The actuators described here are designed to be universally and generally adaptable to a variety of mechanical linear and membrane actuation functions in which it is desired to produce linear, rotary, or out-of-plane membrane displacement, in each case capable to operate against substantial reaction forces. Unlike currently available shape-memory alloy (SMA) wires, sheets and plates, the thin, flat form of the SMA film disclosed herein maximizes heat transfer rate to allow more rapid cyclic actuation. Prior use of thin films of SMA materials has been confined to the deposition of films onto rigid substrates such as silicon, limiting their use to planar elements which are not amenable to the pre-straining required to obtain maximum shape-memory strain recovery. In the present invention, the use of a polymeric substrate gives the actuator a “carrier film” that greatly facilitates handling and installation, while the thin, flat form of the SMA provides a large surface area for easy and secure attachment. Alternatively, two-dimensional membranes may be fabricated that may express a variety of adaptive mechanical responses to environmental stimuli or which may be controlled by externally applied energy input.
Prior SMA cyclic actuator systems using wires, springs or sheets require users to impart strains into the martensitic phase of an SMA material before use, to provide ‘biasing’ forces to reset the actuator to a starting position on cooling. These straining operations often require substantial mechanical complexity and are generally not applicable to thin films deposited onto rigid substrates. The method of pre-straining the actuator prior to delivery to the user allows simplified installation of the actuator element into a mechanical device without the requirement that an external mechanism provide either the biasing forces needed for cyclic actuation, or the pre-strain needed for a cyclic shape-memory effect to be enabled. Furthermore, the prestraining step envisioned in the present invention can be applied for a sufficient number of thermomechanical cycles to stabilize the hysteretic behavior of the actuator, eliminating “shake-down” behaviors such as cyclic creep.
The material system described will allow fabrication of uniaxial, multiaxial or membrane actuators will be readily adaptable to cyclic displacement requiring no additional spring elements. Generation of rotary displacements can be effected by using combinations of three or more linear actuators, and out of plane displacement of membrane actuators can be effected by simple surface modifications of the prestrained sheet material.
The manufacturing process includes thin film deposition, thermomechanical pre-straining, photolithographic patterning, and surface modification, in a continuous process capable of delivering large numbers of finished devices, or large areas of adaptive membrane material, at low cost. For electrically excited linear actuators the provision, by photolithography, of “hairpin” patterns in the SMA metal film simplifies incorporation of the actuator with electrical power supplies. For two-dimensional membrane embodiments, biaxial pre-straining of the SMA metallization allows for the development of cyclic out-of plane displacements through the incorporation of planar elastic layers by surface modification, as described further below.
The disclosed system also takes advantage of the complementary constitutive properties of the SMA metallization and the polymeric carrier film. That is, the SMA layer stiffens and returns to a preset shape when heated, whereas the underlying polymeric layer, when heated, becomes more compliant, and when cooled stiffens and tends to return to its original shape or length. The polymeric web can also function as an electrical and thermal insulator, disposed between SMA metallic layers deposited on both sides of the web, in configurations that allow for large cyclic out-of-plane displacements. In general, the thickness of the polymeric web is such that the mechanical response of the actuator is dominated by the properties and behavior of the SMA metallization.
Processing is described that will allow for low-cost, high-volume manufacture of actuation elements at size scales ranging from sub-millimeter to as much as tens meters in length, or patterned mechanically active membranes whose areas can exceed 1 meter square. Although free-standing shape memory thin films have been previously made, it is difficult to separate such films from their substrates without causing damage to the film. The provision of the polymeric substrate greatly toughens the SMA film, allowing easy handling during subsequent deployment by the end-user. Furthermore, the flexible substrate allows the SMA metallization to be photolithographically patterned into complex functional forms while retaining the sealing integrity of the membrane. Patterning of the active SMA metallization by photolithographic methods also allows, for example, subsequent slitting and/or die cutting operations to be carried out without damaging the edge of the SMA metallization.
After pre-straining and photolithographic patterning, SMA elements can also be provided with local regions having greatly increased elastic stiffness through the use of simple procedures, such as selective-area electroplating, to provide, for example, reinforced end-connection sites and solder tabs.
The fabrication technology used to produce the thin film electrically excited actuators above has excellent potential for ultra-low cost manufacture of small linear and actuators, with enormous flexibility accruing from the batch-style primary materials processing (including the critical pre-stretching step), combined with postprocessing for patterning and end-treatment. The same basic process can also be used to fabricate both discrete linear actuators, complex three-dimensional multiaxial actuation systems, or adaptable “intelligent” membranes, with dimensions that can be measured in meters, having an arbitrarily complex patterned arrays containing high thin film shape-memory “action-centers”. Locally functional thin-film machines can be configured to respond to a variety of potential stimuli, such as thermal gradients across the membrane, applied electrical currents, or even to directed energy from remote irradiation sources such as lasers or electron beams. Alternatively, membranes are disclosed with two-sided metallization that, when supplied with DC power, can use local sensors and control circuitry attached directly to the membrane to control local action functions in response to environmental stimuli.
The membranes can be designed to express such novel properties such as variable heat transfer coefficients, adjustable acoustical response, alterable surface contour (at scales), controllable damping behavior, roll-unroll or fold-unfold functionality, and/or adjustable radio-frequency diffraction/reflection/absorption characteristics.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.