View Full Version : Mecanum wheels
heisenburger
06-01-2014, 17:12
Hi, I was just wondering what is the purpose of the mecanum wheels, how do they work and are they worth the $400 for the set of 4, also if the sell them somewhere else cheaper than that could you give me the link?
Mecanums have some nice maneuverability. They are able to strafe, for example. (you can find some videos of it on YouTube)
However, mecanum wheels will be weak for pushing power and can slide around.
You'll likely be pushed around a lot on the field if you use them
atucker4072
06-01-2014, 17:17
Hi, I was just wondering what is the purpose of the mecanum wheels, how do they work and are they worth the $400 for the set of 4, also if the sell them somewhere else cheaper than that could you give me the link?
Where have you looked?
Also make sure you know what size you need. Don't use mecanum wheels just for a cool factor. Mecanum wheels are great for maneuverability, but when it comes to pushing they are worthless.
Hi, I was just wondering what is the purpose of the mecanum wheels, how do they work and are they worth the $400 for the set of 4, also if the sell them somewhere else cheaper than that could you give me the link?
My team used mecanums last year and they worked well for sliding into place at the feeder station. However, they were terrible when we tried to cycle and were pushed extremely easily. This year may not be the best for mecanums simply because of the lack of safe zones - you can be pushed at any time with nowhere to retreat.
The choice is completely dependent on your team's strategy. Is your shooter reliant on consistent positioning? If so, you may not want to use mecanums because of the pushing issue.
In this game the only big advantage I see for them is with catching the ball from the truss, so unless that's a big part of your strategy, I wouldn't get them. Another downside to them is that they're quite heavy.
brennanvhoek
06-01-2014, 18:23
Mecanum work well if they fit your game strategy. You can get a full set for much less then $400. Vex pro sells a set of 6" for 159.99 http://www.vexrobotics.com/vexpro/wheels-and-hubs/mecanum-wheels.html
Also, in my experience the vex wheels preform much better then the andymark equivalent.
jbsmithtx
06-01-2014, 18:29
Try VEXpro. They have very affordable yet durable mecanum wheels. The other posters are correct in assuming that unless you are trying to catch, mecanums might not work well this year... However don't believe they are easy to push. We had to play defense with our mecanums last year in matches when our shooter failed and were fine. We could block lots of teams. I know this is contrary to popular belief, but a properly done mecanum drive base can in fact be very strong.
For anyone that wants to question our pushing abilites, we have video...
Try VEXpro. They have very affordable yet durable mecanum wheels. The other posters are correct in assuming that unless you are trying to catch, mecanums might not work well this year... However don't believe they are easy to push. We had to play defense with our mecanums last year in matches when our shooter failed and were fine. We could block lots of teams. I know this is contrary to popular belief, but a properly done mecanum drive base can in fact be very strong.
For anyone that wants to question our pushing abilites, we have video...
Mecanum wheels, neglecting friction in the rollers, have an effective coefficient of friction that is a factor of sqrt(2)/2 reduced from that of a traction wheel made of the same material. You can verify this yourself by drawing a free-body diagram of a set of mecanum rollers in contact with the ground.
Whether or not this translates to "being pushed all over the field" depends critically on context (robot weight, weight/drive of opposing robot, weight distribution, manner in which you're hit, etc).
My team has used mecanum wheels for the last three competitions, and each time we had great success with our drivetrain.
The system is meant to give a robot high levels of maneuverability, and when it is allowed to move at high speed, it creates a formidable opponent. Having the range of motion provided by a mecanum drive can allow a driver to jump out of the way of onrushing defenders, all while maintaining a scoring opportunity
When discussing pushing and mecanum, it should not be forgotten that the goal of this drivetrain is maneuverability, not brute strength. Due to this, a pushing match with a robot designed for pushing will be lopsided. But that does not mean that this drivetrain is easy to push. I have seen our past mecanum drives stop six wheel drives dead, and win pushing matches. When we use mecanum, we maintain a balance of speed and pushing power, and ensure we are not compromising one for the other.
This quote sums the entire discussion up, and is good food for thought. "Mecanum drivetrains, like every other, have advantages and disadvantages" - Mr. B
Random42
06-01-2014, 21:19
I have a quick question about mecanum wheels. Can they be placed in a trapezoidal configuration?
