I was looking into West Coast Drives. I was wondering about your opinions of 775 pro drive trains vs 6 mini cims.
In my mind, I think that 775 pros with limited voltage will not have any problems with burning and will save weight. However, I am wondering if 6 mini cims is more reliable in any way or worth the extra weight?
First question, is there a reason you need to save weight? Is weight normally an issue for your team? 6 mini CIMs is definitely more reliable and requires fewer resources.
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I agree that it takes fewer resources. However we had no problems with our 775 drivetrain, so I don’t think we agree with your opinion on reliability.
There should be absolutely no opposition to or controversy around stating that across a large sample size 6 mini-CIM drives will be more reliable than 8 775 pro drives.
6x Mini CIMs and 8x 775pros are at the very opposite ends of the drivetrain weight vs ease/reliability trade-off, which is almost solely based on thermal mass. Thermal mass is the measure of something’s ability to absorb heat energy; the more energy it takes to increase the temperature of the thing by a given amount, the higher its thermal mass. Increased thermal mass is beneficial to a motor’s performance because it can run longer at higher powers without heating up as much. Cooler motors are more efficient, and over a certain temperature the motor will “burn” and permanently lose efficiency. Increased thermal mass, though, is usually caused at least in part by increased mass, which is a downside if your robot is usually close to the weight limit.
Once you account for software protection on the 775pros, 6x Mini CIMs and 8x 775pros will give about the same amount of power. The main difference in performance is based on the fact that 6x Mini CIMs have a relatively high thermal mass, whereas 8x 775pros have a relatively low thermal mass. This is why people above have said 6x Mini CIMs are more reliable, because their increased thermal mass lets them absorb plenty of heat before burning, whereas 8x 775pros could burn up in seconds without proper software protection. This software protection takes resources to write, which could go to other parts of the robot, and is not always foolproof.
As with most engineering questions, there is no one right answer; the answer to your question depends on your design priorities. If you need the extra 6.7 lbs saved by using 8x 775pros and have the resources to dedicate to writing/testing the protection software, then 8x 775pros is the answer. If you aren’t close to the weight limit and would rather dedicate as many resources as possible to other scoring parts of the robot, 6x Mini CIMs is the answer. If you’d like a happy medium, 2x CIMs give about the same power as both other options, have a decent amount of thermal mass, and save ~2 lbs over 6x Mini CIMs.
How big is the sample size on either, really? Nobody was shipping a COTS 775-class gearbox for drivetrains until AndyMark began shipping their EVO gearboxes in the middle of build season this year. (I forget the dates, but I remember they weren’t ready to ship on Kickoff morning. I’m obviously an AndyMark fan, but that alone would’ve taken those gearboxes out of consideration with me even before the merits of 775s on drive or some teams’ not-unreasonable “never bet your season on a first-year product” doctrine.)
It appears that 6-Mini-CIM is trickling down the food chain a little faster, but 3-CIM gearboxes are nothing new and they’ve been bolt-ons for the AM14U drivetrains since day one. I doubt either overtakes 4 CIMs as the most common option unless someone changes the Kickoff kit drivetrain, but I expect the head counts on those two to even out next year or the year after as COTS options spread out in the market.
Generally, I agree with Ari here–Mini CIMs are more idiot-resistant than 775s. (Nothing is idiot-proof, of course.) The Mini CIMs also shave off a little bit of room, which is important if your drivetrain must be particularly skinny. 4901 just barely avoided that fate in 2016, and 1293 had to run Mini CIMs in 2018, because there was no way to package the drivetrain otherwise.
Teams have blown motors on 775 drives before. Maybe not that often, but they have. Has any 6 mini-CIM drive failed a motor yet?
Regardless of pros and cons of either - if absolute idiot-proof reliability is essential (as it should be for maybe… 80% of FRC teams, at least), the miniCIM (or CIM) drive wins on that metric.
We decided to run an 8 775 Pro this season to try to save us weight and give us better performance. By switching to 8 775s, we saved a large amount of weight, around the 6.7 lbs. AriMB mentioned. We always build very heavy robots on accident so this gave us some buffer room to allow us to build more onto the robot.
