This picture shows our new testbed for performing force and motion control experiments on SMA wires. At the top there is a precision linear motion stage that moves a pulley up and down. This pulley has a short chord attached, and this chord ends in two small eyelets for the SMA wires to pull on. For single-wire experiments, the pulley can be locked in place. For antagonistic pair experiments, the pulley is unlocked, and an optical shaft encoder measures the rotation. It is also possible to attach an inertial load to the pulley, like the pendulum visible in this picture.
At the bottom, there is a pair of very sensitive load cells and a strain gauge amplifier. These load cells measure the tensions on the SMA wires. The wires themselves are 80cm long, but are doubled up so that the two ends are down at the load cell and the middle passes through an eyelet. The distance from the load cells to the pulley is therefore a bit more than 40cm.
The linear stage can generate motions with an accuracy of 1 micron and a bandwidth of 30Hz; the optical shaft encoder measures pulley rotation with a resolution of 0.044 degrees; and the load cells can measure forces with a resolution of 0.3mN and a bandwidth of 140Hz. The testbed also has a precision transconductance amplifier (voltage in, current out) to deliver precise heating currents to the wires; and the whole thing is controlled by a DS1104 real-time control board from dSpace.