Has anyone tried this before? How well does it work?
cmrnpizzo14
06-01-2014, 21:32
I have a quick question about mecanum wheels. Can they be placed in a trapezoidal configuration?
Has anyone tried this before? How well does it work?
Could you describe the configuration you are thinking of? Do you mean something like this:
---[]----[]---
[]----------[]
where the [] are wheels and you have the front wheels in narrower (or the back wheels)?
Try VEXpro. They have very affordable yet durable mecanum wheels. The other posters are correct in assuming that unless you are trying to catch, mecanums might not work well this year... However don't believe they are easy to push. We had to play defense with our mecanums last year in matches when our shooter failed and were fine. We could block lots of teams. I know this is contrary to popular belief, but a properly done mecanum drive base can in fact be very strong.
For anyone that wants to question our pushing abilites, we have video...
Our team used the VEXpro wheels last year, and I was very pleased with them. They lasted 2 competitions and multiple fundraisers until they started to fall apart. They are also quiet and light compared to the Andy Mark versions we have used in the past. We also had to be purely defensive in one competition and managed to place high because our mecanum drive allowed us to out-maneuver our opponents and get in front of them to block their shots.
Caleb Sykes
06-01-2014, 22:03
I encourage anyone and everyone who thinks that they understand mecanum physics to read the whitepaper given here: http://www.chiefdelphi.com/media/papers/2759?
Random42
06-01-2014, 23:01
Could you describe the configuration you are thinking of? Do you mean something like this:
---[]----[]---
[]----------[]
where the [] are wheels and you have the front wheels in narrower (or the back wheels)?
Yes. Something very similar to this. The back wheels would be about 12in apart and the front about 24in apart. The front and back wheels would be about 30in apart.
BEN35678
06-01-2014, 23:01
Our team used the VEXpro wheels last year, and I was very pleased with them. They lasted 2 competitions and multiple fundraisers until they started to fall apart. They are also quiet and light compared to the Andy Mark versions we have used in the past. We also had to be purely defensive in one competition and managed to place high because our mecanum drive allowed us to out-maneuver our opponents and get in front of them to block their shots.
Last year we used Andymark mecanum and this year we were looking at the vex pro mecanums, and a couple of our main mentors were worried about them cracking because they are plastic. Did you have anything that looked like it was about to crack or anything like that?
Our team has used a mecanum drive train for the last three years and we've had a lot of fun with them.
As the robot driver for the past two years, I can assure you they're wonderful to control, and driving becomes a lot easier with mecanum. :)
Controlling the a mecanum-powered robot is a lot like moving a character in a first person shooter video game, which I'd assume many students are familiar with already.
Couple a mecanum drive train with gyroscope technology and now your robot can always move relevant to the driver (No matter the orientation), which in my personal opinion makes controlling the robot even easier.
While a downfall may be that you lose a lot of pushing power, the versatility gained to out-maneuver the defense matches said downfall quite well.
Our VexPro mecanums have held up well. They survived one regional and some off season demos with no problems, and in our shop we typically run them on concrete. We are planning to use them again this year.
Last year we used Andymark mecanum and this year we were looking at the vex pro mecanums, and a couple of our main mentors were worried about them cracking because they are plastic. Did you have anything that looked like it was about to crack or anything like that?
Yes, our VEXpro wheels did crack, but that only happened this past December at a fundraiser where we were showing off our robot. We went to two competitions last year and had several other fundraisers and expos before they broke. I believe they are durable enough. You shouldn't have to worry about cracks during competition. I believe they also sell replaceable wheels for the mecanum if any of them seem to have significant wear or break.
Mecanum wheels are very nice for alignment of your robot even when the driver has not had a lot of practice time. I would imagine this year aligning your robot in order to gain possession of a ball would be important to some teams. Practicing gaining possession in a real-game situation is difficult for a lot of teams, so mecanum can be handy here.
