Biomedical Engineering Reference
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locomotion. And so the visco-elastic biomaterials serve several functions during locomotion:
absorbing shock, stabilizing, and improving locomotion energy efficiency (Full and Meijer, 1999).
Moreover, the resonance of the compliant biomaterials appears to perform as the oscillating
elements of analog computers, dynamically stabilizing animal locomotion in response to the
terrain — a low-level intelligence function labeled ''preflex'' by Brown et al. (1996). Note that
such preflex would function separately from nerve activity, involving only well-tuned mechanical
compliance.
In a dramatic display of this principle transferred into a robot, Stanford's Sprawlita robots
traverse highly complex terrain with no sensor feedback at all, stabilized only by the feedforward-
responsive, rubbery compliance of its body and legs (Kim et al., 2004). The latest robot in the
Sprawl series, the small (0.3 kg) autonomous iSprawl runs at 15 body lengths per second — several
times faster than earlier, slower bio-inspired robots (see Figure 6.2). And yet examination and
analysis of iSprawl shows the same bouncing, stable locomotion patterns in as those seen in insects
(Kim et al., 2004).
Comparable to iSprawl, the Mini-Whegs
y
of Case Western Reserve University is smaller and
lighter than iSprawl, predates iSprawl and runs at 10 body lengths per second. Mini-Whegs also run
rapidly over obstacles that are taller than their legs. The Whegs robots are discussed further in the
next section.
Full and Meijer (1999) show that the principles of compliant locomotion hold regardless of leg
number, but with some variations. Full and Meijer also find that in locomotion with four or more
legs, the leading two legs absorb shock and stabilize an animal, while the rear two legs provide
propulsion. In animals with six or more legs, the middle legs provide combined propulsion and
stabilization. In animals with two legs or mono-style hoppers (like a kangaroo), the legs serve for
both shock absorption and propulsion.
Implementing these biological strategies will be more challenging in biped robots, so, many
researchers are initially implementing them in hexapod robots. Such an insect-like hexapod
architecture offers inherent stability, especially when employed with a ground-hugging, sprawl-
legged posture. One sees such a posture in lizards, crabs, cockroaches, and spiders. Both the
Figure
6.2
Stanford's
dynamically
stable
iSprawl.
(Image
courtesy
of
Stanford's
Biomimetic
Robotics
Laboratory.)
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