Purpose: Sprawlita is a Shape Deposition Manufactured platform with six legs of 2 (actuated) DOF each.  Based on the Sprawl 1.0 and Mini-Sprawl prototypes, it is a platform to test ideas about locomotion schemes, leg design and leg arrangement and to build compliant leg structures using SDM.  The size is the same scale as Mini-Sprawl, with slightly more mass (270g vs. 250g) and less stiff compliant hip joints with only one (intended) degree of freedom.

Design, Manufacture, and Assembly Time: ~2 weeks.  The same circuitry and code as in Sprawl 1.0 were used.

Design: Each leg consists of two DOF: a prismatic joint (powered by a pneumatic piston), a rotary joint (powered by an RC servo motor), and a passively compliant hip joint (SDM test specimens - layer 1 of SDM legs).  This new passive hip joint design provides more compliance about the rotary axis of the servo, but is very stiff along the other axes.  Each piston has a spring return.  Like Mini-Sprawl and unlike Sprawl 1.0, only two two-way valves are used here.  Each valve pressurizes the three pistons in one tripod of support. The feet for this version are simply lengths of nylon tubing.

Control: Like Sprawl 1.0 and Mini-Sprawl,  there is very little control involved in the locomotion.  The servos (and therefore the leg angles) are commanded to and held at a certain angle.  The pistons in one tripod of support are simultaneously activated and deactivated.  Both tripods are alternatively activated and deactivated, with a certain duty cycle (a 55% duty cycle means both tripods are activated 10% of the cycle period).  All whole-body motion results from this activation-deactivation and from the compliance at the hips.

Preliminary Experiments:  Experiments with Sprawlita have shown that the increased hip compliance has resulted in locomotion that is not only faster than Mini-Sprawl, but is also lacks an obvious aerial phase, much like the cockroach locomotion used for inspiration.

The videos shown next used a supply pressure of 100psi, a tripod duty-cycle of 35% and a cycle period of 200ms (5 Hz).

Videos: We recommend Quicktime to watch these videos.  For a quick view, download the smaller (lower resolution) versions.  Your processor may have trouble playing the larger versions.
 

.mov file	Description
320x240 Version (27M) 160x120 Version (12M)	2.5 Body lengths per second on a treadmill, including perturbations. Rough Terrain obstacle traversal.
320x240 Version (15M) 160x120 Version (5M)	Close up of Running (~35cm/s).

Shape Deposition Manufacturing: The big difference between this design and Mini-Sprawl (besides the slight hip modification) is the unique method of manufacture - SDM.  The entire body is one piece: servos and wiring are embedded in the plastic of the body.  More impressively, the legs are one piece: the servo attachment, compliant hip joint, and piston are all embedded in the same plastic leg "unit".

Body SDM Process

                     

Leg SDM Process

Click here for a directory of more in-process pictures (in no real order).