pic: 2605 Mecanum Drive

[Ether]

Originally Posted by Twins Inc.

I imagine that the bot with the higher gear ratio, bot A, (or lower fps) would have more control and less acceleration than B. But also more heating, and power consumption because of more motor use/rpm. Or am I mixing things?

Thanks for starting the discussion. I'm going to withhold comment for a while and let others discuss.

[Mr. Lim]

Originally Posted by Ether

Do you have a video you could post.

Yep:
http://youtu.be/pRc0LtIe5VI

It's hard to see, but when the robot sprints straight forwards or backwards, the wheels are spinning faster than the robot is moving. We expected it to happen when you are moving sideways or diagonally, but were surprised to see it happening when we moved straight forwards and backwards too.

This is an old movie from when I was a mentor on 1310. This was the last time I was personally involved with a mecanum drivetrain, so it was some time ago.

[Ether]

Originally Posted by Mr. Lim

It's hard to see, but when the robot sprints straight forwards or backwards, the wheels are spinning faster than the robot is moving.

It looks like the wheels are just losing traction on the floor.

Normal mec behavior in the forward/reverse direction is for the wheels to turn at the same rate as the vehicle. Without losing traction, roller free play and floor compliance will cause some deviation from this, but not to the extent shown in the video.

[Culvan Van Li]

Originally Posted by Ether

Physics Quiz:

Imagine you have 2 robots which are identical in every way, except that Robot A is geared for 8 fps, and Robot B is geared for 16fps but has the voltage limited to 50%.

Describe the performance differences between the 2 robots: acceleration, motor heating, battery drainage, pushing force, fine control of slow maneuvers, etc.

1) Acceleration for Robot B will theoretically be slower. It isn't as bad as you might expect because Meccanum wheels have a lower coefficient of friction than traction wheels, so some of the extra torque is lost to slipping.
2) Motor heating will depend on a large number of variables. In general terms 70.7% voltage will reduce current to 70.7% as well, resulting in only 50% of the power and heat. However the actual answer has more dependencies. If the lower amount of gearing is accomplished by removing a stage of gearing then there will be a ~9% increase in power available. The specifics of where you are on the torque curve may also make a difference.
3) Motor Heating is going to be dominated by the power consumption as listed in the previous answer.
4) Battery Drainage will also be determined by power consumption.
5) Pushing force for a meccanum robot is determined primarily by the coefficient of friction of the wheels. You lose 29.3% right of the bat because of the way the rollers work and the rollers don't have that much traction either. If you chose a meccanum drivetrain you made the choice to go around opponents, not through them.
6) Fine control of slow maneuvers is a matter of driver skill. We've found that very few students naturally think in meccanum driving terms. Even those who do, have trouble with the fine control. That's why we added the half speed mode. Like so many other things this is a trade-off we have chosen. We want to be able to cover a lot of field when we need to and we want to have reasonable control when we need it. We probably would have more control if we geared straight for the lower speed, but then we won't have the higher top end speed. The trade off works for us.

[Culvan Van Li]

Originally Posted by Mr. Lim

Yep:
http://youtu.be/pRc0LtIe5VI

It's hard to see, but when the robot sprints straight forwards or backwards, the wheels are spinning faster than the robot is moving. We expected it to happen when you are moving sideways or diagonally, but were surprised to see it happening when we moved straight forwards and backwards too.

This is an old movie from when I was a mentor on 1310. This was the last time I was personally involved with a mecanum drivetrain, so it was some time ago.

You are absolutely correct. The robot will move slower than the wheels are spinning. The forces are applied at a 45 degree angle to the movement of the wheel so the robot will move at a speed = wheel speed * Sin(45 degrees). That's 70.7% of the speed of the wheels.

If the rollers are too stiff then the robot may move faster than that forwards and backwards and slower sideways. If the rollers have low friction then a meccanum robot will drive the same speed sideways as forward.

[Mr. Lim]

Originally Posted by Ether

It looks like the wheels are just losing traction on the floor.

Normal mec behavior in the forward/reverse direction is for the wheels to turn at the same rate as the vehicle. Without losing traction, roller free play and floor compliance will cause some deviation from this, but not to the extent shown in the video.

Well, the two mecanum drives I personally worked on both did this. We set them up pretty diligently. All rollers ran smoothly, there was minimal axial free play. These were old style AndyMark soft rollers which had great traction, but wore very quickly.

From watching a video it may look like the wheels are losing traction, but they aren't. In person I can tell you the wheels weren't "slipping" on the floor.

When we drove straight forward and backwards, we noticed that the rollers rolled. It was noticeable because when we drove forward or backwards and stopped quickly, you could still see the rollers on the tops of the wheels spinning!

I don't know what a "normal" mecanum is supposed to do, but I do know that's what our mecanums did. If I did it all over again, I would gear for a higher speed knowing that the roller action will reduce your effective top speed.

[Ether]

Originally Posted by Mr. Lim

It was noticeable because when we drove forward or backwards and stopped quickly, you could still see the rollers on the tops of the wheels spinning!

The key here is "stopped quickly". Probably quickly enough to cause the mecs to slide and the rollers to spin.

The affordable mecs with bearingless rollers used for FRC will exhibit some roller rotation (and associated loss of vehicle speed) due to axial free play in the rollers and compliance of the floor material (carpet).