Also, I always throw in my vote for don't believe that mecanum can easily be pushed around. Wait...am i part of the majority on this vote?? :yikes:
At the moment, my team is not a fan of using mecanum, unless it is undeniably the absolute and only way to win matches. With mecanum wheels you can lose up to 50% of your power when doing moves like strafing, whereas with swerve drive, all the power is used more efficiently. Mecanum wheels are indeed expensive, and I don't think that they're worth the price unless you are absolute that they are the best choice. From my experience here in the Californian bay area/central valley, the best teams stick with drop center six wheel drive treaded wheels. Why? They are inexpensive, offer the best traction, best pushing power, least complex in programming and build, easy to fix, and provide all the maneuverability necessary. Name one team that has made it to the Einstein field using mechanum wheels. None, because this has never happened in FRC history, but hey, there's always a first, and it could be anyone!
If your philosophy isn't all about winning, then have at it with the mecanum wheels! Mecanum robots are really fun to drive, too. Still, they are a pretty penny, but 100 bucks per wheel isn't a bad deal. I'm not saying that you can't win with mecanum wheels, many regionals have been won by these types of drive bases, but what I'm saying is that treaded wheels in general are a better choice in my opinion. Treaded wheels are also LIGHTER! And that could be the difference between passing and not passing inspection if you cut it really close with your other robot components.
thefro526
07-01-2014, 08:39
There's a bit of mis-information in this thread, and while I don't want to join in the debate, I'd like to set the record straight for any of the newer teams that might be using this thread for information.
Mecanum Wheels can be purchased from two different FRC Suppliers.
- AndyMark http://www.andymark.com/Mecanum-s/53.htm
AndyMark's Mecanums were first released for the 2007 Season and have been evolving since then. They're offered in FRC relevant sizes of 6" and 8" with a Standard Duty and Heavy Duty version of each. The Pricing Varies from ~$250 for a set of 6" Standard Duty to in excess of $400 for a set of 8" Standard Duty.
- VexPro http://www.vexrobotics.com/vexpro/wheels-and-hubs/mecanum-wheels.html
VexPro's Mecanum Wheels are relatively new compared to AM's Mecanum Wheels, but have proven themselves to hold up reasonably well in the FRC Environment. They're a bit Cheaper than AM's Mecanum Wheels and rely on a primarily plastic construction in place of Metal.
As far as Mecanum's implementation in FRC, they're a decent way to make your drivetrain more maneuverable, as well as adding omni directional functionality within a traditional frame. The increased agility awarded by Mecanum wheels should allow you to move around traditional defense to the point where 'pushing' isn't necessarily required.
All of that being said, when comparing two similar drives, with the only difference being that one Uses Mecanum wheels in place of a traditional wheel, the Mecanum Drive will almost always have less traction than the traditional drive train. This is due in part to a couple of factors, the wheels always being in a constant state of slip (in that they're applying force in a non-fore/aft direction) being one of them, and the other being that each wheel's motor is independent. This means that if one of your wheels leaves the floor, or starts spinning, you're not only putting down the power of (3) motors.
We have used Mecanum wheels the last 3 years with great success.
Great for manouvering in all directions without haveing to turn.
Last year we used the 6" dia heavy ones from AM and could push our way around as well as resist being pushed around. It helps if you have a heavy robot though.
And it's a good idea to buy a spare wheel or two, but they are expensive.
BEN35678
07-01-2014, 09:00
Yes, our VEXpro wheels did crack, but that only happened this past December at a fundraiser where we were showing off our robot. We went to two competitions last year and had several other fundraisers and expos before they broke. I believe they are durable enough. You shouldn't have to worry about cracks during competition. I believe they also sell replaceable wheels for the mecanum if any of them seem to have significant wear or break.
What are the advantages of using the vex pro mecanums then why not just go with the AndyMark then?
efoote868
07-01-2014, 09:56
Mecanum wheels have a short history in FRC. They tend to have bad stereotypes as being easy to push around and difficult to control, particularly among individuals that have never used them.
The way people write about mecanum wheels, you'd think they were describing casters on an ice-rink.
The easiest drive-train I've ever used in FRC was a set of field-centric mecanum wheels. I've watched mecanum wheels be used for defense effectively, and I've seen times where the maneuverability and ease of use of mecanum wheels won matches.
To answer the OP's question, I think $400 is a bargain for a set of mecanum wheels. Whether your team's strategy calls for them is a different question.
purduephotog
07-01-2014, 09:56
Could anyone point me to resources about the maximum speed you can run Mecanum wheels at ?
