Page 25 - Biomimetics : Biologically Inspired Technologies

Bar-Cohen : Biomimetics: Biologically Inspired Technologies DK3163_c001 Final Proof page 11 21.9.2005 6:40pm

Introduction to Biomimetics 11

Figure 1.5 Biologically inspired ground penetrators.

seen in sand-crab habitats, such as the beach. While the ultrasonic/sonic gopher was developed to a
prototype device and was demonstrated to perform its intended function, the ultrasonic/sonic crab has
not yet been produced even though its implementation is not expected to pose any major challenges.

1.5.2.2 Inchworm Motors

The biologic inchworm is a caterpillar of a group of moths called Geomeridae, which has six front
legs and four rear legs. Emulating the mobility mechanism of this larva or caterpillar led to the
development of motors and linear actuators that are known as inchworms. These commercially
available motors are driven by piezoelectric actuators (made by Burleigh Instruments) and they are
capable of moving at a speed of about 2 mm/sec with a resolution of nanometers while providing
hundreds of millimeters of travel. The forces produced by these types of motors can reach over 30 N
with zero-backlash and high stability. Their nonmagnetic content offers advantages for applications
in test instruments such as Magnetic Resonance Imagers (MRI). As opposed to biological muscles,
the piezoelectric actuated inchworms are involved with zero-power dissipation when holding
position. One of the limitations of this mechanical inchworm is its inability to operate at extreme
temperatures that are as low as cryogenic temperatures and as high as 2008C. The brakes and
shaft materials have different thermal expansion coefficients, and as a result, at lower temperatures
the shaft–brake fit becomes tighter breaking the ceramic piezoelectric material that is used. At
higher temperatures, on the other hand, the shaft–brake fit gets loose and the motor stops operating.
Eventually, the curie temperature of the piezoelectric material is exceeded and the motor ceases to
work. Using thermally compatible expansion coefficients is broadening the operating range of
temperatures in which inchworms can be used.
Inchworm mechanisms have many configurations where the unifying drive principle is the use
of two brakes and an extender. An example of the operation of an inchworm is shown in Figure 1.6
where the brakes and clamp are riding linearly on a shaft. These motors perform cyclic steps where
the first brake clamps onto the shaft and the extender pushes the second brake forward. Brake no. 2
then clamps the shaft, brake no. 1 is released, and the extender retracts to move brake no. 1 forward.
Another example of such a motor can be a modification where the brakes and extender operate
inside a tube. The motor elements perform similar travel procedure as shown in Figure 1.6 while
gripping the wall of the internal diameter of the tube in which the inchworm travels. This type of
motion is performed by geometrid larva worms that move inside the ground. Generally, worms use
their head and tail sections as support, similar to the brake in the inchworm, where the legs grab
the ground or the two ends expand sequentially to operate as a brake. A simplified view of the
movement of the millipede (different from that described for the inchworm) is illustrated schemat-
ically in Figure 1.7 showing steps that are made while progressing over the surface of objects such