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moogboy 20-01-2012 21:07
Mechanum Wheels?
 
I've been observing CD for a few days and noticed that amongst all the discussions of drive trains, there has been nothing major said about mechanum wheels. One or two threads mentioned them in passing, but I was curious, as my team decided to go with mechanums, why others were choosing to not go this direction?

the man 20-01-2012 21:13
Re: Mechanum Wheels?
 
We are using mechanum wheels this year, for a few reasons. One we love them. Two the provide great maneuverability. Three there big enough to get over the bump. I really like them.

moogboy 20-01-2012 21:19
Re: Mechanum Wheels?
 
Quote:
Originally Posted by the man (Post 1110118)
We are using mechanum wheels this year, for a few reasons. One we love them. Two the provide great maneuverability. Three there big enough to get over the bump. I really like them.
What size are you using? getting over the bump was something my team thought to be impossible, but our wheels might be a smaller size. do you think you will have any trouble with traction on the bridge?

bearbot 20-01-2012 21:23
Re: Mechanum Wheels?
 
Here is FIRST team 2849 ursamajor and thier sucess with meccaums they workk in bump and terrific on the bridge .We used 8in wheel i am a veteran and mentor on the team.Bumps coming sooon
http://www.youtube.com/watch?v=3Hju0...y7UjRNgBKiTb8I

Chris is me 20-01-2012 21:24
Re: Mechanum Wheels?
 
Quote:
Originally Posted by the man (Post 1110118)
Three there big enough to get over the bump. I really like them.
Have you actually tested this?

372 lives on 20-01-2012 21:27
Re: Mechanum Wheels?
 
Quote:
Originally Posted by bearbot (Post 1110134)
Here is FIRST team 2849 ursamajor and thier sucess with meccaums they workk in bump and terrific on the bridge .We used 8in wheel i am a veteran and mentor on the team
Pics or it didn't happen



Mecanum = chucknorris of wheels(does chuck Norris weigh more than120? :).

the man 20-01-2012 21:29
Re: Mechanum Wheels?
 
We are using the 8 inch. The rollers are incredibly grippy on the bridge, I fore see no issues. More testing will likely occur tomorrow. I will post any thing else we find out about them on here tomorrow. Why would they have a problem with the bump?

SuzyQ.42 20-01-2012 21:30
Re: Mechanum Wheels?
 
The decision to use mecanum wheels is a strategy-based one... if you plan to play a heavy, pushing defense, mecanum is probably the wrong choice, as the diagonal wheels are not so great at traction. If you plan to play a swift and extremely maneuverable offense, then mecanum may be the choice for you. Mecanum wheels are also larger than many plain plaction wheels, which may assist you when climbing the bump. The size can also push your CG up, depending on where you fasten the wheel centers.

bearbot 20-01-2012 21:31
Re: Mechanum Wheels?
 
Quote:
Originally Posted by 372 lives on (Post 1110137)
Pics or it didn't happen



Mecanum = chucknorris of wheels(does chuck Norris weigh more than120? :).

Sorry forgot to add the video

EricH 20-01-2012 21:31
Re: Mechanum Wheels?
 
Quote:
Originally Posted by the man (Post 1110138)
We are using the 8 inch. The rollers are incredibly grippy on the bridge, I fore see no issues. More testing will likely occur tomorrow. I will post any thing else we find out about them on here tomorrow. Why would they have a problem with the bump?
8" wheels, 4" bump, your axle hits smack-dab on the top of the bump. That makes it a little bit tricky to get up and over; I'd put a set of skid plates to knock the bump under the axle to help the robot get over the barrier.

the man 20-01-2012 21:41
Re: Mechanum Wheels?
 
I dont get it. As long as your not going mock 5, ok ok at fast speed in terms of robotics competitions, the back wheels will continue to push as you some what gently push your self over the bump. But we will test this tomorrow.

Andrew Lawrence 20-01-2012 21:43
Re: Mechanum Wheels?
 
I'm not saying no mecanums (or however you spell it), but these are some points made in previous threads:

-Less traction on the key
-As already said, loss in pushing power
-Heavy
-Forces you to use maximum number of motors for drivetrain (not usually an issue)
-While they may not affect your ability to go on the bridge, the ability to stay without being pushed off or falling off of the bridge tips your direction will be tough to deal with
-You won't be driving circles around other's bots unless you have a lot of practice.
-In order to work to their fullest extent, require a lot of thorough engineering
-More complicated to code, even at the minimum level


Like I said, I'm not telling you not to use them, but when you do, be careful and engineer it through (Not meant to sound like most interesting man in the world meme, though I'm tempted. ;))

O'Sancheski 20-01-2012 21:55
Re: Mechanum Wheels?
 
Quote:
Originally Posted by SuperNerd256 (Post 1110153)
-More complicated to code, even at the minimum level
I disagree with this statement. WPI has a complete library for mecanum wheels. My old team used them last year for the first time and had them programmed on the drivetrain the day we received the shipment from AndyMark.

Andrew Lawrence 20-01-2012 21:57
Re: Mechanum Wheels?
 
Quote:
Originally Posted by O'Sancheski (Post 1110162)
I disagree with this statement. WPI has a complete library for mecanum wheels. My old team used them last year for the first time and had them programmed on the drivetrain the day we received the shipment from AndyMark.
I mean when you go into the real nitty gritty coding stuff. THe code they give you is fine, but to make it better takes some work.

This is just what I've been told by our programmers, I'm not a master of code myself, however my trustful programmers are.

