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#76
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Re: Swerve Drive vs Mecanum Wheel drive?
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#77
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Re: Swerve Drive vs Mecanum Wheel drive?
The lag time on getting a swerve drive to start strafing is insignificant. It's 1/2 of a second to 1/8 of a second.(shortest path algorithms help a lot) The drive motors are already ramping up at this point. By the time any other omni directional drive gets to speed the swerve will be their the extra acceleration that a swerve has strafing over mechanum is amazing.
On another note if you have a swerve drive, who spends their time pushing a west coast drive around. Most good teams with swerve know that the whole goal of a pushing match is to waste time. You can role out of pushing matches really easily. We have some really cool code that changes our center of rotation making doing this really easy. If done right you can use the other robots pushing power to help you accelerate past their top speed. (if you have a higher top speed) If a swerve is playing defense just turn your wheels perpendicular to their thrust, Pushing a robot sideways is much much harder then trying to push one head on. With a swerve their is no "foward" making pushing agenst it is hard if the drivers know how the thing works well. If you do try pushing a swerve side ways the lifting motion on the wheels you are next to will cause the outhes to dig in to the carpet making the while thing much more difficult. |
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#78
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Re: Swerve Drive vs Mecanum Wheel drive?
You mention the fact that you have easily plowed through holonomic drivetrains as the proof that ALL holonomic drivetrains will be easy to plow through. I’m sure you have plowed through another bot with tank drive too, so why doesn’t it follow that ALL tank drivetrains will be easy to plow through?
My point is you are overstating your point to the degree of being misinformation. tank is better at pushing than mec or swerve, but let’s not make someone who is looking to get information think that any team with a thrown together tank will be able to easily push around any holonomic drive. Quote:
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statement 2 means that 2 wheels are off the ground. You are saying a typical pushing match ends up with 2 swerve wheels off the ground, and I find that to be misleading Last edited by lcoreyl : 11-04-2014 at 19:10. Reason: forgot a quote |
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#79
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Re: Swerve Drive vs Mecanum Wheel drive?
From what I have seen mechanum has 3 problems.
1: Almost every time I see a team with mechanum I see the having issues with being pushed around and playing bad defense because they can be easily moved. 2: Drifting, I guess drifting can be good sometimes but when you are drifting every time you do a turn at high speed then I can see it being a problem. 3: From what I have heard from people on my team, (maybe true maybe not) they cannot go as fast because their speed is limited. the third problem may not exist, but I think it does because I trust my teammates. |
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#80
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Re: Swerve Drive vs Mecanum Wheel drive?
The third one is both true and false :) The speed is equal to the speed of any other wheel of the same diameter when traveling forwards/reverse, but the speed when just strafing is only sqrt(2)/2 times the maximum forward speed (I believe... its definitely less than 1). Tank doesn't have this option in the first place, and swerve has 100% of its forward speed when traveling sideways.
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#81
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Re: Swerve Drive vs Mecanum Wheel drive?
A lot of misinformation in this thread.
Mecanum drivetrains have a fraction of the traction that skid steers does. This really isn't debatable. There is no difference in traction between a good and a bad mecanum drive, they have the same 4 wheels touching the ground, if they are static there is nothing that makes them any different. The only difference is in gearing but that doesn't matter when the robot is sitting still. Most drivetrains in FRC (especially with the advent of the 6 cim drivetrain) are traction limited, not limited by the power output of the motors. Our drivetrain is geared for ~24 fps in high gear and it's still traction limited. A well designed swerve drive can have just as much pushing power as any tank drive assuming it uses the same tread material. The maneuverability of mecanum or omni wheel drivetrains can be very advantageous for offense, however it's been proven time and again that they really can't do anything on defense. After watching over 400 matches just this season I can't even count the number of times I've seen a mecanum bot just get brushed aside by a tank or swerve drive as if they weren't even there. Robots rarely have their wheels lifted off the ground. Nearly all pushing matches have both robots with all 4 wheels planted on the ground. Despite their weight shifting to the back wheels their force applied on the ground is relatively constant and it marginally affects a robots traction. Mecanum wheels never behave the same as traction wheels. The rollers on mecanums make it so there is always a slight loss in efficiency since those rollers will always spin slightly unless you are going perfectly perpendicular to their direction of rotation. Also the rollers have a relatively smooth surface so even if the wheels are traveling perpendicular to direction of the rollers they still slip and cause a loss in efficiency. It's not any easier to push a robot (on traction wheels) backwards than is to push it sideways assuming that it has enough power to at least keep it's wheels from spinning. It's the same amount of friction in every direction as long as the wheels are locked. Do not take this to mean that mecanums are bad. Just stop saying that they can do things that they can't. Last edited by themccannman : 12-04-2014 at 01:16. |
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#82
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Re: Swerve Drive vs Mecanum Wheel drive?
