Overhead Projector Demonstration of Actuation of NiTi Memory
Metal
NiTi actuator wires contract like muscle fiber when electrically driven.
Batteries resistively heat the wire, causing it to shorten while exerting
considerable force. NiTi can be incorporated into devices that are smaller,
lighter, and easier to use than motors or solenoids. This demonstration
shows some basic ways actuator wires may be used.
This FLEXINOL NiTi memory metal overhead projector demonstration
is based on materials in the Sample Kit available from Dynalloy,
Inc. The description provided here is adapted from the accompanying Dynalloy
documentation. The overhead projector demonstration was developed by Eric J.
Voss.
The NiTi actuator wires contract like muscle fiber when electrically
driven. Electricity (in this cas provided by two AA batteries) resistively
heats the wire, which causes the wire to get shorter while exerting considerable
force (330 grams for a 0.006" diameter w ire). NiTi can be incorporated
into devices that are smaller, lighter, and easier to use than motors
or solenoids. This demonstration shows some basic ways the actuator wires
may be used. Although designed to be employed on the stage of an overhead
projector, this demonstration may also be presented on a tabletop with
a piece of white paper underneath the Plexiglas board. Larger movie (8.3 MB) in a new window
Linear Contraction. This demonstration requires the least force
or strength from the NiTi wire. Force is directly related to the cross
sectional area or diameter of the wire. The alligator clip is attached
to the ring terminal on the left end, and the wire is activated by touching
the other lead to the ring terminal on the right end, causing current
to flow and the NiTi to be resistively heated. As the NiTi warms, it contracts
as it passes into the austenite phase. When the current cases to flow,
the wire cools and transforms back to martensite, allowing the spring
to pull the NiTi wire back to its original length. In this configuration,
the stroke (distance from one end of the limit of its motion to the other)
is approximately 4% of the total length (0.12" change for this 3.00"
wire).
Right Angle Movement. This demonstration shows one way to convert
some of the strength of the NiTi wire into an additional displacement.
The alligator clip is attached to the ring terminal at the top end, and
the wire is activated by touching the other wire to the bottom ring terminal.
In this configuration, the stroke that was hardly visible in the Linear
Contraction demonstration is now almost 20%. This configuration does not
have a reverse bias (the geometry does not make it easier for the wire
to pull as it contracts), but it would make a good latch or lock mechanism.
Double Lever Movement. This demonstration produces the most force.
The alligator clip is attached to the ring terminal on the right end of
the actuator wire, and the wire is activated by touching the other lead
to the ring terminal at the left end. The point where the NiTi wire is
connected to the lever arm moves further away from the pivot point as
the wire contracts, and the pointer sweeps over a range of angles on the
protractor. This gives the NiTi wire more leverage as it contracts, making
it easier for the actuator wire to pull as it contracts. This is called
a reverse bias effect. This is very important for achieving maximum stroke
over very long life (millions of cycles) and is incorporated into optimum
designs in real-life applications.
Materials
All wires in the demonstration are 0.006" diameter, 3.00" +-
0.010" length (working wire length) NiTi with a 90 degree C transition
temperature. The wires have brass ring terminals crimped to both ends
to assist in fastening them to the board. Crimping specifications and
technical characteristics of the NiTi FLEXINOL actuator wires are available
from Dynalloy, Inc.
