CA2725273A1

CA2725273A1 - Shape memory alloy actuator

Info

Publication number: CA2725273A1
Application number: CA 2725273
Authority: CA
Inventors: Isaac A. Shannon
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-12-15
Filing date: 2010-12-15
Publication date: 2012-06-15

Description

SHAPE MEMORY ALLOY ACTUATOR

This invention provides a linear actuator design, more particularly one utilising shape memory alloy wire to provide movement.

BACKGROUND OF THE INVENTION

Mobile robotic systems require powerful, compact and lightweight actuators.
Shape memory alloys (SMA) actuators have a high strength to weight ratio when compared to hydraulic, pneumatic and electric actuators making them desirable for such applications.
Shape memory alloys that exhibit high material strain were discovered in the 1950s. Thin fibers of shape memory alloy can be pretreated to contract when heated. The contraction is a result of the fiber undergoing a phase transition between its martensitic and austensitic phases.
When in the cool phase (martensitic) the alloy is malleable and can easily be stretched by applying external stress.
The original pretrained shape can then be recovered by heating the fiber above its transition temperature. Activation of an actuator is achieved through heating and cooling of the fibers and because of its resistivity the alloy can be heated electrically. There are however several limitations to shape memory alloys. The fiber cannot accomplish shape recovery after strains of more then a few percent, about 5% for for a working life of thousands of cycles. This small strain is a disadvantage that has been overcome in previous designs using mechanical amplification, spring coils, long straight fibers or bistable devices. This means added complexity to the design and a reduction in the power to weight and size ratio. Typical Shape memory alloy actuators can be found in the following Canadian patents:

CA 2149849 Hayward, Grant CA 2391746 MacGregor CA 2439148 Von Behrens , Fairbanks Such actuators have achieved limited popularity and commercial success, there is a need for a simpler and more effective design.

-2-SUMMARY OF THE INVENTION

Considered broadly, this actuator is composed of plurality of stiff and aligned support structures that force one or more fibers of shape memory alloy into to a zigzagging pattern. The structures possess two extremities( A and B) that position the Shape memory alloy fiber.
Extremity B
holds the fiber in a fixed position while extremity A allows the free movement a shape memory alloy(SMA) fiber on a single axis. The fiber is weaved from extremity to extremity of the structures in such a way that when the SMA fibre shortens all the movement occurs in the span between extremity A of the structures and draws said structures together. This allows the fiber to be much longer then the distance between the two points being connected by the actuator. With a longer fiber, the actuator can contract by 30% or more of its initial length without the use of mechanical advantage and also maintaining the full force of the pull of the shape memory alloy's strain.

In the drawings, which form part of this specification, Fig. I is a cross-sectional view of an assembly of 3 members and a shape memory alloy fiber;
Fig. 2 is a semi-digrammatic view of the actuator according to one embodiment of the invention;
Fig. 3 is a plan view of extremity B;

Fig. 4 is a plan view of extremity A;

Fig. 5 is enlarged fragmentary view of the middle support structure and fiber;
Fig. 6 is enlarged fragmentary view of the middle support structure and fiber;;

-3-DETAILED DESCRIPTION OF THE INVENTION

In the particularly advantageous embodiment of the invention illustrated, the device is composed of three aligned identical support structures and a shape memory alloy fiber Fig. 2 .

Each structure is composed of a rod of rigid material 6, in this case high-density polyethylene, with specialised tabs attached to both extremities. The tabs may vary in shape and size according to the strength and heat dissipation requirements of the design. Usually they are fabricated from metal, in this case stainless steel. For ease of description, one of the structure's extremities will be referred to as "A" and the other as "B". Two tabs 3 are attached to extremity "A" on opposing sides of the rod. If the fiber 1 is being activated by electricity then the tabs must be electrically insulated from each other. In each tab there must be a hole that allows free movement of the shape memory fiber in a direction parallel to the structure. A single tab 4 is attached to extremity "B". It protrudes on two opposite sides of the rod, preferably in alignement with both tabs attached to extremity "A". On each protrusion there is a hole large enough to allow the shape memory alloy fiber to penetrate. This tab's purpose is to attach the rod to a fixed position position along the shape memory alloy fiber and provide a point where said fiber can be reorientated in a different direction. Two tabs may be used at this extremity in the fashion described for extremity "A" as long as care is taken to prevent melting of the support rod by the fiber during activation. In both cases friction may be sufficient to prevent the fiber from moving through the tab or tabs.
Otherwise a stopper must be attached to the fiber between the tabs or between the holes in the single tab or the fiber may be knotted to said tabs. The tabs may be attached with adhesif, by melting them into the plastic support structure or by any other means that provides sufficient strength and heat resistance to support the load.

The following is a description of how the fiber is weaved through three support structures. One end of the fiber is terminated with a crimped metal stopper 2. The opposite end goes through a tab on extremity "B" of the first structure in a direction that leads directly to a tab on the same side of extremity "A" of said structure. It will then travel through said tab and span the space between the first and second structure. The fiber will then pass through a tab on extremity "A" of the second structure in a direction pointing towards extremity "B" of the same structure. It will then pass through a corresponding hole in a tab on extremity "A" and bend back into the second

-4-hole of said tab. The fiber will then travel to extremity "B" of the third structure in the same manner as it travelled from extremity "B" of the first structure to extremity "B" of the second structure, and be terminated with a copper crimp once it has passed through the tab of said extremity.

To maintain alignement of all support structures a flexible ribbon 5 of a suitable material, in this case (PVC) is attached to extremity "B" of all support structures in such a way that it will not hinder the movement of the fiber. The amount of ribbon between each structure is roughly equal to that of the span of shape memory alloy fiber between each structure.

Attachment of the actuator to the parts parts being actuated is accomplished using the unused tabs on extremity "A" of the first and last structures. Attachment to extremity "B"
renders the design ineffective because as the fiber contracts only draws extremity "A" of the support structures together.

The shape memory alloy (SMA) fibers should be continuous along the entire length of the device to ensure maximum strength and simplicity. Attaching two or more shorter fibers to attain the required length increases the likelyhood of breakage or wear at the point of attachment.