The only way to resolve this discussion is to take some high speed timestamped video of a mec traveling at a constant speed in a straight line in the forward direction.

If anyone reading this has access to such a video would you please post it?

[Ether]

Originally Posted by Culvan Van Li

the robot will move at a speed = wheel speed * Sin(45 degrees). That's 70.7% of the speed of the wheels.

May I ask where you got this information?

[wireties]

We used mecanums for 4 years and my observations jive with Ether's statements. There is no limit to max fwd velocity related to the angle of the rollers - no question. Our designs were square with even weight distribution and encoder feedback. The forward speed was very close to the calculations (same used for tank). It is very different going sideways of course and it might slip quicker if you accelerate too quickly. We pushed kit bots around, went over bumps, got up on the bridge etc. The 70.7% number is tossed around all the time but it is not true (going straight forward), if mounted properly the rollers do not rotate all the time.

[Culvan Van Li]

Originally Posted by Ether

May I ask where you got this information?

We've used encoders to try to determine our position on the field while using Mecanum. That was a process of calibration because some slip is expected. Adjusting our expectations by sin(45) got us much closer to what we actually saw in 2013, when we used the Vexpro Mecanum. In 2011 & 2010, when we used the Andymark Mecanum, the rollers were stiffer and we didn't see the same behavior. I'm only working from memory for 2011, but when we measured the speed of the robot it was close to what we expected for fixed wheels.

When we get far enough on our robot this year, I'll be sure to repeat the encoder position check. This year's robot may not be a good choice to use as a benchmark though, I'm pretty sure our weight will not be evenly distributed.

[JamesCH95]

Originally Posted by Twins Inc.

I imagine that the bot with the higher gear ratio, bot A, (or lower fps) would have more control and less acceleration than B. But also more heating, and power consumption because of more motor use/rpm. Or am I mixing things?

You're mixing things.

To answer Ether's quiz - A will be superior to B in every performance metric he listed with the possible exception of pushing force, as that would depend greatly on the CoF of the wheels against whatever traction surface they're on.

Assuming the theoretical drives are using CIMs, this chart explains almost everything:



The motors' efficiency is maximized in the 'high speed' regime (left-hand side of the plot), where it will operate when paired with a higher-reduction transmission and no voltage reduction. This means that the input electrical power is converted into more mechanical power and less waste heat as compared to lower-speed operation. There is also more available 'power under the curve' when full-speed operation is allowed, which results in improved drive acceleration (what some might call 'response').

Furthermore, the higher-reduction gearbox that allows full-speed motor control will have better drive resolution. By reducing the allowed output voltage of the motor controller the total range of allowed outputs is reduced.

In my opinion it is very poor design practice to limit motor voltage as a means of maximum speed control, especially when gearing options are so readily available. Not utilizing the motors' full speed capabilities handicaps the drive, which is arguably the most important system on every robot.

[holygrail]

Originally Posted by JamesCH95

To answer Ether's quiz - A will be superior to B in every performance metric he listed with the possible exception of pushing force, as that would depend greatly on the CoF of the wheels against whatever traction surface they're on.

In my opinion it is very poor design practice to limit motor voltage as a means of maximum speed control, especially when gearing options are so readily available. Not utilizing the motors' full speed capabilities handicaps the drive, which is arguably the most important system on every robot.

Two things:
1) A will be superior to B in most performance metrics EXCEPT for top speed. All the driver of B has to do is push a button and A is quickly left behind.

2) It is true that there are an abundance of gearing options including two speed gearboxes (which make no sense on a mecanum drive). The limiting of voltage is only used in this example when high speed maneuvers could be dangerous or strategically bad, like knocking over scored game pieces. I agree that it would be silly to limit voltage without the option of using 100% power, but that is not what we are talking about here.

[Ether]

Originally Posted by holygrail

All the driver of B has to do is push a button and A is quickly left behind.

In the problem statement for the quiz, there was no button for B to push

I agree that it would be silly to limit voltage without the option of using 100% power, but that is not what we are talking about here.

It is what the quiz was talking about, in order to drive home the point that limiting top speed by limiting voltage is not equivalent to limiting top speed via gearing.

I know you realize this, but many rookie teams reading this thread may not.

[JamesCH95]

Originally Posted by holygrail

Two things:
1) A will be superior to B in most performance metrics EXCEPT for top speed. All the driver of B has to do is push a button and A is quickly left behind.

2) It is true that there are an abundance of gearing options including two speed gearboxes (which make no sense on a mecanum drive). The limiting of voltage is only used in this example when high speed maneuvers could be dangerous or strategically bad, like knocking over scored game pieces. I agree that it would be silly to limit voltage without the option of using 100% power, but that is not what we are talking about here.

To address 1) Top speed, by itself, means very little in the context of playing a game. Time-to-target does. 610 epitomized this in 2013 with a 6-CIM drive with a top speed of something around 10-11fps. They 'quickly left behind' many teams geared for a higher top speed.

2) Like Ether said, this is exactly what we are talking about.

Having said that, 95 is experimenting with mecanum wheels and using the joystick throttle to change the joystick's input scaling in order to slow down the drive-train when desired. However, our intention is FAR different from that of the average team who builds a drivetrain and goes "oops, too fast and twitchy to control, better cut down motor output!"