fun with drive trains...

hello again everybody…

I haven’t posted anything on the forum for a little while (more like 9 months). I haven’t qualified for anything (but I did announce middle school division and finals at US nationals). I understand that this might seem elementary, especially for some of the more experienced teams, but I have a HUGE ratio of inexperience to experience this year (about 19 new kids to 2 veterans). Thus, I had a few questions about a very important part of the robot: the drive train. The poll is posted above (or should be), and I have a more specific question below.

The question is this:

This year, I had a 6-wheel drive train, with three on each side of the robot. I had the back wheels and middle wheels powered (all individually, none of them were geared or chained together). Last year, I had 6 wheels, and again, the front two were not powered. I’m drawing out a design now that chains together and powers all 6 wheels. How will this affect (if at all) the robot’s turning/ driving?

Empirical testing has shown that six wheels nearly always works better than four wheels. My assumption is that its due to a better distribution of weight, since friction shouldn’t change with more surface area.

As to your actual question, my team decided to power only the back two wheels out of 3 per side this year, but that was due to concerns about sacks jamming with longer lengths of chain reaching all the way up to the front of the robot.

For your inexperience issue, the best solution to that is more practice, particularly hands on practice. I would recommend not just building the drive train to build the drive train, but to consider all of the parts of the robot when building it. Some of my coolest drive trains couldn’t have lifts easily used on them. Sometimes shorter wheelbases are useful for this, as shown by gateway robots like Jack’s, which had 4 wheels total and was only 12 inches long.

I believe the turning would also be better because the vector forces would would be on the outside of the robot. With 6 wheels, all 6 wheels spin and scrub will reduce. with the back and middle wheels powered, those wheels would spin while the front wheels would sap force.

If the wheels were omni, you should have no worries with turning. The rollers on the wheels will definitely allow you to turn. However, i think if you power all 6 wheels i think the turning scrub would be worse. in this case, if you draw a circle, around all 6 wheels, the wheels would be closer to the front and back. With 4 wheels, the vector forces are closer towards the sides, making turning easier

As for inexperience, my club faces these things commonly (members swap from vex to ftc to frc/ vise versa). Personally, when i was inexperienced, the biggest help to me was learning my materials and watching other teams. I spent about an hour browsing through the vex catalog and i watched numerous games from round up, bowled over, clean sweep, etc. Once you start getting people from abstract to concrete, they can eventually do the reverse. As yonder says, hands on experience is critical. However, it’s hard to get many people working at one time, especially with a team that size. You need computers ready for programming, cad, experimenting, researching, fundraising, calculating, etcetera. Another important factor i believe is important is evolving the robot. Starting with a simple, modular robot, yet completely reliable should be something inexperienced members should be able to do. To stay competitive with that robot, programming and driver practice will give you the edge. As long as your robot is universal, you can continue evolving your bot to be more and more competitive.

I like Mecanum drive, because it only takes 4 motors and you can still strafe. You then have 6 motors to use on the rest of your robot.

I like our 6 motor H-Holonomic Drive.

If we get 2 more 4" omni wheels I would like to put those on each side for a 6 wheel tank drive section + holonomic.

Anyone else thought that this was gonna be another crazy driv thread? :stuck_out_tongue:
(taiwan drive, crab drive, crazy holo, tabor, etc)

We used mechanum for sack attack (4x393), initially geared for torque internally, then for speed, and then when our drive died in most matches, back to torque.

We assumed that mechanum would be helpful, as we saw strafing in front of the troughs as useful for descoring and blocking.

We will never make the same mistake again. The number of times we regretted this season not having a solid 6 wheel drive was countless. We tried to justify their use, however at the end of the season concluded that they were not worth it.

I am glad that there is no legitimate use for mechanums in Toss Up, so other mechanum horny teams do not make the same mistake as us.

Excuse my ranting, but heed this warning: Do not use mechanum!
Cheers.

Mecanum wheels are fantastic. 6 wheel drive is not as important as you make it out to be. A 6 motor drive is, but this can easily be done with mecanums. The traction provided by a 6 wheel drive is not so significant to vex as most people seem to think… Mecanums have served us well even with our gateway wallbot.

Few of the many people I have ever talked to have listed traction as why they use six wheel drives… The real advantage to a six wheel drive is having weight more evenly distributed.

It is not the friction which is a problem. We spoke with one team at champs who informed us that their 8 motor descoring machine could not push their 4 motor mechanum drive.

From our personal experience we found that they slowed turning as well as regular driving, and worked motors much harder. If we were pushed backwards, we would ride harder on our rear wheels, causing those motors to burn out quicker, and we would not strafe properly.

They were however, useful in autonomous and for descoring, and I will say that many teams used them effectively.

But from a team who struggled with them for a year, we would never use them again.

Generally speaking I’ve found mecanum’s incredibly inefficient in both strafing and regular tank driving. I would strongly advise teams to not use mecanums. While it’s entirely possible I “didn’t do it right”, I’ve never seen an efficient mecanum drive work. For example, I don’t recollect seeing any mecanum drives in the round robin finals, nor in the couple division finals video’s I’ve seen from last year.

I think Toss Up will see more holonomic drives however :wink:

I have to agree, I dont really like the vex mecanum drive (although I do see its use in other applications). I personally prefer and X-drive, same basic wheel layout as mecanum and about the same area taken up, but produces identical motion both when driving normally and stafing, unlike the vex mecanum system which strafes slower than it drives forward. In addition, the layout of the wheels creates a small increase in speed of linear movement as compared to a tank drive with the same gearing.

Cough 177 and 1575D cough :eek:

As much as I love the concept of Mecanum Wheels… This post could be my teams life story.

+1

I think the Singapore and Chinese teams are only capable of successfully utilizing the mecanum wheels. [http://youtu.be/GfWBkCLfq3U?t=7s]

From what I have seen, mecanums are very slow and not very good with strafing. Our H-Drive could strafe so much more cleanly than the mecanum strafe (our 2 motors VS 4 motors on mecanum). I have yet to see a beautifully perfected mecanum drive :slight_smile:

Now that we are on this subject, I might purchase the mecanum wheels for some testing :wink:

Your H drive was a 6 motor drive though… it’s 6 motors on H vs 4 motors on mecanum.

I was just talking about the “strafing” part of our drive train. We had 2 motors dedicated to our strafing, then 4 motors to our Tank Drive portion of the drive. So, with driving forward, we had the same amount of motors (4).

Ive found that the mechanums dont strafe as well as a strafe wheel. Prior to using mechanums, our strafe wheel allowed us to “drift” across the field pretty quickly. Although i do agree with the fact that you can distribute more motors to the rest of the robot by using the mechanums. We’ll most likely be using a 6 wheel drive this season.

The Singapore and Chinese teams in the Middle School division used mecanum wheels well. They had 6 motors on HS and they were unstoppable.

Caleb does that mean I don’t have to buy your team omni wheels? :smiley:
I like saving money.