Saturday, December 17, 2011

Synchro Drive in Lego

Here is my take on a synchro drive using Lego:

It's inspired by, but different from, aeh5040's creation in the following ways:

  • I used wheel 3482 and tire 3483 because I have a theory that it will help prevent "walking" when the mechanism is turned for steering. More on that theory later.
  • I used a 12-tooth bevel gear instead of a 20-tooth bevel gear, again to help prevent walking. I believe it is advantageous to employ a reduction in the final driving gears (perhaps because it means less torque in the vertical drive axis), so I may revisit this at another time.
  • The wheel is 1 stud further from the turntable to provide clearance for the wheel and tire. I admire the way aeh5040's design has just the right amount of space for it's pulley and tire. It feels "just so." But I couldn't replicate that with my tire choice. So the supporting triangles had to be attached to the turntable using parts 41678 and 32291.

My theory about walking is this. When the mechanism is turned for steering, the bevel gear attached to the wheel spins as it is rotated around the stationary drive axle. The amount it spins depends on the ratio between the two bevel gears. I chose the 12-tooth double bevel gear over the 20-tooth so the ratio would be 1:1. If the mechanism was "steered" 360 degrees, then the wheel would rotate 360 degrees as well.

I also believe that to prevent walking, the wheel's radius must be equal to its distance from the driving axle. Or, perhaps more correctly, the ratio of the wheel's radius and its distance from the driving axle must be equal(?) to the ratio between the two bevel gears.

I don't have time to draw up a diagram to explain this further, so I'll just leave it be for now.

Advantages of synchro drives in a robotics platform:

  • More energy efficient than scrub steering. This advantage is important if the robot is heavy or the surface has a high coefficient of friction.
  • Robot may be able to change direction quicker. For example, see this video.

Disadvantages of synchro drives in a robotics platform:

  • Robots built on a synchro platform driven by one motor do not rotate about their vertical axis. This is problematic if, for example, the mission involves gathering items off the floor and the intake is located only on one side of the robot. It's less of a problem if the mission allows for a swiveling "head" positioned on top. An obvious workaround to this problem is to employ two drive motors and find a way to counteract their different turning rates.
  • The steering and drive mechanisms take up a lot of room. In addition to the mechanism shown above, you need to add gears and axles to steer and to drive all four wheels from two motors. Compare this to a scrub-steer platform, which can be as simple as putting at each corner of the robot's frame a motor, 2 gears, and a wheel.

Some "final" notes about construction choices:

  • I used a longer-than-strictly-necessary axle for the wheel so it won't pop out during a competition.
  • The triangles can wobble because of the attachment method, so add an axle at each corner of their bases. To help position the triangles better, put bushings on the axles as well.

Additional resource: Sven Bottcher's Principles of robot locomotion

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