Djur 20-01-2012 22:01
Re: Mechanum Wheels?
 
Quote:
Originally Posted by SuperNerd256 (Post 1110153)
I'm not saying no mecanums (or however you spell it), but these are some points made in previous threads:

-Less traction on the key
-As already said, loss in pushing power
-Heavy
-Forces you to use maximum number of motors for drivetrain (not usually an issue)
-While they may not affect your ability to go on the bridge, the ability to stay without being pushed off or falling off of the bridge tips your direction will be tough to deal with
-You won't be driving circles around other's bots unless you have a lot of practice.
-In order to work to their fullest extent, require a lot of thorough engineering
-More complicated to code, even at the minimum level


Like I said, I'm not telling you not to use them, but when you do, be careful and engineer it through (Not meant to sound like most interesting man in the world meme, though I'm tempted. ;))
  • We've been using Mecanums for the past three years and have found no huge loss of traction
  • Usually, but this depends on the drivetrain.
  • Definitely; Mecanums are a lot heavier than conventional wheels.
  • I think most teams use 4 CIMs to drive anyway because it gives the robot a ton of power.
  • Mecanums powered by Toughboxes don't get pushed around as much as people think, but it's defeinitely soemthing to consider.
  • Agreed, a lot of practice is needed with Mecanums to be extremely good at them.
  • Not a lot of engineering, it's really only the initial one-time assembly that matters in the long run.
  • Actually, my team uses all of 20 characters for our Mecanum code, not counting the button to turn it on ;)

Just my experiences with Mecanums.

Ether 20-01-2012 22:03
Re: Mechanum Wheels?
 
Quote:
Originally Posted by SuperNerd256 (Post 1110164)
I mean when you go into the real nitty gritty coding stuff. THe code they give you is fine, but to make it better takes some work.

This is just what I've been told by our programmers
Could you please ask your programmers to tell us how they made it better than the WPI code.

Andrew Lawrence 20-01-2012 22:05
Re: Mechanum Wheels?
 
Quote:
Originally Posted by Djur (Post 1110167)
  • We've been using Mecanums for the psat three years and have found no huge loss of traction
  • Usually, but this depends on the drivetrain.
  • Definitely; Mecanums are a lot heavier than conventional wheels.
  • I think most teams use 4 CIMs to drive anyway because it gives the robot a ton of power.
  • Mecanums powered by Toughboxes don't get pushed around as much as people think, but it's defeinitely soemthing to consider.
  • Agreed, a lot of practice is needed with Mecanums to be extremely good at them.
  • Not a lot of engineering, it's really only the initial one-time assembly that matters in the long run.
  • Actually, my team uses all of 20 characters for our Mecanum code, not counting the button to turn it on ;)

Just my experiences with Mecanums.
Thanks for posting this! I like the other opinions. :)

Andrew Lawrence 20-01-2012 22:06
Re: Mechanum Wheels?
 
Quote:
Originally Posted by Ether (Post 1110171)
Could you please ask your programmers to tell us how they made it better than the WPI code.
The themselves haven't programmed it, but from reading the WPI code and looking at how to make it better, they said getting it perfect (not 100%, but a close 99%) would take some extra code.

I'll ask them at our next meeting, though. Now I'm interested, too. :)

EricH 20-01-2012 22:20
Re: Mechanum Wheels?
 
Quote:
Originally Posted by the man (Post 1110148)
I dont get it. As long as your not going mock 5, ok ok at fast speed in terms of robotics competitions, the back wheels will continue to push as you some what gently push your self over the bump. But we will test this tomorrow.
When you hit the bump with an 8" wheel, that wheel's entire force on the bump is vertical until you get some lift off the ground on that end. So your back wheels have to work twice as hard.

An easy way to deal with this is to put an angle leading down below your axle from the edge of the frame; this allows the front wheels to get some horizontal tractive force in addition to the vertical, which allows them to help the back wheels out a bit. Do that on both sides of all four wheels, and it'll help you out a lot.

Chris Fultz 20-01-2012 22:24
Re: Mechanum Wheels?
 
Quote:
Originally Posted by EricH (Post 1110142)
8" wheels, 4" bump, your axle hits smack-dab on the top of the bump. That makes it a little bit tricky to get up and over; I'd put a set of skid plates to knock the bump under the axle to help the robot get over the barrier.
I don't understand what you are saying here.

When the wheel hits the bump, it will roll up and over. When the wheel is then sitting on top of the bump, the axle is then 4" above the bump. How does the axle hit the top?

EricH 20-01-2012 22:27
Re: Mechanum Wheels?
 
Quote:
Originally Posted by Chris Fultz (Post 1110185)
I don't understand what you are saying here.

When the wheel hits the bump, it will roll up and over. When the wheel is then sitting on top of the bump, the axle is then 4" above the bump. How does the axle hit the top?
On the moment of contact, the front wheels hit with the axle at the top level of the bump. They won't roll up and over without help--their force at that point will be entirely vertical once they leave the ground, but without something to push them horizontally, they'll fall back down. The back wheels are what provide the horizontal force to get the wheel up on top of the bump.

the man 20-01-2012 22:29
Re: Mechanum Wheels?
 
Something else to consider when talking about the bump is the rollers on the mechanum wheels. Oh something else I was wondering, Is the bump 4inches tall measured from the floor or from the lip on the bump?

nitneylion452 20-01-2012 22:37
Re: Mechanum Wheels?
 
