Patent Application
20060201149
Restoring mechanisms, also known as “overcenter mechanisms,” “snap springs,” “snap blades,” and the like, are components of many devices, including valves and electrical switches.
Monostable mechanisms are known. For example, a rigid support can be overlaid by a membrane with projections that restore push buttons, such as those of a telephone keypad, back to an undepressed position. However, such designs lack a second stable position as in a bistable mechanism.
Discontinuous cantilever bistable mechanisms are known, wherein discontinuous cantilevered tongues are held in relation to each other by a surround fashioned from the same sheet as the cantilevers. These discontinuous cantilevers can impart bistable movement to a notched rod captured between the tips of the cantilevers. Discontinuous cantilevers can be undesirable, however, for applications needing a smooth surface on the bistable mechanism.
Dome-like bistable mechanisms, including linear and planar arrays thereof, have been fabricated of thin sheet metal. However, common materials typically limit the height of the dome to about 10% of its diameter, and consequently the maximum throw can be limited to about twice the dome height (hence, about 20% of a diameter).
Disk-like bistable mechanisms are known where a disk is buckled by insertion into a circular housing slightly smaller than the disk. Alternately, or in conjunction, disk mechanisms can be buckled by introduction of a part, such as a rod, that radially displaces portions of the mechanism. These designs can require assembly and one or more additional parts for proper function, and can have limitations similar to dome-like mechanisms.
A micromechanical continuous buckled beam mechanism includes a bistable bridge spanning a recess in an underlying support material. Such a design includes at least two parts (the bridge and the rigid support which must be assembled). Moreover, the rigid support can be unsuitable for applications requiring flexibility and/or for macroscopic applications where the added weight of the rigid support is undesirable.
Piezoelectric actuators are known, but can be expensive and bulky, and can require complicated control electronics. Shape memory alloy actuators are known, but can involve significant amounts of heat generation and can have high power requirements, and can be limited in frequency. For example, maintaining a stable position with existing shape memory actuators can require continuous input of power, which can be undesirable for portable applications and can generate undesirable amounts of heat. Moreover, the operation frequency of shape memory actuators can be limited by heat dissipation because the alloy needs to cool below its activation temperature before the actuator can be operated again.
There is therefore a need in the art for improved bistable mechanisms suitable for actuators, arrays of such actuators, means of operating or controlling actuators, and methods of manufacturing actuators.
An actuator includes a bistable mechanism having a tension beam and a compression beam defined by a relief slit in a flexible substrate; and a first shape memory element that upon heating actuates the bistable mechanism from a first position to a second position. In various embodiments, the tension beam and the compression beam can be substantially parallel. The tension beam can include a permanent out-of-plane deformation. The actuator can include a second tension beam defined by a second relief slit. The first shape memory element can include a shape memory alloy, a bimetallic strip, or a thermally-actuated shape memory polymer. The actuator can include a second shape memory element that actuates the bistable mechanism from the second position to the first position. A heat source can be thermally coupled to each shape memory element that independently heats the shape memory elements to actuate the bistable mechanism. Or, the actuator can include electrical leads coupled to each shape memory element that independently heat the shape memory elements to actuate the bistable mechanism. The first shape memory element can include at least two substantially parallel shape memory alloy wires electrically coupled in series to the electrical leads. The shape memory elements can be mechanically coupled to opposite sides of the compression beam to convert the displacement of each shape memory element into a greater displacement at the compression beam. The flexible substrate can include a material selected from the group consisting of steel alloy, phosphor bronze alloy, aluminum alloy, titanium alloy, carbon fiber/epoxy composite, fiberglass/epoxy composite, Kevlar/epoxy composite, polyimide, polyamide, polyester, polyvinylidene fluoride (PVDF), polypropylene, polyethylene, and urethane. The shape memory element can be in the form of a laminated array of shape memory wires mechanically coupled to the bistable mechanism. A first heat source can be thermally coupled to the first shape memory element. A second heat source can be thermally coupled to the second shape memory element. The shape memory wires can be substantially physically parallel shape memory alloy wires. The wires can include a shape memory alloy selected from the group consisting of NiTi, CuZnAl, and CuAlNi. Preferably, the wires are NiTi. The shape memory wires can have a diameter of less than about 500 micrometers. The ratio of the diameter of the wires divided by the distance between adjacent wires can be less than about 1. The actuator operates in air at 25° C. at a frequency of at least about 2 cycles per second. The actuator can be adapted for automatic control. For example, the shape memory element can be coupled to an open loop automated controller.
