Mini Cim Drive Motors?

[orangemoore]

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?

[Boe]

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

[EricH]

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.

[MichaelBick]

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.

[Nathan Streeter]

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.

[Jonathan Norris]

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.

[Ether]

Originally Posted by Jonathan Norris

Paul told us that they were designed when paired 1:1 with regular CIMs in a drive system

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?

[Jonathan Norris]

Originally Posted by Ether

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.

[Ether]

Originally Posted by Jonathan Norris

Thats the intent that I remember from Paul.

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).

[BenB]

Originally Posted by Jonathan Norris

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 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.

[Ether]

Originally Posted by BenB

Joe Johnson explained why it doesn't matter if motor speeds match in this post.

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?

[Paul Copioli]

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

[Richard Wallace]

Originally Posted by Ether

Does anyone have any data on the relative heat dissipation capability of the mini compared to the CIM?

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.