The two most important aspects of a textile actuator's performance are its range of motion and its torque. It is important to characterize these two aspects of an actuator to get an idea of how much force is being applied for the specific application. When an actuator is inflated, the energy from the air pressure will move the actuator until it is either fully actuated or stopped in its path due to an external constraint. If an actuator is constrained from completing its full range of motion, the actuator will apply torque to the constraint.
Torque can be measured by constraining one section of the actuator to a static cradle and the other section to an arm connected to an assembly that can rotate freely until it is pinned to a static angle for each measurement. A 6 axis force/torque sensor is sandwiched between the arm and the rotating base to measure torque. In this case, both flexion and extension torque in a dual-acting actuator were measured at 25 psi. Measurements were taken at angles ranging from 0 to 120 degrees of rotation; the resultant plot can be seen below, right.
Notice that the flexion torque decreases with an increase in angle while extension torque increases with an increase in angle. This means that the maximum flexion torque is achieved when the actuator is straight (lowest angle) and the maximum extension torque is achieved when the actuator is bent to its highest angle.
Once the actuator is characterized with the method described above, the torque can be determined using an image and a known pressure in the actuator. This can be done by photographing the actuator at various pressures and using image processing techniques to track the angle of the actuator tip relative to the base.
This testing method allows the actuator to have unconstrained motion during inflation. By using this technique of characterizing and visually measuring torque, an actuator's performance can be better understood and tailor made to a specific application. Note that there is often a trade-off between torque and range of motion in textile actuators. Based on the application, it is important to design an actuator to not only produce the desired motion, but also produce desirable torque at intermediate and final states.