CIM Motor for Intense Direction Changes

Hi all, I’m looking to use 4 CIM Motors for a drive-train (possibly x drive) on a tennis launcher robot (turret).

I am planning for the drivetrain on the robot to be able to quickly accelerate from one corner of the court to another (distance of 8 meters) and be able to stop and accelerate again in the opposite direction, this cycle would be repeated multiple times, around 5-6 times before the motor gets a few seconds break for the next rally. The overall duration would be about 45 minutes…

That seems like an awful long time to run these motors at such high acceleration and deceleration… would the internal electrical components such as the armature windings be able to handle all the back emf and heat generated when the motor is braking periodically for 45 minutes, with a few breaks here and there?

I am even considering running water cooling on the outsides of the motors.

There are a lot of factors in play, such as the weight of the robot, gear ratio, wheel size, and where on the motor curve you’re operating in. That said, we’ve run a robot outside, in the sun, in 70-80 degree heat for ~6 hours before doing demos at a local fair. Nothing super intense, but everything (except the wheels) held up just fine.

We’ve also run multi-hour practice sessions, driving around in competition conditions (aka inside) and never had a problem, although they can get awfully hot! The CIM is a beast, and holds up to abuse quite well.

Thx a lot for the reply!

When u ran the robot outside in the sun for “~6”, do u mean about 6 minutes or 6 hours :eyes:?

To be honest, as long as you mount the talons to something metal on the chassis, it’ll probably be fine. In fact, all of our robots have talons mounted to plastic (polycarbonate or otherwise) and we’ve never had any issues with any of them due to overheating, even during long demos or practice. You’re more likely to fry CIMs before you fry Talons (and even that is not very likely).

As far as cooling the motors, CIMs generate most of their heat around the shaft, so cooling the casing isn’t very effective. I’ve seen a few teams use vented screws when mounting the CIMs (to allow airflow into the motor) but I’m not sure how significant of an impact that would make.

Now all that said, I would be curious to know what size wheels and gear ratios you’re thinking about using. The only time I’ve seen people have issues burning CIMs on a drivetrain is when they’re either using wheels that are too large, or reductions that are too high.

On our T-Shirt cannon robot, we basically just use a KoP chassis with the stock gearbox and 8in pneumatic wheels, it’s quite fast, can stop aggressively, and doesn’t overheat.

It might be worth investigating using NEOs instead of CIMs for this. Aside from of the advantages for an in-season frc robot, they stay significantly cooler with CIMs even after prolonged use. I can’t provide any numbers but I’ve seen numerous pieces of anecdotal evidence backing this up. One of the frequent posters from the Killer Bees, possibly Nick? posted about their CIM cooling solution involving the vented screws and how once they switched to NEOs they never once had to cool them even during prolonged practice sessions.


hours. I went back and corrected that!

On the other hand, the key to having the CIM last is the current times the duty cycle. I’ve burned up a CIM stalled at 10% throttle. It took a couple of hours, (used a robot designed for intermittent play and put it in a parade). The outside temperature was rather cool (50’s or 60s Fahrenheit). Anything you can do to get the heat out of the CIM is likely worth it, but the issue is that the heat is generated in the windings on the rotor, and there’s no good heat path out of the motor.
If you can afford it, you may want to consider switching to NEOs, which can be cooled through the mounting face, as the coils of the NEO are on the stator (case). Even better would be an air cooled motor (775pro or redline) which cannot take as much heat at any one time, but which is much better at dissipating heat.

1 Like

Merged two similar threads. Last two posts are from the merged thread.

Thx for the reply, I am planning on using smaller wheels as they are cheaper and will accelerate faster than the larger wheels, ideal for my situation. Working on the gear ratio.

I suspect your issue will be batteries - not motors.

We often (5-7 times a season ) run robots for ~ 2 hours at demos in the summer - but have to change batteries every 10 minutes or less depending on level of abuse. After batteries our next issue is wheels - we often run on asphalt - and the colsens lose all their grey and run on the black hubs - then it is time to swap them out.