Quote:
Originally Posted by EricH (Post 1110188)
On the moment of contact, the front wheels hit with the axle at the top level of the bump. They won't roll up and over without help--their force at that point will be entirely vertical once they leave the ground, but without something to push them horizontally, they'll fall back down. The back wheels are what provide the horizontal force to get the wheel up on top of the bump.
Might it not be fair to assume (or at least hope) that the rollers on mecanum wheels or other deep crevices would catch the lip of the bump and then provide a stronger vertical component and thus be able to pick the robot up?

By virtue of mecanums having rollers, it would also be safe to assume that the roller will roll (gasp!) off the lip. Countered my own argument for mecanums, but not other wheels. Any opinions on that?

DavidGitz 20-01-2012 22:45
Re: Mechanum Wheels?
 
Quote:
Originally Posted by EricH (Post 1110188)
On the moment of contact, the front wheels hit with the axle at the top level of the bump. They won't roll up and over without help--their force at that point will be entirely vertical once they leave the ground, but without something to push them horizontally, they'll fall back down. The back wheels are what provide the horizontal force to get the wheel up on top of the bump.
I think what Chris is saying is that the axle would not hit the bump. Many other parts would hit the bump, and your argument is well founded but the axle itself will not hit, as it is within the perimeter of the mecanum wheel. Assuming of course that a) this is a *standard* mecanum setup b) the robot is attempting to drive straight on to the bump.

BJC 20-01-2012 23:02
Re: Mechanum Wheels?
 
Quote:
Originally Posted by DavidGitz (Post 1110209)
I think what Chris is saying is that the axle would not hit the bump. Many other parts would hit the bump, and your argument is well founded but the axle itself will not hit, as it is within the perimeter of the mecanum wheel. Assuming of course that a) this is a *standard* mecanum setup b) the robot is attempting to drive straight on to the bump.
What I believe he is saying is that the midpoint of the wheel is level with the top of the barrier. Because of this the forces exerted by the robot must be entirely up until the wheel gains some purchase on the barrier and will continue to be able to pull the robot forward (read: over) the barrier.

If when your robot is climbing the barrier where the initial wheel contact with the barrier is lower then the middle of the wheel more forces remain pushing in the forward direction. In the same way, when you are on flat ground the wheels are pushing entirely forward. As a good rule of thumb if you can get the initial contact point inside the bottom 1/3 of the wheel that wheel will fairly easily surmount the point of contact while maintaining its forward force.

This is why it was suggested to put little ramps or something in front of your wheels. So the initial contact point of the barrier to the wheel is as low to the wheel as possible and forward momentum can be maintained.

Regards, Bryan

jvriezen 20-01-2012 23:37
Re: Mechanum Wheels?
 
Quote:
Originally Posted by nitneylion452 (Post 1110200)
Might it not be fair to assume (or at least hope) that the rollers on mecanum wheels or other deep crevices would catch the lip of the bump and then provide a stronger vertical component and thus be able to pick the robot up?

By virtue of mecanums having rollers, it would also be safe to assume that the roller will roll (gasp!) off the lip. Countered my own argument for mecanums, but not other wheels. Any opinions on that?
Our tests with 8" mecanums over the bump is that yes, the crevices between the rollers will catch the top of the bump and aid in lifting the front wheels up. The rear wheels keep the forward force so the rollers don't roll off. However, while we got over the bump (well not really, we got the front wheels over but it was last year's bot and it bottomed due to lack of clearance) I would not recommend relying on just 8" mecanums. It worked much better when we had some help to get the wheels higher up before they encountered the bump.

372 lives on 20-01-2012 23:41
Re: Mechanum Wheels?
 
Quote:
Originally Posted by SuperNerd256 (Post 1110153)
I'm not saying no mecanums (or however you spell it), but these are some points made in previous threads:

-Less traction on the key
-As already said, loss in pushing power
-Heavy
-Forces you to use maximum number of motors for drivetrain (not usually an issue)
-While they may not affect your ability to go on the bridge, the ability to stay without being pushed off or falling off of the bridge tips your direction will beN tough to deal with
-You won't be driving circles around other's bots unless you have a lot of practice.
-In order to work to their fullest extent, require a lot of thorough engineering
-More complicated to code, even at the minimum level


Like I said, I'm not telling you not to use them, but when you do, be careful and engineer it through (Not meant to sound like most interesting man in the world meme, though I'm tempted. ;))

I always laugh when people make posts like these. We played defense in 2010 with mecanums and definetly didn't get pushed around.

And your not going to do well with any robot without practice.

theprgramerdude 21-01-2012 00:08
Re: Mechanum Wheels?
 
Quote:
Originally Posted by SuperNerd256 (Post 1110173)
The themselves haven't programmed it, but from reading the WPI code and looking at how to make it better, they said getting it perfect (not 100%, but a close 99%) would take some extra code.

