John A. Redmond

Behavioral Model and Experimental Validation for a Spool-Packaged Shape Memory Alloy Actuator

Shape memory alloy (SMA) based actuators have the potential to be lower mass, more compact, and more simplistic than conventional based actuators (electrical, hydraulic, etc); however, one of the key issues that plagues their broad use is packaging since long lengths of wire are often necessary to achieve reasonable actuation strokes. Spooling the wire around pulleys or mandrels is one approach to package the wire more compactly and is useful in customizing the footprint of the actuator to the available application space. There is currently a lack of predictive models for actuator designs with spooled packaging that account for the variation of stress and strain along the wires length and the losses due to friction. A spooling model is a critical step toward the application of this technique to overcome the packaging limitations on SMA actuators. This paper presents the derivation of an analytical predictive model for rotary spooled SMA actuators that accounts for general geometric parameters (mandrel diameter, wire length, wire diameter, and wrap angle), SMA material characteristics, loss parameters (friction), and the external loading profile. An experimental study validated the model with good correlation and provided insight into the effects of load and wrap angle. Based upon the model and experimental results, the main limitation to this approach, binding, is discussed. The analytical model and experimental study presented in this paper provide a foundation to design future actuators and insight into the behavioral impact of this packaging technique.

Spool-packaging of shape memory alloy actuators: Performance model and experimental validation:

Shape memory alloy (SMA) actuators in the wire form are attractive because of their simplistic architecture and electrical operation, and their manufacturability at high yields and low cost. While SMA actuators are known for their superior work density among smart materials, packaging long lengths of SMA wire needed for moderate to large motions is an ongoing technical challenge. This article investigates spooling as a packaging approach to provide more compact actuator footprints. An analytical, quasi-static model is derived to provide a foundational tool for the analysis and synthesis of spool-packaged SMA wire actuators. The model predicts motion with respect to a generalized architecture, and specifiable geometric, material, and loading parameters. The model prediction accounts for the effects of local friction loss and bending strains, and for a “binding” limitation due to accumulated friction. An experimental validation study demonstrates the model’s ability to predict actuator motion well in terms of form and magnitude with respect to load and packaging geometry. This model provides a basis for a systematic application of spooled-packaging techniques to overcome packaging limitations of SMA, positioning SMA wire actuators as a viable alternative in many applications.

The design and experimental validation of an ultrafast shape memory alloy resettable (SMART) latch

Latches are essential machine elements utilized by all sectors (military, automotive, consumer, manufacturing, etc.) with a growing need for active capabilities such as automatic release and reset, which require actuation. Shape memory alloy (SMA) actuation is an attractive alternative technology to conventional actuation (electrical, hydraulic, etc.) because SMA, particularly in the wire form, is simple, inexpensive, lightweight, and compact. This paper introduces a fundamental latch technology, referred to as the T-latch, which is driven by an ultrafast SMA wire actuator that employs a novel spool-packaged architecture to produce the necessary rotary release motion within a compact footprint. The T-latch technology can engage passively, maintain a strong structural connection in multiple degrees of freedom with zero power consumption, actively release within a very short timeframe (<20 ms, utilizing the SMA spooled actuator), and then repeat operation with automatic reset. The generic architecture of the T-latch and governing operational behavioral models discussed within this paper provide the background for synthesizing basic active latches across a broad range of applications. To illustrate the utility and general operation of the T-latch, a proof of concept prototype was designed, built, and experimentally characterized regarding the basic functions of engagement, retention, release, and reset for a common case study of automotive panel lockdown. Based on the successful demonstration and model validation presented in this study, the T-latch demonstrates its promise as an attractive alternative technology to conventional technologies with the potential to enable simple, low-cost, lightweight, and compact active latches across a broad range of industrial applications.

Effect of Bending on the Performance of Spool-Packaged Shape Memory Alloy Actuators

Shape memory alloy (SMA) actuation is becoming an increasingly viable technology for industrial applications as many of the technical issues that have limited its use are being addressed (speed of actuation, mechanical connections, performance degradation, quality control, etc.) while increasing production capacities drive costs to practical levels. Shape memory alloys are often selected because of their high energy density which can lead to compact actuators; however, wire forms with small cross-sectional diameters tend to be long (10 to 50 times the length of required stroke). Spooling the wire can be used for compact packaging, but as the spool diameter decreases performance losses and fatigue increase due to bending strains and stresses. This paper presents a simple, design-level model for spooled SMA wire actuators with linear motion outputs that includes the effects of friction and wire bending and accounts for the actuator geometry, applied load, and material friction and constitutive properties. The model was validated experimentally with respect to the ratio of mandrel to SMA wire diameter and agrees well in both form and magnitude with experiments. The resulting model provides the framework for the analysis and synthesis of spooled SMA wire actuators to guide the selection of design parameters with respect to the tradeoffs between performance and packaging.

The Design and Experimental Validation of an Ultrafast SMART (SMA Resettable) Latch

Latches are an essential machine element utilized by all sectors (medical, military, industrial, etc.) and there is a growing need for active latches with automatic release and reset capabilities. Shape memory alloy (SMA), due to its high energy/power densities, is an attractive alternative actuation approach to conventional methods (electrical, hydraulic) because it is inexpensive, lightweight, compact and has a fast heating response times. This paper introduces the T-latch which is based upon a compact spooled SMA rotary actuator. The T-latch can engage passively, maintain a structural connection in multiple degrees of freedom with zero power consumption, actively release very quickly (< 20 ms) and then repeat operation with automatic reset. To provide the basis to apply this latch across sectors, operational behavioral models are summarized for the key states of engagement, retention, release and reset. To demonstrate the technology, a proof-of-concept prototype for automotive panel lockdown was designed, built and experimentally characterized for the basic operational states along with studies of the effects of power, seal and reset force. The results from this study indicate promising suitability of the T-latch technology for a broad range of industrial applications.Copyright

Active distributed attachment surfaces: Distributed latching technique and demonstration

Latches are essential mechanical elements used to controllably connect multiple bodies throughout industry. Latches conventionally attach bodies at a single point or at a few discrete points, and are designed for multiple operation cycles. Actuator controlled latches, minimize the amount of complexity and costs involved with installation and removal by making the attachment between structures controllable, tool-free and fast. However, the use of multiple single point fasteners carries additional part count, material costs, and labor associated with installation and removal, and can creating load concentrations at attachment sites. Alternatively, surface attachments, such as traditional hook and loop mechanisms, distribute structural connection over an area or across many points reducing stress concentrations, allowing engagement of multiple bodies, maintaining structural connections in nonspecific locations and orientations and reducing labor costs to install and detach bodies. However, performance limitations of conventional surface attachments, such as low retention force, restrict potential applications. An active distributed attachment technique has the potential to increase the performance of conventional distributed attachments, as well as overcome the complexity and operation of conventional point attachments. This paper introduces three active distributed latch approaches (Pegboard, Interlocking Teeth, and Active Velcro) that utilize lightweight, compact SMA actuation. Proof-of-concept prototypes were built, and tested experimentally to investigate the engagement, retention, and release performance. The best performing of the three is demonstrated in a full application scale. The first generation prototypes improved upon the performance of conventional surface attachments and show promise in maintaining the necessary structural attachment for industrial applications.Copyright