I'm wondering (and trying) to find out if all the gear boxing is really necessary. Once they're slipping they're slipping, and the friction factor for all four wheels would be ~equal.
Once they're slipping they're slipping, and the friction factor for all four wheels would be ~equal.
What do you mean by "slipping" in this context?
...my team is not a fan of using mechanum...
With mechanum wheels you can lose up to 50% of your power...
...Name one team that has made it to the Einstein field using mechanum wheels...
...If your philosophy isn't all about winning, then have at it with the mechanum wheels...
...Mechanum robots are really fun to drive...
...I'm not saying that you can't win with mechanum wheels...
There is no "h" in mecanum.
Can anyone speak to the wear of VEXPro 4" Mecanum rollers compared to the 6" wheel rollers over the course of multiple competitions?
In an average year we compete in 2-3 official and 2 off-season competitions, perform in several demos & run a lot of driver practice for the next year's team. So the wheels really need to last. Yet it'd be nice to know what spare parts to get ahead of time (i.e. 1 set of spare rollers per 1 event or 2 events?) when comparing wheels. If we have to replace the 4" wheel's rollers every 1 event, we may go the 6" route so we don't have rollers that are wearing thin at the end of an event.
My team's poking at the idea of Mecanum for a few reasons, and it's easy to implement on our current drive frame.
About Mecanum in general:
The only thing I can say without reading a metaphorical earful from Ether (ha, hi Ether) is that the default field-centric (or robot-centric with no deadband) control algorithm robs the drive train of power just by the nature of the code. When translating on the 45-degree relational diagonal (i.e. 0 degrees = full forward, 90 degrees = full right, so at 45 degrees the bot goes forward & right), 2 motors are completely turned off by the algorithm.
If we implemented Mecanum, we'd most likely do a dual-mode driver control. In the first mode, it'd be field-centric holonomic drive. In the 2nd mode, the driver would be limited to robot-centric tank drive with 90-degree strafing. The driver would be in the 2nd mode most of the time.
Hi Jesse :-)
When translating on the 45-degree relational diagonal (i.e. 0 degrees = full forward, 90 degrees = full right, so at 45 degrees the bot goes forward & right), 2 motors are completely turned off by the algorithm.
Yup. And so the available driving torque at the wheels to keep the bot moving against the rolling resistance is half. And so the top speed in that direction will be substantially less than that predicted on the basis of kinematics assuming no load.
In the 2nd mode, the driver would be limited to robot-centric tank drive with 90-degree strafing. The driver would be in the 2nd mode most of the time.
What driver interface do you have in mind for this? Perhaps divide this 2nd mode into 2 modes?
Mode 2a: Standard 2-joystick Y-axis-only tank drive (X-axes disabled)
Mode 2b: Right (or left) X-axis commands strafe (disable Y-axes)
TheMadCADer
07-01-2014, 12:32
In this game the only big advantage I see for them is with catching the ball from the truss, so unless that's a big part of your strategy, I wouldn't get them. Another downside to them is that they're quite heavy.
I'd actually disagree. I wouldn't want a catcher to have mecanum wheels, since they are so easily pushed.
Looking at the scoring for this year, a catch is the same as a high goal score (disregarding assist points, since you can score low and get the same assist score), so if teams know you can catch, they will try to defend it.
Hitting a moving target is added complexity you don't need. I'd pick a catcher that has no wheels at all before I'd pick one with mecanums.
iceBird10
07-01-2014, 15:09
Hi, I was just wondering what is the purpose of the mecanum wheels, how do they work and are they worth the $400 for the set of 4, also if the sell them somewhere else cheaper than that could you give me the link?
My team has used Mecanum since before I joined FIRST, and they are undoubtedly (I think) worth the money. They offer superior maneuverability, specifically in their ability to "track" side-to-side. Their only downside is lack of traction on smooth surfaces (i.e. bridge from 2012) and less stability when pushed. Given this year's game, they would be a great option if you can afford them.
What driver interface do you have in mind for this? Perhaps divide this 2nd mode into 2 modes?
