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:
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.
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.