CIMs are hard to kill - but when you can’t hold your hand on them you might want to let them cool a bit - but have never had them actually fail - completely (loss of some power yes).

I would second the suggestion to use NEOs if you can. They will run significantly cooler and will use less battery; two game changers since heat generation/power consumption are going to be your main enemies here.

1 Like

How many tennis balls will you need to carry to last for 45 minutes? How much will the balls and their container weigh? How much will an adequate ball sequencer and shooter weigh?

It isn’t super effective, but it is better than a poke in the eye per Aren Hill’s testing.

I have to imagine there are techniques to modify a CIM for better cooling outside of FRC rules that haven’t been explored.

Also, cue discussion of 6x Mini CIM drivetrains from years past. Paul Copioli makes the case here, here, and here.

I would also take to heart some of the discussions about all-omniwheel drivetrains in Paul’s posts, assuming the court and your system can accommodate. The drastic reduction in scrub would cut down on current draw significantly as well.

1 Like

If I understand the intended use, I think it’ll just drive back and forth with no turning, so scrub shouldn’t be an issue either way.

Reading OP’s post back, it can certainly be interpreted as “I’m making a dead straight shot”. And if so, you’d be right.

yea Ik, the batteries are a massive pain. I am planning to use three 18 ah Batteries wired in parallel to achieve 54 ah. I’m thinking 54 ah will be enough to power around 6 Cims on base, 1 on linear puncher and 1 on a flywheel for at least half an hour, hopefully more. Considering that the machine won’t also be constantly running (will rest when time spent on picking up the tennis balls). Also pwm will conserve some of the energy as well.

Any thoughts, u think 54 ah will last?

They seem like a great option, just that the sparkmax motor controller for em is like 75 American…so might just stick to the CIMS and run them off a Talon.

1 Like

Are these new batteries, or used ones? I’d be leery of the latter situation, since mixing various ages of battery can result in the batteries trying to charge one another. If boot times are a concern for your system (you didn’t specify roboRIO or a HERO or something else), you could use one battery to power the control system itself and switch out the drive/launch motor battery as appropriate.

I would check my drive base setup against iLITE’s drivetrain simulator. Default reaction is to reach for Mini CIMs as they spin more efficiently (bearing in the output, less rotating mass, etc), but if those aren’t available to you for the same reasons a NEO isn’t you may discover you can reach your target sprint distance at an acceptable time with four (or for a super light robot, two) CIMs. That will cut down on current draw as well.

Regardless of motor choice, I would also pay attention to current draw through the PDP. If it’s going to run this hard for this long, a jump in current draw (or since we aren’t turning much, a significantly different current draw from left to right) is a sign that something has gone wrong or will soon.

Yeah, honestly while the NEOs are basically going to make most other FRC motors obsolete for competition use, you still can’t beat CIMs + a cheap motor controller for price. For an off-season project it usually makes more sense to go cheap.

On the note of cheap, some other things you might want to consider:

  • Do you actually NEED to use Talons for what you’re doing? Would a cheaper controller, like a Spark or a VictorSPX do the job, or do you actually need CAN connectivity and/or sensor feedback?
  • Do you need a full FRC control system, or would something simpler and less expensive like a Cheap & Dirty control system do the job?

Depending on how elaborate your project is, it might make sense to keep your control system as cheap and simple as possible.
Our “Demo Bot” is a dual-barrel T-shirt cannon that we control with a Cheap & Dirty control system and some Spark motor controllers we had kicking around (they send them in the kit every year but we rarely use them, because CAN). We were able to control the drive motors on two axis of the controller, we use another axis to control the shot angle, which is powered by an off-the-shelf (but not FRC legal) linear actuator, and we use the trim dials (which we converted to buttons) to toggle the pneumatic valves for the cannon barrels. The entire system was really simple to set up and use, and the best part is, you turn it on and it’s immediately ready to go (no waiting for the RoboRIO to boot or the AP to connect).

Can you elaborate on what you mean by this?