Mode 2a: Standard 2-joystick Y-axis-only tank drive (X-axes disabled)
Mode 2b: Right (or left) X-axis commands strafe (disable Y-axes)
Unsure, but I think you're on the right track. We're fairly certain which students will drive the robot, so it'll be whatever they're comfortable with. Our most likely candidate is very comfortable with traditional tank controls (and is very good with them), so we'd want to stick with that and create a natural strafe input. Notionally we could implement the x-axis on one or both joysticks with a very large (> 0.33) deadband for strafe detection. When strafing it would ignore the y-axis input.
There is no "h" in mecanum.
Haha my bad, there are days when I spell it mecanum, and days when I spell it mechanum. I wrote that post at like 5 in the morning in my time zone. Gimmie a break with the proofreading!!
I'll be sure to edit that, though. You won't see a single extra "h" in my post when I'm done. Thanks for the correction!
Notionally we could implement the x-axis on one or both joysticks with a very large (> 0.33) deadband for strafe detection. When strafing it would ignore the y-axis input.
Yeah, I thought of that, but then figured the large deadband would make it quite awkward for the driver to do fine-trim left/right strafing when, for example, trying to position the bot for a catch.
With the 2a/2b mode, just hold a button and do your strafing with minimal deadband.
There's a bit of mis-information in this thread
yes... yes there is...
Mecanum Drive will almost always have less traction than the traditional drive train. This is due in part to a couple of factors, the wheels always being in a constant state of slip (in that they're applying force in a non-fore/aft direction) being one of them
I've linked a paper in my signature. In the paper is a link to a video showing the worms eye view of the contact patch of a mecanum wheel while being moved as it would in normal forward motion. The contact point doesn't slip. The rollers don't spin.
With mecanum wheels you can lose up to 50% of your power
the default field-centric (or robot-centric with no deadband) control algorithm robs the drive train of power just by the nature of the code.
While as quoted these statements can both be true, I think it can be mis-interpreted as mecanum has less power than 4WD or 6WD. Driving straight forward or backward there is no power loss in mecanum. In any other situation where mecanum has power loss we are talking about motion that 4WD or 6WD can't do at all, so power loss does not say that 4WD or 6WD is therefore a better choice. (alternatively, I could say 6WD has 100% power loss while moving at a 45 degree angle while mecanum only has a 50% power loss) As long as your driver realizes driving all the way down the field with the robot at a 45 degree angle relative to the field would not be a wise use of mecanum, then power loss is not a real concern when deciding mecanum vs. 4WD/6WD.
Ideal_Nerd
07-01-2014, 17:20
last year our team loved our mecanum wheels. sadly we where borrowing them. this year we are buying them from vex. they are a lot cheaper with similar performance.
http://www.vexrobotics.com/mecanum-wheels.html
What are the advantages of using the vex pro mecanums then why not just go with the AndyMark then?
The VexPro mecanums are cheaper, come pre-assembled (rollers already attached, wheel already together), and are lighter. The last factor is the most significant for me, depending on the size of the wheel you use there can be a 2-4 lb weight difference by going with a set of VexPro over Andymark mecanum wheels. The Andymark mecanum wheels are good but outside of multiple-competition durability I don't know how they are better than the VexPro ones.
Can anyone speak to the wear of VEXPro 4" Mecanum rollers compared to the 6" wheel rollers over the course of multiple competitions?
In an average year we compete in 2-3 official and 2 off-season competitions, perform in several demos & run a lot of driver practice for the next year's team. So the wheels really need to last. Yet it'd be nice to know what spare parts to get ahead of time (i.e. 1 set of spare rollers per 1 event or 2 events?) when comparing wheels. If we have to replace the 4" wheel's rollers every 1 event, we may go the 6" route so we don't have rollers that are wearing thin at the end of an event.
The 4" VexPro Mecanum wheels are a new offering this season, last year they only made 6" and 8" wheels.
cmrnpizzo14
07-01-2014, 18:17
What are the advantages of using the vex pro mecanums then why not just go with the AndyMark then?
Vexpro wheels are less than half the price of the AndyMark ones.
http://www.andymark.com/product-p/am-0732.htm
http://www.vexrobotics.com/mecanum-wheels.html
Vexpro wheels are less than half the price of the AndyMark ones.
http://www.andymark.com/product-p/am-0732.htm
http://www.vexrobotics.com/mecanum-wheels.html
Question: You mentioned the Vex Mechanums cracking in an earlier post. Where did they break, and was the break catestrophic enough to warrant immediate disabling of robot movement (i.e. could the robot finish a match if this occured)?
