toughbox mini mecanum
 

I've search the forum and I haven't found anyone who has done this. We can't be the first!

We have 4 Toughbox Mini transmissions that came with AM14U frames. They take two CIMs each, but if we put only 1 Cim on each one, can we use them for mecanum, given the gear ratios and all?

We are on a limited budget but we might be able to do it this way.

Re: toughbox mini mecanum
 

Not a horrible way to go depending on your wheel size. Just make sure that you calculate out your FPS with our desired wheel size to make sure you have the output to your teams liking.

Re: toughbox mini mecanum
 

Not exactly what you are looking for - but we did run 4 toughbox nano's last year with mecanum's on the AM14U. We used 8 inch mecanum wheels.
It looks like the gear ratios for the nano and mini are about the same so you should be fine there, and we ran one CIM per corner like it sounds like you want to do.

We had to add some gearbox mounting holes and bearing holes in the chassis at one end, because we cut ours to a non-standard length.

We understand the limited budget :)

Re: toughbox mini mecanum
 

We are trying to find 6 inch mecanum wheels for the project as our simple test shows that 4 inch wheels are going to hang because of the length of our chassis when going over the scoring platform.

I'm the poor coder who is *just going to make this work*! The builders intend on covering up one motor mount with duct tape or something like that.

If this works I am going to endorse this method to small teams who are considering, but out of reach of, the 750 dollar upgrade kit.

Re: toughbox mini mecanum
 

It can definitely work. There are three ratios I would consider, 8.46:1 (included), 10.71:1, and 12.76:1. Depending on your strategy, that is your choice to make.

8.46:1


10.71:1


12.76:1

Re: toughbox mini mecanum
 

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?

Re: toughbox mini mecanum
 

Our team did AM14U with Touhgbox Nanos last year. We did have some difficulties with poor tolerances in andymark machining 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.

Re: toughbox mini mecanum
 

has anyone attempted to use the vexpro single reduction clamping gear box? could you get away with this set up. http://www.vexrobotics.com/vexpro/mo.../217-4156.html
11:72 set up? with 8" wheels

Re: toughbox mini mecanum
 

11:72 with 6" wheels is a free speed of 21fps. Way, way too fast. Don't even try 8" wheels there.
11:84 with 6" wheels is good, but if you want to use the clamping gearbox you'll need two stages of reduction. Going 12:72, and then 16:22 via some sprockets to a versablock will net a free speed of 16.8fps, perfect for a 4-cim drivetrain. I don't know how mecanums will affect efficiency or handling at that speed though. You can go to 11:72 for the first stage to back down to 15.3fps if you want.

Re: toughbox mini mecanum
 

This is still way too fast for a mecanum drive in this game. There's no need to go 15 feet per second when your biggest sprints are 1/4 length of the field... you're just wasting current. Mecanum drives do not work as well at very high speeds as west coast tank drives do.

I think for even 6" wheels, to use the clamp on gearboxes you really have to use the second reduction add-on to have good performance. Unless you're racing for the middle bins (and even then...) I would shoot for the 8-11 FPS range this year, but that's just me.

Re: toughbox mini mecanum
 

What do you define as a "very high speed" for mecanum? For me, high speed WCD is 18fps+ on 6 cims. For 4 cims, 17fps. Is mecanum actually that limited in terms of speed? I thought I read that they could go at least 15fps.

I don't think acceleration is a problem. A 15fps bot's time-to-distance is as good as or better than a 10fps bot for anything more than several inches. So you're not wasting current by running at a higher speed- it's very well used current.

EDIT: See here: https://www.youtube.com/watch?v=YbluSr2Mf5Q That's a 15.4fps mecanum bot.

DOUBLEEDIT: I would test both speeds actually. I have never seen anybody even try a mecanum geared for more than that 15.4 one I linked above, so I would try 12:72 x 16:22 AND 11:72 x 16:22 AND 12:72 x 16:32. I would be very interested to see actual results from a team that has tried a fast mecanum.

Re: toughbox mini mecanum
 

This is wonderful help, thanks.

Just for the record, If I had it to do I would strongly consider slide drive. I think mecanum will vibrate too much and if the robot (basically a fork truck) is carrying a stack of totes they could vibrate off.

We won't be having 4 independent suspension wheels with this setup. So that is not going to be a solution.

We also won't have encoders, but I think for us using the built in libraries this is not an issue.

But the drawback of slide drive is that I think the slide wheels will get caught on the lip of the loading platform even knowing it is tapered. In the best case it may cause an unwanted bump.

Re: toughbox mini mecanum
 

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.

Re: toughbox mini mecanum
 

We will be using the clamping gearboxes and 6" mecs with a 20t on the CIM (with 8mm-1/2hex adapter) to a 64t gear. Then the 64t gear's shaft attaches to a #25 16t sprocket which is linked to a 48t sprocket attached to each wheel. The 6" mecanums are attached to WCD bearing blocks tensioned with the cam.

Re: toughbox mini mecanum
 

That certainly does work - that's how we did our robot last year. We used 6 inch mecanums and (I believe) the 10.7 TB minis Using 4 inch mecanum with the 8.45 is a bit lower, but that's good because you'll be carrying a heavier load and this is a slower-moving, though probably much faster scoring, game.
We were conisdering mecanum for our drive this year, but we were worried that the handling would change too much as we picked up the load (we're lifting outside of our chassis). The drivers even noticed it a bit just when we lowered our pickup arm on the practice 'bot (which was wooden and rather heavy) last year. We've opted for an H drive so that at least the direction of thrust will be what we expect, even if the accelerations shift a bit. We're figuring on putting the strafe wheel at the COG when we're carrying four or five totes (we're going to do up to six) so that it behaves well when carrying a full load (during scoring) at the cost of being a bit off-center when empty.

