Alpha Rex Walk – Solution


Preface: A few weeks ago I ripped down and rebuilt my Lego Mindstorms NXT model and replaced it with a by-the-numbers build of the "famous" Alpha Rex humanoid version designed by Lego.

Photo: active-robots.com
The design (Alpha Rex) looks nice, but the model unfortunately doesn’t walk very well.  In fact, you might say it doesn’t walk at all, but sort of shuffles on the spot.  Very annoying.
 
Well, after a couple of hours playing with some different designs, I’ve finally settled on a solution which seems to fix the Alpha Rex walking issue.

Disclaimer: I’ve had to factor in a rechargeable battery (which arrived late last week). 

This means that the weight of the model is slightly forward of center.  If you have an NXT with a battery pack, you’ll understand why it was necessary to make some changes. 

I’ve had to mount the Intelligence Brick upside down (to allow for access to the recharge socket).  The battery pack also adds a little extra size to the rear of the Brick, which means that it had to be mounted slightly forward of the original vertical brace, per this diagram:

 

rear

 
Note the cross brace.  This is to offset the extra padding added to the exterior by the battery pack.  Note also, now the battery can be recharged without needing to remove the brick (or replace batteries!).

So now the Intelligence Brick is mounted upside down.  Makes it (ironically) easier to cable too.  USB connections are still possible (due to the location of the USB port) but it’s recommended that you enable and use Bluetooth instead, if possible.

 

front

Apply these changes (mirrored) for the left leg to achieve the end result.

Now, I’ve included some traction for the legs.  Add per below to both legs which will allow the unit to gain purchase when walking.
 

front_stabilizer

 
The picture below represents a custom bumper implementation.  It was important to use the same logic since the bumper needs to be retractable when a leg is "raised".  The black peg keeps the bumper from swinging forward (down) more than a few degrees (but it’s enough!).
 

new_bumper

 
Attach the bumper in place of the Alpha Rex’s default bumper implementation.  Diagram below shows the original frame with the addition of the custom bumper.
 
 

placement

 
Click on the picture for a larger image.

Give it a try for yourself :)  Remember to mirror the design for the left foot.

Warning: I’ve not tried this on a Brick without the battery pack, so I can not guarantee the results.

I’ll be also posting notes taken from my experiences at getting a Windows computer to communicate with the NXT via Bluetooth – not an easy task.  I’ve also managed to get the NXT to work with the Microsoft Robotics Studio 2008 which is in Beta/CTP (released APril 2008).  Check back for my notes on this.


About Rob Sanders

IT Professional and TOGAF 9 certified architect with over 16 years experience, 14 in commercial software development and 8 years in IT consulting. Check out the "About Rob" page for more information.

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