This picture shows our testbed for performing force and motion control experiments on SMA wires. At the top there is a servo motor (1) which operates a precision linear motion stage (2) that moves a pulley (3) up and down. This pulley has a short chord attached, which ends in two small eyelets for the SMA wires to pass through and pull on. For single-wire experiments, the pulley can be locked so that it does not rotate. For antagonistic-pair experiments, the pulley is unlocked, and an optical shaft encoder (4) measures the rotation. It is also possible to attach an inertial load to the pulley, like the pendulum (5) visible in this picture.
At the bottom, there is a pair of sensitive load cells (6) and a strain gauge amplifier (7). These load cells measure the tensions on the SMA wires (8, but too thin to be visible). The wires themselves are 80cm long, but are folded in half with both ends attached to a load cell and the middle passing through an eyelet.
The servo motor and linear stage can produce 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 (voltage in, current out) amplifier (9) to deliver precise heating currents to the wires; and the testbed is controlled by a DS1104 real-time control board from dSpace (off to the left).