Hey CD i am a member of team 1768. We have been toying with the idea of mecanum drive this year and have read tons of of threads here on CD about it. One thing that was brought to our attention today that we have never heard here on CD is that it kills your battery pretty quickly. It makes sense that driving 4 CIMs would kill the battery faster than two. What has your experience been with mecanum drive and power consumption?
Although I have not run a mecanum drive, I have been involved with a holonomic (omniwheels), which shares pretty much all the characteristics. I do not recall any battery drain issues.
My former team ran 4 CIMs all the time (except last year) without a problem, so I wouldn’t worry about it. Mecanums also have a relatively low coefficient of friction (because of the rolling), so they won’t run into many power sucking situations (like tripping your breakers).
I don’t know how you found out about this, but it isn’t true unless the battery is faulty. 4 CIMs in the drive should last all match and then some on one battery, and throw in other motors and a compressor for good measure. One match shouldn’t drain the battery.
We ran one in 2005 as a test, and never had an issue with the battery.
I think what it really boils down to is how you are running for CIMs. obviously if you are stalling 4 CIMs you are at something like 428 amps, WAY over the breaker. If you can run all four at 15 amps each then you should fine from the graph we were looking at earlier.
I am still interested in hearing about other teams experiences with running 4 CIMs on drive preferably on a mecanum chassis but if not that is fine as well.
Pretty much right on. I don’t remember exactly where on the curve you want to design for with CIMs, but you do want to design them right.
330 ran a 4-CIM drive between 2005 and 2008; I’m not sure about last year. No issues whatsoever with batteries (other than keeping them all charged during hours-long practice sessions in '07 and '08). All the robots were 6WD.
But, in 2005, the competition robot was designed to accept a mecanum drive. The Kitbot was equipped with that system for testing purposes; it didn’t make it onto the competition robot. There were no battery issues on that robot.
With that being said… did it work out in your situation that you were running the CIMs with the desired current and still running at a decent speed? obviously you can design the CIM’s to run at any current but if doing that means you are going to drive at a snails pace then it is kind of pointless. Was the robot that you are talking about able to drive with average speed without drawing too much current or was it slow?
The mecanum robot was normal speed. The others were easily normal speed or fast. No breakers tripping, that I remember.
as an off season project my team built a t-shirt cannon. the cannon and assembly is mounted on a mechanism drive base. we have driven the drive base over 30 minutes on one charged battery. We’ve never had issues with battery life with 4 CIMS
My former team (team 935) has made many robots that use a mecanum drive, and we have never ran into any problems with running out of power. during testing of our robots before competition we could run our robot for a good 5 minutes at full speed before we started having power loss issues. One charged battery should easily last one round, and be able to run your mecanum drive along with your other motors as well.
As others have said, running 4 CIM’s shouldn’t be a problem for a battery. The key is to monitor your current draw in testing, and any areas where it looks high at full speed you need to take a second look at - It means you probably have some unnecessary friction in your drive train somewhere (aka something is out of alignment).
Last year, we used two CIM’s for a shooter. After the initial test and some adjustments, the current draw for those CIM’s was cut in half!
Diner,
The are a variety of reasons that can cause high current in a mecanum drive.
- The gear ratio you have chosen is too low and so the motors are not developing enough torque to drive. That puts the motor in near stall conditions.
- Something is binding in the drive system. You may have left out a required spacer or something else is causing the wheels to bind.
- Your design produces a lot of side load on the CIM motor shafts. These motors cannot handle side loads so any loading will cause high friction on the output shaft and eventual wearing of the motor bearings.
- One or two of the motors are running in the wrong direction. This is especially evident if everything (current and speed) seems normal if you lift the robot off the floor. While it is off the floor note the direction each wheel is turning. If coupled in a two motor transmission, try disconnecting one motor on each side and see if the current is still high.
Without seeing your design it is hard to determine exactly what might be the problem.
Last year we ran two motors for at least three matches. running four for a match won’t be a problem, though if one was careful, he would have another battery ready to go for the next match.
Our robot from last year ran 8" mecanums directly from 12.75:1 toughboxes (cantilevered shafts, two of which definitely experienced some side loading) along with an air compressor. That thing can run a battery down in 10-15 minutes if you drive it continuously and use the pneumatics enough to keep the air compressor running. Even in that situation, we weren’t particularly close to running out of power in one match.
we have used mecanum every year except 2009 (and even then used 4 cims), and have never had battery issues with that, it is only the compressor and vacuums that drain batteries much, unless the battery is bad
If you don’t do it right, yes it will kill your battery. We tried mecanums for the first time last year and were lucky if the battery lasted through one match. Do it right and your golden, do it wrong and you’ll really regret the choice of mecanum.
I’ve used Mecs except 2009.
I would also look and see whether each axle is in alignment with the other ones in a TRI-dimensional environment. Remember that Mecs run on the idea of vectors and so when these vectors are off, the stress on the wheels will increase greatly. I also point out that if you are testing the rubber rollers (plastic less so) on carpet, the frictional force applied to the out-of-line wheels is exponential.
Also, try doing some stress-tests and full power cycles on your battery (don’t hurt yourselves!). Batteries aren’t built for a strenuous 4 weeks and then mild-no work for the other 48 weeks. (Ahh chemistry.)
If you use Mecs and you can’t go to a machine shop, I highly recommend using the supplied chassis with little to no personal drilling. Vectors are fickle!
We have used Andy Mark 8" Mecanum wheels, direct drive on Andy Mark Tough boxes in 2008, 2010, and we will use them again ths year. If you are killing your battery it is not the fault of the wheels. To sum it up they work great.
Yes, that could be true, but (except 2009) I’ve never seen a competitive robot with a 2 CIM drivetrain. 4 CIM drivetrains are perfectly capable of running for several (2-3) matches without killing your battery. There’s almost no situation where I would say a 2 CIM drivetrain would be better than 4 CIM.
A mecanum drive is less likely to “kill your battery” than a traction drive, but even running a very high current single speed traction drive last year we would always make it through a match with 4 CIMs, 2 FPs, and a compressor running. Our batteries left quite low (maybe a second match was possible, probably not) but we made it just fine.