Page 64 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

When the internal bladder is pressurized, it expands and pushes
against the inside of the braided mesh sleeve, forcing the diameter
of the braided mesh to expand. The physical characteristic of the
mesh sleeve is that it contracts in proportion to the degree its diam-
eter is forced to increase. This produces the contractive force of the
air muscle.
It is important to note that to operate properly, the air muscle must
be in a stretched or loaded position when it is in a resting state. If
not, when the air muscle is activated, there will be little if any con-
traction. Typically the air muscle can contract to approximately 25
percent of its length.

Nitinol wire
Nitinol is a metal that belongs to a class of materials called shaped
memory alloys (SMAs). Nitinol is commonly sold in wire form.
When heated, the material can contract up to 10 percent of its
length. The contraction of the material produces linear motion. In
addition to the contraction property, the material also exhibits a
shaped memory effect (SME).
The SME is a unique property of this alloy. When heated to its crit-
ical transition temperature, the material automatically returns to a 43
predefined shape. The predefined shape is one that the material is
trained (heat annealed) to remember. The material is formed into
the training shape. The material is then forcibly confined to the
training shape as the material is annealed (heated) above its tran-
sitional temperature. This realigns the crystalline structure to the
shape. Now the object will return to this shape whenever it is heated
to its transition temperature. So a trained object could be twisted
and folded out of shape and then heated to return the object back
to its original shape.
These unique properties of SMAs rely upon the crystalline structure
of the material. The shape-resuming force approaches 22,000
2
pounds per square inch (lb/in ). It’s very unlikely that anyone will
be working with such large cross sections of material. Even thin
wires of the material produce an impressive force. For instance, a
6-mil wire generates a contractive force of 11 ounces.
When nitinol wire contracts up to 10 percent of its overall length, its
volume remains constant. As the wire contracts, its diameter in-
creases proportionally, keeping the net volume of the wire constant.

The easiest way to heat nitinol wire is by passing an electric DC cur-
rent through it (see Fig. 4.2). However, using a steady DC current

Team LRN Movement and drive systems