly pushing two quite different mechanisms to get everything you can get from each in the short (instantaneous) term and asking which one lasts longer. If you use a CIM and put in some controls so that the CIM does not draw greatly more current than the miniCIM, it will outperform the miniCIM up and down the power curve. Physics lets you fill the gaps in the measurements (subject to later measurements). Physics makes me pretty confident that the heat being generated is quite close to the difference between the electrical energy in and the mechanical energy out. I’m ready to make engineering decisions based on the theory of conservation of energy.

Paul,
Did you post how you did the “max power” test? I could not find a description, and can think of several ways to do the test, though it is likely that none are exactly what you did.

I didn't see a description of the "locked rotor test" either, but that one seems pretty obvious; lock the rotor to a torsiometer, regulate the voltage, measure.  The only variable is the torsion constant of the meter; I'm assuming that it was stiff enough that total rotation was less than a right angle.  If that wasn't it, please elaborate.

Gus,

Right here: http://motors.vex.com/#testing

We explain all three tests exactly how we do them.

Also, I have to make sure the viewers at home understand something about your claims about the spur gear setup you describe here:

If that bushing were such a problem, it should show up in the free run conditions. The CIM dissipates 32.4W running free at 5330 rpm, and the mini-CIM dissipates 35.8W at 5840. If we assume the friction is viscous (proportional to speed), they’re practically twins. Unless you’re putting significant lateral forces on the shaft for long periods of time, I’m dismissing this as a marketing rather than engineering consideration. So far, all but one of our CIM/mini-CIM motors have had spur pinions driving spur gears on well-constrained axles. That one exception directly drove a belt; that case would likely cause me a bit of concern today.

Spur pinions put a significant lateral load on the CIM motor shaft unless you support the other side of the shaft. Even then, it still sees load. Let’s neglect separation forces for now due to the 14.5 deg pressure angle and just deal with loading.

At max power, the CIM motor is spinning at 2670 RPM and has an output torque of 1.2Nm. If you are using the 12T pinion, then the pitch diameter is 0.6", or a radius of 0.3". So Rpinion = .00762m. At 1.2Nm, then the force at the pitch diameter is 157.48N, or 35 lbs. The pressure § on a bronze bushing is then this force divided by the projected area of the bushing, which is 8mm (.315") * .5" (length of bushing). So the pressure is 35/(.315*.5) = 55.56 psi

PV is the rating for bushings and the CIM bushing is oil impregnated bronze, which is rated at max PV = 50,000; max P at 2,000 psi; max V at 1,200 ft/min. The CIM surface speed 220.3 feet per minute at 2670 RPM so the PV is 12,240. Well within the rated range for the bushing, but definitely not 0.

More importantly, the coefficient of friction between a bronze bushing and steel is about .10 so the frictional force on the bushing is about 3.5 lbs, or 15.6N at a distance of 4mm that equates to .0623 N-m or about 5% of the actual output torque at max power.

Not extremely high, but definitely not 0. You’re dismissing the difference between a bushing and bearing at high RPM as marketing vs engineering considerations? I can tell you that is not true, because I made the decision to use the bearing as our testing showed it had much less friction than the bushing.

Maybe I misunderstood.

How much less friction is in the bushing vs. the bearing when running the CIM/miniCIM at max power on a small spur gear? The idea about running the same current on both CIMs and MiniCIMs is a valid one that I want to see the results of.

inb4 someone brings up 6-8x 775pros.

But hey, who likes saving weight anyways.

And if/when they burn out, that’s even more weight saved! Gotta consider the mass of the magic smoke :wink:

What about the mass gain from lifting a heavier robot?

Thanks!

I must have slipped a digit or something when I did the calculation a few years ago; I got something about a tenth of that. Yes, that’s significant.

It would be interesting to test the CIM with the max power test but with the target speed 4664 rpm, where the power output curve crosses 215W on the fast side.

