toughbox mini mecanum

[Hoover]

Originally Posted by Ether

I think Chris' point was that you don't need 15fps for this game. But you do need fine control at near-zero speeds, as when aligning the bot to place totes and bins. A bot geared for lower top speed will have better fine control at low speeds.

In the past, on our drive station I have been using the Z slider for a "throttle" control. Since it really sets the maximum Y value, it really should be called a speed governor.

The original reason for this convention was that when a new student driver took control of the robot they always ended up gunning it right away. With the slider they could get used to driving the robot fast. After a few sessions the students would max out Z and be comfortable with the controls.

I think this won't be maxed out this year. Instead the drivers will keep it from 50 to 75 percent. Even lower if it needs to be slowed to a crawl. This takes the guess work out of how far to push the Y control.

With the Toughbox mini's (standard) gear ratio, that is even more reason to govern speed this year.

[dougwilliams]

Originally Posted by Mecanum Wheel

Hey Doug! Just for future reference, it's spelled mecanum. It's an easy mistake to make. How did the custom bearing holes work out? Did you have to switch to chain due to the difference center to center?

My apologies, I did know the right spelling and I'll carefully blame my computer for "auto-correcting" to the wrong way.

Either way, we like them and have used them for the last 7 or 8 years, and don't subscribe to the pushing match theory, as our Mecanum drive is more maneuverable - we don't get in to many pushing matches.

The custom bearing holes in the chassis worked fine for us. We moved the nano's to the corner spots of the chassis. Two of the nano's were mounted in stock hole locations, and the other shortened end of the side railes we had to drill the nano's hole pattern there. We recessed for the bearings, and removed the two screws so that center bearing was just pressed up against the side rail. We did not use any chains or belts, all direct drive.

Although I would say we were primarily cantilevered, we did also cut bearing holes in the outer rails of the chassis and used long drive shafts and had some extra support on the outside of the Mecanum as well.

Originally Posted by evand4567

...and the lack of a tapped encoder mount, which, because of the poor machining, we were unable to easily remove and tap to add the encoder. It was a mess and very difficult to assemble, although it does, in theory, work. I would not recommend doing it. I'd recommend finding some other gearboxes or make your own.

Agree that having to tap the gearboxes is a weakness of the gearbox, but minor in my opinion. We VHB taped the encoders on last year (and didn't use them anyway) - which I would not recommend. If we decide to use encoders this year we will tap the boxes. I did take one apart last year, and it was simple and readily re-assembled. And, we duct taped the open sides to prevent debris from entering.

We had a rock-solid drive train all year with the (slightly modified) AM14U, Toughbox Nano's, and Mecanum wheels, had it up and running in a few days, and absolutely no flaws through the entire season. I would recommend it without hesitation. Although I'm sure some teams could easily have custom gearboxes, for teams with limited resources (as the original poster implied), I'm guessing that is a stretch and I base my recommendation on that.

[Ether]

Originally Posted by Hoover

In the past, on our drive station I have been using the Z slider for a "throttle" control. Since it really sets the maximum Y value, it really should be called a speed governor.

For purposes of fine control of speeds near zero, that is not as efficacious as reducing top speed via gearing.

[GeeTwo]

Originally Posted by Ether

For purposes of fine control of speeds near zero, that is not as efficacious as reducing top speed via gearing.

But much less expensive than four shifters!

[Ether]

Originally Posted by GeeTwo

But much less expensive than four shifters!

Who said anything about shifting?

[asid61]

Originally Posted by Ether

Who said anything about shifting?

I would assume that shifting would get you finer control over the position in low gear, but a very fast high gear, which may or may not be necessary depending on your strategy.
Fast is good if you are trying to get the cans in the middle quickly, for example.

[Chris is me]

Originally Posted by asid61

I would assume that shifting would get you finer control over the position in low gear, but a very fast high gear, which may or may not be necessary depending on your strategy.
Fast is good if you are trying to get the cans in the middle quickly, for example.

Fast is good, yes, but that doesn't always mean a higher top speed. If you're starting in between the auto and landfill zone, going for the center of the step, you're driving five feet. At distances this short, you can easily gear your robot so quickly that you actually lose the race to a slower top speed robot factoring in acceleration. Even if it's just a few milliseconds slower, you're giving up low speed precision to have a high top speed you'll almost never actually reach. Acceleration models are available on CD to help verify this sort of thing, just keep in mind that the constants people use in these models vary.

What I'm trying to say is that you can't really spout out old "rules of thumb" as fact this year, in a game that isn't open field and for a different drivetrain configuration. Especially when you have never built a mecanum drivetrain. If you give advice without qualifying your lack of experience, teams may be led to make decisions they otherwise wouldn't that could end up hurting them. I've made this mistake before and i'm sure teams have made missteps as a result, hopefully minor.

This might be totally made up, but I've also heard that strafing performance decreases at higher speed gearing. This is probably a function of roller efficiency, if this is the case. I lack experience in this specific aspect of mecanum drives, though.

[Ether]

Originally Posted by Ether

For purposes of fine control of speeds near zero, that is not as efficacious as reducing top speed via gearing.

Originally Posted by GeeTwo

But much less expensive than four shifters!

Originally Posted by Ether

Who said anything about shifting?

Originally Posted by asid61

I would assume that shifting would get you finer control over the position in low gear

I was responding to GeeTwo's implication that reduced top speed gearing requires using a shifting gearbox. For this game, that proposition is arguable. See Chris' post.

[Thromgord]

Originally Posted by Ether

For purposes of fine control of speeds near zero, that is not as efficacious as reducing top speed via gearing.

