CIM Motor for Intense Direction Changes

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.

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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.

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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.

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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?

Yea meant to say the Victor Spx. Thing is the spark controllers are currently out of stock and idk when they would restock. So I might just stick to victor spx.

The control system definitely can be cheap. I was planning on an arduino cleverly controlling multiple mosfet transistors which can each switch up to 15 A. The arduino will provide the PWM signal as well.

The way I know is that when providing motor control with PWM, u are sending current in pulses specified by ur duty cycle (from 1-100%). A smaller duty cycle means for one period of the cycle/square wave (determined by frequency) u are sending current for a smaller fraction of that period.

So in essence saving battery since u are not continuously sending current 100% of the time and also slowing the motor as a result.

Are you referring to the power taken up by PWM signal itself? Not the motors it controls? If so, I seriously doubt you’d even be able to notice the difference between PWM and CAN, such wiring is by far the least significant power draw on any control system.

If you are referring to differences in the power consumption by the motors using PWM vs CAN, could you explain further?

Yea I’m referring to the power consumption by the motors. I don’t know too much about CAN.

I just mentioned earlier in respect to battery consumption that powering motors with PWM is nice for the battery usage since the motors aren’t receiving current 100% of the time depending on the duty cycle.

Parades are hard on FRC robots. Heat. Asphalt. We’ve done a few of these. Much harder on the machine than running near continuous for hours at a STEM night.

TW

You’re basically correct but a little misguided I think. Because the motor acts as an inductor, it tends to resist changes in current passing through it. This has the effect of averaging out the PWM’d voltage we send to it because that’s at such a high frequency (~15kHz.) So in essence we can understand anything less than full throttle as a lower voltage applied to the motor, eg 50% duty cycle is 6V. Because we apply a lower voltage, the motor can draw less current, which in turn draws less from the battery, draining it less.

So PWM isn’t the method by which you’re saving battery, the real saver is running at a lower throttle which we achieve by the method of PWM.

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Because this is a non-FRC environment nothing says you can’t split up your power system.
One battery for each side and one for your other systems.

I would also use Mini-CIMs. Since our switch to them we haven’t had overheating problems.

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