Brackets

Geometry

The brackets make it possible to connect the SMA contractile rings to the drive system without gluing the wires directly to the membrane. This fabrication method allows actuators to be reused or transferred between prototypes with minimal risk of damage. Additionally, the use of brackets provides more control over the interaction between the actuators and the membrane.

Our team hypothesized longer brackets would be more efficient at inverting the torus. It was assumed the long arm of the bracket would act as a second class lever, pushing the torus inward. Brackets were designed with and without the lever arm.

(Left) point bracket, (Right) 0.25	Brackets without the lever arm are referred to as point brackets. Brackets with the lever are referred to as double brackets. The length of a double bracket is considered to be its center distance. Three brackets were tested, point brackets, as well as two lengths of double brackets, 0.25" and 0.375". All brackets were fabricated using an FDM 3D printer.
(Left) 110mm Water Wigglie, (Right) 125mm Water Wigglie	In addition to bracket geometry, two brands of FFT were tested. These FFTs had 125mm and 110mm circumferences.

Experiment

Large FFT with 0.375	An experiment was designed to quantitatively assess the contractile force needed to invert the vinyl toroidal membranes and determine whether this force was affected by bracket geometry. A drawstring-bag-like pulley system was used to simulate contractile force. The force and extension was measured using an instron material tester. In each test, vinyl FFTs were contracted about 45%. Forces were averaged over three tests.

An experiment was designed to quantitatively assess the contractile force needed to invert the vinyl toroidal membranes and determine whether this force was affected by bracket geometry. A drawstring-bag-like pulley system was used to simulate contractile force. The force and extension was measured using an instron material tester.

In each test, vinyl FFTs were contracted about 45%. Forces were averaged over three tests.

Results

Results of the experiment do support the hypothesis that longer brackets invert the torus more effectively. However, despite the apparent correlation, the point brackets on the smaller torus required approximately the same force as the long brackets on the larger torus. This suggests the smaller torus is easier to invert overall. We were unable to evaluate the effect of double brackets on the small torus because of difficulties with membrane adhesion.

Final Bracket Design

Steel point brackets	Both membrane sizes could be inverted using similar amounts of force. However, the larger membrane required the use of double brackets. These were fabricated using a 3D printer and often melted during testing. As a result, they were deemed unsuitable for the prototype. Point brackets were fabricated from thin coils of spring wire. This bracket was impervious to the heat generated by the SMA actuator and selected for the prototype.

Both membrane sizes could be inverted using similar amounts of force. However, the larger membrane required the use of double brackets. These were fabricated using a 3D printer and often melted during testing. As a result, they were deemed unsuitable for the prototype. Point brackets were fabricated from thin coils of spring wire. This bracket was impervious to the heat generated by the SMA actuator and selected for the prototype.

Minimum Force

To establish the minimum force required to invert the torus, a vinyl FFT was lubricated and inverted using progressively weaker actuators. It was determined the required force to reliably invert the torus is 0.3N.

Future Work

Development of SMA compatible double brackets is being investigated. Possible manufacturing processes include SLS 3D printing and resin casting.