Milli (#1) - A Bio-Inspired Millipede Robot

Next in line for bio-inspired robots is the Millipede.
(I know, I know, I am starting a new project without finishing the last ones. But, this one is interesting! ;)

Millipede design that I am looking at is the concept of having many many feet on the ground for the robot.  Similar to the Centi robot concept, I will use a mechanism to transfer the rotational motion of the motors to the feet in contact with the ground.

For this robot build, i am focusing on the work of David Zarrouk.  Zarrouk's SAW robot is a Single Actuator Wave-like robot.  The design uses a single motor that is spinning a helix twisted rod. The rod moves a linked set of treads in a way to create an advancing standing wave.

SAW robot from David Zarrouk

  This is best viewed in his video, from Zarrouk Labs:

For my build, I will start with the wave mechanism to provide the motion for the Millipede robot.  There is a published paper which describes the math put into Zarrouk's robots. I am using this as a starting point in my design. Using this as inspiration, I will first try to replicate his work, then extending on it. I will attempt to use this wave motion, as the driving actuator on the robot.  I am calling this my Worm-Drive, the WildWorm, for Milli the robot.

Initial Design:

The Track - The most complicated part of the worm drive is the tread design. The tread is conceptually the same as a crawling tank caterpillar tread. The only exception to a tank tread is that these treads will not be moved by cog wheels.
Each tread must have a pivot point so that it can be linked with the next tread. When linked together in must be very flexible. The joints must have low friction.  
Part of this design has the helix is spinning inside of it. The inside slot must have clear running path as the helix will actually slide along from side to side. But, as you will see it is not the helix that is moving, but the treads wrapped around the helix.

WildWorm Track design version 3

For this first set of designs, I am using nails to act at the connection axis.  They will be positioned between two pin holes. the inside hole is small and will hold fast to the nail. The outside has a larger radius so that the part rotates around the metal nail.
Finally I have extended a 'tread' to the track link, so that it can directly interact with the surface.

WildWorm 3D printed Tracks
v1 - top left
v2 - top right
v3 - bottom

It might be obvious, the track linkages will need to frictionless. It occurs to me that low friction will need to be designed into the treads. The next iteration of the design will perhaps use a metal washer between the tracks to reduce the friction. Lubricant will also be helpful. Ultimately, the series of track will need to be nearly fluid when shaken.

WildWorm - Helix and Tracks - 1st prototypes

The Helix - The helix is the part in the middle of the assembly that drives the mechanism. There are not a lot of details about the construction of the helix. I have started by using a wire from a coat hanger. I have wrapped the wire around a metal pipe and adjusted to create a consistent helix.  The helix should be smooth to reduce the friction of the track that slides around the helix. It may be possible to 3D print a helix, this would ensure the precision of the design and centering of the motor axis. However, a 3D printed helix may not have the strength needed to support the track linkages.

First Prototype

The first prototype assemblage of the WildWorm, has yielded a lot of insight. That is a nice way of saying, it did not go very smooth as I hoped. There will be a lot of tension between the helix and track. Perhaps this can be relieved by increasing the period of the helix. My first attempt was a 7.5 cm. I see that it should be much longer, maybe 10-12 cm.

WildWorm - 1st assembled prototype of the helix and track (5cm period)

Next steps will be to increase the length (period) of the helix. I will also design the motor housing, stabilizing arms, and a mount for the helix.  The helix will be easier to test, once I can mechanically stabilize (hold horizontal) the tracks.

[UPDATE] Extending the helix out to 10cm period, length of one full cycle, solves many issues. The track slide smoother, and there is significantly less binding.