Hybrid Mecanum 6-wheel Drive Idea

VEX V5 General Discussion General Forum
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Howdy partinoids,

I was thinking the other day about the pros and cons of using mecanum wheels, mostly their flexibility even in normal orientation. However, mecanum wheels keep you from chaining your wheels together to distribute power, and you lose efficiency through the idler wheels. Thinking of a way to solve these problems and looking at our large stock of unused vex differentials, I came up with a way to mix the power from the mecanum wheels into a third omni wheel while maintaining omnidirectional motion. As I can’t think of a specialized use for this, and I think FIRST teams with their radical drive ideas might find this handy, I’ve decided to share it.

The design centers around using a differential gear in reverse. Normally the outer case is driven while the inner gears are split between two back wheels, but in this case the two inner gears are linked to the front and back mecanum wheel shafts. The outer case of the differential drives the center omni wheel, one per side of the robot.

Here’s how it works

  1. Mecanum wheels move normally forward/backward. Idler bevel gears inside the differential are driven both ways at the same time, causing the outer case to spin, powering the omni wheel in the same direction as the mecanums.

  2. Mecanum wheels move in opposite directions to strafe. Bevel gears are driven in the same direction. Outer case of the differential doesn’t move, so the omni wheel just coasts left-right on its rollers.

  3. Diagonal strafing is a bit more complicated, but if assuming 45 degree angles, the omni wheel will be driven at about half speed.

Good demonstration of a differential.

Diagram demonstrating this design. Sorry, I’m still getting used to drawing on a tablet.

1 Like
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WOW
that was a great vid on explaining on how differentials work!
+1

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Great idea! I believe differentials are an underestimated part. I have had some success in making drive bases with them. I will post pictures when I have a chance. The hardest part about them is the time it takes to build complicated drive systems. Detail is critical and it is easier to just direct or chain drive the wheels. I am looking for a equation to describe their motion. The closest I have found is z=c(x +/- y) from a mechanical calculator. You can make your drive line more compact by putting the differential on the same plane as all of the wheels and keeping the wheels lined up. As opposed to having it above the wheels.

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Unfortunately my team only has 2 differentials so I can’t make the differential drive I had wanted to make, but if someone makes a differential drive I’d love to see it :slight_smile:

What I use is the output is the average of the two inputs. If the two inputs are going in the same direction, the output is the same rotational velocity (z=(1+1)/2=1). If directions are opposite, output is 0 (z=(1-1)/2=0).
The torque will be doubled because you have two motors. Even if one motor does not move, the torque is doubled because it’s 1/2 the speed.