While as quoted these statements can both be true, I think it can be mis-interpreted as mecanum has less power than 4WD or 6WD. Driving straight forward or backward there is no power loss in mecanum. In any other situation where mecanum has power loss we are talking about motion that 4WD or 6WD can't do at all, so power loss does not say that 4WD or 6WD is therefore a better choice. (alternatively, I could say 6WD has 100% power loss while moving at a 45 degree angle while mecanum only has a 50% power loss) As long as your driver realizes driving all the way down the field with the robot at a 45 degree angle relative to the field would not be a wise use of mecanum, then power loss is not a real concern when deciding mecanum vs. 4WD/6WD.
I said you can lose up to 50% of power with moves like strafing. I didn't say that you can lose 50% from forward or backwards driving, and you can lose somewhere between >0% to 50% power when strafing diagonally, which is a combination of the forward/back movement and sideways strafing. There is no significant difference in power between forward/back mecanum driving and forward/back tread driving when using the same power source and transmission.
It may be important to be aware that drivers may not have success moving others robots out of the way by turning or strafing, but instead can have success by pushing straight on.
While as quoted these statements can both be true...
Please don't mis-quote me and then poorly attempt to make a counter-point which actually says something completely tangential to what the entire quote said.
cmrnpizzo14
07-01-2014, 19:55
Question: You mentioned the Vex Mechanums cracking in an earlier post. Where did they break, and was the break catestrophic enough to warrant immediate disabling of robot movement (i.e. could the robot finish a match if this occured)?
I personally did not mention it but our team has used mecanum enough for me to answer. Usually it is the roller that cracks I believe, we never have had serious issues with this. Barring a catastrophe of some sort, you can still finish the match driving almost normally. Depending on the severity of the crack you might notice it in your robots driving.
You can purchase additional rollers online to replace old/cracked ones. We did this to all of our rollers to retrofit old mecanum wheels and make them sort of new again before last season. If you check the wheels regularly and make sure you have spares on hand you will be fine.
thefro526
07-01-2014, 20:52
yes... yes there is...
I've linked a paper in my signature. In the paper is a link to a video showing the worms eye view of the contact patch of a mecanum wheel while being moved as it would in normal forward motion. The contact point doesn't slip. The rollers don't spin.
I don't want to debate this too much since it's going to derail the thread, but I'm still not convinced based on the two videos (Glass table, green backlighting) that the rollers do not spin under FRC conditions. Maybe I'm nit picking, but there seemed to be some sort of displacement of the roller (Looks like a few degrees at most) even on the table top test. At FRC speeds, something tells me that this has to add up into the rollers spinning while the robot is moving in standard fore and aft directions. It's been a few years since I've played with Mecanum wheels first hand, but if I remember correctly, rollered seemed to slip while under load.... But I might be wrong.
I'm still not convinced based on the two videos (Glass table, green backlighting) that the rollers do not spin under FRC conditions.
They do spin somewhat under FRC conditions. The roller axial free play in the affordable COTS mec wheels typically used for FRC contributes to spinning. A compliant surface (like carpet) contributes to spinning. This is mentioned on page5 of his paper.
Hitting a moving target is added complexity you don't need. I'd pick a catcher that has no wheels at all before I'd pick one with mecanums.
So ... you'd rely on the 100% accuracy of the throwing robot, rather than provide some flexibility by allowing the catcher to adjust?
Put another way, if the ball is a mere 6" off target, you'd rather have the ball bounce off you (and then have to go chase it) than see a robot with the ability to take a half step to the right? :confused:
TheMadCADer
08-01-2014, 12:58
So ... you'd rely on the 100% accuracy of the throwing robot, rather than provide some flexibility by allowing the catcher to adjust?
Put another way, if the ball is a mere 6" off target, you'd rather have the ball bounce off you (and then have to go chase it) than see a robot with the ability to take a half step to the right? :confused:
Based on initial ball launching prototypes, yes I would.