I'll ask them at our next meeting, though. Now I'm interested, too. :)
Code:
void RobotDrive::MecanumDrive_Polar(float magnitude, float direction, float rotation)
{
        // Normalized for full power along the Cartesian axes.
        magnitude = Limit(magnitude) * sqrt(2.0);
        // The rollers are at 45 degree angles.
        double dirInRad = (direction + 45.0) * 3.14159 / 180.0;
        double cosD = cos(dirInRad);
        double sinD = sin(dirInRad);

        double wheelSpeeds[kMaxNumberOfMotors];
        wheelSpeeds[kFrontLeftMotor] = sinD * magnitude + rotation;
        wheelSpeeds[kFrontRightMotor] = cosD * magnitude - rotation;
        wheelSpeeds[kRearLeftMotor] = cosD * magnitude + rotation;
        wheelSpeeds[kRearRightMotor] = sinD * magnitude - rotation;

        Normalize(wheelSpeeds);

        m_frontLeftMotor->Set(wheelSpeeds[kFrontLeftMotor] * m_invertedMotors[kFrontLeftMotor]);
        m_frontRightMotor->Set(wheelSpeeds[kFrontRightMotor] * m_invertedMotors[kFrontRightMotor]);
        m_rearLeftMotor->Set(wheelSpeeds[kRearLeftMotor] * m_invertedMotors[kRearLeftMotor]);
        m_rearRightMotor->Set(wheelSpeeds[kRearRightMotor] * m_invertedMotors[kRearRightMotor]);
}
This is straight from the RobotDrive source. What could possibly be improved?

Andrew Lawrence 21-01-2012 00:10
Re: Mechanum Wheels?
 
Quote:
Originally Posted by theprgramerdude (Post 1110264)
Code:
void RobotDrive::MecanumDrive_Polar(float magnitude, float direction, float rotation)
{
        // Normalized for full power along the Cartesian axes.
        magnitude = Limit(magnitude) * sqrt(2.0);
        // The rollers are at 45 degree angles.
        double dirInRad = (direction + 45.0) * 3.14159 / 180.0;
        double cosD = cos(dirInRad);
        double sinD = sin(dirInRad);

        double wheelSpeeds[kMaxNumberOfMotors];
        wheelSpeeds[kFrontLeftMotor] = sinD * magnitude + rotation;
        wheelSpeeds[kFrontRightMotor] = cosD * magnitude - rotation;
        wheelSpeeds[kRearLeftMotor] = cosD * magnitude + rotation;
        wheelSpeeds[kRearRightMotor] = sinD * magnitude - rotation;

        Normalize(wheelSpeeds);

        m_frontLeftMotor->Set(wheelSpeeds[kFrontLeftMotor] * m_invertedMotors[kFrontLeftMotor]);
        m_frontRightMotor->Set(wheelSpeeds[kFrontRightMotor] * m_invertedMotors[kFrontRightMotor]);
        m_rearLeftMotor->Set(wheelSpeeds[kRearLeftMotor] * m_invertedMotors[kRearLeftMotor]);
        m_rearRightMotor->Set(wheelSpeeds[kRearRightMotor] * m_invertedMotors[kRearRightMotor]);
}
This is straight from the RobotDrive source. What could possibly be improved?
They said something about advanced controls to make the omnidirectional drive more intuitive and easy to use.

theprgramerdude 21-01-2012 00:28
Re: Mechanum Wheels?
 
Quote:
Originally Posted by SuperNerd256 (Post 1110265)
They said something about advanced controls to make the omnidirectional drive more intuitive and easy to use.
Controls aren't part of the drive code; they're the parameters that are passed into the drive code. What your programmers don't seem to understand is it's their code, not the WPILib code, that can be improved.

It's like saying even though the motor controller is already set to 100%, the code can be improved to set it to 110% via "better controls". That doesn't happen.

Andrew Lawrence 21-01-2012 00:32
Re: Mechanum Wheels?
 
Quote:
Originally Posted by theprgramerdude (Post 1110268)
Controls aren't part of the drive code; they're the parameters that are passed into the drive code. What your programmers don't seem to understand is it's their code, not the WPILib code, that can be improved.

It's like saying even though the motor controller is already set to 100%, the code can be improved to set it to 110% via "better controls". That doesn't happen.
We don't use mecanums. Never have. The point they made was in order to have the best quality controls for the drivers and have the driving experience be the most controllable and intuitive that our drivers can have, more complex programming would be required for mecanums compared to the traditional tank drive.

That's that, let's not derail this thread further.

Joe Schornak 21-01-2012 01:46
Re: Mechanum Wheels?
 
My team has done quite a bit of experimentation with mecanum wheels this season, since we were initially doubtful of their effectiveness in certain critical areas, like traversing the barrier and climbing the bridge. Unfortunately, until we get some videos and pictures up (hopefully this weekend!) you'll have to trust me that our inadequately-described mechanisms actually work.

Surprisingly, the mecanum roller material grips better on plastic than on plywood. I suspect that fine sawdust is the culprit here, as our test of taping a sheet of plastic to one half of the bridge and then driving up resulted in the robot turning away from the side with the plastic, demonstrating higher traction on that side. We would be able to ascend while strafing, but our bridge has a significant lip that makes it very difficult to go from the ground to the bridge while sideways. No problems with turning sideways partway up and balancing that way, though!

We devised a system of raised powered traction wheels to pull the robot onto the barrier and provide a better point of contact to the mecanum wheels. Our prototype long chassis has no problems climbing over the barrier, even from a stop very close to the barrier without a high-speed run-up. The mecanum rollers function exactly as they should, in that the force vectors cancel due to the wheel orientation and no disadvantageous rolling occurs.

We use mecanum wheels because the extra range of motion is highly useful in offensive play. The ability to strafe has been particularly helpful when lining up on the bridge and will likely be very useful when the robot is sideways while trying to balance three robots. From a control perspective, we use flight simulator joysticks with a twist axis, a setup which has proved highly intuitive and easy to explain to 6th-graders during our school's open house.

In short, while mecanum wheels are certainly not perfect for everything, they have none of the negative traits frequently attributed to them, namely low traction, and we expect to use them to great advantage this season.