In some embodiments, an actuator includes a bistable mechanism and a first shape memory element mechanically coupled to the bistable mechanism that upon heating exerts a force that actuates the bistable mechanism from a first position to a second position; in such embodiments, the first shape memory element includes a laminated array of shape memory wires. In various embodiments, a first heat source can be thermally coupled to the first shape memory element, or electrical leads can be coupled to the first shape memory element, whereby the first shape memory element is heated by application of electrical current. The first shape memory element can include at least two substantially parallel shape memory alloy wires electrically coupled in series to the electrical leads. A second heat source can be thermally coupled to a second shape memory element at the bistable mechanism that heats the second shape memory element to exert a force that actuates the bistable mechanism from the second position to the first position. The shape memory wires can be substantially physically parallel shape memory alloy wires. The wires can include a shape memory alloy selected from the group consisting of NiTi, CuZnAl, and CuAlNi, in some embodiments NiTi. The shape memory wires can have a diameter of less than about 500 micrometers. The ratio of the diameter of the wires divided by the distance between adjacent wires can be less than about 1. The actuator can operate in air at 25° C. at a frequency of at least about 2 cycles per second. The bistable mechanism can include a tension beam and a compression beam defined by a relief slit in a flexible substrate, and the first shape memory element can actuate the compression beam from the first position to the second position. The tension beam can include a permanent out-of-plane deformation. Each shape memory element can be coupled to the compression beam to convert the displacement of each shape memory element into a greater displacement at the compression beam. A second tension beam defined by a second relief slit can be included, wherein the beams and the slits can be substantially parallel. The actuator adapted for automatic control, e.g., by coupling to an open loop automated controller. The flexible substrate can include a material selected from the group consisting of steel alloy, phosphor bronze alloy, aluminum alloy, titanium alloy, carbon fiber/epoxy composite, fiberglass/epoxy composite, Kevlar/epoxy composite, polyimide, polyamide, polyester, polyvinylidene fluoride, polypropylene, polyethylene, and urethane.
An actuator array includes two or more of any of the above actuators in the flexible substrate. The flexible substrate can be in the form of a tape including the array of actuators as a linear array; or, the flexible substrate can be in the form of a sheet including the array of actuators as a two-dimensional array. The array can include one or more multiplexing diodes to independently control each actuator. The array can include an open loop automated controller coupled to the actuators.
A method of operating the actuator includes automatically controlling the actuator by heating the first shape memory element to exert a force that actuates the bistable mechanism from a first position to a second position. A second heat source can be heated to actuate a second shape memory element to exert a force that actuates the bistable mechanism from the second position to the first position. In various embodiments, the shape memory elements can be at ambient temperature while the bistable mechanism maintains the first position or the second position, e.g., the heat sources can be deactivated after actuating the actuator.
A method of operating the actuator array includes automatically, independently controlling each actuator.
A method of manufacturing a shape memory element includes wrapping a shape memory wire and an adhesive substrate on a spool to create a layer of substantially physically parallel wire loops adhered to the adhesive layer, and separating a discrete shape memory wire element, the element including an array of substantially physically parallel shape memory wire segments adhered to a discrete portion of the adhesive substrate. The adhesive substrate can include a pattern that defines each discrete shape memory element. The method can include separating each discrete shape memory wire element by mechanical cutting, or by laser cutting. The method can include wrapping the adhesive substrate on the spool and wrapping the wire on the adhesive substrate to contact the wire to the adhesive layer; or, the method can include wrapping the wire on the spool, and wrapping the adhesive substrate on the wire to contact the adhesive layer to the wire. The method can include curing the adhesive layer of the adhesive substrate to create a laminated shape memory element. The wire segments of each discrete shape memory wire element can be cured in a curable matrix to create the laminated shape memory element. The method can include stenciling a conducting adhesive between at least two of the wire segments, whereby the wire segments are conductively linked.
The disclosed inventions have numerous advantages over the prior art. For example, the method of manufacturing the shape memory elements from wire is less expensive than other methods such as sputtering and etching, and creates fewer environmental hazards. Mechanically cutting the wires allows the elements to function without re-annealing, which also allows the use of substrates such as non-polyamide polymers, with melting temperatures below the annealing temperature of shape memory alloys.
