The Eagle Overdrive - time to go straight!

As most FLL teams, the Eagle Unlimited team has been challenged by the difficulty in convincing robots to go straight.  Patrick, one of our team members, did a Science Fair project that concluded that matching the motors and using wheels bound together with a fixed axle both helped to keep the robot going straight (the axle, of course, kept it from turning well).  The dual differential drive design was tested by our team, but rejected for erratic behavior that seemed to be caused by a gear train that was too complex.  The limited slip differential in the "Building LEGO Robots for FLL" guide (page 5-77) put out by the Minnesota FIRST LEGO League (a top quality organization!) was considered by our team, but that would not force the robot to go straight, only straighter.  A locking differential was considered as well (page 5-78), but rejected because it would use the third motor output to control it, and that was usually needed by some competition module. 

A NEW development is the design of the Eagle Overdrive, using clutch gear(s) to improve the straight tracking of a differential drive CoreBot.  Using clutch gear(s) and an axle to link the left and right side gear trains, they will track together while in forward, as with a fixed axle connection.  While differential turning, the opposing forces will exceed the torque capacity of the clutch gear(s), and allow the gear trains to turn in opposite directions without any linkage.  The clutch gear(s) will eventually wear out, but improved performance results in straight tracking without other aids.  The Eagle Overdrive acts as if the axle actual bound the wheels together, so that they must turn at the same rate, discouraging turns.  As the clutch slips when the motors turn in opposite directions, turns are allowed, where a bound axle would always resist turning.

You can use a clutch gear on both sides of the drive train, but the performance will not necessarily match well, as the clutch gears have a fairly wide torque range.  We have found it to be just as well to use one clutch gear and a matching 24 tooth spur gear (same tooth count and diameter).

We feel that this drive innovation will be of benefit to the FLL community, and we welcome all to make use of it, identified as the Eagle Overdrive.

2004 CoreBot Our current modular robot design, adding the Eagle Overdrive.	2004 CoreBot cutaway Bottom view with plates removed.	2004 CoreBot cutaway Bottom view with plates removed.
2004 CoreBot cutaway Just the Eagle Overdrive.

Right click to download ML-CAD file of CoreBot, of the CoreBot cutaway, or of just the Eagle Overdrive; choosing "Save target as".

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