  Handed Shearing Auxetics (HSA) are a promising structure for making
electrically driven robots with distributed compliance that convert a motors
rotation and torque into extension and force. We overcame past limitations on
the range of actuation, blocked force, and stiffness by focusing on two key
design parameters: the point of an HSA's auxetic trajectory that is
energetically preferred, and the number of cells along the HSAs length.
Modeling the HSA as a programmable spring, we characterize the effect of both
on blocked force, minimum energy length, spring constant, angle range and
holding torque. We also examined the effect viscoelasticity has on actuation
forces over time. By varying the auxetic trajectory point, we were able to make
actuators that can push, pull, or do both. We expanded the range of forces
possible from 5N to 150N, and the range of stiffness from 2 N/mm to 89 N/mm.
For a fixed point on the auxetic trajectory, we found decreasing length can
improve force output, at the expense of needing higher torques, and having a
shorter throw. We also found that the viscoelastic effects can limit the amount
of force a 3D printed HSA can apply over time.