372 lives on 21-01-2012 03:07
Re: Mechanum Wheels?
 
Quote:
Originally Posted by SuperNerd256 (Post 1110270)
We don't use mecanums. Never have. The point they made was in order to have the best quality controls for the drivers and have the driving experience be the most controllable and intuitive that our drivers can have, more complex programming would be required for mecanums compared to the traditional tank drive.

That's that, let's not derail this thread further.

just stop...

they aren't that hard to use and if you gave them a chance you would see that. now can you stop posting all of your opinions on something you clearly don't understand because it could mess up peoples decisions on what wheels to use.

jvriezen 21-01-2012 07:13
Re: Mechanum Wheels?
 
Quote:
Originally Posted by theprgramerdude (Post 1110268)
Controls aren't part of the drive code; they're the parameters that are passed into the drive code. What your programmers don't seem to understand is it's their code, not the WPILib code, that can be improved.

It's like saying even though the motor controller is already set to 100%, the code can be improved to set it to 110% via "better controls". That doesn't happen.
One relatively simple improvement you can make for mecanum drive is to add the option for 'field oriented' drive. Using a gyro, you can arrange the code to so that pushing the joystick forward *always* makes the bot move away from you, regardless of which way it is pointing. Moving stick right, the bot always moves directly to the right side wall of the field. This can make driving easier. Our bot last year allowed you to choose via a joystick button which type of drive mode you wanted.

nlknauss 21-01-2012 08:58
Re: Mechanum Wheels?
 
Quote:
Originally Posted by BJC (Post 1110220)
What I believe he is saying is that the midpoint of the wheel is level with the top of the barrier. Because of this the forces exerted by the robot must be entirely up until the wheel gains some purchase on the barrier and will continue to be able to pull the robot forward (read: over) the barrier.

If when your robot is climbing the barrier where the initial wheel contact with the barrier is lower then the middle of the wheel more forces remain pushing in the forward direction. In the same way, when you are on flat ground the wheels are pushing entirely forward. As a good rule of thumb if you can get the initial contact point inside the bottom 1/3 of the wheel that wheel will fairly easily surmount the point of contact while maintaining its forward force.

This is why it was suggested to put little ramps or something in front of your wheels. So the initial contact point of the barrier to the wheel is as low to the wheel as possible and forward momentum can be maintained.

Regards, Bryan
We did some tests with the bump and the AM kit chassis with 8" wheels and found out exactly what you are describing Bryan. Here is a tweet with a picture of the test to put a visual to this part of the discussion. The center of the 8" wheel hit the middle of the bump directly. We ended up putting a slope on the front edge of the chassis to help with horizontal forces.

http://t.co/4GxQFVqU

We used mecanum for the first time last year and had some success with them and the rest of our robot design. I always tell my students that the chassis decision should be based on our robot/game strategy. We built an offensive robot and of course tried to stay in front of our own rack, worked great. We decided not to use mecanum this year for a few reason with the main having to do with high level game strategy.

JaredSanMiguel 21-01-2012 10:09
Re: Mechanum Wheels?
 
Quote:
Originally Posted by SuperNerd256 (Post 1110153)
-You won't be driving circles around other's bots unless you have a lot of practice.
I don't think this is the case at all. I was my teams driver two years ago when we had mecanums in break away. After a couple days of driving the maneuverability of our robot was evident.

Not to discredit what you are saying, if you don't have a couple days to spend on driving then learning at competition may be difficult.

Just don't want to discourage teams, the driving can be mastered in a relatively short amount of time.

Koko Ed 21-01-2012 10:15
Re: Mechanum Wheels?
 
As of last year I still think a robot with Mechanum has never made it to Einstein.

Taylor 21-01-2012 10:27
Re: Mechanum Wheels?
 
Quote:
Originally Posted by Koko Ed (Post 1110393)
As of last year I still think a robot with Mechanum has never made it to Einstein.
Straw man.

That's like saying women are poor drivers because no woman has ever won the NASCAR championship.

As of last year, no robot with a Kinect has ever made it to Einstein either.

thefro526 21-01-2012 10:29
Re: Mechanum Wheels?
 
Quote:
Originally Posted by Koko Ed (Post 1110393)
As of last year I still think a robot with Mechanum has never made it to Einstein.
This is correct.

Though, 2826 made it a stone's throw from Einstein in 2011 with their 'Octocanum' drive. Had 71 not had issues in the Curie finals, mecanums may have made it. We're still quite some time off from seeing a pure mecanum make it to Einstein though.


Quote:
Originally Posted by Taylor (Post 1110398)
Straw man.

That's like saying women are poor drivers because no woman has ever won the NASCAR championship.

As of last year, no robot with a Kinect has ever made it to Einstein either.

Is this really a valid comparison?

Look at the number of women in Nascar vs the Number of teams who use Mecanum wheels in FRC.

And the Kinect point is completely irrelevant here, Mecanum wheels have been in FRC since 2005, at my count that's 7 seasons that they could've and or should've made it to Einstein if they were superior (Though, Mecanum usage really exploded in 2007 and they were illegal in 2009) - the Kinect is new this season, maybe you can make the Kinect comment again in a few years.

The reason a Mecanum robot hasn't made it to Einstein is a really deep topic. I believe, and may be wrong here, but 51 is the only team in the Modern Era to make it to Einstein with any sort of non-swerve omni drive. Maybe Omni Drives that rely on Omni Wheels and Mecanum Wheels just aren't a good fit for FRC, who knows?

Chris is me 21-01-2012 10:33
Re: Mechanum Wheels?
 