My point is that the ball will never be anywhere close to 6" from being caught, it will be much farther away and completely un-catchable because a mecanum robot can not hold field position. If your catching robot uses mecanum wheels, pushing it out of the way will be a common and effective defensive strategy. In a game like this where field position is everything, a mecanum robot is at an extreme disadvantage.
The only role I can see a mecanum robot playing effectively is a robot that cycles the ball by itself with no assists. As soon as it tries to play a role where it has to stop and wait for its alliance to do something it would lose its field position and ability to add assist points.
... a mecanum robot can not hold field position. If your catching robot uses mecanum wheels, pushing it out of the way will be a common and effective defensive strategy. In a game like this where field position is everything, a mecanum robot is at an extreme disadvantage.
Does the same argument apply to giving a healthy bump to the lighter corner of a 6WD drop-center bot?
TheMadCADer
08-01-2014, 15:54
Does the same argument apply to giving a healthy bump to the lighter corner of a 6WD drop-center bot?
To a degree. It requires a more directed bump/push than defending a mecanum robot, and a 6/8 wheel drive train can better resist the imparted turning motion by turning against it, allowing you to hold a stalemate. Trying to drive a mecanum robot that is already slipping is an exercise in futility. The bump also causes rotation more than sliding, which would keep general field position close to static. In this case a good catcher would be orientation-agnostic.
Having a robot that can be pushed in any way is disadvantageous to being a good target for catching, so I'd say the best pure catcher is a robot without wheels. However, making the trade-off of allowing some pushing in exchange for the ability to move and complete other tasks could be a successful strategy. Toggling between a drivetrain and a traction device would give the "best of both worlds" at the expense of complexity (independent swerve being another solution, albeit even more complex).
cadandcookies
08-01-2014, 16:04
Designing to be defended against is a really good way to put a ton of work that may or may not see a benefit in competition.
Remember 2006, when 25 had a robot that was solely human fed, and basically shot from the floor? And went undefeated right up through division elimination rounds?
Ultimately the benefit of mecanum ends up coming down to what your game strategy is. If it's to be a flexible assist bot, it may very well be worth it-- if it's being a thrower bot, it very well may not be. Ultimately it's all just speculation until we get to competition regardless. Just like any other year, there will be successful 6WD robots and successful mecanum robots.
The team makes the tools it uses worthwhile, not the other way around.
...a 6/8 wheel drive train can better resist the imparted turning motion by turning against it, allowing you to hold a stalemate.
I wonder if anyone has ever actually conducted such a test under controlled conditions. The mec bot would have a larger effective wheelbase than 6W drop center. And mec wheels (at least the affordable ones commonly used for FRC) are somewhat harder to push sideways than fwd/rev. Might make an interesting off-season science project to verify this claim or lay it to rest.
Question: You mentioned the Vex Mechanums cracking in an earlier post. Where did they break, and was the break catestrophic enough to warrant immediate disabling of robot movement (i.e. could the robot finish a match if this occured)?
Someone mentioned that they had a VEXpro mecanum wheel crack, but did not provide any additional information. We've done a lot of testing here and not seen any durability issues. We've also not heard very many complaints from users about Mecanum wheels breaking.
We're very happy with how that design turned out, and are confident in the durability, especially in a game with a flat field like Aerial Assist.
If you jump a robot off a loading dock, will the wheels crack? Yes: Based on our testing... not until the 3rd jump. :)
If you slam the wheels into a wall with a rolling robot, will they crack? Not in our testing.
I'm confident that VEXpro Mecanum Wheels will stand-up to normal wear and tear during an FRC season. If a wheel breaks, it is likely that the wheel wasn't damaged due to extended match play, but instead the robot was dropped when being loaded into a mini-van.
Also, I hope it goes without saying for VEX products at this point:
If you have a problem or break, please contact our tech support folks so they can help make it right. prosupport@vex.com
I'm especially excited to see teams using our new 4" Mecanum Wheels (http://www.vexrobotics.com/vexpro/wheels-and-hubs/mecanum-wheels.html). This game lends itself perfectly to small wheels, and I believe there are some strategies which would benefit from holonomic motion.
-John
Is there any information on mecanums wheels and drivetrains? Live Axle vs Dead Axle?