Re: toughbox mini mecanum
 

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.

Re: toughbox mini mecanum
 

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.

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.

Re: toughbox mini mecanum
 

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

Re: toughbox mini mecanum
 

But much less expensive than four shifters!

Re: toughbox mini mecanum
 

Who said anything about shifting?

Re: toughbox mini mecanum
 

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.

Re: toughbox mini mecanum
 

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.

Re: toughbox mini mecanum
 

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.

Re: toughbox mini mecanum
 

[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... :)

Re: toughbox mini mecanum
 

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.

Re: toughbox mini mecanum
 

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?

Re: toughbox mini mecanum
 

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

Is that really what you meant?

Re: toughbox mini mecanum
 

Re: toughbox mini mecanum
 

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.

Re: toughbox mini mecanum
 

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... :confused:

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. :)

Re: toughbox mini mecanum
 

Actually, I was presuming that since Hoover put the throttle control on the Z slider, that it was going to be adjusted during the match, so that you could switch (or actually grade) from "full speed" to "parking maneuvers". To reduce speed through gearing while allowing the high speed would require a shifter, no?

Re: toughbox mini mecanum
 

Yeah, ya know, I am not sure we are seeing how small of space we are in this year. First off we have half the court. Then quite a bit of the court is occupied by playing pieces. And then sharing this space with two other full size robots.

Then there will be things that will be detrimental to a team

1) Out of control driving into objects they don't want to hit.
2) Out of control steering into objects they don't want to hit.
3) Not having the speed to get to objects
4) Not having the finesse to manipulate objects
5) Competition for space or objects by other robots on their own 'alliance'.

All this shows that the game goes fast, slow, fast, slow...

The ideal robot should be able to move in all directions either fast or slow. If mecanum is used then does that mean that shifters are also needed for all 4 transmissions. What a nightmare if just one of the fails, then 2 above is probably going to happen to that team. Shifters are out of the question for out team anyway so my solution then is voltage regulation, i.e. limiting top voltage in a way convenient for the driver (to be determined).

Re: toughbox mini mecanum
 

What does "high speed" mean to you, in the context of this game? How many feet per second?

Re: toughbox mini mecanum
 

The point I was making (for the benefit of rookie teams reading this thread) was simply that reducing top speed by limiting motor voltage does not provide the same kind of fine control of slow small motions that is obtained by reducing the top speed of the bot via gearing.

Gearing a bot for a top speed of 18 fps and limiting the voltage to 6 volts does not provide the same fine control of slow motions as gearing the bot for 9 fps.

Re: toughbox mini mecanum
 

I suspect that most of this game will be played at speeds of less than 1fps. In this context, 2-4fps is high speed - what you'd want to use to carry that stack of totes over to the scoring platform - though you may make a dry run or a single game piece trip faster. Except in the first 30 seconds of the match, anything over 8fps is likely to cause more un-scoring than scoring.

Granted, a shifter is better at fine maneuvers in low gear, but its real purpose is to improve acceleration at low speeds.

Re: toughbox mini mecanum
 

I'm having trouble parsing your post.

Let me rephrase my question: For a robot with a single-speed gearbox, what do you think would be the correct top speed gearing for Recycle Rush?

For Recycle Rush, improving fine control of slow motions is a "real purpose" of gearing for low top speed... even arguably a more important purpose than acceleration.

Re: toughbox mini mecanum
 

More than I've seen in a number of years, ideal top speed is dependent on what a team's strategy is. Our strategy will require getting to a couple places on the floor quickly, so we have geared for a "high" top speed, which is still significantly slower (11 fps) than last year (17 fps); we are also forgoing the shifter. If another strategic approach is taken, 8 or 9 fps maybe plenty or even too much. There are legitimate strategies this year that require 0 fps.

Re: toughbox mini mecanum
 

To answer Hoover'squestion about the Toughboxes, I don't think you will have any problem using them with mecanum. Good for you and your team, for using the resources you have in a smart and creative way.

Re: toughbox mini mecanum
 

The AM14U2 is a nice package. But for FRC purposes what I would really like to see is a "toughbox multi". It would be similar to the mini but would come with alternate gears to give a team a choice of speeds.

When I began this reply I was going to say that I wish this chassis came with shifting transmissions but I know that is more expensive.

In either case, changeable gearing is a great teaching tool. The first year our mechanics mentor drew gearing on the board and asked us which would be for speed and which would be for power, more than half of us (mentors included) got it wrong.

Re: toughbox mini mecanum
 

Of course, it depends on your game strategy. For a robot that stacks on the field and carries the stack to the platform, and does not plan a bunch of trips back and forth from the alliance wall to the landfill, I think 6fps is about right. We plan to either carry a bunch of crates from the human station to the nearest platform, or clear the landfill (at least of the upright totes; we'll work on a tote-flipper after we get the forklift functioning well enough to tune the software practice driving). That said, we're gearing at about twice this speed (13fps), because this is our team's fourth season, and we've mis-analyzed game play three times so far. We want the flexibility to adapt to the game as it turns out to be played as opposed to being optimal for what we expect.