I'm just going to chime in and add that while mecanums aren't ALWAYS bad... 2012 really isn't the year for them.

lcoreyl 21-01-2012 14:05
Re: Mechanum Wheels?
 
Quote:
Originally Posted by Chris is me (Post 1110402)
I'm just going to chime in and add that while mecanums aren't ALWAYS bad... 2012 really isn't the year for them.
for whom? If you are a rookie or just inexperienced team, and you want to be able to shoot, mecanum is pretty simple to implement (if you buy AM wheels anyway) and takes care of your drive and gives you an easy way to line up your shot and your approach to the bridge.

EricH 21-01-2012 14:07
Re: Mechanum Wheels?
 
Quote:
Originally Posted by lcoreyl (Post 1110510)
for whom? If you are a rookie or just inexperienced team, and you want to be able to shoot, mecanum is pretty simple to implement (if you buy AM wheels anyway) and takes care of your drive and gives you an easy way to line up your shot and your approach to the bridge.
Mechanically, yes.

Programming, not really. If you haven't done mecanum in the offseason, it's generally a bad idea to do it in the normal season. The programming can take a while.

lcoreyl 21-01-2012 14:13
Re: Mechanum Wheels?
 
Quote:
Originally Posted by EricH (Post 1110513)
Mechanically, yes.

Programming, not really. If you haven't done mecanum in the offseason, it's generally a bad idea to do it in the normal season. The programming can take a while.
We have no experienced programmers (what is C++?) and received them during week 2 last year, and got them running in one day. We got gyro and encoder feedback working within a week.

Chris is me 21-01-2012 14:16
Re: Mechanum Wheels?
 
Quote:
Originally Posted by lcoreyl (Post 1110518)
We have no experienced programmers (what is C++?) and received them during week 2 last year, and got them running in one day. We got gyro and encoder feedback working within a week.
Oh, I didn't know it was THAT easy. I just figured 6WD takes basically zero setup time.

EricH 21-01-2012 14:23
Re: Mechanum Wheels?
 
Quote:
Originally Posted by lcoreyl (Post 1110518)
We have no experienced programmers (what is C++?) and received them during week 2 last year, and got them running in one day. We got gyro and encoder feedback working within a week.
Back in my day, we didn't get any of this pre-canned code or pre-made wheels. You young whippersnappers don't know how good you have it. It took us several weeks or more to get anything mecanum together!</geezer>

And yes, that is actually a true statement. It took 330 two years of off-and-on work to go from seeing a set for the first time, not in an FRC application, to a full-size, fully-functional set (2003 after Championship to 2005 build season). And they still haven't used mecanums in competition.

Mara 21-01-2012 14:40
Re: Mechanum Wheels?
 
They're expensive and our team doesn't have any experience with them. We always end up sticking with good ol' tank drive.

xXD3sEcR8xX 21-01-2012 14:50
Re: Mechanum Wheels?
 
Quote:
Originally Posted by the man (Post 1110148)
I dont get it. As long as your not going mock 5, ok ok at fast speed in terms of robotics competitions, the back wheels will continue to push as you some what gently push your self over the bump. But we will test this tomorrow.
Mach 5... just saying

Ether 21-01-2012 15:14
Re: Mechanum Wheels?
 

Programming a mec for basic functionality is trivial, even without the WPI library function.

Just take your driver commands for forward, strafe right, and rotate clockwise, and compute the speed of each wheel as follows:

Code:
front_left  = forward + clockwise + right;
front_right = forward - clockwise - right;
rear_left  = forward + clockwise - right;
rear_right  = forward - clockwise + right;
... then normalize the wheel speeds as necessary before sending the commands to the motor controllers.

theprgramerdude 21-01-2012 15:22
Re: Mechanum Wheels?
 
Plus, with the library functions, it's quite literally one line of code.

They also DO have the functionality built in for field oriented drive; the
MecanumDrive commands are overloaded, so it's either a robot oriented control or field oriented, gyro based control.

Programming issues for mecanum (It wants to auto correct to Mexican???) driving really isn't an issue.

Crab/Swerve is another ball game.

apalrd 21-01-2012 18:27
Re: Mechanum Wheels?
 
As a driver who has driven plenty of tank style skid steers, halo style skid steers, swerves, and slide drives (in VRC), this is my opinion on driving:

The third degree of freedom(strafe) takes much longer to master than the first two (forward and rotation). This is true for mecanum as much as it is for swerve and slide drives.

My other opinion to mecanums, or any other omnidirectional drive system, is that the ability to go sideways should be taken with its advantages and disadvantages. Don't ever go with mecanums just because they look cool.

I have not yet seen a video of a mecanum drive sliding around a defender, as many people make mecanums out to be. I have seen a very well driven swerve do it, but I have not yet seen a mecanum gracefully fake out or otherwise evade a somewhat competent defender. If someone can show me this, maybe I'll stop looking so far down on mecanums.

Ether 21-01-2012 18:51
Re: Mechanum Wheels?
 
Quote:
Originally Posted by apalrd (Post 1110728)
halo style skid steers
Could you please explain what you mean by a halo style skid steer? It seems like an oxymoron*.


*I've never played Halo (so my knowledge of it is second-hand) but had been led to believe that the use of "Halo" in FRC context referring to driver interface was related to strafing.


EricH 21-01-2012 18:59
Re: Mechanum Wheels?
 
Quote:
Originally Posted by Ether (Post 1110739)
Could you please explain what you mean by a halo style skid steer? It seems like an oxymoron*.