I have never use them and am looking for information. Transmission? Does and one have some good information to share. Thanks
I have never use them and am looking for information.
Where have you been looking?
Is there any information on mecanums wheels and drivetrains? ...
Sure, there's lots of info. See that search link at the top of this page? Click it, enter "mecanum", and you'll find lots.
Apologies to JesseK and bEdhEd. I was not attempting to say either of you said there was power loss for mecanum moving forward/backward. I was not trying to counter what either of you said, but using your correct statements as an example of the types that are very commonly misinterpreted.
my post was attempting to be pro-active about a future reader's misinterpretation, and was instead itself misinterpreted. I think there is a word for that (other than failure). I guess we can agree that I clearly didn't say it very well.
Apologies to JesseK and bEdhEd. I was not attempting to say either of you said there was power loss for mecanum moving forward/backward. I was not trying to counter what either of you said, but using your correct statements as an example of the types that are very commonly misinterpreted.
my post was attempting to be pro-active about a future reader's misinterpretation, and was instead itself misinterpreted. I think there is a word for that (other than failure). I guess we can agree that I clearly didn't say it very well.
Ah, I understand what you were trying to do. I guess your motive just wasn't clear enough for us to get. Part of it may have been our bad too.
As for your apology, team bEdhEd graciously accepts!
Someone mentioned that they had a VEXpro mecanum wheel crack, but did not provide any additional information. We've done a lot of testing here and not seen any durability issues. We've also not heard very many complaints from users about Mecanum wheels breaking.
We're very happy with how that design turned out, and are confident in the durability, especially in a game with a flat field like Aerial Assist.
If you jump a robot off a loading dock, will the wheels crack? Yes: Based on our testing... not until the 3rd jump. :)
If you slam the wheels into a wall with a rolling robot, will they crack? Not in our testing.
I'm confident that VEXpro Mecanum Wheels will stand-up to normal wear and tear during an FRC season. If a wheel breaks, it is likely that the wheel wasn't damaged due to extended match play, but instead the robot was dropped when being loaded into a mini-van.
Also, I hope it goes without saying for VEX products at this point:
If you have a problem or break, please contact our tech support folks so they can help make it right. prosupport@vex.com
I'm especially excited to see teams using our new 4" Mecanum Wheels (http://www.vexrobotics.com/vexpro/wheels-and-hubs/mecanum-wheels.html). This game lends itself perfectly to small wheels, and I believe there are some strategies which would benefit from holonomic motion.
-John
Wow! I was not expecting a VEX engineer to reply. Thank you, I think that may seal the deal. :]
MrRiedemanJACC
08-01-2014, 23:53
So Mecanum vs 6WD and all things being equal I understand most of the pros and cons.
But if you look at the Robot in 3 Days team Boom Done they have 8 motors on their drivetrain - 4 cim and 4 minicim. What if instead of having a straight gear reduction, a team were to use a direct drive shifting transmission such as the Vex Pro ball shifter on each Mecanum wheel with a Cim and Mini-Cim as the input to each wheel? And you took that against a 6WD drivetrain with the three CIM Vex Pro Shifting transmission per side?
Does that muddy the water a bit?
So Mecanum vs 6WD and all things being equal I understand most of the pros and cons.
But if you look at the Robot in 3 Days team Boom Done they have 8 motors on their drivetrain - 4 cim and 4 minicim. What if instead of having a straight gear reduction, a team were to use a direct drive shifting transmission such as the Vex Pro ball shifter on each Mecanum wheel with a Cim and Mini-Cim as the input to each wheel? And you took that against a 6WD drivetrain with the three CIM Vex Pro Shifting transmission per side?
Does that muddy the water a bit?
What would lead you to believe that it would muddy the water? Can you provide reasons why, in your estimation, the mecanum drive implementation you're suggesting here would fare better against the 6WD robot you're describing than a more typical mecanum drive?
Do the math and you'll have your answer. This stuff is pretty cut and dry.
Wow! I was not expecting a VEX engineer to reply. Thank you, I think that may seal the deal. :]
That's one of the things I love about CD and the FIRST community as a whole. Despite reaching across the world, the community is so close-knit that you can always be heard.
I hope you have good luck with your mecanum drive this year. Please post back your results!
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