Dang it Paul. I thought we were done innovating in the drivetrain gearbox. It’s pretty hard for us to refute the logic of a 6 Mini CIM drivetrain, especially since we see so much heat fade. Sounds like a great off-off-season project to get the students ready for the season.

All,

I made a calculator error in my calculations below. Ari Meles-Braverman pointed it out to me in a PM. The pressure for the PV calculation is actually 222 psi, not 55.56 psi. see the bolded items in my quote below for the corrections.

For us viewers at home, what would the math be if the CIM uses a bearing instead?

Paul can correct me if I’m mistaken somewhere, but the equivalent CoF of normal ball bearings* is usually somewhere between 0.001 and 0.0015 (compared to 0.1 for the bushing). So the bearing friction force would be between 0.05% and 0.075% of the CIM torque at max power, compared to 5% for the bushing.

Those numbers increase when you take into account the fact that the bearings are actually being used on a MiniCIM, which has a lower torque at max power. Multiplying by the ratio of torques of the two motors, it should work out to about somewhere between 0.086% and 0.129% of the MiniCIM’s torque at max power.

*If the MiniCIM uses roller bearings, they usually have a equivalent CoF of about 0.002, so they would have a friction force of 0.174% of the MiniCIM’s torque at max power. Even then, that’s still much less than the CIM with bushings.

Just to understand where you’re coming from in the bulk of these posts… Are you comparing 1 CIM to 1 miniCIM, or 1 CIM to 1.5 minCIMs.

Are you factoring in some assumed heading as well? Shifting down the peak power over time curve.

If this becomes a priority, and you don’t have a well-worn CIM handy for a dyno test, I’m sure I could convince my team to swap 2 off of our 2017 drive train.

Does anyone have spec data on CIM or miniCIM rotor inertia?

I ran a quick calc using the AndyMark 2013 FRC drivetrain kit and the rotor inertia spec for the 2018 FRC BLDC motor. I assumed 6 motors, a 150 lb robot, and ignored the moment of inertia of the wheels and gears (just wanted to compare rotor inertia to robot mass inertia).

  M = mass of robot in kg

  R = radius of wheel in meters

  G = motor speed divided by wheel speed

  J = rotor inertia Nm/sec2

  N = number of motors

  Jm = inertia of robot mass reflected back to rotor
*
*
M: 150 * 0.4535924$
R: 6/2 * 0.0254$
G: 11.53 $ "AndyMark 2013 drivetrain"$
J: 0.0013 * 0.00706155$ "2018 FRC BLDC rotor"$
N: 6$
J;
9.1800149999999996*10^-6
Jm: (M*R^2)/G^2;
0.0029717232112796
(N*J)/Jm;
0.018534730889787

254 has found that swapping in new CIMs for old ones after a couple of events (or a few dozen hours of practice driving) leads to noticeable improvements in acceleration. Can’t say how much of that is due to the bushings vs. the brushes or other factors, but I’d suspect it’s mostly the former.

This is really interesting. Out of curiosity, have you quantified it in any way? E.g. time-to-distance before and after swapping motors? I realize the actual effect will manifest differently with different robots/drivetrain configurations, but I’m wondering if this is something that more teams should be considering/chasing down or if this is more like a few percentage points of efficiency that is only going to be apparent once many other drivetrain factors are optimized.

We haven’t quantified it, but we’ve noticed the same trend.

Older CIMs also start to definitely smell like burning (very scientific).

Ever taken apart a CIM from a well used drivetrain? The brushes and commutator get pretty filthy with carbon buildup. I don’t know how much it affects performance, but apparently RC hobbyists clean their motors regularly to keep them in peak condition. I think for most of our applications, that performance hit is pretty negligible. Optimizing other parts of the robot is usually a higher priority.

If VEX still has their motor testing setup available, it would be interesting to compare a fresh motor to itself after a lot of use. Maybe a CIM degrades faster than a miniCIM due to its bushing, or maybe they degrade at the same rate?

Out of curiosity, how does inertia “propagate” through gear ratios? You define G but it seems to go unused?