Here are two somewhat minor advantages to mecanum drives vs tank drives that I have never seen discussed.
1. I have never ever seen a mecanum robot tip over because they braked too fast, but I have seen numerous tank drive bots do this. Given, this can be corrected fairly simply with some ramp-up code, but most teams don't have anything like this in their code. 2. It is not possible to (for lack of a better word) pin a good mecanum bot in the center of the field with nothing around. Almost every tank drive I have seen gets trapped when a robot side loads them. They can still spin in circles, and maybe manage to break free this way, but a good mecanum robot can spin off quickly in this situation. Value these things as you will, but keep them in mind. |
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#83
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Re: Swerve Drive vs Mecanum Wheel drive?
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The only time the rollers do not spin1 is when going forward and backward, is which case the vehicle is not going perfectly perpendicular to the direction of rotation of the rollers. Quote:
1In theory, with no roller axial free play and a non-compliant floor surface. |
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#84
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Re: Swerve Drive vs Mecanum Wheel drive?
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The reason most mecanums have noticeably slower strafing speed is due to roller friction, roller axial free play, and floor surface compliance. |
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#85
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Re: Swerve Drive vs Mecanum Wheel drive?
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I'm not trying to stir up an argument, I'd just like to see something the supports these claims. |
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#86
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Re: Swerve Drive vs Mecanum Wheel drive?
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And even if they were the same part number, who said they were all assembled and adjusted with the same care and attention to detail? Differing amounts of friction in the rollers (due to wheel design, manufacturing tolerances, and care in assembly and adjustment) affects the wheel-to-floor traction of the wheels. A detailed explanation of the physics can be found here. Quote:
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#87
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Re: Swerve Drive vs Mecanum Wheel drive?
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Hope that clears things up. |
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#88
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Re: Swerve Drive vs Mecanum Wheel drive?
Jake, I agree that if you assume there are no differences between "good" and "bad" mecanum bots then there will be no differences between them. I just think that's a bad assumption, in practice. When you said "The rollers on mecanums ... will always spin slightly unless you are going perfectly perpendicular to their direction of rotation" I read that to mean that the only time the rollers on a mec wheel on a mec vehicle would not spin is when said wheel was going in a direction perfectly perpendicular to the direction of rotation of the roller presently touching the ground (i.e., going in the direction of that roller's axis). If that is what you meant, it is not correct. If that is not what you meant, could you please clarify what you did mean? In practice it is possible to build a mecanum vehicle which, when operated in the forward (or reverse) direction on a hard surface, will exhibit virtually no motion of the rollers. In other words, spinning rollers in the forward/reverse direction is not an inherent property of mecanum wheels: it is a mostly due to roller axial free play and operating on a compliant surface. |
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#89
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Re: Swerve Drive vs Mecanum Wheel drive?
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As a driver from 2996, I would have to say that I personally don't like the robot auto correcting for me, It slows me down. I prefer no sensors during driver control because at least for me, it isn't as responsive in a controlling sense. Accurate, yes. Responsively accurate, no. Just my opinion for the way I drive. Last edited by lgphoneeric : 13-04-2014 at 16:49. Reason: I failed at quoting the first time. |
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#90
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Re: Swerve Drive vs Mecanum Wheel drive?
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