I am currently trying to design a robot in CAD and I was trying to decide what type of motors should be for the drivetrain. Currently you can use 4 mini cims so 2 per side. Would that be strong enough?
It would probably be strong enough (havent done the math but i would think it would work) but i would use four regular CIMs in your drive unless you have a good reason not to
It should work. However, VEXPro is quite clear in the motor description that it is only 2/3 of the power of the CIM (and a higher output speed, meaning less available torque). For an FRC-season drivetrain, you will need to make the decision as to whether the lower power in drive is worth the lower weight of the drive and higher power available elsewhere for yourself.
That said, why not try going with 1 of each? They use the same mounting, and aside from the speed difference (which is pretty easy to deal with by gearing), they should work well together.
Based on the current motor allotments there isn’t much reason to switch to mini-cims. You have 6 CIMs and 4 mini CIMs, which gives you plenty of power if you need it. Unless you have a game with low traction surfaces or wheels(like lunacy), I would recommend running 4 CIMs because it helps getting through defense and allows you to play defense if needed. The weight loss is worth it. I would reccomend always running at least 4 CIMs on your drive.
4 Mini CIMs would probably be very noticeably weaker on the drivetrain than 4 CIMs.
According to the VexPro specs, the Mini CIMs have 66% of the power and 50% of the torque (when adjusted for their different free speed rpms) of the standard CIM. As an educated guess, the torque is probably a more important factor for acceleration and pushing than the max power is… So the 4 Mini CIMs would be pretty similar to a 2 CIM drive. In a nutshell, 2 CIMs is “inadequate” for a ~100+ pound that drives on an ordinary surface and will be accelerating beyond 10fps, pushing robots around, or driving over field obstacles. Given that a few teams are even using 6 CIMs now, I’d say you’re best off with 2 CIMs & 2 Mini CIMs or 4 CIMS. The extra .5# per CIM on the drivetrain is well worth it for >95% of FRC robots.
As some anecdotal stuff: 1519’s used 2 CIMs on three of our competition robots… one was Speedracer, a fast lapbot from 2008; the other was Fezzik, our hurdling robot from 2008; and one was Colonel York, our 2009 robot.
Speedracer from 2008: was a ~40# robot that had 2 CIMs driving the rear wheels and ackerman steering in the front. It was geared fast, but not excessively so. It accelerated well and never had overheating issues. Definitely no regrets on using only 2 CIMs. We have video of it on our team webpage (our more recent videos are on youtube, though).
Fezzik, our hurdler from 2008: was initially designed to meet all robot rules (weight, volume, interchangeable electronics, bumpers) together with Speed Racer… so we took a couple weight-saving compromises… one was to use only 2 CIMs. It experienced significant over-heating issues after matches - in elims they got hot enough that you couldn’t keep your hand on them! After our first event (and the interchangeable robot thing was ruled illegal - see thread about it) we put the extra 2 CIMs on there. It was probably was ~11-14fps… moderately fast but nothing noteworthy.
Colonel York, our 2009 robot: ran on the low-traction surface using the required slick wheels… as a result there was no need for more than 2 CIMs of torque. I’d venture you could’ve used 1 RS550 on each side of a tank drive that year, given that the wheels had some <0.1 coefficient of friction on the field surface.
Every other year we’ve used 4 CIMs in either single-speed or two-speed transmissions. Again, if you really need the weight maybe switching 2 of the CIMs for MiniCIMs would be worthwhile; however, I’d venture for most robots there are better weight saving measures.
They are certainly a possibility for drive motors, our team had a great opportunity to talk to Paul Copioli at length about the Mini Cim’s (and other Vex Pro products) at Waterloo this year. Paul told us that they were designed when paired 1:1 with regular CIMs in a drive system, a 4 CIM 2 Mini CIM drive could be quite powerful. We ran a 6 CIM drive this year and loved the acceleration and power it gave us with a single speed gearbox.
The meaning of the above is a bit unclear to me.
It sounds like it’s saying “no gear compensation for the different free speeds”. That’s not the intent, is it?
