Our team has as an issue with our drivetrain. We find that when we try to turn every once in a while the robot tends to stutter and it looks like the lights (RSL & camera ring) start to pulse or dim. Even with a full battery.
We have a West Coast Drivetrain with 4 CIMs and 2 Mini CIMs. It’s geared for 15.8 feet per second. The robot weighs close to the max at around 116 lbs. We’re using six Talon SRXs in Voltage mode. Two masters, one on each side, with two additional ones on both sides powering the front and rear motors as slaves.
We haven’t looked at the current draw, but can check that at our next regional.
I was wondering if anyone has seen anything like this and if there are any ideas how to diagnose what the issue could be? Maybe gearing, Mini Vs. regular CIMs, bad PDP, etc.
What wheels/tread are you using? Do you have a center drop in the drivetrain and if so, how large of a drop? It sounds like if you are experiencing high current draw while turning, it’s probably a result of the wheel’s scrubbing while turning (inherent to any WCD). Usually a center drop is added so that only 4 wheels out of 6 are contacting the ground at any time, effectively lowering the turning scrub. Another solution could be to use omni wheels on either the front or back of the drivetrain.
The gearing certainly is not helping ( I am one to talk though my team is geared to 16.5ft/sec). One issue might be the mini CIM it runs at a higher speed for a given voltage so you either need to change the gearing respectively or drive it with a separate command for the motor control on the miniCIM. That will improve your efficiency.
Watch the voltage readout on the driver station, you’re for sure browning out. Although I’m not sure how so many manage this, we’re geared for 7 fps and 18 fps, and can dead spin in high gear. With a good 1/8th-inch center drop it’s not bad at all, we’re using blue nitrile grip on the AM performance wheels.
Sounds like a friction issue on your robots part. If you don’t have a center drop, I would recommend artificially creating one by either making your outer wheels slightly smaller or making your center wheel slightly larger (probably no more than 1/8th.) I’m also assuming you have a 6 wheel config.
We’ve seen this a few times before tuning both mechanical and code.
You said it’s in Voltage mode - is that voltage ramping mode? The input for that is Volts/sec.
Your drive train pulls TONS of current for 0.X seconds. If the mechanical workings of your gearboxes are correct then conservatively, in theory the high current draw is for somewhere around 0.1 seconds. Since your motors are geared based upon a 12V motor, in order to prevent massive current draw you want to spread the change from 0V to 12V across >= 0.1 seconds. Or, 12/0.1 = 120volts / sec. Adjust this number up or down to get more or less aggressive acceleration. We tuned this number until aggressive acceleration of our robot maintained greater than 9V on the driver’s station.
We also designed our robot so that during acceleration, the only other motor running is the compressor. Most everything else is pneumatic, reducing battery draw.
To test the mechanical side, you simple need to put the robot on blocks and run the wheels. If you apply full power to one direction and the compressor audibly reduces in power for an extended period, check the driver’s display battery voltage. When running at full speed on blocks, the battery voltage of your drive train should still be above 12V. If it isn’t, then double check that your drive train gearboxes aren’t binding.
The gearing sounds fine - I’d check for mechanical resistance in your drivetrain and for any potentially dead motors (or motors turning the wrong way).
You could potentially do this now. The logs on the DS laptop should still be there. And if you know where everything is plugged into the PDP you can track everything pretty nicely there. Perhaps I am missing something.
Our 6 CIM drive did some weird things this season. For a short period of time something changed in the code or talon settings, and one motor in each of our gearboxes was being powered in reverse of the other two. I would have suspected this to cause a pretty drastic change in driving characteristics, but it was fairly subtle in our test environment (just driving about, not much load).
Cross slaving talons can also be a bugger. One motor from each side of the drive could be slaved to the master from the other side, which would basically cause an intermittent case of the problem discussed above.
We kept a spreadsheet of each motor which listed its CAN ID and PDP Channel as well as its purpose, physical location, and the physical location of its Talon on the E Board to help diagnose these issues. Even at that, we had a bunch of times when Talon IDs didn’t “stick” and caused these types of issues.
