Our team is nearing design completion on an off-season drivebase project… We’d decided to use a sheet metal drivebase a while back, but have been toying with different gearbox/transmission ideas. We’ve used Super Shifters for the past 4 years… and while we did consider trying the new Sonic Shifter and the Ball Shifter, we decided to broaden our experience by trying a single-speed 6 CIM drivetrain.
So, have you tried using 6 CIMs on your drivetrain? Do you know of other teams that have? What top speed have you geared for? Did you struggle with too much current-draw/voltage-drop in a pushing match and/or in acceleration? If so, did you use any software limits to minimize it (i.e. a voltage ramp, mentioned here, or a cut-off if the robot detects a voltage drop)? Also, what did your robot weigh?
We’re currently planning to gear for about 10.5 fps (about a 10.5:1 reduction), but are primarily concerned about drawing too much current in a pushing match. If we have current draw issues we can either try software limits/overrides and/or try removing the two “extra” CIMs or swap them for miniCIMs or RS550s, which each draw around 2/3 the current of a CIM.
So far, I only know of 254 using 6 CIMs in 2013 with their 2-speed+PTO gearbox and of 610 using them this year in their 1-speed+PTO gearbox. I heard though that when 610 set aside their climber mechanism they also removed the 2 “extra” CIMs… according to their website they designed for a top speed of about 9fps.
Any recommendations, cautions, or further information?
We used the 7:1 (84T) single speed gearbox from VexPro, on a sheet metal wide 4WD chassis with 4 inch AM performance wheels and roughtop. The only thing I would change is our choice of tread – I think nitrile would have held up better. We ended up changing treads after every match in which we played defense. That was most of our matches because our scoring mechanism suffered from complexity (another story). The AM 1 inch wide roughtop that we used de-laminated (i.e., tread separated from woven structure) whenever we got into a good pushing contest. We ended up winning several of those contests anyway, but spending several minutes changing wheels in the pits was not fun. Especially in District competition with frequent quick-turn matches when there is no time to return to the pit at all.
TL;DR – if you use 6 CIMs, make sure your treads can take the torque.
We have used six CIMs in our drivetrain. However, our drivetrain never used more than four of them at full power at once which probably helped avoid any current/voltage problems. No special software was needed.
Our gearboxes were geared at 9.7:1 although the effective gear ratio from motors to groud speed varied somewhat. Either way, we probably should have gone with slightly more reduction.
We were a few pounds underweight and couldn’t push worth beans, which was expected given our design. As for random tips, stability is more about how far away your points of contact are from each other than about how many there are.
We used 4 CIMs 2 Mini CIMs on our offseason bot this year and loved it. Never had any current issues at all. We ran a 6wd WCD with 4"x1.5" Colson wheels through a WCP single speed I modified to fit a 3rd motor geared for 14 ft/s on a belt drive, and it was beautiful. In the upcoming season, I’d like to run a similar setup using modified WCP shifters at 16 ft/s and 6 ft/s. We were extremely light, but were still able to push 120 lb robots without a problem and never tripped a breaker.
6 CIM drive trains are a great feature, but I don’t know if they are worth it unless you have a power takeoff. With the power takeoff, you get 50% more power than with 4 CIMs, and you run it single speed and still have decent traction and decent pushing force.
However, if there is no power takeoff, it’s easy to justify. With 2 CIMs, you can have a traction limited slow gear easily, so you won’t have better pushing power. The only real benefit would be acceleration, but even then, the added weight of two motors is really, really hard to justify just for improved acceleration.
I think the main reason the GDC went to 6 CIMs this year was because of climbing. They wanted teams to have 4 CIM drives and 2 CIM + 2 miniCIM climbers or 6 CIM drive with PTO. I predict that if next year we don’t see a game that involves pulling something heavy, lifting something heavy, or something that requires a ton of power, we’ll be back to 4 CIMs.
What’s the math for a traction limited drive? Accounting for the fact that the CIMs are run through motor controllers with a 40A max (probably closer to 20-30A in competition), I have trouble imagining a traction limited drive with blue nitrile wheels.
While we have never used a 6 CIM drive before, we did once try a 4 CIM + 2 Large CIMs (FP CIMs?) drive system back in 2006. As a whole the system worked well but wasn’t really anything special to talk about as far as performance (the drive was a fairly low geared tank tread system).
A CIM motor can draw more than 40 amps. The victors are rated for 60 amps continuous draw and are rated for 120 amps surge current.
During a pushing match, your CIM motor will draw much more than 40 amps. You can draw 80 amps for 1.5-4.9 seconds and you can draw 70 amps for around 8 seconds on the snap action breaker before they start clicking.
You should probably read the datasheet for the breakers.
If you would like to do the math, search for JVN’s mechanical design calculator, but many teams base their low gear speed at the point where added more torque would just make the wheels spin. This is why nobody puts together 10 motors on a gearbox and just pushes everybody around.
We had 6 CIMs this year, in a two speed configuration. The drive was awesome and we really enjoyed the pushing power, speed, and acceleration of the extra motors. However, we did a lot of pushing this year, and the constant strain on the batteries is resulting in them not being able to hold much of a charge, to the point that we cannot re-use this year’s batteries next year.
I’m not really much of an electrical guy, but my interpretation of the data sheet shows that you could draw 480 amps for 5 seconds. However, I will guarantee that with 4 CIMs geared at 5.5 fps, you WILL slip way before the motor is even close to stalling, regardless of which wheels you use, blue nitrile, AM high grip, or any other.
I think that’s a legitimate reading of the graph - and if you can pull that much power then yeah you would be slip limited at that gearing - at least for 5 seconds…
An equally pertinent question might be, what steps can a team take to minimize drivetrain mechanical power losses? There are many factors to consider, including alignment, lubrication, fits/clearances, materials, etc.; however, the concept is simple. If it rolls straight and easily, the losses are low. Many of us learned this as Cub Scouts building pinewood race cars.
80% to 85% is a figure often “plugged in” when using drivetrain calculators (e.g., JVN) – however, some teams build drive trains that are better than that, and quite a few build them worse.
One good test to try is measuring the free current of the completed drivetrain (wheels up).
