So my team wants to build several different chassis this fall to find which we think is best. Anyways, I am leading the 6-CIM drive-train group. The idea I’m working on is a 6 wheeled design with a dropped center wheel direct driven by a 3-CIM gearbox.
I went on VEX to look at my options for gearboxes, and now I’m stuck. I found these gearboxes online, and I think they would work well. The only problem is that I have no idea what gear reductions I should buy. Basically, I want it to be two speeds, one for driving fast, and one (more importantly) with a lot of torque to push other bots around. What’s your opinion on what I should get?
And thanks in advance to anyone who posts an answer
You need more quantifiable goals for the drivetrain. When you can put things in numbers, it’s much easier to choose. For example, the calculator that thatprogrammer posted has a rough calculation on time it would take to cover a certain distance at every gear ratio. If it was 2013, a logical choice would be to pick your high gear to minimize the time between the feeder station and your shooting location. If it was 2014, what distance would you want to cover in the least amount of time? Or would there be some other factor that you would want to maximize your drivetrain toward? There is no correct answer; it is both dependent on the game and your strategic choices for the game.
It’s great to see that your team wants to experiment in the offseason with different drive trains! My team tried building a 6 wheel WCD in the off season a few years ago and probably wont go back for a long time.
It’s hard to say what gear ratios you need without knowing anything else about your drive train plans. Are you planning to have a directly driven center wheel? Live axle/dead axle? And most important for determining your gearing is what wheel size are you planning on using? Before we recommend a gear box or a gear ratio we need to know more about your set up.
If you are asking for generalized speeds for how fast your high and low gear speeds should be, it is generally considered for a low gear the gear ratio should be set for somewhere around 4-6 (maybe even a little higher) feet per second. And a high gear around 14 -16 feet per second. (Some teams gear their high gear for 18 - 20 fps even). It really depends on the game to determine the exact ratios. If you feel that there will be a lot of defense like this last year then having a low gear on the bottom end could be of value, but if the field is also open and you need to move fast having a upper end of the high gear values is also very realistic.
First off, thanks for that calulator, it’s very helpful.
And now for a another question.
The only thing that seems to change between the 2-CIM and 3-CIM gearboxes is the Force @ 40 amps box. (If I’m wrong in my next assumption, please feel free to correct me. I’m a mechanical engineer, not electrical) Now if there are less motors on the robot (I.E. 4 CIMs vs. 6), wouldn’t there be a higher amperage for those less motors? If so, wouldn’t the 2-CIM gearbox be producing more than the 3-CIM?
My plan so far is a 1/8th dropped center wheel direct driven by the gearbox and the two other wheels either belted or chained to the center wheel. All wheels would be 6".
What I really want is a gear ratio that can give me a lot of torque. I’ve noticed over my years in FIRST that most teams either get pushed around or stalemate with other robots on the field. Teams that can push others around are always recognizable and are almost always picked for alliances, even if it’s just to play defense (Team 2013 was a perfect example of that this year). And even in games where speed is important (2013), I always see cases where being able to downshift would have given a team a boost to victory.
True. AN advantage of most 3 CIM gearboxes (If not all) is that you can also test using a mini CIM and two CIM motors. Test and see what amount of torque works for you.
The main breaker has to be over 120 amps for a while before it trips. If I recall correctly, it will take about 40 seconds for it to trip at 240 amps and about 5 seconds at 500 amps. The motors in your drive train will probably each spend most of their times under 40 amps of current in a match.
The CIMs don’t need to run at full power all the time, only when they are in a pushing match or similar situation. When a robot is at rest, the CIMs actually use no power.
The main breaker actually trips based on temperature. Saying xx amps will trip the main breaker in xx seconds means that xx amps for xx seconds will heat up the breaker enough for it to trip. This time will be shorter if the breaker starts hot and longer if the breaker starts colder.
So if it’s about temperature, would the various components near the breaker on the bot heating up affect weather or not it trips? Also, once the breaker does trip, is it true that it becomes more likely to trip again at a lower temp?
Very little heat is going to be transferred through air to the breaker. So no, as long as your breaker is thermally isolated you won’t see any difference.