Moreover, the separate wires can have more surface area which can allow better contact with laminating adhesive to avoid wire pull-out, and can dissipate heat more rapidly compared to larger pieces of shape memory alloy. High surface area per unit volume can allow a higher actuation frequency.
Also, the shape memory elements in the disclosed inventions are discrete. Compared to devices wherein adjacent actuators are formed from a continuous piece of shape memory alloy, the disclosed inventions can be more isolated and thus can experience less thermal cross talk.
Moreover, coupling two shape memory elements with a bistable mechanism allows the actuator to maintain a position while shut off after actuation, which can minimize power consumption and heat production compared to existing devices. This can be particularly beneficial for devices intended to operate in power or temperature sensitive environments, such as handheld massagers, massage chairs, massaging foot spas, massaging car seat covers, and similar products intended to operate near the human body.
Also, the elastic energy stored in the bistable mechanism can provide a restoring force to return a shape memory element to its original length after contraction.
Further, the bistable mechanism can transform the short (4%) contraction typical of shape memory alloys into a displacement large enough to be useful, when shape memory elements are coupled to the compression beam of the bistable mechanism; or can transform the contraction into a shorter but more forceful motion when the shape memory elements are coupled to the tension beam.
Another benefit of the bistable mechanism is that it enables simple, robust, open-loop control, whereas other devices can require complex closed-loop control because the resistance and Young's modulus of shape memory alloys change nonlinearly with heating, and the work cycle has hysteresis.
Yet another benefit of the bistable mechanism is that mechanisms of different orientations, size, mechanical characteristics, spacing, and the like can be combined in the same array of actuators.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. A description of preferred embodiments of the invention follows.
Flexible substrate 102 can include a material selected from the group consisting of steel alloy, phosphor bronze alloy, aluminum alloy, titanium alloy, carbon fiber/epoxy composite, fiberglass/epoxy composite, Kevlar/epoxy composite, polyimide, polyamide, polyester, polyvinylidene fluoride (PVDF), polypropylene, polyethylene, polyurethane, and the like.
Bistable actuator 100 includes a separate shape memory element 110, which can be made of a shape memory alloy, a bimetallic strip, thermally-actuated shape memory polymers (such as styrene-based thermally-actuated shape memory polymers and oligo(e-caprolactone) dimethacrylate+n-butyl acrylate thermally-actuated shape memory polymers), or the like.
Typically, shape memory element 110, can be made of a shape memory alloy. As used herein, shape memory alloy can include any such alloy know to the art, for example, NiTi, CuZnAl, CuAlNi, and the like. More preferably, shape memory element 110 is NiTi.
As used herein, a bimetallic strip can be any combination of two metals that expand differently in response to increasing temperature. Such strips are well-known to the art, for example, bimetallic strips employed in thermostats, and the like. Shape memory element 110 can be a bimetallic strip separate from flexible substrate 102. Or, flexible substrate 102 can function as one metal of the bimetallic strip. In embodiments wherein two shape memory elements, as bimetallic strips, are employed on opposing sides of the bistable mechanism, flexible substrate 102 can function as one metal of each bimetallic strip.
Shape memory element 110 can be actuated by heating, e.g., by resistive heating through a current passed through the element, by application of a voltage, by thermal contact with a separate heating element, by radiative or convective heat transfer from an external heat source, or the like. Typically, shape memory element 110 is actuated by heating with a resistive heating element that is in thermal contact with the shape memory element.
Shape memory element 110 can be in the form of a strip or sheet, or more preferably is in the form of a set of substantially physically parallel shape memory alloy wires, typically in a laminated array; see also features 510/511 in
In various embodiments, the shape memory element can be actuated by resistive heating through a current passed through the element, wherein typically, the element is in the form of a set of at least two substantially physically parallel shape memory alloy wires 12′ as shown in shape memory element 110′ in
The frequency of operation of shape memory devices such as shape memory element 110 can be determined in part by the ability of the device to dissipate heat. For example, shape memory element 110 must be below its critical temperature before it can be actuated again. Thus, when shape memory element 110 is in the form a set of substantially physically parallel shape memory alloy wires, the extra surface area of the wires and the distance between adjacent wires can allow it to dissipate heat more rapidly than if shape memory element 110 was the same mass of shape memory alloy in a monolithic form such as a strip or sheet. Thus, in preferred embodiments, shape memory element 110 can be actuated at a frequency at 25° C. in air of at least about 2 cycles per second.
