pic: 2605 Mecanum Drive



Here is an example of our Mecanum Drive this year. We all agree that this by far the simplest drive we have made. The wheel-gear is made of phenolic, which was machined along with the CUI-encoder mount/cap on the CIM. I featured this mount late last year, and this is their intended use. We also made encoder cables out of ethernet cable, which wind nicely. I would highly suggest doing this. We are using them with CAN Jags at the moment.

Do you have something keeping the belts tensioned apart from the screws to tighten the cim down. If it doesn’t I think it will gradually get loose and the belts might fall off. What I’m thinking of is something like the WCP cam tensioner.

Really love the simplicity of this, and it’s a perfect use for your encoder mounts.

What’s the overall reduction, and what size wheels are those?

We’ve done chain drivetrains like this and found that they slip unless you use something like star washers on the bolts. This was sixteenth inch sheet metal though, these tubes might work better.

+1 on the slippage issue. That was our primary problem last year. Add a WCP cam or something to keep it tensioned or it will slip.

I believe the reduction is 9:1, and those are 6 inch AndyMark Wheels (not HD). We’ll be cutting a smaller reduction soon, because these gears actually pinch the belts on the ground and cause problems.

Thanks for the input! We’ll look into adding a cam next to each motor.

At 9:1 with 6" wheels the bot free speed is already on the high side for this year’s game, no?

We had the exact same gearing and wheel size last year, and it was only a moderate speed on the field.

Am I the only one a little worried about the side load you’re putting on those CIMs? Direct belt drive off a CIM just doesn’t seem like a great idea, even if they’re super robust

9:1 on a 6 inch wheel is 15.4 fps free speed. Twins Inc indicated they’re going to gear it even faster. Some would say that’s a bit too fast for this year’s game. How fast do you plan to gear your bot this year?

We usually gear for around a 15 to 17 fps free speed but then we use code to tone it down a bit so that when you push the joystick forward you only get 50 to 70% of the full speed. We have a turbo button on the joystick that the driver has to push for full speed. Works well.

With mecanums it’s not a bad idea to gear a bit on the fast side anyways. Even though your wheels are spinning at a particular speed, doesn’t mean your robot will move at the same speed - especially if your mecanum rollers don’t have a lot of friction, which is ideal for moving sideways.

It’s one of the strangest things to see a mecanum drivetrain drive, and watch the wheels spin faster than the robot is travelling… :mad:

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.

Do you have a video you could post.

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.

Yep:

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.

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.

  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.

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.