Sprawl Version 4 (Sprawlita with leg extensions)

Motohide Hatanaka  02/26/2001

Angled back view   Close-up of the hind leg with extension


Sprawlex is a variation of the Sprawlita hexapod with an SDM-manufactured linkage to increase the stroke length of the rear legs. The motivation for Sprawlex is found in our ISER 2000 paper, in which we observed that Sprawlita has a short phase in which the rear legs appear to drag on the ground because they run out of stroke length. Our hypothesis was that by increasing the stroke length of the hind legs, we would achieve the following objectives:
  1. achieve a running motion that more closely matches the inverted spring/pendulum dynamics seen in most running animals
  2. better match the ground speed of the rear feet at contact (to reduce slippage)
  3. make the robot run faster.


Leg extensions are attached to the hind legs of Sprawlita. The extensions are compliant pantograph linkages that extend the stroke from 20mm to 32mm.*  Like the original Sprawlita legs, they are fabricated using the Shape Deposition Manufacturing method. A series of experiments were conducted with variations on the linkage geometry and flexure material. Best performance and fatigue life were obtained using a combination of hard polyurethane links with fiber reinforced soft polyurethane living hinges. Details are provided on the leg extensions page.


The hypothesis that motivated the leg extensions is currently being tested. Preliminary results indicate that objective #2 has been achieved. However, objectives #1 and #3 will require modification of the front and middle legs of the robot. The leg extensions also have the effect of increasing the compliance in the rear legs and of decreasing the thrust force for a given piston diameter. As a result, the robot is now being propelled mainly by the middle pair of legs. To overcome these effects the front and middle legs should be made more compliant.

Ongoing and future work(as of Feb 26, 2001)

We are testing variations on the design that use either larger bore pneumatic cylinders or two cylinders per leg. These cylinders require more air than the original cylinders and we will therefore have to modify the valving as well. We are also exploring more rigid hind-leg hip joints and more compliant fore and middle-leg hip joints.

*(The stroke of the pneumatic piston is restricted to 16mm from 20mm but then it is mechanically doubled to provide 32mm of effective stroke.)

Comments and questions to: Motohide Hatanaka