6 CIM Drivetrains... What's Your Experience?

[thefro526]

We ran a 6 CIM Drivetrain in 2013, and other than a couple of hiccups, it worked really well for us.

We were geared a bit on the higher side of things, 6:1 for an effective max speed of ~12.5-13fps. Compared to a 4 CIM Drive with Similar Gearing or a 4 CIM 2-Speed Drive, it seemed like our acceleration performance and/or general "Zippy-ness" of the drive was on par with most of the front-runners in the field.

We did experience current draw problems a handful of times over the course of the season, most of which were triggered by defense being played by, or on us. It's also worth noting that we had 5 other motors on our robot, (4) of which were RS550's, and one of which was an AM-9015, so if any of them were running (especially being pulsated or stalled) it didn't exactly help the problem. In either case, even though we thought that the drive should have popped the PD Breakers (40A) prior to popping the 120A main, this was rarely, if ever, the case.

After the season, we did some digging into the problem, and found that the cause was due to a misunderstanding (or assumption) made about the PD and Main Breakers. As Mark posted above, the 40A Snap action breakers are capable of handling some amount of current in excess of 40A for some finite period of time - which basically means that a drive can draw more than enough current to pop the 120A Main breaker well before any of the 40A PD Breakers pop.

If you reference the data sheets for the 120 (http://www.waytekwire.com/datasheet/...UITBREAKER.pdf) and the 40A Snap Action breaker (http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf) you'll be able to fairly accurately estimate when your breakers will trip, if ever, during a match. In the case of an aggressively geared robot (like ours) the drive motors can pull somewhere between 60 and 70a each, for a total of between 360A and 420A. If you reference the curves for the snap action breakers, you'll see that at 150% rated current (60A for a 40A) the breaker can last between 3.9 and 47 seconds, but the Main (loaded at 300%) should pop in 5 to 15 seconds. The numbers are even worse at 420A, the 40A PD Breakers will only hold for 2.2 - 9.2 seconds, and the main will last for 2.5 - 8 seconds. It's also worth nothing that these calculations don't take any other current draws into account, so if were running a 6 CIM Drive and had some sort of RS550 powered load pulsating (stop/start) or a compressor kick on, you'd more than likely spend the rest of the match with a robot turned off.

The easiest way to get around the current draw problem seems to be to gear the drive more conservatively, use a wheel with a lower CoF, or to avoid pushing matches... But none of those are really that 'fun', so the other method would be/could be to use a 2-speed transmission along with a 6 Motor Setup. If you were to gear for ~40A Current Draw at maximum load in low gear, you should see very few problems with extreme current draw - if anything, you may have issues with voltage drops and minor brown outs - but those are still better than looking at your robot while it sits dead on the field.

FWIW, circuit breakers are also very, very susceptible to temperature, so keep that in mind. If the venue is abnormally hot, or abnormally cold, or your robot is still warm from the previous match, the breaker performance can change. More often than not, a hot breaker will be able to hold less current, and a cold breaker is able to hold more. As we monitored breaker performance throughout the off season, I noticed that the main breaker was 'warm' to the touch after the majority of our elimination matches.... Anyway, should you decide that monitoring the temperature of the breakers is something you'd like to do, it's best done with proper supervision and safety equipment (Always) - and non-contact tools (think IR Thermometer) when possible.