Coil Actuators

Nitinol

Phases of NiTi.	The contractile rings are composed of coils of Nitinol (NiTi) wire. NiTi is a nickel titanium alloy that changes shape when heated. It accomplishes this though phase transition between austenite and martensite. In austenite, NiTi has an organised crystal structure where nickel and titanium atoms are held tightly in place. In martensite, the atoms are more loosely held than austenite and the structure is able to deform slightly without breaking any atomic bonds. When deformed martensite is heated it transitions into austenite. As the atoms reform into austenite the NiTi returns to its original shape.

niti_structure_transformation — Phases of NiTi.

The contractile rings are composed of coils of Nitinol (NiTi) wire. NiTi is a nickel titanium alloy that changes shape when heated. It accomplishes this though phase transition between austenite and martensite. In austenite, NiTi has an organised crystal structure where nickel and titanium atoms are held tightly in place. In martensite, the atoms are more loosely held than austenite and the structure is able to deform slightly without breaking any atomic bonds. When deformed martensite is heated it transitions into austenite. As the atoms reform into austenite the NiTi returns to its original shape.

The original shape can become stabilized through a process called shape setting. In shape setting, the NiTi is fixed in a deformed state and annealed to reset the structure of the austenite. The contractile rings were annealed at 390 degrees C for 20 minutes. [7]

Coil Actuators

Diagram of coil geometry.	NiTi can only recover from about 6% strain. In straight NiTi wire the stroke length is equivalent to strain; however, in a helical wire, the relationship between stroke length and strain is nonlinear. In helical wire, stroke lengths of 200% to 1600% can be achieved while deforming the wire 6%. The stroke length and strength of the coil actuator is largely dependent on four parameters: diameter of actuator wire (d), diameter of coil (D), initial pitch angle (⍺), and number of active coils (n). [8] There are caveats when designing SMA coil actuators. Stroke length of an actuator is typically inversely related to its strength. [8] Additionally, NiTi actuators are monodirectional and require an external bias force to be reset following actuation. SMA coil actuators need enough strength to overcome bias force in addition to the strength needed to perform the desired task. The net force is the force available to perform work.

SpringDiagram2 — Diagram of coil geometry.

NiTi can only recover from about 6% strain. In straight NiTi wire the stroke length is equivalent to strain; however, in a helical wire, the relationship between stroke length and strain is nonlinear. In helical wire, stroke lengths of 200% to 1600% can be achieved while deforming the wire 6%. The stroke length and strength of the coil actuator is largely dependent on four parameters: diameter of actuator wire (d), diameter of coil (D), initial pitch angle (⍺), and number of active coils (n). [8] There are caveats when designing SMA coil actuators. Stroke length of an actuator is typically inversely related to its strength. [8] Additionally, NiTi actuators are monodirectional and require an external bias force to be reset following actuation. SMA coil actuators need enough strength to overcome bias force in addition to the strength needed to perform the desired task. The net force is the force available to perform work.

Designing The Actuator

The contractile ring is simply a coil actuator bent in a circle. Through observations of the FFT it was determined the relaxed ring actuator should have a circumference of 110mm and contract down to 30mm. The ring must also be capable of maintaining a net contractile force of 0.3N throughout actuation.

The Actuators

The acutaor ring will contract until the net force falls below the 0.3N needed to invert the FFT. Through some trial and error we were able to design a coil actuator that roughly met all of the requirements outlined in the section above. The final actuator design has an actuation strength 0.62N and requires 0.28N to deform to a length of 110mm. Actuators were fabricated out of 0.15mm NiTi and had a coil diamiter of 0.9mm. They have an initial length of 22mm and can elongate to 110mm while experiencing 4% strain. The contractile ring is made by connecting both ends of the actuator to a loop of magnet wire. The magnet wire allows the two ends to be held together without shorting.

Electrical

The contractile rings are actuated by 20 volts from a regulated DC power supply. NiTi is extremely difficult to solder. Therefore, power cables were connected mechanically to the contractile rings though the metal brackets. Each ring is powered by an individual mosfet and the entire prototype is controlled using an external Arduino Mega 2560.

Prototype connected to electrical board.

TestSetup — Prototype connected to electrical board.

Future Work

Currently, the prototype relies on the inversion of the torus to re-expand the contractile rings on the front of the drive system. This strategy is unreliable. The team is currently working to develop contractile rings that passively expand when not activated.