Leg extensions

Shape Deposition Manufactured (SDM) linkages to increase the stroke length of the hind legs of the Sprawlita hexapod.

Motohide Hatanaka 02/26/01

Above: Version0 (top), Version1 (right), Version2 (left)
Right-top: Version2 side view
Right-bottom: Version2 angled view

Design and fabrication

Leg extensions are designed to allow longer stroke lengths on the hind legs of Sprawlex, allowing it to travel at faster velocities. The leg extension is an SDM fabricated six bar pantograph linkage which doubles the stroke length.
Version0was a prototype built with plastic bars, bearings, and regular steel fasteners to test the kinematic requirements of the mechanism.
Version1 consisted of solid polyurethane links and soft polyurethane living hinges. The flexures had to be relatively thick (t=1.6mm) to meet the strength requirement without reinforcements. Unfortunately, they turned out to be too stiff for fast actuation of the legs.
Following tests with Version1, an array of different prototypes was fabricated to test the effects of various reinforcing materials including cotton and synthetic fabrics, fibers, etc. The best combination of free movement, strength and fatigue life were obtained using a soft urethane plastic and a fabric of cotton and polyester mixture.
Version2 consists of solid polyurethane links and fiber reinforced soft polyurethane living hinges. The fiber reinforcement allows the fabrication of strong yet thin (t=0.8mm) and highly flexible hinges. It also alows the living hinges to be short, reducing undesirable compliance in directions other than the primary bending axis.

Observations and possible improvements:

Although Version2 is largely successful, a few improvments are possible:
  1. In one specimen, a failure occured at a joint due to debonding between the reinforcement fabric and the soft polyurethane. This can be avoided by unraveling the edges of the fabric for better penetration of the urethane.
  2. The flexible joints have slight compliance in addition to the primary bending direction (see diagram below). To reduce this compliance the flexures can be made even shorter.

Questions and comments to: Motohide Hatanaka