Shape memory elements 510 and 511 can be mechanically coupled to opposite sides of compression beam 108 to convert the displacement of each shape memory element into a greater displacement at the compression beam.
The actuator can be adapted for automatic control. For example, the shape memory element can be coupled to an open loop automated controller 526. Dielectric layers 528 and 530 can be included that can separate heat sources 524 and 525 from flexible substrate 102, e.g., when flexible substrate 102 is a conductor. Electrical leads 532 can be included to power heat sources 524 and 525. Leads 532 can be coupled to. automated controller 526.
Actuator 500 can be automatically controlled by operating controller 526 (e.g., an open loop automated controller) to heat first shape memory element 510 via heat source 524 to exert a force that actuates the bistable mechanism from a first position to a second position. The second shape memory element can be heated via heat source 525 to exert a force that actuates the bistable mechanism from the second position to the first position. Heat sources 524 and 525 can be deactivated after actuating the mechanism between the first and second positions after actuation, e.g., the shape memory elements can be at ambient temperature while the bistable mechanism maintains the first position or the second position. Controller 526 can be employed to control the displacement of shape memory elements 510 or 511 to give a greater displacement at compression beam 108, wherein shape memory elements 510 or 511 are mechanically coupled to compression beam 108. Controller 526 can be employed to operate the actuator at a frequency at 25° C. in air of at least about 2 cycles per second.
When bistable mechanisms or actuators are elements in an array, the elements can be the same or different in size, construction, mechanical characteristics, orientation, spacing, and the like. For example, in typical embodiments, such array elements are the same size, have the same mechanical characteristics, are oriented in the same direction, are regularly spaced on the array, as depicted in arrays 600 and 700 in
The wire can be wrapped on the spool at a desired pitch or loop spacing using, e.g., micro-controlled rotary and linear stages. The pitch or loop spacing can be set so that the ratio of the diameter of the wires divided by the distance between adjacent wire loops on the spool is less than about 1. The wires can include a shape memory alloy selected and sized as described above under
The combination of spooled wire 942 on adhesive substrate 944 can be removed from the spool 946 by cutting via a keyway 956 in the spool.
The method can include separating discrete shape memory wire elements 950, 950′, 950″ . . . by mechanical or laser cutting, preferably mechanical cutting. Discrete shape memory wire elements 950, 950′, 950″ . . . each include an array of substantially physically parallel shape memory wire segments 952 adhered to the adhesive substrate
The adhesive layer of adhesive substrate 944 can be cured to create laminated shape memory element 910. Or, wire segments 952 of each discrete shape memory wire element 950 can be cured in a separately applied curable matrix (e.g., a polymer curable by exposure to radiation (light, heat, ultraviolet light, electron beam radiation) curing agents, catalysts, and the like) to create laminated shape memory element 910. For example, the wire element 950 can be aligned in a jig, at least a portion of the wire segments 952 can be embedded in a uncured adhesive matrix, which can be applied with a stencil; the adhesive can be cured; and the laminated shape memory element 910 can be released, optionally with solvent.
In another embodiment, adhesive substrate 944 can include a pattern that defines each discrete shape memory element, e.g., a pre-printed or pre-cut pattern that facilitates separating discrete shape memory wire elements 950, 950′, 950″ . . . . For example, the pattern can be an interdigitating pattern that can lead to less waste, as shown by dotted line 958. The pattern can preferably be laser cut. Moreover, the interdigitating pattern is cut, two interdigitating strips of adhesive substrate comprising discrete shape memory wire elements 950, 950′, 950″ are released. These interdigitating strips can be mounted on an alignment frame 960 which can facilitate alignment of the elements for application of the curable adhesive matrix to each element. After curing, individual laminated shape memory elements 910 can be separated and released. Or, alignment frame 960 can hold the discrete shape memory wire elements 950, 950′, 950″ and the uncured matrix in a position to contact an array of bistable mechanisms (e.g., arrays 600 and 700 in
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
The invention was supported, in part, by grant N66001-02-C-8802 from the Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in the invention.