Thats the intent that I remember from Paul (I might be totally wrong), but to be honest I haven’t worked on motor dynamics in a couple years. Obviously at free speed and stall the mini Cim wouldn’t match the CIM, but the dynamics of how the load is shared in a gearbox with motors of non-matching free speeds is escaping me right now. I’ll shoot Paul an Email and see if he can correct me.
I had forgotten that the mini CIM free speed is 6200. I guess that’s close enough to 5310 that 1:1 would work. The mini-CIM would be carrying just a bit more than its “fair share” of the load (relatively speaking).
I wouldn’t be surprised if this was the design intent. Joe Johnson explained why it doesn’t matter if motor speeds match in this post. For packaging reasons, we geared 2 CIMs 1:1 with 2 mini-CIMs in our climbing mechanism this year and it seemed to work fine.
To be clear: Joe did not say “it doesn’t matter”. He said:
The question of whether to put multiple motors in parallel is not a simple one to answer. … I don’t not agree with those who say that you must always match free speeds of the motor
There are times when you do want to match free speeds. It depends on the application and the expected operating conditions.
Here’s a simple chart which shows the difference between running a CIM and a mini 1:1 vs running them with matched free speeds. Column H “relative heat” shows the mini’s heat generation relative to the CIM. It’s an indication of how “hard” the mini is working relative to the CIM. The mini is smaller than the CIM and cannot be expected to sustain the same rate of heat dissipation as the CIM.
Does anyone have any data on the relative heat dissipation capability of the mini compared to the CIM?
*
Ok so the Mini CIM was designed specifically do mate 1:1 with the CIM motor. The design goals were:
(1) Same exact mounting as CIM motor
(2) Have current draw of approximately 40 amps at max power.
(3) Optimized (while keeping constraint #2) to mate 1:1 with the CIM motor to maximize the Mini CIM contribution at maximum power. We are about 10% off from this goal.
We utilized a Magtrol automated Dynamometer setup to test various conditions to make sure the contribution of the mini CIM when combined with a CIM motor was as expected.
My team ran mini CIMs along side CIM motors in our drive gearbox (4 CIMs and 2 Mini CIMs) and they performed as expected.
I hope this sheds some light on the mini CIM performance.
Paul
Only a little data now, will try to get more later:
We fitted a CIM and a mini-CIM with thermocouples to measure temperature on their brush guides; this is the hottest non-moving internal part – the moving armature windings get a few degrees hotter. On a Magtrol brake dynamometer, we ran the CIM at its normal load, which is about 0.45 N-m drawing about 27 Ampere, per the data sheet. We ran the mini-CIM at 2/3 of that load, 0.30 N-m drawing about 17 Ampere.
As has already been noted, the mini-CIM operating at 2/3 the torque load of a CIM is actually running more efficiently; thus its power losses (waste heat) are lower than the CIMs in both relative and absolute terms.
Our thermocouple measurements showed the CIM brush guide reaching 125 degrees Celsius after eight minutes, while the mini-CIM brush guide took twelve minutes to reach the same temperature.
Did you record the CIM RPM under those test conditions?
Only a little data now, will try to get more later
This would be interesting: apply whatever load is required to pull down the mini (with 12V applied) to the same RPM as the CIM (1:1 mechanical pairing simulation) and see how many minutes it takes to reach 125C.
At 12 volts, a CIM goes 3790 rpm at 40 amps and a miniCIM goes 3375 rpm at 40 amps. Therefore, designing a drive train to pull 40 amps per motor at the traction limit with CIMs and miniCIMs in parallel is near achievable with a 1:1 pairing of the motors.
Running the miniCIM at the same speed as the CIM at the traction limit (1:1 pairing) would mean running the CIM at 40 amps and the miniCIM at 34.3 amps, at 12 volts.
Based on these results, I would say it makes sense to run CIMs and miniCIMs together 1:1 in the drive train if design calls for it. So if you need extra power elsewhere, need weight savings that can’t be found anywhere else, or have other design criteria, it should work well. It has worked well for many teams.
