UFO JOE: The First Steps

 

3D Printed Chassis and Walking Mechanism

After receiving all the mechanical parts from ServoCity and 3D printing the chassis and legs I was ready to finally test the theory behind UFO JOE.  If this walking mechanism worked it would mean that I could finally start to properly design the full body spinner.  Now this wouldn't be a perfect demonstration of the walking mechanism, as the drive motor needs to be exchanged for a brushless system, and the front and back legs need to be linked with pullies.  I will go over how these affected the walking later in the post.  For more details on the walking mechanism check out my last post on UFO JOE.

UFO JOE Takes It's First Steps

In the video you can see that the mechanism worked... mostly.  This aspect of the drive is only meant to move the robot forwards and backwards, and that works almost perfectly.  While it may appear slow, the speed of the walking isn't the focus, the torque is.  By heavily gearing down the motors used, speed is given up in exchange for higher torque, which is needed to fully lift the 6-lb robot off the ground.  Since the only motion that will be needed on UFO JOE will be whatever's necessary to comply with basic translational movement rules, a low speed is perfectly fine.

Video Demonstrating The Back Axle Rocking

Now for the problems.  In the video you can see the back axle of the legs rocking back and forth through the lower half of the rotation instead of completing it fully.  To combat this, the design of the walking mechanism included belts and pullies to properly convert the angular motion of the front axle to the back.  However, I hadn't ordered the belts yet.  Without the belts, the only way for the motion of the walker to be translated to the back is through the pushing and pulling of the legs horizontally.  This means that the rotation of the back axle can only occur through the torque created by this horizontal force.

Diagram Showing The Change In Force Strength Through A Rotation

The first issue with this method of transferring motion is shown above.  As the front portion of the leg makes it's way through its rotation, the horizontal force it produces down the leg varies.  At the bottom and top of the rotation, all force would be transmitted horizontally, while on the left and right all force would be transmitted vertically.  When all force is transmitted vertically, no force is left to drive the back axle.  Due to the tolerances in the assembly, the back axle has an easier time rocking back and forth instead of completing a full rotation.

Torque Equation Demonstration

In addition to the weakening force being applied to the back pulley as the leg rotation reaches the failure point, the influence of the force over the torque that could be generated diminishes.  As shown in the quick example above, as a force is applied towards the axis of rotation, instead of a preferably perpendicular angle, the amount of force converted into torque is lowered to zero.  This means that as the leg reaches the point where it should transfer upwards it no longer has any torque being applied to it.  Both of these issues should be resolved once the pulley belts arrive.

UFO JOE Heavy Weight Test

I discovered the last issue with the current design during the test of the walking mechanism's ability to lift things of substantial weight.  The idea of the robot is to have as heavy of a weapon as possible, so being able to lift 5-6 pounds would be good for the reliability of drive train during fights.  In this video, you can hear a clicking noise as the robot is attempting to lift off the ground.  This is due to a skipping of the gears, which shouldn't be happening due to the exact tolerance of the gear meshing in the design.

The Gearing Of The Drive Motor To The Pulley

In future tests I noticed the issue was a corner between the electronics housing and the base of the chassis.  As the gears encountered any stress the corner would turn into a joint and would bend enough that the gears would separate, losing all torque and causing the clicking noise.  By thickening up this corner (and possibly making the base thicker) this issue will be removed and the robot should be able to lift enough weight to enable the weapon to be placed on top.

That's all for this update on the walking mechanism.  Once the belts arrive I will do a small update which will include the design and testing of a simple steering mechanism.