To your question about the 40 seconds being cumulative, that number is an estimate based on continuous stall conditions. If you stall for 20 seconds, then wait for a minute and stall for another 20 seconds, it likely won’t trip but it is absolutely possible. Giving the breaker time to cool down will help, but each breaker is going to be manufactured with slight differences. Doing your best to avoid stall conditions (ie. ensure that you wheels will slip on the carpet instead of freezing) is the best thing you can do to avoid tripping.
As for your second question, I don’t believe there have been experiments to track the propensity for earlier tripping after a breaker has tripped once. I believe, though, that anecdotal evidence suggests that they DO trip quicker after tripping once.
So if the breaker trips, would I be better off letting it cool and then continuing on or should I try and replace it? The answer to this would factor into the placement of the electronics (I.E. how hard it is to get to each part)
Re: main breaker trips, our team had a lot of experience with this over the past season with an aggressively-geared 6-CIM SS drive. Way more than we would have liked. Here’s a rough outline of what we discovered:
Six stalled CIMs will pull around 40 amps/CIM from a fully-charged FRC battery. Note that this is significantly lower than the rated stall current of a CIM - this is because as you draw more current, the effective voltage of your battery drops. The standard FRC main breaker, from our testing, could sustain this for anywhere from 10-30 seconds. The effects are “quasi-cumulative,” in that since the breaker is triggered by heat buildup, and heat dissipates over time, you regain some of that time when you remove the current draw, but in our experience, not very much over the time scales involved in an FRC match.
Six slipping CIMs on our robot (120lbs w/ bumpers, drive geared 6.11:1) pulled around 30 amps/CIM and could be sustained almost indefinitely without breaker trip over the course of an FRC match. Our robot in this configuration was just on the very edge of being able to slip the wheels, so this was essentially the “worst-case” scenario for wheel-slipping, and other configurations would likely draw less.
What can be discerned from this is that as long as you can be absolutely sure that you will not stall your drive motors while pushing, you should not have breaker problems with a 6CIM drive. Unfortunately, figuring out precisely what gearing gives you that certainty is nontrivial, as it depends critically on several subtle factors (how much current your robot is drawing apart from the drive, how long the wires leading to your drive motors are, how fresh you expect your battery to be in-match, among others). You could try to calculate this beforehand (the math is not hard), but I’d recommend simply erring on the side of caution and picking a relatively slow low-gear (no faster than 7FPS*). Do that, and you should have no worries about breaker trouble.
Your fast gear can be however fast your drivers feel comfortable with. Just make sure you do not get into a pushing match while in your fast gear, or you can (and will) trip the breaker.
How I arrived at that number: Our robot was just able to slip the wheels at 14fps gearing while weighing 120lbs. Our wheel COF was 1.1. For a “worst-case” scenario, assume a 150lb robot with a 1.5 wheel COF. Both factors scale linearly with required torque, so our gearing would have to be 14FPS * (120/150) * (1.1/1.5) = ~8 FPS. Reducing this to 7 FPS gives you a bit of overhead for not-quite-charged batteries/other stuff pulling current/whathaveyou. You could probably claw back a bit by having better (read: shorter and/or fatter) wiring than what we had, or by having exceptional drive efficiency (though, subjectively, I thought ours was pretty good on the robot in question).
Using the calculator from earlier in the thread, the gearbox I’m currently thinking of buying is going to have a top speed of 4.8FPS, so I guess that works out fine for me.
The only thing that worries me is that the calculator says the maximum current draw at maximum is 155 amps. Inj your opinion, is this number realistic or is it underestimated?
Also, your FIRST (hah, pun) hand experience with a 6-CIM chassis is very helpful.
I don’t know how detailed the calculator you’re using is, but that sounds realistic. That’s not at all a worrisome number, either - the FRC breaker can take 155 amps for essentially a whole match. During the times last year when our robot was not running into problems, we were pulling ~180 amps in pushing matches, and that was about the worst-case possible while still slipping the wheels. It was when we got near the 240-amp mark that things went south, and I only ever saw that type of current draw when the motors were stalled. You will not stall your drive motors when geared for 5FPS, barring some sort of cataclysmic failure.
Make sure that if you’re going for a 4.8FPS gearbox that it’s a two-speed. That’s far too slow to function well as a single-speed drive.