Power wise, the drivetrain was geared faster and felt like it had more pushing power compared to our 4 cim drivetrain in 2017. We definetly noticed a difference in power but throughout the season, we noticed a decline in power. We are going to try to rebuild part of the drivetrain at state to keep it running good through worlds and off season, including swapping motors to make sure accidental damage is taken care of.
If we were always light on weight, my team would consider doing a minicim driveteam.
This is a pretty perfect post that exemplifies the tradeoffs.
Should every team do 775 Pro drives? definitely not.
Should teams that have the bandwidth to make sure precautions are made to not burn out motors and are able to keep an eye on any degradation do it if said teams generally hit 120lbs every year and would like an easy way to save enough weight for an entire mechanism (think can-grabber, 2012 stinger, etc.) yes!
This conversation is not black and white, despite it being treated as such most of the time on CD. Like most things in FRC, it is really about your teams resources when it comes to time, weight, programming bandwidth, etc.
That being said, if you aren’t comfortable with 775 Pros for your drivetrain, fine. Anyone not switching at least from 4X CIM to 6X MiniCIM at this point doesn’t make much sense to me. Only logical reason I can see is to save 2 PDP slots, but I would think 10 PDP slots for everything non-drivetrain on the robot should be enough for even elite robots.
And now for something completely different…
Which channels of the PDP will you be connecting the motor controllers to? The 40A channels or the 20/30A channels or a mix? Will you need some of those channels for other functions on your robot? Will not having some of those channels make other functions on your robot sub-optimal?
Engineering requires one to look at the whole problem to achieve the correct balance of compromises. Sometimes “the best” may not be the best.
The answer isn’t obvious. 8x775 pro drives can put all their motors safely on 30A slots, freeing all 8 40A slots. Whereas 4 cim drives would only leave you 4. Not sure what people do for 6 (mini) cim, but without current limiting, you’ll also want to use the 40A slots, leaving only 2.
If you are asking the question, you should be using 6 Mini-cims.
If you haven’t purchased or manufactured 775pro gearboxes by this point.
If you haven’t configured motor controllers to limit current or voltage limit by this point.
If you don’t know what voltage ramp rate is.
If you haven’t done off season testing on a full weight robot, pushing it into walls, pushing other robots, running it for 2 minutes at full speed with multiple direction changes, etc.
If you don’t know what a traction limited drivetrain is.
If you plan on running 20fps with no low gear.
You should not be using a 775Pro drivetrain this coming season.
Even if you did do and know all the above, you can still burn up a motor.
We did at Champs this year, it sucked.
The reason our motor burned up?
Not 100% sure which is the worst part.
Possible causes:
We damaged some motors during initial startup during build season and we didn’t replace all the motors.
The air cooled motors have vents that allow contamination to enter the motors.
Our current limiting was set much higher than others using 775pros this year.
We got into many pushing matches this year, maybe we overtaxed the motors.
We competed at three regionals and didn’t have an issue until towards the end of our qualification matches at Champs.
Will Pwnage be using a 775pro drivetrain this coming year?
The answer is maybe.
How did 33 like using the 6 775 drivetrain this year? Many issues?
The only reason for possibly switching to mini cims is the ability to stall them and not have to worry (as much, over time it may add up with too much stalling), even though we very rarely get involved in pushes matches. Extra PDP spots are a plus, but we have never reached the limit since I’ve been involved.
And we did push the the 775s a little much this year. For the next season, we do plan to add a little more current limiting, and add more ramping to limit drivers from going instantly forward 100% to reverse 100%, which I’d like to believe wasnt the healthiest. Along with this, we are working to develop some sort of diagnostics to run tests to determine the health of the motors.
A lot more programming than a CIM, but when controlled, we have had less issues all the way around.
What issues did you have with CIMs/mCIMs that you didn’t have with 775’s?
I would say that what you are “forced” to learn when developing a 775 drivetrain maybe of great use no matter the drivetrain.
It may be that due to how much more attention to detail you have to have to successfully use the 775 in a drivetrain the better the overall drivetrain will be.