From what I’ve seen, miniCIMs are best suited of any of the 2013 legal motors to supplement or replace CIMs in the drive train since they are designed to take that type of abuse. Most other FRC motors simply are not.
Yes, but that data doesn’t matter for the question you posed: compare rates of temperature rise. Waste heat dissipated by the CIM and mini-CIM depends almost entirely on torque loading.
My test was intended to check Paul’s design goal for the mini-CIM, which he has stated was to share load with a CIM (at 1:1 gearing) while taking about 2/3 of the CIM’s torque; i.e., the mini-CIM takes 2/3 N-m for each 1 N-m taken by the CIM, so the motors share the torque loading about 60:40.
My results indicate that Paul actually exceeded his design goal (:)) since the mini-CIM’s internal temperature is rising more slowly than the CIM’s while it delivers 40% of the total load torque.
Conclusion: if your drivetrain needs more power than one CIM can deliver, but not as much as two CIMs, then one CIM and one mini-CIM is an excellent combination. You can run them at the same speed, and mini-CIM internal temperature rise will NOT be a limiting factor.
*For the sake of the discussion below, let’s agree that the mini-CIM meets the stated design goal for FRC competition use, but pursue the heating question just a bit more.
Given these two motors, how would one gear them if the goal were instead to share the load in such a way as to balance the motor temperatures as much as possible?
There is no single answer. It depends on the location of the temperature you are trying to balance, and it depends on the duty cycle.
Using the CIM motor curves, at 27A at 12V (Richard’s test), the CIM would be spinning at 4320 RPM and generating 119 watts of waste heat. Only 66 of those watts, or 55%, is due torque-producing current (I2R heating). The other 45% (54 watts) is due to eddy currents, hysteresis, internal friction and windage, etc, which also cause motor heating.
If the mini-CIM is constrained to the same speed as the CIM (1:1), then at 12V and 4320 RPM it would be generating 133 watts of waste heat.
Under these conditions, the mini would be producing just slightly less torque than the CIM.
So it appears at first blush that the mini-CIM would run hotter than the CIM under these conditions.
However, these numbers obviously change as the motor heats up due to changing coil resistance and magnetic properties. And the temperature has a distribution within the motor due to internal structure and the different sources of heat.
It’s possible (but quite a bit of work) to model all these parameters accurately. With the right equipment and setup, a test would yield the answer.
For sake of discussion it would be interesting to include non-competition duty cycles. Are there any teams out there who have a CIM+mini 1:1 on a practice/demo bot? Is either motor hotter than the other after a good hard workout?
This is exactly how we ran our drivetrain as well. 2 cims + 1 minicim per side. It worked exceptionally well.
In answer to Ether’s question: We had them 1:1 but the minicim was unpowered except when we got into pushing matches, so we never heated the minicim up.
One possible test set-up could be a brake dynamometer, a two-motor gearbox (e.g., AM CIMple Box included in recent FRC kits), thermocoupled CIM and mini-CIM motors, and a laboratory dc supply capable of feeding the motors at 12 Volts, with combined current draw up to 84 Ampere. I have all that on hand. The CIMple Box ratio is 4.67:1. Test loads of interest might be 4, 5, and 6 Newton-meter at the CIMple box output shaft – loads beyond that would require more current than my supply can deliver. At each load, the test procedure would be to record temperature rise until the brush guide reaches 125 degrees Celsius and compare the plots, CIM vs. mini-CIM.
Please check my figures above and suggest changes.
Sounds like a good test. Thank you for doing this.
I would suggest only 2 changes:
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record the outside case temperature of each motor (in addition to the brush guides), and
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add a lower torque (higher speed) test point of 2.5 Nm (at the gearbox output shaft).
Rationale:
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Since motor case temperature is the metric most familiar to FRC students, it would be enlightening to record that and show how it does (or doesn’t) correlate with brush guide temperature.
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Higher speeds are where the mini might be expected to fare worse than the CIM. Useful to have a data point there.