[EDIT: The below comments are referring to variable-speed gearboxes, not gearboxes in general. -Thromgord]

For this year's challenge, gearing systems are completely unneeded to control speed. Last year, our team had quite a bit of success by keying buttons on our controller to 'reduce speed by 50%' and 'increase speed by 50%.' It allowed us to pick up hypothetical* exercise balls with great precision, speed to the other end of the field really quickly, and then get to the precise place to shoot the hypothetical* balls into the goal. It wasn't even difficult to code!

*Our robot never actually did this, which is why I'm considering this "hypothetical."

It wasn't difficult to program, and while it added no extra torque, no extra torque was needed that year. Except for when we were getting pushed around backwards. You won't require much torque at all this year, though, so it shouldn't be a problem.

By the way, one comment I haven't heard is that mecanum wheels don't work as well under very light loads; we built a mecanum 'bot in the off-season two years ago, and without putting about 40lbs of weight on it (initial weight was about 60lbs), the mecanum wheels wouldn't work properly (it had difficulty going in even the vague direction we wanted to, and after blaming the programmer like we always do, we added more weight and it helped tremendously). I know that you'll have no trouble making a heavy-enough robot, but just be sure to keep this in mind.

I just realized that our team wasn't all that great last year. Ah well, we're off to a good start so far...

[asid61]

Originally Posted by Chris is me

Fast is good, yes, but that doesn't always mean a higher top speed. If you're starting in between the auto and landfill zone, going for the center of the step, you're driving five feet. At distances this short, you can easily gear your robot so quickly that you actually lose the race to a slower top speed robot factoring in acceleration. Even if it's just a few milliseconds slower, you're giving up low speed precision to have a high top speed you'll almost never actually reach. Acceleration models are available on CD to help verify this sort of thing, just keep in mind that the constants people use in these models vary.

What I'm trying to say is that you can't really spout out old "rules of thumb" as fact this year, in a game that isn't open field and for a different drivetrain configuration. Especially when you have never built a mecanum drivetrain. If you give advice without qualifying your lack of experience, teams may be led to make decisions they otherwise wouldn't that could end up hurting them. I've made this mistake before and i'm sure teams have made missteps as a result, hopefully minor.

This might be totally made up, but I've also heard that strafing performance decreases at higher speed gearing. This is probably a function of roller efficiency, if this is the case. I lack experience in this specific aspect of mecanum drives, though.

I am factoring in acceleration. I use a drivetrain calculator that was on CD a year or two ago to determine the effectiveness of different speeds. Of course, the "speed loss constant" is somewhat of a fudge factor (80%), but that's hard to avoid. If somebody could supply the Kro and Krv to use, that would be pretty useful, as right now I'm just using the stock values.
Five feet is more than enough for a 15fps bot to gain the advantage over a 10fps bot, and as the distance gets larger the 15fps bot gets faster. The crossover point is at ~2.2ft, when both a 10.5fps and 15.0fps bot take ~.41 seconds. Of course my constants are probably throwing it off a little, but if you could supply your own calculated numbers that would be really helpful.
Because we will likely need to drive quickly at the start of the match/ auton, I feel like the minor time advantage we get at distances lower than 2ft is not worth it, especially because at those distance we probably don't want to go too fast anyway.

[Whippet]

Originally Posted by Thromgord

By the way, one comment I haven't heard is that mecanum wheels don't work as well under very light loads; we built a mecanum 'bot in the off-season two years ago, and without putting about 40lbs of weight on it (initial weight was about 60lbs), the mecanum wheels wouldn't work properly (it had difficulty going in even the vague direction we wanted to, and after blaming the programmer like we always do, we added more weight and it helped tremendously). I know that you'll have no trouble making a heavy-enough robot, but just be sure to keep this in mind.)

Our robot last year weighed in at 75 lbs with the battery and bumpers installed, and had no issue whatsoever with driving in any desired direction. Was your robot properly balanced so that there was even weight distribution between all 4 wheels? Did you use a gyro for field-centric drive?

[Ether]

Originally Posted by Thromgord

For this year's challenge, gearing systems are completely unneeded to control speed.

It almost sounds like you are advocating driving the wheel at the same rotational speed as the motor.

Is that really what you meant?

[Whippet]

Originally Posted by Ether

It almost sounds like you are advocating driving the wheel at the same rotational speed as the motor.

Is that really what you meant?

http://www.chiefdelphi.com/media/photos/36127

[Ether]

Originally Posted by Whippet

http://www.chiefdelphi.com/media/photos/36127

I am well aware that this can be done.

The question I was asking Thromgord is whether his post was to be construed as advocating doing such a thing for the 2015 game.

[Thromgord]

Originally Posted by Whippet

Our robot last year weighed in at 75 lbs with the battery and bumpers installed, and had no issue whatsoever with driving in any desired direction. Was your robot properly balanced so that there was even weight distribution between all 4 wheels? Did you use a gyro for field-centric drive?

Yes for your first question, no for the second. If I remember correctly (which I likely don't), the wheels were spinning in place a lot, and when sideways translation was tried, the robot still just had its wheels spinning in place. Maybe if the wheels were turning slower, or maybe if our carpeting was different...

Originally Posted by Ether

It almost sounds like you are advocating driving the wheel at the same rotational speed as the motor.

Is that really what you meant?

No, not at all. I was intending to state that a variable-speed gearbox wasn't needed. I personally thought that I was being reasonably clear, but then again, I'm new here. Changing my first post to be a little more clear, now.