We had a bad PDB that caused very similar issues on our robot. We chased electrical issues all day friday until we replaced it. We have pretty much the same drivetrain as you except the minicims, but we’ve done that in the past with no problems.
+1
Gearing is not to ridiculous. Likely you have binding in your drivetrain.
The other thing you should verify is that EVERY electrical connection from the battery terminals to the motor are TIGHT. Loose connections in a high current path causes considerable voltage drop. With the lights dimming, it makes sense it is somewhere before the speed controllers, like at the main breaker.
This advice is wrong. You do not need to make any adjustments for a mini-CIM in a drivetrain in code or gearing, and any attempt to do this will make your problem either worse, or no better. Mini-CIMs should be run at the same gearing and voltage as a CIM for more or less optimal results.
OP: This sounds like a pretty classic example of high turning scrub. Please post a picture of your chassis, particularly your drive wheels. How much drop center do you have, if any? How long and how wide?
The motor combination and gearing should be fine from a power standpoint.
Suggestions -
Put the robot on a flat, hard floor. How much rock do you have. A good recommendation is that if the back wheels are on the floor, the front ones should be up 1/4". That is 1/8" drop.
Check the center wheels for wear. How does the center wheel tread look compared to the front and back. The center wheel tread will wear faster than the front and back, and your “drop” will go away. You may need to sway out wheels or replace the tread.
Pull breakers and run one motor at a time. Make sure the wheels always go the same way.
Turn the wheels by hand. Everything should spend freely.
Don’t change the pinion on the mini-CIM. A mini-CIM with full CIM combination with the same reductions works well. Search for a post from Paul Copioli @ VEX for the details.
I still don’t buy that the gearing is correct because we don’t know the gear ratio, only the ‘ft/s’ - which could be actual or adjusted. Can you provide your exact gearbox setup including gear ratios?
I’d still contend that anytime 6 CIMs get involved (or 4 CIMs and 2 Mini-CIMs) and you aren’t doing anything but applying 100% power when driving, you’re setting yourself up for issues like this. Make sure you’re doing some kind of voltage ramping to help your robot not draw too much current when you go from 0 to 100% power.
It took me 1 season of 6 CIM drive issues to never touch them again. I’m probably superstitious now but 4 CIMs should work just fine for 99.5% of FRC teams.
This is not true. In fact the minicim was originally designed to be a drop in lower-powered alternative for a normal CIM. The miniCim and Cim combination work well together with no gear change between them.
I guess I misunderstood some advice I had previously read. I though since the CIM and the Mini CIM had a 9% diffrence in speed that they would benefit from having a system that effectively set them to the same speed whether that be mechanical or programing.
The mechanical system that sets them to the same speed is the gearbox. Two motors geared to the same gear literally cannot go at different speeds. The motors will adjust by providing different amounts of torque and pulling different amounts of current (which they’d be doing anyway because they’re different motors).
Okay, we’re getting closer. My guess is this is just one stage of your gearbox though or you have further reduction going from the output of the gearbox to sprockets on each of your wheels.
Lets try this, answer these questions and we will have a much better picture of your potential issues just in case you’re drawing too much current -
Which gearbox are you using - AndyMark, Vex, West Coast Products, etc. The specific model would be helpful i.e. this year’s kit gearbox, a Vex single speed double reduction gearbox, etc.
List each stage of gearing - count number of teeth on each gear. For example, first stage may be 12T pinion on your CIM motor driving a 40T gear, 2nd stage is likely your 44:30 stage you mentioned. Also include any sprocket / belt reductions occurring off of the output of the gearbox.
Which wheels are you using - colsons, other? And what is their diameter?
If you have no rock at all, there as way too much wheel contact while turning, and that will drive up your power requirements. Skidding 6 wheels is difficult.
If you are using treaded wheels (like the AM performance), you can wrap them first with a thin layer of material, and then add the tread strip on top of that. If you rivet through both layers it will stay on (our prototype is running this set up because we did not have enough drop in the original build).
Another option is to check the specs on several 4" wheels. They are not all exactly 4" diameter, so you might be able to swap out the center wheels for something slightly larger, or the front/back wheels for something slightly smaller, to get the center drop you need.