*I've never played Halo (so my knowledge of it is second-hand) but had been led to believe that the use of "Halo" in FRC context referring to driver interface was related to strafing.
As I understand the Halo controls (not having played it), one stick is motion and the other is rotation. In an FRC application, it would work better on a mecanum or swerve. The other similar type is a throttle stick/steering wheel setup.

Ether 21-01-2012 19:09
Re: Mechanum Wheels?
 
Quote:
Originally Posted by EricH (Post 1110744)
As I understand the Halo controls (not having played it), one stick is motion and the other is rotation.
Yes, but the "one stick" which is motion commands Y and X translation, i.e., forward/reverse and strafing.

So I'm trying to imagine what a Halo-style skid-steer would be, since you can't strafe a skid-steer.

The only mental image I can conjure up is arcade drive with 2 joysticks; one of which uses only the Y-axis (for forward/reverse) and the other uses only the X-axis (for rotation). But I can't see how that's better than a one-stick arcade.

apalrd 21-01-2012 19:13
Re: Mechanum Wheels?
 
The "Halo" HMI uses the left stick as translation and right as rotation (4 DOF - forward, strafe, look side, look up). We just don't use any of the DOF's that the robot can't drive in, so for a skid-steer we use the Y axis of the left stick as the "throttle" and X axis of the right stick as "wheel" in an arcade or cheesy style drive. We use a cheesy style drive but automate the "quick-turn" input based on wheel values (more later in the post).

We've also implemented Halo controls on slide robots (3 DOF), and find them very nice to drive. However, while facing yourself, it takes much longer to get used to the orientation and slide the opposite way, something some of our VRC drivers handled very well, and something one of our VRC drivers never really handled. This is by far the best HMI for a 3DOF robot.

The "Cheesy Drive" is 254's arcade drive which uses "quick turn" and "speed turn" algorithms to find a much better solution than a standard arcade drive:
-When in quick turn, the output is a standard arcade drive -> Left = throttle + wheel, right = throttle - wheel
-When in speed turn mode, left = throttle + (wheel * throttle * gain) and right = throttle + (wheel * throttle * gain), so it handles speed adjustments properly.
-Quick Turn is only used when throttle is at or near 0, so you can turn without applying forward power.

Ether 21-01-2012 19:54
Re: Mechanum Wheels?
 
Quote:
Originally Posted by apalrd (Post 1110752)
The "Cheesy Drive" is 254's arcade drive which uses "quick turn" and "speed turn" algorithms to find a much better solution than a standard arcade drive:
-When in quick turn, the output is a standard arcade drive -> Left = throttle + wheel, right = throttle - wheel
-When in speed turn mode, left = throttle + (wheel * throttle * gain) and right = throttle + (wheel * throttle * gain), so it handles speed adjustments properly.
-Quick Turn is only used when throttle is at or near 0, so you can turn without applying forward power.
This table might be of interest.

The first column is for the simple L=Y+X, R=Y-X with normalization. The second column is the same as the first except it uses clipping instead of normalization. The third column is a slightly (very slightly) more complicated algorithm which I believe gives the same results as what you get from the WPI library (and may indeed even be mathematically identical... I haven't checked).

ThirteenOfTwo 21-01-2012 21:25
Re: Mechanum Wheels?
 
Quote:
Originally Posted by SuperNerd256 (Post 1110153)
I'm not saying no mecanums (or however you spell it), but these are some points made in previous threads:

-Less traction on the key
-As already said, loss in pushing power
-Heavy
-Forces you to use maximum number of motors for drivetrain (not usually an issue)
-While they may not affect your ability to go on the bridge, the ability to stay without being pushed off or falling off of the bridge tips your direction will be tough to deal with
-You won't be driving circles around other's bots unless you have a lot of practice.
-In order to work to their fullest extent, require a lot of thorough engineering
-More complicated to code, even at the minimum level


Like I said, I'm not telling you not to use them, but when you do, be careful and engineer it through (Not meant to sound like most interesting man in the world meme, though I'm tempted. ;))
Most of these are fairly situational arguments. We're going with mecanum this year and I'm fairly happy with the decision, though I suppose that requires some justification. A little history is in order. This is gonna be long...

We, like many other Hawaii teams, started in Overdrive. We built an 8-wheel tank drive and did fairly well, though our claw was too easily damaged and we didn't make it very far in the competition. We observed that some of the drives that were highly successful in Overdrive were omnidirectional drives that could abuse the "lap" rule and kept it in mind for later use.

Lunacy was Lunacy, of course. We didn't get a chance to try omnidirectional movement, though we actually ended up being a really solid defensive robot because of our uncommon three-wheel single-swerve drive. We misanalyzed the game and didn't build a floor collection device, so we didn't make it out of Hawaii. Our driver loved being more mobile and better at turning than all of the other robots at the competition.

In Breakaway we again misanalyzed the game and thought that crossing the bumps was going to be an important part of gameplay, which it really wasn't. But we didn't want to be able to be blocked coming off of the bump, so we figured we needed horizontal movement on top of the bump or right behind it. So we picked mecanum. We did fairly well that year, captaining a semifinalist alliance, but were knocked out by the eventual champions. However, or "ball magnet" was very substandard and we couldn't strafe while using it, so we ended up not using the strafing at all. It was pretty bad, honestly; we had no room to use mecanum's maneuvering and we couldn't even really strafe. However, our driver got some practice maneuvering outside of the competition and we got software for mecanum developed.

