toughbox mini mecanum

[coachgallina]

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

[asid61]

Originally Posted by coachgallina

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

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.

[Chris is me]

Originally Posted by asid61

11:84 with 6" wheels is good

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.

[asid61]

Originally Posted by Chris is me

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.

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.

[Hoover]

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.

[Ether]

Originally Posted by asid61

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.

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.

[jman4747]

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.

[GeeTwo]

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.

[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.

[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.

[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.