The CIM being super friendly to the average person may lead to engineering “laziness” because its forgiving.
Maybe they had less issues because they built a better drivetrain with what they learned using 775. But the actual motors had nothing to do with the issues.
In 2017 we were geared roughly 13 fps using a 4 cim drivetrain (#35 chain, WCP SS). Throughout a typical match, it wouldn’t be surprising to completely lose one side of the drivetrain, and then lose both (not all the time did we lose both though). This is back before we knew about current limiting also just as a note so this may have been able to fix it. We would essentially just be driving and the robot would just stop driving one side, essentially just trapping us in one spot. We replaced the breakers on the drivetrain throughout season which helped for a little bit but still didnt fully prevent the issue. We never tried replacing the motor due to our robot being cramped and requiring nearly all the electronics to be removed to pull the gearbox. Watching some of our matches during Einsteins 2017 showed the issues, we were able to kind of cover it up when we only lost one side by looking like we were trying to play defense (typically at that stage in the match we were).
Essentially, we were constantly losing control of our 4 cim drivetrain in 2017.
We believe the 775s helped solved the issue by spreading the load across 8 motors rather than 4.
I urge everyone to make their own list of engineering driven (and if possible quantitative) trade-offs.
That said… every time 148 does this kind of analysis we can’t get past the reliability concern. Cory hit it on the head:
The 775pro is a great motor. For the drivetrain, we prefer the reliability of a sealed-case motor which in our experience and testing provides a reduced risk of failure in that application.
Note: I’m not saying 775pro drivetrains will fail… I’m saying they have an increased risk of failure, and require additional levels of complexity (i.e. software limiting, etc) to mitigate that risk.
For what it’s worth: the Robowranglers will continue to use CIM or MiniCIM powered drivetrains for the foreseeable future, and we will breathe a small sigh of relief when our partners do the same.
-JVN
We burned out one 775 Pro motor on our drive during the regular season. This was at our first district event (Southfield) and was caused by a programming issue we had in a match that caused the robot to drive into a wall for more then half the match.
We had one of our 550 steering motors burn out at our third district event (Windsor), as something caused the VP it powers to jam (we think due to poor initial assembly).
Our drive performed poorly at our second district event (Milford) due to an MA3 encoder failure on the back-left module. We speculate this was due to ESD, but really have no proof of that, just deductive reasoning that nothing else appeared wrong with any wires or anything mechanically involved. We would likely switch to using Mag encoders or some other solution in the future to avoid using the MA3, which is more exposed to potential issues like that.
Those were the only failures in the competition season. We have had three drive motors fail at off-season events this year, however all had the fans torn apart on the inside. We have at least one occurrence that we think we can trace back to a cube landing right on top of the motor and likely slamming the wire leads further into the casing, interfering and imploding the fans. So we think the cause of these motor failures in the off-season has been due to poor design on our part of having the vertical mounted motors so exposed to contact with things.
If we were to run this or a similar swerve in the future, we would definitely find a way to more soundly protect the top of the motor. In a skid steer drive, this generally would not be an issue at all.
I will say that overall at competitions, even going through playoffs, we never really saw our drive motors heat up excessively. The steering motors got way hotter (we will be tinkering with ratios for that if we do it again). The only times our drive motors really got hot was after a lot of practice cycles at the shop.
I know that these motors will degrade over time and use, other teams have tried to post their findings and experiences. We have not done much testing to quantify this at all as of yet.
For those curious, the drive was 6x 775 Pros total single speed, with a 12:100 32DP stage, a 16:26 20DP stage, and a 15:15 bevel gear stage. Overall reduction of 13.54:1, and a theoretical free speed of 18.1fps with 3" colson wheels.
We would likely run a little slower theoretical free speed in the future, but of course this depends on the game.
I’ve burned all 6 CIM motors in a 6 CIM drive before, yet we didn’t really have any issues burning motors in our 775 pro drive this year (we may of broken a fan at some point). I recognize that these are isolated experiences and I still think a CIM drive train is more reliable for the average team.