In Logomotion we analyzed the game perfectly well, but the design we set out to build was beyond our capabilities to achieve. We decided on mecanum because we had had experience with it the previous year, we thought that strafing in the zones would be useful, and we liked the added maneuverability. The drive was actually great, but our arm was finished too late and we were unable to use it at competition, rendering us a defensive/feeder robot. We used the newly-lightning-fast mecanum drive to great effect in feeding, but because we were a feeder and not a scorer, we once again failed to make it to Worlds. However, we improved our mecanum software, got used to how strategy plays with mecanum, and saw how useful it can be on a big field.

This year, we saw the field and immediately thought "mecanum". The wide-open spaces, no-pushing zones, and small game objects just scream for a robot that uses maneuverability rather than power to avoid opponents. We'd gotten used to the idea of the mecanum drive, we had a very good one from the previous year sitting in our shop to model off of, and our analysis showed that bump-crossing (sort of like in Breakaway) is nice but not necessary. Our driver, who had gotten very good with mecanum, has sadly graduated, but our new trainees have taken to it quickly. We're pretty happy with our decision.

So with regards to your points...
1) Precise motion on the key is fairly irrelevant if you turret your ball-scoring device.
2) The loss in pushing power actually isn't as severe as people make it out to be. We've never been pushed around by a tank drive and we've even pushed a few (sideways) ourselves.
3) It's possible to get an effective robot very light this year by consolidating mechanisms. We are estimating that we'll come in way light even with mecanums. We need the extra weight.
4) Not an issue because there are so many motors this year.
5) Refuted by our testing of the bridge balancing two mecanum robots. Sorry, we don't have video.
7) Already done in previous years.
8) Already done in previous years. (Breakaway was sort of a happy mistake...)

You may have noticed that I skipped 6. I did it because I wanted to add even more text to an already unnecessarily overlong post.

Too many people think of mecanum in ways that are fundamentally wrongheaded. Mecanum isn't going to "run circles around" a competent defensive robot. Honestly, if you're running circles around anyone, regardless of your drive, their drive is probably atrocious. Neither is pure horizontal strafing very useful. The actual value of mecanum is in the more complex maneuvers that few first-time-with-mecanum teams discover (we certainly didn't discover them in '10). The bootleg turn, circle strafing, and impact redirection are three examples of genuinely valuable things that mecanum enables you to do much more easily than tank drive does, but you hardly ever hear about them in mecanum discussion. Pro-mecanum people and anti-mecanum people alike often don't know what they're talking about.

Other assorted thoughts on mecanum drives, because apparently I don't feel like I've typed enough yet...

The pushing-power thing is blown way out of proportion. Unless your drive is stupidly weak or your opponent's is stupidly strong, you're not going to get shoved around, and mecanums can push tanks sideways (and occasionally head-to-head).

Mecanum can maneuver well on the bridge and the key alike, based on our tests. Super-precise maneuvers on the key don't seem all that useful anyways.

The fragility thing is blown way out of proportion, too. As long as you don't do anything stupid (like drive over the bump this year without wheel guards), you'll be fine.

Not everyone can drive a mecanum well. Some people who pick them up drive them like tank drives that can strafe horizontally, which is dumb, because that just makes them weaker tank drives with a bonus feature that your driver is likely to forget about. It does take practice, and it also takes a different way of thinking.

The no-mecanums-on-Einstein thing is a silly myth. FRC is fairly stratified and teams feel comfortable doing what they've done before; teams who have been around for a while (which tend to be the best teams) have been doing tank for a long time. Many teams who have the resources to get to Einstein have the resources to build a swerve instead of a mecanum. Which is better.

Even though we've used mecanums each of the last three years, I'd like to think we're not set in our ways. This year, we essentially knew going in that we wanted to use mecanum because it was the only thing that we've really refined. However, this was a good year to be forced to use mecanum. I've always had some doubts--a lot of very smart mentors from very good teams swear off mecanum, and I figure they can't all be wrong and we can't be the only ones who are right. We're planning to branch out next summer when we have the resources and the time to develop a good new drive.

In conclusion, mecanums are good when:
-there are wide-open spaces on the field
-game objects are fairly small
-your programmers have a modicum of experience
-the game is not pushing-heavy

Mecanums are bad when:
-your driver is entrenched into "tank-thinking"
-your programmers are very inexperienced
-the field contains many obstacles or small spaces
-you need to push to get access to a zone
-you have the resources to build a swerve

I hope that this post didn't bore anyone to tears and that it was informative, if only to help you see the (maybe misguided from your point of view) thought process of someone who's coached a mecanum robot for all of his time in FRC.

pfreivald 21-01-2012 21:39
Re: Mechanum Wheels?
 
Quote:
Originally Posted by EricH (Post 1110744)
As I understand the Halo controls (not having played it), one stick is motion and the other is rotation. In an FRC application, it would work better on a mecanum or swerve. The other similar type is a throttle stick/steering wheel setup.
This is why I chuckle (not out loud, due to an overbearing sanity) every time I read something about mecanum being hard to program or to use. Mecanum straight out of the box works very well (as long as your CG is in the center of the wheels), and -- this is the big part -- controls EXACTLY like a first-person shooter. This means that your average driver has a gazillion hours of practice controlling an out-of-the-box mecanum-type system, even if they've never done a minute of FRC. We love mecanum drive, and this will be our third year using it.

That said, IMNHO straight-up mecanum will have a much bigger problem this year than barriers and bridges, and those using it will find that other robots are a great impediment to success.


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